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Managing Hazardous Waste — RCRA Subtitle C SECTION III SECTION III

Chapter 7: Hazardous Waste Combustion

In this chapter...

Overview........................................................... III-113

Which are the Regulated Units? ....................... III-114

- Incinerators.................................................... III-115

- Boilers and Industrial Furnaces ..................... III-115

Regulatory Requirements ................................. III-116

- Performance Standards ................................ III-117

- Operating Requirements ............................... III-119

Additional Requirements .................................. III-120

Future Integrated Regulations .......................... III-121

Summary .......................................................... III-121

OVERVIEW

A large number of TSDFs use combustion, the

controlled burning of substances in an enclosed

area, as a means of treating and disposing of

hazardous waste. Approximately 2.5 percent (6

million tons) of the hazardous waste generated

in the United States in 1995 was treated

using combustion. As a hazardous waste

management practice, combustion has

several unique attributes. First, if properly

conducted, it permanently destroys toxic

organic compounds contained in

hazardous waste by breaking their

chemical bonds and reverting them to

their constituent elements, thereby

reducing or removing their toxicity.

Second, combustion reduces the

volume of hazardous waste to be

disposed of on land by converting solids and

liquids to ash. Land disposal of ash, as opposed to

disposal of untreated hazardous waste, is in many

instances both safer and more efficient.

Combustion is an intricate treatment process.

During burning, organic wastes are converted

from solids and liquids into gases. These gases

pass through the flame, are heated further, and

eventually become so hot that their organic

compounds break down into the constituent

atoms. These atoms combine with oxygen and

form stable gases that are released to the

atmosphere after passing through air pollution

control devices.

The stable gases produced by combustion of

organics are primarily carbon dioxide and water

vapor. Depending on waste composition,

however, small quantities of carbon monoxide,

nitrogen oxides, hydrogen chloride, and other

gases may form. These gases have the potential to

cause harm to human health and the

environment. The regulation of these

emissions is the primary focus of the

RCRA combustion unit standards.

The management or disposal of

metals and ash, other by-products of

the combustion process, also causes

concern. Ash is an inert solid

material composed primarily

of carbon, salts, and metals.

During combustion, most

CHAPTER 7

HAZARDOUS WASTE COMBUSTION
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 SECTION III SECTION III Managing Hazardous Waste — RCRA Subtitle C

Chapter 7: Hazardous Waste Combustion

ash collects at the bottom of the combustion

chamber (bottom ash). When this ash is removed

from the combustion chamber, it may be

considered hazardous waste via the derived-from

rule or because it exhibits a characteristic. Small

particles of ash (particulate matter that may also

have metals attached), however, may be carried

up the stack with the gases (fly ash). These

particles and associated metals are also regulated

by the combustion regulations, as they may carry

hazardous constituents out of the unit and into the

atmosphere. Since combustion will not destroy

inorganic compounds present in hazardous waste,

such as metals, it is possible that such compounds

may also end up in bottom ash and fly ash at

harmful concentrations. Ash residue is subject to

applicable RCRA standards and may need to be

treated for metals or other inorganic constituents

prior to land disposal (see Figure III-23).

In the early years of RCRA, EPA’s idea was to

combust as much hazardous waste as possible and

landfill the resultant ash. This process destroyed

the majority of the waste, thus reducing the

volume requiring disposal. However, it was

determined that incomplete or improperly

conducted combustion had the potential to

present a major public health risk, and therefore,

became the topic of much public outcry. This

public concern, coupled with EPA’s advancements

in assessing potential risks arising from combustion,

caused a shift in EPA’s strategy on combustion.

This shift in thinking resulted in the increasing

stringency of combustion requirements over time.

WHICH ARE THE REGULATED UNITS?

Hazardous wastes are combusted for various

purposes. The purpose of combustion is directly

related to the type of unit used. There are two

classes of combustion units, those that burn waste

for energy recovery and those that burn waste for

destruction. At the present time, the regulations

that apply to each activity vary with the type of

waste that is burned, the type of combustion

device, and the purpose of the burning. Under

proposed regulations, known as the maximum

achievable control technology (MACT) proposed

rule, expected to be finalized in late 1998, the

COMBUSTION CHAMBER

BOTTOM ASH 

(Carbon, Salts, and Metals)

STABLE GASES 

(Carbon Dioxide, Water 

Vapor, Carbon Monoxide, 

Nitrogen Oxide, Hydrogen 

Chloride, and Chlorine)

FLY ASH 

(Particulate Matter, 

Carbon, Salts, and 

Metals)

AIR POLLUTION 

CONTROL DEVICE

WASTE

Atoms combine with 

oxygen to form 

stable gases

Gases pass through flame 

and break down into atoms

Waste is converted 

to gases

Figure III-23: THE COMBUSTION PROCESS
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Chapter 7: Hazardous Waste Combustion

regulations would be harmonized and the purpose

of burning would no longer be a key factor. The

MACT proposal is fully discussed later in this

chapter.

■ Incinerators

The first class of combustion units are

hazardous waste incinerators. Incineration is the

combustion of hazardous waste primarily for

destruction (i.e., disposal). Incineration is a

method of thermal destruction of primarily organic

hazardous waste using controlled flame

combustion (see Figure III-24). This process can

reduce large volumes of waste materials to ash

and lessen toxic gaseous emissions. An

incinerator is an enclosed device that uses

controlled flame combustion and does not meet

the more specific criteria for classification as a

boiler, industrial furnace, sludge dryer (a unit that

dehydrates hazardous sludge), or carbon

regeneration unit (a unit that regenerates spent

activated carbon). Incinerators also include

infrared incinerators (a unit that uses electric heat

followed by a controlled flame afterburner) and

plasma arc incinerators (a unit that uses electrical

discharge followed by a controlled flame

afterburner).

■ Boilers and Industrial Furnaces

The second class of combustion units are BIFs.

Boilers are used to recover energy from hazardous

waste, while industrial furnaces are used primarily

to recover material values.

PA defines boilers as enclosed devices that use

controlled flame combustion to recover and

export energy in the form of steam, heated fluid,

or heated gases. A boiler is comprised of two

main parts, the combustion chamber used to heat

the hazardous waste and the tubes or pipes that

hold the fluid used to produce energy (see Figure

III-25). The regulatory definition of boiler requires

that these two parts be in close proximity to one

Figure III-25:

CROSS-SECTION OF A BOILER

Coal 

Burner

Steam 

Water To Be Output

Heated

Hazardous 

Waste

Water

Water 

Tubes

Secondary 

Chamber

Primary 

Chamber

Auxiliary Fuel 

and/or 

Liquid Waste

Combustion 

Air Hazardous 

Waste

Feed Ram

Air

Ash

Transfer Ram

Ash

Discharge Ram

Ash Discharge

Auxiliary Fuel 

and/or 

Liquid Waste

Figure III-24:

CROSS-SECTION OF AN INCINERATOR
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Chapter 7: Hazardous Waste Combustion

another to ensure the effectiveness of the unit’s

energy recovery system and to maintain a high

thermal energy recovery efficiency. In addition,

the unit must export or use the majority of the

recovered energy for a beneficial purpose.

Industrial furnaces are enclosed units that are

integral parts of a manufacturing process and use

thermal treatment to recover materials or energy

from hazardous waste (see Figure III-26). These

units may use hazardous waste as a fuel to heat

raw materials to make a commodity (e.g., a

cement kiln making cement) or the unit may

recover materials from the actual hazardous waste

(e.g., a lead smelter recovering lead values). The

following 12 devices meet the definition of an

industrial furnace:

• Cement kiln

• Aggregate kiln

• Coke oven

• Smelting, melting, and refining furnace

• Methane reforming furnace

• Pulping liquor recovery furnace

• Lime kiln

• Phosphate kiln

• Blast furnace

• Titanium dioxide chloride process oxidation

reactor

• Halogen acid furnace

• Combustion device used in the recovery of

sulfur values from spent sulfuric acid.

After notice and comment, EPA may add other

devices to this list of industrial furnaces upon

consideration of factors related to the design and

use of the unit.

Not all units that meet the definition of boiler

or industrial furnace are subject to the BIF

standards. Each individual unit must first be

evaluated against a number of exemptions from

the BIF requirements. For a variety of reasons

(e.g., to avoid duplicative regulation), EPA

exempted the following units from the BIF

regulations:

• Units burning used oil for energy recovery

• Units burning gas recovered from hazardous

or solid waste landfills for energy recovery

• Units burning hazardous wastes that are

exempt from RCRA regulation, such as

household hazardous wastes

• Units burning hazardous waste produced by

CESQGs

• Coke ovens burning only K087 decanter tank

tar sludge from coking operations

• Certain units engaged in precious metals

recovery

• Certain smelting, melting, and refining

furnaces processing hazardous waste solely for

metals recovery

• Certain other industrial metal recovery

furnaces.

REGULATORY REQUIREMENTS

The RCRA regulation of combustion units is

composed of two types of requirements,

performance standards and operating

requirements. Performance standards are the

numerical pollutant emission limits developed by

Figure III-26:

CROSS-SECTION OF AN INDUSTRIAL FURNACE

Stack

Clinker

 Cooler

Kiln

Preheater

Dust

Collector
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Managing Hazardous Waste — RCRA Subtitle C SECTION III SECTION III

Chapter 7: Hazardous Waste Combustion

EPA or states. Operating requirements, on the

other hand, are parameters established by the

facility and written into a permit that will ensure

that the unit meets the numerical performance

standards.

■ Performance Standards

Emissions from combustion units may be

comprised of a variety of hazardous pollutants. To

minimize potential harmful effects of these

pollutants, EPA developed performance standards

to regulate four pollutant categories: organics,

hydrogen chloride and chlorine gas, particulate

matter, and metals. Both incinerators and BIFs

have performance standards that they must meet.

For each category or type of emission, the

regulations establish compliance methods and

alternatives.

Organics

Because the primary purpose of a combustion

unit is to destroy the organic components found in

hazardous waste, it is essential to verify that the

unit is efficiently destroying organics in the waste.

This is determined based on the unit’s organic

destruction and removal efficiency (DRE) as

demonstrated in a trial burn. Since it would be

nearly impossible to determine the DRE results for

every organic constituent in the waste, certain

principal organic hazardous constituents

(POHCs) are selected for this demonstration.

These POHCs are selected for each facility based

on their high concentration in the wastestream

and their greater difficulty to burn. If the unit

achieves the required DRE for the POHCs, then it

is presumed that it will achieve the same (or

better) DRE for all other easier-to-burn organics in

the wastestream. At least one POHC will be

selected from each wastestream that the facility

manages. The facility designates the selected

POHCs in their permit application (the permitting

process for combustion units is fully discussed in

Section III, Chapter 8).

The combustion unit must demonstrate a DRE

of 99.99 percent for each POHC in the hazardous

wastestream. This means that for every 10,000

molecules of the POHC entering the unit, only

one molecule can be released to the atmosphere.

In addition, due to an increased threat to human

health and the environment posed by certain

dioxin-containing wastes (F020, F021, F022,

F023, F026, and F027), the required DRE for

POHCs in these units has been established at

99.9999 percent, or one released molecule for

every one million burned (see Figure III-27).

These DRE standards must be met by both

incinerators and BIFs.

Hydrogen Chloride and Chlorine Gas

Hydrogen chloride and chlorine gases form

when chlorinated organic compounds in

hazardous wastes are burned. If uncontrolled, this

chlorine can become a human health risk and is a

large component in the formation of acid rain.

EPA has developed different requirements to

control the emissions of chlorine from the

different classes of combustion units.

Figure III-27:

PERFORMANCE STANDARDS FOR ORGANICS

WASTE FEED 

POHCs are chosen based on their high concentration in the 

wastestream and their greater difficulty to burn. If the unit 

demonstrates the required DRE for the POHCs, then it is 

presumed that it will be able to achieve the same (or better) 

DRE for all other easier-to-burn organics in the wastestream. 

For every 10,000 molecules of POHCs that enter a combustion 

process, the unit must destroy 9,999 of them. 

1 molecule of 

the POHC

Measurement of 

99.99 percent 

organic DRE

10,000 

molecules of 

POHCs
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Chapter 7: Hazardous Waste Combustion

Incinerators currently must control the

emissions of hydrogen chloride, but are not

required by the regulations to control chlorine gas.

An incinerator burning hazardous waste cannot

emit more than 1.8 kg of hydrogen chloride per

hour or more than one percent of the total

hydrogen chloride that is found in the stack gas

prior to entering any pollution-control equipment,

whichever is larger.

Hazardous waste BIFs must follow a tiered

system for the regulation of both hydrogen

chloride and chlorine gas. The owner and

operator determines the allowable feed or

emission rate of total chlorine by selecting one of

three approaches, called tiers. Each tier differs in

the amount of monitoring, and in some cases, air

dispersion modeling (i.e., modeling the air

pathways through which pollutants may travel),

that the owner and operator is required to

conduct (see Figure III-28).

Each facility can select any of the three tiers.

Factors that a facility may consider in selecting a

tier include the physical characteristics of the

facility and surrounding terrain, the anticipated

waste compositions and feed rates, and the level

of resources available for conducting the analysis.

The main distinction between the tiers is the point

of compliance. This is the point at which the

owner and operator must ensure that chlorine

concentrations will be below EPA’s acceptable

exposure levels. The owner and operator must

determine if the cost of conducting monitoring

and modeling is worth the benefit of possibly

combusting waste with a higher concentration of

chlorine (see Figure III-29).

Particulate Matter

The third combustion unit performance

standard is for particulate matter. Particulate

matter consists of small dust-like particles emitted

from combustion units. The particles themselves

are not normally toxic, but may become caught in

the lungs (causing respiratory damage) if inhaled,

or may enter into the environment where they can

cause either ecological damage or, via food chain

intake, can reenter the human health exposure

pathway. In addition, particulate matter may

provide a point of attachment for toxic metals and

organic compounds. To minimize these adverse

conditions, EPA developed an emission limit of

180 milligrams per dry standard cubic meter

(dscm) for all classes of combustion units.

Incinerators and BIFs are subject to this same

numerical standard.

Metals

The final performance standard is for toxic

metals. For BIFs, metals, both carcinogenic and

noncarcinogenic, are regulated under the same

type of tiered system as chlorine. The facility

determines an appropriate tier for each regulated

metal and assures that the facility meets these feed

rate and emission standards. A different tier may

be selected for each metal pollutant (see Figure III28). The regulations do not require hazardous

waste incinerators to meet the metal performance

standards.

Figure III-28:

PERFORMANCE STANDARDS FOR HYDROGEN

CHLORIDE, CHLORINE GAS, AND METALS

Tier III monitoring 

involves limiting the 

risks that hydrogen 

chloride, chlorine gas, 

and metals will pose to 

the surrounding 

TIER I environment.

TIER III

Tier I monitoring 

involves limiting 

the hourly waste 

feed rate of 

hydrogen chloride, 

chlorine gas, and 

metals.

Tier II monitoring involves 

limiting stack emissions of 

hydrogen chloride, 

chlorine gas, and metals.

WASTE FEED

TIER II
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Additional Performance Standards

EPA may require owners and operators of

hazardous waste combustion units to comply with

additional performance standards by virtue of the

omnibus authority. This authority allows EPA to

incorporate additional terms and conditions into a

facility’s permit as necessary to protect human

health and the environment. For example,

owners and operators may be required to limit

metal emissions from their incinerators. (The

omnibus authority is fully discussed in Section III,

Chapter 8.)

EPA frequently requires that site-specific risk

assessments, incorporating direct and indirect

exposures, be conducted during the combustion

unit’s permitting process. These risk assessments

ensure that the unit’s impact on the surrounding

environment is minimized to the extent possible.

■ Operating Requirements

The second type of combustion unit standards

are the operating requirements. The goal of

setting operating requirements for hazardous

waste combustion units is to ensure that the unit

will operate in a way that meets the performance

standards for organics, chlorine, particulate matter,

and metal pollutants. The unit’s permit will

specify the operating conditions that have been

shown to meet the performance standards for

organics, chlorine gas, particulate matter, and

metals (permit requirements for combustion units

are fully discussed in Section III, Chapter 8).

A RCRA permit for a hazardous waste

combustion unit sets operating requirements that

specify allowable ranges for, and requires

continuous monitoring of, certain critical

parameters that will ensure compliance with the

performance standards. Operation within these

parameters ensures that combustion is performed

in the most protective manner and the

performance standards are achieved (see Figure

III-30). These parameters, or operating

requirements, may include:

• Maximum allowable carbon monoxide levels

in stack emissions

• Allowable ranges for temperature

• Maximum waste feed rates

• Combustion gas velocity

• Limits on variations of system design and

operating procedures.

Figure III-29:

THE TIERED SYSTEM OF

MODELING AND MONITORING

Level of Monitoring and Modeling

Feed Rate/Emission Rate

Low

Low High

High

Tier I

Tier III

Tier II

Owners and operators can choose any Tier for their hydrogen 

chloride, chlorine gas, and metals monitoring; however, the 

level of monitoring and modeling increase with each tier.

Tier I, which requires the lowest level of monitoring 

through adjusting waste feed rates, assumes that 100 

percent of the pollutants will escape into the environment. 

Tier II requires increased 

monitoring to ensure that 

pollutants have been 

eliminated through either 

partitioning to bottom ash or 

products, or collected by the 

air pollution control device.

Tier III requires the most 

monitoring by using air 

dispersion modeling to ensure 

that pollutant exposure does 

not pose a threat to human 

health and the environment.
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 SECTION III SECTION III Managing Hazardous Waste — RCRA Subtitle C

Chapter 7: Hazardous Waste Combustion

CARBON 

MONOXIDE 

EMISSIONS

WASTE FEED RATE

GAS 

VELOCITY

Combustion facilities must operate in accordance with certain conditions that specify allowable 

ranges and limits for waste feed rates, temperature, gas velocity, and carbon monoxide emissions.

TEMPERATURE

Figure III-30: OPERATING REQUIREMENTS

• Sampling and analyzing the waste and exhaust

emissions to verify that the operating

requirements established in the permit

achieve the performance standards

• Conducting visual inspections of the

combustion unit and its associated equipment

• Testing the emergency waste feed cut-off

system and associated alarms

• Placing monitoring and inspection data in the

operating log.

Residues from the combustion of hazardous

waste are also potentially subject to RCRA

regulation. If a combustion unit burns a listed

hazardous waste, the ash is also considered a

listed waste via the derived-from rule. The owner

and operator must also determine whether this

ash exhibits any hazardous waste characteristics.

The same is true if a unit burns waste that only

exhibits a characteristic. Ash that exhibits a

characteristic must be managed as a hazardous

waste.

ADDITIONAL REQUIREMENTS

Because hazardous waste combustion units

are a type of TSDF, they are subject to the general

TSDF standards (as discussed in Section III,

Chapter 5) in addition to combustion unit

performance standards and operating

requirements. Combustion units are also subject

to specific waste analysis, inspection and

monitoring, and residue management

requirements.

While combusting hazardous waste, the

combustion process and equipment must be

monitored and inspected to avoid potential

accidents or incomplete combustion. The

monitoring and inspection requirements for

incinerators are detailed in the regulations, while

the requirements for BIFs are determined on a

site-specific basis. Possible inspection and

monitoring requirements include:

• Monitoring the combustion temperature, and

hazardous waste feed rate
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Chapter 7: Hazardous Waste Combustion

FUTURE INTEGRATED REGULATIONS

On April 19, 1996, EPA published a proposed

rule under the joint authority of RCRA and CAA to

upgrade the emission standards for hazardous

waste combustors. These emission standards were

developed based on the maximum achievable

control technology process commonly employed

by CAA. This process develops technology-based,

concentration limits for individual constituents.

This rule proposes to eliminate the current

inconsistencies between the BIF and incinerator

regulations by subjecting all hazardous waste

combustors to one set of emissions standards.

Specifically, this rule will affect incinerators,

cement kilns, and lightweight aggregate kilns. EPA

plans to address boilers and other industrial

furnaces in a future rulemaking.

Consistent with EPA’s trend of gradually

increasing the stringency of standards over time,

the rule proposes more stringent emissions

standards for dioxins, furans, mercury, cadmium,

lead, particulate matter, hydrogen chloride,

chlorine gas, hydrocarbons, carbon monoxide,

and several low-volatile metals. This rule also

makes several significant changes to the existing

combustion regulations.

SUMMARY

Combustion, the controlled burning of

hazardous substances in an enclosed area, has the

potential to adversely affect human health and the

environment, and it is therefore subject to strict

regulation. As a result, the burning of hazardous

waste in incinerators and BIFs is regulated by two

methods, performance standards and operating

requirements.

The performance standards are intended to

regulate four pollutant categories:

• Organics

• Hydrogen chloride and chlorine gas

• Particulate matter

• Metals.

These performance standards may include a

demonstration of the unit’s DRE for certain

POHCs, and emission standards for hydrogen

chloride, chlorine gas, metals, and particulate

matter.

The operating requirements are intended to

ensure that the combustion unit will operate in a

way that meets the performance standards for

these pollutants. Operating conditions may

include:

• Maximum allowable carbon monoxide levels

in stack emissions

• Allowable ranges for temperature

• Maximum waste feed rates

• Combustion gas velocity

• Limits on variations of system design and

operating procedures.

In addition to operating and performance

requirements, combustion units are subject to

specific waste analysis, inspection and monitoring,

and residue management requirements.

EPA has developed these requirements in

order to efficiently control the emission of

hazardous pollutants without being overly

burdensome on the regulated community. The

combustion unit regulations are in the process of

being updated based on CAA’s MACT process,

and therefore, may change significantly in the

future.