i

Draft

Technical Support Document for 

HWC MACT Standards

U.S. Environmental Protection Agency

Office of Solid Waste and Emergency Response (5305)

401 M Street, SW

Washington, D.C. 20460

February 1996

Volume I:

Description of Source Categories
ii

ACKNOWLEDGMENTS

This document was prepared by the U.S. EPA’s Office of Solid Waste, Waste Management

Division. Energy and Environmental Research Corporation (EER) provided technical support under

EPA Contract No. 68-D2-0164.
iii

ABSTRACT

EPA is revising hazardous air pollutant (HAP) emissions regulations for hazardous waste

burning combustors, specifically incinerators, cement kilns, and light weight aggregate kilns. The

new emissions standards are being developed using the “maximum achievable control technology”

(MACT) approach defined in Title 3 of the 1990 Clean Air Act Amendments. MACT standards are

set for the hazardous air pollutants (HAP) of PCDD/PCDF, mercury, semi volatile metals (cadmium

and lead), low volatile metals (antimony, arsenic, beryllium, and chromium), total chlorine

(considering both HCl and Cl2), CO and HC (not strictly HAPs, but used as surrogates for toxic

organic HAPs), and particulate matter (again, not itself strictly a HAP, but used as a surrogate for

both condensed metals and organics). This volume provides a description of source categories.

Provided are: (1) process descriptions of major design and operating features; (2) description of air

pollution control devices including design principles, performance, and operating efficiency, process

monitoring, and upgrade/retrofit options; and (3) major source determination for all sources including

a discussion on the methodology used to estimate annual emissions, assumptions used, and an

emissions summary for each source listing total HAP emissions.
iv

TABLE OF CONTENTS

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

2.0 Process Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Hazardous Waste Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Rotary Kiln Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

Operating Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

Liquid Injection Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5

Fluidized Bed Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Operating Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Fixed Hearth Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Operating Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

Air Pollution Control Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

Cement Kilns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10

Process Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10

Wet Process Kilns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

Dry Process Kilns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

Semi-Dry Process Kilns . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13

Clinker Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13

Hazardous Waste Feeding Techniques . . . . . . . . . . . . . . . . . . . . . . 2-14

Air Pollution Control System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-15

Lightweight Aggregate Kilns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-16

Process Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-16

Operating Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-18

3.0 Air Pollution Control Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Cyclones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Fabric Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Electrostatic Precipitators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Wet Electrostatic Precipitators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

Venturi Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

Flux Force Condensation Collision . . . . . . . . . . . . . . . . . . . . . . . . 3-16

Free Jet Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17

Rotary Atomizing Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17

Rotor Filter Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17

Electrodynamic Venturi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18

Wet Scrubbers 3-20

Spray Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21

Tray Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21

Packed Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22

Ionizing Wet Scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22

Dry Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26

Spray Dryers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27
v

Flue Gas Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-30

Activated Carbon Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23

Other Metals Control Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-35

Activated Carbon Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-36

Earth-based Sorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-40

Selenium Coated Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-41

Sodium Sulfide Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-41

Mercury Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-42

Catalytic Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-42

Sulfur Based Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-42

Air Pollution Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-43

Wet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-43

Dry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-44

Hybrid Dry/Wet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-44

4.0 Major Source Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Incinerator Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Kiln Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-1
vi

LIST OF FIGURES

2-1 Typical rotary kiln incinerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-19

2-2 Typical liquid injection incinerator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20

2-3 Typical fluidized bed incinerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21

2-4 Typical fixed hearth incinerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-22

2-5 Wet cement kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-23

2-6 Typical gas (dashed line) and solids (solid line) temperature profiles for a long

wet-process cement kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-24

2-7 Typical gas velocities for a long wet process cement kiln . . . . . . . . . . . . . . . . . . . 2-25

2-8 Dry kiln process material (solid line) and gas temperatures (dashed line). . . . . . . 2-26

2-9 Dry preheater/precalciner cement kiln schematic . . . . . . . . . . . . . . . . . . . . . . . . . 2-27

2-10 Typical gas (dashed line) and solids (solid line) temperature and gas velocities

for a preheater cement kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-28

2-11 Typical gas velocities through a preheater cement kiln . . . . . . . . . . . . . . . . . . . . . 2-29

2-12 Typical gas (dashed line) and solids (solid line) temperature for a preheater/

precalciner cement kiln. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-30

2-13 Semi-dry cement kiln schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-31

2-14 Semi-dry kiln material (solid line) and gas (dashed line) temperatures. . . . . . . . . 2-32

2-15 Light weight aggregate kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33

3-1 Cyclone particle collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-45

3-2 Pulse jet cleaning baghouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-46

3-3 Reverse air cleaning baghouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-47

3-4 Shaker cleaning baghouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-48

3-5 Electrostatic precipitator particle collection procedure . . . . . . . . . . . . . . . . . . . . . 3-49

3-6 One and two-stage electrostatic precipitator operation . . . . . . . . . . . . . . . . . . . . . 3-50

3-7 Plate type ESP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-51

3-8 Tubular type ESP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-52

3-9 Wet ESP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-53

3-10 Variable throat venturi scrubber with wetted elbow . . . . . . . . . . . . . . . . . . . . . . . . 3-54

3-11 Performance principles of a venturi scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-55

3-12 Non-wetted, fixed throat venturi scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-56

3-13 Force flux/condensation/collision scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-57

3-14 Free jet scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-58

3-15 Venturi scrubber with different flow control throat mechanisms . . . . . . . . . . . . . . 3-59

3-16a Two-stage wet scrubbing process schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-60

3-16b Venturi and tray tower wet scrubbing system arrangement . . . . . . . . . . . . . . . . . . 3-61

3-18 Tray tower wet scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-62

3-19 Packed bed wet scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-63

3-20 Ionizing wet scrubber schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-64

3-21 Limestone reagent preparation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-65

3-22 Impact of slurry pH (acidity) on SO2 removal efficiency . . . . . . . . . . . . . . . . . . . 3-66

3-23 Impact of liquid to gas ratio on SO2 removal efficiency . . . . . . . . . . . . . . . . . . . . 3-66

3-24 Dry scrubbing sorbent injection process schematic . . . . . . . . . . . . . . . . . . . . . . . . 3-67

3-25 Semi-dry spary drier absorption process schematic . . . . . . . . . . . . . . . . . . . . . . . . 3-68

3-26 Spray dryer designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-69

3-27 Air dilution cooling procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-70

3-28 Example of the amount of dilution air required for dilution cooling . . . . . . . . . . . 3-71

3-29 Heat exchanger schematics (a) Gas-gas recuperator (b) Shell and tube type. . . . . 3-72

3-30 Water quench cooling procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-73

3-31 Example of amount of water required to cool flue gas. . . . . . . . . . . . . . . . . . . . . . 3-74

3-32 Hybrid wet/dry system schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-75
vii

4-1 Estimated annual HAP emissions for incinerators including HCl and Cl2 . . . . . . . . . . 4-5

4-2 Estimated annual HAP emissions for incinerators excluding HCl and Cl2 . . . . . . . . . . 4-6

4-3 Estimated annual HAP emissions for kilns including HCl and Cl2 . . . . . . . . . . . . . . . . . 4-7

4-4 Estimated annual HAP emissions for kilns excluding HCl and Cl2 . . . . . . . . . . . . . . . . . 4-8
viii

LIST OF TABLES

3-1 Sorbents Used for Removal of Metals from Combustion Source Flue Gases . . . . 3-76

4-1 Major Source Determination Summary for Commercial and On-Site Incinerators . 4-9

4-2 Major Source Determination Summary for Cement and LWA Kilns. . . . . . . . . . . 4-10
ix

ACRONYMS

AB Afterburner

ACS Acid scrubber

APCD Air pollution control device

BIF Boiler and Industrial Furnace

C Cyclone

CAAA Clean Air Act Amendment

CCS Calvert collision scrubber

CKD Cement kiln dust

CO Carbon monoxide

DA Dilution air

DI Dry injection

DM Demister

EDV Electrodynamic venturi

EPA U.S. Environmental Protection Agency

ESP Electrostatic precipitator

FF Fabric filter

FSI Furnace sorbent injection

GC Gas cooler

HAP Hazardous air pollutant

HC Hydrocarbons

HE Heat exchanger

HES High energy scrubber

HF HEPA filter

HS Hydrogen chloride scrubber

HTHE High temperature heat exchanger

IWS Ionizing wet scrubber

KOV Knock out vessel

LTHE Low temperature heat exchanger

LWA Light weight aggregate

LWAK Light weight aggregate kiln

MACT Maximum achievable control technology

PBC Packed bed condenser

PCDD/PCDF Polychlorinate dibenzofurans and dibenzo-p-dioxins

PIC Product of incomplete combustion

PM Particulate matter

Q Quench

QC Quench column

RCRA Resource Conservation and Recovery Act

S Scrubber

SCA Specific collection area

SDA Spray dryer absorber

VQ Venturi quench

VS Venturi scrubber

WHB Waste heat boiler

WS Wet scrubber
1-1

SECTION 1

INTRODUCTION

The U.S. Environmental Protection Agency (EPA) regulates the burning of hazardous waste

in incinerators under 40 CFR Part 264/265, Subpart O and in industrial furnaces under 40 CFR Part

266, Subpart H. The Agency is proposing revised regulations applicable to these hazardous waste

combustion (HWC) devices. This document provides the description of the source categories and

the associated pollution control devices currently in use or applicable to the process. It is the first

in a series of seven volumes of technical background documents for the rule. These include:

Technical Support Document for HWC MACT Standards, Volume I: Description of

Source Categories, which provides process descriptions of major design and operating

features including different process types and air pollution control devices currently in use

and potentially applicable to various combustion source categories; description of air pollution

control devices including design principles, performance and operating efficiency, process

monitoring options, and upgrade/retrofit options; and major source determination for all

sources including a discussion on the methodology used to estimate annual emissions,

assumptions used, and an emissions summary for each source listing total HAP emissions.

Technical Support Document for HWC MACT Standards, Volume II: HWC Emissions

Data Base, which contains a summary of the emissions information on toxic metals,

particulate matter (PM), HCl and Cl2, hydrocarbons, carbon monoxide, semi-volatile and

volatile organic compounds, and dioxins/furans from HWCs. Other detailed information

encompassed in the data summary include company name and location, emitting process

information, combustor design and operation information, APCD design and operation

information, stack conditions during testing, feed stream feed rates, and emissions rates of

HAPs by test condition.

Technical Support Document for HWC MACT Standards, Volume III: Selection of

Proposed MACT Standards and Technologies, which identifies the MACT floor for each

HAP and source category for existing sources and new sources and discusses the approach

used to define the floor and beyond-the-floor alternatives considered for the proposed rule.

Technical Support Document for HWC MACT Standards, Volume IV: Compliance with

the Proposed HWC Standards, which contains detailed discussions of continuous emissions

monitors and operating limits for the proposed rule.

Technical Support Document for HWC MACT Standards, Volume V: Engineering Costs,
1-2

which contains the cost estimates for APCD requirements for existing and new facilities to

meet the proposed emissions standards.

Technical Support Document for HWC MACT Standards, Volume VI: Development of

Comparable Fuels Specifications, which summarizes the composition including hazardous

species in benchmark fossil fuels such as gasoline, #2 fuel oil, #4 fuel oil, and #6 fuel oil.

This information is being used to develop specifications which EPA is considering to allow

comparable fuels to be excluded from the definition of hazardous waste.

Technical Support Document for HWC MACT Standards, Volume VII: Miscellaneous

Technical Issues, which provides additional information on several topics such as the

treatment of measurements below analytical detection limits, the procedures for handling

missing data, and the rationale for grouping metals of similar volatility. The impact of these

methodologies on the proposed MACT limits, the cost estimates, and the national emissions

estimates are also discussed.

Emission standards are being proposed for three types of hazardous waste incineration

facilities:

• Cement Kilns

• Lightweight Aggregate Kilns

• Incinerators (On-site and Commercial)

The hazardous air pollutants for which emission standards are proposed are:

• Mercury (Hg)

• Low Volatility Metals (LVM)

• Semi-Volatile Metals (SVM)

• Particulate Matter (PM)

• Hydrogen Chloride and Chlorine as Total Chlorine (HCl/Cl2)

• Carbon Monoxide (CO)

• Hydrocarbons (HC)

• Dioxins/Furans (PCDD/PCDF)

These emission standards are being developed through the “maximum achievable control

technology” (MACT) approach defined in Title 3 of the 1990 Clean Air Act Amendments (CAAA).

In this approach the MACT floor standard for existing facilities is established at the level of the

average performance of the best 12% of existing sources. Depending on the additional benefits and

costs, EPA may elect to set more stringent, but technically achievable, beyond-the-floor (BTF)

standards for specific HAPs. In the proposed rule, BTF standards have been set for PCDD/PCDF

and for Hg for all HWC source categories, and BTF standards have been proposed for HCl/Cl2 for

LWAKs only.

The proposed floor and BTF standards have been selected based on a database (described in

Volume II) of trial burn and compliance test emissions measurements from 77 incinerators, 35

cement kilns, and 12 lightweight aggregate kilns using a process described in detail in Volume III.
1-3

In this volume, the three different source categories (hazardous waste incinerators, cement

kilns, and light weight aggregate kilns) are described. This description includes process operating

characteristics. Additionally, the air pollution control systems that are used to control the hazardous

air pollutants (HAPs) are discussed. The report also describes the methodology used to determine

major sources within the hazardous waste combustor community.
2-1

SECTION 2

PROCESS DESCRIPTIONS

Incineration has developed as a means of treating various types of waste materials, including

hazardous wastes, municipal wastes, medical wastes, and sewage sludges. Incineration may be

effective for destroying hazardous organic wastes and reducing waste volume and mass, thus aiding

in safe ultimate disposal. Additionally, the heat generated from waste combustion may be used to

produce electricity or process heat, or may be used to fuel industrial processes such as cement or

light weight aggregate production or industrial boilers. Three important operating conditions for

proper incineration are incineration temperature, gas residence time at temperature, and waste,

supplemental fuel, and combustion air mixing and turbulence rates. These conditions vary with the

chemical structure and physical form of the waste as well as the incinerator design. Process

descriptions for equipment which burns hazardous waste are included in this section. These include

hazardous waste incinerators, cement kilns, and lightweight aggregate kilns. Process components,

characteristics, and operating parameters are discussed for each of the different types of units.

2.1 HAZARDOUS WASTE INCINERATORS

Hazardous waste incinerators are used at both “commercial” and “on-site” hazardous waste

treatment facilities. Commercial facilities accept and treat wastes, for a tipping fee, which have

been generated off-site. The sole purpose of commercial incinerators is to generate a profit from

treating hazardous wastes. There are approximately 22 commercial hazardous waste incineration

facility sites run by 17 companies in the United States. On-site facilities treat only wastes which

have been generated at the facility to avoid the costs of off-site treatment. There are approximately

168 on-site hazardous waste incineration facilities in the United States.

Different types of technologies are used for hazardous waste incineration; the majority of

which include rotary kilns, liquid injection incinerators, fluidized bed incinerators, and fixed hearth

incinerators. The majority of the commercial incinerators are rotary kilns. Additionally, there are

three liquid injection, one fluidized bed, and four fixed hearth facilities. The on-site facilities

consist of approximately equal numbers of rotary kilns and liquid injection facilities, with a few

additional fixed hearths and fluidized beds. The following incinerator designs are discussed in this

section: rotary kiln, fluidized bed, liquid injection, and fixed hearth. In general, each of the systems

have common exhaust gas air pollution control requirements. Air pollution control devices and

systems used on hazardous waste incinerators are discussed in the Section 2.1.5. Specific design,

operating, and performance characteristics of the individual air pollution control devices is discussed

in detail in Section 3.
2-2

2.1.1 Rotary Kiln Incinerators

Rotary kilns are the commonly used technology for hazardous waste incineration. This is

due to their ability to handle almost any conceivable waste form (e.g., bulk and containerized

solids, sludges, slurries, bulk and containerized liquids, and less common wastes such as Department

of Defense propellants, ammunitions, and nerve agents) as well as a variety of waste compositions

(e.g., wide ranges of waste organics, halogens, heating value, and principal organic hazardous

constituents).

2.1.1.1 Process Description

Rotary kiln systems used for hazardous waste incineration typically consist of two incineration

chambers: the rotary kiln and an afterburner. A typical rotary kiln system is shown in Figure 2-1.

The rotary kiln itself is a cylindrical refractory-lined steel shell typically with a diameter which is

typically less than 15 to 20 feet (to allow for truck or rail shipment) and a lengthto-diameter ratio of

2:1 to 10:1. The shell is supported by two or more steel “trundles” that ride on rollers, allowing the

kiln to rotate around its horizontal axis. The refractory is typically made of an acid resistance brick.

Rotary kiln incinerators are typically sized around 60 million Btu/hr, but may be as large as 150

million Btu/hr.

Usually, the inside of the kiln is lined with a smooth refractory, although recent designs

have included internal vanes or paddles to encourage solids mixing along the kiln length. The kiln

is oriented on a slight incline from the horizontal, known as the “rake”. The rake is less than 5° and

typically ranges from 2 to 4°. The kiln rotation rate typically ranges from 0.5 to 2 rotations per

minute. Mixing may be improved by increased rotation rate; however, this also acts to reduce

solids residence time. Kiln rotation and incline serve to promote the mixing of wastes with

combustion air, as well as to facilitate the heat transfer between the waste and hot flames and

refractory, and to move the wastes through the rotary section.

Almost all hazardous waste rotary kiln incinerators are of the “co-current” design because

they are better suited toward treating combustible solid wastes. In the co-current design, as shown

in Figure 2-1, wastes and auxiliary fuels are fed at the same upper kiln end (i.e., the waste and flue

gas travel in the same direction). Co-current designs provide for rapid ignition of the cold wastes

and maximum gas residence time for the products of combustion and thus achieve the largest

amount of volatile organic destruction in the rotary section.

Solid and liquid wastes are fed directly into the rotary kiln. Solids can be fed, on a continuous

or semi-continuous basis, through a variety of waste feed mechanisms such as a ram feeder, auger

screw feeder, or belt feeder for waste contained in drums. For batch feeding, an air lock is usually

used to reduce the amount of air infiltration through the feeding chute. Liquid wastes may also be

injected, with steam or air assisted atomizing nozzles, directly into the kiln through the main burner.

These liquid wastes may also be injected through a waste lance, and/or mixed with the solid wastes.

In most cases, rotary kilns can handle unprocessed wastes. However, waste pretreatment may

include mixing of liquid and solid wastes, as well as neutralizing corrosive wastes prior to being

fed to the kiln. Waste grinding and sizing may aid in smooth kiln operation.
2-3

Wastes are heated by the primary flame, bulk gases, and refractory walls. Through a series

of volatilization and partial combustion reactions, combustible fractions of the wastes are gasified.

The solids continue to heat and burn as they travel down the kiln. Typically, solids retention time in

the kiln is 0.5 to 1.5 hours, while gas residence time through the kiln is usually around 2 seconds.

Waste feed to the kiln is controlled so that the waste contributes no more than 20% of the kiln

volume. Typical flame/solids temperatures in the rotary kiln range from 1200 to 3000°F.

An auxiliary gas or oil burner, located at the feeding end of the kiln, is used for start-up

(bring unit up to temperature) and to maintain desired kiln temperature when sufficient heat input is

not available from the waste. Wastes with an average heating value of 4500 Btu/lb are typically

adequate to sustain combustion at kiln temperatures between 1600 to 1800°F. Combustion air is

provided through ports on the face of the kiln, as well as through rotary seal leakage. The kiln

typically operates at 50 to 200% excess air levels.

Kiln operating pressure is maintained at negative to the atmosphere, typically -0.5 to 2.0 in.

H2O gauge, through the use of an induced draft fan which is located downstream of the air pollution

control system. Operation at negative pressure assures that fugitive emissions of combustion gases

to the atmosphere through rotary seal leaks are avoided. Seals on ends of the rotary section provide

possible kiln gas leakage points. There is potential for kiln gas fugitives to escape through leaks in

rotary seals when excessive kiln pressure develops. This “over-pressure” can occur when highly

combustible or explosive wastes are fed into the kiln. The resulting increase in combustion gas

volume can not be handled by the downstream exhaust system, creating an over-pressure of the

kiln. Also, if seals become worn or damaged, ambient air infiltration through them may make it

difficult to maintain temperature at an acceptable kiln gas flow rate.

Overcharging of highly combustible or explosive wastes may also lead to kiln “puffing”.

Volatiles are released quickly at high temperature thus depleting local oxygen, creating pyrolysis

conditions and the formation of products of incomplete combustion. Higher rotations rates also

increase the puff magnitude because devolatilization rates are enhanced by bed mixing. Puffing

can be avoided by operating at a charging rate which will reduce the hydrocarbon release rate to

avoid consuming available oxygen. Also, transient puffs can be minimized by operating at lower

rotational speeds and lower kiln temperatures.

To avoid over-pressurization caused by feeding highly combustible wastes, a thermal

emergency relief vent, typically located between the afterburner and the air pollution control system,

is used to protect the equipment. Flue gases are released through this vent when overpressurizations

occur. The use of the emergency relief vent is minimized to avoid the direct release of untreated

flue gases to the atmosphere (i.e., flue gases which have not passed through the air pollution control

system).

Inorganics, ash, slag, and other incombustible items that remain when the waste reaches the

end of the kiln are removed by gravity into an ash pit. Typically, the ash is water quenched with the

ash removal system normally provided with a water seal trough. Dry collection systems allow for

undesirable air leakage into the kiln. Dropping of solids into the water seal trough can result in

steam explosions, often resulting in positive pressure excursions.
2-4

Exhaust gas from the kiln is usually routed to a secondary refractory lined combustion

chamber, referred to as an “afterburner”. The afterburner is typically operated at 2000 to 2500°F,

with 100 to 200% excess air, turbulent mixing flow, and a gas residence time from 1 to 3 seconds to

ensure complete combustion of the remaining volatile gas phase unburned components in the kiln

flue gas. An auxiliary fuel, and sometimes pumpable liquid hazardous wastes, are used to maintain

the afterburner temperature. A hot cyclone may be positioned between the kiln and afterburner for

removal of entrained solid particles that may cause slagging problems in the afterburner.

Design modifications to the rotary kiln have included: 1) a “fast” rotary kiln which rotates

at a rate greater than 20 rotations per minute to provide efficient mixing, 2) starved air and oxygen

assisted kilns to reduce flue gas volume and auxiliary fuel requirements, and 3) “slagging” kilns.

The slagging kiln operates above the ash melting point (2600 to 2800°F) to generate a molten ash.

Eutectic properties of slag are controlled using additives to the feed. Slagging kilns typically have

a negative rake, permitting for the accumulation of slag in the kiln. In addition, these kilns have the

ability to accept metal drums and salt laden wastes, provide better destruction efficiency and generate

lower particulate emissions compared with normal rotary kilns, and produce of a slag product

known as “frit” which is less leachable than non-slagging kiln ash. However, slagging kilns have

increased NOx emissions, shorter refractory lifetime, and potential for the slag to solidify in the

kiln, creating operational difficulties.

2.1.1.2 Operating Parameters

Rotary kiln incinerator operating parameters include:

• Kiln and afterburner exit temperature — This is typically maintained above 1500°F.

Temperature must be maintained above the minimum demonstrated for adequate destruction

of POHCs and avoidance of product of incomplete combustion (PIC) formation. Monitoring

limitations include errors in measurement from furnace and flame radiation. The temperature

may be controlled through adjustment of waste, auxiliary fuel, and combustion air feed

rates.

• Kiln pressure — Kiln pressure must be maintained at negative to atmosphere and the

afterburner pressure must be below that of kiln to prevent kiln fugitive emissions through

kiln rotary seals. Pressure is controlled by use of an induced draft fan and damper system

downstream of the combustion chambers.

• Combustion gas velocity (flowrate) — This is controlled to ensure proper combustion gas

residence time at operating temperature for complete destruction of volatile PICs. Gas

flowrate is dependent on waste feed composition and feed rate, as well as auxiliary fuel and

combustion air rates.

• Waste feed rate — This is adjusted to avoid over-loading, over-pressuring, and depleting

kiln oxygen, which may result in kiln fugitives and excessive kiln flue gas volatiles. If kiln

is fed semi-continuously, the maximum size of each batch must be controlled.
2-5

• Oxygen level at kiln and afterburner exit — Controlled to assure availability of oxygen, and

thus the potential for complete combustion (i.e., lower CO, HC, and possibly PIC levels).

The kiln is typically operated at 50 to 100% excess air, while the afterburner is typically

operated at 100 to 200% excess air.

• CO and HC combustion gas levels — CO and HC levels are monitored to ensure

satisfactory operation of kiln and afterburner and possible avoidance of PIC formation.

• Kiln solids residence time — This is typically maintained at 0.5 and 1.5 hours and is

controlled by rotation rate and kiln rake to ensure that the waste spends enough time in the

kiln to be thoroughly treated. Controlling residence time is important for treatment of

relatively non-combustible wastes.

• Kiln solids and combustion air mixing — Assures that volatiles are thoroughly combusted.

2.1.2 Liquid Injection Incinerators

Liquid injection combustors, like rotary kiln incinerators, are commonly used for hazardous

waste incineration. Liquid injection combustors can be used to dispose of virtually any combustible

liquid or liquid-like waste (e.g., liquids, slurries, and sludges).

Typical liquid injection incinerator systems, which are possibly the simplest type of

combustion device, include a waste burner system, an auxiliary fuel system, an air supply system,

a combustion chamber, and an air pollution control system. A typical liquid injection incinerator is

shown in Figure 2-2. Liquid wastes are fed and atomized into the combustion chamber through the

waste burner nozzles. These nozzles atomize the waste and mix it with combustion air. Atomization

is usually achieved either by mechanical methods such as a rotary cup or pressure atomization

systems, or by twin-fluid nozzles which use high-pressure air or steam. With a relatively large

surface area, the atomized particles vaporize quickly, forming a highly combustible mix of waste

fumes and combustion air. This mixture ignites and burns in the combustion chamber. Typical

combustion chamber residence time and temperature ranges are 0.5 to 2 seconds and 1300°F to

3000°F, respectively, in order to ensure complete liquid waste combustion. Typical liquid feed

rates are as high as 200 cubic feet per hour. If the energy content of the waste is not high enough to

maintain adequate ignition and incineration temperatures, a supplemental fuel such as fuel oil or

natural gas is provided. In some cases, wastes with high solids are filtered prior to incineration to

avoid nozzle plugging.

The liquid injection incinerator may dispose of aqueous or non-aqueous waste which can be

atomized through a burner nozzle (i.e., a viscosity less than 10000 SSU). The waste must be

atomized to small droplets of 40 μm or less. Atomization is usually accomplished mechanically

using rotary cup or pressure atomization systems. Wastes with high solids are filtered prior to

entering the feed tank. The liquid waste fuel system transfers the waste from drums into a feed

tank.
2-6

The combustion chamber can be as simple in design as a refractory-lined cylinder, or it can

be relatively complex with combustion air preheat around the chamber and firing multiple fuel

streams. The incinerator can be designed using either a horizontal or vertical orientation. In either

case, flame impingement on the combustion chamber wall is undesirable because it can lead to

refractory corrosion and loss of heat. Thus, the burner(s) are located to prevent flame impingement

on the walls.

2.1.3 Fluidized Bed Incinerators

Fluidized beds are occasionally used for hazardous waste combustion, although not nearly

as frequently as rotary kilns or liquid injection types.

2.1.3.1 Process Description

A fluidized bed incinerator consists of a fluidized bed reactor, fluidizing air blower, waste

feed system, auxiliary fuel feed system, and an air pollution control device system. A typical

reactor is shown in Figure 2-3. The fluidized bed reactor is essentially a vertical cylinder containing

a bed of granular material (typically sand or some other inert material) at the bottom. Combustion

air is introduced at the bottom of the cylinder and flows up through openings in a distribution plate;

the distribution plate is designed to allow sufficient air to pass upward into the bed but to prevent

bed media from falling through when the bed is at rest. The air flows up through the bed material,

suspending the granular particles.

Solid, liquid, and gaseous waste fuels may be injected into the bed, where they mix with the

combustion air and hot bed media, and burn. The waste is put in direct contact with the bed media,

causing heat transfer from the bed particles. At the proper temperature, waste ignition and combustion

occur. The bed media acts to scrub the waste particles, exposing fresh surface by the abrasion

process which encourages rapid combustion of the waste. Waste and auxiliary fuel are injected

radially into the bed and react at temperatures from 840 to 1500°F. Further reaction occurs in the

volume above the bed, known as the “freeboard”, which is maintained at temperatures up to 1800°F.

An auxiliary burner is located above the bed to provide heat for start-up, reheat, and maintenance of

bed temperature. Waste ash and bed materials are removed periodically from the bottom of the bed

as well as entrained out of the bed with the exhaust gas.

Typical reactors have an inside diameter of 26 feet and elevation of 33 feet. Silica beds are

commonly used and have a depth of 3 feet at rest and extending up to 6.5 feet in height when

fluidizing air is passed through the bed.

Generally, fluidized bed incinerators have two separate waste preparation/feed systems —

one for solids and one for liquids. In some cases, four feed systems are employed: wet solids, dry

solids, viscous fluids, and non-viscous fluids. Solid wastes are usually fed into a coarse shredder.

The coarsely shredded waste falls into a classifier which separates the light and dense particles.

The lighter particles are transferred to a secondary shredder from which they are conveyed to the

hopper for feeding the fluidized bed. Liquid waste is pumped into a larger holding tank. To ensure

the mixture is as homogeneous as possible, the liquid waste is continuously pumped through a
2-7

recirculating loop from the bottom of the tank to the top. A metering pump draws the waste fuel to

be burned from the tank to the primary reactor. Nozzles are used to atomize and distribute the

liquid waste within the bed.

Due to efficient mixing, fluidized beds in general can operate at lower temperatures compared

with other incineration technologies. This potentially results in lower NOx and volatile metals

emissions, as well as lower potential for ash agglomeration and lower auxiliary fuel requirements.

This also allows for the treatment of low heating value wastes (below 4500 Btu/lb). The use of

alkali containing bed materials such as lime of sodium allows for “in-situ” acid gas control; hence,

fluidized beds may be efficient at treating high sulfur or halogenated wastes due to reduced

downstream acid as emissions control requirements.

Maintaining proper bed fluidization is important for effective waste treatment. Solid waste

size reduction (shredding or grinding) may be required. Fluid beds are not well suited for irregular,

bulky wastes, tarry solids, or wastes with a fusible ash content. Improper bed fluidization may lead

to bubbles of gas that “short circuit” the bed, allowing for potentially unburned wastes to escape the

bed. Particulate emissions in the flue gas may be high compared to other incinerator designs due to

the high fluidization velocity, erosion of bed material, bed turbulence, and the small size of waste

and bed particles.

2.1.3.2 Operating Parameters

Fluidized bed operating parameters include:

• Bed temperature — The fluidized bed temperature must be monitored and controlled by

waste feed and auxiliary burner adjustments, as well as adjustments of the input rate of

combustion air and inert fluidizing gases, to assure that the temperature remains above an

established minimum. Operating temperatures are normally maintained in the 1400 to 1600°F

range.

• Oxygen level — The oxygen level in the bed must be maintained to ensure the potential for

complete combustion.

• CO and HC flue gas levels — Monitoring the CO and HC concentration in the flue gas is

also an indication of complete combustion.

• Bed retention time — Solids retention time in the bed is a measure to the thorough treatment

of the waste. Retention time is especially important for relatively non-combustible wastes.

• Bed fluidization — Uniform bed fluidization must be maintained to properly treat the waste.

• Bed gas residence time — The combustion gas residence time must be monitored to assure

that the combustion gas has been exposed to volatile destruction temperature for a sufficient

period of time.
2-8

2.1.4 Fixed Hearth Incinerators

Fixed hearth incinerators, which are used extensively for medical waste incineration, are

less commonly used to incinerate hazardous wastes. Fixed hearths can handle bulk solids and

liquids, however, this design does not have the versatility of a rotary kiln.

2.1.4.1 Process Description

Fixed hearth incinerators typically contain two furnace chambers: a primary and secondary

chamber. A typical fixed hearth system is shown in Figure 2-4. Solid and liquid wastes may be

charged into the primary chamber; small units are normally batch-fed, while larger units may be

continuously fed with a screw feeder or moving grate, or semi-continuous fed with a raw pusher. In

some designs, there may be two or three step hearths on which the ash and waste are pushed with

rams through the system. In other designs, rotating rabble arms stir the solid waste material on the

grate.

A controlled flow of “under-fire” combustion air is introduced, usually up through the hearth

on which the waste sits. In some designs, combustion air may also be provided from the wall over

the waste bed. In many fixed hearth incinerators, known as controlled air or starved air incinerators,

70 to 80% of the stoichiometric air required is provided in the primary chamber, thus the primary

chamber is operated in a “starved-air” mode. In this situation the waste is pyrolyzed and partially

combusted. In some cases steam injection into the primary combustor is used to enhance waste

fixed carbon burnout (fixed carbon is non-volatile carbon which will burn only when exposed to

combustion air). Because fixed hearth incinerators have relatively poor mixing of waste with the

combustion air, they are not effective at processing waste which require turbulence for effective

combustion, such as powdered carbon, pulp wastes, sludges, and viscous wastes.

Primary chamber temperature is controlled by the under-fire air supply. Temperature in the

primary chamber is maintained high enough to destroy hazardous organics in the waste (typically

1000°F) but low enough to reduce the potential for slagging and refractory damage (1800°F). Waste

slagging may clog the under-fire air ports. Bottom ash is removed on a continuous or semi-continuous

method, depending on the unit design. The ash is usually dumped into a water bath, which provides

a seal between the primary unit and the atmosphere.

For controlled air units, due to insufficient supply of air, the primary chamber flue gas will

contain unburned hydrocarbons, and high levels of CO and H2. These volatiles are burned out in a

secondary or afterburner chamber. In the secondary chamber, 140 to 200% excess air is provided,

as well as sufficient residence time at temperature for complete volatiles burnout. Liquid wastes

may be injected either in the primary or secondary chamber. Supplementary fuel may be provided

in both chambers for the maintenance of temperature. Secondary chamber temperature may be as

high as 2000°F. Temperatures beyond this are avoided to prevent refractory damage, decreased gas

residence time, and usage of auxiliary fuel.
2-9

2.1.4.2 Operating Parameters

Critical fixed hearth operating parameters include primary chamber and afterburner

temperature. Primary chamber temperature is controlled by the under-fire air supply. Afterburner

temperature can be controlled by auxiliary fuel firing and primary waste feed rate adjustments.

Note that the capacity of afterburner or secondary chamber limits the primary chamber burning

rate. The afterburner must have adequate volume to accept and oxidize all volatile gases generated

in the primary chamber. Additionally, the afterburner must be kept operating at excess air conditions

to ensure complete volatile burnout.

2.1.5 Air Pollution Control Techniques

The exhaust combustion gas leaving the primary (or secondary if used) incinerator

combustion chamber will be composed primarily of CO2, O2, and N2, and H2O vapor. It may also

contain undesirable constituents that are a byproduct of the combustion process such as: (1) acid

gases (HCl, HF, SO2, and NOx), (2) entrained suspended solid particles (potentially containing

condensed metals and unburned non-volatile organics), (3) inorganic vapors (e.g., volatile metals

such as mercury), and (4) products of incomplete combustion such as unburned organics and CO.

Levels of these combustion byproducts is highly site specific, and depends on a variety of factors

such as waste composition and incinerator system design and operating parameters (e.g., temperature

and exhaust gas velocity).

Most hazardous waste incinerators use air pollution control systems to remove undesirable

components such as suspended solid particles (“particulate matter”) and acid gases from the exhaust

gas prior to release to the atmosphere. The air pollution control system is typically made up of a

series of different devices that work as a unit to clean the exhaust combustion gas. Unit operations

usually include exhaust gas cooling, following by particulate matter and acid gas control.

Exhaust gas cooling can be performed many different ways including using a waste heat

boiler or heat exchanger, mixing with cool ambient air, or injection water into the exhaust gas. For

PM and acid gas control, many different types of air pollution control devices can be used, including

wet scrubbers (such as venturi and packed bed type wet scrubbers, ionizing wet scrubbers), and

electrostatic precipitators and fabric filters (sometimes used in combination with dry acid gas

scrubbing). In general, the control systems can be grouped into the following three categories:

“wet”, “dry”, and “hybrid wet/dry” systems.

In wet systems, a wet scrubber is used for both particulate and acid gas control. Typically,

a venturi scrubber and packed bed scrubber are used in a back-to-back arrangement. Ionizing wet

scrubbers, wet electrostatic precipitators, and innovative venturi-type scrubbers may be used for

more efficient particulate control. Wet systems generate a wet effluent liquid waste stream (scrubber

blowdown), are relatively inefficient at fine particulate control compared to dry control techniques,

and have equipment corrosion concerns. However, wet scrubbers typically provide efficient control

of acid gases and have lower operating temperatures (compared with dry systems) which may help

control the emissions of volatile metals.
2-10

In dry systems, a fabric filter or electrostatic precipitator is used for particulate control,

sometimes in combination with dry scrubbing for acid gas control. Dry scrubbing systems in

comparison with wet scrubbing systems are inefficient at the control of acid gases.

In hybrid systems, a dry technique (ESP or fabric filter) is used for particulate control (and

possibly acid control with use of dry scrubbing) followed by a wet technique (venturi and packed

bed scrubber) for acid gas control. Hybrid systems have the advantages of both wet and dry systems

(lower operating temperature for capture of volatile metals, efficient collection of fine particulate,

efficient capture of acid gases), while avoiding many of the individual disadvantages. In some

hybrid systems, known as “zero discharge systems”, the wet scrubber liquid is used in the dry

scrubbing operation, thus minimizing the amount of liquid byproduct waste.

In a separate and unique category are facilities which do not use any air pollution control

devices. Some liquid injection and fixed hearth facilities, which generate low levels of particulate

when treating low ash content wastes, and/or may not generate acid gases when burning nonhalogen

containing wastes, do not utilize post-combustion air pollution control devices.

2.2 CEMENT KILNS

The manufacture of cement is one of the largest mineral products in the United States. In

1990, 111 plants in the United States produced about 80 million tons of cement. There are about 34

separate sites where cement kilns are burning hazardous waste derived fuel in the United States.

2.2.1 Process Components

Cement is made from a carefully proportioned mixture of raw materials containing calcium

(typically limestone, although oyster shells and chalk may also be used), silica and alumina (typically

clay, shale, slate, and/or sand), and iron (typically steel mill scale or iron ore). These materials are

ground to a fine power (80% passing 200 mesh), thoroughly mixed, and heated to a very high

temperature to produce a cement “clinker” product. The raw feed material, known as “meal”, is

heated in a kiln; a large, inclined, rotating cylindrical steel furnace lined with refractory materials.

Heating of the meal as it moves through the kiln drives a number of chemical and physical processes

which are necessary to form the clinker. In the first “drying and preheating zone”, occurring in a

temperature range of 70 to 1100°F, residual water is evaporated from the raw meal feed, and clay

materials begin to decompose and are dehydrated (removal of bound water). Next, in the “calcining

zone” (with materials temperatures of from 1100 to 1650°F) the material is “calcined”; that is

calcium carbonate in the limestone is dissociated producing calcium oxide (“burnt lime”) and carbon

dioxide Finally, in the “burning zone”, also known as the “clinkering” or “sintering” zone, calcium

oxide reacts with silicates, iron, and aluminum to form “clinker”. The clinker is a chemically

complex mixture of calcium silicates, aluminates, and alumino-ferrites. A minimum meal

temperature of 2700°F is necessary in the burning zone of the kiln to produce the clinker.

Heating of the raw meal to produce cement clinker can take place in three different types of

arrangements: “wet”, “dry”, or “semi-dry” processes. The hardware used to dry, calcine, and clinker

the raw meal is difference between the three set-ups; the physical and chemical processes to produce

the cement are similar. A rotary kiln is the common element in all systems, and always contains the
2-11

burning zone and all or part of the calcining zone.

2.2.1.1 Wet Process Kilns

A schematic of the wet cement making process is shown in Figure 2-5. In wet process

kilns, ground raw materials are mixed with water (about 30% by weight) to form a slurried meal.

The slurry is ground in wet mills to required size and pump fed to the elevated end of the rotary

cement kiln. The slurry is fed to the kiln through a flow metered pump. The kiln is a long (450 to

750 ft. in length with a length-to-diameter ratio of about 30:1 to 40:1), inclined rotating cylindrical

steel furnace. Refractory material internally lines the kiln to protect the steel shell from the high

temperature and to retain heat within the kiln. The kiln slope (3 to 6°) and rotation (50 to 70

revolutions per hour) cause the meal to move toward the “hot” lower end of the kiln. The kiln is

fired at the hot end, usually with coal or petroleum coke as the primary fuel; natural gas, fuel oil, or

hazardous waste derived fuel may also be used as a supplemental energy source. This energy is

required to heat the raw materials to clinker formation temperature. As the raw meal moves down

the kiln it is heated by hot fuel combustion gases moving in a “counter-current” direction. This

configuration (exhaust gases and solids flowing in opposite directions through the kiln) provides

for more efficient heat transfer compared with “co-current” operations. Kiln rotational speed, raw

meal feed, and fuel firing rates are adjusted to maintain clinker quality and heat transfer and to limit

the raw meal from occupying more than 15 to 20% of the internal kiln volume. To improve the

energy efficiency of the process, steel chains are typically attached to the inside of the kiln shell in

the upper kiln drying zone. The chains act as a heat transfer mechanism. As the kiln rotates, the

chains hang by gravity and pick up heat from the kiln flue gases. When the hot chains settle back

into the meal after further rotation, heat is transferred from the chains to the meal.

The wet cement making process is the older traditional process, characterized in part by

handling, mixing, and blending of the raw materials in the slurry form, and lower emissions of kiln

dust. However, because all water must be evaporated out of the slurry mixture, wet process kilns

require greater energy input than other types of cement kilns; typically, from 5 to 7 million Btu/ton

of clinker product is required. Although, because some raw materials are quarried in a wet form,

the wet process may be preferred.

A typical solids and gas temperature profile of a wet kiln is shown in Figure 2-6. The feed

does not begin to heat up until all of the water in the raw meal slurry is evaporated. The gas

temperature in the burning zone approaches 4000°F, while the clinker reaches a maximum

temperature of approximately 2700°F, which is required for the formation of clinker. Flue gas

velocity is also shown in Figure 2-7 as a function of kiln axial location. This relates to a kiln gas

residence time of over 10 seconds. Typical solids residence time in the kiln is from 2 to 3 hours.

2.2.1.2 Dry Process Kilns

Due to higher energy efficiency compared with wet kilns (3.4 to 4.5 million Btu/ton clinker),

and improvements in dry grinding and blending technology, dry processes have become popular.

In the dry process, the ground raw material is dried and pneumatically transported to the kiln, or

preheater if used. Kiln exhaust flue gases or hot clinker cooler air are typically used for drying the

raw materials. There are three different dry process configurations: long, preheater, or preheater/
2-12

precalciner.

Long Dry Process Kilns — A long dry process kiln has a same configuration as the wet

type; the only difference is that raw meal is fed in a dry form. Long dry kilns have typical lengthtodiameter ratios of about 30:1; they are typically shorter than wet kilns since slurried meal water

evaporation is not required, allowing for the meal to be heated more rapidly. A typical long dry kiln

solid/gas temperature profile is shown in Figure 2-8. Kiln gas and solids residence times are similar

to those of wet kilns (on the order of 10 seconds and 2 to 3 hours respectively). Internal chains are

also used in dry process long kilns (similar to wet kilns) to increase process energy efficiency.

Preheater Dry Process Kiln — The preheater arrangement process is shown in Figure 2-9.

Preheaters are used to further increase the thermal efficiency of the cement making process. In the

preheater system, kiln exhaust gas is used to increase meal temperature and partially calcine the

meal before feeding it to the rotary kiln. The preheater accomplishes the same tasks as the back end

of a long kiln, but more energy efficiently compared with the rotary kiln. There are two types of

preheater arrangements: the traveling grate type, and more commonly, the suspension type. A

suspension preheater (“Humboldt” Design) consists of a vertical tower containing a series of cyclonetype vessels (typically containing four stages). Raw meal is introduced at the top of the tower. Hot

kiln exhaust flue gases pass counter-current through the downward moving meal to heat the meal

prior to introduction into the kiln. The cyclones are used to separate the meal from the kiln flue

gases; collected meal in the cyclones is sequentially dropped into the next lower stages. Because

the meal enters the kiln at a higher temperature than that of the conventional long dry kilns, the

length of the pre-heater kiln is shorter; kilns with preheaters typically have length-todiameter ratios

of about 15:1.

A typical solid/gas temperature profile is shown in Figure 2-10. The preheater kiln typically

has a gas residence time of about 6 seconds and a solids residence time of about 30 minutes. The

gas residence time through the preheater cyclones (typically four stages) is usually about 5 seconds

(Figure 2-11 shows typical gas velocities through the preheater kiln system).

With preheater systems, it is often necessary to utilize an “alkali” bypass, in which a portion

of the kiln flue gases are routed away from the preheater tower at a location between the feed end of

the rotary kiln and the preheater tower. The bypass is used to remove undesirable components,

such as certain alkali constituents, that may accumulate in the kiln due to an internal circulation

loop caused by volatilization at high temperatures in the kiln and condensation in the lower

temperatures of the preheater. Accumulated alkali salts may cause preheater operating problems

such as clogging of the cyclones, and an increase in fine NaCl or KCl fume in the emissions gases.

Typically 10 to 15% of the flue gas is routed through the bypass. Systems without bypasses are

limited with respect to raw meal and waste concentrations of alkali metals, chloride (greater than

0.015% Cl by weight), and sulfur that can be tolerated in the raw materials.

The internal circulation of alkali components is greater in systems with preheaters compared

to systems without preheaters due to the filtering effect of feed material flow in the preheater

cyclones. Systems without preheaters have a kiln dust with a high content of alkali salts, which can

be removed from the internal cycle when caught in the air pollution control device. However, for

the preheater arrangement, to reduce the alkali buildup, a bypass is required.
2-13

Preheater/Precalciner Dry Process Kiln — A preheater/precalciner process is also shown in

Figure 2-9. A preheater/precalciner is similar to the preheater arrangement described above with

the addition of an auxiliary firing system to further increase the raw materials temperature prior to

introduction into the kiln. An additional precalciner combustion vessel is added to bottom of preheater

tower, immediately upstream of the kiln. Typical systems use 30 to 60% of the kiln fuel in the

precalciner to release up to 95% of the CO2 from the raw material. Precalciner air can be supplied

either directly with the precalciner fuel, or else supplied at hot end of the kiln. In another arrangement,

the kiln flue gas may be routed around the calciner directly to the preheater. Kilns with preheater/

precalciners can be even shorter than those with preheaters only (length-to-diameter ratio of 10:1).

The primary advantage of using the precalciner is to increase the production capacity of the

kiln. since only the clinker burning is performed in the kiln. The use of the precalciner also increases

the kiln refractory lifetime due to reduced thermal load on burning zone. These configurations also

require a bypass system for alkali control.

A typical solid/gas temperature profile of a kiln with a preheater and precalciner is shown in

Figure 2-12. It is similar to that of the preheater kiln; however, the time at which the gas temperature

is above 1650°F is slightly less in a precalciner type kiln.

2.2.1.3 Semi-Dry Process Kilns

In the semi-dry process, the ground feed material is pelletized with 12 to 14% water. The

pellets are put on a moving “Lepol” grate on which they are dried and partially calcined by hot kiln

exhaust gases before being fed to the rotary kiln. A semi-dry process schematic is shown in Figure

2-13. A typical solids/gas temperature diagram is given in Figure 2-14.

2.2.2 Clinker Handling

In all arrangements, the clinker is removed from the kiln at the hot end. After it passes

through the burning zone and by the kiln flame, it enters a short cooling area where the clinker melt

begins to solidify; the cooling rate from the burning zone to the kiln exit is important since it

determines the microstructure of the clinker. The clinker leaves the kiln at about 2000°F and falls

into a clinker cooler. The cooler is typically a moving grate onto which the clinker sits. Cooling air

is blown through the clinker bed. The cooled clinker consists of grey colored nodules of variable

diameters, typically up to two inches. The clinker is blended with gypsum and ground in a ball mill

to produce the final product, cement. Hot exhaust air produced from the clinker cooler is either

directed to the kiln where it is used as combustion air or used to pre-dry the raw feed material in the

case of dry process kilns.
2-14

2.2.3 Hazardous Waste Feeding Techniques

The feeding of the hazardous waste into the kiln is a primary factor in determining the

destruction of the waste which is dependent on temperature and residence time. The method of

introducing the waste also depends on its physical state, as discussed in the following sections.

Liquid wastes are either blended directly with the conventional fuels which are provided at

the hot end of the kiln, or they are injected separately through a separate burner/atomizer into the

primary kiln flame. Extreme combustion conditions of high temperature (greater than 3000°F),

turbulent mixing, and long as phase residence time (from 4 to as high as 16 seconds in long kilns)

have been demonstrated to overcome any possible deficiencies in local oxygen availability and

potential atomization problems.

In the past, solid wastes were fed at the cold end of the kiln with the raw meal. However,

the addition of wastes at cold end is no longer practiced because as the wastes move down the kiln

and get hotter, volatile components are driven off at temperatures below those required for complete

oxidation and destruction (note that the current Boiler and Industrial Furnaces Rule no longer allows

this, requiring that at the location of solid waste feeding, the combustion gas temperature must

exceed 1800°F and that adequate oxygen must be present to ensure complete combustion of organic

constituents in the waste). Thus, significant emissions of unburned volatiles may occur without the

use of an afterburner or other add-on volatile emissions control techniques.

Alternatively, solid wastes are not usually charged directly into the hot end burning zone for

a variety of reasons. If introduced too close to the end of the kiln, the solid wastes may leave the

kiln with the clinker in an unburned state due to insufficient time in the hot conditions. Their

presence can create local reducing conditions which adversely effect clinker quality such as strength,

stability, set-up time, and color. Oxidizing conditions must be maintained in the burning zone. If

insufficient oxygen is present, a key component of cement clinker, tetra calcium alumino ferrite

does not form; instead Fe2O3 is reduced to FeO. This leads to a clinker product that produces a

quick setting cement with decreased final strength. Additionally, the presence of unburned carbon

in the burning region produces a clinker with an undesirable brown color. To overcome these

feeding location limitations, a variety of techniques are used for the introduction of solid wastes

into cement kilns. The choice depends on the solid waste type (composition and physical attributes)

and kiln configuration (dry long, dry with preheater, wet, etc.).

Method which are used rely on the introduction of the solid waste into the calcining zone.

For long kilns, this means that the waste is introduced mid-kiln, while for preheater/precalciner

kilns, it is introduced onto the feed shelf. Introduction in the calcining zone allow the solids to have

sufficient residence time for the organics to be driven off so that the presence of waste combustibles

do not reach the burning zone creating reducing conditions which are detrimental to clinker quality,

and yet far enough away form the cold end to provide sufficient temperature and residence time for

complete destruction of volatile gaseous organics. These methods include:

• For long type wet and dry kilns, and dry kilns with preheaters, a method has been developed

for charging solid wastes directly through a hatch on the rotating kiln wall at a intermediate
2-15

location within the calcining zone. At each rotation, the hatch is opened, and containerized

waste solids are fed down a drop tube that is inserted through the hatch and into the rotating

kiln. The drop tube prevents hot mineral material from escaping through the port or contacting

the enclosure. It is important that the volume of the volatile components does not exceed

the capacity for their complete combustion in the gas stream. Thus, wastes are containerized

to minimizing the potential for overloading the combustion capacity (create local reducing

conditions).

• As mentioned, for preheater or precalcining kilns, the solid waste may be injected directly

into the precalciner vessel or preheater inlet.

• Containerized solid wastes may also be injected at the hot end of the kiln at high enough

velocity so that they are projected into the calcining zone. An “air cannon”, which is mounted

to the kiln hood, is used to propel the waste containers.

Other methods used for solid waste treatment in cement kilns can be used. If the waste is in

powdered form, it may injected directly into the primary burning zone coal flame of the cement

kiln. In another set up, solid waste is burned in a separate kiln or combustor; the flue gas is routed

to the hot end of the cement kiln, which is used as an afterburner and for acid gas control.

2.2.4 Air Pollution Control System

Kiln combustion gases are drawn through the system with an induced draft fan; this maintains

kiln pressure under atmospheric to prevent fugitive emissions. Kiln (or preheater if used) exhaust

gases contain large amounts of suspended fine solid particles, known as “cement kiln dust” (CKD),

contain significant amounts of entrained particulate matter due to the turbulence in the kiln from

the rotary action and from the use of finely ground feed material. The CKD is composed of fuel

and waste ash and meal raw materials. The CKD is removed from the exhaust gas in either ESPs or

FFs. ESPs have been the more common choice due to their reliability and low maintenance

requirements. Recently though, with increasingly strict particulate control standards, FF have also

become common. Typically, fabric filters are more efficient at collection of submicron particulate

and provide lower guaranteed particulate matter emissions levels. ESPs are more common on wet

kilns since the higher exhaust gas moisture content is difficult for FFs to handle. FF more common

at dry process kilns. Depending on the facility, the kiln exit gas may need to be cooled prior to

being introduce into the air pollution control device. Air dilution or water spray may be used for

this purpose. In another arrangement, hot gases leaving the kiln are cooled in a waste heat boiler

which produces steam; the steam is converted to electricity this is used on-site. Many plants return

a portion of the CKD to the raw feed materials or directly into the hot end flame (known as

“insufflation”). However, some must be removed to lower the buildup of alkali salts. CKD can be

used in other industries as neutralizers or additives, however, usually the excess CKD is land disposed.

Due to the nature of the cement process, add-on acid gas air pollution control devices, such

as wet or dry scrubbers, are not used. The cement feed raw materials are high in alkaline content

(limestone in particular), which provides for “in-situ” absorption of chlorine and other halogens

and sulfur, which may originate in either the coal or the hazardous waste), preventing the formation

and release of acid gases.
2-16

CO and HC main stack gas emissions levels from cement kilns are typically higher than

those encountered in hazardous waste incinerators. CO is believed to be high due to the limestone

calcination process and the decomposition of CO2 into CO due to elevated temperatures and moisture

and metal catalysts in the cement kiln. HCs are volatilized from the raw meal at the cold end of the

kiln at temperatures below those necessary for complete destruction and oxidation. Additionally,

although excess oxygen conditions (oxidization atmosphere) are required for quality cement

production, low kiln levels (1 to 4% in cement kilns, compared with 5 to 10% in hazardous waste

incinerators) are desired for high temperature, energy efficient operation. Low oxygen levels may

lead to local oxygen deficient regions and incomplete destruction of volatiles.

2.3 LIGHTWEIGHT AGGREGATE KILNS

“Light weight aggregate” (LWA) is a high strength, light weight material, with a density

much less than ordinary aggregates such as gravel, sand, or stone. Its primary use is in concrete,

made by combining LWA with cement. LWA concrete blocks and structural concrete are frequently

used in the construction of roads, bridges, and buildings. There are over 36 LWA producing facilities

in the United States. Of these, there are six which are burning hazardous waste derived fuel.

2.3.1 Process Components

A lightweight aggregate plant is composed of a quarry, a raw material preparation area, a

kiln, a cooler, and a product storage area. The material flows from the quarry to the raw material

preparation area. From there, the material is fed into the rotary kiln.

LWA is made by heating raw materials such as clay, shale, and slate to high temperature in

a rotating light weight aggregate kiln (LWAK), as shown in Figure 2-15. When the highly siliceous

raw material is heated, it bloats and expands forming a strong light weight material when cooled.

The kiln is a long steel cylinder (typically 120 to 300 feet in length with a 7 to 10 foot diameter),

lined internally with refractory bricks. It rotates about its axis at 3 to 5 revolutions per minute, and

is inclined at an angle of 3 to 5° from the horizontal. It operates in a manner similar to previously

described for cement kilns. Crushed raw material is continuously fed to the kiln at the upper “cold”

end. Gravity and kiln rotation cause the material to travel downward through the kiln toward its

bottom end. At the bottom “hot” end, fuels such as coal, petroleum coke, natural gas, fuel oil, and/

or hazardous waste are burned with air to provide energy to heat the raw materials. Combustion air

is provided in an excess (80 to 100% of minimum requirements) to aid in “expanding” the raw

materials. The raw material travelling down the kiln (from the upper to lower end) is heated by hot

fuel combustion gases moving in an opposite direction (from the lower to upper end). This “countercurrent” operation provides for efficient heat transfer between the hot gases and raw material,

leading to a minimization of fuel energy requirements. Energy requirements are typically 3 to 6

million Btu/ton of LWA product.

The raw material goes through various temperature regimes as it is heated in the kiln which

cause the transformation of the material into the LWA product. In the first preheating stage, raw

material moisture is driven off. As the temperature increases, the material becomes soft and pliable

and decomposes. Formation of internal gases (such as SO2, SO3, CO2, and O2) also begins. These
2-17

gases cause the material to expand. When the material temperature approaches the “expanding”

zone temperature of 1900 to 2100°F, it becomes sufficiently viscous to internally trap the expanding

gases. This bloating action produces small unconnected gas cells, which the material retains after

it cools and solidifies; these internal gas cells give the aggregate its characteristic light weight

property. Maintaining the proper temperature is critical for making LWA. If the temperature is too

low, the clay will not expand to form the desired structure. However, if the temperature is too high,

the material fuses, leading to the formation of a heavy coating on the kiln interior that must be

removed after shutting down and cooling the kiln. Kiln temperature is controlled by adjusting fuel

and raw material feed rates, kiln rotation rate, and the amount of combustion air. The hot LWA

product is removed at the bottom of the kiln, and cooled. The LWA may be graded and crushed

after cooling. LWAK production rates typically range from 200 to 1000 tons of LWA/day.

An induced draft fan is used to pull combustion gases up through the kiln and to maintain a

negative kiln pressure with respect to the atmosphere to prevent fugitive gas emissions from leaking

through the rotary kiln seals. Kiln exhaust gases leave the cold upper end of the kiln at a temperature

from 400 to 1000°F. The kiln exhaust gas contains suspended dust particles (consisting potentially

of raw material, waste ash, and fuel ash) and may also contain volatile inorganics such as acid gases

(including HCl and SO2 produced as a byproduct of the combustion of chlorine and sulfur containing

wastes and fuels) and certain volatile metals. The LWAK exhaust gas may also contain elevated

levels of unburned organics and CO compared with hazardous waste incinerators due to “countercurrent” operation; at the upper kiln end, as the raw material is heated, organics may be vaporized

at temperatures below those required for their complete oxidation and destruction.

LWAKs historically have used both wet scrubbers and dry systems such as fabric filters and

ESPs for exhaust gas dust emissions control. All hazardous waste burning LWAKs currently use

fabric filters for the removal and collection of dust contained in the exhaust gas. The exhaust gas

may require cooling (with either a water spray, heat exchanger, or cold air dilution) prior to the

fabric filter due to fabric operating temperature limitations. The collected dust can be recycled

back into the kiln (at the hot or cold end) or mixed into the final LWA product. Some LWAKs also

use dry scrubbing (by the addition of an acid gas absorbing alkali upstream of the fabric filter) or

wet scrubbing techniques for acid gas emissions control.

In currently operating hazardous waste burning LWAKs, only liquid hazardous wastes are

treated. Liquid wastes are either blended directly with conventional fuels provided at the hot end of

the kiln, or they are injected separately through a lance or atomizer into the hot end flame. The high

combustion gas flame temperature (greater than 3000°F) and kiln gas residence time (greater than

2 seconds) is used to destroy hazardous organics; waste in-organics are either incorporated into the

LWA product or leave the kiln with the exhaust gas (in the form of either vapors or solids). Similar

to that discussed for cement kilns, LWAKs may potentially be used to treat solid wastes. Fine

powder combustible solids may be injected similar to liquids directly into the hot end flame. Bulk

solids may be introduced at mid-kiln through mid-kiln hatches or with air-cannon injection from

the hot end.
2-18

2.3.2 Operating Parameters

Kiln operating parameters such as flame temperature, excess air level, feed size, material

flow, gas and solids residence time, and speed of rotation vary from plant to plant and are determined

by the characteristics of the raw material. Raw materials include clay, shale, or slate which have

varying properties linked to their geological formation. As mentioned, maximum temperature in

the rotary kiln varies from 2050 to 2300°F, depending on the type of raw material being processed

and its moisture content. Kiln exit gas temperatures may range from 300 to 1200°F, again depending

on the raw material and on the kiln’s internal design. Approximately 80 to 100% excess air is

forced into the kiln to aid in expanding the raw material. A typical lightweight aggregate rotary

kiln has a combustion gas residence time of greater than 4 seconds, based on a kiln gas temperature

at the hot end of 2100°F and a temperature at the cold end of 700°F.
2-19

Rotary kiln incinerator.

Liquid Rotary seals

waste 

and/or 

auxiliary 

fuel

Solid 

waste

Combustion air

Kiln 

shroud

Rotary kiln

Ash discharge

(to ash quench)

Air

Liquid 

waste 

and/or 

auxiliary 

fuel

Afterburner

Support a

Figure 2-1. Typical rotary kiln incinerator.

aa

aarollers

apollution control system)

heat recovery and/or air 

aa(to dump stack, waste 

aaExhaust gasaa

a

a

a
2-20

Auxiliary 

fuel

Atomizing 

steam or air

Liquid 

waste

Swirl 

vanes

Primary 

combustion 

air

Burner

Aqueous 

waste

Atomizing 

steam or air

Exhaust gas

(to waste heat recovery 

and/or air pollution 

control system)

Liquid injection incinerator.

Figure 2-2. Typical liquid injection incinerator.
combustion 

Fluidizing 

air

a

2-21

aAsh and bed 

removal

Exhaust gas

(to waste heat recovery 

and/or air pollution 

control system)

Auxiliary fuel 

and air

Solid and 

liquid waste

Waste, ash 

recycle, and 

bed make-up 

feed

Fluidized 

bed

Preheater 

burner

Distributor 

plate

Freeboard 

"disengagement" 

section

Windbox

Fluidized bed incinerator.

Auxiliary 

burner

Figure 2-3. Typical fluidized bed incinerator.
2-22

Combustion 

air

Ash 

transfer 

ram

Ash 

discharge 

ram

Feed 

ram

Ash discharge 

(to ash quench)

Solid 

waste

Auxiliary fuel 

and/or liquid 

waste

Auxiliary fuel 

and/or liquid 

waste

Underfire 

combustion 

air

Primary 

chamber

Secondary 

chamber

Exhaust gas

(to waste heat recovery 

and/or air pollution 

Air

Figure 2-4. Typical fixed hearth incinerator.

acontrol system)

aa

a

a

Fixed hearth incinerator.

aa

a

aaa

aa

a

a

a

aa
2-23

Particulate 

Control 

Device

Rotary Cement Kiln

Kiln Gas

Meal

Waste 

Dust

Clinker 

Cooler

Kiln 

Exhaust 

Stack

Flue Gas 

Cooling 

(water 

spray, air 

dilution)

Raw Materials

 Limestone Silica Clay Iron Ore

Firing 

Hood

Feed 

House

Chains

Gas, Oil, 

Coal, and/or 

Coke Liquid and Solid 

(Powder) 

Waste

Solid Waste 

Cannon

Calcining 

Zone

Clinker Cooler Gas

(May be used to 

preheat raw feed or 

primary flame air)

Air

Grinding 

Raw Meal 

Feed Silo

and 

Blending

Slurried Meal

Kiln 

Flue 

Gas

Cooled 

Clinker 

(to ball mill 

and storage 

silo)

Solid Waste (mid 

kiln entry)

Particulate 

Control 

Device

Clinker 

Cooler 

Exhaust 

Stack

Solid 

Waste

Figure 2-5. Wet cement kiln.

Cement Kiln 

Dust 

Recycle

Dust to 

Ball Mill

To kiln burner 

(hot end)

Cement 

Kiln Dust 

aa

a

aInsufflation aa
2-24
2-25
2-26
2-27

Exhaust gas

By-pass gas

Solid waste

Liquid waste

Coal, oil, and/or gas

Air

Precalciner

Preheatertower

Raw materials

 Limestone

 Silica Clay Iron ore

Dry meal

Particulate 

control 

device

Rotary cement kiln

Kiln gas

Meal

Waste dust

Clinker 

cooler

Main 

exhaust 

stack

Flue gas 

cooling 

(water 

spray, air 

dilution)

Firing 

hood Gas, oil, coal, and/or 

coke

Liquid and 

solid (powder) 

waste

Clinker cooler 

exhaust gas

(preheat raw feed 

or primary flame 

or precalciner air)

Cooled clinker 

(to ball mill and/or 

storage silo)

Particulate 

control device

Clinker 

cooler 

exhaust 

stack

Cement kiln dust recycle

Dust to ball mill

To kiln burner 

(hot end)

Cement kiln 

dust 

insufflation

Particulate 

control device By-pass exhaust 

stack

Waste dust

Preheater/precalciner cement kiln.

Precalcined meal

Grinding Raw 

and meal feed 

blending silo

Air

aa

aa

a

aa

a

a

aa

a

aa

aa

Figure 2-9. Dry preheater/precalciner cement kiln schematic.
2-28
2-29
2-30
2-31
2-32
2-33

Particulate 

Control 

Device

Rotary Light Weight Aggregate Kiln

Raw material

Kiln Gas Waste Dust

LWA 

Cooler

Kiln 

Exhaust 

Stack

Flue Gas 

Cooling 

(water 

spray, air 

dilution)

Raw Materials

 Clay Shale Slate

Firing 

Hood

Feed 

House

Gas, Oil, 

Coal, and/or 

Coke

Liquid and 

Solid (Powder) 

Waste

Solid Waste 

Cannon

Cooler Gas

(May be used to 

preheat raw feed or 

primary flame air)

Air

Grinding 

Raw Meal 

Feed Silo

and 

Blending

Kiln 

Flue 

Gas

Cooled LWA

(to grinding 

and sizing)

Solid 

Waste

Figure 2-15. Light weight aggregate kiln.

Kiln Dust 

Recycle

To mix with 

LWA product

aAcid

aGas 

Control

aaa
3-1

SECTION 3

AIR POLLUTION CONTROL TECHNIQUES

The flue gas from hazardous waste burning incinerators, cement kilns, and light weight

aggregate kilns can contain a variety of contaminants, including particulate matter, acid gases,

metals (such as mercury, cadmium, lead, etc.), and trace organics. This section presents information

on the design, performance, process monitoring requirements, and the necessary inspection and

maintenance procedures for maintaining proper operation of common air pollution control devices

designed for the removal of these contaminants from the flue gas.

As discussed in Section 2, cement kilns currently utilize electrostatic precipitators (ESP)

and fabric filters (FF) for particulate (containing condensed metals and organics) control. Lightweight

aggregate kilns utilize FFs (sometimes in combination with dry scrubbing and wet scrubbing) for

acid gas and particulate control. Hazardous waste incinerators in the United States operate with a

variety of different air pollution control systems, as discussed in more detail in Volume 3. These

include:

• Dry scrubbing in combination with dry particulate control with FF or ESPs (“dry” air

pollution control systems).

• FF or ESPs (dry PM control devices) followed by wet scrubbing (“wet/dry hybrid” air

pollution control systems).

• Wet scrubbing systems for both PM and acid gas control, including various different

combinations of wet scrubbers such as ionizing wet scrubbers, wet ESPs, venturi scrubbers,

packed beds, and spray towers.

The following air pollution control techniques that are used on hazardous waste burning combustors

are discussed:

• Cyclones — Used for coarse particulate removal.

• Fabric filters — Used for coarse and fine particulate removal.

• Electrostatic precipitators — Used for coarse and fine particulate removal.

• Venturi scrubbers — Used for both particulate and acid gas control.
3-2

• Wet scrubbers — Including ionizing wet scrubbers. Used primarily for acid gas control.

• Spray dryers and dry scrubbers — Used for acid gas control.

• Hybrid wet/dry scrubbing systems — Used for both acid gas and particulate control.

• Flue gas cooling — Includes techniques such as water quench, air dilution, or heat

exchangers.

• Other metals control techniques — Including activated carbon used for metals and organics

control, earth-based sorbents for semi-volatile metals control, and specific mercury control

techniques including selenium coated filters, sodium sulfide injection, and mercury scrubbers.

• Catalytic oxidation — Used for organics control.

• Sulfur-based control — Used for organics control, in particular PCDD/PCDF.

• Air pollution control systems types — Discussion of typical intergrations of various

individual air pollution control devices that are used, including “dry”, “wet”, and “hybrid

wet/dry” scrubbing systems.

3.1 CYCLONES

Cyclones are typically incorporated as a pre-filtering process for removing large particles

from the flue gas. Cyclones can operate at relatively high temperatures and function without moving

parts, making operation simple. The cyclone chamber provides angular (swirling) gas flow which

causes the suspended particles to accelerate toward the chamber walls. Particles, being denser than

gas, separate from the gas stream and impact the cyclone chamber walls. The cyclones are erected

vertically and the particles fall under gravity to the collection hopper.

3.1.1 Design Principles

The cyclone device is typically positioned immediately downstream of the primary

combustion chamber. The cyclone’s service environment is dependent on its construction material.

Cyclones can be constructed to withstand harsh environments such as extreme weather conditions

and high temperatures since they have no moving parts.

The cyclone is typically a vertically erected conical or cylindrical shaped chamber which

receives particle laden flue gas in the upper chamber region, as shown in Figure 3-1. The gas enters

either tangentially or axially in a downward spiral path around the chamber walls. The spiralling

motion causes the particles in the gas flow to accelerate to the chamber walls, where their momentum

causes them to separate and fall out of the gas stream. The particles drop to the bottom of the

cyclone by gravity and downward motion of the gas. The particles fall into the collection hoppers

and are removed by an ash removal system. The cleaned gas still carries significant levels of fine

particulate which remain suspended in the flow due to their airborne characteristics. The treated
3-3

gas reverses direction at the bottom of the chamber and returns up the center of the chamber and

exits out the top.

3.1.2 Performance

Cyclone removal efficiency is dependent on the gas velocity, rate of change of gas direction,

and the particle size distribution, density, and composition. The main limitation of the cyclone is

the inability to effectively remove small particles less than 5 μm in diameter. For small particles,

the inertial separating force (particle momentum) is low and the particles are more prone to remain

suspended in the gas stream.

The removal efficiency of a cyclone can be improved by increasing the swirling velocity of

the gas, which increases the inertial separating force or particle momentum. This is effectively

accomplished by reducing the diameter of the cyclone chamber or increasing the gas flow rate.

Cyclone performance is affected by gas flow rate, since this affects the swirling velocity in the

cyclone. Cyclone efficiency is relatively insensitive to dust loading, and in fact, the efficiency can

increase with higher loading due to particle interactions.

Cyclone efficiency is generally poor compared with the efficiency of electrostatic precipitators

and fabric filters. Cyclone efficiency is less than 20% for sub-micron particulate. Efficiency varies

linearly with particle size, typically ranging from 5% for 0.5 μm particles to 50% for 3 μm particles.

3.1.3 Process Monitoring

The pressure drop and inlet gas temperature are monitored to assure proper operation. The

pressure drop is monitored so that any tendency of the cyclone to plug can be signaled by high

pressure drop. Also, leaks and cracks in the cyclone can be detected by reduced pressure drop.

Pressure drop is also a function of gas flow and may be used as an indicator of capture efficiency.

Generally, efficiency increases with higher pressure (i.e., higher gas flow). Monitoring of the inlet

gas temperature may be required, depending on the cyclone’s application and construction material,

to operate above the acid dew point and avoid corrosive conditions.

3.1.4 Inspection and Maintenance

Cyclones have minimal maintenance requirements due to the lack of moving parts. Cyclone

wall corrosion, leakage, particle deposits and plugging should be regularly checked. Life expectancy

of cyclones is long and is only limited by material corrosion, erosion and thermal stresses and

cracking. Overall, the cyclone requires very little routine maintenance and off-line maintenance is

not conducted in a hostile environment.

3.2 FABRIC FILTERS

Fabric filters (FF), also commonly known as baghouses, are used to remove gas suspended

particles much like the filtration device on a common household vacuum cleaner. The fabric filter

is utilized in relatively low temperature environments (around 350°F) but always above the dew

point of water and common acid gases. The particle laden gas stream enters the FF chamber and
3-4

passes through vertically suspended filter bags. Particles in the flue gas are collected on the filters.

Particle build-up is periodically removed from the filters by one of a variety of methods. The

collected particles fall to a collection hopper situated below the filter bags. Fabric filter characteristics

include:

• High capture efficiency (including fine particulate).

• Capture efficiency relatively independent of particle characteristics.

• Frequent routine maintenance is required.

• Monitoring, inspection, and maintenance is simple.

3.2.1 Design Principles

Fabric filters are used to remove suspended particles from flue gas by capturing the particles

on the surface of a porous fabric. Particle laden gas enters the collection device and passes through

an array of cylindrical filtering bags which retain the particles; and the clean gas exits through the

outlet duct. The design of the FF system slows the gas velocity and evenly distributes the gas to all

the filter bags. Particles collect on the filters and form a dust cake on the surface of the filters. As

the dust layer builds, it becomes more difficult for particles to penetrate. This increases both the

pressure drop across the filters and the collection of the particles. It is actually the dust cake which

achieves the high efficiency collection of particulate. For this reason, unless the filters are

preconditioned (i.e., an artificial filter cake is built-up) prior to operation, filter efficiency is lowest

at start-up and immediately after the bags are cleaned.

Fabric filters are usually externally heated and/or insulated to ensure that the device remains

above the minimum required operating temperature to prevent condensation which can plug and

corrode the filter bags (typically greater than 250°F). This is especially important during start-up

and shutdown operations, when temperatures are likely to drop below the gas dew point. A FF’s

maximum operating temperature is limited by the working temperature of the fabric. For many

common filtering fabrics, the maximum operating temperature is below 500°F. Fabric filters are

equipped with spark arresters upstream to prevent fugitive sparks and hot flyash from burning the

filters.

Filters are most effective at collecting particles when coated with dust, however, too high a

coating will create a high pressure drop across the filter. The pressure drop across the FF is an

important parameter since it relates directly to energy cost for the induced draft fan. Typical

differential pressures for FFs are 1 to 6" w.c.. The filters need to be cleaned periodically. When

removing the dust build-up, it is important not to remove too much dust cake or else excessive dust

leakage will occur while fresh cake develops. There are four principal types of cleaning systems:

pulse jet, reverse gas, shaker, and sonic.
3-5

The pulse jet system utilizes high pressure air to clean the filters, as shown in Figure 3-2.

The high pressure air inflates the bags, cracking the external dust cake. When the air is removed,

the bags return to their original shape and the dust cakes drop into the collection hopper. Pulse jet

systems can be cleaned while on-line, allowing continuous operation of the unit. Filters can be

maintained from outside the collector, which allows the maintenance to be performed in a clean,

safe environment. This vigorous cleaning technique tends to limit filter bag life.

In reverse air systems, the dirty gas enters the fabric filter and passes from the interior to the

exterior of the bags, as shown in Figure 3-3. The FF must be divided into modules which are taken

off-line during cleaning. Low pressure air is introduced from the exterior of the bags, collapsing

the bags and cracking the interior dust cake which falls to the collection hopper. The cleaning is

accomplished with relatively low air pressure, resulting in maximum bag life. This method is

preferred for easily damaged fabrics.

Shaker systems, shown in Figure 3-4, move the tops of the bags in a circular path, causing

a wave motion through the bag length. This causes the dust to crack and fall to the hopper. Bags

must have high abrasion resistance for this cleaning technique. In addition, individual bags must

be taken off-line for the cleaning process.

Sonic cleaning, if used, usually augments another cleaning method. Sonic energy is

introduced into the filtering device. The sonic waves generate acceleration forces that tend to

separate the dust from the fabric.

Fabrics used for the filter bags are usually woven with relatively large openings in excess of

50 μm in diameter. Fiber types include natural fibers such as cotton or wool (not often used), and

synthetic fibers such as nylon, acrylic, Dacron, Teflon, Nomex, Polyproplene, Goretex, and fiberglass.

The synthetic fibers are typically resistant to moisture and acid/alkali components in the flue gas,

however they have limited operating temperatures, typically around 500°F. The fibers may be

either woven or felted. Woven fibers have a definite pattern, while felted fibers are randomly

compressed into a dense mat. Newly developed ceramic and metal fiber bags have improved

temperature resistance (greater than 1000°F operations) and increased structural durability compared

with conventional fabric media. The selection of the fiber bag material and construction is important

to fabric filter performance. Deciding factors for fabric selection include:

• Temperature — Ability to withstand maximum continuous service temperature, and

temperature surges. For typical fiberglass or teflon bags, the maximum rating is about

500°F. New ceramic fibers are being developed that are able to withstand a much higher

temperature level.

• Corrosion resistance — Ability to resist degradation from acids, alkalis, solvents, or

oxidizing agents.
3-6

• Hydrolysis — Ability to handle effects of humidity. High moisture can cause blinding of

the fabric.

• Dimensional stability — Ability to tolerate shrinking and stretching fabric.

• Fabric mechanical strength — Ability to withstand the structural demands exerted on the

fabric by the gas flow and cleaning system.

3.2.2 Performance

Fabric filters are very efficient at removal of particles of all sizes. Fabric filter collection is

reported to be relatively independent of inlet mass loading. Three primary mechanisms are

responsible for particulate capture on a relatively clean fabric: inertial impaction, direct interception,

and Brownian diffusion. Both inertial impaction and direct interception are associated with relatively

large particles, whereas Brownian diffusion is associated with smaller submicron particles. As

discussed above, after a period of operation, a cake forms on the fabric surface, and “sieving”

becomes a dominant collection mechanism. Sieving occurs when the particle is too large to pass

through the surface cake which has built up on the fabric. Emissions of particulate through a fabric

filter are due to particulate seepage through the dust cake and fabric, gas flow through gaps or tears

in the fabric, and gas flow between gaps between the bags and the tubesheet. Due to these

mechanisms, the particle size distribution of the particulate escaping the fabric filter has been shown

to be similar to that in the inlet gas stream. Particle leakage through a fabric cake occurs due to the

cake build up process:

• Individual particles strike edges of pores and form chains that bridge across the pores.

• As more and more pores become covered with cake, the gas velocity through the remaining

pores increases significantly.

• The large gas velocity through the uncovered pores prevents the pores from being bridged.

• These uncovered pores become leakage points.

Fabric filter efficiencies typically range between 99 and 99.9% for particle sizes as low as

0.1 μm. Particle capture is relatively insensitive to particle and dust physical characteristics such as

particle resistivity and dust loading. Efficiency is decreased as the air-to-cloth ratio increases as

gas velocity rises. Air-to-cloth ratio is a ratio of the gas volume flow rate to the filter surface area

and is a measure of the superficial gas velocity through the filter. Typical air-to-cloth ratios range

from 3 to 5 acfm/ft2, with lower air-to-cloth ratios of 2 acfm/ft2 providing improved performance.

Particle capture efficiency is also dependent on the frequency of bag cleaning, cake build-up, and

fabric type and weave, as well as on the physical condition of the bags.

Factors which specifically affect the efficiency of a baghouse include:

• Holes and tears in the bag and fabric abrasion mainly due to too vigorous a cleaning cycle.
3-7

• Chemical attack of the fabric.

• Inadequate cake buildup.

• Thermal degradation of the bag fabric.

• Gas-to-cloth ratio (superficial flue gas velocity).

• Frequency of cleaning.

• Fabric specifications ( i.e., fabric type and weave).

Fabric filter technology has made significant advances with the improvement in FF bag

materials. As a result, FFs are quickly becoming a preferred APCD, especially since ash effluent is

dry and does not require the handling and disposal processes of liquid waste. Recently developed

techniques to improved fabric filter collection efficiency that have been demonstrated include:

• Use of electrostatically enhanced filters; shown to have higher collection efficiency, lower

pressure drops, and lower lifetime costs compared with conventional units. This techniques

is still in the design and development stage.

• Use of a fabric material made of polytetrafluoroethylene (PFTE) membrane. The PTFE has

a smaller pare size compared with conventional felt fabrics, resulting in increased particulate

matter collection. Levels below 0.001 gr/dscf have been demonstrated.

• Flue gas conditioning with ammonia and SO3 has been shown to improve fabric filter

collection efficiency by increasing particulate cohesivity which improves the pore bridging

ability, leading to reduced direct particle penetration. It also enhances the ability of the

particulate to remain in place once caught, reducing the indirect penetration loses.

3.2.3 Upgrade Options

Upgrades to improve FF performance may involve enlarging the device to achieve a higher

air-to-cloth ratio and selecting improved filtering materials and pre-coating agents. Three options

are available for improving the performance of fabric filters including:

• Replacing existing FF unit with larger unit to reduce air-to-cloth ratio.

• Adding to the existing FF a parallel FF and thereby reducing the air-to-cloth ratio.

• Upgrading the existing FF material to high performance filtering materials and the use of

precoating agents. This option typically represents the least costly upgrade procedure.

3.2.4 Process Monitoring

Fabric filter operations are monitored by flue gas inlet temperature, gas flow rate, and pressure

drop across the system. The baghouse temperature must be maintained above the acid condensation

point in order to reduce corrosion and fabric wear. This is most important during system start-up

and shutdown conditions. If acid deposition occurs after shutdown, the acid moisture will settle on

the fabric and eventually leave behind a residue which may contribute to the brittleness of the bags

and cause failure when put into operation again. An atypically high pressure drop can signify that
3-8

bags are binding or plugging, gas flow is excessive, or fabric cleaning is inadequate. Low pressure

drop signifies possible filter holes and leakage, leakage between the bag and bag supports, or

inadequate dust cake formation. The FF performance can be monitored with an opacity meter.

Intermittent “puffing”, which is an indication of pinhole leaks in the filter bags, after cleaning, can

be detected with opacity monitoring. Additionally, holes in bags can be detected using fluorescent

powders. The powder is injected into the inlet of the fabric filter to inspect for leaks. Leaks are

detected by inspection for the powder on the clean side of the bag using ultraviolet light.

3.2.5 Inspection and Maintenance

Fabric filters are relatively simple to operate and require minimal maintenance and repair.

However, they require frequent routine inspection and maintenance. Several FF inspections and

maintenance items are required on a daily and weekly basis. Filter bags require periodic inspection

for correct tensioning and conditions such as tears, holes due to abrasion, and dust accumulation on

the surface. The typical filter bag life is as much as 10 years using reverse air cleaning, but this can

be reduced to 2 years for improperly operated systems. Filter bags are fragile and prone to hole

formation if not handled carefully.

3.3 ELECTROSTATIC PRECIPITATORS

Electrostatic precipitators (ESPs) are common air pollution control devices which have

been utilized for many years in both the United States and abroad. Electrostatic precipitators are

generally efficient at removing fine particles from flue gas. Electrostatic precipitators are available

in a variety of designs and can operate in a dry or wet mode and in hot or cold gas conditions. ESPs

are known for the following characteristics:

• High capture efficiency.

• Lowest capture occurs with 0.1 to 1 μm particles.

• Extensive monitoring requirements.

• Automatic controls.

• Low routine maintenance.

ESPs have several advantages, but most significant is the high collection efficiency and low gas

pressure drop.

3.3.1 Design Principles

Electrostatic precipitation is a process by which gas suspended particles are electrically

charged and passed through an electric field which propels the charged particles towards collecting

plates, as shown in Figure 3-5. The charged particles stick to the plates, and periodically a rapping

(impact) mechanism dislodges the collected particles from the plates. The dislodged particles drop

into the collection hopper for removal.
3-9

In the particle charging step, particles in the gas stream are charged by the development of

a current of negative ions generated from a high-voltage corona discharge. The corona is established

between an active electrode (usually a fine wire or plate of negative polarity) maintained at a high

voltage, and a smooth cylindrical or plate electrode at ground potential. The corona ionizes the flue

gas as it passes around the electrode, forming a large number of ions in the region near the electrode

surface. Negative ions are attracted to the ground pipe or plate. Particles passing through the

corona field are subjected to intense bombardment by the negative ions and become charged.

The negatively charged particles are electrically attracted to the grounded collection surface.

The particle “migration velocity” toward the collection surface, which is influenced by the particle

diameter, particle charge, and collecting field intensity, typically ranges from about 0.1 to 3 ft/sec.

Electrostatic precipitators can be found in both high and low temperature applications from

the material temperature limits of 1300°F and down to the condensation point of flue gas constituents.

Precipitators are commonly designed for either hot- or cold-side treatment. Hot-side precipitators

have larger collecting plate areas and are constructed of suitable steels to handle continuous high

temperature operation. The “cold-side” categorization refers to the ESPs operating at gas

temperatures below 500°F.

The general configuration of an ESP is a wire (or rod) discharge electrode positioned at an

equal distance between two collecting plates. The electrostatic precipitator casing houses many

passages of parallel collecting plates with wires (or rods) suspended at regular intervals through the

passage. The plates are further compartmentalized into fields, with each field typically energized

by its own set of power supplies. A negative high-voltage direct-current power is applied to the

discharge electrodes, and the collecting plates are grounded. The high-voltage direct-current power

produces a corona in the wire-to-plate spacing by ionizing the gas. The corona then creates an

avalanche of negative ions traveling from the negative discharge electrode to the grounded collecting

plates. Suspended particles passing through the wire-to-plate spacing are bombarded by negative

ions and become charged. The high voltage electricity produces an electric field between the wire

and the plate which provides the electromotive force to attract the charged particles to the collecting

plates.

Electrostatic precipitators can be classified into three types: tubular, wire-to-plate, and flat

plate. A wire-to-plate type ESP is shown in Figure 3-6. It consists of parallel plates of sheet metal

collection surfaces with wire electrodes hanging between the plates. Flat plate ESPs, as shown in

Figure 3-7, differ from wire-to-plate types in that the electrodes consist of flat plates rather than

wired. The plates increase the electric field used to collect the particles, and provide increased

surface collection area. However, a flat plate ESP operates with little or no corona, which leads to

large rapping losses, especially if the flue gas velocity is high. The tubular type, shown in Figure

38, consists of an array of tubes, round or multi-sided, which act as the collection surface, with a

discharge electrode located in the center of each tube.

Also, there are two typical arrangements of the electrodes used in the ESP: single-stage or

two-stage, as shown in Figure 3-6. In the single stage arrangement, the discharge and collecting

electrodes are located in the same chamber. In the two stage arrangement, the discharge electrodes
3-10

are placed in the first chamber, where particulate charging is accomplished. Collection electrodes

are placed in a second chamber located downstream of the first chamber. Particles are removed

from the gas in the second chamber.

The following process characteristics dictate the specific ESP design:

• Gas volume flow rate.

• Particle loading.

• Gas temperature.

• Particle size distribution.

• Particle composition.

To accommodate these process characteristics, the following physical and electrical characteristics

of an ESP may be varied:

• Number of fields.

• Number of passages per field.

• Wire-to-plate spacing.

• Collection plate surface area.

• Wire (or rod) diameter.

• Aspect ratio (length to height).

• Maximum secondary voltage.

• Maximum secondary current.

• Number of sparks per minute.

The charged particles which accumulate on the collecting surfaces must be periodically

dislodged. Dislodging the collected particle layer is accomplished in several ways using either

mechanical rapping devices or water for wet ESPs as discussed in a following section. Typically,

dry electrostatic precipitators utilize mechanical rapping devices which strike the collecting plates

and dislodge the particles. Rappers are classified into two categories:

• Impulse types which produce an impact or hammer blow. A magnetic system provides

controlled blows (essentially continuous rapping) which eliminates rapping puffs

characteristic of intermittent rapping systems. Impulse rappers are most useful for cleaning

collecting plates.

• Vibrator types which produce a continuous vibration for some duration. These are usually

operated intermittently. They are usually used for corona-electrode cleaning.

The particles fall into the collection hoppers and are removed for disposal. Cleaning of the ESP

collecting plates is critical because collected particles may be lost by reentrainment into the gas

stream while still inside the precipitator.

Newly designed ESPs have larger wire-to-plate spacings and use rigid rod discharge

electrodes instead of wire. The newer ESPs are capable of operating at higher voltage, which

increases the electric field strength and the subsequent particle capture. Many ESPs are equipped
3-11

with advanced controlled power supplies which monitor electrical condition and maximize

performance while minimizing power consumption. Advanced control units provide features such

as intermittent energization and spark rate control. Intermittent energization switches off power

repeatably for extremely short durations and maintains ESP performance while minimizing power

consumption. Spark rate is important in the electrical operation. Sparking is a phenomenon which

occurs because an ionized discharge to the collecting surface resembles an electrical arc. Excessive

sparking will reduce the applied voltage and waste power; insufficient sparking may indicate that

the ESP is not operating near its full potential. With automatic controls, a desired spark rate can be

specified and will automatically be monitored and maintained.

3.3.2 Performance

Operating efficiency of the ESP increases with increasing plate area, increasing voltage and

decreasing gas flow rate. Capture efficiency is dependent on:

• Specific collection area (SCA).

• Operating voltage.

• Particle characteristics.

The precipitator’s size is measured in terms of specific collection area (SCA), which is the

ratio of collection surface area to gas volume flow rate. Older ESP designs have specific collection

areas (SCAs) which are typically under 300 ft2/kacfm with a plate-to-plate spacing of about 9

inches. The newer designs incorporate wider plate-to-plate spacings with rigid rod discharge

electrodes replacing weighted wire type electrodes. The newer ESPs have state-of-the-art

microprocessor power controls, and the geometry permits higher voltage potentials. The particle

collection is also subsequently improved.

Electrostatic precipitators are least efficient at capturing particles in the 0.1 to 1 μm diameter

size range; however, when designed and operated properly, the ESP is still capable of excellent

collection of particles in this size range. Electrostatic precipitators capture submicron particulate

with approximately 90 to 95% efficiency. From the efficiency on submicron particles, the efficiency

increases linearly to 99.9% for particles of 5 μm. Operation is sensitive to fluctuations in gas

flows, temperature, particulate and gas composition, and particle loadings. This is because the

electrostatic precipitator’s effectiveness is strongly influenced by the resistivity of the particles in

the gas stream.

The ESP is very efficient at capturing particles of all sizes, however, it is not efficient at

capturing particles that have either high or low resistivities. Resistivity is a measure of the resistance

of the particulate matter to being charged and is a function of particle composition and gas

temperature. Particles with high resistivity (greater than 2x1011 ohm-cm) are difficult to charge

and collect. Also, once collected, they do no readily give up charge to the collector plate and tend

to build up on the electrode. The ion current generated by the electrodes must pass through the

particle layer before reaching the grounded plate. The current gives rise to an electric filed in the

layer, and when the dust layer builds, can become large enough to cause sparking, known as “back

corona.” When sparking occurs the electrical power input to the ESP must be reduced. The reduction

limits the collection efficiency of the process. Low resistivity particles (less than 2x108 ohm-cm)
3-12

can also be a problem. At this resistivity, particles are easily charged and collected, but the low

resistivity will cause the collected particles to give up their charge to the collector plate and easily

become re-entrained in the flue gas. Particles with resistivities between 2x108 and 2x1011 ohm-cm

are most efficiently collected. Parameters which influence particle resistivity include mineral

composition, density, and flue gas temperatures, moisture, and sulfur content. Resistivity increases

with increasing temperature and decreasing moisture and sulfur content. Effective acid gas removal

upstream of the ESP results in an increase in resistivity and a corresponding decrease in ESP

efficiency. Resistivity can be reduced by adding a conditioning agent such as sulfur trioxide,

ammonia, or water vapor to the flow.

Other factors which may adversely affect ESP performance include:

• Flyby of the particle due to exceptionally high operating flow rates in which the particle

travels past the collection plates with enough speed so as not to be affected by the charged

field.

• Misalignment of the electrodes and collectors plates.

• Dust buildup on the electrodes or collection plates.

• Erosion or corrosion of the collection plates.

• Leakage of scrubber liquid when pH is too high (wet ESPs only).

• Re-entrainment occurring from redispersion of collected particles during rapping of the

electrodes, seepage of collected dust from the hoppers, and direct scouring of collecting

electrode surfaces by the gas streams.

• Undersized equipment.

• Presence of “back corona”, as discussed above, which is due to particle buildup on the

collection plates.

Within the last 2 to 3 years, manufacturers (for example BHA/Preciptech in Kansas, MO)

have developed advanced PC-based ESP controllers which regulate ESP control to improve power

delivery. The advanced controllers are limited to the same peak power, however, the controllers are

capable of managing power delivery and avoiding slow power ramping, such that over time an

higher average power through put is achieved. In addition, these “smart” ESP controllers can

incorporate a PM setpoint and use data from an opacity type meter to provide feedback to control

PM emissions directly. In addition, these controllers can be readily modem linked to allow remote

ESP performance monitoring, potentially reducing maintenance costs. Purchase costs estimates

for these type controllers are on the order of $70,000.
3-13

3.3.3 Upgrade Options

ESP retrofit upgrade enhancement options include:

• Rapping — Improve rapping cycle frequency and procedures to minimize PM reentrainment

during rapping.

• Electrical power control system — As discussed above, the addition of advanced controlled

power system which monitors the electrical field condition can significantly improve ESP

performance. Systems may include “intermittent energization”, “pulse energization”, and

spark rate control systems. Intermittent energization acts to rapidly shut off and on the

electric power to the ESP for short durations. This cycling limits the available time for

detrimental back corona to form. Spark rate control optimizes the operating voltage to as

high as possible for more efficient particle capture. Pulse energization consists of

superimposing narrow high voltage pulses onto a base DC voltage as a way of overcoming

high resistivity particle problems.

• Geometry — Modification of internal plate and electrode geometry to increase plat to

electrode spacing, allowing for high voltage potentials and increased PM capture.

• Flue gas conditioning — Addition of water into the flue gas will act to lower the gas volume

flowrate and gas temperature, reducing particle resistivity and increasing particle collection.

Also, reagents such as sulfur trioxide or ammonia may be injected into the flue gas to

condition particles for lower resistivity.

• Gas distribution — Improve flue gas distribution so as to provide a uniform velocity

distribution to all sections of the ESP. Acts to minimize particle reentrainment and gas

sneakage, and reduces local occurrences of high flue gas velocities where PM capture may

be low due to low particle residence time. This may be accomplished with the use of flow

straighteners such as screens and turning vanes.

• Specific collection area — An increase in the size of the ESP, usually done by making the

existing unit taller, acting to reduce the gas velocity, increase particle residence time, thus

increasing the capture efficiency.

Note that specific retrofit procedures which are most applicable to a given facility must be chosen

on a case-by-case basis, depending on the current condition of the existing facility.

3.3.4 Process Monitoring

To assure proper operation, the inlet gas temperature, flue gas flow rate and electrical

conditions are monitored. On-line electrostatic precipitator performance is typically monitored

with an opacity meter, however, continuous and reliable monitoring of electrostatic precipitator

performance is difficult. An electrostatic precipitator’s performance is sensitive to flow rate, particle
3-14

composition and temperature, and the electrostatic precipitator’s response to these parameters is

relatively slow.

Temperature has a direct effect on particle resistivity, and therefore electrostatic precipitators

must be operated within the designed temperature range. For cold-side ESPs, the temperature

range is limited to temperatures above the condensation point and below the electrostatic precipitator’s

material limit usually specified by the manufacturer. For hot-side electrostatic precipitators, high

temperature is preferred within the maximum operating range specified by the manufacturer. Particle

resistivity peaks at around 400°F and is lowest at high and low temperature.

The electrical conditions do depend on the type of ESP. For the conventional cold-side

wire-to-plate ESP, the voltage will range from 25 to 50 kV with even higher voltage for wider

wireto-plate designed electrostatic precipitators. Spark rate is often optimized on site but is generally

between 50 and 150 sparks per minute. The higher the secondary current the better. Secondary

current may be quenched by excessive particle loading. The secondary current density, which is

the secondary current divided by the collecting plate surface area, is often less than 100 nA/cm2.

The secondary current density can be calculated for each field by dividing the field secondary

current by the plate surface area in that field. Generally, the first fields have the lowest current

density because a significant percentage of the current goes into charging the particles which are

captured in fields downstream.

To summarize, the following monitoring activities are required for optimum ESP operation:

• Inlet gas temperature.

• Gas flow rate.

• Electrical conditions.

• Rapper intensity.

• Hopper ash level.

3.3.5 Inspection and Maintenance

Maintenance is required for cleaning carbon deposits on plates which can cause short circuits,

corrosion of plates, and erosion of electrodes. Electrostatic precipitators are fairly sophisticated

devices requiring automatic controls for rectifier equipment, measurements systems for rapper

intensity, hopper dust level, and metering systems of gas process variables such as gas flow and

temperature. Because of this complexity, highly trained maintenance personnel are required.

However, routine maintenance requirements for electrostatic precipitators are low compared with

fabric filters. The frequency of inspections and maintenance are typically monthly for external

systems such as power supplies, monitors, etc., and annually for internal systems such as electrode

conditions, electrical connections, etc. Internal inspections and maintenance are conducted in hostile

environments due to the potential mechanical and electrical energy of the electrostatic precipitator.

Annual inspections of the following are necessary: electrode alignment; electrode pitting; ash

build-ups; ash hardening; hopper performance; electrode insulation; and rapper operations. To

summarize, the following inspection and maintenance activities are required for optimum ESP

operation:
3-15

• Highly trained personnel.

• Low routine maintenance.

• Inspection of electrode misalignment, pitting, ash build-up, ash hardening, hopper blockage,

electrode insulation cracks, and rapper performance.

3.3.6 Wet Electrostatic Precipitators

Wet electrostatic precipitators, shown in Figure 3-9, utilize water to rinse the collected

particle layer off the collecting surface. Wet ESPs must be operated at or below the adiabatic

saturation temperature of the irrigating fluid (water) so that the fluid is not evaporated. There are

several types of wet ESPs, depending on the irrigation method:

• Self irrigating — Captured droplets wet the collecting surface.

• Spray irrigation — Spray nozzles continuously irrigate the collecting surface.

• Intermittently flushed irrigation — The ESP operates as a dry precipitator with periodic

rinsing to replace rapping.

• Film irrigation — A continuous liquid film flushes the collecting surface.

The wet electrostatic precipitator is used effectively in the following conditions:

• After flue gas has been through a wet scrubber.

• Low gas temperature and high moisture content.

• High sub-micron particle concentration.

• Liquid particle collection.

• For handling dust wet.

3.4 VENTURI SCRUBBERS

Venturi scrubbers are a commonly used for particulate and acid gas control on hazardous

waste incinerators.

3.4.1 Design Principles

The conventional venturi scrubber consists of a converging section, a throat section (either

rectangular or circular), and an expansion section, as shown in Figure 3-10. An alkaline recirculating

liquid is usually injected at an angle, in one or more streams, into the throat section or just upstream.

The engineering principle, as shown in Figure 3-11, is to accelerate the gas to a high velocity in the

throat section, causing atomization of the liquid. In some cases, however, spray nozzles are used to

atomize the liquid. The solution droplets are then accelerated to their terminal velocity. These

droplets provide a large surface area for collection of fine particles and are many times larger than

the particles in the flue gas. The particles are captured when they collide with the slower moving

droplets. As the mixture decelerates in the expanding section, further impaction occurs and causes
3-16

the droplets to agglomerate. Particle capture is primarily related to the amount of liquid atomization

achieved, which is proportional to the pressure drop (gas velocity) across the device.

Once the particles have been trapped by the liquid, a separator (e.g., cyclone, demister,

swirl vanes) is used to remove the liquid from the cleaned gas. Mist eliminators consist of a mesh

of fine wire or wave plates. They provide a large area of collection surface area while maintaining

a high void space. Their use minimizes the risk of blockage by entrained solid particles and moisture

in potential downstream operations.

The most prominent feature of the venturi design is a converging throat section which

causes acceleration of the flue gas flow. The scrubbing slurry is introduced at the inlet of the throat

and is sheared into fine droplets by the high velocity flue gas stream. A turbulent zone downstream

of the throat promotes thorough mixing of the gas and slurry droplets. As the droplets slow down

through the diverging section, they collide and agglomerate and are separated from the cleaned gas

stream by gravity as the gas passes on to the stack.

Venturi scrubbers use one of two approaches: wetted or non-wetted, as shown in Figures

310 and 3-12. When the gas entering the venturi is not at saturated conditions, the wetted approach

is used. In this approach, a protective film of liquid is established on the convergent portion of the

venturi. This liquid is introduced upstream of the throat and flows down the converging sides into

the throat, where it is atomized. The gas is cooled by evaporation and reaches the throat at near

saturation conditions. When inlet gases are hot and a significant amount of liquid needs to be

evaporated, the wetted approach is preferred. The non-wetted approach is restricted to gases with

subsaturation temperatures entering the throat. If the hot gases enter the throat, they will vaporize

the water droplets and reduce the device’s ability to capture particles.

Scrubbers work best when the flue gas entering the scrubber is fully saturated with water so

that none of the injected water droplets tend to evaporate. Evaporation produces a flux of water

vapor out from the surface of the target droplets that inhibits the capture of smaller flue gas particles.

If the liquid is cooled to a temperature below the gas temperature, the capture of small particles will

be enhanced. This occurs due to condensation of water vapor from the flue gas onto the target

droplets coupled with the tendency of particles to migrate toward and collect on cool surfaces.

3.4.2 Innovative High-Energy Scrubbers

3.4.2.1 Flux force/Condensation/Collision

Flux force/condensation/collisions scrubbers are a variation on the venturi type scrubber.

The Calvert Collision Scrubber (CCS), shown in Figure 3-13, is an example of this type of scrubber.

Initially, the flue gas is quenched with cooled scrubber water. Gases entering the scrubber are split

into two streams which are directed to separate venturi throats where scrubber liquid is injected

into the gas steam. The discharged gas/liquid streams from the two venturis are directed toward

each other, causing them to collide at a high relative velocity. The head-on collision of the streams

causes the droplets to shear into fine droplets, thus improving the efficiency of the device. Mist

eliminator is used to remove entrained droplets. Through the use of cooled scrubber liquid, a “flux

force condensation” technique is utilized. Flux force condensation is based upon the principle that
3-17

when a gas stream is saturated with water and then cooled, a portion of the moisture will condense,

and the fine particles in the gas stream serve as condensation nuclei. As moisture condenses on the

particles, they grow in mass and are more easily collected by conventional impaction. Therefore,

the condensation enhances the scrubbing system’s collection of fine particles, acid gases and metals.

3.4.2.2 Free-Jet

Free jet scrubbers have the same basic configuration of venturi type scrubbers (i.e., quench,

scrubber, and moisture separator). Figure 3-14 shows several types of free jet scrubbing systems.

The energy for moving the gases through the system and cleaning the gases is provided by the

injection of a compressible fluid (typically steam or air) from a supersonic ejector nozzle which is

located inside the flue gas duct. The amount of fluid added through the ejector is proportional to

the mass of gas flowing through the system. The turndown capacity of the system is high because

the ejector supplies the energy required to move the gases through the system. At the exit of the

ejector nozzle, water is injected into the high velocity flow. The velocity of the steam or air breaks

the water into small droplets. The flue gas and ejector fluid mixture then passes through a subsonic

nozzle in which additional water spray is injected. Finally, the gas passes into an expansion section

where free jet mixing takes place, aiding in further particulate agglomeration and capture. The

primary advantage is improved capture efficiency compared with conventional venturi scrubbers

and lower total energy requirements. Disadvantages include the potential need for on-site steam

supply for the ejector driven systems.

3.4.2.3 Rotary Atomizing

In rotary atomizing scrubbers, the flue gas is saturated and passed through a “curtain” of

liquid droplets, generated by a rotary atomizer disk. Rotary atomizer spins at speeds above 10000

rotations per minute, creating the droplet curtain. Advantages include efficient collection of

submicron particulate compared to venturi type scrubbers and independence on the flue gas flow

rate. Disadvantages temperature limitations of rotary mechanical equipment and lack of operating

experience.

3.4.2.4 Rotor Filter

Flue gas passes through two rotors, each made up of five counter-rotating wheels and driven

by a separate electrical motor (Valenti, 1993). The wheels project V-shaped airfoil-type spokes that

generate vacuum and centrifugal forces when they are in rotation. Particulates are mechanically

removed from the flue gas by the centrifugal force of rotation, which will funnel them into the cape,

or wall of the Rotor Filter. The leading edge of the spoke causes the particles to conglomerate into

larger particles, which facilities their removal. Additionally, it has been shown to remove gaseous

pollutants such as sulfur oxides by conglomeration. Rotor filters have been shown to achieve

performance levels of 0.0067 gr/dscf on a medical waste incinerator.
3-18

3.4.2.5 Electrodynamic Venturi

The Electrodynamic Venturi (EDV) concept involves a series of steps consisting of saturation,

expansion, condensation, ionization, and filtration (Pilney et al., 1993; Foisy and Karell, 1993).

The flue gas is saturated in a water spray tower and passed into an “Agglo-filtering Module” in

which the gas is expanded; expansion of a gas saturated with water vapor produces a state of

supersaturation. Particulate matter act as condensation nuclei and a film of water is formed on the

surface of the particulate. The supersaturation gas then enters an “Electro-filtering Module”; a

high voltage electrode is mounted on the axial centerline of the module to charge the wetted dust

particles and droplets prior to filtration. In the final stage, he flue gas containing the ionized

encapsulated particulate is passed through a curtain of positively charge water droplets. Particulate

matter is removed by impaction with the water droplets, which is boosted by the electrostatic attraction

effect. EDVs have been shown to achieve performance levels of 0.005 gr/dscf.

3.4.3 Performance

To obtain a high collection efficiency, the flue gas throat velocity must be maintained at a

specified level. Approaches that can be used to adjust for varying flue gas flow conditions include:

• Variable throat geometries with adjustable throat inserts (shown in Figure 3-15).

• Adjustable butterfly valves, dampers, or baffles in the throat region (shown in Figure 3-15).

• Adjustment of the elbow area in the throat section. Most scrubbers have a “flooded

elbow” which helps collect entrained droplets before exiting the venturi.

The liquid surface tension and liquid turbidity are important to capture efficiency. If the

surface tension is too high, small particles will “bounce” off the water droplets. High surface

tension also has an adverse effect on droplet formation. High liquid turbidity will cause erosion

and abrasion of the venturi section.

High pressure differential venturi scrubbers achieve higher particle removal, however, capture

of submicron particles is still relatively inefficient. High pressure differential venturi scrubbers,

also known as high energy scrubbers, are often equipped with variable throats for tuning the capture

performance and minimizing pressure drop. The high pressure differential is costly in terms of

electrical power requirements and is impractical for larger units.

The capture efficiency of venturi-type scrubbers is affected by the following:

• Inadequate scrubber liquid feed due to erosion and corrosion of the scrubber liquid

transport system.
3-19

• Erosion of the venturi wall dimensions.

• Erratic pressure drops due to changes in the gas flow rate.

• Scaling of internal parts.

• High temperatures resulting in atomized scrubber liquid droplets evaporation.

Advantages to the use of venturi scrubbers include:

• Secondary capture of acid gases.

• Performance is independent of the chemical and physical properties of the particles.

• Can handle high humidity flue gas streams.

• Operates at low temperature thus increasing the change of condensing and capturing volatile

metals and organic constituents.

Disadvantages to the use of venturi scrubbers includes:

• Steam plume visibility. Flue gas heating may be required to eliminate the steam plume.

• Generation of secondary liquid waste stream. The particulate matter collected in a venturi

is

wet. This may be advantageous since it minimizes the potential for particulate blow-off

during handling and disposal. However, it creates a secondary liquid waste stream with an

increased volume compared to dry collection techniques.

3.4.4 Process Monitoring

The essential parameters which should be monitored for optimum venturi scrubber

performance include:

• Scrubber blowdown frequency.

• Scrubber water suspended solids.

• Liquid pH.

• Gas inlet temperatures.

• Pressure drop across the venturi.

• Gas flow rate.

• Liquid feed rate.

• Liquid-to-gas ratio.

The scrubber operation is monitored by liquid spray rate, gas flow rate, liquid pH, scrubber

blowdown rate and inlet gas temperature. The pressure drop is a good indication of system
3-20

performance (particle capture efficiency). The pressure drop is usually controlled with a variable

venturi throat. A pH monitor is used to measure the scrubber liquid pH in order to control the

amount of caustic material added to the scrubber sump. A low scrubber liquor pH will cause

corrosion, while a high pH will cause scaling.

3.4.5 Inspection and Maintenance

Maintenance concerns include prevention of corrosion and scaling on all scrubber internal

surfaces, excessive dust build-up, nozzle damage (abrasion/erosion), plugging and fluid leakage.

However, because the venturi is self cleaning, it has high resistance to fouling. Visual inspection is

usually required for the throat, nozzles and liquid pump. Venturi scrubbers are often applied to

hazardous waste incinerator facilities since they are easily operated and integrated with acid gas

control technologies. A more detailed discussion of the inspection and maintenance of all types of

wet scrubbers in included in Section 3.5.4.

3.5 WET SCRUBBERS

Wet scrubbers appear in a variety of configurations with widely variable efficiencies for

controlling acid gas and particulate emissions. Wet scrubbers are used for the simultaneous removal

of soluble gaseous pollutants and particulates. The majority of hazardous waste incinerators use

packed bed scrubbers, plate tower scrubbers, and/or venturi scrubbers for air pollution control.

In wet scrubbing, the flue gas enters the absorber where it is saturated and contacted with

the sorbent solution. The sorbent reacts with the acid gases to form salts, which are generally

insoluble and may be removed by clarifying, thickening, and vacuum filtering of the scrubber

blowdown. Typically, wet scrubbers have a 2-stage arrangement, as shown in Figures 3-16a and

316b: the first stage (typically a venturi type scrubber) for HCl and HF removal and a second stage

for SO2 removal (typically a packed or spray tower type). This is done to avoid the build-up of

chlorides in the scrubber liquor, which can be highly corrosive. In the first stage, water is used to

quench the gas to saturation conditions and HCl and HF are captured in a venturi type set up. In the

second stage, a sorbent slurry is used for SO2 removal in a packed, tray, or spray tower.

The primary advantages of wet scrubbers are that they can typically provide higher levels

of acid gas control compared with dry scrubbers and they generate low amounts of waste for treatment

and disposal because acid gas absorption is more efficient when the gases are saturated (Sedman

and Brna, 1987). However, wet scrubber system disadvantages include:

• Require waste water treatment facilities or drying equipment for scrubber byproducts

processing prior to disposal.

• Not as efficient at particulate removal compared to dry scrubbers.

• May require stack gas reheat requirements prior to exhausting to atmosphere.
3-21

• Prone to corrosion problems and may require expensive materials of construction.

• Higher operating and maintenance requirements compared with dry scrubbers.

• Accumulate of suspended and dissolved solids in scrubber liquors. To keep from being

entrained by the gas flow, liquid must be replaced and replaced with fresh liquor.

3.5.1 Design Principles

Wet scrubbers remove particles from the gas by capturing the particles in liquid droplets

and separating the droplets from the gas stream. The liquid injected to capture the pollutants can be

water or a solution. This solution is often referred to as scrubbing liquor or scrubbing slurry. The

majority of operating scrubbers use lime or limestone as the scrubbing solution because of its wide

availability and low cost. Sodium-based scrubbing solutions are also used, but to a lesser extent.

This section will focus on lime slurry injection type wet scrubbers which have been proven effective

in the removal of particulate, HCl, SO2, and volatile metals.

3.5.1.1 Scrubber Designs

There are a number of scrubber designs currently used on hazardous waste incinerators.

These include types such as spray tower, tray tower, packed tower, and ionizing wet scrubbers.

Spray Tower — A common wet scrubber design is the spray tower, shown schematically in

Figure 3-17. Spray scrubbers consist of a vessel with a bank of liquid spray nozzles which are

mounted across the vessel cross section. Atomized scrubbing liquid (usually water or a dilute

solution of an alkaline agent in water) produced by the nozzles fall through the gas stream, which

travels up through the vessel. The scrubbing slurry absorbs flue gas acid gases, forming a disposable

sludge. The cleaned gas passes through the mist eliminators to removed entrained liquid and

particulate. The liquid droplets must be large enough so that their terminal velocity is larger than

the gas velocity, otherwise the droplets will be carried out of the vessel.

Tray Tower — Another scrubber design is the tray tower (also known as an

“impingementtype”) scrubber, shown in Figure 3-18. The flue gas enters at the base and passes

upward through holes in one or more perforated plates mounted horizontally across the scrubber.

Scrubbing slurry is sprayed onto the top tray from above, and then overflows down onto the

subsequent bottom trays. Each of the trays has many small openings; the flue gas is forced up

through the openings, forming small jets. The slurry on the tray becomes a “froth” due to the gas

passing through it, providing very good contact of the flue gas with the slurry. The jets prevent the

passage of the liquid layer through the tray openings. The main disadvantage of tray tower scrubbers

is that they cannot handle load variations (flue gas flow variations). At low flue gas flow rates, the

slurry will drop through the sieve (weeping), while at too high of a flow rate, the slurry mixture is

blown out of the scrubber. Additionally, the tray holes are prone to plugging, and the scrubber must

be shut down and cleaned periodically.
3-22

Packed Tower — Packed tower scrubbers, shown in Figure 3-19, incorporate a bed of

packing material mounted across the scrubber vessel. A liquid is injected down through the packing

material, which is usually a random mixture of individual pieces such as balls, saddles, rings, or

shaped solids. The flue gas enters at the base of the tower and flows up through the packing against

the slurry flow which is introduced at the top of the scrubber. The packing slows the flue gas down,

distributes the gas and liquid slurry flows, and provides increased surface area for the gas to contact

the slurry, resulting in high removal efficiencies. Operational problems of packed beds include

changes in gas velocity hinders liquid movement through the packed bed, and plugging of packed

bed with solid deposits.

Ionizing Wet Scrubbers — Ionizing wet scrubbers (IWS) combine the principles of

electrostatic particle charging, inertial impaction, and gas absorption to simultaneously collect

submicron particles, liquid droplets, and acid gases. As shown in Figure 3-20, the IWS consists of

two sections: a high voltage ionization section and a packed scrubber section. The gas stream

passes through the ionization section where the particles are electrostatically charged. The charged

particles then enter the packed bed section where particles are removed by attraction to neutral

surfaces (collection plates) and impaction on the packed bed materials. A water or caustic

recirculating stream continuously washes the packed bed to remove collected particles and acid

gases. Because the IWS is a fractional removal device, particle removal efficiency can be increased

by employing multiple units in series. IWS advantages include increased particulate matter as well

as acid gas collection efficiency compared to typical venturi type and other wet scrubbers, and low

pressure drops.

3.5.1.2 Reagent Preparation and Injection Equipment

The scrubber slurry side of the process starts with the fresh scrubber slurry being pumped

into the scrubber. The portion of the scrubber that contains the slurry solution is often referred to as

the reaction tank. Next, the scrubbing slurry is pumped from the bottom of the tank to the slurry

nozzles located approximately two-thirds of the way up the tower. The nozzles spray down against

the flue gas flow. The scrubbing solution reacts with the flue gas and most of the particles fall back

into the reaction tank, but some of the slurry (the smaller particles) can become entrained in the flue

gas flow and travel up toward the mist eliminators. The mist eliminators serve to trap most of the

particles entrained in the flue gas, and they are then washed down to the reaction tank by wash

water spray.

Not all of the slurry pumped from the bottom of the reaction tank is sent to the slurry

nozzles. A small percent is removed as waste and is made up for by the fresh slurry feed. The spent

slurry is diverted to a thickener or hydroclone which removes moisture. The dewatered slurry is

disposed of, and the excess moisture is recycled back into the reaction tank and is also used as mist

eliminator wash water.

Limestone and lime-based scrubbing slurries are by far the most commonly used scrubbing

reagents. The main advantages of using lime or limestone include:
3-23

• The process is simple and has few process steps.

• Capital and operating costs are low and limestone is abundant.

• SO2 removal efficiency can be as high as 95%; HCl and HF removal efficiency can exceed

99%.

The main disadvantages of lime and limestone based scrubbing systems include:

• Large quantities of waste must be disposed of in an acceptable manner.

• Limestone systems have a tendency for scaling, plugging, and erosion.

• Large slurry flows are needed, resulting in large pumps with high electrical consumption.

In addition to lime and limestone scrubbing slurries, dual alkali systems are also in use

which utilize a mix of sodium carbonate (NaCO3) and lime or limestone. The main advantage of

the dual alkali process is that scaling (deposits of calcium solids on the scrubber surfaces) is

minimized. The main disadvantage is that sodium may leak out of the system with the waste and

potentially contaminate ground water.

Typically, wet limestone and lime scrubbers employ on-site wet grinding (slaking for lime)

for slurry preparation. Figure 3-21 shows a typical limestone reagent preparation system. The raw

limestone with a diameter of approximately 1 inch is fed through a weigh belt feeder to a ball mill.

Water is added at the feed chute of the mill. From the mill, the limestone is sent to a

classifier which separates the coarse limestone from the fine limestone. The classifier sends the

fine limestone to the limestone feed tank to be made into scrubber slurry. The coarse limestone is

sent back to the mill for more grinding. Grinds ranging from coarse (70% of the limestone passing

through a 200 mesh sieve) to fine (95% passes through a 325 mesh sieve). The finer the grind, the

better the reduction will be, but the energy consumption for fine grinding is high.

The spray nozzles used to control the slurry mixing with the flue gas typically operate

between 5 and 20 psi and have corkscrew tips. The nozzles produce many droplets of approximately

2500 to 4000 μm in diameter. The SO2 reduction chemistry occurs on the droplet surfaces, so the

smaller the droplets, the greater the amount of surface area that is available for reaction, improving

SO2 removal efficiency. However, to decrease the droplet size, nozzles with smaller openings are

required. The pressure required to push the slurry through smaller nozzles is high, and smaller

nozzles tend to plug.

3.5.1.3 Mist Eliminators

Mist eliminators are used on wet scrubbers to collect slurry droplets entrained in the scrubbed

flue gas stream and return them to the scrubbing liquor at the bottom of the scrubber. Mist eliminators

are located at the exit to the scrubber. Most of the droplets leave the flue gas flow due to gravity,

but the small droplets can be carried out with the gas. If these droplets are not removed from the

gas before it exits the scrubber, it can deposit on the ductwork, the induced draft fan, and the walls

of the stack. This may lead to pluggage and corrosion.
3-24

Most mist eliminators are of the impingement type: the small liquid droplets impact a

collection plate, coalesce, and fall by gravity back into the scrubbing liquor. The mist eliminators

are sprayed with wash water to remove accumulated solids. The wash water is generally a mixture

of fresh water and clear water from the slurry dewatering system.

3.5.1.4 Waste Treatment and Disposal

There are several types of waste treatment and disposal methods currently in use. The

earliest method of waste disposal was simply to discharge the spent wet slurry to a settling pond.

However, site availability and pond management costs have limited ponding in recent years. The

most popular disposal approach in practice today is to perform two separate stages of dewatering

and then sending the filter cake to a landfill.

The first stage of dewatering (primary dewatering) can use thickeners or hydroclones. After

the primary dewatering, the sludge is between 20% and 30% solids. Secondary dewatering is

accomplished with vacuum filters, or centrifuges. Vacuum drum filters make up 80% of the secondary

dewatering equipment population. Normally, before the dried sludge goes to a landfill, the sulfites

must be mixed with flyash and lime. Since gypsum needs no further treatment before it can be

landfilled or sold to wallboard manufacturers, some scrubbers employ forced oxidation equipment

in the reaction tank to convert the calcium sulfite semihydrate to gypsum.

3.5.2 Performance

To achieve optimum performance, several design and operating parameters must be

considered including:

• Concentration and pH of caustic (such as limestone or sodium) in scrubbing slurry.

• Scrubbing slurry flow rate compared to gas flow rate.

• Uniform distribution of the scrubbing slurry.

• Uniform distribution of the flue gas.

The amount of limestone in the slurry controls the amount of calcium ion available to react

with the bisulfite ion. The chemical equations show that, theoretically, one molecule of limestone

is required to remove one SO2 molecule. However, due to real world limitations on how well the

limestone and flue gas can mix, up to 1.2 molecules of limestone are typically used for every

molecule of SO2.

To achieve very good contacting of limestone to acid gases, one should simply increase the

concentration of limestone in the scrubber solution. However, if too much limestone is added, the

pH of the solution will rise (the solution will become less acidic). The acid gas removal reactions

work most efficiently at pH levels below approximately 6.3. Therefore, the amount of limestone

added must be high enough to allow contacting with the acid gases, but not so high as to raise the

pH above 6.3. Figure 3-22 shows the effect of scrubber solution pH on SO2 removal.
3-25

In addition to limestone concentration in the scrubbing solution and scrubbing solution pH,

the slurry solution flow rate also determines what level of acid gas reduction can be achieved. The

slurry flow rate is related to the flue gas flow rate and is referred to as the liquid to gas ratio (l/g). In

general, the higher the liquid to gas ratio, the better the acid gas removal efficiency will be. Typical

limestone scrubbers operate at liquid to gas ratios of approximately 30 gallons of slurry for every

1000 cubic feet of flue gas. Since the flue gas flow rate varies with load, so must the slurry flow

rate if the design liquid to gas ratio is maintained. Figure 3-23 demonstrates how the liquid to gas

ratio (at a constant pH) impacts the SO2 removal efficiency. As may be seen, liquid to gas ratios

above 30 gallons per 1000 cubic feet of flue gas do not significantly impact SO2 reduction efficiency.

Even if the concentration of limestone, the pH, and the liquid-to-gas ratio are optimum,

good acid gas removal efficiencies will not be achieved unless the scrubbing slurry is sprayed

evenly into the flue gas. To mix the slurry with the flue gas as rapidly and evenly as possible, it is

best to have many small nozzles distributed across the scrubber cross section. If only one or two

nozzles are used, pockets of flue gas can escape the scrubber without being treated. Unfortunately,

small nozzles have a tendency to plug, so the final design must be a compromise between many

small nozzles that evenly distribute the scrubbing slurry but have a tendency to plug and a few large

nozzles that do not evenly distribute the slurry but will not plug.

Finally, care must be taken to ensure that the flue gas flow is evenly distributed across the

scrubber. For example, if most of the flue gas goes up the right side of the scrubber and the

scrubbing solution is evenly sprayed across the scrubber, there will not be enough scrubbing solution

to react with all the acid gas on the right side, causing poor acid gas reduction. The flue gas inlet

duct at the bottom of the scrubber is designed to promote even distribution of the flue gas across the

scrubber cross section. In some cases, perforated plates are used to help distribute the flow, but

these cause the pressure drop across the scrubber to increase and results in a higher electricity

consumption by the fan.

3.5.3 Process Monitoring

For the reasons previously stated, wet scrubber operation is monitored by the following

key parameters:

• Pressure drop across the scrubber.

• Concentration and pH of scrubbing slurry.

• Liquid-to-gas ratio via gas flow rate and slurry flow rate.

• Accumulation of solids within the scrubber.

• Induced draft fan power consumption.

• Reagent preparation system operation.

• Inlet and outlet gas temperatures.

• Dewatering process.

3.5.4 Inspection and Maintenance
3-26

Scrubber operators must constantly monitor and control the system to ensure proper

performance. For example, when load changes occur, the scrubber operator needs to reset the

limestone feed rate and use the pH monitor as a backup. In addition to good operation and

communication, a preventive maintenance program specified by the manufacturer should be

implemented; this should include inspection of scrubber module, mist eliminators, dampers, ducts,

fans, slurry equipment, valves, and instrumentation for corrosion, erosion, scale deposits, leaks,

and plugging. Visual inspection can allow identification of small problems before damage becomes

so extensive that major repair is required.

Mist eliminators tend to be subject to buildup of slurry solids and chemical scale, causing

the passages to restrict the flow of the flue gas. The first sign of scale buildup is typically noticed

by an increase in pressure drop across the scrubber. Water washing is typically sufficient to prevent

serious buildup problems.

Because the scrubber control system is based on flue gas and scrubber flow rates, the

operating staff should routinely monitor and record readings from all instruments used to measure

these flows. If any of the readings appear abnormal, they should be investigated. To verify liquid

flow rates or evaluate pump/nozzle erosion, the operator should monitor pressure in the slurry

header and the recirculation pump discharge. An increase in the pressure usually indicates plugging

of nozzles. A decrease can indicate wear of the nozzles or pump impellers. Slurry flow in pipes can

be checked by touching the pipe. If it is cold at the normal operating temperature, then the line is

plugged.

The slurry feed requirement is usually controlled by the pH indicator. The sensor lines

where pH measurement elements are used should be frequently backflushed and calibrated with

buffer solutions to ensure reliable operation.

3.6 DRY SCRUBBING

In some respects, dry scrubbing systems represent a less complicated and cleaner control

system than wet scrubbers. This is because no mist eliminator is required, the number of pumps

and amount of piping are greatly reduced, and the waste material is in the form of dry particulates,

which eliminates the need to treat liquid scrubber wastes and significantly reduces the waste volume.

Dry sorbent injection, shown schematically in Figure 3-24, involves the injection of a dry

sorbent fine powder into the flue gas. Acid gas removal occurs as the sorbent reacts with the acids

in the flue gas to form solid salts. The solids salts are collected in a downstream particulate collector.

The particulate collector can be either an ESP or a fabric filter, however, a fabric filter is typically

preferred due to increased capture efficiency and sorbent utilization. This is because the removal

process continues in the fabric filter; captured sorbent, fly ash, and condensed volatile matter is
3-27

held on the fabric and remains exposed to the flue gas, thus allowing for further reaction of unused

sorbent with the flue gas in the particulate collector. Dry sorbent injection is usually performed

after the flue gas has been cooled, typically with a water quench.

Typically, hydrated lime or sodium carbonate (nacholite or trona ores) sorbents are used.

The choice of sorbent depends on the preferred operating temperature. Lime is effective at high

temperature whereas sodium must be used at lower temperatures. For lime, SOx removal is optimized

at 1750°F while HCl and HF is best at 800°F. In cases where efficient capture is required, this may

require the use of multiple injection locations. Typically, large quantities (50 to 300% excess) or

sorbent are required for efficient control in dry systems. Increased control efficiency and decreased

consumption of sorbent can be achieved by increasing flue gas humidity and recycling the reaction

products captured in the particulate control device back into the flue gas stream.

In some cases, the sorbent may be injected directly into the furnace section of the combustor,

known as “furnace sorbent injection” (FSI). Typically, FSI provides extended sorbent/flue gas

contact times compared with injection just upstream of the particulate collector. Due to the high

temperatures, limestone is exclusively used; the limestone calcinates to produce lime which then

reacts with the acids. The primary disadvantage of FSI is that poor sorbent utilization is achieved

due to the hindrance of the lime-acid (in particular chlorine) reaction at high temperature. In some

innovative arrangements, FSI is used in combination with injection upstream of the particulate

collector to provide optimum acid gas removal.

The primary advantage is the production of a dry secondary waste. Additionally, it has

been shown that mercury may be removed from the flue gas when calcium hydroxide is present.

Disadvantages include low utilization of sorbent, high particulate loadings, and the encouragement

of fouling and erosion of the heat transfer surfaces for FSI.

3.7 SEMI-DRY SCRUBBING

3.7.1 Design Principles

The “spray dryer absorption” (SDA) process, shown in Figure 3-25, is considered a “semidry” process since a wet sorbent material is used, but the residue produced is dry. In the SDA

process, sorbent is mixed with water to form a slurry solution. The flue gas is dispersed, usually by

vanes, through a reactor vessel. There are many reactor designs; cocurrent or countercurrent; upor down-flow; single or multiple gas entrances. The slurry is injected into the reactor as a finely

atomized spray. Acid gases present in the flue gas are absorbed into the small liquid droplets and

reacted with the sorbent to form a salt. As the droplets pass through the reactor, heat from the flue
3-28

gas evaporates the water, cooling the flue gas and forming solid particles of salt. Similar to the dry

process, dry solids, including salt, unreacted sorbent, and fly ash, are collected downstream of the

reactor in a particulate removal device, which is typically a fabric filter, although ESPs can also be

used. In some systems, a portion of the dried product is removed from the bottom of the spray

dryer, while in others, it is carried over to and collected in the downstream particulate removal

device. As in the dry process, the feed is sometimes recycled to decrease reagent consumption.

The SDA process is capable of higher acid removal efficiencies (typically greater than 90%

for HCl and 80% for SO2) compared with the dry process (Sedman and Brna, 1987). However, the

following operations are critical to its success:

• The reactor temperature must be maintained high enough to ensure complete sorbent-slurry

evaporation and to ensure the production of a free flowing product.

• Lower flue gas temperature favors efficient acid capture, however, the flue gas outlet gas

temperature must be maintained well above the saturation level, otherwise corrosion

problems will occur.

• Fine atomization of slurry spray and good flue gas/slurry droplet mixing is required.

Additionally, if droplets are too small, a very fine particulate is produced, making particulate

removal difficult.

• The flue gas must be well dispersed throughout the reactor vessel.

• Usage of large amounts of sorbent; typically, 50 to 100% excess of the sorbent is required.

As with dry sorbent injection, to improve sorbent usage, the collected solids may be either

recycled as a dry powder or dissolved and injected thorough the atomizing nozzle.

• Also, to avoid corrosion or product stickiness, the particulate collector and collection hopper

must be either well insulated or heated, and air leakage must be controlled to minimize cold

spots.

For spray dryers, also known as wet/dry scrubbers, hot flue gas enters the top of the dryer

(scrubber) vessel where it is intimately mixed and cooled with a finely atomized lime slurry spray.

Figure 3-26 illustrates several spray dryer arrangements. The moisture is completely evaporated,

leaving the solids as suspended particulates. Mixing occurs in a turbulent flow field. The quenched

and chemically conditioned flue gas then flows to a fabric filter and/or an electrostatic precipitator.

The major equipment found in a typical spray dryer scrubbing system includes the spray dryer

absorber, the particulate-collection system, reagent and slurry preparation and handling equipment,

solids transfer, and process control and instrumentation. In spray dryer systems, the particulate

collector is downstream and is considered an integral part of the system.
3-29

The spray dryer absorber provides the initial contact between the atomized reactive alkali

and the acid-gas contaminants. There are two types of atomizers illustrated in Figure 3-26: rotary

disks (or wheels) and dual-fluid pneumatic nozzles. In each case, the slurry is atomized as droplets

into the dryer, reacts with the acid gases, and dries to a fine powder which is then carried over into

the ESP or fabric filter and removed. The particulate collector also serves as an additional contact

point between the dried reactants and acid gases, providing additional removal. Some spray dryer

systems operate with partial recycling of the particulates by mixing the captured particulates with

the fresh lime in either slurry or dry form before being reinjected into the associated scrubber.

The chosen atomization method affects the design of the spray dryer absorber vessel,

including the physical dimensions. For a rotary-atomizer type of spray dryer, which projects the

droplets radially outward and perpendicular to the gas flow, the length-to-diameter ratio of the

dryer (L/D) is typically 0.8 to 1. The droplets decelerate rapidly owing to the drag forces of the

downward-moving flue gas and eventually attain the speed and direction of the flue gas. To avoid

wall deposition, the designed radial distance between the atomizer and the dryer wall must be

sufficient to allow for adequate drying of the largest droplets. In a dual-fluid pneumatic nozzle type

of spray dryer, which atomizes the droplets in the direction of the gas flow (downward), the L/D is

typically 2:1. In this design, the sidewall deposition is a minor problem. For either design, optimum

spray dryer performance is achieved through proper choice of the L/D, droplet size, and residence

time. The designed residence time for most spray dryers is 10 to 12 seconds, however few systems

operate at 100% of the design flow rate. Thus, actual residence time for most systems is 12 to 15

seconds. Spray dryers range from 25 to 50 feet in diameter.

3.7.2 Performance

The application of a spray dryer results in: a flue-gas temperature of 200 to 300°F;

approximately 20% less gas flow through the particulate-matter collector due to lower gas

temperatures; a particle size distribution with a mean diameter of 20 to 30 μm; and a significantly

increased dust loading. The increased particle size distribution and decreased flow rate are beneficial,

but the decreased temperature and increased dust loading tend to increase pressure drops across

fabric filters and to decrease efficiency across ESPs. The increased dust loading caused by the

reacted products ranges from as little as 1.5 gr/acf at 500 ppm of SO2 to as high as 14 gr/acf at 3000

ppm of reacted products. Therefore, increased dust loading must be taken into account in the

design of the particulate-matter collector and the associated dust-handling support system.

3.7.3 Process Monitoring

The following process monitoring parameters are the same as those for wet scrubber

applications, with the exception of waste treatment system.

• Pressure drop across the scrubber.

• Concentration and pH of scrubbing slurry.

• Liquid-to-gas ratio via gas flow rate and slurry flow rate.

• Accumulation of solids within the scrubber.

• Induced draft fan power consumption.

• Reagent preparation system operation.

• Inlet and outlet gas temperatures.
3-30

Most operational problems associated with spray dryers are due to low temperatures at the

exit of the dryer. When the gas approaches the water dew point, acid condensation may result in the

downstream equipment. Therefore, the flue gas exit temperature is a critical process monitoring

parameter.

3.7.4 Inspection and Maintenance

Spray dryer inspection and maintenance is very similar to that required by wet scrubbers.

Operators must constantly monitor and control the system to ensure proper performance. For

example, when load changes occur, the scrubber operator needs to reset the limestone feed rate and

use the pH monitor as a backup. In addition to good operation and communication, a preventive

maintenance program specified by the manufacturer should be implemented which is similar to the

wet scrubber activities.

Visual inspection of the scrubber section and tanks should be performed on a regular basis

to identify leaks, scaling, corrosion and erosion problems. Visual inspection can allow identification

of small problems before damage becomes so extensive that major repair is required.

Because the scrubber control system is based on flue gas and scrubber flow rates, the

operating staff should routinely monitor and record readings from all instruments used to measure

these flows. If any of the readings appear abnormal, they should be investigated. To verify liquid

flow rates or evaluate pump/nozzle erosion, the operator should monitor pressure in the slurry

header and the recirculation pump discharge. An increase in the pressure usually indicates plugging

of nozzles. A decrease can indicate wear of the nozzles or pump impellers. Slurry flow in pipes can

be checked by touching the pipe. If it is cold at the normal operating temperature, then the line is

plugged.

The slurry feed requirement is usually controlled by the pH indicator. The sensor lines

where pH measurement elements are used should be frequently backflushed and calibrated with

buffer solutions to ensure reliable operation.

3.8 FLUE GAS COOLING

The objectives of flue gas cooling are to:

• Protect low-temperature air pollution control equipment such as fabric filters and cod-side

ESPs.

• Saturate the flue gas prior to wet scrubbing.

• Condense vaporized pollutants such as volatile toxic metals.

• Prevent the formation of certain classes of organics due to fly ash catalyzed reactions. For

example, by cooling the flue gas rapidly, the formation of PCDD/PCDFs can be reduced.

• Recover waste heat.
3-31

Flue gas cooling is required prior to particle and acid gas removal in low temperature devices

such as baghouses, cold ESPs, and wet scrubbers. There are four methods for cooling the flue gas

leaving the incinerator:

• Air dilution.

• Heat exchanger.

• Water quench.

• Radiation and convection duct cooling.

Radiation and convection cooling utilizes heat transfer from the flue gas flow in a long

uninsulated duct. This is the simplest method in principle. However, this method is not considered

for further review due to large space requirements, particle sedimentation in the duct, and the lack

of the ability for precise temperature control during flue gas temperature and flow fluctuations.

The three other methods are discussed below.

3.8.1 Air Dilution

3.8.1.1 Design Principles

For air dilution cooling, shown in Figure 3-27, cool air is injected directly into the hot flue

gas stream. The amount of dilution air required is dependent on the initial temperature of the flue

gas and the amount of temperature reduction desired, as shown in Figure 3-28. The resulting mix

of the two gas streams produces a stream at an intermediate temperature. Air is usually injected

tangentially into the axially flowing flue gas stream. Special air mixing jets may be used so that

thorough mixing of the two streams is obtained within a relatively short distance.

3.8.1.2 Performance

The use of air dilution as a cooling method has certain disadvantages, including:

• Air dilution creates a substantial increase in the total flue gas volume flow rate. This

additional flow requires downstream pollution control equipment to be considerably larger.

• The control of both flue gas temperature and velocity is not possible. Downstream devices

which are affected by velocity (e.g., pressure drop and efficiency for filters and venturi

scrubbers) may be adversely affected by changes in quench air requirements to maintain a

target flue gas temperature.

• There is potential for the intake of ambient moisture and dust if the dilution air is not

preconditioned. This may cause problems in the downstream equipment and may introduce

additional pollutants into the flue gas.
3-32

• Potential for either exceeding the capacity of the blower, causing the exiting flue gas stream

to be too hot, or the blower flow rate cannot be turned down sufficiently to provide minimum

air flow, causing the flue gas stream to be too cold.

Because of these limitations, air dilution cooling is not generally used as the only means of

gas cooling; however, it is often used to make small changes in flue gas temperature, such as

precooling of the flue gas upstream of a heat exchanger to protect the heat exchanger tubes from

high temperature degradation. Advantages of air dilution cooling include ease of use and

implementation and low maintenance requirements.

3.8.1.3 Process Monitoring

The outlet gas temperature and blower flow rate are easily monitored by thermocouple and

flowmeter, respectively, to assure proper operation.

3.8.2 Heat Exchanger

Heat exchangers can also be used for cooling of the hot flue gas stream. In a heat exchanger,

heat is transferred from the flue gas through a common wall to a cooling fluid. Note that boilers are

a type of heat exchanger.

3.8.2.1 Design Principles

Heat exchangers transfer heat through a combination of convective and conductive processes.

A wide variety of heat exchangers are commercially available, including gas-to-gas and gas-toliquid types, as shown schematically in Figure 3-29. In a gas-to-gas heat exchanger, the cooling

fluid is typically air (e.g., a recuperator used to preheat combustion air). In a gas-to-liquid heat

exchanger, the cooling fluid is a liquid, typically water. The liquid is usually contained in a closed

circuit, and liquid cooling is often provided by ambient air blowing.

3.8.2.2 Performance

Liquid heat exchangers are more efficient than gas-to-gas heat exchangers due to their

increased capacity to remove heat. However, as mentioned earlier, they have increased in complexity

since they typically utilize a closed loop cooling fluid. Liquid heat exchangers also have an increased

tendency to promote the condensation of acid gases and subsequent corrosion of common walls.

Advantages to the use of heat exchangers for flue gas cooling include:

• No increase in flue gas volume. Unlike air dilution, and due to the cooling fluid and hot

flue gas streams being separated, the volume of the flue gas remains the same. Therefore, it

is not necessary to increase the size of flue gas cleaning equipment.
3-33

• Recovery (and potential use) of waste heat.

Disadvantages to the use of heat exchangers include:

• The heat exchanger cooling capacity is limited by the heat transfer rate, which is determined

by the cooling fluid flow rate, the inlet temperature, and the thickness of particulate matter

deposits on the cooling surface. Note that if the flue gas contains significant amounts of

particles, particle buildup on the tubes act as an insulator and impair the heat transfer

efficiency, known as tube “fouling”. This can be mitigated by the use of high temperature

particle removal devices upstream of the heat exchanger or by the use of soot blowers to

keep the tubes clean.

• Tube corrosion from condensation of flue gas components (chlorine, sulfur, metals).

• Mechanical failure due to thermal expansion of different exchanger parts at different rates.

• Hot-side tube blockage from soot and flyash deposits.

• Tube erosion in heavily dust laden gas streams.

• Loss of coolant from fire, explosion, power outage, system leak, or mechanical failure of

the

pump, resulting in a system failure. For heat exchangers using liquid cooling, however,

loss of coolant will not cause an immediate rise in temperature due to the remaining thermal

inertia of liquid coolant. Additionally, liquid heat exchangers can avoid the immediate

effects of a power or pump failure by the use of an elevated supply tank to provide the head

necessary to push the fluid through the system.

• Increased flue gas flow rate and temperature, resulting in an inability to provide sufficient

cooling liquid, causing a possible system failure due to the flue gas temperature levels.

3.8.2.3 Process Monitoring

The outlet gas temperature and coolant flow rate are easily monitored by thermocouple and

flowmeter, respectively. The temperature of the cooling fluid is also monitored to assure proper

operation, especially if the system has one pass.

3.8.2.4 Maintenance

If the hot flue gas contains significant amounts of particles, the heat exchanger tubes will

require frequent cleaning. As mentioned above, this problem can be mitigated by the use of a high

temperature particle removal device upstream of the heat exchanger or by the use of soot blowers to

keep the tubes clean. Other maintenance procedures include service of the pumps and blowers.
3-34

3.8.3 Quench

In a water-quench cooler, as shown in Figure 3-30, the hot flue gas is cooled by injection of

water into the gas stream. The gas temperature is reduced as the water spray evaporates.

3.8.3.1 Design Principles

The quench is typically performed in a dedicated cooling vessel. Hot gases enter the vessel

and are decelerated so that water droplet evaporation occurs completely within the vessel. To

ensure evaporation, the water spray must be atomized into small droplets in sufficient quantities to

maintain a constant outlet gas temperature. The amount of injected water must be controlled in

response to fluctuations in the flue gas temperature. Because it is difficult for the water nozzles to

maintain a small droplet size over a wide range of flow rates, nozzles that are used have either

variable flow areas or utilize two-phase flow (high pressure air or steam can atomize water over a

wide range of flow rates). Rotary nozzles can also be used to maintain a uniform distribution of

water droplets over a wide range of flows.

A water quench is almost always used upstream of a gas scrubber when the inlet flue gas

temperature is above the saturation point. In this situation, to ensure that the temperature is lowered

to its saturation temperature (which is required by the scrubber for effective scrubbing efficiency),

a surplus of water is introduced. Because droplets and particles will tend to migrate to the cooler

wall, an inside film of water is often used to wash collected particles from the walls. This water is

drained from bottom of the cooling vessel and discarded or recycled. The amount of water typically

required to cool the flue gas is shown in Figure 3-31.

When a water quench is used prior to a fabric filter or dry ESP, it is critical that the gas

temperature remain above the saturation level and that all the water droplets completely evaporate.

Otherwise, corrosion and plugging problems may occur in these devices from condensed moisture

and other gases. Complete evaporation is assured by fine atomization and large gas residence times

in the cooler vessel.

3.8.3.2 Performance

When excess moisture in the flue gas is undesirable, the most important consideration is

that the injected droplets have a uniformly small size range. Large unevaporated droplets may exit

the cooling vessel before they have time to completely evaporate. These droplets may cause problems

with downstream equipment and may create a non-uniform temperature distribution in the flue gas.

Also, droplets may migrate to the vessel wall, reducing the cooling potential of the device.

Water quench cooling has the following advantages:

• Provides the finest and quickest temperature control of the three cooling methods.
3-35

• Potential for capturing acid gases and particles if reagents are injected with the water.

• Low maintenance requirements.

Disadvantages to using water quench cooling include:

• It has the potential to produce a secondary liquid waste stream when the liquid spray is not

fully evaporated.

• It has an increased chance of a visible steam plume.

• It has difficulty in accommodating low flow rates and variations in moisture feed.

• Failure modes such as:

— Plugged spray nozzles, especially if a dry reagent is used for acid gas control.

— Corrosion of spray nozzles. This becomes more of a problem if the spray

liquor is

collected and recycled and/or if acid gases are present in the flue gas.

— Loss of quench water from fire, explosion, power outage, system leak, or

mechanical

failure of the pump, resulting in system failure.

— Increased flue gas flow rate and temperature result in an inability to provide

sufficient quench liquid, causing a possible system failure due to flue gas temperature

levels.

3.8.3.3 Process Monitoring

Quench operation is indicated by monitoring the outlet gas temperature, water flow rate,

and nozzle pressure. The outlet gas temperature is monitored with thermocouples. Water flow and

nozzle pressure are monitored with pressure and flowrate gauges.

3.8.3.4 Maintenance

Maintenance requirements include checking (and occasionally replacing) spray nozzles for

corrosion and pluggage, and inspecting the water pump system.

3.9 OTHER METALS CONTROL TECHNIQUES

New techniques are being developed specifically for the control and capture of individual

metals species from combustion sources. Mercury, in particular, due to its toxicity and volatility

(making it difficult to capture in conventional air pollution control devices), has received special
3-36

attention. Techniques for its control include the use of activated carbon, selenium filters, special

scrubbers, and sodium sulfide. Additionally, special sorbents have been found effective at controlling

metals such as cadmium and lead.

3.9.1 Sorbents

A variety of sorbents have and are being developed that have shown the ability to

independently remove trace metals from combustion source flue gases. Sorbents having

demonstrated metal capturing potential are shown in Table 3-1, and include zeolites, activated

carbon, calcium-based, alumina-based, and silicates. Solid sorbents capture and immobilize the

metal by a combination of adsorption and chemical reaction; the metal is reacted and/or absorbed

on the surface of the sorbent, which is then more easily removed from the flue gas. The actual

mechanism depends on the particular metal and sorbent. Some metals react with the sorbent to

form a stable compound; other presumably inert materials, such as silica sand, appear to pick up

metals by some type of adsorption mechanism. Note that the adsorption procedure is important

since it determines the potential leachability of metal from the captured product. The non-leachable

water insoluble fraction is surmised to be that which is chemisorbed or reactively adsorbed, while

the leachable water soluble fraction is that which is merely physisorbed.

3.9.1.1 Activated Carbon Adsorption

Activated carbon has been used for the “add-on” control of organics, in particular PCDD/

PCDF, and mercury from combustion source flue gases, including municipal waste, medical waste,

and hazardous waste incinerators. Various mechanisms have been suggested for the capture on

activated carbon; these include physical adsorption in the pores of the carbon, catalytic oxidation to

mercury oxides, and/or chemically mediated adsorption based on the presence of oxygen and/or

chlorine (Krivanek, 1993). The presence of chlorine has been demonstrated to augment mercury

capture efficiency (Felsvang, 1993).

Design Principles — Activated carbon has a very large inner pore surface (large surface to

volume ratio), which can adsorptively bond a very broad range of substances. Typical activated

carbon has a surface area of from 300 to 800 m2

/g; high grade carbon can have surface area of 1500

m2

/g. Additionally, it posses catalytic properties that can be used for simultaneous NOx control if

required. Two types of activated carbon are commonly used: formation coke (based on coal) and

lignite coke. Lignite coke is more commonly used due to its low cost. In some attempts, impregnation

agents have been added to the activated carbon to enhance its control efficiency.

Activated carbon can be used in a couple of different arrangements for the control of mercury

and organics in combustion system flue gases. These include:

• Fixed bed process — In the fixed bed process, flue gas flows through one or more beds of

activated carbon arranged in a series; spent activated carbon is withdrawn from the bottom

of each bed segment. Carbon monoxide levels are continuously monitored across bed to

minimize risk of fire. To prevent plugging and contamination, the bed is usually placed

after a flue gas particulate control device. Although capture performance tends to increase

at lower temperatures, the gas should not be below saturation temperature to avoid
3-37

condensation of moisture in the bed. Activated carbon tends to lose its adsorption capacity

when it gets contaminated with moisture (Frillici, 1993). Typical bed operating temperatures

range between 250 to 300°F.

• Fluidized bed process — In the fluidized bed process, flue gas fluidizes a bed containing

activated carbon. The fluidized bed arrangement consumes less coke compared to the fixed

bed, however, has a higher pressure drop. Circulating beds have also been used, and have

demonstrated very high mercury and dioxin removal. Typical bed operating temperature is

similar to fixed bed arrangements, ranging between 250 to 300°F.

• Duct injection — In the duct injection process, activated carbon is injected into the flue gas

upstream of a particulate control device, typically a fabric filter or ESP. Because of the

corona discharge phenomena of an ESP and combustible nature of activated carbon, an ESP

is not considered the best choice for activated carbon collection from carbon injection systems.

Additionally, a fabric filter provides for extended hold-up of the injected activated carbon

in the flue gas, allowing for additional mercury removal compared to an ESP; for the same

level of control, a higher activated carbon feed rate may be required for ESPs compared to

fabric filters. However, relatively no difference between capture using ESP and fabric filter

has been demonstrated (Felsvang, 1992). In some versions, a cyclone reactor is used to

improve the mixing of the flue gas and the activated carbon before it passes to the particulate

control device. Activated carbon may also be injected upstream of a rotary atomizer, similar

in design to conventional spray drying units. The activated carbon may be injected either

by itself as a powder or in a dry or wet solution in combination with an acid gas removing

sorbent such as lime, calcium hydroxide, or sodium sulfide. The duct injection process

consumes the least activated carbon of the three methods, typically from 50 to 400 mg/m3

of flue gas for municipal waste combustors.

Note that duct carbon injection is not recommended for facilities with “wet” pollution control

systems (e.g., a type of wet scrubber) since at this time, the two have not been successfully integrated.

Performance — Literature reviews and discussions with technology vendors have indicated

that the use of activated carbon with either direct duct injection or in bed arrangements can control

PCDD/PCDF flue gas emissions levels from hazardous waste burning facilities to below 5 ng/dscm

of total PCDD/PCDF, and TEQ levels to below 0.1 ng/dscm. Additionally, levels consistently

below 0.05 TEQ ng/dscm have been achieved, and as low as 0.005 TEQ ng/dscm in select cases;

although, surveyed vendor performance guarantees have been limited to 0.05 ng/dscm @ 11% O2.

Even higher control levels may be possible by optimizing critical operating parameters.
3-38

As mentioned, in addition to toxic organics control, activated carbon can also effectively

control mercury. Fixed bed, fluidized bed, and duct injection arrangements have all been

demonstrated to perform at 90+% mercury removal, with some as high as 99% or greater. These

systems without carbon injection have poor mercury capture efficiency ranging from 0 to 40%.

Again, highest control levels are achieved by optimizing critical operating parameters.

Operating parameters that effect activated carbon performance include the following:

• Activated carbon type and specifications — Carbon type may be defined by general chemical

and physical properties. Carbon specifications such as chemical properties (e.g., composition,

use of additives or enhancers such as iodine or sulfur impregnation) and physical properties

(e.g., particle size, specific surface area, pore size) can significantly affect performance.

However, experiments have shown that activated carbon type has little effect on mercury

capture; coal-based, lignite-based, and wood-based activated carbon all performed at similar

levels (White, 1992).

• Activated carbon feedrate — Removal rate increases with increasing activated carbon

feedrate.

• Injection location for duct injection process — No noticeable difference in performance

when activated carbon was injected either downstream of the municipal waste combustor

economizer, at spray dryer outlet, or along with lime used for the spray dryer (White, 1992).

• Temperature — PCDD/PCDF capture efficiency tends to increase with decreasing flue gas

temperature, especially at temperatures below 425°F (Christiansen, 1992). Mercury capture

efficiency has also been shown to improve at lower temperatures (Christiansen, 1992),

however, there may be a tradeoff; other research has shown that at higher temperatures, the

bond between the mercury and the activated carbon is stronger compared to the bonding

occurring at lower temperature (Gullett, 1993).

Flue gas temperature is especially important in carbon bed applications since a temperature

spike in the bed may cause adsorbed PCDD/PCDF, mercury, and other heavy metals and organics

to desorb and reenter the stack gas emissions stream. However, since most facilities utilize some

type of PM control device upstream of the carbon bed, and the inlet temperature to the PM control

device must be maintained at a certain level to avoid PCDD/PCDF formation, temperature spikes

in the carbon bed should not occur.

Retrofit applicability to existing sources of cement kilns, light weight aggregate kilns and

hazardous waste incinerators is discussed in Section 5 of Technical Support Document for HWC

MACT Standards Volume III: Selection of MACT Standards and Technologies. Also discussed is

detailed activated carbon performance data for medical waste incinerators, municipal waste

combustors, and hazardous waste incinerators. Note that to date, activated carbon systems have

not been applied to cement or light weight aggregate kilns.
3-39

Process Monitoring — Activated carbon processes are often used in conjunction with other

air pollution control technologies. Monitoring for these other technologies should be reviewed in

the previous sections. The following describes process monitoring to be performed for each of the

carbon adsorption applications:

• Fixed bed process — Fixed bed operations are monitored by flue gas inlet temperature, gas

flow rate, carbon monoxide levels, upstream and downstream pollutant concentrations, and

pressure drop across the system. It is recommended that flue gas temperatures remain at

least 100°F above dew point temperature, and at least 40°F above acid gas dewpoint (Steag,

1994). The bed temperature typically ranges between 250 and 300°F. Carbon monoxide

levels are monitored to minimize the risk of fire in the bed.

• Fluidized bed process — Fluidized bed operations are monitored by bed temperature, bed

pressure drop, and flue gas flow rate.

• Duct injection — Duct injection operations are monitored by flue gas temperature, flue gas

flow rate, upstream and downstream pollutant concentrations, and activated carbon injection

rate.

Inspection and Maintenance — One maintenance requirement of activated carbon systems

is the treatment or disposal of the used carbon. Current treatment ideas include a wash process to

remove metals, followed by destruction in the primary combustion source, or vitrifying the activated

carbon and fly ash into a glass substance. Destruction in the primary combustion source may be a

problem since the carbon may contain elevated levels of mercury and other volatile heavy metals

which will be re-released when the carbon is burned. Alternatively, the used carbon may be directly

landfilled, or landfilled after stabilization by techniques such as cementation or polymerization.

Recent studies have shown that post-treatment may not be necessary for captured ash and

carbon contaminated with mercury (Felsvang, 1993). Captured activated carbon and fly ash were

stored for 3 years in different arrangements: open container, plastic bag, solidified by water addition,

and stored in open container at elevated temperature (50°C). No significant revolatilzation of

mercury was seen in any of the situations during the three year period. Additionally, leaching

studies performed on by-product ash gave results that were typically below EPA levels. However,

the oxygen deficient methane rich environment of a landfill may enhance the desorption of previously

captured mercury.

To date, no work has been performed to determine the leachability of PCDD/PCDF adsorbed

on the carbon. Thus potentially, the carbon injection technique may only serve to transfer the PCDD/

PCDF from the flue gas to a landfill leachate if disposed of directly.

Inspection requirements include close monitoring of carbon monoxide levels, and ash and

carbon build-up to prevent fire hazards. Use of combustible substances like activated carbon requires

consideration due to risk of fire and explosion. When present with oxygen, activated carbon may

self-ignite at temperatures as low as 200°C.
3-40

3.9.1.2 High Temperature Earth Based Sorbents

The use of sorbents such as sand, limestone, alumina, silica, bauxite, and dolomite at

temperatures in the range of 500 to 900°C have been demonstrated to capture metals such as lead

and cadmium. Some of these efforts include:

• Gao and Silcox (1993) studied the capture of lead and cadmium by use of a diatomaceous

earth sorbent (composed of silica, alumina, and iron oxide) in lab scale fixed and fluidized

beds, and a pilot scale rotary kiln. Sorbent capture took place in a temperature range of 600

to 870°C. Lead was retained at 100% efficiency. Cadmium was not efficiently captured.

Leachability of the capture metal decreased with increasing operating temperature.

• Ho et al. (1992) studied the capture of lead in a lab scale fluidized bed combustor with the

use of sorbent bed materials including sand, limestone, and aluminum oxide. A wide range

of lead control was achieved, from 4 to 90%, depending primarily on sorbent material and

bed temperature. Limestone demonstrated the best capture ability. A bed temperature of

700°C was shown to provide best capture; capture fell off as temperature increased due to

sorbent agglomeration and vaporization of metals; at lower temperature, due incomplete

combustion, metals were not release and were carried away with unburned soot particles.

Also, a smaller sorbent size was shown to have higher capture efficiency; however, smaller

sorbent was most difficult to keep in bed.

• Scotto et al. (1992) investigated the capture of lead by injection of kaolinite into the post

flame region in a bench scale combustor burning lead containing wastes. Kaolinite was

shown to capture lead by 99%. Additionally, the sorbent reduced the submicron particulate

distribution and lead formerly found in the submicron size range was now associated with

larger kaolinite particles. At high chlorine levels, the addition of sorbent was not effective

at scavenging the lead vapor.

• Eddings and Lighty (1992) demonstrated that both metallic lead and lead oxide failed to

vaporize when heated on a grid in the presence of a clay material.

• Hassel et al. (1990) conducted work in a pilot scale fluidized bed to evaluate use of sorbent

bed materials such as silica sand, bauxite, dolomite, and aluminosilicate for the capture of

copper, zinc, and bismuth. Capture efficiencies of from 60 to 90% were demonstrated for a

bed operating temperature of 1600°F. Silica sand and dolomite performed the best. In case

of copper, a significant portion of the captured material was present as chlorine salt, which

is readily leachable. Leachability of metals from bed materials were below regulatory limits.
3-41

• Uberoi and Shadman (1991) studied the high temperature (800°C) removal of cadmium

from simulated flue gases using silica, alumina, kaolinite, emathlite, and lime. Bauxite and

alumina are best performers, however, bauxite products were leachable. The overall sorption

process was determined to be a complex combination of adsorption, condensation, diffusion,

and chemical reactions. Others were not effective at cadmium capture.

• Uberoi and Shadman (1990) studied high temperature (700°C) removal of lead from

simulated flue gases using silica, alumina, kaolinite, bauxite and emathlite. All except

limestone were effective for lead removal; kaolinite and bauxite were the most effective.

• Srinivasachar et al. (1992) performed a laboratory scale study of fixed bed of proprietary

sorbent for the capture of Pb, Cd, As, and Zn. Capture efficiencies of from 65-100% were

demonstrated with a bed temperature of from 700 to 900°C. The sorbent/metal product was

shown to be non-leachable. X-ray fluorescence analysis indicated that the metal reacted

chemically with the sorbent rather than being just physically adsorbed by heterogeneous

condensation.

• Crosley and Kedl (1992) studied the adsorption of cesium with the use of silica at

temperatures of 1000 to 1300°F. Cesium was efficiently adsorbed; most of the adsorbed

cesium was non-leachable. Developed of method for selecting the appropriate sorbent based

on the existence of a stable phase of metal oxide/sorbent oxide combination.

3.9.2 Mercury Specific Controls

3.9.2.1 Selenium Coated Filter

Mercury capture with selenium coated filters was originally developed for metallurgical

smelting operations in Europe (Lindquist, 1991). They consist of a cylindrical shell containing

porous material impregnated with selenium. Selenium has a strong affinity for mercury; it combines

to form mercury selenide (HgSe), a highly stable compound. The filters are position downstream

of a primary particulate control device to avoid plugging. Problems with the use of selenium filters

include that the gas temperature must be 140°F or less and the gas must be clean of particulate,

while keeping the gases from becoming saturated, the use of selenium produces a double threat in

terms of disposal of the spent material, and filter blinding and poisoning are potential problems.

Spent filters are returned to the manufacturer for recharging, although they have lives of several

decades.

3.9.2.2 Sodium Sulfide Injection

Sodium sulfide injection has been successfully used for mercury control from municipal

waste combustors in Europe and Canada and has been tested on one unit in the United States

(White and Nebel, 1991). Sodium sulfide is a crystalline solid that is dissolved into a water solution,

and is sprayed into the flue gas upstream of a particulate control device. The solution reacts with

mercury to form HgS, a high stable solid reaction product. Injection is typically performed at a flue

gas temperature of 260 to 300°F. It has been shown to have a capture efficiency of from 50 to 90%.
3-42

Problems with its use include the potential release of hydrogen sulfide fumes from bags of sodium

sulfide as they are opened, deposits on nozzles and duct work, proper disposal of resultant secondary

waste, proper solution atomization, HgS generated as a fine particulate that is difficult to capture in

a particulate control device, and the fact that it can not be used with a spray drying sorbent feed

because of potential CaS scaling of the sorbent feed line (Andersson and Weimer, 1991).

3.9.2.3 Mercury Scrubbers

Mercury scrubbers have been proposed that rely on the reaction of mercury with chemicals

such as sodium hypochlorite or with a chelating agent and cupric chloride, to form water-soluble

species of mercury which can then be removed in conventional wet scrubbers (Dalton et al., 1992).

90-95% mercury removal has been demonstrated. The primary problem is treatment of the

contaminated scrubber liquors. Additional research is needed to determine optimum operating

conditions and effective chemical additives.

3.10 CATALYTIC OXIDATION

Recent work by Japanese and European researchers has shown that PCDD/PCDF may be

controlled by catalytic oxidation using metal (such as vanadium and tungsten oxides on a platinum

oxide based substrate) catalysts that are presently commonly used for reducing NOx emissions

through the Selective Catalytic Reduction technique (SCR) (Fouhy, 1992; Hagenmaier, 1992; Ok,

1993; Hiraoka, 1990). The catalytic oxidation of PCDD/PCDF has been shown to occur in a

temperature range of about 480 to 660°F. PCDD/PCDF destruction efficiencies of 95 to 98% have

been demonstrated, with controlled levels below 0.1 ng/dscm TEQ. For example, operation on a

full scale MWC controlled levels of PCDD/PCDF to below 0.1 ng/dscm TEQ, with most levels at

or below 0.05 (Boos, 1993). See Section 5 of Technical Support Document for HWC MACT

Standards Volume III: Selection of MACT Standards and Technologies for further discussion on

MWC data. Catalysts are currently available from many vendors including Lentjes, Deutsche

Babcock, Siemens, BASF, Huls, Degussa, and SGP-VA. Problems with this method include catalyst

fouling and contamination from metals, sulfur, or chlorine compounds, leading to deactivation.

3.11 SULFUR-BASED CONTROL

Source testing has shown that PCDD/PCDF are either not detected or present at very low

levels in coal fired boiler flue gas emissions (Junk and Richard, 1981). This has been surmised to

be due to the presence of relatively higher levels of sulfur which may act to suppress PCDD/PCDF

formation. This is supported by additional work on the co-firing of coal with municipal solid

waste. Full scale testing on a MWC has shown that the addition of coal (containing 19 weight

percent sulfur) to the municipal solid waste stream acted to reduce PCDD/PCDF levels from 10 to

less than 1 TEQ ng/dscm (Lindbauer, 1992). Other studies on the co-combustion of coal with

plastics and bark have shown very low PCDD/PCDF levels below 0.1 TEQ ng/dscm (Mattila,

1992; Frankenhaeuser, 1993). Interestingly, with the addition of limestone to the combustion region,

as expected sulfur flue gas emissions decreased, but PCDD/PCDF emissions increased substantially,

further lending evidence to the fact that the presence of sulfur suppresses PCDD/PCDF formation

(Frankenhaeuser, 1992). Additional data substantiating sulfur-based control is discussed in Section
3-43

5 of Technical Support Document for HWC MACT Standards Volume III: Selection of MACT

Standards and Technologies.

It has been postulated that the presence of sulfur as SO2 suppresses Cl2 formation, thus

reducing PCDD/PCDF formation. However, controlled laboratory scale studies determined that

gas phase SO2 has little apparent effect on PCDD/PCDF formation, and that the presence of sulfur

as an upstream reaction product acting to alter the form of the metal catalyst required for low

temperature (300 to 400°C) catalytic formation (e.g., CuSO4 instead of CuO) may be attributable to

low PCDD/PCDF levels in systems with sulfur (Gullet, 1992).

3.12 AIR POLLUTION CONTROL SYSTEM TYPES

As discussed above, the choice of flue gas cleaning components and arrangement depends

on characteristics of the flue gas such as:

• Particle size distribution and loading.

• Acid gas content (nitrogen, sulfur, and chlorine).

• Organics content.

• Metals content.

• Radioactivity type and level (transuranic, alpha, beta or gamma contaminated, low level,

etc.).

Note that these parameters are influenced by both the incinerator type and waste composition and

type. Generally, all air pollution control system can be classified as either dry, wet, or hybrid dry/

wet.

3.12.1 Wet

Wet system are typically selected when the flue gas has significant levels of moisture, “sticky”

compounds, and/or when acid gas control is required. In the wet system, a quench cooler is used to

saturate the flue gas stream. Venturi and other novel scrubbers may be used for particulate control;

additional packed bed, spray or tray type scrubber may be used for additional acid gas and volatilized

component capture. The flue gas stream must then be conditioned (heated or dried) if final polishing

(e.g., with HEPA filtration) to avoid moisture condensation in the filters. Advantages of wet systems

include rapid and predictable gas cooling, low temperature operation aiding in capture of volatilized

metals and acid gases, and ease of byproducts handled in wet form. However, disadvantages include:

• Generation of contaminated secondary liquid stream.

• Rapid degradation of dry filters located downstream of wet scrubbing form moisture carry

over from cooling and scrubbing steps.
3-44

• Inefficient fine particle collection compared with dry filter systems.

• Increased corrosion potential.

3.12.2 Dry

Dry systems are preferred when the flue gas leaving the incinerator does not contain acid

gases. Dry systems do not saturate the flue gas to prevent condensation in the downstream devices,

although water injection may be used for cooling. Dry systems utilize either high temperature

coarse particle removal devices such as cyclones and ceramic or sintered metal filters, or lower

temperature devices such as baghouses and ESPs. Dry systems can use spray driers to control acid

gases, although large amounts of secondary wastes (captured dry solids) are generated due to their

relative inefficiency.

Advantages of using dry systems include efficient capture of fine particles and avoidance of

contaminated liquid waste generation, efficient control of fine particulate, and minimization of

corrosion potential. Disadvantages include inefficient or nonexistent capture of acid gases and

generation of large amounts of dry scrubbing product.

3.12.3 Hybrid Wet/Dry

The third type of system, a “hybrid dry/wet” combination, uses a combination of both dry

and wet components, as shown in Figure 3-32. Dry/wet systems perform dry initial and fine particle

removal in a manner similar to that of dry systems; however, the flue gas is passed through a wet

scrubber/absorber, which is used to remove acid gases and other volatilized components. Typically,

a SDA/FF and wet scrubber are used in series. The SDA is located upstream of the wet scrubber,

and is utilized to dispose of the liquid effluent produced from the downstream two-stage wet scrubber.

Hybrid systems may be designed to include the advantages of both wet and dry system including

generation of dry secondary waste, efficient capture of fine particulate, as well as low temperature

capture and control of acid gases and volatile metals. The primary disadvantage is increased capital

cost and system complexity. Other potential disadvantages include the re-release of contaminants

caught in the wet scrubber liquid when reinjected back into the flue gas stream to cool the flue gas,

and the potential for recycling and buildup of trace constituents due to this recycling of scrubber

liquor blowdown.
3-45

Clean Gas

Dirty Gas

Dust Retrieval

Gas Outlet

Gas

Inlet

Gas Outlet

Dust Retrieval Dust Retrieval

Gas

Inlet

Figure 3-1. Cyclone operations.

(a) Particle collection 

procedure

(b) Flue gas inlet 

options
3-46

Figure 3-2. Pulse-air-jet type bag filter.

Induced Flow

Collars

Venturi

Nozzles

Wire Retainers

Filter

Cylinders

Collector

Housing

Fume Blow Pipe Sheet

To

Atmoshere

Material Discharge

Manometer

Ash

Laden

Air

Inlet

Hopper

Air

Lock

Exhaust Duct
Valve Clean

Closed Gas

Bags DistendedDue To Inflation

3-47

aaaaaaaaaaDirty

aGas aaaaaa

a

Dust Collecting Mode

Dirty Gas ToOther ModulesFor Cleaning

ValveClosed

TensionAdjustment

Cleaning 

Gas

Sewn In

Rings

Dust Out

Figure 3-3. Reverse air cleaning baghouse.
3-48

aaFigure 3-4. Shaker cleaning baghouse. Clean Gas Out Bags Distended Due To Inflation Dirty Gas Inlet Open Dust Collecting ModeSlidegate ClosedTube Sheet SIDE VIEW Bag Cleaning Mode FRONT VIEW Dirty Gas Inlet Closed Slidegate Open Dust Being Removed
3-49

Particulate-Laden

Gas Flow

Uncharged Particles

Particles Attracted to Collector Electrode

and Forming Dust Layer

High Tension Supply

from Rectifier

Clean Gas Exit

Discharge Electrode

at Negative Polarity

Charged Particles Electrical Field

Figure 3-5. Electrostatic precipitator particle collection procedure.
3-50

Figure 3-6. One and two-stage wire-to-plate ESP operation.

(A) Single Stage Precipitator

Collector

Plates

Discharge

Electrode

Frames &

Wires

ParticulateLaden 

Gas Inlet

(B) Two Stage Precipitator

Alternative

Plates

Grounded

Pre-Ionizing

Wires Pre-Ionizing

Stage

ParticulateLaden 

Gas Inlet

Tubes

Precipitating

Stage

Ground
3-51

Discharge Electrode

Rapper

Insulator

Compartment

Collecting

Plate Rapper

Cover Plate

Top End

Panel

Perforated Gas

Distribution Plate

Gas Flow

Bottom

End Panel

Electrode

Hopper

Hopper Baffle

Discharge

Electrode

Weight

Collecting

Plate

Side Panel

Figure 3-7. Plate type electrostatic precipitator.
3-52

. .

Insulator

Precipitator

Shell Clean

Gas

Exit

Discharge

Electrode

Dust on

Precipitator Wall

Gas

Entrance

Collected Dust

a - c

Input

Rectifier

Set

High-Voltage

Cable

.

...

.. .

.

. .

. .

.

. ..

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.. . .. . . .

.

.

.

.

.

.

.

. .. .

. ... ..

.

.

.

..

. .

...

.

.

...

.

.

...

. ..

.

..

. . ..

.

...... . . . . .

.

.

. . .

.

. . .. . . . . . . . . . . . .

.

.

.

.

.

.

.

....

.

.

.

.

.

..

. . . . .

. . .

. . . .

. .

. . .

.

. .. . .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

..

.

. .

.

..

..

.

.

.

.

.

.

.

.

.

. .

. . .

.

.

.

.

.

.

.

.. .

.

. .

.

.

.

...

.

..

.

.

.

...

. ..

. .

.

.

.

..

.

.

..

.

.

.

...

...

.

.

.

.

.

..

.

.

.

..

.

.

...

...

.

.

.

.

.

...

.

..

.

.

.

...

. ..

.

.

.

.

.

...

.

..

.

.

.

...

. ..

.

.

.

.

.

...

.

...

.

.

...

. ..

.

.

.

.

.

...

.

...

.

.

...

. ..

.

. . . . ..

. . ..

. . . . ..

. . ..

.

....

.

.

..

..

. .

....

.

.

....

.

.

....

.

.

..

..

.

.

....

.

.

..

..

..

..

..

.

.

..

..

.

.

....

.

.

....

.

.

....

.

.

..

..

.

.

..

..

.

.

.

..

.

.

.

..

..

.

.

.

.

.

...

.

...

.

.

...

. ..

.

.

.

.

.

...

.

...

.

.

...

. ..

.

.

. . . ..

.

.

.

.

.

..

.

.

..

.

.

.

...

...

.

.

.

.

.

..

.

.

..

.

.

.

...

...

.

.

.

.

.

..

.

.

.

..

.

.

...

...

. .

.

.

.

..

.

.

..

.

.

.

...

...

. .

.

.

.

..

.

.

...

.

.

...

...

.

.

.

.

.

..

.

.

.

.

.

.

.

...

...

.

.

.

.

.

..

.

.

...

.

.

...

...

.

. . ...

. ..

.

.

.

. ...

. ..

.

.

. . ...

. ..

.

.

. . ...

. ..

.

.

. . ...

. ..

.

.

.

....

.

.

....

.

. . ...

. ..

.

.

.

....

.

.

... .

. ..

.. .

.. .

.

....

.

... .

. ..

. .. .

.

. .. .

.

.

Figure 3-8. Tubular type ESP.
3-53

High Voltage

Grid

High Voltage

Connection

High Voltage

Insulator

Compartment

High Voltage

Support

Insulator

Drain

Intermittent

Flushing Header

Gas Distribution

Devices

Purge Air

Connection

Rigid Mast

Alignment

Mechanism

Sonic Charge

Discharge Electrode

Precipitating

Electrode

Collecting

Electrode Tube

Discharge Drain

Figure 3-9. Wet ESP.

Flue Gas
3-54

Hot Gas

Inlet

Alternate or Supplemental

Nozzle Location for Very

High Temperature Gases

Tangential Liquid Inlets

(40% of Liquid)

Converging Inlet-Wetted

with Tangential Liquid

Scrubbing Liquor

to Throat

Throat Insert

Throat-Cross Section

Varies with Insert

Position

Expansion Section

Wetted Elbow 

(Fills with Liquid)

Hydraulic or Mechanical

Adjustment for Throat Scrubbing Liquor

to Throat

Figure 3-10. Variable throat venturi scrubber with wetted elbow.
3-55

Particles

accelerate

Particles collide

with droplets

Droplets have

no axial velocity

Particles

decelerate

Droplets

accelerate

Spray nozzle

Diverging 

region

Throat

Converging 

region

Figure 3-11. Performance principles of a venturi scrubber.

Flue gas

Water injection
3-56

Venturi Spray

Nozzle

Convergent Inlet

Fixed Throat Venturi

Expansion Section

Hot Gas Inlet

.

.

Figure 3-12. Non-wetted, fixed throat venturi scrubber.

Cold Gas Outlet
3-57

Figure 3-13. Force flux/condensation/collision scrubber.

Primary Droplet Eliminator

Most of the water removed.

Gas Inlet

Flanged inlet for dust,

fume and acid gas flow.

a

f

Throat Inlet

The gas is split into two equal flows.

b

Throats

Point of liquid injection

(adjustable throats available).

c

Collision Zone

Submicron particulate and acid 

gases removed from the gas stream.

d

Energy Recovery Diffuser

Gas and liquid stream expanded.

e

High Efficiency Waveforms

Fine droplet eliminators polish the gas.

g

Gas Outlet

Flanged connection for

clean gas flow exit.

h

Pump

Returns water to the throat 

via distribution headers.

i

a

b

c

d

e

f i

g

h
3-58

Figure 3-14. Free-jet scrubbers.

Dirty Gas Inlet

Scrubbing Liquid 

Injection

Mixing 

Free-Jet Tube

Nozzle

Fan-Driven Free-Jet Scrubber

. . . .

. . . .

. . . .

. Dirty Gas Inlet . . .

Subsonic 

Nozzle

Water Injection

Tandem Nozzle Fan-Driven Free-Jet Scrubber

Dirty Gas Inlet

Supersonic 

Ejector Nozzle

Water Injection

Subsonic Nozzle

Steam or 

Compressed Air Water

. . . . . . . .

Ejector Driven Free-Jet Scrubber

. . .

. . .

. .

. . .

.

Clean Gas Outlet

Clean Gas Outlet

Clean Gas Outlet
3-59

Gas Inlet

Recycle Slurries

for Wetted Approach

Side Wall Injection

in Throat Region

Expansion

Section

Leaf Dampers

Center Baffle

Removable Insert

Figure 3-15. Venturi scrubber with different flow control throat mechanisms.

Variable 

Configuration
3-60

Flue Gas

H2O

Blowdown

Sorbent

Venturi

Scrubber

Tray, Packed or 

Spray-Type 

Scrubber

Blowdown

Sorbent

Clean Gas

Figure 3-16a. Two-stage wet scrubbing process schematic.
3-61

Inlet

Quench Section

Liquid Feeds

Venturi

Chevron Mist

Eliminator

Liquid Feed

Wetted Elbow

Disc and Donut

Baffle Stages

Combination Cyclonic

Seperator and Countercurrent BaffleType Acid Gas Chamber

Liquid Discharge

Figure 3-16b. Venturi and tray tower wet scrubbing system arrangement.

Liquid 

Flow

Gas

Flow

Gas

Flow

Outlet
a

3-62

Solids Disposal

Spent 

Slurry Thickener

Recycle Liquid

To Slurry

Nozzles

Wash

Water

Cleaned

Flue Gas

To Stack

Mist

Eliminators

Flue Gas From

Dust Collector

Limestone

Slurry Feed Absorber

Reaction

Tank

Figure 3-17. Schematic of wet scrubbing spray tower system.
3-63
3-64
3-65

Figure 3-20. Ionizing wet scrubber schematic.

Stage 1 Stage 2

Ionizer Section

Recycle 

Spray 

Header Packed Bed

Ionizer Wires

Collection 

Plates

Inlet 

Transition 

Section

Defuge 

Flash 

Header

Outlet Transition Section

Sump
3-66

Dry Limestone

Feed Bin

and Gate Grinding

Water Supply

Weigh Feeder

Hydroclone

Classifier

Overflow

Launder

Underflow

Launder

Ball Mill

Limestone

Feed Tank

Dilution Water

Mill

Product 

Tank

Mill

Product

Pump Main Process

Water

Figure 3-21. Limestone reagent preparation system.
3-67





 

 









 

          









 



50

60

70

80

90

100

2345678

Percent SO2 Removed

Scrubber Effluent pH

l/g=2.5 liters/cubic meter

∆P = 4 inches water

Two Stage Absorber

70

75

80

85

90

95

100

0 10 20 30 40

Percent SO2 Removal

Liquid to Gas Ratio (gal/1000 acf)

∆P = 7” water

∆P = 10”

water

pH = 5.8 to 7.1

Single Stage Absorber

Figure 3-22. Impact of slurry pH (acidity) on SO2 removal efficiency.

Figure 3-23. Impact of liquid to gas ratio on SO2 removal efficiency (Elliott, 1989).
3-68

Flue Gas 

From 

Incinerator

Gas Cooling 

(e.g., Quench 

Tower, Heat Exchanger)

Dry

Sorbent

Sorbent

Recycle

Dry Product

Disposal

Clean Gas

Particulate

Collector

Figure 3-24. Dry scrubbing sorbent injection process schematic.

Sorbent 

Storage

Reaction 

Chamber
3-69

Flue Gas

Spray

Absorber

Particulate

Collector

Clean Gas

Dry Product

Sorbent Recycle

Sorbent

H2O

Sorbent

Slurry

Figure 3-25. Semi-dry spray drier absorption process schematic.

Sorbent 

Storage

Slurry 

Mixing 

Tank
3-70
3-71

Hot flue 

gas

Cool

flue gas

Dilution air Air injection 

nozzles

Figure 3-27. Air dilution cooling procedure.
3-72

1

1

1

1

1

1

1

1

1

2 2 2 2 2 2 2 2 2

E E E E E E E E E

F F F F F F F F F

300 350 400 450 500 550 600 650 700

0

5

10

15

20

25

Mass Flow Ratio *

Tin(gas) (°C)

1

2

E

F

Tout

150°C

200°C

100°C

50°C

* Mass Flow Ratio =

mass flow of dilution air

mass flow of hot flue gas

Note:

Figure 3-28. Example of the increase in flue gas flow rate using air dilution cooling.
3-73

Hot flue gas

Cool flue gas

Cooling fan

Outlet 

cooling air

Cooling air

Hot flue 

gas

Cool flue 

gas

Inlet cooling fluid

Outlet cooling fluid

Shell Tube

Heat transfer surface

(b)

(a)

Figure 3-29. Heat exchanger schematics: (a) Gas-gas recuperator; (b) Shell and tube type.
3-74

Hot flue gas

Cool

flue gas

Unevaporated 

quench liquid

Quench liquid 

spray nozzles

liquid spray

Figure 3-30. Water quench cooling procedure.
3-75

1

1

1

1

1

1

1

C

C

C

C

C

C

C

E

E

E

E

E

E

E

2

2

2

2

2

2

2















400 450 500 550 600 650 700

0.1

0.15

0.2

0.25

0.3

0.35

Mass Flow Ratio*

T(flue gas)in (°C)

1 60

C 80

E 100

2 120

 140

T(out) °C

* Mass Flow Ratio = mass flow of quench water

mass flow of hot flue gas

Note:

Figure 3-31. Example of increase in flue gas flow rate using water quench cooling.
3-76

Flue Gas

Spray Dryer

Absorber

Particulate

Collector

Wet Scrubbing

Sorbent

H2O

Dry

Waste

Clean Gas

Sorbent

Dry Scrubbing

Figure 3-32. Hybrid wet/dry system schematic.

Venturi

Scrubber

Tower-Type

Scrubber
3-77

Sorbent Primary Temperature Removal

Removal Range Efficiency

(°C) (%)

Zeolites Hg < 100

 Sulfur impregnated

Activated carbon Hg < 200 90+

Calcium-based

 Calcium hydroxide/fly ash Hg < 300

 Limestone/fly ash As, Cd, Pb 600-900 

Alumina-based Cd, Pb 500-700

 Bauxite

Silicates Pb, Cd, As 600-900

 Silica sand

 Kaolinite

 Emathlite

 Aluminum silicates

Based from that of Clarke and Sloss (1992).

TABLE 3-1. SORBENTS USED FOR REMOVAL OF METALS FROM 

COMBUSTION SOURCE FLUE GASES
4-1

SECTION 4

MAJOR SOURCE DETERMINATION

Title 3 of the 1990 Clean Air Act Amendments requires that “major sources” of hazardous

air pollutants meet emission standards based on the reduction capability of the maximum achievable

control technology (MACT). The amendments define major sources as follows:

“Any stationary source or group of stationary sources located within a contiguous

area and under common control that emits or has the potential to emit considering

controls, in the aggregate, 10 tons per year or more of any hazardous air pollutant or

25 tons per year or more of any combination of hazardous air pollutants.”

The objective of this task was to identify the incinerator and industrial kiln (cement and

light weight aggregate) facilities that meet the major source criteria. This section describes the

methodology used to evaluate the facilities and the number of facilities in each source category that

meet the major source definition.

4.1 METHODOLOGY

To determine which facilities are major sources, the following procedure was followed:

1. Emission rates for individual incinerators/kilns.

Emission rates for individual incinerators and kilns were obtained from trial burns conducted

to determine compliance with the Resource Recovery and Conservation Act (RCRA). These tests

were performed while burning hazardous waste at worst case operating conditions. The tests were

designed to represent the maximum emissions conditions expected to be achieved by the given

facility. These maximum conditions are suitable for the major source determination since the CAAA

reads that a major source has the “potential” to emit the quantitative amounts stated above.

For compounds measured below the laboratory detection limit (i.e., non–detect values), the

emission rate is unknown. Assumptions were made to estimate the emission rate for these compounds.

For this study, three different cases were evaluated:

i. Full-detection: emitted value is assumed to be the laboratory detection limit value,

ii. Half-detection: emitted value is assumed to be half the laboratory detection limit value,

iii. Zero-detection: emitted value is assumed to be zero.
4-2

It should be noted that trial burns are conducted at either worst case metal emission conditions

or organic emission conditions. In actual operation, however, a given incinerator or kiln does not

always operate under worst case conditions. Therefore, using trial burn test data to estimate emissions

will provide worst case determinations of major sources. For this study, two emission rate values

were used for each CAAA substance including:

iv. maximum emission rate measured during the compliance test; and

v. average emission rate measured during the compliance test.

Using the procedures described above provides six sets of emissions profiles for each incinerator

and kiln tested. The worst case set being a combination of using full detection limits for nondetected

data (i) and maximum emission rates for detected data (iv).

2. Annual emissions for individual incinerators and kilns

Determination of the annual emission rates (tons/year) for CAAA substances required several

assumptions about the annual operating schedules, since most test reports did not supply this

information. Two scenarios were considered. A worst case scenario, which addressed the phrase

“potential to emit” in the amendment, assumes the units operate at 365 days/year. EPA information,

however, indicates that facilities rarely operate at this capacity. Limited information from various

facilities indicate that an average operating schedule is approximately 315 days/year per unit. This

scenario was also addressed to represent a more realistic situation.

3. Annual emissions for incinerators on a “Contiguous Area” under common ownership

Since trial burns conducted to determine compliance with RCRA provide data for individual

units (incinerators or kilns) only, total emissions for all units at a facility (units located on a

“contiguous area” under common ownership) had to be determined, as required by the major source

definition. The number of hazardous waste burning units at a given facility was obtained from the

trial burn reports and EPA supplied information. At some facilities, emissions data were available

for all units. However, a couple of the facilities only tested selected units whose emissions were

representative of all the units on site. In these cases, the emission rates from the tested units were

applied to the units that were not tested. In some cases, emissions from only one unit of many on a

given site were evaluated.

4.2 INCINERATOR EVALUATION

18 commercial incinerators and 28 on-site incinerators were evaluated. Since in some cases

there were more than one incinerator per facility, this amounted to 16 commercial incinerator facilities

and 23 on-site incinerator facilities.

Table 4-1 summarizes the results of the study, assuming an operating schedule of 315 days/

year. Columns 3 through 6 demonstrate whether the facility is a major source under the specific

conditions (“Y” indicates that a facility is a major source under the given conditions, “N” indicates

that it is not). The first of these columns show which facilities meet the major source criteria while

considering HCl and chlorine emissions. This category is broken down to indicate which facilities
4-3

are major sources when considering the maximum and average emission measured during the trial

burn. Columns 5 and 6 show the same information when excluding HCl and chlorine from the data

list. Table 4-1 applies to all three detection limit assumptions (i, ii, iii).

Also shown in Table 4-1 are the number of organic, metal, and chlorine compounds considered

in the study. Lack of measurements may be a reason some facilities did not meet the major source

criteria. The next column of the table lists the facility’s air pollution control devices (APCD).

APCD acronyms are described in the glossary at the beginning of this report. The last two columns

indicate the number of incinerators within the facility, and comment on how the facility’s total

emissions were calculated using the available information.

11 of the 39 evaluated facilities meet the major source criteria when including HCl and

chlorine in the emissions summation using maximum emissions. As might be expected, a greater

fraction of the commercial incinerators meet the major source criteria than of the on-site incinerators.

The number of “major source” facilities drops to 5 when using the average emission of each

compound measured. When excluding HCl and chlorine from the emissions list, only 1 site remains

a major source (Atochem, Carrollton, KY). This facility meets the criteria because antimony

emissions exceeded 10 tons/yr during the high metals feed testing.

Dropping the operating schedule from 365 days/year to 315 days/year affected the

classification of only one facility. Rollins Environmental in Deer Park, Texas, dropped below the

major source criteria when evaluating the average emissions and including HCl and chlorine.

Figures 4-1 and 4-2 show total hazardous air pollutant (HAP) emissions per year from each

facility with and without HCl/Cl2. The type of incinerator, commercial (COMM), or on–site (OS),

and the type of control equipment (see list of acronyms for descriptions) are provided in the figures.

It should be noted that Figures 4-1 and 4-2 can only be used to determine if a facility can be

classified under the 25 tons/yr or greater major source category. Reference to the original report

needs to be made to view data representing the 10 tons/yr criteria. In summary, only Allied Chemical

(Birmingham, AL), Atochem (Carrollton, NY), Trade Waste Incineration (Sauget), and 3M (Cottage

Grove, MN) were classified as major sources based on 10 tons/yr or greater emissions of a single

substance. For each of these facilities HCl emissions exceeded 10 tons/yr, except Atochem where

antimony was in excess of the 10 tons/year criteria. The figures clearly show that using maximum

emissions instead of average emissions can have a significant impact on total facility emissions.

4.3 CEMENT AND LIGHTWEIGHT AGGREGATE KILN EVALUATION

53 cement kilns and 13 light weight aggregate kilns (LWAK) were evaluated (U.S. EPA,

1994). Since in many cases there are more than one kiln per facility, this amounts to 26 cement kiln

facilities and 6 LWAK facilities.

Table 4-2 summarizes the results of the study, assuming an operating schedule of 315 days/

year. Columns 3 through 6 demonstrate whether the facility is a major source under the specific

conditions (“Y” indicates that a facility is a major source under the given conditions, “N” indicates

that it is not). The first of these columns show which facilities meet the major source criteria while

considering HCl and chlorine emissions. This category is broken down to indicate which facilities
4-4

are major sources when considering the maximum and average emission measured during the trial

burn. Columns 5 and 6 show the same information when excluding HCl and chlorine from the data

list. Table 4-2 applies to all three detection limit assumptions (i, ii, iii).

Also shown in Table 4-2 are the number of organic, metal, and chlorine compounds considered

in the study. Lack of measurements may be a reason some facilities did not meet the major source

criteria. The next column of the table lists the facility’s air pollution control devices (APCD). The

last two columns indicate the number of kilns within the facility, and comment on how the facility’s

total emissions were calculated using the available information.

All but 4 facilities meet the major source criteria when including HCl and chlorine in the

emissions summation using maximum emissions. The 4 that do not meet the criteria, Essroc

Corporation (Dorado, PR), Lone Star (Greencastle, IN), National Cement Company (Lebec, CA),

and Southdown (Fairborn, OH) all have extremely low HCl and chlorine emissions, and (except for

Lone Star) have relatively few organic measurements. When excluding HCl and chlorine from the

emissions list, only 2 facilities meet the major source criteria: Ash Grove (Foreman, AR) and Holnam

Inc. (Clarksville, MO). There is reason to believe that the Ash Grove data may include a toluene

outlier for a single point. For this reason, calculations were performed for this site with and without

the suspected outlier. Table 4-2 shows that when the outlier is excluded, and when HCl and chlorine

emissions are also excluded, Ash Grove at Foreman is no longer a major source. The Holnam

facility meets the major source criteria even when excluding HCl and chlorine because of it’s

consistently high organics emissions.

Dropping the operating schedule from 365 days/year to 315 days/year affected the

classification of only one facility. Lafarge Corporation in Paulding, Ohio, dropped below the major

source criteria when evaluating the average emissions and including HCl and chlorine.

Figures 4-3 and 4-4 show total hazard air pollutant (HAP) emissions per year from each

facility with and without HCl/Cl2, respectively. The type of kiln, dry cement kiln (DCK), wet

cement kiln (WCK) or lightweight aggregate kiln (LWAK), and the type of control equipment,

fabric filter (FF) or electrostatic precipitator (ESP), also are provided in the figures. Again, note

that Figures 4-3 and 4-4 can only be used to determine if a facility can be classified under the 25

tons/yr or greater major source category. Reference to the original report must be made to see data

that classifies a facility under the 10 tons/yr or greater major source category. In summary, the

following facilities were classified as major sources based on 10 tons/yr or greater maximum

emissions of a single substance: Lafarge Corp. (Demopolis, AL), Southdown (Kosmosdale, KY),

North Texas (Midlothian, TX), Keystone Cement (Bath, PA). For each of these facilities, HCl or

chlorine emissions exceeded 10 tons/yr. The figures clearly show that using maximum emissions

instead of average emissions can have a significant impact on total facility emissions.
4-5

Iowa Army Amm, Middletown, IA (OS): GC/C/FF

Radford Army, Radford, VA (OS): QC/FF/QC/S

Upjohn, Kalamazoo, MI (OS): QC/VS/S/IWS

Lake City Army, Independence, MO (OS): HTHE/LTHE/FF

Occidental Chem, Niagara Falls, NY (OS): QC/AS/IWS

DuPont, La Porte, TX (OS): WHB/HE/FF

DuPont, Orange, TX (OS): QC/FF/SS/C/HES/DM

M&T Chemicals, Carrolton, KY (OS): WHB/FF/S

Dept. of Army, Johnston Atoll (OS): QC/VS/S/DM/C

Dow Chemical, Freeport, TX (OS): WHB/QC/WS/IWS

US. Dept of Energy, Oak Ridge, TN (OS): WHB/QC/S/VS/DM

Olin Corp, East Alton, IL (OS): WHB/DA/DI/FF

Glaxo, RTP, NC (OS): DA/DI/FF/HF/CA

Ross Incineration, Grafton, OH (COMM): WS/IWS

Eli Lilly and Co, Lafayette, IN (OS): QC/VS/C/CT/S/DM

Chevron, Philadelphia, PA (OS): QC/VS/KOV/DM

Rollins, Baton Rouge, LA (COMM): S

Waste Tech, East Liverpool, OH (COMM): SD/ESP/WS

DuPont, Deepwater, NJ (OS): AT/PTS/RJS/ESP

Pfizer, Groton, CT (OS): WHB/QC/PBC/VS/ES

Miles, New Martinsville, WV (OS): WHB/ESP/WS

Vulcan Materials, Wichita, KS (OS): WHB/ACS/HS/CS

Thermal Oxidation, Roebuck, SC (OS): WHB/FF/VQ/ACS/DM

General Electric, Pittsfield, MA (COMM): WS

Rollins, Bridgeport, NJ (COMM): HES/WS

Aptus, Aragonite, UT (COMM): SD/FF/WS/ESP

CWM Chemical, Chicago, IL (COMM): WS/IWS

ThermalKEM, Rock Hill, SC (COMM): WS

3 M, Cottage Grove, MN (OS): WS/ESP

Atochem, Carrollton, KY (COMM): FF/WS

Trade Waste, Sauget, IL (COMM): SD/FF

Dow Chemical, Midland, MI (OS): QC/VS/DM/ESP/AS/IWS

Aptus, Coffeyville, KS (COMM): SD/FF/WS/IWS

Laidlaw, Roebuck, SC (COMM): FF/VQ/WS/DM

Rollins, Deer Park, TX (COMM): WS

Allied, Birmingham, AL (COMM)

LWD, Calvert City, KY (COMM): FF/S

Chevron Chemical, Richmond, CA (OS): VS/C

Marine Shale, Amelia, LA (COMM): AB/WS/SD/FF

0

50

250200150100

Total HAP Emissions, t/y

MaxAve

Figure 4-1. Estimated annual HAP emissions for incinerators including HCl and Cl

2.

Max = 2330 t/yAve = 2100 t/y
4-6

Iowa Army Amm, Middletown, IA (OS): GC/C/FF

Occidental Chem, Niagara Falls, NY (OS): QC/AS/IWS

Glaxo, RTP, NC (OS): DA/DI/FF/HF/CA

Trade Waste, Sauget, IL (COMM): SD/FF

Radford Army, Radford, VA (OS): QC/FF/QC/S

Upjohn, Kalamazoo, MI (OS): QC/VS/S/IWS

US. Dept of Energy, Oak Ridge, TN (OS): WHB/QC/S/VS/DM

Dow Chemical, Freeport, TX (OS): WHB/QC/WS/IWS

CWM Chemical, Chicago, IL (COMM): WS/IWS

DuPont, Deepwater, NJ (OS): AT/PTS/RJS/ESP

Lake City Army, Independence, MO (OS): HTHE/LTHE/FF

General Electric, Pittsfield, MA (COMM): WS

Olin Corp, East Alton, IL (OS): WHB/DA/DI/FF

M&T Chemicals, Carrolton, KY (OS): WHB/FF/S

Miles, New Martinsville, WV (OS): WHB/ESP/WS

Chevron Chemical, Richmond, CA (OS): VS/C

Pfizer, Groton, CT (OS): WHB/QC/PBC/VS/ES

Vulcan Materials, Wichita, KS (OS): WHB/ACS/HS/CS

ThermalKEM, Rock Hill, SC (COMM): WS

Laidlaw, Roebuck, SC (COMM): FF/VQ/WS/DM

Thermal Oxidation, Roebuck, SC (OS): WHB/FF/VQ/ACS/DM

DuPont, La Porte, TX (OS): WHB/HE/FF

LWD, Calvert City, KY (COMM): FF/S

Eli Lilly and Co, Lafayette, IN (OS): QC/VS/C/CT/S/DM

DuPont, Orange, TX (OS): QC/FF/SS/C/HES/DM

Cheveron, Philadelphia, PA (OS): QC/VS/KOV/DM

Dow Chemical, Midland, MI (OS): QC/VS/DM/ESP/AS/IWS

Dept. of Army, Johnston Atoll (OS): QC/VS/S/DM/C

Waste Tech, East Liverpool, OH (COMM): SD/ESP/WS

Marine Shale, Amelia, LA (COMM): AB/WS/SD/FF

Aptus, Aragonite, UT (COMM): SD/FF/WS/ESP

Rollins, Bridgeport, NJ (COMM): HES/WS

Rollins, Baton Rouge, LA (COMM): S

Ross Incineration, Grafton, OH (COMM): WS/IWS

Aptus, Coffeyville, KS (COMM): SD/FF/WS/IWS

Allied, Birmingham, AL (COMM)

3 M, Cottage Grove, MN (OS): WS/ESP

Rollins, Deer Park, TX (COMM): WS

Atochem, Carrollton, KY (COMM): FF/WS

50

1510

Total HAP Emissions, t/y

Max

Ave

Figure 4-2. Estimated annual HAP emissions for incinerators excluding HCl and Cl2.
4-7

Essroc Dorado DCK FF

Southdown Fairborn DCK FF

Lone Star Greencastle WCK ESP

National Lebec DCK FF

Larfarge Demopolis DCK ESP

Keystone Bath WCK ESP

Southdown Kosmosdale DCK FF

North Texas Midlothian WCK ESP

Larfarge Paulding WCK ESP

Holnam Clarksville WCK ESP

Norlite Cohoes LWAK FF

Heartland Independence DCK FF

Southdown Knoxville DCK FF

Giant Harleyville WCK FF

Larfarge Alpena DCK FF

Essroc Logansport WCK ESP

Ash Grove Chanute WCK ESP

Larfarge Fredonia WCK ESP

Solite Brooks LWAK FF

Ash Grove Louisville WCK ESP

Continental Hannibal WCK ESP

Holnam Artesia WCK ESP

Texas Midlothian WCK ESP

Ash Grove Foreman WCK ESP

River Festus DCK ESP

Solite Green Cove LWAK FF

Holnam Holly Hill WCK ESP

Solite Arvonia LWAK FF

Medusa Wampum DCK ESP

Solite Cascade LWAK FF

Lone Star Cape Girardeau DCK FF

Solite Norwood LWAK FF

0

900800700600500400300200100

Total HAP Emissions, t/y

MaxAvg

Figure 4-3. Estimated annual HAP emissions for kilns including HCl and Cl

2.
4-8

Solite Green Cove LWAK FF

Southdown Knoxville DCK FF

Solite Norwood LWAK FF

Larfarge Alpena DCK FF

Solite Brooks LWAK FF

Southdown Fairborn DCK FF

Larfarge Demopolis DCK ESP

Heartland Independence DCK FF

Southdown Kosmosdale DCK FF

Solite Cascade LWAK FF

North Texas Midlothian WCK ESP

Essroc Dorado DCK FF

Solite Arvonia LWAK FF

Texas Midlothian WCK ESP

Keystone Bath WCK ESP

Lone Star Cape Girardeau DCK FF

Holnam Artesia WCK ESP

Larfarge Fredonia WCK ESP

Holnam Holly Hill WCK ESP

River Festus DCK ESP

Medusa Wampum DCK ESP

Giant Harleyville WCK FF

Larfarge Paulding WCK ESP

Ash Grove Louisville WCK ESP

National Lebec DCK FF

Essroc Logansport WCK ESP

Norlite Cohoes LWAK FF

Lone Star Greencastle WCK ESP

Ash Grove Chanute WCK ESP

Continental Hannibal WCK ESP

Holnam Clarksville WCK ESP

Ash Grove Foreman WCK ESP

0

755025

125100

Total HAP Emissions, t/y

MaxAvg

Figure 4-4. Estimated annual HAP emissions for kilns excluding HCl and Cl2.
4-9

TABLE 4-1. MAJOR SOURCE DETERMINATION SUMMARY FOR COMMERCIAL AND ON-SITE INCINERATORS

(315 operating days/year assumption)

 Major Source Major Source No. of Organic No. of Metals HCl/Cl # of

Facility Type (incl. HCl/Cl) (No HCl/Cl) Compounds Compounds Consideration APCD Units Comment

Max. Ave. Max. Ave. Considered Considered On Site

Allied Chemical: Birmingham, AL Commercial Y Y* N N 2 5 HCl ? 1Aptus, Inc.: Aragonite, UT Commercial N N N N 30 8 HCl/Cl2 SD/FF/WS/ESP 1

Aptus, Inc.: Coffeyville, KS Commercial Y N N N 30 9 HCl SD/FF/WS/IWS 1

Atochem: Carrollton, KY Commercial Y* N Y* N 0 6 HCl FF/WS 1

CWM Chemical Services: Chicago, IL Commercial NNNN 2 0 HCl/Cl2 WS/IWS 1

General Electric: Pittsfield, MA Commercial N N N N 3 9 HCl WS 1Laidlaw Environmental: Roebuck, SC Commercial Y N N N 2 7 HCl/Cl2 FF/VQ/WS/DM 1

LWD Inc.: Calvert City, KY Commercial Y Y N N 3 0 HCl/Cl2 FF/S 3 Data entered for 3 indiv. units

Marine Shale Proc.: Amelia, LA Commercial Y Y N N 0 7 HCl/Cl2 AB/WS/SD/FF 2 Data for unit #1. Unit #2 not operating.

Rollins Environmental: Baton Rouge, LA Commercial N N N N 52 9 HCl S 1Rollins Environmental: Bridgeport, NJ Commercial N N N N 11 9 HCl HES/WS 1Rollins Environmental: Deer Park, TX Commercial Y N** N N 14 7 HCl/Cl2 WS 3 Data and info. available for 1 unit only

Ross Incineration: Grafton, OH Commercial N N N N 1 7 –– WS/IWS 1

ThermalKEM: Rock Hill, SC Commercial NNNN 3 0 HCl/Cl2 WS 1Trade Waste Incineration: Sauget, IL Commercial Y* Y* N N 3 0 HCl/Cl2 SD/FF 4 Data and info. available for 1 unit only

Waste Tech. Ind,: East Liverpool, OH Commercial NNNN 7 0 HCl/Cl2 SD/ESP/WS 13 M: Cottage Grove, MN On-Site Y* N N N 4 7 HCl WS/ESP 1Chevron: Philadelphia, PA On-Site N N N N 0 7 HCl VS/C 1Chevron Chemical Co.: Richmond, CA On-Site Y Y N N 22 8 HCl/Cl2 QC/VS/KOV/DM 1

Dept. of Army: Johnston Atoll On-Site N N N N 26 7 HCl QC/VS/S/DM/C 4 Data and info. available for 2 units only

Dow Chemical Co.: Freeport, TX On-Site N N N N 2 0 HCl WHB/QC/WS/IWS 1Dow Chemical Co.: Midland, MI On-Site Y N N N 3 7 HCl/Cl2 QC/VS/DM/ESP/AS/IWS 2 Data entered for 2 indiv. units

DuPont: Deepwater, NJ On-Site NNNN 2 0 HCl/Cl2 AT/PTS/RJS/ESP 1DuPont: La Porte, TX On-Site N N N N 1 0 –– WHB/HE/FF ? Data entered for 1 indiv. unitDuPont: Orange, TX On-Site NNNN 3 7 HCl/Cl2 QC/FF/SS/C/HES/DM 1Eli Lilly and Co.: Lafayette, IN On-Site N N N N 3 0 HCl QC/VS/C/CT/S/DM ? Data entered for 1 unit

Glaxo, Inc.: RTP, NC On-Site N N N N 2 7 HCl DA/DI/FF/HF/CA 1

Iowa Army Amm. Plant: Middletown, IA On-Site N N N N 0 1 –– GC/C/FF 1Lake City Army Amm. Plant: Independence, MO On-Site N N N N 0 7 –– HTHE/LTHE/FF 1

M&T Chemicals Inc.: Carrolton, KY On-Site N N N N 2 0 HCl WHB/FF/S 1

Miles, Inc.: New Martinsville, WV On-Site N N N N 2 7 HCl WHB/ESP/WS 1

Occidental Chem. Corp: Niagara Falls, NY On-Site NNNN 0 4 HCl/Cl2 QC/AS/IWS 1

Olin Corp: East Alton, IL On-Site NNNN 1 7 HCl/Cl2 WHB/DA/DI/FF 2 Data for 1 unit applied to 2nd identical unit

Pfizer, Inc.: Groton, CT On-Site N N N N 14 3 HCl WHB/QC/PBC/VS/ES 2 Data and info. available for 1 unit only

Radford Army Amm. Plant: Radford, VA On-Site N N N N 0 1 –– QC/FF/QC/S 2 Data for 1 unit applied to 2nd identical unit

Thermal Oxidation Corp: Roebuck, SC On-Site N N N N 3 0 HCl WHB/FF/VQ/ACS/DM 1

US. Dept of Energy: Oak Ridge, TN On-Site N N N N 1 0 HCl QC/VS/S/IWS 1

Upjohn Co: Kalamazoo, MI On-Site N N N N 2 5 HCl WHB/QC/S/VS/DM 1

Vulcan Materials Co: Wichita, KS On-Site N N N N 30 3 HCl WHB/ACS/HS/CS 1

* Site meets major source criteria due to emission of single compound > 10 tons/yr (total compounds < 25 tons/yr).** 365 operating days/year assumption would denote this value as Y
4-10

TABLE 4-2. MAJOR SOURCE DETERMINATION SUMMARY FOR CEMENT AND LWA KILNS

(315 operating days/year assumption)

 Major Source Major Source No. of Organic No. of Metals Chlorine # of 

Facility Type (incl. HCl/Cl2) (No HCl/Cl2) Compounds Compounds Compounds APCD Kilns Comments

Max. Ave. Max. Ave. Considered Considered Considered On-Ste

Ash Grove: Chanute, KS Cement Kiln Y Y N N 26 7 HCl/Cl2 ESP 2 Data for 1 kiln applied to 2nd identical kiln.Ash Grove: Foreman, AR Cement Kiln Y Y Y Y 28 7 HCl/Cl2 ESP 3 Data entered for 3 individual kilns. Toluene outlier included.

(w/o Toluene outlier) Y Y N N Toluene outlier not included.

Ash Grove: Louisville, NE Cement Kiln Y Y N N 28 7 HCl/Cl2 ESP 2 Data for 1 kiln applied to 2nd identical kiln.Continental: Hannibal, MO Cement Kiln Y Y N N 20 7 HCl/Cl2 ESP 1Essroc Corp.: Logansport, IN Cement Kiln Y Y N N 4 6 HCl/Cl2 ESP 2 Data for 1 kiln applied to 2nd identical kiln.Essroc Corp.: Dorado, PR Cement Kiln NNNN 4 7 HCl/Cl2 FF 1Giant Cement: Harleyville, SC Cement Kiln Y Y N N 0 9 HCl/Cl2 FF 4 Data entered for 2 kilns. Data for 1 kiln applied to 2 other identical kilns.Heartland Cement: Independence, KS Cement Kiln Y Y* N N 0 7 HCl/Cl2 FF 4 All kilns exit to common stack. All kilns operating during test.Holnam Inc.: Artesia, MS Cement Kiln Y Y N N 1 7 HCl/Cl2 ESP 1Holnam Inc.: Clarksville, MO Cement Kiln Y Y Y Y 19 7 HCl/Cl2 ESP 1 Holnam Inc.: Holly Hill, SC Cement Kiln Y Y N N 2 9 HCl/Cl2 ESP 2 Data entered for 2 individual kilns.Keystone Cement: Bath, PA Cement Kiln Y* Y* N N 1 8 HCl/Cl2 ESP 2 Data entered for 2 individual kilns.Larfarge Corp.: Alpena, MI Cement Kiln Y Y N N 0 5 HCl/Cl2 FF 5 Data for 1 kiln applied to 2nd identical kiln. 3 kilns not in operation.Larfarge Corp.: Demopolis, AL Cement Kiln Y* N N N 0 5 HCl/Cl2 ESP 1Larfarge Corp.: Fredonia, KS Cement Kiln Y Y N N 1 5 HCl/Cl2 ESP 2 Data entered for 2 individual kilns.Larfarge Corp.: Paulding, OH Cement Kiln Y N** N N 0 5 HCl/Cl2 ESP 2 Data for 1 kiln applied to 2nd identical kiln.Lone Star Ind.: Cape Girardeau, MO Cement Kiln Y Y N N 3 7 HCl/Cl2 FF 1 Lone Star Ind.: Greencastle, IN Cement Kiln N N N N 23 7 HCl/Cl2 ESP 1Medusa Cement: Wampum, PA Cement Kiln Y Y N N 1 7 HCl/Cl2 ESP 3 Data entered for 2 kilns. Data for 1 kiln applied to another identical kiln.National Cement Co.: Lebec, CA Cement Kiln NNNN 1 8 HCl/Cl2 FF 1Norlite Corp.: Cohoes, NY LWA Y Y* N N 4 9 HCl/Cl2 FF 2 Data for 1 kiln applied to 2nd identical kiln.North Texas Cement: Midlothian, TX Cement Kiln Y* Y* N N 3 5 HCl/Cl2 ESP 3 Data for 1 kiln applied to 2 other identical kilns.River Cement: Festus, MO Cement Kiln Y Y N N 9 7 HCl/Cl2 ESP 2 2 kilns exit to common stack. Both kilns operating during test.Solite Corp.: Arvonia, VA LWA Y Y N N 0 7 HCl/Cl2 FF 2 Data entered for 2 individual kilns.Solite Corp.: Brooks, KY LWA Y Y N N 0 7 HCl/Cl2 FF 1Solite Corp.: Cascade, VA LWA Y Y N N 16 7 HCl/Cl2 FF 3 Data entered for 3 individual kilns.Solite Corp.: Green Cove, FL LWA Y Y N N 0 7 HCl/Cl2 FF 1Solite Corp.: Norwood, NC LWA Y Y N N 0 7 HCl/Cl2 FF 4 Data entered for 4 individual kilns.Southdown, Inc.: Fairborn, OH Cement Kiln NNNN 0 5 HCl/Cl2 FF 1 Southdown, Inc.: Knoxville, TN Cement Kiln Y Y N N 1 5 HCl/Cl2 FF 1Southdown, Inc.: Kosmosdale, KY Cement Kiln Y* Y* N N 10 5 HCl/Cl2 FF 1Texas Industries: Midlothian, TX Cement Kiln Y Y N N 0 5 HCl/Cl2 ESP 4 Data for 1 kiln applied to 3 other identical kilns.* Site meets major source criteria due to emission of single compound > 10 tons/yr (total compounds < 25 tons/yr).** 365 operating days/year assumption would denote this value as Y
R-1

REFERENCES

Air Pollution Control Consultant, “Ceramic Monolith Filter Removes Particulate From Hot Gases,”

The Air Pollution Control Consultant, pp. 1.10-1.11, September/October 1992.

Andersson, C., and B. Weimer, “Mercury Emission Control - Sodium Sulfide Dosing at the Hogdalen

Plant in Stockholm,” Proceedings of the 2nd Annual International Specialty Conference on Municipal

Waste Combustion, Air and Waste Management Association, VIP-19, pp. 664-674, Tampa, FL,

April 1991.

Barton, R., G. Hassel, W.S. Lanier, and W.R. Seeker, State-of-the-Art Assessment of Medical

WasteThermal Treatment, EPA Contract No. 68-03-3365, June 1990.

Barton, R., W.R. Seeker, and H. Bostian, “The Behavior of Metals in Municipal Sludge Incinerators,”

Trans IChemE, Vol. 69, Part B, pp. 29-36, February 1991.

Baturay, A., and J. Bruno, “Reduction of Metal Emissions from Sewage Sludge Incinerators with

Wet Electrostatic Precipitators,” Proceedings of the 83rd Annual Meeting of the Air and Waste

Management Association, Pittsburgh, PA, 90-30.8, June 24-29, 1990.

Benoit, M., E. Hansen, et al., U.S. Patent No. 4,850,290, “Method for Energy Recovery from Solid

Hazardous Wastes,” July 25, 1989.

Blizard, G. and R. Tidona, “Design, Installation and Start-Up of a Rotary Kiln Hospital Waste

Incineration System with Heat Recovery and SDA/FF APC System,” Proceedings of the 1992

Incineration Conference, Albequerque, NM, May 1992.

Blumbach, J., and L. Nethe, “Sorbalit - A New Economic Approach Reducing Mercury and

DioxinEmissions,” Proceedings of 85th Annual Air and Waste Management Association Meeting,

Kansas City, MO, 92-41.09, June 21-26 1992.

Bonner, T., et al., Hazardous Waste Incineration Engineering, Noyes Data Corporation, 1981.

Boos, R., R. Budin, et al., “PCDD- and PCDF Destruction by A SCR-Unit in a Municipal Waste

Incinerator,” Chemoshpere, Vol. 25, No. 3, pp. 375-382, 1993.

Braun, H., and A. Gerig, “Mercury Emission Monitoring on Municipal Waste Combustion,”

Proceedings of the Municipal Waste Combustion International Specialty Conference, Air and Waste

Management Association, VIP-19, Tampa, FL, April 1991.

Brna, T., “Toxic Metal Emissions from MWCs and Their Control,” Proceedings of the Municipal

Waste Combustion International Specialty Conference, Air and Waste Management Association,

VIP-19, Tampa, FL, pp. 145-161, April 1991.

Brown, B., and K. Felsvang, “Control of Mercury and Dioxin Emissions from United States and

European Municipal Waste Incinerators by Spray Dryer Absorption Systems,” Proceedings of the

Municipal Waste Combustion International Specialty Conference, Air and Waste Management

Association, VIP-19, Tampa, FL, pp. 675-705, April 1991.

Buonicore, A.J., and W.T. Davis, (editors), Air Pollution Control Engineering Manual, Air and

Waste Management Association, Van Nostrand Reinhold, New York, 1992.
R-2

Buonicore, A., L. Theodore, R. McKinney, and G. Tchobanoglous, “Waste Management,”

Perry’sChemcial Engineer’s Handbook - 6th Edition, R. Perry, Editor, McGraw Hill, New York,

pp. 26-31, 1984.

Buonicore, A., “Experience With Air Pollution Control Equipment on Hazardous Waste Incinerators,”

83rd Annual Meeting of the Air and Waste Management Association, Pittsburgh, PA, 90-33.2, June

24-29, 1990.

Calvert, product literature, San Diego, CA, phone (619) 272-0050, 1992.

Ceilote, product literature, Berea, OH, 44017, phone (216) 243-0700, 1992.

Cheremisenoff, P.N., “Pollution Engineering Guide to Fine Particulate Control in Air Pollution,”

Pudvan Publishing Co., Northbrook IL, 1988.

Christiansen, O., and B. Brown, “Control of Heavy Metals and Dioxins from Hazardous

WasteIncinerators by Spray Dryer Absorption Systems and Activated Carbon Injection,” Proceedings

of 85th Annual Air and Waste Management Association Meeting, Kansas City, MO, 92-41.06, June

21-26, 1992.

Clarke, L., and L. Sloss, Trace Elements: Emissions from Coal Combustion and Gasification,

International Energy Agency Coal Research, IEACR/49, London, July 1992.

Clarke, M., “The Nation’s First Mercury Emissions Standard: An Assessment of New Jersey’s

Standard for Municipal Waste Combustors,” Proceedings of the 86th Annual Air and Waste

Management Association Meeting, Denver, CO, 93-RP-154.01, June 13-18, 1993.

Clarke, M., “A Review of Activated Carbon Technologies for Reducing MSW Incinerator

Emissions,” Proceedings of the Municipal Waste Combustion International Specialty Conference,

Air and Waste Management Association, VIP-19, Tampa, FL, pp. 975-994, April 1991.

Christiansen, O., and B. Brown, “Control of Heavy Metals and Dioxins from Hazardous Waste

Incinerators by Spray Dryer Absorption Systems and Activated Carbon Injection,” Proceedings of

85th Annual Air and Waste Management Association Meeting, Kansas City, MO, 92-41.06, June

21-26, 1992.

Crosley, S., and R. Kedl, “Laboratory Measurements of Metal Adsorption from Simulated Incinerator

Flue Gases: Sorbent Selection for Cesium Capture,” Proceedings of the 1992 Incineration

Conference, Albuquerque, NM, pp. 605-609, May 11-15, 1992.

Dalton, D., R. Gillins, T. Harris, T., and A. Wollerman, “An Assessment of Off-Gas Treatment

Technologies for Application to Thermal Treatment of Department of Energy Wastes,” Prepared

for U.S. DOE under contract DE-AC05-84OR21400, DOE/MWIP-1, September 1992.

DeWees, W., C. Davis, S. McClintock, A. Cone, H. Bostian, E. Crumpler, and S. Steinsberger,

“Sampling and Analysis of Municipal Wastewater Sludge Incinerator Emissions for Metals, Metal

Species, and Organics,” 83rd Annual Meeting of the Air and Waste Management Association, Pittsburgh, PA, 90-26.7, June 24-29, 1990.

Dellinger, H., D. Pershing, and A. Sarofim, “Evaluation of the Origin, Emissions, and Control of

Organic and Metal Compounds from Cement Kilns Co-fired With Hazardous Wastes,” Scientific

Advisory Board on Cement Kiln Recycling, June 8, 1993.
R-3

Dempsey, C., and T. Oppelt, “Incineration of Hazardous Waste: A Critical Review,” Journal of the

Air and Waste Management Association, Vol. 43, p. 35, January 1993.

Donnelly, J., and F. Felsvang, “Joy/Niro SDA MSW Gas Cleaning Systems New Developments,”

Proceedings of the 82nd Annual Air and Waste Management Association Meeting, Anaheim, CA,

89-109.6, June 25-30, 1989.

Durkee, K., and J. Eddinger, “Status of EPA Regulatory Development Program for Medical

WasteIncinerators — Results of Emission Test Program,” Proceedings of the 1992 Incineration

Conference, Albuquerque, NM, pp. 447-456, May 11-15, 1992.

Eddings, E., and J. Lighty, “Fundamental Studies on Metal Behavior During Solids Incineration,”

Combustion Science and Technology, Vol. 85, pp. 375-385, 1992.

EER (Energy and Environmental Research Corporation), Air Pollution Control and

MonitoringTechnologies for Rocky Flats Fludized Bed Unit: State of the Art Assessment, EPA

Contract No. 68-CO-0094, 1993.

Elliott, T.C., Standard Handbook of Powerplant Engineering, McGraw-Hill Publishing Co., 1989.

England, G., et al., “Field Evaluation of Humidification for Precipitator Performance Enhancement,”

Proceedings of the 7th Symposium on the Transfer and Utilization of Particulate Control Technology,

EPRI/EPA, Vol. 1, May 1989.

EPA (U.S. Environmental Protection Agency), U.S. Patent No. 4,904,283, “Enhanced Fabric

Filtration Through Controlled Electrostatically Augmented Dust Deposition,” February 27 (1990).

EPA (U.S. Environmental Protection Agency), Handbook: Operation and Maintenance of Hospital

Medical Waste Incinerators, EPA/625/6-89/024, January 1990.

EPA (U.S. Environmental Protection Agency), Handbook: Control Technologies for Hazardous

Air Pollutants, EPA/625/6-91/014, June 1991.

EPA Contract 68-D2-0164, “Commercial and On-Site Incinerators, First Draft,” EPA Work

Assignment 1-9, Prepared by Energy and Environmental Research Corporation, June 1994.

EPA Contract 68-D2-0164, “Cement and Lightweight Aggregate Kiln Major Source Report, Second

Draft,” EPA Work Assignment 1-10 and 1-11, Prepared by Energy and Environmental Research

Corporation, June 1994.

Epner, E., and J. Yackiw, “Locating and Estimating Air Toxic Emissions from Medical

WasteIncinerators,” EPA No. 68D90054, September 1993.

Expanded Shale Clay and Slate Institute, Lightweight Concrete - History, Applications and

Economics, Washington, D.C., 1971.

Felsvang, K., R. Gleiser, G. Juip, and K. Nielsen, “Activated Carbon Injection in Spray Dryer/ESP/

FF for Mercury and Toxics Control,” Trace Elements Transformation in Coal-Fired Power Systems

Workshop, Scottsdale, AZ, April 19-21, 1993.

Fouhy, K., “Legislation Scrubs Flue Gas Emissions,” Chemical Engineering, pp. 31-35, June 1992.
R-4

Frankenhaeuser, M., H. Manninen, I. Koho, et al., “Organic Emissions from Co-Combustion of

Mixed Plastics with Coal in a Bubbling Fluidized Bed Boiler,” Chemoshpere, Vol. 27, Nos, 1-3, pp.

309-316, 1993.

Frillici, P., R. Li, G. Bacon, and D. Hayes, “Evaluation of Mercury Emission Reduction Alternative

for the Fort Dix Resource Recovery Facility,” Proceedings of the 86th Annual AWMA Meeting, 93-

RP-154.03, Denver, CO, June 1993.

Gaige, C., and R. Halil, “Clearing the Air About Municipal Waste Combustors,” Solid Waste and

Power, pp. 12-17, January/February 1992.

Gao, D., and G. Silcox, “Metal Capture by Mineral-based Sorbent Added to the Solids of RotaryKiln

Incinerators,” 1993 Spring Meeting of the Wester States Section of the Combustion Institute, Salt

Lake City, UT, March 1993.

Gennrich, T., “Nextel High Temperature Filter Bags,” Proceedings of The User and Fabric Filtration

Conference VI, Air and Waste Management Association, SP-84, pp. 125-147, Toronto, Ontario,

September 1992.

Gooch, J., A Manual on the Use of Flue Gas Conditioning for ESP Performance Enhancement,

EPRI, CS-4145, August 1985.

Guest, T., “Mercury Control in Canada,” 86th Annual Meeting of the Air and Waste Management

Association, Denver, CO, 93-WP-109.01, June 21-26, 1993.

Gullet, B., K. Bruce, and L. Beach, “Effect of Sulfur Dioxide on the Formation Mechanism of

Polychlorinated Dibenzodioxin and Dibenzofuran in Municipal Waste Combustors,” Environmental

Science and Technology, Vol. 26, No. 10, 1992.

Gullett, B., and W. Jozewicz, “Bench-Scale Sorption and Desorption of Mercury with Activated

Carbon,” Proceedings of the Municipal Waste Combustion International Specialty Conference, Air

and Waste Management Association, VIP-32, Williamsburg, VA, pp. 903-917, March 1993.

Gullett, B.K., P.M. Lemieux, J.D. Kilgroe, and J.E. Dunn, “Formation and Prevention of

Polychlorinated Dibenzo-p-Dioxin and Poly chlorinated Dibenzofuran During Waste Combustion:

The Role of Combustion and Sorbent Parameters,” Environmental Science and Technology, Vol.

51, No. 2, 1994.

Hagenmaier, K., K. Tichaczek, H. Brunner, and G. Mittelbach, “Application of De-Nox Catalysts

for the reduction of Emissions of PCDD/PCDF and Other PICs from Waste Incineration Facilities

by Catalytic Oxidation,” Proceedings of the Municipal Waste Combustion International Specialty

Conference, Air and Waste Management Association, VIP-19, Tampa, FL, p. 382, April 1992.

Hartenstein, H., “A Fixed Bed Activated Coke/Carbon Filter as a Final Gas Cleaning StageRetroffited

for a Hazardous Waste Incineration Plant - The First 6 Months of Operating Experience,” Proceedings

of 85th Annual Air and Waste Management Association Meeting, 9248.04, Kansas City, MO, June

21-26, 1992.

Hassel, G., T. McGrath, W.R. Seeker, and D. Taylor, “Capture of Volatile Metals in a

FluidizedBedAfterburner,” Proceedings of the Fall Meeting of the Western States Section of the

Combustion Institue, La Holla, CA, October 1990.
R-5

Hiraoka, M., N. Takkeda, T. Kasakura, et al., “Catalytic Destruction of PCDDs in Flue Gas,” presented

at Dioxin ‘90 EPRI Seminar, Bayreuth, F.R.G., 1990.

Ho, T., C. Chen, H. Hopper, and D. Oberacker, “Metal Capture During Fluidized Bed Incineration

of Wastes Contaminated with Lead Chloride,” Combustion Science and Technology, Vol. 85, pp.

101-116, 1992.

Hyland, G., W. Cramer, G. Faloon, and T. Holder, “State of the Art, Small-Scale Medical

WasteIncinerator Operating Case History,” Proceedings of the 1993 Incineration Conference,

Knoxville, TN, pp. 355-362, May 3-7, 1993.

IAEA (International Atomic Energy Agency), Treatment of Off-Gas From Radioactive Waste

Incinerators, Vienna, Technical Report Series No. 302, 1989.

Jaasund, S.A., “Electrostatic Precipitators: Better Wet Than Dry,” Chemical Engineering, pp. 159163,

November 23, 1987.

John Zink, product literature, Tulsa, OK, (708) 656-7800, 1992.

Junk, G., J. Richard, Chemosphere, Vol. 10, pp. 1237-1241, 1981.

Kilgroe, J., T. Brna, D. White, et al., “Camden County MWC Carbon Injection Test Results,”

Proceedings of the Municipal Waste Combustion International Specialty Conference, Air and Waste

Management Association, VIP-32, Williamsburg, VA, pp. 123-142, March 1993.

Krivanek, S., “Mercury Control Technologies for MWC’s; The Unanswered Questions,” Proceedings

of the Municipal Waste Combustion International Specialty Conference, Air and Waste Management

Association, VIP-32, Williamsburg, VA, pp. 824-840, March 1993.

Lamb, L., C. Brickley, and R. Dykes, Locating and Estimating Air Toxics Emissions from

SewageSludge Incinerators, EPA-450/2-90-009, May 1990.

Lancia, A., D. Musmarra, F. Pepe, and G. Volpicelli, “Adsorption of Mercuric Chloride Vaporsfrom

Incinerator Flue Gases on Calcium Hydroxide Particles,” Combustion Science and Technology,

Vol. 93, pp. 277-289, 1993.

Laudal, D., and S. Miller, “Enhanced Fine Particulate Control for Reduced Air-Toxic Emissions,”

Proceedings of Managing Hazardous Air Pollutants - State-of-the-Art, EPRI, Washington, DC,

November 1991.

Lea, F.M., The Chemisty of Cement and Concrete, Chemical Publishing Co., Inc., New York, NY,

1970.

Lindbauer, R., F. Wurst, and T. Prey, “Combustion Dioxin Suppression in Municipal Solid Waste

Incineration with Sulphur Additives,” Chemosphere, Vol. 25, Nos. 7-10, pp. 1409-1414, 1992.

Lindquist, B., “Gas Cleaning in Connection with Waste Incineration,” Proceedings of the Municipal

Waste Combustion International Specialty Conference, Air and Waste Management Association,

VIP-19, Tampa, FL, pp. 300-311, April 1991.

Litt, R., and T. Tewksbury, “Trace Metal Retention When Firing Hazardous Waste in a FluidizedBed

Incinerator,” EPA-600/2-84-198, 1984.
R-6

Malcolm Pirnie, Inc., TRP Environmental Associates, Inc, and Clement International Corporation,

A Report on Merucry Control System for the Lee County Resource Recovery Facility, prepared for

the Florida Department of Utilities, Lee County, Florida, April 1992.

Marschall, H., H. Spencer, H., and H. Elliott, “New Oregon Incinerator Demonstrates Process

Innovation to Protect Both Air and Water,” Proceedings of Specialty Conference - Thermal Treatment

of Municipal, Industrial, and Hospital Wastes II, Air and Waste Management Association, SP-69,

pp. 60-73, Pittsburgh, PA, November 1989.

Mattila, H., T. Virtanen, et al., “Emissions of Poluchlorinated Dibenzo-p-dioxins and Dibenzofurans

in Flue Gas from Co-Combustion of Mixed Plastics with Coal and Bark,” Chemoshpere, Vol. 25,

No. 11, pp. 1599-1609, 1992.

McDonald, J.R., and A.H. Dean, Electrostatic Precipitator Manual, Noyes Data Corporation, 1982.

McKeena, J.D., and J.H. Turner, Fabric Filter - Baghouses I: Theory, Design, and Selection, ETS

International, Inc., 1989.

Morun, B., and F. Schwarzkopf, “Use of High-Surface-Are-Hydrate-Additive in a Two Stage

Process,” Proceedings of the 1992 Incineration Conference, Albuquerque, NM, pp. 133-139, May

11-15, 1992.

Natale, F., K. Callahan, and G. Whalen, “Pilot Plant Testing of a Wet Electrostatic Precipitator at

the GCAU Fluid Bed Sludge Incinerator,” 83rd Annual Meeting of the Air and Waste Management

Association, Pittsburgh, PA, 90-30.7, June 24-29, 1990.

Ok, G., Y. Hanai, and T. Katou, “Decomposition of Chlorinated Dioxins, Odorous Compounds and

NOx from MSW Incineration Plant by Oxidizing Catalysts,” Chemosphere, Vol. 26, No. 12, pp.

2167-2172, 1993.

Otani, Y., Emi, H., Kanaoka, C., Uchijima, I., and H. Nishino, “Removal of Mercury Vapor from

Air with Sulfur-Impregnated Adsorbents,” Environmental Science and Technology, Vol. 22, No. 6,

pp. 708-711, 1988.

Peray, K.E., The Rotary Cement Kiln, 2nd Edition, Chemical Publishing Co., New York, NY, 1986.

Pethnick, F., “High Temperature Baghouse Operation Using a Unique Metallic Fiber Filter Design,”

Proceedings of The User and Fabric Filtration Conference VI, Air and Waste Management

Association, SP-84, pp. 99-110, Toronto, Ontario, September 1992.

Pilney, J., G. Gathje, H. Vincent, and G. Jones, “EDV Field Testing at the 3M Corporate Incinerator,”

Proceedings of the 1993 Incineration Conference, Knoxville, TN, pp. 227-231, May 3-7, 1993.

Politi, D., A. Harleston, and E. Mull, “Control of Flue Gas Emissions at the Healthwaste Medical

Waste Incineration Facility Bronx, New York: A State of the Art Medical Waste Disposal Facility,”

Proceedings of the 1993 Incineration Conference, Knoxville, TN, pp. 389-394, May 3-7, 1993.

Richman, M., D. Fickling, and J. Hahn, “Mercury Removal Studies at a Municipal Waste Combustor

in Marion County, Oregon,” Proceedings of the Municipal Waste Combustion International Specialty

Conference, Air and Waste Management Association, VIP-32, Williamsburg, VA, pp. 918-932,

March 1993.

Saxena, S., R. Henry, and W. Podolski, “Particle Removal From High-Temperature, High-Pressure

Combustion Gases,” Progress in Energy and Combustion Science, Vol. 11, No. 3, p. 193, 1985.
R-7

Schoner, P., “An Innovative System for the Emission Control of Heavy Metals and Dioxin,”

Proceedings of the 1992 Incineration Conference, Albuquerque, NM, pp. 129-132, May 1992.

Scotto, M., T. Peterson, and J. Wendt, “Hazardous Waste Incineration; The In Situ Capture of Lead

by Sorbents in a Laboratory Scale Combustor,” 24th International Symposium on Combustion, pp.

1109-1118, The Combustion Institute, Pittsburgh, PA, 1992.

Sedman, C., and T. Brna, Municipal Waste Combustion Study: Flue Gas Cleaning Technology,

EPA/530-SW-021d, June 1987.

Shamekhi, R., H. Petersen, and W. Panknin, “Research into Activated Carbon Technology on Harmful

Organic Substances, Heavy Metals and NOx Control,” Proceedings of the 83rd Annual Air and

Waste Mangement Association Meeting, Pittsburgh, PA, 90-25.1, June 24-29, 1990.

Sheppard, S., “Operating Experience With the Ionizing Wet Scrubber on Hazwaste Incinerators,”

Proceedings of the 1992 Incineration Conference, Albuquerque, NM, pp. 87-92, May 11-15, 1992.

Sprung, S., Technological Problems in Pyroprocessing Cement Clinker: Cause and Solution, BertonVerlag, Dusseldorf, Germany, 1985.

Srinivasachar, S., J. Morency, and B. Wyslouzil, “Heavy Metal Transformations and Capture During

Incineration,” 85th Annual Meeting of the Air and Waste Management Association, Kansas City,

MO, 92-41.07, June 21-26, 1992.

Steag, telephone conversation on July 25, 1994 with Volker Rummenhohl, (202) 338–8767, 1994.

Stohr, J., J. Furrer, J. Hunsigner, and S. Kreisz, “Final Cleaning of Flue Gas from Municipal Waste

Incinerators,” Proceedings of the 1993 Incineration Conference, Knoxville, TN, pp. 203206, May

3-7, 1993.

Theodore, L., and A. Buonicore, Air Pollution Control Equipment: Selection, Design, Operation,

and Maintenance, Englewood Cliffs, NY, 1982.

Troxler, W., G. Smith, C. Henke, J. Lauber, J. Czapla, and J. Segada, “Ionizing Wet Scrubber Pilot

Test Results for Controlling Emissions from a Liquid Hazardous Waste Incinerator,” 85th Annual

Meeting of the Air and Waste Management Association, Kansas City, MO, 92-41.05, June 21-26,

1992.

Uberoi, M., and F. Shadman, “Sorbents for the Removal of Lead Compounds from Hot Flue Gases,”

AIChE, Vol. 36, No. 2, pp. 307-309, 1990.

Uberoi, M., and F. Shadman, “High Temperature Removal of Cadmium Compounds Using

SolidSorbents,” Environmental Science and Technology, Vol. 25, No. 7, pp. 1285-1289, 1991.

Valenti, M., “Tougher Standards for Burning Hazardous Waste,” Mechanical Engineering, Vol.115,

p. 69, August 1993.

Weitzman, L., “Cement Kilns as Hazardous Waste Incinerators, Environmental Progress, Vol. 2,

No. 1, pp. 10-14, 1983.
R-8

White, D., T. Brna, et al., “Parametric Evaluation of Powdered Activated Carbon Injection of Control

of Mercury Emissions form a Municipal Waste Combustor,” Proceedings of 85th Annual Air and

Waste Management Association Annual Meeting, Kansas, MO, 92-40.06, June 1992.

White, D., K. Nebel, and M. Johnston, “Municipal Waste Combustors: A Survey of Mercury

Emissions and Applicable Control Technologies,” Proceedings of the Municipal Waste Combustion

International Specialty Conference, Air and Waste Management Association, VIP-19, Tampa, FL,

pp. 652-662, April 1991.

White, H., Industrial Electrostatic Precipitation, Addison-Wesley Publishing Company, Reading,

MA, 1970.

Zievers, E. C., Zievers, J., and Eggerstandt, P., “Cylindrical Porous Ceramic Elements for Hot Gas

Filtration in the Chemical Industry,” Proceedings of the 1990 Incineration Conference, San Diego

CA, May 14-18, 1990.

Zievers, E., Zievers, J., Eggerstedt, P., and Aguilar, P., “Porous Ceramics in Medical Waste

Incineration,” Proceedings of the 1991 Incineration Conference, pp. 183-188, Knoxville, TN, May

13-17, 1991.