Nazih K Shammas and Lawrence K Wang

23.1 Incinerators 956

23.1.1 Incinerator Design and Operating Considerations 957

23.1.2 Regulatory Summary 960

23.1.3 Regulatory Developments 965

23.1.4 Special Issues 966

23.2 Boilers and Industrial Furnaces 967

23.2.1 Regulatory Summary 967

23.2.2 Regulatory Developments 977

23.3 Emission Standards for Combustors 979

Acronyms 979

References 983

In the United States, more than 3.1 million tons of hazardous waste were disposed of through combustion in 2005. This figure represents 7.2% of the approximately 44 million tons of hazardous waste generated.1 The state of Texas, which ranked first in hazardous waste production, generated 15.2 million tons, while the state of Louisiana that ranked second produced 5.5 million tons.2 These two states alone contributed almost 50% of the total U.S. hazardous waste generation.

There are two categories of combustion units for solid and liquid hazardous wastes:

1. Incinerators—used primarily for waste destruction.

2. Boilers and industrial furnaces (BIFs)—used primarily for energy and material recovery.

Incinerators are used to burn hazardous waste primarily for waste destruction/treatment purposes; however, some energy or material recovery can occur. When performed properly, incineration destroys the toxic organic constituents in hazardous waste and reduces the volume of the waste.3 Since metals will not combust, incineration is not an effective method for treating metal-bearing hazardous wastes.

BIFs are typically used to burn hazardous waste for the significant energy and material recovery potential, with waste treatment being a secondary benefit. Boilers typically combust waste for energy recovery, whereas industrial furnaces burn waste for both energy and material recovery.3 A boiler is defined as an enclosed device that uses controlled flame combustion to recover and export energy in the form of steam, heated fluid, or heated gases. An industrial furnace is a unit that is an integral part of a manufacturing process and uses thermal treatment to recover materials or energy.

Resource Conservation and Recovery Act (RCRA) regulations governing incinerators can be found at 40 CFR Part 264/265, Subpart O-Incinerators.4 RCRA regulations governing BIFs can be found at 40 CFR Part 266, Subpart H-Hazardous Waste Burned in BIFs.5

CONTENTS

Hazardous waste combustors (HWCs) also are regulated under the Clean Air Act (CAA).6 The CAA protects human health and the environment from the harmful effects of air pollution by requiring significant reductions in the emissions of the most dangerous air pollutants. These pollutants are known or suspected to cause serious health problems such as cancer or birth defects, and are referred to as hazardous air pollutants (HAPs).

As originally enacted, the CAA required that the U.S. Environmental Protection Agency (U.S. EPA) establish National Emission Standards for Hazardous Air Pollutants (NESHAPs) on a chemical-by-chemical basis. Under this mandate, U.S. EPA established NESHAPs for seven HAPs. However, the 1990 amendments to the CAA changed U.S. EPA's approach to regulating HAPs, so that NESHAPs are now established based on the maximum achievable control technology (MACT) for an industry group or source category (e.g., HWCs). These standards are based on emission levels that are already being achieved by the better-performing sources within the group.

The NESHAP was developed in two phases. Phase I addresses hazardous waste burning incinerators, cement kilns, and lightweight aggregate kilns and was originally promulgated on September 30, 1999. Hazardous waste burning industrial boilers, process heaters, and hydrochloric acid production furnaces were addressed in Phase II, which was signed on September 14, 2005. Replacement standards for Phase I also were signed on this date.

23.1 INCINERATORS

Incineration destroys organic compounds contained in hazardous wastes and reduces the volume of the wastes by removing liquids. To achieve those goals, the incinerator must be able to provide controlled burning (combustion) conditions that ensure the proper mixing of air, temperature, and gas, and adequate time to allow a thorough destruction of organic constituents to take place. A deficiency in any of those requirements can result in incomplete combustion and the production of smoke and possibly harmful air emissions. Such emissions are a potential public health hazard because nearby communities may be exposed to site contaminants via the air transport pathway. It should also be recognized that human exposure to airborne incinerator contaminants can occur indirectly by consumption of animals or plants raised in areas where deposition of emissions takes place.7

The specific equipment used for each step depends on the incinerator type and the physical and chemical characteristics of the wastes the incinerator is designed to burn. Wastes are fed into the incinerator in batches or in a continuous stream. Liquid wastes are often pumped and atomized into fine droplets that burn more easily. Solid wastes may be fed into the incinerator in bulk or in containers using a conveyer, a gravity system, or a ram feeder.8

As the wastes are heated, they are converted from solids or liquids into gases. The gases are mixed with air and pass through a hot flame. As the temperature of the gases rises, the organic compounds in the gases begin to break down and recombine with oxygen to form carbon dioxide and water. Depending on the waste composition, other organic and inorganic compounds may form.

In most hazardous waste incinerators, combustion occurs in two combustion chambers. Combustion is completed in the secondary combustion chamber after the compounds have been converted to gases and partially combusted in the first chamber.

Incineration produces gases and solids, in the form of ash and slag. Combustion gases are composed primarily of carbon dioxide and water, as well as small quantities of carbon monoxide, nitrogen oxides, and small concentrations of organic and inorganic compounds.

Following combustion, the combustion gases move through various devices that cool and cleanse the gases. A fan is typically used to pull the gases through the incinerator and the air pollution control equipment. Gases are quenched with a water mixture to reform any particulate matter (PM). Acids can be removed with wet or dry scrubbers. PM can be removed using either dry (bag-house) or wet systems.

When Congress enacted the RCRA in 1976, it directed U.S. EPA to establish performance, design, and operating standards for all hazardous waste treatment, storage, and disposal facilities

(TSDFs). U.S. EPA promulgated both general facility standards that apply to all TSDFs and requirements for specific types of units (e.g., incinerators, landfills, and surface impoundments) in 40 CFR Parts 264 and 265. The regulations under Parts 264 and 265, Subpart O, apply to owners and operators of facilities that incinerate hazardous waste.4

23.1.1 Incinerator Design and Operating Considerations 23.1.1.1 Overview of Combustion

Incineration is the controlled burning of substances in an enclosed area. During a burn, wastes are fed into the incinerator's combustion chamber. As the wastes are heated, they are converted from solids and liquids into gases. These gases pass through the flame and are heated further. Eventually, the gases become so hot that the organic compounds in the gases break down into their constituent atoms. These atoms combine with oxygen and form stable gases that are released to the atmosphere after passing through air pollution control devices (APCDs).

For incineration to be an effective method for destroying the hazardous properties of wastes, combustion must be complete. Three critical factors ensure the completeness of combustion in an incinerator8:

1. The temperature in the combustion chamber.

2. The length of time wastes are maintained at high temperatures.

3. The turbulence or degree of mixing of the wastes and the air.

23.1.1.2 Design Considerations

To minimize the public's potential exposure to site emissions, an incinerator must be designed and operated properly. The incinerator must be designed to burn waste materials thoroughly. The combustion chambers must be of a size and arranged in a way to provide adequate time for the gases produced by burning waste to mix with proper amounts of combustion air, and to maintain the high temperatures needed to ensure that the burning is completed.9

When an incinerator is designed, the waste to be burned must be characterized for properties such as heat content (fuel value), percent moisture, chlorine content, metals content, and physical characteristics. The size and physical layout of the incinerator should be based on those waste properties.

The incinerator must be designed and operated in a manner that minimizes production of nonstack, fugitive emissions. This can be accomplished by ensuring proper seals at all system connections, maintaining negative gas pressures throughout the combustion gas flow path, and by limiting the waste feed to prevent excessive and rapid releases of volatile compounds. Careful attention must also be given to the design and operation of waste handling systems to minimize fugitive emissions. Agency for Toxic Substances and Disease Registry (ATSDR) public health assessors have found that excavation and handling of soils at some Superfund sites, and waste unloading and repackaging operations at some RCRA facilities, have been major sources of airborne contaminants that have resulted in exposure of workers and/or nearby residents10 to hazardous wastes.

Another critical part of the incinerator design is the pollution control system.11 Pollution control systems directly influence the levels and kinds of pollutants that are released and that can potentially reach the public. Most modern hazardous waste incinerators are designed with extensive air pollution removal systems. For example, a common pollution control system might include a system that cools or "quenches" gases produced by burning waste, followed by a system that reduces acid gas emissions, and ultimately followed by a particulate removal system such as fabric filters (bag-houses), electrostatic precipitators, venturi scrubbers, and others.10

Finally, current design of hazardous waste incinerators includes various safeguards, such as process monitoring devices (to monitor parameters such as temperature, air flow, and operating pressures); continuous emission monitoring systems (to measure air emissions of carbon monoxide, gas flow rates, and possibly other combustion performance indicators); and automatic waste feed shutoff devices (AWFSOs). AWFSOs, as required by RCRA regulations (40 CFR 264), automatically stop the waste feed to the incinerator when specified monitoring parameters exceed or fall below limits specified in the permit. The parameters that trigger the AWFSOs are established based on successful trial burns. AWFSOs are critical to ensure that the incinerator cannot operate in an improper condition for extended periods of time).12

23.1.1.3 Operating Considerations

Operating considerations of importance in protection of public health are described in the following subsections.10

23.1.1.3.1 Training of Operators

Even with all the proper design features, skilled operators are essential for a safe, effective incineration program. Operators should understand the principles of good combustion and be thoroughly familiar with all major and support systems at their plants. Careful attention to proper waste burn rates and waste blending, as needed, helps to ensure that the combustion systems are not overloaded and that the AWFSOs are not activated excessively. Routine maintenance, inspection, and instrument calibrations should be conducted and recorded. Safety and emergency response plans that thoroughly address likely failure scenarios (including power, systems, and operational failures) must be in place, documented, and shared with local officials. Emergency "release" drills should be conducted periodically with the knowledge and involvement of local emergency response personnel. In addition, all employees should be adequately trained in appropriate health and safety procedures for the safe day-to-day operation of the incinerator.

Figure 23.1 provides a compilation of information on reported emergency incidents at hazardous waste combustion facilities and other TSDFs regulated under the RCRA. It covers emergency incidents such as fires, explosions, hazardous waste spills, or unauthorized releases of hazardous waste. The reported incidents at 24 hazardous waste combustion units and 26 other TSDFs

1977-80 1981-85 1986-90 1991 1992 1993 1994 1995 1996 1997 1998

Years

FIGURE 23.1 Number of incidents at combustion facilities and TSDFs. (Adapted from U.S. EPA, Report on Emergency Incidents at Hazardous Waste Combustion Facilities and Other Treatment, Storage and Disposal Facilities (TSDFs), EPA530-R-99-014, U.S. Environmental Protection Agency, Washington, DC, June 1999.)

1977-80 1981-85 1986-90 1991 1992 1993 1994 1995 1996 1997 1998

Years

FIGURE 23.1 Number of incidents at combustion facilities and TSDFs. (Adapted from U.S. EPA, Report on Emergency Incidents at Hazardous Waste Combustion Facilities and Other Treatment, Storage and Disposal Facilities (TSDFs), EPA530-R-99-014, U.S. Environmental Protection Agency, Washington, DC, June 1999.)

emphasize the importance of safety procedures for the safe day-to-day operation of incinerators. Of the incidents that have occurred, nine facilities experienced incidents that were directly related to the actual combustion of hazardous waste. The remaining incidents involved were not combustion related and consist of activities such as waste handling and storage. Detailed incident reports of the various combustion units can be found in a U.S. EPA Report.13

23.1.1.3.2 Siting of the Incinerator

Another consideration relevant to public health and frequently raised by the public is the location of the incinerator with respect to the community. More specifically, consideration should be given to the possible health impacts associated with living or working people in the path of incinerator emissions. To address those concerns, when reviewing the location of an incinerator, regulatory agencies use generally accepted air dispersion models14 in conjunction with local meteorologic data to determine the permit conditions necessary to protect human health and the environment. Such modeling results can be particularly helpful in identifying prevailing wind transport patterns and their effect on downwind pollutant concentrations. Ideally, the site should not be where modeled high ground-level concentrations of stack emissions coincide with population centers. Dispersion models can also help evaluate the need for, and the location of, off-site air monitors used to detect fugitive emissions associated with incinerator operations and related hazardous materials-handling activities. If there is concern about the impact of incineration on a specific major food resource, such as a fish hatchery, and ATSDR has data regarding the uptake of the contaminants of concern by the particular food chain species, dispersion modeling can serve to estimate the concentration of emissions that would be available at ground level for food chain uptake.

23.1.1.3.3 Storage of Materials

In addition to the aforementioned issues regarding the incineration process, other concerns of relevance to public health need to be addressed. For example, hazardous waste to be fed to the incinerator and process effluents resulting from the incinerator should be stored in a manner that does not allow for uncontrolled environmental releases of potentially harmful substances. Dry, dusty materials should be enclosed or otherwise stored to prevent windborne transport of contaminated particulates. Wastes containing volatile organic compounds should be stored under conditions that safely collect and remove gases released from the wastes.

Similarly, wet wastes or process effluents should be stored in chemically compatible, leak-resistant containers. Storage areas for such liquid-bearing materials should have dikes or be designed to contain leakage. Processing of wastes, such as blending or shredding operations, may provide opportunities for aerosolization of contaminants. Such conditions should be adequately considered and waste-processing areas are designed to minimize the potential exposure to workers on-site, as well as to people living or working nearby.

23.1.1.3.4 Transportation of Wastes to the Incinerator

The means of transporting hazardous waste into the incinerator plant should be carefully considered. Routes of access should be selected to minimize accident (release) potential and to avoid residential and play areas if possible. For the remediation of Superfund sites, for which no over-the-road hauling is required, care is still needed to avoid spills and releases when transporting the wastes on-site.

23.1.1.3.5 Maintaining Good Performance

Some considerations relevant to public health concerns about modern and effective incineration systems have been described. However, local health officials and citizens of communities with hazardous waste incinerators have expressed to ATSDR their concern that they may not be able to judge a good operation, or that, once the initial trial burns and inspections are completed, the system may not be operated in the same manner as during the testing phase. Citizens have also expressed concern that burning rates will be exceeded or monitoring systems will be overridden.

ATSDR believes that one way to ensure that the system continues to operate in a manner consistent with operating conditions specified in the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) contract15 is for U.S. EPA to conduct frequent, random, unannounced facility inspections and to routinely provide the results to the public. Under some circumstances, permanent on-site inspectors might be advisable.

Another way to ensure continued satisfactory operation is to retest the incinerator periodically. This would be appropriate if the CERCLA incinerator operates at the site for an extended period of time, or there are other indications that it may not be operating properly.

Each time a CERCLA incinerator is relocated, ATSDR recommends that it be retested. A less rigorous trial burn may be appropriate if the incinerator has successfully passed a full trial burn on similar wastes at another site.

23.1.1.3.6 Community Right-to-Know

ATSDR also strongly recommends that information and data for an incinerator's design, testing, operation, and monitoring be shared with the public. The Agency endorses this approach as being consistent with the community right-to-know requirements already in place for industries that use and store hazardous substances in the community.

In addition, as a final caution, it must be remembered that each site is unique, and must be carefully evaluated individually and not by generic extrapolation of data from other sites or studies.

23.1.1.3.7 Summary of Public Health Considerations

The following is a summary of items that should be considered when evaluating proposed or existing hazardous waste incinerators for public health acceptability10:

1. The technology used or proposed to be used at a site is proven to be appropriate for, and compatible with, the materials to be burned.

2. In selecting a site for an incinerator, proximity to residential and other populations and local meteorologic conditions is considered to ensure a location that minimizes the prevailing wind transport of air emissions to affected populations.

3. Recognized, acceptable, and when possible, U.S. EPA-approved air modeling is used to help screen and identify potentially impacted areas as mentioned previously.

4. Trial burns, with appropriate stack sampling and analysis, and subsequent continuous emissions monitoring are conducted to demonstrate that the incinerator performs as specified.

5. Adequate training is provided to incinerator operators to ensure that the incinerator is operated in a manner that does not adversely affect the operators' or the community's health.

6. An active inspection program is instituted.

7. Where the incinerator must be at a site close to neighboring populations, local ambient air monitors are used to detect possible site releases to the air requiring corrective or emergency action.

8. Proper management of residual ash is part of the design and operation of the incinerator.

9. Procedures consistent with the community right-to-know philosophy are instituted.

23.1.2 Regulatory Summary

The standards for hazardous waste incinerators primarily regulate the emissions that result from the combustion process. Specifically, the regulations restrict the emissions of organics, hydrogen chloride (HCl), and PM, as well as fugitive emissions. A very important aspect of the regulations is that compliance with operating conditions specified in the permit is deemed to be compliance with the limits for organics, HCl, and PM.16

Incinerators in existence on May 19, 1980, were allowed to continue burning hazardous waste if the units complied with the Part 265, Subpart O,4 interim status standards. On November 8, 1989, however, interim status was terminated for all existing hazardous waste incinerators unless the owner/operator had submitted a Part B permit application by November 8, 1986. Due to this deadline, there are very few incinerators currently operating under interim status. This chapter, therefore, focuses primarily on the requirements for permitted, rather than interim status, incinerators. There is a comparison of the requirements for permitted and interim status incinerators at the end of Section 23.1.

The stable gases produced by incineration are primarily carbon dioxide and water. Depending on waste composition, however, small quantities of carbon monoxide, nitrogen oxides, HCl, and other gases may form. Also, if combustion is not complete, compounds known as products of incomplete combustion (PICs) may be emitted. RCRA regulations control the amount of HCl released from the APCD.

Another by-product of the combustion process is ash. Ash is an inert solid material composed primarily of carbon, salts, and metals. During combustion, most 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.17 Some ash, however, is carried up with the gases as small particles, or PM. These particles are also collected in the APCD in accordance with RCRA-established limits.

As a hazardous waste management practice, incineration has two unique attributes16:

1. 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.

2. Incineration reduces the volume of hazardous waste by converting solids and liquids to ash. Land disposal of ash, as opposed to untreated hazardous waste, is therefore both safer and more efficient. Incineration, however, will not destroy inorganic compounds, such as metals, present in hazardous waste. Residue ash from incinerators is subject to applicable RCRA standards and may need to be treated for metals or other nonorganic constituents prior to land disposal.

23.1.2.1 Subpart O Applicability

The Subpart O standards apply to units that treat or destroy hazardous waste and which meet the definition of an incinerator. An incinerator is any enclosed device that uses controlled flame combustion and does not meet the criteria for classification as a boiler, sludge dryer, carbon regeneration unit, or industrial furnace. Typical incinerators3 include rotary kilns, liquid injectors, fixed hearth units, and fluidized bed incinerators (Table 23.1). The definition of an incinerator also includes units that meet the definition of an infrared incinerator or plasma arc incinerator. An infrared incinerator is any enclosed device that uses electric-powered resistance as a source of heat and which is not listed as an industrial furnace. A plasma arc incinerator is any enclosed device that uses a high-intensity electrical discharge as a source of heat and which is not listed as an industrial furnace.

All devices classified as incinerators that burn hazardous waste must follow the Subpart O standards, with the following exception. The Regional Administrator must exempt an owner/operator applying for a permit from all of the incinerator standards in Subpart O, except waste analysis and closure, if the hazardous waste fed into an incinerator is considered as low-risk waste. The criteria for defining a waste as low risk are as follows4:

1. A waste is a hazardous waste if it is listed or is identified for ignitability, corrosivity, or both.

2. A waste is a hazardous waste if it is listed, or is identified for reactivity, and will not be burned with other hazardous wastes (this exemption does not apply to wastes that are reac tive for generating toxic gases when mixed with water or cyanide or sulfide gases and to a waste contains none of the listed hazardous constituents).

TABLE 23.1

Lists of Devices That Are Considered to Be Incinerators and Industrial Furnaces

Incinerators

Industrial Furnaces

1. Rotary kilns

2. Fluidized bed units

1. Cement kiln

2. Lime kiln

3. Aggregate kiln

4. Phosphate kiln

5. Coke oven

6. Blast furnace

7. Smelting, melting, and refining furnace

8. Titanium dioxide chloride process oxidation reactor

9. Methane reforming furnace

10. Halogen acid furnace

11. Pulping liquor recovery furnace

12. Combustion device used in the recovery of sulfur

3. Liquid injection units

4. Fixed hearth units values from spent sulfuric acid

Source: U.S. EPA, Hazardous Wastes Combustion, U.S. Environmental Protection Agency, Washington, DC, 2008. Available at http://www.epa.gov/epaoswer/hazwaste/hazcmbst.htm.

23.1.2.2 Performance Standards

The Subpart O standards4 for hazardous waste incinerators set performance standards that limit the quantity of gaseous emissions an incinerator may release. Specifically, the regulations set limits on the emission of organics, HCl, and PM. The following section outlines the requirements for each of these substances.

23.1.2.2.1 Organics

To obtain a permit, an owner/operator must demonstrate that emission levels set for various hazardous organic constituents are not exceeded. U.S. EPA's principal measure of incinerator performance is its destruction and removal efficiency (DRE). A 99.99% DRE means that one molecule of an organic compound is released to the air for every 10,000 molecules entering the incinerator. A 99.9999% DRE means that one molecule of an organic compound is released to the air for every 1 million molecules entering the incinerator.

Since it would be impossible to monitor the DRE results for every organic constituent contained in a waste, certain principal organic hazardous constituents (POHCs) are selected for monitoring and are designated in the permit. POHCs are selected based on high concentration in the waste feed and difficulty in burning compared with other organic compounds. If the incinerator achieves the required DRE for the selected POHCs, then it is presumed that the incinerator should achieve the same or better DRE for organic compounds that are easier to incinerate.

RCRA performance standards require a minimum DRE of 99.99% for POHCs designated in the permit and a minimum DRE of 99.9999% for dioxin-bearing wastes.

23.1.2.2.2 Hydrogen Chloride

HCl is an acidic gas that forms when chlorinated organic compounds in hazardous wastes are burned. An incinerator burning hazardous waste cannot emit more than 1.8 kg of HCl/h or more than 1% of the total HCl in the stack gas prior to entering any pollution control equipment, whichever is larger.

23.1.2.2.3 Particulate Matter

PM are tiny particles of ash that are carried along with the combustion gases to the incinerator's stack. The incinerator regulations control metal emissions through the performance standard for particulates, since metals are often contained in or attached to the PM. A limit of 180 mg PM/dscm of gas emitted through the stack has been discussed in section.

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