Incineration: Introducing the technical issues

Incineration: Introducing the technical issues

ENVIRON IMPACT ASSESS REV 1989;9:163-180 163 INCINERATION: INTRODUCING THE TECHNICAL ISSUES JOSEPH J. SANTOLERI Municipal Solid Waste Generation T...

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ENVIRON IMPACT ASSESS REV 1989;9:163-180

163

INCINERATION: INTRODUCING THE TECHNICAL ISSUES

JOSEPH J. SANTOLERI

Municipal Solid Waste Generation The rapidly increasing amounts of municipal waste generated in high technology "throwaway" societies are threatening modern society everywhere. American households and neighborhood businesses discard over 160 billion pounds of solid waste per year. This was the equivalent of 3.6 pounds per person each day in 1986 (and is expected to reach almost 4 pounds by the year 2000). West Germans throw away less than 2.5 pounds per day, Oslo residents 1.7 pounds. Every year we throw away 16 billion disposable diapers, 2 billion razors and blades, and mountains of automobile tires, construction and demolition debris, sludge, automobile bodies, nonhazardous industrial wastes, incinerator residues, household hazardous wastes, and nonfood products such as detergents or cosmetics that may be left inside discarded containers. One third of the commercial and industrial trash generated by Americans per year is packaging materials. Within the next 10 years, more than one half of US cities will run out of dump site capacity. Waste minimization, recycling, reclamation, and incineration are necessary for survival.

Waste Minimization Many states are implementing legislation to aid in the reduction of solid waste production. Reduction in solid waste production decreases the volume of municipal solid waste (MSW) produced and increases the lifetime of existing landfills without the disadvantages of other proposed methods. This reduction is achievable by manufacturing products using recyclable material, eliminating unnecessary packaging, and in general by modifying the purchaser's habits that are accepted in today's society. © 1989 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010

0195-9255/89/$3.50

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JOSEPH J. SANTOLERI

Recycling Recycling is the next best alternative to reduction of MSW. Today, Norwegians recycle approximately 40% of the total waste stream and use mass-burn incineration to dispose of another 40%. In Germany, large receptacles are conveniently located on street corners for deposit of separated materials. The early acceptance of and widespread support for recycling have resulted in a 20% to 25% recovery rate. Another 50% of the waste stream is handled by incineration. Particle and gaseous emissions are closely monitored and strictly controlled. In Denmark, the return rate for bottles is a remarkable 99.6%. Ninety percent of all Danish cardboard is made from recyclable material, and 45% of photocopy paper is produced using recycled paper and straw. Japan effectively manages its wastes through recycling, incineration, and landfilling. Fifty percent of all paper and 42% of glass is recycled. Japan incinerates 67% of the 50% of unrecyclable materials in the waste stream. Rigorous safety and environmental regulations require use of equipment such as dry scrubbers, electrostatic precipitators, and air filters that can remove 98% or more of potentially harmful substances from stack emissions.

Landfills In the United States, current EPA data show that more than 80% of our trash is landfilled in about 6500 facilities, another 10% is incinerated (with or without energy recovery), and the last 10% is recycled. The advantage of using the waste-to-energy facilities is that they reduce the volume of refuse by 90%, a reduction that no other waste disposal option can offer. Of the existing landfills, almost 2000 will close within five years, causing an overall yearly capacity loss of 56 million tons. Even with new landfill construction and required siting, permits, etc., an optimum assumption is that additional landfills will be made available for only 20 million tons of this amount. To the public, landfills and incinerators are unneeded and unwanted in their neighborhoods. Many communities, particularly in the Northeast, must ship their trash more than 300 miles to find a permanent disposal site. Sending waste to someone else's backyard is not a solution. The major objections to incinerators and landfills are air emissions and groundwater contamination.

Incineration Incineration is not a new technology; it has been used for many years. In fact, some MSW incineration facilities were in operation as long as 50 years ago and before. The revolution in municipal refuse combustion has advanced from open dump fires through enclosed refractory incinerators, the development of fixed grate and moving grate stokers, and ultimately the provision of forced draft and induced draft fans and furnace temperature controls before stack emissions be-

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came a matter of concern. In those times, the concern was to get rid of the garbage and preferably not allow too much visible smoke to discharge from the stack. Early regulations of particulate emissions forced communities to choose between shutting down their incinerators and retrofitting them with electrostatic precipitators. Most were shut down since landfill was less expensive. During the 1970s, more incinerators were shut down than were being considered for new MSW plants. In recent times, the concept of generating energy and power from the combustion of refuse coupled with the increasing costs and distances of landfills has revived interest in combustion as a means of reducing the quantity of waste to be landfilled. This revival is taking place in a climate of increased concern for environmental quality and increasingly stringent guidelines and regulations. In addition to particulate control, acid gases and heavy metals are now included in the guidelines, and a heightened concern has developed for trace organic emissions such as dioxins and furans. All of these concerns must be addressed today before new municipal waste combustors can emerge from the regulatory process and obtain construction permits.

Technology for the Incineration of Municipal Solid Waste MSW can be combusted by numerous methods. Two methods of producing energy from waste have emerged as those most preferred by communities and industry. These are the mass-burn system, in which unprocessed waste is incinerated and energy recovered, and the refuse derived fuel (RDF) system, in which unprocessed waste is processed into a usable fuel. RDF is burned within a dedicated boiler or as supplemental fuel in a coal- or oil-fired boiler. The incineration types used for mass-burn fuel and RDF are starved-air modular, stoker grates, rotary kiln, and fluidized bed units. The number and types of MSW combustion facilities located in the United States are presented in Table !. This table indicates the capacity ranges, the number of facilities, and their capacity in tons per day for systems with and without heat recovery. The modular units are limited to facilities with a maximum of 300 tons per day (TPD). Mass-burn and RDF facilities are normally found in plants producing more than 250 TPD. Heat recovery is utilized on approximately one half of the mass-burn facilities that produce more than 250 TPD (Rood 1988).

Processes Associated with Total Incineration Systems The basic process associated with an incinerator facility is combustion. This may be related directly to any energy-producing furnace whether it be a boiler generating steam, a process furnace in a refinery or chemical plant that produces a product, or a kiln in the minerals industry producing cement, lime, or aggregate. In each case, proper control of the fuel rate, air rate, and temperature is fundamental. Production of energy or salable products demands that this be accom-

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JOSEPH J. SANTOLERI

TABLE 1. Summary of Existing Municipal Solid Waste Combustion Facilities in the United States* Number of installed facilities

Total installed capacity (tons per day)

Capacity range (tons per day)

With heat recovery

Without heat recovery

Mass burn Modular RDF

<250

8 37 1

4 17 0

1291 3292 200

748 610 0

Mass burn Modular RDF

250 to <500

4 2 3

2 0 0

1820 570 1100

900 0 0

Mass burn Modular RDF

500 to < 1000

4 0 1

11 0 0

2740 0 600

7150 0 0

Mass burn Modular RDF Totals

/> 1000

8 0 5 73

4 0 0 38

14250 0 9500 35,363

4200 0 0 13,608

Design type

111

With heat recovery

Without heat recovery

48,971

*Greene (1987).

plished with minimum fuel costs. In an incineration system, the fuel is waste material. Operation at high efficiency is not critical. What is important is that the system be capable of operation at the maximum waste feed rate, achieve optimum temperatures to ensure complete combustion, and minimize the emissions of pollutants created in the combustion process. Most waste materials that are disposed of by incineration are organic compounds with or without inorganics. Many of these materials contain elements which are not combustible, for example, salts, metals, water, and ash. The organic elements carbon and hydrogen react with oxygen at elevated temperatures to form carbon dioxide, water vapor, and other byproducts. If the wastes contain chlorides, hydrogen chloride and chlorine will be formed in the combustion reaction. If sulfur or phosphorous are present in the waste, sulfur dioxide and phosphorous pentoxide will result. It is, therefore, very important to determine the composition of the waste delivered to the MSW plant from the local area. This may vary considerably across the nation and may also vary depending on the season. The moisture content of the waste also varies from one country to another, depending on the types of materials used by the community. The approximate heat content of the waste stream is also important in sizing

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the air volume required for the incinerator. Incinerator chambers are designed for maximum throughput (in pounds per hour or TPD) as well as maximum heat release (Btu per hour). Both are critical in determining the size of the major components of the system as well as its utilities. Figure 1 is a schematic of a total system, including the feed system, incinerator, waste heat recovery, quench, air pollution control, residue control, and stack discharge.

Combustion Process Adequate furnace temperature is fundamental to good combustion. Heat is the driving force which sustains the combustion process. This heat is generated by waste alone or in combination with auxiliary fuel firing. Preheating of the furnace is accomplished by use of auxiliary fuels. Once proper temperature levels are reached, waste is introduced. Air carrying oxygen provides the oxidant that reacts with the organics (CxHy) to release energy and generate the products of combustion (CO2 and H20). If sufficient air is not available, or if there is inadequate mixing (turbulence), incomplete products of combustion (PICS) will result. If too much air is used, causing a drop in temperature, PICS will also be generated along with excessive carryover of particulates into the pollution control system. The volume of the combustion chambers determines the total time that the waste materials and products of combustion are at the incineration temperature. If the chamber volume is too small for the amount of waste being burned, insufficient residence time will result. This creates unburned waste in the ash from the system and exhaust gases will contain carbon monoxide and unburned hydrocarbons. Proper combustion occurs when the three "t"'s are maintained in the combustion zone. These are temperature, turbulence, and time. The three "t" 's plus adequate oxygen ensure that combustion is complete. This is needed no matter what type of combustion system is selected.

Modular Incinerators Modular incinerators are prefabricated units that burn several tons of waste per hour. As the name implies, they require a minimum of field work. One or more units are supplied to form a facility which will satisfy the needs of the community. They are generally of relatively small capacity. Initially, the waste is placed on a tipping floor. Large, noncombustible materials are separated from the refuse. The combustible refuse is transferred to the combustor with a front-end loader or a conveyor system. Hydraulic rams feed the refuse into the primary combustion chamber of the modular unit. Refuse undergoes chemical decomposition via pyrolysis in starved-air conditions as the

Wasla Fuel

Air

:1 -I

Incinerator

I Ileal

Recovery Syslem

I

Quench System

L I

L

Effluent Control System

>

L

Emission Conlrol Medium

Emission Conlrol System

Li~)uld Solid Discharge

Quench Medium

FIGURE I. Generalized Waste Incineration System

_I ¸ -I

[

Float Recovery Medium

Clean Gas To Exhausl Slack

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waste is moved through the primary chamber. This is effected by either a traveling grate or multiple ram feeders. When greater capacities are needed, a number of modular incinerators may be installed.to operate in parallel. The primary chamber operates at approximately 30% to 60% of the theoretical air requirements. Turbulence is minimal as a result, which prevents carryover of ash and particulates into the downstream chamber. Gas exit temperatures of 1400-1700°F will cause volatilization of the organics contained in the solid refuse. A secondary combustion chamber, or afterburner, follows the primary chamber. The exhaust gases from the primary will consist of unburned hydrocarbons, carbon monoxide, hydrogen, nitrogen, acid gases, and water vapor. The secondary chamber operates at temperatures of 1800-2200°F with sufficient excess air (oxygen) to ensure complete oxidation of the organic gases and volatilized compounds. Combustion air is introduced at the crossover duct between the primary and secondary chambers. An auxiliary fossil fuel (natural gas or oil) burner will fire into the secondary to provide the mixing (turbulence) and temperature needed to achieve complete combustion; the three "t" 's of combustion are also required to optimize the process. Also critical to the proper combustion of the wastes is the excess air rate (the amount of oxygen in stack gases). Normal oxygen levels for refuse incineration are in the range of 6% to 10% dry volume. This ensures that the oxygen is available throughout the afterburner and minimizes zones or pockets lacking oxygen ("rich" zones), which generate PICs. Once the gases are combusted, they pass through either a waste heat r~covery system or a quench system. Temperatures of the combustion gases drop. It is critical, therefore, that the design of the combustion zones (primary and secondary) provides the needed mixing, heat transfer, and oxidation to combust the refuse to its inert products: carbon dioxide, water vapor, nitrogen, and oxygen. The primary chamber design provides the necessary time to ensure that the organic components are volatilized, reducing the carbon content of the ash discharge. In this process, the inert components and metals are concentrated as the volume and weight of refuse are reduced by 60% to 90%. Since ash and metals do not burn, they will exit at a much higher concentration than that of the incoming feed materials. The final disposal of the bottom ash has become a major concern of the opponents to MSW incineration. Heavy metals have the capability to migrate through landfills and contaminate groundwater.

Mass Burn Combustion Technology The total installed capacity (TPD) is highest with the mass-burning system of MSW (Table 1). "Mass burn" refers to the combustor's ability to incinerate MSWs with a minimum amount of preprocessing. Large objects such as refrigerators and engine blocks are removed prior to entry into the feed systems. These plants are field-erected with individual units capable of burning as much as 1000

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TPD. Several mass-bum units may be included in a single facility, providing an even larger capacity. In a mass-burn facility, a large receiving warehouse is constructed for trucktipping; a crane is used to transfer the MSW to a feed chute assembly. The MSW enters the combustion chamber of the mass-burn unit. Waterwall, refractorylined, and rotary kilns have been used as combustion chambers for mass-burn incinerators. Waterwall systems have furnace walls lined with tubes containing water which absorbs the heat of combustion. The waterwall design decreases the need for a refractory system since the walls are in direct contact with the combustion gases. The waste heat recovery boiler is an integral part of the system wall. In refractory systems, waste gases are directed to a boiler to generate steam. The combustion products are contained within the refractory walls to ensure that no cooling occurs until the combustion process is complete. Good refractory design is necessary to prevent spalling and chemical attack that may occur at the higher-temperature operation. The rotary kiln utilizes a refractory-lined steel shell which is mounted in steel tires to provide rotation. The axis of the kiln is sloped to allow materials to move from the inlet to the exit as the combustion process occurs. The rotation provides exposure of the materials to the hot refractory walls and to the flames created by the burning refuse. Ash in a dry or slag form is discharged at the back end of the kiln. Exhaust gases enter the secondary combustion chamber. The MSW is fed through the combustors by various rams, moving grates, or gravity chutes into the rotary kilns. Mass-burn systems employ a slightly different principle of combustion than the modular incinerators. Combustion air is supplied in excess of that required for theoretical (stoichiometric) combustion. These units are often referred to as "excess air system." Operating conditions that can be controlled with mass-burn facilities include MSW feed rate, oxygen concentration in the combustion zones, temperature within the combustion zones, auxiliary fuel firing rate, and refuse residence time. It is important to monitor the operating conditions of the incinerator because of the highly variable composition and heat content of MSW. The combustion gases formed in the primary combustion chamber pass through a secondary combustion chamber and into waste heat recovery and air pollution control systems. The ash generated in the primary chamber is removed by the ash collection system, which may be dry or utilize a water quench. Mass-burn units will generate a higher quantity of ash since little, if any, preprocessing of the MSW takes place. The volume reduction will not be as great because of the metal, glass, and inert material that are contained in the MSW. Temperatures in the primary chamber must be controlled to prevent slagging of the inert compounds (metals, salts, etc.). This creates problems in the rams, grates, or refractories in the combustion chamber zone as well as in the ash removal port. Because of some of the aforementioned problems, the Department of Energy

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has supported research on the potential benefits to mass-burn technology of burning a presorted MSW fuel. Comparative boiler efficiency tests at three massburn incineration sites utilizing as-received MSW and presorted MSW were reported in The Journal of the Air and Waste Management Association by Sommer et al. (April 1989). Test results indicate that waste presorting can benefit the mass-burn process. Flue gas and ash heavy metals were found to be significantly reduced; discarded automobile batteries contributed to the lead levels in the waste stream. Reduction in emissions of carbon monoxide, hydrogen chloride, hydrochloric acid, hydrogen fluoride, and nitrous oxide were reported, and increases in facility boiler efficiency and MSW disposal capacity were measured (Neulicht 1987; Sommer et al. 1989).

Refuse-Derived Fuel RDF technology processes MSW to create a relatively homogeneous product prior to combustion. Processing includes the removal of large nonprocessable objects, separation of recoverable materials such as metal and glass, shredding, size segregation, and/or compaction. RDF can be burned in a dedicated boiler or transported to a separate boiler to be burned in combination with fuel oil, coal, or wood. In some instances, a specially constructed boiler will burn 100% RDF. A large Northeast industrial facility combusts plant wastes that are shredded and conveyed into existing burner windboxes to serve as the primary fuel. No. 6 fuel oil serves as standby fuel. The shredded refuse operates with 100% excess air to provide the mixing and temperature control to prevent slagging and corrosion of the tubes. As a result, the capacity of the boiler is reduced by 40% to 50% compared to the capacity using fuel oil firing with 5% to 10% excess air. There are generally three types of processes for preparing RDF. They are as follows. FluffRDF systems reduce waste size in a series of shredders. Magnetic separation removes ferrous metals. An air classifier separates heavy materials, such as glass, from lighter, burnable materials such as paper and cloth. The final RDF is stored as fluff. Shelf time is limited to prevent the material from setting up and becoming a solid. D-RDF is a fuel created by densifying processed RDF into cubes and pellets. Transportation problems and costs are reduced, and shelf life is extended. Densifted RDF contains less water with resultant higher heating value. The costs of capital equipment and maintenance have been a drawback to rapid development of D-RDF. A method of producing easily handled RDF is taken from the pulping process in the paper industry. Water is added to the separated materials, creating a pulp which may be pressed into pellets. Moisture content up to 50% reduces the effective heat content of the fuel. Critical to the success of RDF systems is the quality of the fuel produced.

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Noncombustibles and impurities in the fuel mixture may cause problems with boiler operation. A high-quality RDF will be relatively free of metals, grit, glass, and other noncombustible material. The fuel is shredded to a small size (fluff) or densified (D-RDF) into cubes. Nine percent of the MSW incinerators within the United States use RDF technology; facilities that use RDF technology supply 23% of the total refuse combustion capacity. One half of these RDF facilities have combustion capacities greater than or equal to 1000 TPD of MSW, and all are designed with heat recovery (Table 1).

Energy Recovery Various designs for heat recovery have been provided in the MSW facilities. The majority are water tube boilers rather than fire tube ones. Some units utilize a large radiant furnace similar to coal- or oil-fired boilers. The gases are cooled from the combustion zone (secondary chamber) temperature of 1800-2400°F to temperatures in the range of 1200-1400°F. This cools any slag or molten salts carried over from the primary combustor prior to entering the convection bank of the boiler. The molten materials are cooled to a dry solid before contacting the colder tube surface. The dried particles are carried by the combustion gases through the boiler. Any materials that do collect on the radiant tubes are intermittently blown off by soot blowers or fracture due to normal expansion and contraction of the tubes or vibration created by gas flow. Some boiler designs are all-convection tube banks. To ensure that the molten particles are cooled, flue gases are recycled from the discharge of the pollutioncontrol system (400°F) into the high-temperature gases exiting the secondary combustion chamber (1800-2400°F). Gases are cooled to 1200-1400°F. This increases the size and cost of the heat recovery boiler. It also adds a hot recirculation fan to the system. Typical boiler auxiliaries are supplied with MSW heat recovery units. Close control, observation, and inspection are required due to variations in waste composition, operating temperatures, and excess air levels. Since MSW contains components which generate a wide variety of acid gases--hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.--the MSW boiler is more susceptible to corrosion. Close control of incineration temperatures and boiler metal temperatures is required. Experience has shown that carbon steel tubes will survive in this atmosphere. Corrosion occurs if metals rise to the hot acid gas corrosion level or the acid dewpoint level. If metals are maintained within this temperature range, long life can be achieved. However, due to upsets, lack of monitoring, and inadequate inspection and maintenance, boilers in MSW facilities can fail, leading to downtime. The system must be designed to allow for the necessary inspection, maintenance, and repair. This is why most

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systems are equipped with parallel (redundant) units providing for annual shutdowns. The end use of the steam must be carefully reviewed when waste heat recovery is considered. There must be a continuing need for the energy recovered. Intermittent operation of a boiler leads to rapid failure. The various uses for steam from MSW plants have been as follows: • industrial plant needs for process steam, • industrial park with a central steam distribution loop, and • cogeneration providing power to the grid of the local utility (Cross et al. 1987).

Environmental Concerns The emissions generated by the combustion of the MSW are complicated. They must be controlled in order to achieve compliance with state and federal air pollution control regulations. Emissions occur because of variations in the composition of MSW, incomplete combustion, the design and operation of the combustors, and the design, operation, and monitoring of the air pollution control devices. The emissions include those exiting the stack, considered air pollutants, and the residue created by the bottom ash from the primary chamber and scrubber collection system (wet or dry) (Table 2) (Rood 1988).

Air Pollutants The pollutants generated by the combustion of MSW are categorized as EPA criteria pollutants, acidic gases, heavy metals, and organic material. The criteria pollutants of interest include sulfur dioxide, nitrogen dioxide, carbon monoxide, lead, and particulate material. These pollutants are subject to EPA primary and/or secondary National Ambient Air Quality Standards (NAAQS). Uncontrolled emissions of sulfur dioxide and nitrogen dioxide cause the formation of acid due to cooling, and this combines with water vapor and air to form sulfuric acid and nitric acid (acid rain). Air pollution controls are necessary to minimize these emissions when the waste streams contain high levels of sulfur and nitrogen. Carbon monoxide results when all of the carbon in the waste is not oxidized to carbon dioxide. It is very difficult to eliminate all the carbon monoxide. Natural gas fuel, which is considered one of the easiest fuels to burn, generates carbon monoxide at levels from 5 to 500 ppm, depending on the type of combustor and combustion zone conditions. Carbon monoxide and carbon dioxide stack monitors are an excellent means of determining the com-

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TABLE 2. Pollutants Generated by the Combustion of Municipal Solid Waste Air pollutants

Solid waste

Criteria pollutants Sulfur dioxide Nitrogen dioxide Carbon monoxide Particulate material Lead Acidic gases Hydrogen chloride Hydrogen fluoride Heavy metals Arsenic Cadmium Mercury Chromium Nickel Lead Organic material Polychlorinated dibenzo-p-dioxins (PCDD) Polychlorinated dibenzofurans (PCDF) Polynuclear aromatic hydrocarbons

Organic material Heavy metals

bustion efficiency of a waste incinerator. This can be determined by the following relationship:

Combustion efficiency

-

CO2 + CO × 100.

CO 2

Efficiency levels of 99.9% or greater provide assurance that the organic constituents in the waste have been oxidized and will not create hydrocarbon emissions. Lead exists in the flue gas as a result of its vaporization in MSW during combustion. Lead condenses from the flue gases and forms particulate material downstream of the primary/secondary chambers. Separating the sources of lead (most importantly, batteries) from the waste stream aids in reducing emissions levels. Particulate material that forms as a result of MSW combustion is categorized as bottom ash and fly ash. Bottom ash is the residue that exits the primary and secondary chamber. Fly ash is formed by the entrainment of particles in the flue gas, by gas phase compounds that condense at lower temperatures beyond the waste heat recovery and quench systems, and by the condensation of volatile inorganic compounds that vaporize in the combustion zones and recondense in

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the lower temperature zones. The resultant particulate distribution from MSW incinerator types varies in mass concentration, particle size distribution, and chemical composition.

Acid Gases The primary acid gases generated by MSW incinerators are hydrogen chloride and hydrogen fluoride. Wastes contain both chlorine and fluorine. The concentrations of chlorine and fluorine in typical MSW are approximately 0.5% and 0.01% by mass, respectively. Most of the chlorine in MSW is from plastics such as polyvinyl chloride. Free halogens (chlorine or fluorine) are generated when the combustion process is not operated properly. Many systems use very high excess air and low temperatures. Both of these combine to reduce the conversion of the chlorine in the waste to hydrogen chloride. Chlorine must be hydrolyzed to hydrochloric acid, and in order to do this, the temperature must be in the range of 1600-1800°F and above, the oxygen level must be in the range of 2% to 6% and there must be sufficient hydrogen to react with the chlorine. As temperatures are reduced and as oxygen increases, more chlorine is formed, decreasing the amount of hydrochloric acid that is generated. This requires careful review of the scrubber system required. Hydrogen chloride is scrubbed very effectively by water. However, chlorine requires a caustic or lime solution for neutralization to a salt. This adds to the cost of operation of the system. These halogens are much more aggressive in their corrosive action. Emissions of metallic compounds from incinerators are of interest because of the health effects associated with trace level concentrations of heavy metals. Heavy metals exist in MSW before combustion. They are conserved during the combustion process and are emitted in the flue gas or as bottom ash. Numerous organic compounds may be emitted from MSW combustors. These compounds, if present, are due to incomplete combustion in the primary and secondary zones of the incinerator. Groups of organic compounds that are of interest because of their health effects are polychlorinated dibenzo-p-dioxins (PCDD), tetrachlorodibenzo-p-dioxins (TCDD), polychlorinated dibenzo-furans (PCDF), tetrachlorodibenzofurans (TCDF), polychlorinated biphenyls (PCB), chlorobenzenes (CB), chlorophenols (CP), and polycyclic aromatic hydrocarbons (PAH). Numerous isomers of these groups of organic compounds can exist and have different levels of toxicity. Mechanisms that produce chlorinated organic compounds in MSW combustors include the penetration of already existing chlorinated organic compounds into the combustor, the production of structurally similar organic precursor during combustion that react to form chlorinated organic compounds, and catalyze reactions of chlorine with organic compounds on the surface of ash.

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JOSEPH J. SANTOLERI

Solid Residue The residue produced by the combustion of MSW is 20% to 40% of the initial volume of unprocessed MSW. This solid waste is in the form of fly ash and bottom ash. Fly ash emissions have already been discussed. Bottom ash emissions are important to consider because of the concentrations of toxic organic compounds and heavy metals that are present in the ash, which have the capability to migrate through landfills and contaminate groundwater. Organic compounds may be present in the bottom ash due to improper combustion in the primary zones or to an overloading condition allowing less exposure of the refuse to the temperatures and oxidizing conditions in the combustion zones. Hea~(y metals have been detected in MSW incinerator bottom ash. This is important with respect to the toxicity of the bottom ash and the transfer of these metals from the ash to landfill leachate. If this is considered hazardous because of the concentration of heavy metals, the final disposal of this bottom ash must take place in a hazardous waste landfill.

Air Pollution Control Technology There are numerous approaches to controlling air emissions from MSW incinerators. One must first look at the design of the incinerator proper to determine if this is the major cause of high air emissions. Many systems operate with a draft created by an induced draft fan at the stack. A high draft condition creates velocity in the combustion zones which causes carryover of particulates in the flue gas stream. Sealing the openings into the primary and secondary zones aids in reducing the total air-in leakage as well as the total volume of flue gases. The combination of an increased volume of flue gases causing high velocities within the incinerator will carry particulates into the downstream systems. Improving combustion to reduce the generation of nitrogen oxides, hydrocarbons, and carbon monoxide also lessens the requirements of the air pollution control system. Reducing the total amount of excess air of the combustion also aids in reducing the amount of air pollution control equipment needed. In any case, the prime purpose of air pollution controls is minimization of the two major pollutants: acidic gases and particulates. Control systems may be classified as wet, dry, dry-wet, or dry-dry. Their selection depends on the need for both acid gas and particulate removal, Most systems are chosen on the basis of the latest regulations developed by the individual states or localities. As concern grows regarding the risks associated with metal emissions, the regulations for particulate emissions become more stringent. The standard of 0.08 grains per dry standard cubic foot is being lowered to levels of 0.015 to 0.02; the selection of the proper pollution control system is dependent on the level of control required by the regulations.

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Wet systems include the venturi scrubber and the packed tower absorber. A wet electrostatic precipitator may also be used as the particulate control device. This equipment is used for acid gas control. Dry systems use either a baghouse filter or an electrostatic precipitator. These are most effective for particulate control. In some cases, a combination of wet and dry systems is used to provide both acid gas and particulate removal. Residues are generated by these scrubber systems and must also be treated. In a wet system, the scrubber discharge is a salt solution. This may be treated on site in a solar lagoon or sent to a wastewater treatment system. The residue from the dry system is a dry powder collected from the baghouse or the electrostatic precipitator.

Wet Scrubber Gaseous pollutants are water-soluble and can be removed from flue gas via absorption with an aqueous wet scrubber. These acid gases react with alkaline liquid scrubber medium, which thus removes the pollutants from the gas phase. Flue gases are typically saturated with water vapor as the gas is emitted from the wet scrubber. The typical alkaline absorbents used to enhance a wet scrubber's ability to react with sulfur dioxide, hydrogen chloride, and hydrogen fluoride are sodium hydroxide and sodium carbonate. The collection efficiency for these acid gases in a wet system is high relative to that using other control devices. Typically, 99% of hydrochloric acid and hydrogen fluoride, and over 90% of the sulfur dioxide, will be removed. The major disadvantage of this type of scrubber is the production of liquid effluent that must be processed before disposal. If the scrubber is also used as a particulate removal device, the pressure drops across the scrubber, which creates a need for high-energy fans; this is another disadvantage.

Spray Dryer Acid gases and sulfur dioxide may also be neutralized in a spray dryer. An aqueous slurry of lime is injected into a large chamber to contact the acid gases in the flue gas. The water in the lime slurry is evaporated, leaving a solid which is a combination of the necessary excess lime and the calcium salt compound, either calcium chloride or calcium sulphate. Beyond the spray dryer, the gases and solids exit at a temperature of 400-500°F. A baghouse filter or an electrostatic precipitator is used to remove the particulate matter from the flue gas. The particles come from the refuse combustion as well as from the solids created by the acid gas absorption. The major advantage of the spray dryer-baghouse or spray dryer-electrostatic precipitator is the ability to remove both the acidic gases as well as the particulate. This is done at low energy cost. The pressure drop requirements are low. The final residue is a dry product which must be disposed of in an appropriate landfill.

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The disadvantages of these methods are the high capital cost associated with combined systems and the maintenance costs of baghouse operation. The final particulate emissions resulting from this type of device are well within the level established by the best available control technology. A well-designed system will meet the most stringent state codes.

Dry-Dry Scrubber In systems where no liquid is used, the acidic gases and particulates are controlled by a combined dry scrubber followed by a baghouse or precipitator. The alkaline material which would be lime, or calcium hydroxide, is injected into the flue gas downstream of the secondary combustion chamber. The absorbent reacts with the acidic gases before the particulate material is separated and removed from the gas stream with a fabric filter or electrostatic precipitator. The advantage of this system is that the capital cost is less than that of a spray dryer; however, the collection efficiency of the acidic gases is far less than that in either a wet scrubber or spray dryer.

Environmental Protection Regulations Control of air pollutants is regulated by both federal and state environmental protection agencies. Various regulations have been promulgated singe the passage of the Clean Air Act of 1963. Federal regulations are contained in Title 40, Chapter I, Subchapter C, Air Programs, Parts 50 to 81 of the Code of the Federal Register. The applicable federal regulations are listed below: • 40 CFR 50: National Primary and Secondary Ambient Air Quality Standards (NAAQS), • 40CFR52.21: Prevention of Significant Deterioration of Air Quality (PSD), • 40 CRF 60: Standards of Performance for New Stationary Sources (NSPS), and • 40 CFR 61: National Emissions Standards for Hazardous Air Pollutants (NESHAP). Title II of the Act addresses the issue of air pollution at the state level and is contained in Sections 8-10. In particular, Section 9.4 ensures that emissions from new municipal waste incineration facilities which burn a total of 25 tons or more of municipal waste per day are adequately controlled. The Act also stipulates that an MSW plant burning a total of 25 tons or more is subject to emissions limits and operating standards based on the application of Best Available Control Technology (BACT), as determined by the Agency. BACT is an emission limitation derived from the maximum degree of pollutant reduction.

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On January 1, 1970, the Congress passed the National Environmental Policy Act (NEPA). This law was intended to achieve environmental protection, as were the various laws that followed. In 1976, the Resource Conservation and Recovery Act (RCRA) was promulgated. Federal regulations and guidelines pertaining to the handling of solid wastes are contained in Title 40, Chapter 1, Subchapter I--Solid Wastes, Parts 240 to 280 of the CFR. Both federal and state regulations contain provisions excluding solid waste produced by MSW combustion facilities from the Hazardous Regulatory System scheme contained in the RCRA. These provisions are found in Title 40 CFR 261(b)(1) and in Title 35 IAC 721.104(b)(1). They exclude "household waste that has been collected, transported, stored, treated, disposed, recovered (e.g., RDF), or reused" from the definition of hazardous waste. Unless an owner/operator is willing and able to comply with hazardous waste regulations, it is important that the MSW combustion facility be designed to accept only appropriate types of wastes. Even though the solid waste generated by MSW combustion facilities is not defined as a hazardous waste, it is classified as a special waste in many states. As such, the disposa! of the solid waste requires special handling. There are a variety of potential wastewater discharges from a MSW combustion facility, depending on its design. Potential sources of wastewater are: • ash transport water, • screen backwash, • equipment and facility washwater, • site drainage, and • sanitary water. Control of wastewater discharges is regulated by both the federal and state environmental protection agencies. Various regulations have been promulgated as a result of the passage of the Federal Water Pollution Control Act (Clean Water Act) of 1972. Federal regulations and guidelines pertaining to the control of wastewater discharges are contained in Title 40, Chapter I, Subchapter D - Water Programs, Parts 104 to 147 of the CFR. There are a few ancillary environmental impacts which must be reviewed and mitigated when necessary. The two major issues are noise and odor. Control of noise pollution is regulated by the federal and state environmental protection agencies. Regulation have resulted from the Noise Control Act of 1972. Concerns about odor are similar in nature to those about noise. Odors have a potential to cause enough nuisance to require regulation. The odor regulations are part of the air pollution regulations. Operating practices are typically contained in the permits to control odors. Complying with MSW storage time requirements and following good housekeeping practices will provide adequate control of odor emissions.

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Conclusion A properly designed and operated incinerator is an effective way to dispose of the household trash and most of the waste products of small industries located within a city. All of the individuals responsible for authorizing the construction of incinerators must understand that combustion is a very complex chemical process involving pyrolysis, reduction, volatilization, oxidation, recombination of chemical compounds, and many other chemical processes. With the composition of the materials being burned having varying heating values, densities, and physical makeups, the possibilities of poor performance become great indeed. In a properly designed incinerator, any known toxic material can be converted into harmless combustion products. This depends on the unit being maintained and operated properly. Toxic materials such as arsenic, lead, and mercury cannot be made harmless by burning. This applies as well to other inorganic material such as glass and metal. These will not burn, but will cause problems in the combustion zones as well as in the heat recovery and air pollution system. These materials must be segregated under proper supervision. With a combination of recycling and proper preprocessing of the MSW, incineration can be made a cheap and safe way to dispose of more than 98% of all the waste generated by a city.

References 1. Cross, F., O'Leary, P., and Walsh, P. January 1987. Waste Incineration and Energy Recovery. Waste Age. 2. Greene, S, June 1987. Municipal Waste Combustion Study: Report to Congress, EPA/ 530 SW 87 021A. 3. Hasselriis, F. June 1985. Relationship Between Municipal Refuse Combustion Conditions and Trace Organic Emissions. APCA Meeting, Detroit, Michigan. 4. Neulicht, R. June 1987. Results of the Combustion and Emissions Research Project at the Vicon Incinerator Facility in Pittsfield, Massachusetts. NYSERDA. 5. Pearce, A. Waste Management Option. Pennsylvania Environmental Council, Inc. 6. Rood, M. J. September 1988. Technological and Economic Evaluation of Municipal Solid Waste Incineration. University of Illinois at Chicago. 7. Sommer, E. J., et al. April 1989. Mass Burn Incineration with a Presorted MSW Fuel. Journal of the Air Pollution Control Association. 8. Wingate, P. J. November 28, 1987. Trash Incineration Can Bc Safe and Cheap. Philadelphia Inquirer.