Incineration Technologies for Managing Solid Waste Modern particularly waste-totechnologyy can be a viable option for solid waste management. by Bruce Bawkon Most solid waste management planning recognizes incineration as a viable alternative, although the public perception of waste incinerators as a major source of air pollution and hazardous emissions has made it a less popular alternative than the realities justify. The technologies available and the emissions standards mandated by the Clean Air Act Amendments of 1990, however, mitigate the risks to public health and make incineration an alternative worth exploring. Incineration can reduce solid waste to 85 percent to 90 percent of the incoming volume, or 65 percent to 80 percent of the incoming weight. With some modification, a waste incinerator can be designed to recover energy in the form of steam, hot water or electricity. This latter type is known as a resource recovery or waste-to-energy facility. Most newer waste incinerators are the waste-to-energy type. Incineration facilities can be very costly to develop, but once operational, waste disposal costs tend to remain fairly stable. Also, life-cycle costs can be significantly reduced through the recovery and use or sale of thermal and/or electric energy. Although all incinerators now require pollution control devices, there remain issues and concerns about air quality, ash disposal and environmental impacts. Technology Regardless of the specific technologies used, all incinerator types have common features. Each has a receiving area where waste is deposited and mechanical processing systems remove non-combustible material and provide for a level of material recovery before combustion. Waste is then fed into a combustion chamber. The combustion gases pass through heat exchangers and pollution control equipment before being discharged through a stack into the atmosphere. The most common and proven methods used to combust solid waste are mass burn, refuse derived fuel (RDF) and starved air modular combustors. 96 POLLUTION ENGINEERING SEPTEMBER I99 1
In a waste-to-energy facility, combustion heat is used to produce steam, hot water or electricity for use in the plant or for sale to outside energy markets. Bottom ash or residue, approximately 10 percent of the original waste volume, is removed from the combustion chamber. Mass burn combustion In this country, most large incinerators are mass bum facilities. Refuse is burned in the same form as it is delivered with the exception that some large metal items are removed from,the waste stream. This technology has been used sine the 1970s and has experienced the greatest technical and financial operating success. Typical unit size is in the range of 400 to 1000 tons per day (TPD) with some facilities as large as 3000 TPD. The air required for combustion is supplied by air ducts below the directcombustion grate system, as well as by secondary air injectors above the grate system around the fire zone. The underfire primary air system is usually sectioned so that a series of supply points and quantities of underfire air (the primary combustion air) can be sectionally adjusted to aid in combustion control. The overfire air jets provide oxygen to complete the combustion of gases expelled from the primary combustion area. Air jets also allow, in conjunction with the furnace and boiler size, the proper time, temperature and turbulence necessary for complete combustion of the gas stream. Two arrangements can be used for steam generation. In a waterwall unit, shown in figure 1, the furnace walls are lined with boiler water tubes in which water is heated to the boiling point by the combustion process. The other arrangement involves a dedicated boiler where steam is generated by hot combustion gases after leaving the furnace. Key advantages of mass bum facilities relate to their well established and proven technology, demonstrated longterm reliability, good thermal efficiency and minimal refuse processing requirements. Disadvantages relate to the long lead times required to design and build plants and their significant capital construction cost. Starved air (modular) combustion Starved air modular incinerators are relatively small combustion and heat recovery systems typically ranging in size from five to 100 TPD capacity. See Figure 2. Multiple units can be installed when greater capacity is needed. Starved air incineration of solid waste is achieved in either two- or three-stage systems. Partial combustion occurs in the primary chamber producing a gas with a low energy content. A starved air environment (less than stoi- chiometric gas conditions) is created in the primary chamber restricting the amount of air fed into the chamber. Combustible gases produced in the primary chamber are then completely burned in the secondary chamber. A waste heat boiler typically is used to recover the energy of combustion. Once waste enters the primary chamber of a typical controlled-air system, a three-stage reaction occurs: Drying Gasification. Burnout. A two-chamber configuration supports the three-stage reaction. Combustion temperature typically ranges from 1500"F.to 1800 F. Two-stage controlled air combustion technology limits air pollutant emissions when burning solid waste. The relatively low combustion temperature in the primary chamber aids' in pollution control by minimizing the vaporization of the metallic components of the waste, as well as slagging of the glass components. Gases generated in the primary chamber are transferred to the secondary chamber for high-temperature burning to allow for more complete combustion. Figure 1. The furnace walls of a waterwall unit are lined with boiler water tubes. SEPTEMBER 199 I POLLUTION ENGINEERING 97
Other key advantages relate to fast construction time, relatively low construction cost and flexibility. Disadvantages are limited size, lower thermal efficiency, higher maintenance costs and shorter equipment life. Refuse derived fuel combustion Refuse derived fuel (RDF) is a system whereby refuse is used as fuel for a plant producing energy. This reduces or eliminates the plant s need for other fuels. RDF systems are different from mass bum technologies because they are designed to separate solid waste into combustible and non-combustible factions. The waste stream is sorted and processed, with non-combustibles, such as metal and glass, removed to be recycled or landfilled. The combustible portion of the waste has a higher energy content and is a more efficient fuel. There are two RDF technologies currently available in this country: RDF combusted in grate boilers, where RDF is burned on the surface of a grate. RDF combusted in fluidized-bed boilers, where RDF is burned in suspension by an upward flow of combustion air in a bed of sand or limestone. reducing a plant s need for other fuel sources. An RDF facility is actually two facilities: a preprocessing or fuel plant; and a combustion and energy recovery plant. These facilities do not have to be located at the same site. A processing facility removes non-combustibles and prepares a fuel by shredding the combustible fraction. Additional processing of the fuel, including adding fuel amendments and pelletizing the RDF fuel, would be performed only if required by the combustion facility. An RDF facility can be designed to bum RDF exclusively at a dedicated waste combustion facility or combined with other fuels to co-fire it with sludge, wood chips, peat or coal. Typically, an RDF plant produces steam or electricity. RDF with dedicated boilers involves both production and combustion of RDF fuel. This combined function requires the construction of a dedicated boiler to burn the RDF. RDF with fluidized-bed combustion units burn finely processed refuse in a turbulent bed. The bed contains a fluidizing medium of inert particles kept in a state of agitation and fluidity by a high velocity flow of combustion air introduced into the bottom of the combustor through a series of nozzles. The boiler section is located above the com- bustion process. The bed media may be sand or limestone. Limestone beds also help decrease acid gas emissions. There are two types of fluidized-bed combustors used with RDF. The first, bubbling-bed, suspends the RDF along with ash and inert material such as sand, with an upward flow of combustion air. The combustion process and bed media are contained in a combustion vessel usually designed to recover energy through a system of boiler tubes. The bed material, ash and other inert material remain in the combustor. Hot gases are ducted to a boiler and air pollution control system. The second type, circulating fluidized-bed, shown in Figure 3, combusts the RDF in a medium of ash and inert material with an upward flow of combustion air that forces some of the bed material to pass out of the primary combustion into a cyclone separator. The cyclone separates the inert particles from the hot exhaust gas. The particles are returned to the combustion chamber to be added to the bed material. The hot gases are ducted to the boiler Figure 2. Starved air modular incinerators involve either two- or threestage systems. 98 POLLUTION ENGINEERING SEPTEMBER 1991
Figure 3. Circulating fluidized-bed incincerators combust refuse derived fuel in a medium of ash and inert material. land air pollution control system. Fluidized-bed combustion units require a high quality RDF product with nearly all ferrous materials, glass and aluminum removed. Several units are operating suceessfully in Europe with loose and pelletized RDF, as well as other fuels. Bubbling-bed combustion offers several advantages over conventional mass 1 burn combustion processes. A more homogeneous fuel is burned and the fuel is more completely mixed in the combustion chamber for more efficient!combustion. Addition of lime to the bed media can promote desulfurization. The combustion process is more stable as a tremendous amount of heat is absorbed in the bed media and can maintain a constant combustion temperature as variations in the fuel occur. Therefore, a more steady control of nitrogen oxide emissions can be maintained. Combustors represent a major departure from the waste-to-energy technologies in that the circulating fluidized-bed allows for several advantages over the more common mass burn and bubblingbed combustors. The circulating flu- idized-bed can accommodate greater fluctuations in fuel heat content by varying the density of the bed. The turbulent mixing promotes an improved combustion efficiency and can impact reduced dioxin emissions. Boiler eficiency can approach 80 percent dm to low excess air requirements, which ape greater than that typically expected from mass bum and bubbling-bed combustors. Turn down capabilities are significantly greater, with operating capacity ranging from 25 percent to 100 percent of capacity. Advantages of RDF systems relate to more homogeneous fuel stock, higher SEPTEMB~R 1991 POLLUTION ENGINEERING 99
combustion heat to produce thermal and/or electric energy. sure), or extraction turbines. The type of turbine selected will be dedicated by the requirements of power or thermal energy markets. In tons of actual energy production, each ton of solid waste can produce approximately 5500 pounds of exportable steam or 550 kilowatt-hours of exportable electricity. The energy equivalence of solid waste and fossil fuels is shown in Table 1. Particulate material (PM) Lead (Pb) Acidic acids Hydrogen chloride Hydrogen fluoride Chromium Nickel Lead heat content, ability to be burned as supplemental fuel and compatibility with recyclable material recovery. Key disadvantages deal with extensive and expensive equipment needs for processing and lack of demonstrated long-term reliability. Disadvantages of the fluidized-bed combustors include a requirement for a refined, preprocessed fuel, with a controlled size for the RDF. This includes a preprocessing facility that efficiently removes glass to prevent slagging in the bed, and control and monitoring of erosion in the combustor walls caused by the recirculation of the sand bed material. This can be accomplished through design of the duct work and periodic inspection. Finally, the most important disadvantage is the limited operating experience in the U.S., as compared to mass burn and RDF combustion on a grate. Historically, facilities using fluidizedbed combustion have been more expensive than conventional boiler facilities. Resource recovery and cogeneration Most modern waste incineration plants are designed to use combustion heat to produce thermal and/or electric energy. Resource recovery facilities vary with respect to the energy products produced and the required energy generation systems. Organic material Polychlorinated dibenzo-pdioxins (PCDD) Polychlorinated dibenzofurans (PCDF) Polynuclear aromatic hydrocarbons (PNAH) Thermal energy generated from solid waste in the form of steam, hot water or hot air, may be used for space heating and cooling, process heating and for the production of power, including electricity. The most common form of thermal energy from waste is steam. Steam temperature requirements generally range from 250 F to 1000 F. Steam produced from solid waste is identical to, and indistinguishable from, steam produced by other fuels. Electrical energy is produced at a waste-to-energy facility by passing steam produced by the incinerator's boiler system through a turbine generator. Within the turbine, entering steam is expanded through nozzles, transforming the steam's energy to velocity. This expanded steam jet then strikes blades on a turbine wheel, forcing the wheel to revolve. This mechanical power is used to turn a generator, producing electricity. Electrical energy can be used in the facility, sold to nearby industries, sold to the local utility or transmitted to other utilities or customers. The design of the turbine system must match the types and specifications of the energy products to be sold, including electricity and steam. Generally, turbines can be classified as condensing, non-condensing (back pres- Ash and residue disposal Incineration still leaves a residue of ash and inert material that comprises 10 to 15 percent of incoming volume or 20 to 35 percent of incoming weight. This residue must be landfilled. Due to concerns about content and hazard potential, incinerator ash is classified as a special waste in most states and cannot be co-disposed with other solid waste at a conventional landfill. See Table 2. The safest current disposal mode is a lined monofill (only incinerator ash is disposed in the landfill cell, and is not mixed with other refuse) equipped with leachate collection and monitoring systems. Impact of recycling on waste-to-energy A popular myth is that recycling undermines waste-to-energy facilities. Both field tests and quantitative derivations show that combustion plants benefit from the removal of recyclable materials from the waste stream prior to incineration. Recycling increases the heat content of municipal solid waste while reducing air pollution. Removal of batteries from the waste stream is particularly beneficial because it can significantly reduce heavy metal emissions. A recent study for the University of Illinois shows that if 50 percent of the paper and all plastic, metal and glass were removed from the waste srream, its heat value would increase from 5500 Btu/lb to 6148 Btu/lb. Similar conclusions were reached from a series of tests run by National Recovery Technologies Inc. (NRT). In addition to increasing heat value, recycling increased boiler efficiency and reduced ash quantities. Also, it resulted in the reduction of carbon monoxide, hydrocarbon, heavy metal and acid gas emissions. Air emissions control The Clean Air Act Amendments of 1990 establish area classifications based 100 POLLUTION ENGINEERING SEPTEMBER 1991
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The Clean Air Act Amendments require EPA to establish waste incinerator standards. i. I Target I i on ambient air quality, and identify the performance and criteria required for threshold controls for major sources. The Act calls for the Environmental Protection Agency (EPA) to establish by Nov. 15, 1991, waste incinerator standards that provide maximum reductions in air emissions, taking into account costs, health and environmental impacts and energy requirements. Standards for new sources must be no less stringent than those achieved in practice by the best controlled similar unit or Best Available Control Technology as determined by EPA. Combus- 102 POLLUTION ENGINEERING SEPTEMBER 1991 tion practices recommended by EPA are listed in Table 3. Incinerator standards must include numerical limits for particulate matter, opacity, sulfur dioxide, hydrogen chloride, oxides of nitrogen, carbon monoxide, lead, cadmium, mercury, and dioxins and dibenzofurans. For other pollutants, EPA may identify numerical limits or require monitoring of surrogate substances, parameters or residence times. The Act requires EPA to establish monitoring and operation guidelines for incinerators. EPA must issue a model state program for operator training. Bruce Bawkon is senior environment scientist with Envirodyne Engineers Inc., Chicago, Ill. Reader Interest Review Please circle the appropriate number on the Reader Service Card to indicate your level of interest in this article. High 471 Medium 472 Low 473