United States Office of Air Quality EPA 456/F-99-006R

Environmental Protection Planning and Standards November 1999

Agency Research Triangle Park, NC 27711

Air
EPA-456/F-99-006R

November 1999

Nitrogen Oxides (NOx),

 Why and How They Are Controlled

Prepared by

Clean Air Technology Center (MD-12)

Information Transfer and Program Integration Division

Office of Air Quality Planning and Standards

U.S. Environmental Protection Agency

Research Triangle Park, North Carolina 27711
ii

DISCLAIMER

This report has been reviewed by the Information Transfer and Program Integration

Division of the Office of Air Quality Planning and Standards, U.S. Environmental Protection

Agency and approved for publication. Approval does not signify that the contents of this report

reflect the views and policies of the U.S. Environmental Protection Agency. Mention of trade

names or commercial products is not intended to constitute endorsement or recommendation for

use. Copies of this report are available form the National Technical Information Service, 

U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161, telephone

number (800) 553-6847.

CORRECTION NOTICE

This document, EPA-456/F-99-006a, corrects errors found in the original document, 

EPA-456/F-99-006. These corrections are:

Page 8, fourth paragraph: “Destruction or Recovery Efficiency” has been changed to

“Destruction or Removal Efficiency;”

Page 10, Method 2. Reducing Residence Time: This section has been rewritten to correct for an

ambiguity in the original text.

Page 20, Table 4. Added Selective Non-Catalytic Reduction (SNCR) to the table and added

acronyms for other technologies.

Page 29, last paragraph: This paragraph has been rewritten to correct an error in stating the

configuration of a typical cogeneration facility.

Page30, Internal Combustion Reciprocating Engines: A sentence has been added to the end of

this section to refer the readers to Table 13 for more information;

Page 41, third through seventh paragraphs: These paragraphs were renumbered to correct for a

numbering error (numbers 6 and 7 were used twice).
iii

FORWARD

The Clean Air Technology Center (CATC) serves as a resource on all areas of

emerging and existing air pollution prevention and control technologies, and provides public

access to data and information on their use, effectiveness and cost. In addition, the CATC will

provide technical support, including access to EPA’s knowledge base, to government agencies

and others, as resources allow, related to the technical and economic feasibility, operation and

maintenance of these technologies.

Public Access and Information Transfer

INTERNET / World Wide Web Home Page

http://www.epa.gov/ttn/catc

Communications

CATC Info-Line: (919) 541-0800 (English)

CATC/CICA Info-Line: (919) 541-1800 (Spanish)

FAX: (919) 541-0242

E-Mail: catcmail@epamail.epa.gov

Data Resources

 RACT/BACT/LAER Clearinghouse (RBLC)

Query, view and download data you select on 

- Source Specific Technology Applications

- Air Pollution Regulatory Requirements 

 CATC PRODUCTS

download technical reports, cost information and software 

Related Programs and Centers

 CICA - U.S.-Mexico Border Information Center on Air Pollution / 

Centro de Información sobre Contaminación de Aire Para la Frontera 

entre EE.UU. Y México

 SBAP - Small Business Assistance Program

 International Technology Transfer Center for Global Greenhouse Gasses
iv

ACKNOWLEDGMENTS

This technical bulletin was made possible through the diligent and persistent efforts of

Lyndon Cox, Senior Environmental Employee with the Clean Air Technology Center (CATC). 

Lyndon did an exceptional job identifying information sources, gathering relative data and

putting this bulletin together. The CATC also appreciates the helpful and timely comments and

cooperation of the following peer reviewers:

Jim Eddinger, Combustion Group, Emission Standards Division, Office of Air Quality Planning

and Standards, Office of Air and Radiation, U.S. EPA.

Doug Grano, Ozone Policy and Strategies Group, Air Quality Strategies and Standards Division,

Office of Air Quality Planning and Standards, Office of Air and Radiation, U.S. EPA.

William Vatavuk, Innovative Strategies and Economics Group, Air Quality Strategies and

Standards Division, Office of Air Quality Planning and Standards, Office of Air and Radiation,

U.S. EPA.

Ravi Strivastava, Air Pollution Technology Branch, Air Pollution Prevention and Control

Division, National Risk Management Research Laboratory, Office of Research and

Development, U.S. EPA.

In addition, the CATC thanks the individuals, companies and institutions who supplied

information on nitrogen oxide abatement technology used to prepare this Technical Bulletin. 

Contributors are indicated in the REFERENCES section of this bulletin.
v

TABLE OF CONTENTS

TOPIC Page

WHY SHOULD WE CONTROL NOx? ......................................1

WHAT IS NITROGEN OXIDE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

WHERE DOES NOx COME FROM? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

HOW DOES NOx AFFECT THE ENVIRONMENT? . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

ARE THERE OTHER NOx RELATED ISSUES? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

WHAT ABATEMENT AND CONTROL PRINCIPLES APPLY? .................8

WHAT ABATEMENT TECHNOLOGIES ARE AVAILABLE? . . . . . . . . . . . . . . . . . 11

EXTERNAL COMBUSTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

EXTERNAL COMBUSTION: POLLUTION PREVENTION METHODS . . . . 15

LESS EXCESS AIR (LEA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

BURNERS OUT OF SERVICE (BOOS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

OVER FIRE AIR (OFA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

LOW NOx BURNERS (LNB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

FLUE GAS RECIRCULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

WATER OR STEAM INJECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

REDUCED AIR PREHEAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

FUEL REBURNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

COMBUSTION OPTIMIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

AIR STAGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

FUEL STAGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

OXYGEN INSTEAD OF AIR FOR COMBUSTION . . . . . . . . . . . . . . . . . . 17

INJECTION OF OXIDANT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

CATALYTIC COMBUSTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

ULTRA-LOW NOx FUELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

NON-THERMAL PLASMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

EXTERNAL COMBUSTION: ADD-ON CONTROL TECHNOLOGY . . . . . . 18

SELECTIVE CATALYTIC REDUCTION (SCR) . . . . . . . . . . . . . . . . . . . . 18

SELECTIVE NON-CATALYTIC REDUCTION (SNCR) . . . . . . . . . . . . . . 18

SORPTION - BOTH ADSORPTION AND ABSORPTION . . . . . . . . . . . . 19

COMBINED TECHNOLOGY APPROACHES . . . . . . . . . . . . . . . . . . . . . . 19
vi

TABLE OF CONTENTS (continued)

TOPIC Page

INTERNAL COMBUSTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

INTERNAL COMBUSTION: POLLUTION PREVENTION METHODS . . . . 20

LOW NOx BURNERS (LNB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

STEAM / WATER INJECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

CATALYTIC COMBUSTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

AIR-FUEL RATIO AND IGNITION TYPE . . . . . . . . . . . . . . . . . . . . . . . . . 21

PRE-STRATIFIED CHARGE (PSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

LEAN BURN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

INTERNAL COMBUSTION: ADD-ON CONTROL TECHNOLOGY . . . . . . . 21

SELECTIVE CATALYTIC REDUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . 21

NON-SELECTIVE CATALYTIC REDUCTION (NSCR) . . . . . . . . . . . . . . 22

NON-THERMAL PLASMA REACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . 22

DO FUELS AND COMBUSTION TYPE AFFECT ABATEMENT? . . . . . . . . . . . . . . 22

SOLID FUELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

LIQUID FUELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

SEMI-SOLID FUELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

GAS FUEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

COMBUSTION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

DRY BOTTOM BOILERS - WALLED FIRED, FRONT-FIRED

or OPPOSED-FIRED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

DRY BOTTOM BOILERS - TANGENTIALLY FIRED . . . . . . . . . . . . . . . . . . 26

WET BOTTOM (SLAG TAP) BOILERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

FLUIDIZED BED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

STOKERS WITH TRAVELING GRATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

STOKERS WITH SPREADERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

GAS TURBINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

INTERNAL COMBUSTION RECIPROCATING ENGINES . . . . . . . . . . . . . . 30

WHAT DOES NOx ABATEMENT AND CONTROL COST? . . . . . . . . . . . . . . . . . . . 30

ARE THESE METHODS SUFFICIENT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
vii

FIGURES

1. NOx Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2. Ozone Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

TABLES

1. Nitrogen Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. NOx Control Methods ........................................9

3. External Combustion NOx Limiting technologies . . . . . . . . . . . . . . . . . . 12

4. Internal combustion NOx Limiting Technologies . . . . . . . . . . . . . . . . . . 20

5. Common Combustion Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6. NOx technologies currently used for dry bottom wall-fired,

front-fired or opposed fired boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7. NOx technologies currently used for dry bottom tangentially 

fired boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8. NOx technologies currently used for wet bottom (slag tap) boilers . . . . . 27

9. NOx technologies currently used for fluidized bed combustion . . . . . . . . 28

10. NOx technologies currently used for stokers with traveling grates . . . . 29

11. NOx technologies currently used for stokers with spreader grates . . . . . 30

12. NOx technologies currently used for gas turbines . . . . . . . . . . . . . . . . . 31

13. NOx technologies currently used for stationary internal 

 combustion engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

14. 1993 Costs of NOx Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

15. 1997 Costs of NOx Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

16. Unit Costs for NOx Control Technologies for Non-Utility 

Stationary Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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1

Nitrogen Oxides (NOx),

 Why and How They Are Controlled

When we try to look only at one thing in Nature, 

we find it connected to everything else.

John Muir

Nitrogen oxides (NOx) are a very interesting and important family of air polluting chemical

compounds. This bulletin explains why NOx are important air pollutants and how NOx are

formed and react in the atmosphere. This bulletin also discusses the principles on which all NOx

control and pollution prevention technologies are based; available NOx technologies for various

combustion sources; and performance and cost of NOx technologies..

WHY SHOULD WE CONTROL NOx?

NOx represent a family of seven compounds. Actually, EPA regulates only nitrogen dioxide

(NO2) as a surrogate for this family of compounds because it is the most prevalent form of NOx

in the atmosphere that is generated by anthropogenic (human) activities. NO2 is not only an

important air pollutant by itself, but also reacts in the atmosphere to form ozone (O3) and acid

rain. It is important to note that the ozone that we want to minimize is tropospheric ozone; that

is, ozone in the ambient air that we breathe. We are not talking about stratospheric ozone in the

upper atmosphere that we cannot breathe. Stratospheric ozone protects us and the troposphere

from ionizing radiation coming from the sun.

EPA has established National Ambient Air Quality Standards (NAAQS) for NO2 and

tropospheric ozone. The NAAQS define levels of air quality that are necessary, with a

reasonable margin of safety, to protect public health (primary standard) and public welfare

(secondary standard) from any known or anticipated adverse effects of pollution. The primary

and secondary standard for NO2 is 0.053 parts per million (ppm) (100 micrograms per cubic

meter), annual arithmetic mean concentration. 

Tropospheric ozone has been and continues to be a significant air pollution problem in the

United States and is the primary constituent of smog. Large portions of the country do not meet

the ozone NAAQS and thereby expose large segments of the population to unhealthy levels of

ozone in the air. NO2 reacts in the presence of air and ultraviolet light (UV) in sunlight to form

ozone and nitric oxide (NO). The NO then reacts with free radicals in the atmosphere, which are

also created by the UV acting on volatile organic compounds (VOC). The free radicals then

recycle NO to NO2. In this way, each molecule of NO can produce ozone multiple times.40 This

will continue until the VOC are reduced to short chains of carbon compounds that cease to be 

photo reactive (a reaction caused by light). A VOC molecule can usually do this about 5 times.

In addition to the NO2 and Ozone NAAQS concerns, NOx and sulfur oxides (SOx) in the
2

atmosphere are captured by moisture to form acid rain. Acid rain, along with cloud and dry

deposition, severely affects certain ecosystems and directly affects some segments of our

economy. All of these facts indicate an obvious need to reduce NOx emissions. However, to

successfully do so, we must understand the generation and control of the NOx family of air

pollutants. 

WHAT IS A NITROGEN OXIDE?

Diatomic molecular nitrogen (N2) is a relatively inert gas that makes up about 80% of the air we

breathe. However, the chemical element nitrogen (N), as a single atom, can be reactive and have

ionization levels (referred to as valence states) from plus one to plus five. Thus nitrogen can

form several different oxides. Using the Niels Bohr model of the atom, valence state relates to

the number of electrons which are either deficient (positive valence) or surplus (negative

valence) in the ion when compared with the neutral molecule. The family of NOx compounds

and their properties are listed in Table 1.

Table 1. Nitrogen Oxides (NOx)

Formula Name Nitrogen

Valence

Properties

N2O nitrous oxide 1 colorless gas

water soluble

NO

N2O2

nitric oxide

dinitrogen dioxide

2 colorless gas

slightly water soluble 

N2O3 dinitrogen trioxide 3 black solid

water soluble, decomposes in water

NO2

N2O4

nitrogen dioxide

dinitrogen tetroxide

4 red-brown gas

very water soluble, decomposes in water

N2O5 dinitrogen pentoxide 5 white solid

very water soluble, decomposes in water

Oxygen ions are always at valence minus 2. Depending upon the number of oxygen ions (always

balanced by the valence state of nitrogen), NOx can react to either deplete or enhance ozone

concentrations. The nitrogen ion in these oxides really does a dance in which it has (at different

times) various numbers of oxygen ions as partners. Nitrogen changes its number of partners

when it changes its ionization energy level. This happens whenever NOx: (1) is hit with a

photon of ionizing radiation (UV or a shorter wavelength light); (2) is hit with enough photons

that together transfer enough energy to change its ionization level; (3) is catalyzed; (4) is

stimulated sufficiently by thermal (IR) energy; (5) reacts with a chemically oxidizing or reducing

radical (an ionized fragment of a molecule); or (6) reacts with a chemically oxidizing or reducing
3

ion (an atom with unbalanced electrical charge).

When any of these oxides dissolve in water and decompose, they form nitric acid (HNO3) or

nitrous acid (HNO2). Nitric acid forms nitrate salts when it is neutralized. Nitrous acid forms

nitrite salts. Thus, NOx and its derivatives exist and react either as gases in the air, as acids in

droplets of water, or as a salt. These gases, acid gases and salts together contribute to pollution

effects that have been observed and attributed to acid rain. 

Nitrous oxide (N2O), NO, and NO2 are the most abundant nitrogen oxides in the air. N2O (also

known as laughing gas) is produced abundantly by biogenic sources such as plants and yeasts. It

is only mildly reactive, and is an analgesic (i.e., unlike an anaesthetic you still feel pain, but you

feel so good that you just don’t mind it). N2O is an ozone depleting substance which reacts with

O3 in both the troposphere (i.e., below 10,000 feet above sea level) and in the stratosphere

(50,000 - 150,000 feet). N2O has a long half-life, estimated at from 100 to 150 years. 

Oxidation of N2O by O3 can occur at any temperature and yields both molecular oxygen (O2) 

and either NO or two NO molecules joined together as its dimer, dinitrogen dioxide (N2O2). The

NO or N2O2 then oxidizes quickly (in about two hours) to NO2. The NO2 then creates an ozone

molecule out of a molecule of oxygen (O2) when it gets hit by a photon of ionizing radiation from

sunlight. N2O is also a “Greenhouse Gas” which, like carbon dioxide (CO2), absorbs long

wavelength infrared radiation to hold heat radiating from Earth, and thereby contributes to global

warming. 

Emissions of NOx from combustion are primarily in the form of NO. According to the

Zeldovich equations, NO is generated to the limit of available oxygen (about 200,000 ppm) in air

at temperatures above 1,300C (2,370F). At temperatures below 760C (1,400F), NO is either

generated in much lower concentrations or not at all. Combustion NO is generated as a function

of air to fuel ratio and is more pronounced when the mixture is on the fuel-lean side of the

stoichiometric ratio50 (the ratio of chemicals which enter into reaction). The Zeldovich equations

are:

N2 + O  NO + N

N + O2  NO + O

N + OH  NO + H

Except for NO from soils, lightning and natural fires, NO is largely anthropogenic (i.e., generated

by human activity). Biogenic sources are generally thought to account for less than 10% of total

NO emissions. NO produces the same failure to absorb oxygen into the blood as carbon 

monoxide (CO). However, since NO is only slightly soluble in water, it poses no real threat

except to infants and very sensitive individuals.

NO2 is present in the atmosphere and in acid rain. It produces nitric acid (HNO3) when dissolved

in water. When NO2 reacts with a photon to make O2 become O3, NO2 becomes NO. This NO is

then oxidized within hours to NO2 by radicals from the photo reaction of VOC. Therefore, our
4

present ozone concentration is the product of both NOx and VOC pollution. 

Dinitrogen trioxide (N2O3) and dinitrogen tetroxide (N2O4) exist in very small concentrations in

flue gas. However, they exist in such low concentrations in the atmosphere that both their

presence and their effect are often ignored. N2O4 is two NO2 molecules joined together (another

dimer) and reacts like NO2; so, the presence of N2O4 may be masked by the more abundant NO2.

Dinitrogen pentoxide (N2O5) is the most highly ionized form of nitrogen oxide. It is generated in

air in a very small concentration, unless it is emitted from a process (such as a nitric acid

production facility) that is specifically designed to generate it. N2O5 is highly reactive, and

forms nitric acid (HNO3) when it decomposes in water. 

Some experts feel that NO2 is a good surrogate for NOx because NO is rapidly converted to NO2,

and N2O has such a long life because it is not highly reactive. Others feel that due to their role in

forming ozone, both NO and NO2 should be considered NOx. Still others feel that all nitrogen

oxides (including N2O) need to be regulated. NO and NO2 are certainly the most plentiful forms

of NOx and they are largely (but not exclusively) from anthropogenic sources. N2O is largely

biogenic, and as such is not subject to regulation. For environmental purposes, using the

concentration of NO2 as a surrogate for the concentration of NOx has seemed to suffice, for it is

the precursor for ozone.

WHERE DOES NOx COME FROM?

Automobiles and other mobile sources contribute about half of the NOx that is emitted. Electric

power plant boilers produce about 40% of the NOx emissions from stationary sources.34

Additionally, substantial emissions are also added by such anthropogenic sources as industrial

boilers, incinerators, gas turbines, reciprocating spark ignition and Diesel engines in stationary

sources, iron and steel mills, cement manufacture, glass manufacture, petroleum refineries, and

nitric acid manufacture. Biogenic or natural sources of nitrogen oxides include lightning, forest

fires, grass fires, trees, bushes, grasses, and yeasts.1 These various sources produce differing

amounts of each oxide. The anthropogenic sources are approximately shown as:

Mobile Sources Electric Power

Plants

Everything Else

50% 20% 30%

This shows a graphic portrayal of the emissions of our two greatest sources of NOx. If we

could reduce the NOx emissions from just these two leading categories, we might be able to live

with the rest. However, don’t expect either of these categories to become zero in the foreseeable

future. We cannot expect the car, truck, bus, and airplane to disappear. The zero-emission car is

still on the drawing board and not on the production line. Also, social customs will have to

change before consumption of electricity can be reduced.
5

In all combustion there are three opportunities for NOx formation. They are:

1. Thermal NOx - The concentration of “thermal NOx” is controlled by the nitrogen and

oxygen molar concentrations and the temperature of combustion. Combustion at temperatures

well below 1,300C (2,370F) forms much smaller concentrations of thermal NOx. 

2. Fuel NOx - Fuels that contain nitrogen (e.g., coal) create “fuel NOx” that results from

oxidation of the already-ionized nitrogen contained in the fuel.

3. Prompt NOx - Prompt NOx is formed from molecular nitrogen in the air combining with fuel

in fuel-rich conditions which exist, to some extent, in all combustion. This nitrogen then oxidizes

along with the fuel and becomes NOx during combustion, just like fuel NOx. The abundance of 

prompt NOx is disputed by the various writers of articles and reports - probably because they

each are either considering fuels intrinsically containing very large or very small amounts of

nitrogen, or are considering burners that are intended to either have or not have fuel-rich regions

in the flame.

HOW DOES NOx AFFECT THE ENVIRONMENT?

Because NOx are transparent to most wavelengths of light (although NO2 has a brownish color

and the rare N2O3 is black), they allow the vast majority of photons to pass through and,

therefore, have a lifetime of at least several days. Because NO2 is recycled from NO by the photo

reaction of VOC to make more ozone, NO2 seems to have an even longer lifetime and is capable

of traveling considerable distances before creating ozone. Weather systems usually travel over

the earth’s surface and allow the atmospheric effects to move downwind for several hundred

miles. This was noted in EPA reports more than twenty years ago. These reports found that each

major city on the East coast has a plume of ozone that extends more than a hundred miles out to

sea before concentrations drop to 100 parts per billion (ppb). Another report cited the same

phenomenon for St. Louis. Therefore, this problem was not just on the sea coast. Since ozone in

clean air has a lifetime of only a few hours, this phenomenon is a measure of the effect and the

persistence of both VOC and NOx. 

Differences in the distance estimates between the emission of NOx and the generation of ozone

may be related to differences in plume transport (wind) speeds as well as other meteorological

and air quality factors. It is important to note that, under the right conditions, power plant plumes

may travel relatively long distances overnight with little loss of VOC, NO and NO2 . These

pollutants can thus be available to participate in photochemical reactions at distant locations on

the following day.41 Figure 1 shows a map of NOx concentration drawn by the Center for Air

Pollution Impact and Trend Analysis (CAPITA) at Washington University in St. Louis and

reported to the Ozone Transport Assessment Group, a national workgroup that addressed the

problem of ground-level ozone (smog) and the long-range transport of air pollution across the

Eastern United States. OTAG was a partnership among the EPA, the Environmental Council of

the States (ECOS) and various industry and environmental groups with the goal of developing a

thoughtful assessment and a consensus agreement for reducing ground-level ozone and the
6

Figure 2 NOx Map

Figure 3 Ozone Map

pollutants that cause it. The animated version of Figure 1 shows the trajectory of NOx emissions

moving with the weather over an 8 day period. 

Figure 2 is a map of ozone concentration that shows the same trajectory over the 8 day period. 

The animated version shows concentrations of both NOx and ozone moving with the weather for

several hundred miles.5

Ozone is the primary constituent of smog. Between 1970 and 1990, we in the United States have

tried to control ozone primarily by controlling the emissions of VOC. However, we have had

mixed results, for although some areas reduced their VOC emissions and attained their ozone

goals, others have not. It now appears that the communities that failed to meet their ozone goals
7

may not be completely at fault, for they appear to be affected by NOx and VOC emissions in the

air coming to them. To meet the ozone NAAQS, EPA must now regulate emissions of NOx

regionally.

ARE THERE OTHER NOx RELATED ISSUES?

Yes. Nutrient enrichment problems (eutrophication) occur in bodies of water when the

availability of either nitrates or phosphates become too large. As a result, the ratios of nitrogen

to phosphorus, silicon, and iron and other nutrients are altered. This alteration may induce

changes in phytoplankton, produce noxious or toxic brown or red algal blooms (which are called

“red tides”), or stimulate other plant growth. The algal blooms and plant growth produce a

shadow and cause the death of other plants in the water, which depletes the oxygen content of the

water (hypoxia) when the plants die, sink, and decay. Such eutrophication can make the bottom

strata of water unihabitable for both marine animals (such as fish and shellfish) and aquatic

plants. It can progress to virtually the complete depth of the water. It is estimated that between

12% and 44% of the nitrogen loading of coastal water bodies comes from the air.40 Inland lakes

are also affected in this way.

Another dimension of the problem is that high temperature combustion can convert sulfur in fuel

to SO2 and SO3. While SO2 is toxic and forms sulfurous acid when dissolved in water, SO3 is

both toxic and hygroscopic (moisture absorbing) and forms sulfuric acid by combining with

moisture in the atmosphere. SO2 and SO3 form sulfites and sulfates when their acids are

neutralized. Both of these acids can form solid particles by reacting with ammonia in air. SO2

and SO3 also contribute to pH (acidity) changes in water, which can adversely affect both land

and aquatic life. Therefore, both NOx and SOx from combustion can kill plants and animals.

CAPITA has shown that there are about equal amounts by weight of sulfate/sulfite, nitrate and

organic particles making up 90% of Particulate Matter less than 2.5 microns in aerodynamic

diameter (PM-2.5). This was confirmed by Brigham Young University researchers. The Six

Cities Study, published in the New England Journal of Medicine in 1990, has shown that illness

and premature death are closely correlated with the amount of PM-2.5 in the air. Therefore, there

is epidemiological data indicting nitrogen oxides, sulfur oxides, and/or organic compounds as 

PM-2.5 aerosols. There is currently no evidence that separately examines the health effects of

each of these substances. PM-2.5 usually appear as smog, smoke, white overcast, haze, or fog

which does not clear when air warms up. Brown smog is colored by nitrogen dioxide. 

Because the nitric acid, sulfurous acid and sulfuric acid react with ammonia in air to form solid

crystals that are much smaller than 2.5 microns and can be nucleation sites for particle growth,

we need to be concerned about each of these pollutants. Some research indicates that even

insoluble particles much smaller than 2.5 microns in size can exhibit severe toxic effects.38 The

smallest particles that have shown toxicity have a diameter of about 3% to 5% of the wavelength

of any color of visible light. Therefore, these particles are too small to even scatter light and

cannot even be detected optically.
8

Acid deposition occurs from airborne acidic or acidifying compounds, principally sulfates (SO4

-2) 

and nitrates (NO3

-1), that can be transported over long distances before returning to earth. This

occurs through rain or snow (wet deposition), fog or cloud water (cloud deposition), or transfer

of gases or particles (dry deposition). While severity of damage depends on the sensitivity of the

receptor, acid deposition and NOx “represent a threat to natural resources, ecosystems, visibility,

materials, and public health.”(section 401(a)(1) of the Clean Air Act).40

WHAT ABATEMENT AND CONTROL PRINCIPLES APPLY?

NOx abatement and control technology is a relatively complex issue. We shall try to provide a

structure to the spectrum of NOx pollution prevention and control technologies by first giving the

principles that are used. Then we shall describe the more successful pollution prevention and

emission control technologies and strategies. 

Please note that abatement and control of NOx from nitric acid manufacturing and “pickling”

baths differs from abatement and control at combustion sources. Combustion sources all have

NOx in a large flow of flue gas, while nitric acid manufacturing plants and pickling baths try to

contain the NOx. Wet scrubbers (absorbers) can control NOx emissions from acid plants and

pickling, and can use either alkali in water, water alone, or hydrogen peroxide as the liquid that

captures the NOx.3 The wet scrubber operates by liquid flowing downward by gravity through a

packing medium, opposed by an upward flow of gas. Scrubbers operate on the interchange of

substances between gas and liquid. This requires that the height of the absorber, type of packing,

liquid flow, liquid properties, gas properties, and gas flow should collectively cause a scrubber to

have the desired control efficiency. Chapter 9 of the OAQPS Control Cost Manual provides

guidance on the application, sizing, and cost of these scrubbers (referred to as gas absorbers). 

Also, Table 16 in this Bulletin presents some information for non-combustion NOx sources. 

Other then that, non-combustion NOx sources are not addressed in this Bulletin. 

For combustion sources, this Bulletin defines abatement and emission control principles and

states the Destruction or Removal Efficiency (DRE) that each successful technology is capable of

achieving. The effectiveness of pollution prevention measures in reducing NO and NO2

generation also is expressed in terms of relative DRE; i.e., the amount NOx generation is reduced

by using a prevention technology compared to NOx generation when not using that technology. 

Then, specific boiler types and combustion systems and applicable NOx technologies for each

system are discussed. Finally, the cost of these technologies is considered.

Many new combustion systems incorporate NOx prevention methods into their design and

generate far less NOx then similar but older systems. As a result, considering DRE (even a

relative DRE) for NOx may be inappropriate. Comparing estimated or actual NOx emissions

from a new, well-designed system to NOx emitted by a similar well-controlled and operated

older system may be the best way of evaluating how effectively a new combustion system

minimizes NOx emissions.

Table 2 lists principles or methods that are used to reduce NOx. Basically there are six
9

principles, with the seventh being an intentional combination of some subset of the six. 

Table 2. NOx Control Methods 6,7

Abatement or Emission

Control Principle or

Method

Successful Technologies Pollution Prevention

Method (P2) or Addon Technology (A)

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning

Low NOx Burners (LNB)

Combustion Optimization

Burners Out Of Service (BOOS)

Less Excess Air (LEA)

Inject Water or Steam

Over Fire Air (OFA)

Air Staging

Reduced Air Preheat

Catalytic Combustion

P2

P2

P2

P2

P2

P2

P2

P2

P2

P2

P2

2.Reducing residence time

 at peak temperature

Inject Air 

Inject Fuel

Inject Steam

P2

P2

P2

3. Chemical reduction of

NOx

Fuel Reburning (FR)

Low NOx Burners (LNB)

Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction

(SNCR)

P2

P2

A

A

4. Oxidation of NOx with 

subsequent absorption

Non-Thermal Plasma Reactor

Inject Oxidant

A

A

5. Removal of nitrogen Oxygen Instead Of Air

Ultra-Low Nitrogen Fuel

P2

P2

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

A

A

7. Combinations of these

 Methods

All Commercial Products P2 and A

Method 1. Reducing Temperature -- Reducing combustion temperature means avoiding the

stoichiometric ratio (the exact ratio of chemicals that enter into reaction). Essentially, this

technique dilutes calories with an excess of fuel, air, flue gas, or steam. Combustion controls use

different forms of this technique and are different for fuels with high and low nitrogen content. 
10

Control of NOx from combustion of high nitrogen content fuels (e.g., coal) can be understood by

the net stoichiometric ratio. Control of the NOx from combustion of low nitrogen fuels (such as

gas and oil) can be seen as lean versus rich fuel/air ratios. Either way, this technique avoids the

ideal stoichiometric ratio because this is the ratio that produces higher temperatures that generate

higher concentrations of thermal NOx. 

Combustion temperature may be reduced by: (1) using fuel rich mixtures to limit the amount of

oxygen available; (2) using fuel lean mixtures to limit temperature by diluting energy input; 

(3) injecting cooled oxygen-depleted flue gas into the combustion air to dilute energy; 

(4) injecting cooled flue gas with added fuel; or (5) injecting water or steam. Low-NOx burners

are based partially on this principle.8,9,10 The basic technique is to reduce the temperature of

combustion products with an excess of fuel, air, flue gas, or steam. This method keeps the vast

majority of nitrogen from becoming ionized (i.e., getting a non-zero valence). 

Method 2. Reducing Residence Time -- Reducing residence time at high combustion

temperatures can be done by ignition or injection timing with internal combustion engines. It can

also be done in boilers by restricting the flame to a short region in which the combustion air

becomes flue gas. This is immediately followed by injection of fuel, steam, more combustion

air, or recirculating flue gas. This short residence time at peak temperature keeps the vast

majority of nitrogen from becoming ionized. This bears no relationship to total residence time

of a flue gas in a boiler.

Method 3. Chemical Reduction of NOx – This technique provides a chemically reducing (i.e., 

reversal of oxidization) substance to remove oxygen from nitrogen oxides. Examples include

Selective Catalytic Reduction (SCR) which uses ammonia, Selective Non-Catalytic Reduction

(SNCR) which use ammonia or urea, and Fuel Reburning (FR). Non-thermal plasma, an

emerging technology, when used with a reducing agent, chemically reduces NOx. All of these

technologies attempt to chemically reduce the valence level of nitrogen to zero after the valence

has become higher.11 Some low-NOx burners also are based partially on this principle.

Method 4. Oxidation of NOx -- This technique intentionally raises the valence of the nitrogen

ion to allow water to absorb it (i.e., it is based on the greater solubility of NOx at higher valence). 

This is accomplished either by using a catalyst, injecting hydrogen peroxide, creating ozone

within the air flow, or injecting ozone into the air flow. Non-thermal plasma, when used without

a reducing agent, can be used to oxidize NOx. A scrubber must be added to the process to

absorb N2O5 emissions to the atmosphere. Any resultant nitric acid can be either neutralized by

the scrubber liquid and then sold (usually as a calcium or ammonia salt), or collected as nitric

acid to sell to customers.12, 49

Method 5. Removal of nitrogen from combustion -- This is accomplished by removing

nitrogen as a reactant either by: (1) using oxygen instead of air in the combustion process; or 

(2) using ultra-low nitrogen content fuel to form less fuel NOx. Eliminating nitrogen by using

oxygen tends to produce a rather intense flame that must be subsequently and suitably diluted. 

Although Method 2 can lower the temperature quickly to avoid forming excessive NOx, it cannot
11

eliminate nitrogen oxides totally if air is the quench medium. Hot flue gas heats the air that is

used to quench it and this heating generates some thermal NOx. This method also includes

reducing the net excess air used in the combustion process because air is 80% nitrogen. Using

ultra-low-nitrogen content fuels with oxygen can nearly eliminate fuel and prompt NOx.13 

Method 6. Sorption, both adsorption and absorption -- Treatment of flue gas by injection of 

sorbents (such as ammonia, powdered limestone, aluminum oxide, or carbon) can remove NOx

and other pollutants (principally sulfur). There have been successful efforts to make sorption

products a marketable commodity. This kind of treatment has been applied in the combustion

chamber, flue, and baghouse. The use of carbon as an adsorbent has not led to a marketable

product, but it is sometimes used to limit NOx emissions in spite of this. The sorption method is

often referred to as using a dry sorbent, but slurries also have been used. This method uses either

adsorption or absorption followed by filtration and/or electrostatic precipitation to remove the

sorbent. 

Method 7. Combinations of these methods -- Many of these methods can be combined to

achieve a lower NOx concentration than can be achieved alone by any one method. For example,

a fuel-rich cyclone burner (Method 1) can be followed by fuel reburn (Method 3) and over-fire

air (Method 1). This has produced as much as a 70% reduction in NOx.55 Other control

technologies that are intended to primarily reduce concentrations of sulfur also strongly affect the

nitrogen oxide concentration. For example, the SOx-NOx-ROx-Box (SNRB) technology uses a

limestone sorbent in the flue gas from the boiler to absorb sulfur. This is followed by ammonia

injection and SCR using catalyst fibers in the baghouse filter bags. The sulfur is recovered from

the sorbent and the sorbent regenerated by a Claus process. This has demonstrated removal of up

to 90% of the NOx along with 80% of the SOx.39, 42 EBARA of Japan reported that an electron

beam reactor with added ammonia removed 80% of the SO2 and 60% of the NOx for a utility

boiler in China.54 FLS Milo and Sons reported at the same symposium that 95% of the SO2 and

70%-90% of the NOx were removed in several demonstrations of their SNAP technology, which

is based upon an aluminum oxide adsorber with Claus regeneration.56

WHAT ABATEMENT TECHNOLOGIES ARE AVAILABLE?

In this report existing NOx abatement technologies are divided into two categories, external

combustion applications (e.g., boilers, furnaces and process heaters) and internal combustion

applications (e.g., stationary internal combustion engines and turbines). These categories are

further subdivided into pollution prevention (which reduces NOx generation) and add-on control

technologies (which reduces NOx emissions). 

EXTERNAL COMBUSTION

For external combustion applicable technologies are shown in Table 3 (based on Table 2 in

Select the Right NOx Control Technology, Stephen Wood, Chemical Engineering Progress,

January 1994).
12

Table 3. External Combustion NOx Limiting Technologies

Technique Description Advantages Disadvantages Impacts Less Excess Air (LEA) Reduces oxygen

availability

Easy modification Low NOx reduction High CO 

Flame lengthFlame stability

Off Stoichiometrica. Burners Out ofService (BOOS)b. Over Fire Air (OFA)

staged combustion Low cost

No capital costfor BOOS

a. Higher air flow forCOb. high capital cost

Flame lengthFan capacityHeader pressure

Low NOx Burner (LNB)

Internal stagedcombustion

Low operating costCompatible FGR

Moderately high capitalcost

Flame lengthFan capacityTurndown stability

Flue Gas Recirculation(FGR)

<30% flue gasrecirculated with air,decreasing temperature

High NOx reductionpotential for lownitrogen fuels

Moderately high capitalcost and operating costAffects heat transferand system pressures

Fan capacityFurnace pressureBurner pressure dropTurndown stability

Water/Steam Injection Reduces flame

temperature

Moderate capital costNOx reduction similarto FGR

Efficiency penaltyFan power higher

Flame stabilityEfficiency penalty

Reduced Air Preheat Air not preheated,

reduces flametemperature

High NOx reductionpotential 

Significant efficiencyloss (1% per 40(F)

Fan capacityEfficiency penalty

Selective Catalyticreduction (SCR)(add-on technology)

Catalyst located in theair flow, promotesreaction betweenammonia and NOx

High NOx removal Very high capital cost

High operating costCatalyst sitingIncreased pressure dropPossible water washrequired

Space requirementsAmmonia slipHazardous wasteDisposal
Table 3. External Combustion NOx Limiting Technologies

Technique Description Advantages Disadvantages Impacts 

13

Selective Non-CatalyticReduction (SNCR)(add-on technology)a. ureab. ammonia

Inject reagent to reactwith NOx

a. Low capital costModerate NOx removalNon-toxic chemicalb. Low operating costModerate NOx removal

a. TemperaturedependentNOx reduction less atlower loadsb. Moderately highcapital costAmmonia storage,handling, injectionsystem

a. Furnace geometryTemperature profileb. Furnace geometryTemperature profile

Fuel Reburning Inject fuel to react with

NOx

Moderate costModerate NOx removal

Extends residence time Furnace temperature

profile

CombustionOptimization

Change efficiency ofprimary combustion

Minimal cost Extends residence time Furnace temperature

profile

Catalytic Combustion Catalyst causes

combustion to be atlow temperature

Lowest possible NOx Very high capital cost

High operating costCatalyst siting

Space requirementsDisposal

Non-Thermal Plasma Reducing agent ionized

or oxidant created inflow

Moderate costEasy sitingHigh NOx removal 

Fouling possibleOzone emissionpossible

Uses electrical power 

Inject Oxidant Chemical oxidant

injected in flow

Moderate cost Nitric acid removal Add-on 

Oxygen instead of Air Uses oxygen to oxidize

fuel

Moderate to high costIntense combustion

Eliminates promptNOxFurnace alteration

Equipment to handleoxygen

Ultra-Low NitrogenFuel

Uses low -nitrogen fuel Eliminates fuel NOx

No capital cost

Slight rise in operatingcost

Minimal change 
Table 3. External Combustion NOx Limiting Technologies

Technique Description Advantages Disadvantages Impacts 

14

Use Sorbents (add-ontechnology) in:a. Combustionb. Duct to Baghousec. Duct to ElectrostaticPrecipitator

Use a chemical toabsorb NOx or anadsorber to hold it

Can control otherpollutants as well asNOxModerate operatingcost

Cost of handlingsorbentSpace for the sorbentstorage and handling

Add-on 

Air Staging Admit air in separated

stages

Reduce peakcombustiontemperature

Extend combustion to alonger residence time atlower temperature

Adds ducts anddampers to control airFurnace modification

Fuel Staging Admit fuel in separated

stages

Reduce peakcombustiontemperature

Extend combustion to alonger residence time atlower temperature

Adds fuel injectors toother locationsFurnace modification
15

EXTERNAL COMBUSTION: POLLUTION PREVENTION METHODS

LESS EXCESS AIR (LEA)

Excess air flow for combustion has been correlated to the amount of NOx generated. Limiting

the net excess air flow to under 2% can strongly limit NOx content of flue gas. Although there

are fuel-rich and fuel-lean zones in the combustion region, the overall net excess air is limited

when using this approach.43

BURNERS OUT OF SERVICE (BOOS)

Multiple-burner equipment can have part of an array of burners with some “burners out of 

service” (not feeding fuel, but supplying air or flue gas). This allows the burners around them to

supply fuel and air to air or flue gas flowing from the BOOS. The result is combustion by stages

with temperature always lower than when all burners are in service. Thus, thermal NOx is lower. 

The degree to which NOx generation is reduced depends upon the spatial relationship of the

BOOS to the other burners.44

OVER FIRE AIR (OFA)

When primary combustion uses a fuel-rich mixture, use of OFA completes the combustion. 

Because the mixture is always off-stoichiometric when combustion is occurring, the temperature

is held down. After all other stages of combustion, the remainder of the fuel is oxidized in the

over fire air. This is usually not a grossly excessive amount of air.

LOW NOx BURNERS (LNB)

A LNB provides a stable flame that has several different zones. For example, the first zone can

be primary combustion. The second zone can be Fuel Reburning (FR) with fuel added to

chemically reduce NOx. The third zone can be the final combustion in low excess air to limit the

temperature. There are many variations on the LNB theme of reducing NOx. The LNB has

produced up to 80% DRE.17,18, 32, 33 This can be one of the least expensive pollution prevention

technologies with high DRE. LNB have had problems with designs that had flame attaching to

the burners, resulting in a need for maintenance. We believe that these design problems should

now be a thing of the past. 

FLUE GAS RECIRCULATION (FGR)

Recirculation of cooled flue gas reduces temperature by diluting the oxygen content of

combustion air and by causing heat to be diluted in a greater mass of flue gas. Heat in the flue gas

can be recovered by a heat exchanger. This reduction of temperature lowers the NOx

concentration that is generated. If combustion temperature is held down to below 1,400F, the

thermal NOx formation will be negligible.50
16

WATER OR STEAM INJECTION

Injection of water or steam causes the stoichiometry of the mixture to be changed and adds steam

to dilute calories generated by combustion. Both of these actions cause combustion temperature

to be lower. If temperature is sufficiently reduced, thermal NOx will not be formed in as great a

concentration. 

REDUCED AIR PREHEAT

Air is usually preheated to cool the flue gases, reduce the heat losses, and gain efficiency. 

However, this can raise the temperature of combustion air to a level where NOx forms more

readily. By reducing air preheat, the combustion temperature is lowered and NOx formation is

suppressed. This can lower efficiency, but can limit NOx generation.

FUEL REBURNING (FR)

Recirculation of cooled flue gas with added fuel (this can be natural gas, pulverized coal, or even 

oil spray) causes dilution of calories, similar to FGR, and primary combustion temperature can be

lowered. Also, when added as a secondary combustion stage, the presence of added fuel

chemically reduces newly generated NOx to molecular nitrogen. Added fuel is only partially

consumed in reducing NOx and burning is completed in a later stage using either combustion air

nozzles or over-fire-air. This technique has been demonstrated to be effective with residence

times from 0.2 seconds to 1.2 seconds and has achieved up to 76% DRE. 17

COMBUSTION OPTIMIZATION

Combustion optimization refers to the active control of combustion. In a natural gas fired boiler,

by decreasing combustion efficiency from 100% to 99%, NOx generation dropped to a much

more acceptable level.14,15 For coal-fired boilers a 20% to 60% reduction in NOx has been

experienced. These active combustion control measures seek to find an optimum combustion

efficiency and to control combustion (and hence emissions) at that efficiency. Another approach

uses a neural network computer program to find the optimum control point.16 Still another

approach is to use software to optimize inputs for the defined output.52,53

One vendor decreases the amount of air that is pre-mixed with fuel from the stoichiometric ratio

(ratio that produces the hottest flame) to lengthen the flame at the burner and reduce the rate of

heat release per unit volume. This can work where the boiler tubes are far enough away from the

burner. Carbon monoxide, unburned fuel, and partially burned fuel that result can then be

subsequently oxidized in over-fire-air at a lower temperature. Combustion must be optimized

for the conditions that are encountered. 50% DRE has been reported.14

AIR STAGING

Combustion air is divided into two streams. The first stream is mixed with fuel in a ratio that
17

produces a reducing flame. The second stream is injected downstream of the flame and makes

the net ratio slightly excess air compared to the stoichiometric ratio. DRE up to 99% have been

reported.51

FUEL STAGING

This is staging of combustion using fuel instead of the air. Fuel is divided into two streams. The

first stream feeds primary combustion that operates in a reducing fuel to air ratio. The second

stream is injected downstream of primary combustion, causing the net fuel to air ratio to be only

slightly oxidizing. Excess fuel in primary combustion dilutes heat to reduce temperature. The

second stream oxidizes the fuel while reducing the NOx to N2. This is reported to achieve a 50%

DRE.51

OXYGEN INSTEAD OF AIR FOR COMBUSTION

An example of this is a cyclone burner where the flame is short and intense. Excess fuel air or

steam, injected just after the combustion chamber per Method 2 is sufficient to rapidly quench

the flue gas to below NOx formation temperature. Combustion can then be completed in overfire air. Oxygen can now be separated from air at a low enough cost to make this economical.13

This technique has reduced NOx by up to 20%23 in burners using conventional fuel. This

technique also is usable with low-NOx burners to prevent the prompt NOx from being formed.

INJECTION OF OXIDANT

The oxidation of nitrogen to its higher valence states makes NOx soluble in water. When this is

done a gas absorber can be effective. Oxidants that have been injected into the air flow are

ozone, ionized oxygen, or hydrogen peroxide. Non-thermal plasma generates oxygen ions within

the air flow to achieve this. Other oxidants have to be injected and mixed in the flow. Nitric

acid can be absorbed by water, hydrogen peroxide, or an alkaline fluid. Calcium or ammonia

dissolved in the water can make an alkaline fluid that will react with nitric and sulfuric acids to

produce a nitrate or sulfate salt that can be recovered. Alternatively, using water or hydrogen

peroxide to absorb NOx can provide nitric acid for the commercial market.

CATALYTIC COMBUSTION

Use of a catalyst to cause combustion to occur below NO formation temperatures can provide a

suitable means of limiting temperature. This technique is not used often because it is very load

sensitive. However, where it is used, catalytic combustion can achieve less than a 1 ppm

concentration of NOx in the flue gas. 

ULTRA-LOW NITROGEN FUELS

These fuels can avoid NOx that results from nitrogen contained in conventional fuels. The result

can be up to a 70% reduction in NOx emissions.43 Now there are ultra-low-nitrogen liquid fuel
18

oils. These oils contain 15-20 times less nitrogen than standard No. 2 fuel oil. This oil is now

commercially available and competitively priced. Ultra-low-nitrogen oil is most frequently used

in Southern California where the air pollution is particularly a problem. Natural gas can be

considered a low-nitrogen fuel. Coke (the quenched char from coal) can also be an ultra-lownitrogen fuel because nitrogen in the volatile fraction of the coal is removed in making coke. 

NON-THERMAL PLASMA

Using methane and hexane as reducing agents, non-thermal plasma has been shown to remove

NOx in a laboratory setting with a reactor duct only 2 feet long. The reducing agents were

ionized by a transient high voltage that created a non-thermal plasma. The ionized reducing

agents reacted with NOx and achieved a 94% DRE. There are indications that an even higher

DRE can be achieved. A successful commercial vendor uses ammonia as a reducing agent to

react with NOx in an electron beam generated plasma. Such a short reactor can meet available

space requirements for virtually any plant. The non-thermal plasma reactor could also be used

without reducing agent to generate ozone and use that ozone to raise the valence of nitrogen for

subsequent absorption as nitric acid.

EXTERNAL COMBUSTION: ADD-ON CONTROL TECHNOLOGY

Add-on controls are applicable to a broad range of sources and fuels. This differs from the

pollution prevention techniques listed above in that the prevention techniques must be adapted to

the circumstances of their use. 

SELECTIVE CATALYTIC REDUCTION (SCR)

SCR uses a catalyst to react injected ammonia to chemically reduce NOx. It can achieve up to a

94% DRE34 and is one of the most effective NOx abatement techniques. However, this

technology has a high initial cost. In addition, catalysts have a finite life in flue gas and some

ammonia “slips through” without being reacted. SCR has historically used precious metal

catalysts, but can now also use base-metal and zeolite catalysts. The base-metal and zeolite

catalysts operate at much different temperatures then the precious metal catalysts.11

SELECTIVE NON-CATALYTIC REDUCTION (SNCR)

In SNCR ammonia or urea is injected within a boiler or in ducts in a region where temperature is

between 900C and 1100C. This technology is based on temperature ionizing the ammonia or

urea instead of using a catalyst or non-thermal plasma. This temperature “window“ – which is

reported differently by various authors -- is important because outside of it either more ammonia

“slips” through or more NOx is generated than is being chemically reduced. The temperature

“window” is different for urea and ammonia. Reduction of the NOx by SNCR can have up to a

70% DRE.23,35,43
19

SORPTION – BOTH ADSORPTION AND ABSORPTION

Several methods are used to inject and remove adsorbent or absorbent. One method sprays dry

powdered limestone into the flue gas. The limestone then reacts with both sulfuric acid and

nitric acid. There also is a spray dryer approach that sprays a slurry of powdered limestone and

aqueous ammonia into the flue gas. The limestone preferentially reacts with the sulfur while the

ammonia preferentially reacts with the NOx. In-duct injection of dry sorbents is another example

of this technique and can reduce pollutants in three stages: (1) in the combustion chamber, (2) in

the flue gas duct leading to the baghouse, and (3) in the flue gas duct leading to the electrostatic

precipitator. The by products formed by sorption are gypsum (calcium sulfate) that is sold to

make wallboard, and ammonium nitrate that can be sold to make either an explosive or a

fertilizer. Sorption is reported to have up to a 60% DRE.23, 31 Another version uses carbon

injected into the air flow to finish the capture of NOx. The carbon is captured in either the

baghouse or the ESP just like other sorbents. There are many absorbents and adsorbents

available.

COMBINED TECHNOLOGY APPROACHES

Very seldom is only one method or principle used alone. The choice depends upon the type of

combustion system, type of boiler or other energy conversion device, and type of fuel used. 

Available technologies will be narrowed by consideration of turndown ratio, stability of

combustion, availability or access to burners, air supply controls, fuel impurities, and cost among

other factors.

There are many examples and here are a few of them. Selective catalytic reduction of NOx to N2

can be followed by selective oxidation of sulfur dioxide to sulfur trioxide. Then sulfuric acid is

formed followed by scrubbing sulfuric acid from the flue gas.30 

LNB can be used in conjunction with SCR or SNCR to achieve a greater overall DRE than any of

these can achieve alone. Water/steam injection can be used with SCR to achieve a DRE greater

than SCR can achieve alone. Fuel reburning and SCR can be used together as well as separately,

to get the maximum NOx reduction.57

INTERNAL COMBUSTION

Now we turn to internal combustion, which usually occurs at elevated pressures. Again, we

divide the technologies between pollution prevention techniques and add-on technologies. This

is shown in Table 4.

These techniques can be used in combination. Pollution prevention techniques do not have to be

used separately. Add-on techniques could be used sequentially after a pollution prevention

technique when they do not impose conflicting demands on the process.
20

Table 4. Internal Combustion NOx Limiting Technologies

Pollution Prevention Add-On Control

Low-NOx Burners (LNB) Selective Catalytic Reduction (SCR)

Steam/Water Injection Selective Non-Catalytic Reduction (SNCR)

Catalytic Combustion Non-Selective Catalytic Reduction (NSCR)

Air-Fuel Ratio and Ignition Type Non-Thermal Plasma

Pre-Stratified Charge

Lean Burn

INTERNAL COMBUSTION: POLLUTION PREVENTION METHODS

LOW NOx BURNERS (LNB)

Combining the use of LNB with closely controlled air/fuel ratio and water/steam injection can

yield emissions as low as 10 ppm from gas turbines.46

STEAM/ WATER INJECTION

To reduce combustion temperature, steam or water can be mixed with the air flow. This lowers

combustion temperature to below 1,400F to limit NOx generation to about 40 ppm.46 This can

cause the concentration of CO and unburned hydrocarbons emitted from a turbine to be

increased. However, these can be burned by either a catalyst bed, afterburner, or another stage of

combustion. This otherwise wasted fuel and heat can also be recovered in co-generation boilers.

CATALYTIC COMBUSTION

A catalyst is used to react fuel with air at a lower temperature than normal combustion at which

generation of significant amounts of NOx does not occur. Emissions under 1 ppm NOx have

been reported.46 However, if this combustion is for a turbine, turbine efficiency may depend

upon achieving a higher temperature. When a catalyst is present, you also need to assure that

NOx will not be formed at the combustion temperature that results. 

This technology has a relatively high capital and operation and maintenance cost because there is

both a substantial initial investment and a replacement cost for the catalyst. The need for

replacement and, therefore, replacement cost are usually driven by impurities in the fuel. 

However, catalytic combustion generates possibly the lowest level of thermal NOx.
21

AIR-FUEL RATIO AND IGNITION TYPE

For internal combustion reciprocating engines, retardation of injection or spark ignition, or an

air-fuel ratio that departs from stoichiometric conditions will reduce peak temperature. Lower

peak temperature will limit the amount of NOx formation. This technique can achieve up to 50%

control efficiency.19, 48 

When a three-way catalyst is used for spark ignition engines, exhaust gas must have no more than

0.5% oxygen. This technique can be up to 98% effective.

The use of plasma ignition (an alternating current or AC system) instead of a direct current (DC)

spark ignition system can also allow a greater fuel-lean departure from the stoichiometric ratio. 

NOx emissions from internal combustion engines using plasma ignition have been reported to be

reduced by up to 97%.20,21, 45 

Delaying injection of fuel in a compression ignition (diesel) engine can reduce the NOx

emissions. The amount of this reduction will depend upon the engine, valving, and fuel. 

Excessive timing retard can cause combustion instability or misfire.48 However, some claims of

high effectiveness are to be found with ostensibly excessive retard.

PRE-STRATIFIED CHARGE (PSC)

PSC refers to an engine equipped with a pre-combustion chamber that receives a rich enough

air/fuel mixture to ignite dependably. This pre-combustion chamber fires a jet of flame into the

main combustion chamber (cylinder). The main combustion chamber has a fuel-lean mixture

that needs pre-combustion flame to ignite it reliably. The injected flame also produces a swirl in

the main combustion chamber that acts like stratified charge combustion. This dependably

ignites the lean main cylinder mixture. The PSC can achieve NOx emissions of 2

grams/horsepower-hour (g/hp-hr) or 140 ppm.48

LEAN BURN

Natural gas fueled engines that operate with a fuel-lean air/fuel ratio are capable of low NOx

emissions. These can achieve less than 1.0 gram/brake horsepower-hour according to the RACTBACT-LAER Clearinghouse (RBLC) (http://www.epa.gov/ttn/catc, then select RBLC).

INTERNAL COMBUSTION ADD-ON CONTROL TECHNOLOGY

SELECTIVE CATALYTIC REDUCTION (SCR)

As with boilers, SCR can be used to obtain up to a 90% DRE of NOx. When used with a LNB

or steam/water injection, NOx can be reduced to 5-10 ppm.46 With compression ignition

engines, zeolite catalysts achieve a DRE of 90+%, while base-metal catalysts can achieve a 80%

to 90% DRE.48
22

NON-SELECTIVE CATALYTIC REDUCTION (NSCR)

NSCR is the same technique used in automobile applications as a three-way catalytic converter.

It does not require injection of a reducing agent because it uses unburned hydrocarbons as a

reducing agent. The catalyst requires that exhaust have no more than 0.5% oxygen. This

technique uses a fuel rich mixture that, combined with back pressure from exhaust flow through

the catalyst, increases the brake specific fuel consumption of the engine. However, NOx control

of 90% to 98% can be achieved.48

NON-THERMAL PLASMA REACTORS

This approach uses a non-thermal plasma to ionize ammonia, urea, hexane, methane or other

reducing agents injected into a flue gas. Combined with the effect of temperature, non-thermal

plasma ionizes the reducing agent that reacts with nitrogen oxides achieving a 94% DRE. This

decreases the amount of reducing chemicals that “slips” through unreacted.20,44 The use of nonthermal plasma was developed to ionize pollutants and act as a catalyst to control NOx in diesel

exhaust.36

DO FUELS AND COMBUSTION TYPE AFFECT ABATEMENT?

Yes they do. Here again, we find a spectrum of types, almost enough to make every gas turbine,

internal combustion engine, boiler, or furnace seem unique. The type of fuel can vary with the

vein of the mine from which coal was obtained, the well in the oilfield from which crude oil

came, the refinery for petroleum based fuels, or the supplier of natural gas. Thus the

concentration of impurities will vary between sources, refineries, and suppliers. Even “natural

gas” (methane) may contain some “supplier gas” (propane, butane, and carbon monoxide) which

will cause the composition of “natural gas” to vary. 

The type of combustion system (low-NOx burner, over-fire air, tangential firing, wall firing, etc)

also will sometimes limit options. Each type of boiler, each type of fuel, each combustion

system, and each construction of a boiler puts constraints on what is possible. It is not possible

to treat each combination of combustion system and fuel in detail in this Technical Bulletin;

however, we will try to show the picture while painting with broad strokes.

The choice of fuel and combustion system often depends upon: (1) what can or cannot be

adjusted; (2) whether ducts are suitable for sorption; (3) what the effect on boiler maintenance

will be; (4) the temperature profile in the flow: (5) how the combustion system can be modified;

(6) what types of burners can be used; and (7) what can either be added or modified. The list

does not end there, but continues. Let us consider some fuels with these limitations in mind.

SOLID FUELS

In burning a solid fuel (such as coal), combustion control is achieved by first getting the primary

burner to gasify the volatile fraction of a fuel. The volatile fraction is carried away from char by
23

air flow, oxidized in the air flow, and becomes flue gas. Char needs more combustion air to burn

and provide further heat, part of which is used to volatilize additional fuel. To control

combustion temperature, you traditionally would limit combustion air through the char fraction. 

The volatile fraction is oxidized in over-fire-air or a secondary stage of burner and must have its

air separately controlled. The balance of combustion air between these stages must be adjusted

for composition of fuel being used, boiler loading, and transient loads. Because all of these

parameters will vary continually, provisions to make balancing adjustments dynamically are

recommended.

Pulverized coal can be burned similar to oil. The flame is usually well defined and, depending on 

particle size, char may remain in suspension in flue gas throughout burning. The volatile fraction

burns in air even as char is burned. If the particles are too coarse, char will continue burning on

its trajectory after leaving the flame, but will stop burning at some point. The trade jargon for this

is “ unburned carbon (UBC),” “carbon in the ash (CIA),” or “loss of ignition (LOI).” These terms

refer to carbon in char that does not burn along the trajectory. UBC is minimized by grinding

particles finer and classifying particles so that larger ones are returned to the roller mill or grinder.

Particles will become fly ash if they are small enough. UBC ranging from 0.5% to 5% is

considered acceptable. Therefore, particle size at ignition is important. The major concerns are to

control stoichiometry and combustion temperature to minimize unburned carbon in ash.

Biomass is another solid fuel, but burning biomass char is less of a concern than with coal. 

Biomass cannot be pulverized to small particles, but can burn to ash in a short time. As with the

burning of all char, ash and fly ash are problems, but can be treated with a slag tap or ash pit, 

baghouse, and/or electrostatic precipitator.

LIQUID FUELS

Liquid fuels burn like the volatile fraction of solid fuel provided that the droplets are small

enough. Liquid fuels usually have less nitrogen content than solid fuels. Combustion of liquids

and gases can be controlled much more readily than char from solid fuel because combustion is

less dependent on the history of the past few minutes of demand. Combustion is also completed

essentially without residual ash. The fuel-air ratio can be used to control combustion temperature

and can be adjusted to minimize NOx generation. The flame can be well-defined and

combustion is essentially completed within the flame. Therefore, burning oil or liquid-from-coal

or liquid-from-gas is different from burning coal because there is usually less nitrogen in the fuel,

a lack of char, complete burning within the flame and a lack of ash.

SEMI-SOLID FUELS

Semi-solid fuels are residuals from refineries. They are not clean burning like distillates and

often are not even liquid at room temperature. Many impurities typically found in crude oil are

concentrated in semi-solid residual fuel. These fuels can contain more nitrogen than coal, but

usually contain less sulfur.50 Therefore, semi-solid fuels are intermediate between coal and oil. 

They often have somewhat less impurities than coal (although they can have more impurities),
24

but they do produce ash. 

GAS FUEL

Natural gas is desulfurized before it is sent in a pipeline. Therefore, natural gas has almost no

sulfur, essentially no impurities, and no ash. The only thing that varies is heat content per cubic

meter. This variance is caused by natural gas producers supplementing natural gas with propane,

liquified petroleum gas (butane), carbon monoxide, or other gaseous fuel. As a result, air to fuel

ratio must be controllable to allow for changes in the stoichiometric ratio. 

COMBUSTION SYSTEMS

To take advantage of a specific NOx abatement technology, a combustion system must either

have certain features in place, or needed system modifications must be technically and

economically feasible. Therefore, when identifying applicable pollution prevention and emission

control technologies, we must first consider combustion system design. The major types of

combustion systems are shown in Table 5.23 

Table 5. Common Combustion Systems

Type of Combustion Unit Fuel

Dry bottom boilers - wall-fired, front-fired or

opposed-fired

pulverized coal, gas, or liquid

Dry bottom boilers - tangentially fired pulverized coal, gas, or liquid

Wet bottom (slag tap) boilers - cyclone-type

burners

pulverized coal, gas, or liquid

Fluidized bed coal

Stokers with traveling grate. crushed coal

Stokers with spreader grate crushed coal.

Gas turbines gas and liquid

Internal combustion engines gas and liquid

Each NOx abatement technology has different implementations, development histories, and,

therefore, commercial status. Selection of a technology must occur after an engineering study to

determine technical and economic feasibility of each NOx technology. This includes how each

technology can be implemented and its cost. Options may be limited by inability to adjust

combustion system air flow appropriately, ducts that are at the wrong temperature, or ducts that
25

are too short to provide adequate mixing. These problems can be solved, but may require too

much modification to make them economical. 

DRY BOTTOM BOILERS - WALL-FIRED, FRONT-FIRED or OPPOSED-FIRED

Dry bottom pulverized coal, gas, and liquid fuel wall-fired boilers have used low-NOx burners to

inject fuel and air from lower walls. Front-fired boilers have burners on one wall. Opposed-fired

boilers have burners on front and back walls. These boilers typically use methods that reduce

peak temperature, reduce residence time at peak temperature, or chemically reduce NOx

(Methods 1, 2 & 3). These methods are used for large utility boilers in which combustion

efficiency is all-important. NOx oxidation with absorption and removal of nitrogen (Methods 4

and 5) represent newer technologies that may be applied in the future. Using a sorbent (Method

6) is already in use for some boilers. See Table 6 for NOx technologies used for dry bottom

wall-fired, front-fired or opposed-fired boilers.

Table 6: NOx technologies currently used for dry bottom 

wall-fired, front-fired or opposed-fired boilers.

NOx Abatement Method Techniques Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Low NOx Burners (LNB)

Combustion Optimization

Burners Out Of Service (BOOS)

Over Fire Air (OFA)

Less Excess Air (LEA)

Inject Water or Steam

Reduced Air Preheat

50-70%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Fuel Staging of Combustion

Inject Steam

50-70%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction (SNCR)

Fuel Reburning (FR)

Low NOx Burners (LNB)

35-90%

4. Oxidation of NOx with 

subsequent absorption

Inject Oxidant

Non-Thermal Plasma Reactor (NTPR)

60-80%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%
26

DRY BOTTOM BOILERS - TANGENTIALLY FIRED

Dry bottom pulverized coal, gas, or liquid fuel tangentially-fired boilers use jets from each corner

of a furnace to inject fuel and combustion air in a swirl. The injected mix of fuel and combustion

air forms a fireball in the center of the boiler. This firing configuration is used in medium sized

utility and large industrial boilers. This combustion technique holds flame temperatures down

(Method 1). In addition, chemical reduction of NOx (Method 3) is frequently used. NOx

oxidation (Method 4) techniques may be used in the future. Sorbents (Method 6) are already

used for some boilers. See Table 7 for NOx technologies used for dry bottom tangentially fired

boilers.

Table 7: NOx technologies currently used for dry bottom tangentially fired boilers.

NOx Abatement Method Techniques Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Over Fire Air (OFA)

Less Excess Air (LEA)

Inject Water or Steam

Reduced Air Preheat

50-70%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Fuel Staging of Combustion

Inject Steam

50-70%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction (SNCR)

Fuel Reburning (FR)

35-90%

4. Oxidation of NOx with

subsequent absorption

Non-Thermal Plasma Reactor (NTPR) 60-80%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%

WET BOTTOM (SLAG TAP) BOILERS

Wet bottom (slag tap) boilers use cyclone burners to create an intense flame. The flame is so hot

that it melts ash, which then becomes slag that must be removed via a slag tap. These boilers are

known to have higher NOx generation because combustion temperature is so high. As a result,

this high temperature combustion technique is not widely used because the NOx concentration

necessarily must be greater. Removal of non-fuel nitrogen as a reactant from the combustion

process (Method 5) applies here. Reducing residence time at peak temperature, chemical
27

reduction of NOx, and NOx oxidation with absorption (Methods 2, 3 & 4) also apply to this

combustion system. In addition, some slag tap boilers may be using sorbents (Method 6). There

are recent reports that reducing peak temperature (Method 1) so that ash just melts has been used. 

See Table 8 for NOx technologies used for slag tap boilers.

Table 8: NOx technologies currently used for wet bottom (slag tap) boilers.

NOx Abatement Method Techniques Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Over Fire Air (OFA)

Less Excess Air (LEA)

Inject Water or Steam

Reduced Air Preheat

30-70%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Fuel Staging of Combustion

Inject Steam

20-50%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction (SNCR)

Fuel Reburning (FR)

35-90%

4. Oxidation of NOx with

subsequent absorption

Non-Thermal Plasma Reactor (NTPR) 60-80%

5. Removal of nitrogen Use Oxygen Instead Of Air

Ultra-Low Nitrogen Fuel

No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%

FLUIDIZED BED

Fluidized bed combustion occurs in a bed of crushed coal that has air flowing upward through it

to make coal particles behave like a fluid. Boiler pipes can be either submerged in the bed or

exposed to the hot gases after they leave the bed. The fluidized bed is temperature controlled

(Method 1). The bed also is a chemically reducing region in which available oxygen is

consumed by carbon (Method 3) that reduces ionization of nitrogen. Excess air is injected

(Method 2) over the fluidized bed to complete combustion of CO and other burnables. This

allows for the addition of pulverized limestone (Method 6) to coal in the fluidized bed. Sulfur

oxides then react with the limestone to form gypsum, a marketable product. Gypsum must be

separated from the ash. As a result, NOx generation can be essentially limited to prompt NOx

and fuel NOx. See Table 9 for NOx technologies used for fluidized bed combustion units.
28

Table 9: NOx techniques currently used for fluidized bed combustion.

NOx Abatement Method Techniques Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Over Fire Air (OFA)

Less Excess Air (LEA)

Reduced Air Preheat

No Data

2.Reducing residence time

 at peak temperature

Inject Steam No Data

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction(SNCR)

Fuel Reburning (FR)

35-90%

4. Oxidation of NOx with 

subsequent absorption

Non-Thermal Plasma Reactor (NTPR) 60-80%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%

STOKERS WITH TRAVELING GRATE

Stokers with traveling grate cause the coal to move as it burns. Thus, char combustion is in one

zone while volatiles are liberated and combusted in another zone. These stokers are commonly

used with industrial boilers that are smaller than utility boilers. Reducing peak temperature,

chemical reduction of NOx, and sorbents (Methods 1, 3 & 6) usually are applied. Perhaps NOx

oxidation (Method 4) also will apply in the future. See Table 10 for NOx technologies used for

stokers with traveling grates.

STOKERS WITH SPREADERS

Stokers with spreaders throw coal over the grate in a controlled manner. Coal is crushed, but

particles are typically larger than pulverized coal. Therefore, combustion of volatiles begins

while coal is in flight and combustion of char occurs on the grate. This system is used with

somewhat larger boilers than stokers with traveling grates. It can be used in power plants, but

this combustion system is used mainly for industrial boilers. Like stokers with traveling grates,

reducing peak temperature, chemical reduction of NOx, and sorbents (Methods 1, 3, and 6)

usually are applied. Perhaps NOx oxidation (Method 4) also will apply in the future. See Table

11 for NOx technologies used for stokers with Spreader grates.
29

Table 10: NOx technologies currently used for stokers with traveling grates.

NOx Abatement Method Technique Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Combustion Optimization

Over Fire Air (OFA)

Less Excess Air (LEA)

Inject Water or Steam

Reduced Air Preheat

35-50%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Fuel Staging of Combustion

50-70%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction (SNCR)

Fuel Reburning (FR)

55-80%

4. Oxidation of NOx with 

subsequent absorption

Inject Oxidant

Non-Thermal Plasma Reactor (NTPR)

60-80%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%

GAS TURBINES

Gas turbines use the Brayton Cycle with a burner to raise temperature of gas after compression

and before expansion through the turbine. Turbines mainly use reducing peak temperature and

reducing residence time (Methods 1 and 2) approaches to limit NOx emissions. Because

addition of particles to air flow entering the turbine would accelerate erosion of turbine blades,

sorbents (Method 6) could only be applied after the expansion in the turbine. NOx reduction

(Method 3) has been used to treat exhaust gases. Many turbine operators claim that they use

“good combustion practices” that do reduce the particles that produce visible emissions (which

they equate with pollution), but say nothing about the NOx emissions which are not visible. 

Cogeneration units use a gas turbine to generate electricity and provide preheated combustion air

for a boiler. Gas turbine exhaust is typically 10-15% oxygen and can be used to provide

combustion air for a low pressure boiler. That boiler can be used to provide steam for another

turbine, a process heater, a space heater, or some combination of these. If a steam turbine is used

to generate electricity, it may constrain what can be done. Sorbent particles can be introduced to

a flow after it leaves a gas turbine in order to control NOx. There has also been some success in

reducing NOx concentrations when burning biomass fuels in a boiler. See Table 12 for NOx

technologies used for gas turbines.
30

Table 11: NOx technologies currently used for stokers with spreader grates.

NOx Abatement Method Technique Now Available Efficiency

1. Reducing peak temperature Flue Gas Recirculation (FGR)

Natural Gas Reburning (NGR)

Low NOx Burners (LNB)

Combustion Optimization

Over Fire Air (OFA)

Less Excess Air (LEA)

Inject Water or Steam

Reduced Air Preheat

50-65%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Fuel Staging of Combustion

Inject Steam

50-65%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction

(SNCR)

Fuel Reburning (FR)

35-80%

4. Oxidation of NOx with 

subsequent absorption

Inject Oxidant

Non-Thermal Plasma Reactor (NTPR)

60-80%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Combustion Chambers

Sorbent In Ducts

60-90%

INTERNAL COMBUSTION RECIPROCATING ENGINES

Internal combustion engines use air-to-fuel ratio and ignition/injection timing to control maximum

temperature and residence time. This can reduce the concentration of NOx that is generated by 

reducing peak temperature (Method 1). Valve timing adjustments can reduce residence time at

peak temperature (Method 2) to control NOx formation. Chemical reduction of NOx (Method 3)

is used in catalytic converters to reduce NOx to N2. Some stationary engines use both Method 3

and NOx oxidation (Method 4). A non-thermal plasma reactor was developed for treatment of

diesel exhaust, but is not yet marketed to our knowledge. A plasma ignition system allows greater

freedom in the air-fuel ratio and the ignition timing of spark ignition engines. See Table 13 for

NOx technologies used for stationary internal combustion engines.

WHAT DOES NOx ABATEMENT AND CONTROL COST?

The cost of NOx abatement and control has been changing rapidly with dramatic reductions in

recent years. Table 14 gives the 1993 cost as given in the Alternative Control Techniques 
31

Table 12: NOx technologies currently used for gas turbines.

NOx Abatement Method Technique Now Available Efficiency

1. Reducing peak temperature Natural Gas Reburning (NGR)

Low NOx Burners (LNB)

Inject Water or Steam

Reduced Air Preheat

Catalytic Combustion

70-85%

2.Reducing residence time

 at peak temperature

Air Staging of Combustion

Inject Steam

70-80%

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Selective Non-Catalytic Reduction (SNCR)

Fuel Reburning (FR)

Low NOx Burners (LNB)

70-90%

4. Oxidation of NOx with 

subsequent absorption

Non-Thermal Plasma Reactor (NTPR) No Data

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Ducts 60-90%

Table 13: NOx technologies currently used for stationary internal combustion engines.

NOx Abatement Method Technique Now Available Efficiency

1. Reducing peak temperature Air/fuel Ratio

Timing of Ignition/Type of Ignition

Pre-Stratified Combustion

20-97%

2.Reducing residence time

 at peak temperature

Valve Timing No Data

3. Chemical reduction of NOx Selective Catalytic Reduction (SCR)

Non-Selective Catalytic Reduction (NCSR)

80-90%

4. Oxidation of NOx with

subsequent absorption

Non-Thermal Plasma Reactor (NTPR) 80-95%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel No Data

6. Using a sorbent Sorbent In Exhaust Ducts

Adsorber in fixed Bed

60-90%
32

Table 14. 1993 Costs of NOx Controls

Cost of NOx Controls in 1993 Dollars

Control Low -Cap. High -Cap. Low - Oper. High -Oper. Low High

Device $/MMBTU $/MMBTU $/MMBTU $/MMBTU $/ton $/ton

LNB 650 8,300 340 1,500 240 4,300

LNB + FGR 650 7,630

SNCR 1,600 3,300 680 1,200 N/A N/A

(1994 ESTIMATE) 700 1,300

SCR 2,400 20,000 1,500 5,800 1,810 10,900

(1994 ESTIMATE) 500 2,800

Document NOx Emissions from Industrial/Commercial/Institutional Boilers (EPA 453/R-94-

022).

The EPA Region III Low-NOx Control Technology Study in 1994 said that low-NOx burners

had both beneficial effects on operating costs and detrimental effects on the burners, their life

expectancy, and the boilers in which they were installed. Coal quality and some boiler designs

caused NOx to remain high even after low-NOx burners were in place. Capital costs ranged

from $1.91 to $54.24 per kW. Operating costs ranged from $-23,000 (a profit) to $1,113,750 per

year. Thus, no reliable cost estimates could be obtained regarding low-NOx burner operation.

Coal quality, boiler capacity, and burner design were among the variables influencing this cost. 

Many plants could not even give an estimate. SNCR cost between $700 and $1,300 per ton of

NOx reduced. SCR cost between $500 and $2,800 per ton of NOx reduced.25 However, cost per

ton of NOx removed for all technologies is apparently becoming smaller.

These costs vary by control technique; type of fuel; grade of fuel; size of boiler, engine or

turbine; type of boiler, or turbine; and other factors. Other costs were also changing with time.

Therefore you need to examine the costs of these NOx control technologies for a specific

application and at a particular time. 

These preliminary cost estimates will also be further reduced as operating experience is gained,

competition sharpens, and design iterations eliminate the high-maintenance or life-shortening

features. Confidence in this view of the future is based upon reports that some users of low-NOx

burners had already seen in 1994 that operating costs could be reduced to make the changeover

yield a net profit. SCR and SNCR costs may also have declined further, since there is now

competition for these technologies. 
33

This analysis was supported by 1997 cost figures. These are presented in Table 15. This table is

from the Analyzing Electric Power Generation Under the CAAA 26. It appears that competition

and improved designs are still driving the costs downward. The Table was presented in that

publication and is presented here for your convenience.

Table 15. 1997 Costs of NOx Controls

Analyzing Electric Power Generation Under the CAAA -- Cost Estimates26

Boiler Type Control Type Capital Cost Fixed O & M Variable O& M % Control

$/kW $/kW/yr mils/kWh

Wall Fired LNB w/o OFA 16.8 0.25 0.05 67.5

Wall Fired LNB w/ OFA 22.8 0.35 0.07 67.5

Tang-Fired LNB w/ OFA 32.3 0.49 0 47.3

Tang-Fired LNB w/ SOFA 34.7 0.53 0 52.3

Tang Fired LNB w/ BOFA 46.7 0.71 0.02 57.3

Cell Burners Non Plug-In Comb. Ctl. 22.8 0.34 0.07 60

Cyclone Coal Reburning 70.7 1.07 0.25 50

Wet Bottom NOx Comb. Ctl. 9.6 0.14 0.05 50

Vert. Fired NOx Comb. Ctl. 10.8 0.17 0.05 40

SCR -Low NOx Rate 69.7 6.12 0.24 70

SCR- High NOx Rate 71.8 6.38 0.4 80

SNCR-Low NOx Rate 16.6 0.24 0.82 40

SNCR-Cyclone 9.6 0.14 1.27 35

SNCR-High NOx Rate 19 0.29 0.88 35

Nat.Gas Reburn-Low 32.4 0.49 40

Nat.Gas Reburn-High 32.4 0.49 50

Note that in Table 8 the following acronyms are used:

LNB is low-NOx burner BOFA is both close coupled and over fire air

OFA is closed coupled overfire air Comb. Ctl. is combined controls

SOFA is separate over fire air
34

The Institute of Clean Air Companies also suggests that in 1999 SCR will cost $50 - $80 per kW

for retrofit to units, which relates to $400-$1800 per ton of NOx that is destroyed.34 Their cost

estimate for SNCR ranges from $5-$15 per kW, which relates to $400-$2000 per ton of NOx that

is destroyed.35 Cost-effectiveness does not correlate with boiler capacity alone - other variables

such as type and quality of fuel, type of boiler, SNCR/SCR design, etc enter into the analysis. 

While the “Performance of Selective Catalytic Reduction on Coal-fired Steam Generating Units”

study was for Germany, it cited a recent cost range for SCR from $52 to $77 per kW with retrofit

units tending to have lower costs. Thus, the costs in Germany were very similar to costs in the

U.S.A., except that retrofit costs usually are greater in the United States.

Table 16, Unit Costs for NOx control Technologies for Non-Utility Stationary Sources, is from

the report “Ozone Transport Rulemaking Non-Electricity Generating Unit Cost Analysis.” The

report was prepared for EPA by the Pechan-Avanti Group. It indicates efficiencies and cost

estimates for various NOx technologies for the year 2007. This appears to be a conservative

estimate of efficiency and cost. Efficiencies indicated in the table tend to be lower than currently

demonstrated efficiencies. Costs were based on historical information and, therefore, should be

high estimates because NOx technology costs appear to be declining over time. The table was

included here to indicate the relative efficiency and cost of NOx technologies for specific types

of combustion systems. 

The Alternative Control Techniques Document NOx Emissions from Stationary Reciprocating

Internal Combustion Engines contains cost algorithms for the pollution prevention techniques

and control technology applied to internal reciprocating engines. Costs for NOx elimination run

from $250/ton to $1,300/ton for engines larger than 1,000 horsepower. For smaller engines the

cost runs from $400/ton to over $3,500/ton. 

ARE THESE METHODS SUFFICIENT?

YES, these methods are sufficient to meet the present goal of reducing NOx emissions 2 million

tons below 1980 emissions.47 The goal appears to be set at a level that can be achieved based on

the state of NOx technology in 1996. However, we will have to see whether achieving our

present goal provides the needed relief. A still more stringent set of standards may become

necessary at sometime in the future. 

As a high-technology nation and with a large urban and suburban population, we depend on

automobiles, buses, airplanes, railroads, and trucks for transportation. We also depend upon

electric power for computers, lights, air conditioning, and commerce. Therefore, we seem caught

in a dilemma, for, while we depend on transportation, vehicles produce most of our NOx. While

we also consume electric power as part of our market economy, electric power generation also

generates over 40% of our NOx from stationary sources in 1995.34

We also need to generate less NOx without regard to the ionization level of the nitrogen. In

1999, we are currently not capable of doing that. Therefore, we must do the best that we can,
35

Table 16: Unit Costs for NOx Control Technologies for Non-Utility Stationary Sources

Ozone Season

Cost Effectiveness ($1990/ton)

Source Type/Fuel Type Control Technology

Percent

Reduction (%) Small* Large*

ICI Boilers - Coal/Wall SNCR 40 1,870 1,380

ICI Boilers - Coal/Wall LNB 50 3,490 2,600

ICI Boilers - Coal/Wall SCR 70 2,910 2,450

ICI Boilers - Coal/FBC SNCR - Urea 75 1,220 910

ICI Boilers - Coal/Stoker SNCR 40 1,810 1,350

ICI Boilers - Coal/Cyclone SNCR 35 1,480 1,110

ICI Boilers - Coal/Cyclone Coal Reburn 50 3,730 710

ICI Boilers - Coal/Cyclone NGR 55 3,730 710

ICI Boilers - Coal/Cyclone SCR 80 1,840 1,560

ICI Boilers - Residual Oil LNB 50 940 1,020

ICI Boilers - Residual Oil SNCR 50 5,600 1,950

ICI Boilers - Residual Oil LNB + FGR 60 2,670 920

ICI Boilers - Residual Oil SCR 80 3,460 1,840

ICI Boilers - Distillate Oil LNB 50 2,810 4,950

ICI Boilers - Distillate Oil SNCR 50 10,080 3,520

ICI Boilers - Distillate Oil LNB + FGR 60 5,960 1,810

ICI Boilers - Distillate Oil SCR 80 6,480 3,460

ICI Boilers - Natural Gas LNB 50 1,950 1,560

ICI Boilers - Natural Gas SNCR 50 8,400 2,930

ICI Boilers - Natural Gas LNB + FGR 60 6,110 1,420

ICI Boilers - Natural Gas OT + WI 65 1,620 760

ICI Boilers - Natural Gas SCR 80 5,190 2,770

ICI Boilers - Wood/Bark/Stoker SNCR - Urea 55 2,090 1,430

ICI Boilers - Wood/Bark/FBC SNCR - Ammonia 55 1,660 1,210

ICI Boilers - MSW/Stoker SNCR - Urea 55 2,610 1,830

ICI Boilers - Process Gas LNB 50 1,950 1,560

ICI Boilers - Process Gas LNB + FGR 60 6,110 1,420

ICI Boilers - Process Gas OT + WI 65 1,620 760

ICI Boilers - Process Gas SCR 80 4,990 2,570

ICI Boilers - Coke SNCR 40 1,870 1,380

ICI Boilers - Coke LNB 50 3,490 2,600

ICI Boilers - Coke SCR 70 2,910 2,450

ICI Boilers - LPG LNB 50 2,810 4,950

ICI Boilers - LPG SNCR 50 10,000 3,440

ICI Boilers - LPG LNB + FGR 60 5,960 1,810

ICI Boilers - LPG SCR 80 6,240 3,220

ICI Boilers - Bagasse SNCR - Urea 55 2,090 1,430

ICI Boilers - Liquid Waste LNB 50 940 1,020

ICI Boilers - Liquid Waste SNCR 50 5,560 1,910

ICI Boilers - Liquid Waste LNB + FGR 60 2,670 920

ICI Boilers - Liquid Waste SCR 80 3,320 1,710

Internal Combustion Engines - Oil IR 25 1,840 1,160
Ozone Season

Cost Effectiveness ($1990/ton)

Source Type/Fuel Type Control Technology

Percent

Reduction (%) Small* Large*

36

Internal Combustion Engines - Oil SCR 80 4,690 1,850

Internal Combustion Engines - Gas IR 20 2,430 1,320

Internal Combustion Engines - Gas AF RATIO 20 3,730 900

Internal Combustion Engines - Gas AF + IR 30 3,430 1,080

Internal Combustion Engines - Gas L-E (Medium Speed) 87 890 N/A

Internal Combustion Engines - Gas L-E (Low Speed) 87 4,000 1,500

Internal Combustion Engines - Gas SCR 90 5,547 1,075

IC Engines - Gas, Diesel, LPG IR 25 1,840 1,160

IC Engines - Gas, Diesel, LPG SCR 80 4,690 1,850

Gas Turbines - Oil Water Injection 68 3,080 1,540

Gas Turbines - Oil SCR + Water Injection 90 4,240 1,860

Gas Turbines - Natural Gas Water Injection 76 3,590 1,750

Gas Turbines - Natural Gas Steam Injection 80 2,490 1,190

Gas Turbines - Natural Gas LNB 84 1,170 240

Gas Turbines - Natural Gas SCR + LNB 94 4,850 1,140

Gas Turbines - Natural Gas SCR + Steam Injection 95 3,750 1,570

Gas Turbines - Natural Gas SCR + Water Injection 95 5,040 2,060

Gas Turbines - Jet Fuel Water Injection 68 3,080 1,540

Gas Turbines - Jet Fuel SCR + Water Injection 90 4,240 1,860

Process Heaters - Distillate Oil LNB 45 8,290 2,320

Process Heaters - Distillate Oil LNB + FGR 48 10,130 4,000

Process Heaters - Distillate Oil SNCR 60 6,210 3,230

Process Heaters - Distillate Oil ULNB 74 5,110 1,450

Process Heaters - Distillate Oil SCR 75 18,970 12,520

Process Heaters - Distillate Oil LNB + SNCR 78 7,160 3,630

Process Heaters - Distillate Oil LNB + SCR 92 18,770 10,910

Process Heaters - Residual Oil LNB + FGR 34 8,330 3,290

Process Heaters - Residual Oil LNB 37 6,010 1,690

Process Heaters - Residual Oil SNCR 60 3,730 2,050

Process Heaters - Residual Oil ULNB 73 3,080 860

Process Heaters - Residual Oil LNB + SNCR 75 4,730 2,510

Process Heaters - Residual Oil SCR 75 10,560 7,170

Process Heaters - Residual Oil LNB + SCR 91 11,170 6,550

Process Heaters - Natural Gas LNB 50 5,250 4,290

Process Heaters - Natural Gas LNB + FGR 55 7,610 5,890

Process Heaters - Natural Gas SNCR 60 5,560 3,740

Process Heaters - Natural Gas ULNB 75 3,580 2,870

Process Heaters - Natural Gas SCR 75 24,840 16,760

Process Heaters - Natural Gas LNB + SNCR 80 6,960 5,080

Process Heaters - Natural Gas LNB + SCR 88 23,880 16,500

Process Heaters - Process Gas LNB 50 5,250 4,290

Process Heaters - Process Gas LNB + FGR 55 7,610 5,890

Process Heaters - Process Gas SNCR 60 5,560 3,740

Process Heaters - Process Gas ULNB 75 3,580 2,870

Process Heaters - Process Gas SCR 75 24,840 16,760

Process Heaters - Process Gas LNB + SNCR 80 6,960 5,080

Process Heaters - Process Gas LNB + SCR 88 23,880 16,500
Ozone Season

Cost Effectiveness ($1990/ton)

Source Type/Fuel Type Control Technology

Percent

Reduction (%) Small* Large*

37

Process Heaters - LPG LNB 45 8,290 2,320

Process Heaters - LPG LNB + FGR 48 10,130 4,000

Process Heaters - LPG SNCR 60 6,210 3,230

Process Heaters - LPG ULNB 74 5,110 1,450

Process Heaters - LPG SCR 75 18,970 12,520

Process Heaters - LPG LNB + SNCR 78 7,160 3,630

Process Heaters - LPG LNB + SCR 92 18,770 10,910

Process Heaters - Other Fuel LNB + FGR 34 8,330 3,290

Process Heaters - Other Fuel LNB 37 6,010 1,690

Process Heaters - Other Fuel SNCR 60 3,730 2,050

Process Heaters - Other Fuel ULNB 73 3,080 860

Process Heaters - Other Fuel LNB + SNCR 75 4,730 2,510

Process Heaters - Other Fuel SCR 75 10,560 7,170

Process Heaters - Other Fuel LNB + SCR 91 11,170 6,550

Adipic Acid Manufacturing Thermal Reduction 81 1,000 1,000

Adipic Acid Manufacturing Extended Absorption 86 210 210

Nitric Acid Manufacturing Extended Absorption 95 840 840

Nitric Acid Manufacturing SCR 97 1,010 1,010

Nitric Acid Manufacturing SNCR 98 940 940

Glass Manufacturing - Container Electric Boost 10 17,050 17,050

Glass Manufacturing - Container Cullet Preheat 25 2,240 2,240

Glass Manufacturing - Container LNB 40 4,040 4,040

Glass Manufacturing - Container SNCR 40 3,320 3,320

Glass Manufacturing - Container SCR 75 4,550 4,550

Glass Manufacturing - Container OXY-Firing 85 10,960 10,960

Glass Manufacturing - Flat Electric Boost 10 5,540 5,540

Glass Manufacturing - Flat LNB 40 1,660 1,660

Glass Manufacturing - Flat SNCR 40 1,380 1,380

Glass Manufacturing - Flat SCR 75 1,490 1,490

Glass Manufacturing - Flat OXY-Firing 85 4,530 4,530

Glass Manufacturing - Pressed Electric Boost 10 20,910 20,910

Glass Manufacturing - Pressed Cullet Preheat 25 1,930 1,930

Glass Manufacturing - Pressed LNB 40 3,570 3,570

Glass Manufacturing - Pressed SNCR 40 3,080 3,080

Glass Manufacturing - Pressed SCR 75 5,170 5,170

Glass Manufacturing - Pressed OXY-Firing 85 9,310 9,310

Cement Manufacturing - Dry Mid-Kiln Firing 30 1,110 1,110

Cement Manufacturing - Dry LNB 30 1,340 1,340

Cement Manufacturing - Dry SNCR - Urea Based 50 1,280 1,280

Cement Manufacturing - Dry SNCR - NH3 Based 50 1,490 1,490

Cement Manufacturing - Dry SCR 80 6,850 6,850

Cement Manufacturing - Wet Mid-Kiln Firing 30 1,010 1,010

Cement Manufacturing - Wet LNB 30 1,260 1,260

Cement Manufacturing - Wet SCR 80 5,840 5,840

Iron & Steel Mills - Reheating LEA 13 3,160 3,160

Iron & Steel Mills - Reheating LNB 66 720 720

Iron & Steel Mills - Reheating LNB + FGR 77 900 900
Ozone Season

Cost Effectiveness ($1990/ton)

Source Type/Fuel Type Control Technology

Percent

Reduction (%) Small* Large*

38

Iron & Steel Mills - Annealing LNB 50 1,350 1,350

Iron & Steel Mills - Annealing LNB + FGR 60 1,790 1,790

Iron & Steel Mills - Annealing SNCR 60 3,130 3,130

Iron & Steel Mills - Annealing LNB + SNCR 80 3,460 3,460

Iron & Steel Mills - Annealing SCR 85 8,490 8,490

Iron & Steel Mills - Annealing LNB + SCR 90 9,070 9,070

Iron & Steel Mills - Galvanizing LNB 50 1,170 1,170

Iron & Steel Mills - Galvanizing LNB + FGR 60 1,370 1,370

Municipal Waste Combustors SNCR 45 2,140 2,140

Medical Waste Incinerators SNCR 45 8,570 8,570

Space Heaters - Distillate Oil LNB 50 2,810 4,950

Space Heaters - Distillate Oil SNCR 50 10,000 3,440

Space Heaters - Distillate Oil LNB + FGR 60 5,960 1,810

Space Heaters - Distillate Oil SCR 80 6,240 3,220

Space Heaters - Natural Gas LNB 50 1,950 1,560

Space Heaters - Natural Gas SNCR 50 8,330 2,860

Space Heaters - Natural Gas LNB + FGR 60 6,110 1,420

Space Heaters - Natural Gas OT + WI 65 1,620 760

Space Heaters - Natural Gas SCR 80 4,990 2,570

Ammonia - NG-Fired Reformers LNB 50 1,950 1,560

Ammonia - NG-Fired Reformers SNCR 50 8,330 2,860

Ammonia - NG-Fired Reformers LNB + FGR 60 6,110 1,420

Ammonia - NG-Fired Reformers OT + WI 65 1,620 760

Ammonia - NG-Fired Reformers SCR 80 4,990 2,570

Ammonia - Oil-Fired Reformers LNB 50 940 1,020

Ammonia - Oil-Fired Reformers SNCR 50 5,560 1,910

Ammonia - Oil-Fired Reformers LNB + FGR 60 2,670 920

Ammonia - Oil-Fired Reformers SCR 80 3,320 1,710

Lime Kilns Mid-Kiln Firing 30 1,110 1,110

Lime Kilns LNB 30 1,340 1,340

Lime Kilns SNCR - Urea Based 50 1,280 1,280

Lime Kilns SNCR - NH3 Based 50 1,490 1,490

Lime Kilns SCR 80 6,850 6,850

Comm./Inst. Incinerators SNCR 45 2,140 2,140

Indust. Incinerators SNCR 45 2,140 2,140

Sulfate Pulping - Recovery Furnaces LNB 50 1,950 1,560

Sulfate Pulping - Recovery Furnaces SNCR 50 8,330 2,860

Sulfate Pulping - Recovery Furnaces LNB + FGR 60 6,110 1,420

Sulfate Pulping - Recovery Furnaces OT + WI 65 1,620 760

Sulfate Pulping - Recovery Furnaces SCR 80 4,990 2,570

Ammonia Prod; Feedstock Desulfurization LNB + FGR 60 6,110 1,420

Plastics Prod-Specific; (ABS) Resin LNB + FGR 55 7,610 5,890

Starch Mfg; Combined Operations LNB + FGR 55 7,610 5,890
Ozone Season

Cost Effectiveness ($1990/ton)

Source Type/Fuel Type Control Technology

Percent

Reduction (%) Small* Large*

39

By-Product Coke Mfg; Oven Underfiring SNCR 60 3,130 3,130

Pri Cop Smel; Reverb Smelt Furn LNB + FGR 60 1,790 1,790

Iron Prod; Blast Furn; Blast Htg Stoves LNB + FGR 77 900 900

Steel Prod; Soaking Pits LNB + FGR 60 1,790 1,790

Fuel Fired Equip; Process Htrs; Pro Gas LNB + FGR 55 7,610 5,890

Sec Alum Prod; Smelting Furn/Reverb LNB 50 1,350 1,350

Steel Foundries; Heat Treating Furn LNB 50 1,350 1,350

Fuel Fired Equip; Furnaces; Natural Gas LNB 50 1,350 1,350

Asphaltic Conc; Rotary Dryer; Conv Plant LNB 50 5,250 4,290

Ceramic Clay Mfg; Drying LNB 50 5,250 4,290

Coal Cleaning-Thrml Dryer; Fluidized Bed LNB 50 3,490 2,600

Fbrglass Mfg; Txtle-Type Fbr; Recup Furn LNB 40 4,040 4,040

Sand/Gravel; Dryer LNB + FGR 55 7,610 5,890

Fluid Cat Cracking Units; Cracking Unit LNB + FGR 55 7,610 5,890

Conv Coating of Prod; Acid Cleaning Bath LNB 50 5,250 4,290

Natural Gas Prod; Compressors SCR 20 5,547 1,075

In-Process; Bituminous Coal; Cement Kiln SNCR - urea based 50 1,280 1,280

In-Process; Bituminous Coal; Lime Kiln SNCR - urea based 50 1,280 1,280

In-Process Fuel Use;Bituminous Coal; Gen SNCR 40 1,420 1,060

In-Process Fuel Use; Residual Oil; Gen LNB 37 6,010 1,690

In-Process Fuel Use; Natural Gas; Gen LNB 50 5,250 4,290

In-Proc;Process Gas;Coke Oven/Blast Furn LNB + FGR 55 7,610 5,890

In-Process; Process Gas; Coke Oven Gas LNB 50 5,250 4,290

Surf Coat Oper;Coating Oven Htr;Nat Gas LNB 50 5,250 4,290

Solid Waste Disp;Gov;Other Incin;Sludge SNCR 45 2,140 2,140

NOTE: *Small source cost per ton values are used to estimate control costs for all sources with 1995 NOx emissions below 1 ton per day. 

If the ozone season daily 1995 baseline NOx value is 1 ton or more, the cost per ton value for large sources is used.

N/A = not applicable. The population of medium speed gas-fired IC engines are all considered small.
40

and hope that we can endure while looking for the discovery and cost-reduction of the “better

technology” that is capable of meeting the goal of sustainability. The reference literature

suggests that the leading edge of pollution prevention and control technology in 1999 is capable

of about 94%-99% control of NOx or 1 ppm-5 ppm of NOx.27,28

As we all know, the leading edge technology always costs more because engineering has not yet

shifted from feasibility to cost-reduction, and competition in the technology is not fully

developed. Therefore, we can expect technology to be more affordable with time and to also

improve its capability. We should expect to see the goal change with time as the technology

advances, and as the achievable goal becomes both more stringent and more economically

feasible. At one time, the capabilities that we now have seemed beyond imagining, and the costs

seemed prohibitive. We live in an interesting time when the NOx pollution prevention,

abatement, and control technology is becoming more capable and more affordable.

CONCLUSIONS

1. Different fuels require different combustion, abatement and control techniques. Different

coals have a varying content of volatile ingredients. The nitrogen content of fuel is important, as

are the content of sulfur, lead, mercury and other contaminants. Ultra-low nitrogen content fuels

have been developed and are already cost competitive. Thus, we can achieve some control of

NOx from the lowered concentration of nitrogen in the fuel without investing in changed burner

designs.29

2. The design of the boiler, internal combustion engine, or gas turbine has a major effect on the

operation. NOx formation tends to increase with an increase in boiler capacity, because larger

boilers tend to have more intense combustion with higher combustion temperatures and longer

residence time for flue gases. The same appears to be true of engines and turbines.

3. Staging of the combustion is implicit in several pollution prevention techniques. Tandem 

application (or use of hybrid control technology) of NOx control techniques (first SNCR, then

SCR in the duct, and then sorption before the ESP which is referred to as “polishing”) have been

used to achieve an overall reduction of 90+% in NOx and 80% in SOx, even without using lowNOx burners to lower NOx generation. 

4. Combustion of natural gas and petroleum distillates can be controlled in much the same way

as pulverized coal. The major differences between coal and natural gas or oil are that gas and oil:

(1) generally are lower in sulfur and ash; (2) usually are lower in nitrogen; and (3) probably are

lower in lead and mercury. Thus, gas and oil do not deactivate a catalyst used in Selective

Catalytic Reduction (SCR) at the same rate that coal or semi-solid fuels do. 

5. The semi-solid petroleum products can actually have higher levels of sulfur, nitrogen and other

impurities than coal. They do not have as much char or ash as coal, but have more than the

lighter distillates. 
41

6. We should expect the declining cost trend of control technology to continue as operating

experience is gained, firing techniques are adapted to fuels, design flaws are corrected, and

new designs appear. We should expect to see costs become less as they are driven down by

competition between suppliers of successful technologies. 

7. NOx control technology appears capable of more than just meeting EPA’s present goal and

this should provide emission credits that can be traded to those firms that choose to continue

emitting poorly controlled emissions. The sale of these credits by those that over-corrected for

the present goal should further offset any net costs in adopting these control technologies.

8. Commercially available NOx control systems are already available. The availability of these

technologies was part of the basis of the recent NOx SIP Call and Title IV regulations.

9. There has been an economic incentive to make combustion more efficient and to innovate

ways to control nitrogen oxides. However, the amount of time and money that must be invested

for a full-scale test of a strategy is significant. Acceptance requires that a technology be tried and

succeed before a larger scale test is even contemplated. This delays acceptance of improved

techniques.

10. There seems to be no control technology which is clearly superior for all combustion systems,

boilers, engines, or fuels. Lacking a clear winner, one must select fuels and control technology 

either from among those already proven, or from a growing number of new and promising ideas. 

11. The end of the search for control technologies is not yet in sight, and the search must

continue. Past research must of necessity give ambivalent answers, because there are so many

conflicting factors. However, at under $150 per ton of NOx prevented and with up to 80%

control efficiency, the low-NOx burner, where applicable, appears to be among the least

expensive emission control technologies. SCR is more expensive, but can obtain up to 94%

control efficiency. SNCR can be adopted without the initial cost of catalyst, although it is

somewhat less effective. LNB, SCR and SNCR are all viable technologies across a wide

spectrum of applications.

12. Research and development will have to continue to seek more effective answers and try to

balance them against cost and efficiency. The cost will decrease as technology advances,

operating experience is gained, competition becomes sharper, design flaws are corrected, and

better designs become available. Reliability can only be gained with time. Cost will be reduced

with time and experience. We also must expect that the level of pollution prevention and control

technology effectiveness will improve with time.
42

REFERENCES:

1. Nitrogen Oxides

www.ccinet.ab.ca/casa/nitrogen.htm

2. Pollution Engineering Magazine Online

www.manufacturing.net...eng/archives/1995/pol10401.95/04adp3f0.htm

3. Alternative Control Techniques - Nitric Acid and Adipic Acid Manufacturing Plants

EPA 450/3-91-026

4. Bay Journal, July/August 1996

www.gmu.edu/bios/Bay/journal/96-08/ozone.htm

5. A Graphical Explanation of a regional Ozone Episode

capita.wustl.edu/otag/reports/03epi95/03epi95.html

6. XONON

www.catalytica-inc.com/cs/nox_control.html

7. Summary of NOx Control Technologies and Their Availability and Extent of Application,

EPA 450/3-92-004

8. Sourcebook: NOx Control Teschnology Data, EPA 600/2-91-029 

9. Fully Engineered Approach to Acid Rain Control, F. Bauer, Energy Engineering, Vol. 91,

No.4, 1994

10. Alternate Control Techniques Document - NOx Emissions from Cement Manufacturing

EPA-453/R-94-004

11. Put a Lid on NOx Emissions, J. Czarnecki, C. Pereira, M. Uberoi, K. Zak, Pollution

Engineering, November 1994

12. Combined Corona/Catalyst Process for Low-Temperature NOx Control

es.epa.gov/ncerqa_abstracts/sbir/other/air/helfritch.html

13. Reducing NOx Emissions, B. Chambers, Glass Industry, May 1993

 

14. Marathon Monitors

www.marathonmonitors.com/glassnox.htm
43

15. Active Control of a Natural Gas-Fired Burner

ucicl.eng.uci.edu/research/active-control/

16. NICE3: More Electricity, Fewer Emissions from Coal-Fired Boilers

es.epa.gov/program/p2dept/energy/nice3/nice3-2.html

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T. Sommer, D. Engelhardt, and S. Freedman, 96-RP139.04, Air & Waste Management Assn.

18. Regulatory Developments in NOx Controls for Utility Boilers, C. Harrison, 96-RP139.03, Air

& Waste Management Assn.

19. Alternative Control Techniques Document -- NOx Emissions from Stationary Reciprocating

Internal Combustion Engines, EPA 453/R-93-032

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K. Chess, S. Yao, A. Russell and H. Hsu, Journal of the Air & Waste Management Association,

August, 1995, p.627

21. CNG Installing NOx-reduction System in Compressor Stations, Oil & Gas Journal, Sept. 26,

1994

22. NOx Control Technology Requirements Under the United States 1990 Clean Air Act

Amendments Compare to Those in Selected Pacific Rim Countries, C. A. Miller, R. Hall, R. Stern,

EPA 600/A-94-259

23. Nitrogen Oxides Control Technology Fact Book, Leslie L. Sloss, Noyes Data Corp., 1992

24. Alternate Control Techniques Document -- NOx Emissions from Industrial/Commercial/

Institutional (ICI) Boilers, EPA 453/R-94-022

25. EPA Region 3 Low-NOx Control Technology Study, K. Bruce, C. Castaldi, J. Cook, D.

Lachapelle, Acurex Environmental Corporation, Acurex Report FR-97-116

26. Analyzing Electric Power Generating Under the CAAA, Office of Air and Radiation, 1998 

27. SNRB Catalytic Baghouse Laboratory Pilot Testing, G. Akudlac, G. Farthing, T. Szymanski, R.

Corbett, Environmental Progress, Vol.11, No. 1, 1992

28. Cement Kiln NOx Control, A. McQueen, S. Bortz, M. Hatch, and R. Leonard, IEEE

Transactions on Industry Applications, Vol. 31, No. 1 January/February 1995

29. States’ Report on Nitrogen Oxides Reduction Technology Options fpr Application by the

Ozone Transport Assessment Group, OTAG, 1996
44

30. Retrofit control technology reducing NOx emissions, S. Kuehn, Power Engineering,

February, 1994

31. Advanced emissions control brings coal back to New Jersey, Power. 1994

32. Overview of NOx Emission Control for Utility Boilers, J. E. Staudt, Proceedings of the

American Power Conference, 1993

33. Select the Right NOx Control Technology, S. Wood, Chemical Engineering Progress,

January 1994

34. Selective Catalytic Reduction Control of NOx Emissions, SCR Committee of Institute of

Clean Air Companies, November 1997

35. Selective Non-Catalytic Reduction for Controlling NOx Emissions, SNCR Committee of

Institute of Clean Air Companies, October 1997

36. Emission Control Potential for Heavy-Duty Diesel Engines, OMS Fact Sheet, EPA 420-F95-009b, June 1996, www.epa.gov/omswww/noxfact2.htm 

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Strategies, S. Roselle, Atmospheric Sciences Modeling Division, Air Resources Laboratory,

National Oceanic and Atmospheric Administration, Atmospheric Environment, Vol. 28, No. 10, 

pp 1757-1772, 1994, Elsevier Science Ltd.

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39. The Sox-NOx-Rox Box Flue Gas Cleanup Demonstration Project

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Workgroup, D. Guinnup and B. Collom

42. Combined SO2/NOx

www.worldbank.org/html/fpd/.../EA/mitigatn/thermair/aqsocbsn.htm

43. Cleaver-Brooks, Emissions

www.cleaver-brooks.com/Emissions1.html
45

44. Methods for Reducing NOx Emissions, C. Latta, Roy F. Weston Inc., Plant Engineering,

September 1998

45. Plasma Ignition System Reduces NOx Emissions, M. Manning, Pipeline & Gas Journal,

October 1995, pp. 26-30

46. Low-Emission Gas Turbines Using Catalytic Combustion, S. Vatcha, Energy Conversion

Management, Vol. 38 No. 10-13, pp. 1327-1334 

47. Factsheet: PhaseI of the NOx Reduction Program

nsdi.epa.gov/acidrain/nox/noxfs.html

48. Alternative Control Techniques Document -- NOx Emissions from Stationary Reciprocating

Internal Combustion Engines. EPA-453/R-93-032, July 1993

49. The Economic Feasibility of using Hydrogen Peroxide for the Ehanced Oxidation and

Removal of Nitrogen Oxides from Coal-Fired Power Plant Flue Gases, J. Hatwood, C. Cooper, 

AWMA Journal, March 1998

50. Fossil Fuel Combustion, A Source Book, W. Bartok and A. Sarofim, Wiley-Interscience,

John Wiley & Sons

51. Maximum Achievable Control Technology for NOx Emissions from Thermal Oxidation, P.

Nutcher and D. Lewandowski, 94-WA74A.03, Air and Waste Management Association, Annual

Meeting, Cincinnati, OH, June 19-24 1994

52. GNOCIS - 1999 Update on the Generic NOx Control Intelligent System, G. Warriner, AJA.

Sorge, M. Slatsky, J. Noblett, J. Stallings, EPRI-DOE-EPA Combined Utility Air Pollution

Control Symposium: The MEGA Symposium, Atlanta, August 1999

53. Obtaining Reduced NOx and Improved Efficiency Using Advanced Empirical Optimization

on a Boiler Operated in Load-Following Mode, P. Patterson, EPRI-DOE-EPA Combined Utility

Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999

54. Simultaneous SO2, SO3, and NOx Removal by Commercial Application of the EBA Process,

S. Hirano, S. Aoki, M Izutsu, and Y. Yuki, EPRI-DOE-EPA Combined Utility Air Pollution

Control Symposium: The MEGA Symposium, Atlanta, August 1999

55. The State of the Art in Cyclone Boiler NOx Reduction, D. O’Connor, R. Himes, T,

Facchiano, EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA

Symposium, Atlanta, August 1999
46

56. Update on SNAP Technology for Simultaneous SOx and NOx Removal, K. Felsvang, V.

Boscak, S. Iversen, P. Anderson, EPRI-DOE-EPA Combined Utility Air Pollution Control

Symposium: The MEGA Symposium, Atlanta, August 1999

57. Advanced Reburning for SIP Call NOx Control, EPRI-DOE-EPA Combined Utility Air

Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999

The Following are General Background References, which were used for general information. 

They were used in multiple places. The facts presented were also used to develop a consistent

picture of NOx generation and abatement technology in the face of many conflicting claims and

pieces of data. They also were used to understand and reconcile the conflicts in data from

different combustion devices.:

Performance of Selective Catalytic Reduction on Coal-fired Steam Generating Units, U.S. EPA.,

Acid Rain Division,1997

Alternate Control Techniques Document - NOx Emissions from Iron and Steel Mills

EPA-453/R-94-065

Nitrogen Chemistry and NOx Control in a Fluid Catalytic Cracking Regenerator, X. Zhao, A.

Peters, G. Weatherbee, Industrial Engineering Chemical Research, Vol.36, No. 11, 1997,

American Chemical Society

Control of Combustion-Generated Nitrogen Oxide Emission: Technology Driven by Regulation,

C. T. Bowman, Twenty-fourth Symposium (International) oc Combustion/ The Combustion

Institute, 1992, pp. 859-878

Advanced Biomass Reburning for High Efficiency NOx Control

es.epa.gov/ncerqa_abstracts/sbir/other/air/seeker.html

The Regional Transport of Ozone

www.epa.gov/ttn/otag/about_1.html

OTAG Analysis with PPM

www.epa.gov/capi/otagmain.html

About OTAG

envpro.ncsc.org/OTAGDC/about_otag.html

Air Quality Analysis Working Group 

capita.wustl.edu/otag

Clean Air Technology News, Institute of Clean Air Companies, Summer 1998 and Winter 1998
47

Post Combustion Control of Oxides of Nitrogen Emissions from a 900 TPD Processed Refuse

Fuel Boiler, J. Zakaria, L. Wolfenden, J. Mc Conlogue, G. Pierce, The 1994 National Waste

Processing Conference, ASME, 1994

Cost Estimates for Selected Applications of NOx Control Technologies on Stationary

Combustion Boilers (Draft Report), The Cadmus Group, Inc. and Bechtel Power Corporation for

U.S.EPA, March 1996.

Ozone Transport Rulemaking Bob-Electric Generating Unit Cost Analysis, Prepared by PechanAvanti Group for U.S. EPA (Contract Nos. 68D40102 & 68D98052), Setember 17, 1998.
48

TECHNICAL REPORT DATA

(Please read Instructions on reverse before completing)

1. REPORT NO.

EPA-456/F-99-006R

2. 3. RECIPIENT'S ACCESSION NO.

4. TITLE AND SUBTITLE

Nitrogen Oxides (NOx) Why and How They Are Controlled

5. REPORT DATE

November 1999

6. PERFORMING ORGANIZATION CODE

7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT NO.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Clean Air Technology Center (MD-12)

Information Transfer and Program Integration Division

Office of Air Quality Planning and Standards

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

10. PROGRAM ELEMENT NO.

11. CONTRACT/GRANT NO.

12. SPONSORING AGENCY NAME AND ADDRESS

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Research Triangle Park, NC 27711

13. TYPE OF REPORT AND PERIOD COVERED

Final

14. SPONSORING AGENCY CODE

15. SUPPLEMENTARY NOTES 

For more information, call the CATC Information Line at (919) 541-0800 or access the CATC Web page at 

www.epa.gov/ttn/catc 

16. ABSTRACT

The purpose of this document is to educate people about nitrogen oxides, how they are formed, the danger

that they represent, and how emissions can be controlled. This knowledge is needed to make an informed

choice of the control technology that is to be used. 

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

nitrous oxide, nitric oxide, nitrogen dioxide,

dinitrogen pentoxide, nitrous acid, nitric acid,

ozone, volatile organic compounds, VOC,

generation, pollution prevention, control

technology, control technologies, emission

control, acid rain, combustion, boilers, gas

turbines, internal combustion engines, cost of

emission controls, air pollution 

air pollution control technology,

combustion, pollution prevention,

nitrogen oxides, boilers, internal

combustion engines, diesel engines 

18. DISTRIBUTION STATEMENT

Release Unlimited, Available from the OAQPS

TTN Web Page and NTIS

19. SECURITY CLASS (Report)

Unclassified

21. NO. OF PAGES

20. SECURITY CLASS (Page)

Unclassified

22. PRICE

EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE