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2 Coal Combustion Residues T his chapter provides an overview of the basics of CCRs, including their production, characteristics, and disposal and use options (see Figure 2.1). It then examines how CCRs are generated, including the combustion technologies used and the pollution control equipment utilized, which contribute to the type, quantity, and characteristics of CCRs generated. Finally, it considers the possible options for CCR management, which include disposal in landfills or surface impoundments, use of the CCR as a component of an engineered product, or use or disposal in a coal mine. Although placement of CCRs in coal mines is the focus of this report, a brief presentation of the alternatives to mine placement is included in this report to illustrate the available CCR management alternatives. TYPES OF COAL COMBUSTION RESIDUES Coal does not completely convert to a gas upon combustion; therefore, all coal-fired boilers produce solid materials in the form of CCRs. The amount of CCRs produced by utilities has increased as the demand for energy in the United States has grown. A variety of solid materials may be generated from the combustion of coal, including fly ash, bottom ash, boiler slag, and residues from air pollution control technologies, such as flue gas desulfurization (FGD) materials (Figure 2.1). Fly ash represents a major component (62 percent) of CCRs, followed by FGD mate- rial (19 percent), and bottom ash and boiler slag (18 percent) (USDOE, EIA, 2003b). The major types of CCRs are described in detail below. An overview of common coal combustion technologies is provided in Sidebar 2.1. 27
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COAL COMBUSTION RESIDUES 29 SIDEBAR 2.1 Technologies for Coal Combustion Many industrial and utility boilers use coal as the primary source of fuel. The boiler is the unit that encloses the furnace, where the fuel is combusted. When coal is fed into the furnace, the heat generated is used to heat water circulating in tubes surrounding the furnace. As the water heats, it turns to steam. The steam is cap- tured and used within the facility to turn the blades of an electricity generator or a compressor for refrigeration, to heat a process or a building, or for many other uses. There are three primary coal combustion technologies used in boilers: 1. Grate firing, where coal is combusted while residing on a grate within the furnace; 2. Suspension firing (e.g., pulverized coal (PC) firing), where coal is crushed to a fine powder prior to entering the boiler's furnace and subsequently combusted in suspension with the combustion air; and 3. Fluidized bed combustion (FBC), where coal is combusted in a sus- pension with a solid sorbent (usually limestone) or an inert material such as sand (Davis, 2000). Utility boilers generate steam to drive turbine generators for the production of electricity. Utility boilers are commonly suspension-fired boilers, such as pulver- ized-coal boilers. The coal-refuse-fired facilities generally use FBC technology. Fly Ash Fly ash consists of fine particles carried out of the boiler by the flue gases. Most fly ash is captured by dust-collecting systems before it escapes the boiler's stack. Common particulate matter control devices include mechanical collectors, electrostatic precipitators, and fabric filters (Sidebar 2.2). Other constituents mobilized in the coal combustion process may be associated with fly ash. For example, mercury tends to adsorb to fly ash unless another material, such as activated carbon, is added to the flue gas to capture the mercury preferentially. Bottom Ash and Boiler Slag Bottom ash typically consists of large ash particles that accumulate at the bottom of the boiler. Boiler slag is a molten inorganic material that is collected at the bottom of the boiler and discharged into a water-filled pit, where it is cooled with water (quenched) and removed as glassy particles resembling sand. The form of the ash or slag produced is dependent on the type of furnace and the fusion temperature (or melting point) of the ash generated from the coal. Some pulverized coal (PC) furnaces (see Sidebar 2.1) fire coals of high ash-fusion
30 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 2.2 Particulate Matter Control Devices There are three common particulate matter control devices used with coal-fired furnaces, described below. Mechanical Collectors, most commonly known as cyclones or multicyclones, force a cyclonic flow of the exit gas. This flow causes ash particles to be thrown against the walls of the collector and to drop out of the gas. Cyclones are most effective for larger particles; collection efficiency drops well below 90 percent for the smallest particles. Electrostatic Precipitators (ESPs) are the most common particulate control technology used by coal-fired utilities. An ESP generates a high-intensity electrical field that causes ash particles to acquire an electrical charge and migrate to an oppositely charged collection surface. For typical coal-fired utilities, this process results in a collection efficiency of greater than 99 percent. Fabric Filters, also known as baghouses, capture ash as the exit gas passes through a series of porous filter bags. Baghouses have an efficiency of greater than 99 percent. SOURCE: USEPA, 1999b. temperatures and use a dry ash removal technique (Davis, 2000). Others fire coal with a low ash-fusion temperature causing much of the ash to form a liquid slag, which is then drained from the bottom. Boiler slag is a CCR that is expected to be produced in diminished quantities in the future because of the retirement of the older boilers that produce liquid slag in significant quantities. Residues from Air Pollution Control Technologies Several air pollution control regulations have been enacted to improve air quality in the United States. To implement these regulations, many coal-fired plants use pollution control devices, in addition to particulate matter controls, which can generate their own type of residue or change the characteristics of existing residues. The characteristics of the residue generated are dependent on the type of pollution control equipment installed, which varies widely between plants (and even between units at the same plant) depending on space constraints, compatibility with existing equipment, and regulatory performance requirements. Sulfur Dioxide Emissions Control Technology Sulfur dioxide (SO2) emissions controls are the most common devices added to augment the control of particulate matter. SO2 is a component of fine airborne particulate matter in the form of aerosols and is the primary component of acid
COAL COMBUSTION RESIDUES 31 SIDEBAR 2.3 Desulfurization Technologies Post-combustion desulfurization technologies (or SO2 scrubbers) are catego- rized as recovery systems and non-recovery systems. Recovery systems are those that produce FGD wastes that are suitable for use in engineered products, such as wallboard. Non-recovery systems produce FGD waste that must be disposed of. Non-recovery systems are further classified as wet and dry systems. Wet systems scrub and saturate flue gas with a slurry of water and a sorbent (usually lime or limestone) that reacts to remove sulfur from the gas in the form of a sludge. Dry systems typically contact flue gas with a sorbent slurry in a spray dryer without saturating the gas with water. The dry reaction product is then collected along with fly ash in a fabric filter or ESP. Wet systems are more effective at removing sulfur dioxide and, therefore, are used by a larger proportion of generators. However, because of their use of liquids, wet systems produce more waste than do dry systems (USEPA, 1999b). Desufurization can also be accomplished within the coal combustion process itself. In systems utilizing FBC technology, desulfurization can be accomplished by co-firing the coal with limestone. The limestone then serves a dual purpose: a bed material for the furnace and an SO2 sorbent (Woodruff et al., 1998). rain. Units that remove SO2 emissions from flue gas are referred to as flue gas desulfurization (FGD) systems (see Sidebar 2.3). Since the implementation of the Clean Air Act's Acid Rain Program (40 CFR 72-75) in 1990, FGD technologies have added a significant non-ash component to CCRs (Figure 2.2). In 2005, the Environmental Protection Agency enacted the Clean Air Interstate Rule (70 FR 25162) establishing a new emission reduction program for SO2 and NOx (nitro- gen oxide) generating reductions of these pollutants in 28 states and the District of Columbia. The Clean Air Interstate Rule incorporates and goes beyond the existing Clean Air Act Acid Rain Program and may lead to more FGD materials being produced or to a new material produced by the introduction of new tech- nologies. Nitrogen Oxide Emissions Control Technology There are several types of NOx emissions control technologies. The simplest is called a low NOx burner, which reduces the formation of NOx by controlling the environment in which the coal combusts (flame temperature and chemical environment). Selective Catalytic Reduction and Selective Non-Catalytic Reduc- tion are post-combustion control technologies used for NOx emission reduction. These processes utilize ammonia reacted with the flue gas to convert it to elemen- tal nitrogen and water (CURC, 2005). These processes may increase the ammo- nia content of CCRs making them less marketable (Butalia and Wolfe, 2000).
32 MANAGING COAL COMBUSTION RESIDUES IN MINES 140 (Millions)70 Tons 0 1966 1970 1980 1990 2003 Fly Ash Bottom Ash Boiler Slag Flue Gas Desulfurization FIGURE 2.2 Generation of fly ash, bottom ash, boiler slag, and FGD by utilities (1966- 2003). NOTE: This figure does not include the approximately 5 million short tons of CCR produced by independent power producers firing coal refuse. SOURCE: American Coal Ash Association, Aurora, CO, written communication, Octo- ber 2005. Courtesy of the American Coal Ash Association. Mercury Emissions Controls Technology The implementation of the Clean Air Mercury Rule in 2005 (70 FR 28606) is expected to increase the use of mercury control technologies. The Clean Air Mer- cury Rule is intended to reduce nationwide utility emissions of mercury by creating a market-based cap-and-trade program occurring in two distinct phases. The first- phase cap of 38 tons will likely be achieved by taking advantage of "co-benefit" reductions--mercury reductions achieved by reducing SO2 and NOx emissions under the Clean Air Act Amendments and the Clean Air Interstate Rule. The second phase, due in 2018, caps coal-fired power plant emissions at 15 tons and will likely necessitate installation of controls specific to mercury capture. Some of these technologies, such as activated carbon injection, will result in a separate waste stream, but it is also possible that emerging technologies may simply change the characteristics of existing CCRs by increasing their mercury content. The characteristics and potential environmental impact of residues generated from mercury control is currently being studied by the EPA's National Risk Management Research Laboratory. Preliminary studies indicate
COAL COMBUSTION RESIDUES 33 that leaching of mercury from activated carbon injection materials may not be of concern. Preliminary results of Heebink et al. (2004) show that leachate mercury concentrations were low, regardless of the concentration of mercury in the original sample. All concentrations were below the primary drinking water standard of 2 µg/L. Early results from studies of mercury leachates from FGD associated with mercury controls, however, show that there is potential for undesirable release of mercury into the environment from this type of CCR (Thorneloe, 2005). PHYSICAL AND CHEMICAL CHARACTERISTICS OF COAL COMBUSTION RESIDUES The chemical and physical characteristics of CCRs vary widely. For ex- ample, a dry scrubber FGD material may contain a relatively low concentration of metals, but a high concentration of sulfur compounds. Alternatively, a fly ash collected with a baghouse after being treated with activated carbon may have a relatively high concentration of mercury as well as carbon. This section describes the factors influencing the characteristics of CCRs and presents information on the physical and chemical characteristics of various CCRs. Factors Influencing the Characteristics of Coal Combustion Residues There are several factors that influence the physical and chemical character- istics of the CCRs produced, including 1. Chemical characteristics of the source coal, 2. Chemical characteristics of any co-fired materials, 3. Combustion technology, 4. Pollution control technology used by the CCR producing facility, and 5. Residue handling technology used by the CCR producing facility. Source Coal Because CCRs largely represent the noncombustible constituents in coal, their characteristics are strongly influenced by the source coal itself. As described in Chapter 1, coal is comprised of carbonaceous materials and a complex mixture of various minerals. Both the major and the minor mineral constituents of coal contain metals and other elements that could be of concern if they were released in the environment in the proximity of sensitive receptors (Schweinfurth, 2003). Both the form and the concentrations of these trace elements vary with coal type (e.g., lignite, bituminous) and coal region. The United States Geological Survey (USGS) maintains an extensive database of coal quality characteristics of the major coal basins throughout the United States (Bragg et al., 2005).
34 MANAGING COAL COMBUSTION RESIDUES IN MINES Co-Fired Materials Some facilities co-fire coal with other fuels such as wood, biomass, plastics, petroleum coke, tire-derived fuel, refuse-derived fuel, and peat or manufactured gas plant wastes. Fifty-nine percent of non-utilities, encompassing industrial, commercial, and institutional facilities, co-combust other fuels with coal (e.g., oil, gas, wood chips; Carrell, 2002). Co-firing coal with other materials can result in a variety of chemical constituents in the final CCR. In its 1999 report to Congress, the EPA examined data provided by the Electric Power Research Institute regarding the residues generated from these co-fired fuels and deter- mined that there was a potential for some of the mixtures to contain elevated levels of metals in the bulk material. The organic chemical constituent composi- tion of the CCRs generated from co-fired fuels was generally below detection limits (USEPA, 1999a). Other facilities, such as the independent power produc- ers in Pennsylvania, utilize fluidized bed combustion (FBC) boilers and co-fire coal refuse with limestone, resulting in a highly alkaline CCR. Combustion Technology The effects of combustion technology on the characteristics of CCRs vary based on the source coal and the operating conditions. However, different technolo- gies (Sidebar 2.1), especially FBC, can have an effect on the ash characteristics. Generally, given the same source coal and operating conditions, an FBC boiler will yield CCRs with a higher calcium concentration (as an oxide or sulfate) and lower silicon dioxide and aluminum oxide concentrations than a suspension-fired com- bustion boiler due to the addition of limestone during combustion (Sellakumar et al., 1999). Fluidized bed combustion also operates at a lower combustion tempera- ture than PC combustion technology, resulting in different mineral transformations in the ash (discussed in more detail later in this chapter). Several utility-scale technologies are emerging in the commercial market to allow the combustion of coal without the addition of post-combustion pollution controls, including integrated gasification combined cycle (IGCC) and pressur- ized fluidized bed combustion (PFBC). These emerging technologies have low air emissions relative to conventional coal-firing technologies and may also al- low for capture of CO2 from the exhaust gases (Booras and Holt, 2004). Today, the use of these technologies is minor relative to the use of standard combustion technologies; however, they hold promise for expanded use in the future. Re- search has shown that the characteristics of CCRs from IGCC differ markedly from those from traditional combustion technologies. Specifically, IGCC pro- duces primarily slag, elemental sulfur, and sulfuric acid, all of which may hold economic value as salable by-products (Shilling and Lee, 2003). However, addi- tional processing may be needed to remove excess carbon in IGCC slag, before it can be used in cement (Ratafia-Brown et al., 2002).
COAL COMBUSTION RESIDUES 35 Pollution Control Technology As mentioned earlier in this chapter, air emissions control technology has the potential to affect the characteristics of an exiting CCR stream. It may improve or diminish the marketability of CCRs for productive uses, and it may change the profile of the toxic constituents of the CCRs. For example, NOx emission controls by themselves do not cause the production of a solid residual stream, but their use may lead to high ammonia content in the resulting fly ash, thereby changing the opportunities for utilization as opposed to disposal (Rathbone and Robl, 2002). For this reason, regulatory agencies responsible for imposing pollution control standards should carefully consider the implications of air pollution control re- quirements for the marketability of CCRs to ensure that the full suite of environ- mental consequences is analyzed and understood. Residue Handling Technology Residue collection systems from the boiler and its auxiliaries vary between facilities and from unit to unit. Some units use a collection system that results in a combined residual in either a dry or a wet form. The type of materials that may be combined prior to leaving a plant is a function of individual plant collection logis- tics and/or any requirements to facilitate final disposal. Because residues are being produced constantly during the combustion process and must be removed regu- larly, facilities usually have a storage system such as a silo for dry materials or a surface impoundment (pond) for wet materials. Whether a CCR is in a wet or dry form and whether several CCR streams have been commingled are important fac- tors in the management opportunities that may be available to the CCR generator. Physical and Chemical Characteristics Understanding the physical and chemical properties of CCRs is important because these properties influence the opportunities for CCR use and disposal and affect the leachability of contaminants from CCRs. The physical and chemi- cal properties discussed include mineralogy, grain size, bulk chemical content, trace element content, organic chemical content, and radioactive content. Mineralogy The mineralogical characteristics of CCRs reflect the source coal, the com- bustion process itself, and any pollution control technologies used. Pulverized coal combustion occurs at high temperature (typically above 1400ºC) and there- fore causes significant transformations of the inorganic minerals in coal (e.g., clay minerals, carbonates, sulfides, quartz) (Kim, 2002a). At such temperatures, minerals may decompose or oxidize (Clarke and Sloss, 1992). Amorphous alumi-
36 MANAGING COAL COMBUSTION RESIDUES IN MINES nosilicate glass typically represents more than 60 percent of the mineral mass in PC fly ash (Hower et al., 1997; McCarthy et al., 1999). Other major mineral phases in PC fly ashes may include mullite (Al6Si2O13), quartz (SiO2), lime (CaO), anhydrite (CaSO4), periclase (MgO), hematite (Fe2O3), magnetite (Fe3O4), and tricalcium aluminate (Ca3Al2O6). Coal combustion residues containing sub- stantial quantities of lime will have high levels of alkalinity, because lime forms a strong base, Ca(OH)2, upon reaction with water. The lower temperature of the FBC process (approximately 800ºC), combined with the added limestone pro- duces different assemblage of minerals in the fly ash and bottom ash. The pri- mary minerals in FBC ash are anhydrite, lime, iron oxides, and quartz. Flue gas desulfurization residues consist primarily of gypsum (CaSO42H2O) and calcium sulfite hemihydrate (CaSO30.5H2O) (Tishmack, 1996). Grain Size The grain size of CCRs is related to where the residues are collected (e.g., fly ash versus bottom ash). Both PC and FBC fly ash are fine grained, with a mean particle size of approximately 20-30 mm (Chugh et al., 2000). Pulverized coal fly ash particles tend to melt at high combustion temperatures and condense as spheres, resulting in relatively low surface area for this small grain size (0.7 to 37 m2/g) (Nagataki et al., 1995), while FBC fly ashes maintain a more irregular shape (Figure 2.3). The FGD residues are also fine grained, with a mean particle size of 20-40 µm (Tishmack, 1996). Boiler slag particles are typically the size of fine gravel to coarse sand with 90 to 100 percent passing a 4.75 mm sieve, 40 to 60 percent passing a 2.0 mm sieve, and 10 percent or less passing a 0.42 mm FIGURE 2.3. Scanning electron microscopy images of (left) pulverized coal fly ash and (right) fluidized bed combustion fly ash. SOURCE: Chugh et al., 2000.
COAL COMBUSTION RESIDUES 37 TABLE 2.1 Typical Bulk Chemical Compositions of PC and FBC Fly Ash, FBC Bed Material, and FGD Scrubber Sludge FBC FGD Scrubber PC Fly Ash FBC Fly Ash Bed Material Sludge (% by wt) (% by wt) (% by wt) (% by wt) SiO2 55.90 22.10 09.7 00.45 Al2O3 15.40 06.80 03.69 BDL Fe2O3 16.10 06.67 02.16 BDL SO3 01.15 15.67 24.42 58.73 CaO 05.06 38.70 53.10 041.0 MgO 00.78 01.29 00.88 BDL Total Na2O 01.48 00.50 00.16 BDL Total K2O 01.93 01.12 00.39 00.02 Loss on ignition 00.58 05.46 00.80 00.00 NOTE: These data reflect the weight percent of major elements as oxides; they do not describe the actual mineralogy in the CCRs. BDL= below detection limit. SOURCE: Chugh et al., 1998. sieve. Bottom ash is predominantly sand sized, although bottom ash particles range in size from a fine gravel to a fine sand with very low percentages of silt- clay-sized particles (usually with 50 to 90 percent passing a 4.75 mm sieve and 10 percent or less passing a 0.075 mm sieve) (Moulton, 1973). Bulk Chemical Content Typical bulk chemical compositions of several common CCRs are presented in Table 2.1. Silicon, aluminum, and iron are major constituents in both PC and FBC fly ash, while calcium content varies substantially with source coal type. The FBC residues from bituminous coal combustion are typically higher in cal- cium and sulfur than PC CCRs because of the co-combustion of limestone for SO2 control in FBCs. The pH of CCRs is primarily a factor of the amount of alkaline metal oxides (e.g., calcium oxide, magnesium oxide) present (Daniels et al., 2002). Although many CCRs are alkaline, Furr et al. (1977) reported pH values of 23 fly ashes across the United States ranging from 4.2 to 11.8. The acidic fly ashes generally came from power plants burning bituminous coal ex- tracted from southeastern or mid-Atlantic states. Trace Element Content The trace elements contained in CCRs are derived from naturally occurring minerals present in the source coal. Non-volatile constituents (e.g., lead, cadmium) tend to be concentrated in CCRs as a result of the combustion process. The extent
38 MANAGING COAL COMBUSTION RESIDUES IN MINES of concentration is related to the ash content (percentage of non-combustible mate- rial) in the coal. For example, with an ash content of 12.5 percent, nonvolatile metals should be found at eightfold higher concentrations in bulk CCRs than in the source coal. The trace element content of coal varies across coal types (Figure 2.4), which results in regional variations in the trace element content of CCRs produced, based on the primary coal source. For example, bituminous coals generally contain higher quantities of arsenic and selenium relative to other ranks of coal, but they have the lowest boron, mercury, and cadmium content. Lignite coals tend to have the highest mercury and lead contents. Trace element content also varies with the individual types of CCRs coming out of a single boiler (Figure 2.4). Fly ash, in particular, tends to be enriched in arsenic, boron, and lead, whereas FGD and boiler slag residues tend to be the most enriched in mercury. FBC residues are less enriched than traditional CCRs in selenium, lead, cadmium, boron, and arsenic. Concentration data for an ex- panded list of trace elements in fly ash, bottom ash, boiler slag, and FGD are presented in Table 2.2. The modes of occurrence of trace elements vary in different CCRs and ultimately influence the leachability of these constituents. For example, trace elements may be sorbed to particle surfaces or associated with surface coatings on CCR grains. They may be evenly distributed throughout glassy fly-ash grains or tightly bound within the mineral structure itself (USGS, 2002). Although some of these trace metals have nutrient value at low concentra- tions, they can also present toxicity problems at higher concentrations (see Chap- ter 4). One example of a metal with a fairly narrow difference between concentra- tions that are nutritionally essential and those that are toxic is selenium. Thus, small enrichments of an element such as selenium can pose risks to human health and the environment. Organic Chemical Content Coal combustion residues may contain a variety of organic chemicals, al- though many of the organic compounds in coal are volatilized or destroyed by high combustion temperatures. The EPA (USEPA, 1999a) reported that "based on available information, total and leachable organics are generally reported to be at or below analytical detection limits." Research on the concentrations of or- ganic chemicals in CCRs is fairly limited and focused primarily on organic constituents in fly ash. Dioxins. The Electric Power Research Institute (EPRI, 1998) conducted a study of dioxins in CCRs from 11 sites at which the CCRs were co-managed with other power plant wastes. The most toxic of the dioxins (2,3,7,8-tetrachlorodibenzo-p- dioxin; 2,3,7,8-TCDD) was not detected in any of the samples. For each of the samples, researchers calculated toxicity-weighted composite concentrations con-
COAL COMBUSTION RESIDUES 39 sidering 17 dioxins of interest. They observed that the composite dioxin concen- trations for the CCRs tested were well below EPA risk-based concentrations for soil ingestion at residential and industrial areas (4 and 40 ng/kg, respectively). Polycyclic Aromatic Hydrocarbons. Polycyclic aromatic hydrocarbons (PAHs) form during the combustion of coal and adsorb onto fly ash particles. Gohda et al. (1993) determined the concentrations of 16 PAHs in coal fly ash samples from a coal-gas production plant. The total PAH concentration detected was 184 mg/kg. Gohda et al. (1993) speculated that the occurrence of PAHs in the coal-gas plant fly ash was due to incomplete combustion or low combustion temperatures. PAHs have low solubilities in water and tend to sorb to solids (Smith et al., 1988). Therefore, leaching of PAHs from CCRs is anticipated to be low. Elevated risks from PAHs will likely require direct exposure of biota to CCRs, although the exposure risk would depend on the bioavailability of PAHs (see NRC, 2003). Radioactive Content A few trace elements found in source coal are inherently radioactive; there- fore, concern has been raised that CCRs may also be radioactive. The most common potentially radioactive elements found in coal are uranium and thorium and their decay products radium and radon (USGS, 1997). The range of uranium concentrations in source coal is 1-4 parts per million (ppm), which is similar to that in many common rocks and soils (USGS, 1997). Radon gas present in the source coal is transferred almost entirely to the stack gases. Uranium and thorium are less volatile and are therefore almost completely captured in the solid-phase particulate matter resulting from combustion. The uranium concentration that may be found in a fly ash (~10-30 ppm) is similar to that of many shales, granites, and phosphate rocks (USGS, 1997a). A German study of health effects of FGD reported lower levels of radium and potassium-40 and equal levels of thorium- 232 in FGD gypsum compared to natural gypsum (Beckert et al., 1991; EPRI, 1994). DISPOSAL AND USE OPTIONS FOR COAL COMBUSTION RESIDUES As shown in Figure 2.1, once CCRs are generated at a coal-fired facility, the facility can pursue a variety of management options that may or may not include placement in a coal mine. For example, CCRs may be disposed of in landfills or surface impoundments. CCRs may be used as raw materials for the manufacture of products (e.g., wallboard) or for civil engineering applications (e.g., friction agents on snow). CCRs may also be disposed of in mines as a fill material for reclamation or used for other applications in mines, such as subsidence control. Many factors enter into the decision-making process when weighing the manage-
40 MANAGING COAL COMBUSTION RESIDUES IN MINES Boron 350.00 300.00 (ppm) 250.00 200.00 150.00 Concentration 100.00 50.00 0.00 Ash Ash Ash Ash Slag FGD LigniteFly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Cadmium 45.00 40.00 35.00 (ppm) 30.00 25.00 20.00 Concentration 15.00 10.00 5.00 0.00 Ash Ash Ash Ash Slag FGD Lignite Fly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Mercury 10.00 9.00 8.00 (ppm) 7.00 6.00 5.00 4.00 Concentration 3.00 2.00 1.00 0.00 Ash Ash Ash Ash Slag FGD Lignite Fly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Types of Soils, Source Coal and CCRs FIGURE 2.4 Bulk selected trace metal constituent concentrations in soils, source coal, and CCRs. For comparison with a familiar natural material, trace metal concentrations in soil are also presented. NOTE: All graphs show concentration data in parts per million (ppm), however the scales vary
COAL COMBUSTION RESIDUES 41 Lead 60.00 50.00 (ppm) 40.00 30.00 20.00 Concentration 10.00 0.00 Ash Ash Ash Ash Slag FGD Lignite Fly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Selenium 9.00 8.00 7.00 (ppm) 6.00 5.00 4.00 3.00 Concentration 2.00 1.00 0.00 Ash Ash Ash Ash Slag FGD LigniteFly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Arsenic 50.00 45.00 40.00 35.00 (ppm) 30.00 25.00 20.00 15.00 Concentration 10.00 5.00 0.00 FGD Ash Ash Ash Ash Slag FGD Lignite Fly Fly footnote)AnthraciteBituminous Bed Boiler Coal: Bottom (see FBC FBC FBC Coal: Subbituminous Coal: Soils Coal: Types of Soils, Source Coal and CCRs between graphs. Soil data reflect a median value from the USGS soils database of the following states: Texas, New Mexico, Pennsylvania, Louisiana, Oklahoma, West Virginia, Maryland, Michigan, Arizona, Kentucky, New Jersey, Illinois, Indiana, New York, Tennessee. SOURCE: USDOE, EIA, 2001; USGS, 2001.
42 MANAGING COAL COMBUSTION RESIDUES IN MINES TABLE 2.2 Ash Constituent Table Fly Ash (ppm) Bottom Ash (ppm) Boiler Slag (ppm) Constituent Median Range Median Range Median Range Aluminuma -- -- -- -- -- -- Antimonyb 4.6 0.2-205 4.0 0.18-8.4 0.8 0.25-1.0 Arsenicb 43.4 0.0003-391.0 4.7 0.80-36.5 4.5 0.01-254 Bariumb 806.5 0.02-10,850 633 24-9,630 413 6.19-1,720 Berylliumb 5.0 0.200-2,105 2.2 1.4-2.9 7.0 7.0-7.0 Boronb 311 2.98-2,050 90.0 1.79-390 49.5 0.10-55.0 Cadmiumb 3.4 0.01-76.0 003.1 0.050-5.5 40.5 0.01-40.5 Chromiumc 136 3.6-437 120 3.4-350 -- -- ChromiumVIb 90 0.19-651 121.0 3.41-4,710 158 1.43-5,981 Cobaltc 35.9 4.90-79.0 24 7.1-60.4 -- -- Copperc 112 0.20-655 61.1 2.39-146.3 32.0 1.37-156 Fluorinec 29.0 0.40-320 50.0 2.5-104 -- -- Irona -- -- -- -- -- -- Leadb 56.8 0.02-273 13.2 0.86-843.0 8.0 0.40-120 Manganesec 250 24.5-750 297 56.7-769 -- -- Mercuryb 0.1 0.013-49.5 0.009 0003-0.040 9.5 0.016-9.5 Molybdenuma -- -- -- -- -- -- Nickelb 77.6 0.1-1,270 79.6 1.9-1,267 83.0 3.3-177 Potassiuma -- -- -- -- -- -- Seleniumb 7.7 0.0003-49.5 0.8 0.007-9.0 4.5 0.10-14.0 Silverb 3.2 0.01-49.5 3.0 0.06-7.1 37.0 0.01-74.0 Strontiumc 775 30.0-3,855 800 170-1,800 -- -- Thalliumb 9.0 0.15-85.0 na 2.0 38.5 33.5-40.0 Vanadiumb 252 43.5-5,015 141 24.0-264 75.0 75.0-320.0 Zincb 148 0.28-2,200 52.6 3.80-717 35.8 4.43-530 NOTE: FGD = flue gas desulfurization; ppm = parts per million. aCIBO, 1997. b1993 regulatory determination in USEPA, 1999b. cTetratech analyses in USEPA, 1999c. ment options and economic impacts of CCR utilization or disposal. Such factors include the characteristics of the CCR, local applications for utilizing the CCR, costs and demands for alternate uses (considering the availability of virgin mate- rial), transportation distance to industries able to use CCRs, location and costs of CCR disposal options, and the local regulatory environment. Therefore, under- standing both the characteristics of CCRs and the options available for disposal and use is critical to sound CCR management. The various alternatives for the disposal and use of CCRs are discussed in more detail below. For the purpose of this study, data were sought on the amounts of CCR generated and how these CCRs are subsequently disposed of or used, including how much is placed in coal mines. In the process of gathering these data, it
COAL COMBUSTION RESIDUES 43 FGD (ppm) FBC: Fly Ash (ppm) FBC: Bed Ash (ppm) Median Range Median Range Median Range -- -- 42,300 20-88,900 18,000 9-68,800 6.0 3.65-90.0 7.75 0.125-259 10 0.125-361 32.5 0.0075-341.0 27.55 2.8-176 14.6 2.5-80 162.5 0.08-2,280 348 31.3-2,690 184 7.3-453 29.3 0.900-49.5 2.23 1.08-11.5 1.21 0.5-8 60.0 5.00-633 39.1 0.025-2,470 14.1 0.025-304 3.9 0.005-81.9 1.25 0.013-6.68 1.02 0.0125-7.16 -- -- 44.8 5.17-97.1 37 4.1-86 73.0 0.17-312 -- -- -- -- -- -- 19 2.5-79.8 11.3 1.4-75.8 46.1 0.04-251.0 41.1 2-99 13.8 1.65-37.1 -- -- -- -- -- -- -- -- 25,300 22.2-76,500 11,100 6.2-19,300 25.3 0.01-527.0 25 1.03-105 12.5 0.848-58 -- -- 165 0.05-548 241 52.2-751 4.8 0.073-39.0 0.323 0.00005-129 0.05 0.00005-16.2 -- -- 6.25 2.35-48.6 14.7 6-63.4 68.1 3.7-191.0 41.4 6.25-923 22 1-945 -- -- 3510 1.13-10,200 584 1.3-8,980 4.5 0.0150-162.0 8.36 0.47-166 0.952 0.152-45 3.3 0.01-10.3 1.03 0.05-11.6 1 0.05-87.6 -- -- -- -- -- -- 9.0 9.0-9.0 3.28 1.25-39 3.03 0.5-25 65.0 0.01-302.0 194 36.4-3,830 69 12-5,240 90.9 0.01-5,070 38.5 25-143 34 17.4-399 became evident that the data-gathering instruments currently used are varied and inconsistent. As a result, the committee was not able to collect accurate and inclusive data regarding CCR generation and subsequent disposal or use. The data contained in this report are based on the best available information, although the numbers are likely to be underestimates due to incomplete reporting and the fact that all major generators of CCRs are not included in the surveys (see Sidebar 2.4). The committee concludes that the available data regarding CCR generation and disposal or uses are inadequate. The committee recommends that existing data-gathering mechanisms be expanded to include comprehensive reporting of CCR generation quantities and classifications, and clarified to allow for a clear determination as to its disposal or use.
44 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 2.4 Data Gathering Mechanisms for Tracking CCR Generation, Disposal, and Use The three main sources of CCR generation and disposal information are the U.S. Energy Information Administration (EIA) plant-level annual report (F767), an annual voluntary survey of utilities conducted by the American Coal Ash Associa- tion (ACAA), and state-level information. The EIA requires any fossil fuel facility with a generation greater than 100 megawatts to report how much of the major types of CCRs the facility generates (e.g., fly ash, bottom ash, FGD). Even though independent power producers who fire coal refuse contribute a significant annual tonnage of CCRs to the total placed in mines for reclamation, most coal refuse-fired facilities are smaller than 100 MW; thus, their data are not contained in the EIA database. Additionally, facilities that report to the EIA are required to report only the amount of CCR placed in landfills and surface impoundments and the amount sold. Facilities do not have to report the purposes for which the CCRs were sold, and minefill is not listed as a reporting option. Facilities that give away their CCRs differ in how they report to EIA; some record given-away material as "off-site disposal," whereas others record it as "sold" but with a footnote indicating that no money exchanged hands. ACAA's annual survey of utilities is voluntary, and on average, only 65 percent of U.S. utilities report to ACAA. The survey requests specific information on the final disposition of CCRs in a variety of categories, making it a fairly thorough report regarding utilization activities. ACAA's survey includes a category for mining applications; however, the category does not differentiate between placement in minefills and other uses in mine settings. ACAA's survey also does not include non-utilities (e.g., coal refuse-fired facilities). Individual states may also collect data regarding the generation or disposition of CCRs. For example, the committee received information from the State of Penn- sylvania on the annual quantity of CCRs generated from coal refuse-fired facilities. Because of the large volumes of CCRs generated at Pennsylvania's coal refuse- fired facilities that are subsequently used in mine reclamation, those numbers were included in this report. However, the committee recognizes that these data repre- sent an underestimate of the total amount of CCRs generated from coal refuse- fired facilities in the United States, since data from other states were not readily available. Non-Mine Disposal and Use Options There are many disposal options and uses for CCRs outside the mine setting. Disposal in landfills and surface impoundments is the most commonly used CCR management option (see Chapter 1); however, there are many alternative uses, such as the use of fly ash in cement. The utilization of CCRs in these productive alternatives has been increasing steadily. The cumulative CCR utilization rate increased from 24.8 percent in 1995 (ACAA, 1995) to 38.1 percent in 2003
COAL COMBUSTION RESIDUES 45 60 40 (Millions) Tons 20 0 1966 1970 1980 1990 2003 Fly Ash Bottom Ash Boiler Slag Flue Gas Desulfurization FIGURE 2.5 Alternative uses of CCR by year (for purposes other than disposal). Note that the figure data may also include CCR mine placement that has been classified as "beneficial use." SOURCE: American Coal Ash Association, Aurora, CO, written communication, Octo- ber 2005. Courtesy of the American Coal Ash Association. (ACAA, 2005a) as markets for CCRs increased.1 Figure 2.5 illustrates the steadily increasing amounts of CCR products in the United States that are being utilized for purposes other than disposal. Alternative uses of CCRs may help to conserve resources by reducing the consumption of virgin materials (e.g., gypsum for wallboard production) and thereby lessen the impacts of associated mining operations (e.g., gypsum mines). It should be noted that many states refer to these alternative uses as "beneficial uses" (see Chapter 5 for a more complete discussion of the term "beneficial use"). In its 2000 regulatory determination EPA determined that, with the exception of minefilling, these uses are not likely to present significant risks to human health or the environment. Of the reported 126 million short tons of CCRs produced in 2003 by utilities and independent power producers, approximately 44 million short tons were used outside of mine settings for a variety of alternative applica- tions such as concrete, structural fill projects, or waste stabilization (ACAA, 2005a). The sections below describe a few of the uses of CCRs that can occur outside the mine setting. 1 These percentages include CCRs used in what ACAA defines as "mining applications", which may include alternative uses or minefilling.
46 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 2.5 Fly Ash Classifications Fly ash is commonly used for construction purposes in structural fills, cement, and concrete. To help the concrete industry ensure that the use of a particular type of fly ash meets applicable concrete performance standards, the American Society for Testing and Materials (ASTM) has developed classifications for fly ash in its circular C618 (Specification for Coal Fly Ash and Raw or Calcined Natural Poz- zolan for Use as a Mineral Admixture in Concrete): Class C and Class F. It should be noted that many fly ashes do not meet either ASTM designation, rendering them unsuitable for commercial use in concrete. Class C fly ash is generally produced during the burning of lignite or subbitumi- nous coal most likely from deposits in the western United States. This ash gener- ally contains more calcium, less iron, and a lime (CaO) content in the range of 15 to 30 percent. This classification may include fly ashes with either pozzolanic or cementitious properties (ASTM, 2002a). Class F fly ash is produced during the burning of anthracite or bituminous coal most likely from deposits in the eastern and midwestern United States. It contains silica, aluminum, and iron in combinations greater than 70 percent. This class of fly ash has pozzolanic properties (ASTM, 2002a). Definitions Pozzolanic fly ash has little or no cementitious properties unless chemically reacted with calcium hydroxide and water, resulting in a compound with cementi- tious properties (ASTM, 2002a). Cementitious fly ash has the properties of, or acts like, a cement (American Geological Institute, 1997) when mixed with water. Cement and Concrete Perhaps the most widely known use of CCRs is the application of fly ash to replace natural materials in the production of portland cement. Fly ash that con- tains the silica, alumina, calcium, and iron oxides needed in portland cement are sometimes used as raw materials (Sidebar 2.5). Fly ash also lowers the heat of hydration and can contribute to the long-term strength of the cement product. CCRs can also be used as an aggregate addition in the production of concrete blocks and other pre-cast concrete products (ACAA, 1998). Engineered Fill Fly ash and bottom ash can be used to produce road base materials, manufac- tured aggregates, flowable fills, structural fills, and embankments. CCRs can also be used as flowable fill for civil engineering applications where conventional backfilling may be difficult or undesirable and minimal subsequent settlement of the fill material is desired.
COAL COMBUSTION RESIDUES 47 CCRs can be used as engineered fill to alter a site's topography; for example, in urban areas or areas where borrow material is in limited supply or expensive, CCRs are commonly used for embankment construction on roadways (ACAA, 1998). Although FBC ash is not suitable for large-volume use in cement and con- crete because of its high sulfur content, its calcium content and cementitious or pozzolanic behavior make it well suited for other applications. The FBC ash can sometimes be substituted for lime in cement for road base construction and also shows potential for use as a synthetic aggregate (Conn et al., 1999). The FBC and FGD residues also have characteristics that make them usable for the construc- tion of low-permeability liners or caps (Wolfe et al., 2000). To ensure that material or structures containing CCRs meet or exceed indus- try performance standards for traditionally used materials, many technical orga- nizations issue standards or guidelines for the use of CCRs in their applications (e.g., Sidebar 2.5). These include the ASTM, the American Concrete Institute (ACI), the Federal Highway Administration (FHWA), and the American Society for Civil Engineers (ASCE), among others (ACAA, 2005c). Wallboard Increasing amounts of FGD residues are used as synthetic gypsum in the wallboard industry. The use of synthetic gypsum in the wallboard industry is often economically attractive, which has resulted in several gypsum companies opening wallboard manufacturing plants near utility generating facilities (Kalyoncu, 1999). Soil Amendments Coal combustion residues can also be used to modify soils chemically or physically. Chemically, they may be used to add micronutrients and to change the pH of soils. Studies have shown that fly ash may be used to improve nutrient- deficient soils, providing a source of essential nutrients for plants (e.g., boron) and animals (e.g., selenium); however, elemental concentrations must be moni- tored closely to prevent toxicity to both plants and animals (Carlson and Adriano, 1993). Alkaline fly ash can be used to reduce soil acidity (Adriano et al., 1980; Carlson and Adriano, 1993). Physically, CCRs can increase the water-bearing capacity and increase water infiltration. Fly ash can increase aeration in clay soils or increase the water-bearing capacity of sandy soils (Carlson and Adriano, 1993). It may also increase the water-bearing capacity of soils because of its tendency to cause cementation, which can be especially useful in geotechnical applications (Carlson and Adriano, 1993; ACAA, 1998). Other Applications There are also numerous other uses of CCRs. For example, boiler slag is commonly used as a component in the manufacture of roofing tile and shingles.
48 MANAGING COAL COMBUSTION RESIDUES IN MINES Boiler slag, when processed, also generates a material that can be used for sand- blasting abrasives, which does not contain the free silica of sand, making it safer for workers. The abrasive quality of many CCRs makes them suitable for use as traction control materials on snow- and ice-covered roadways, and the dark color of the materials aids in the absorption of radiant energy, which enhances the melting process. A portion of fly ash, called cenospheres, can be used as a performance enhancing product in paints, coatings, and adhesives. Mine-Specific Disposal and Use Options As the consumption of coal for electric power generation has increased, so has the demand for disposal sites for CCRs. Although the recycling of various CCRs into engineered products is the preferred alternative, conditions do not always lend themselves to such a solution. In these cases, CCR disposal alterna- tives are usually limited to surface impoundments, landfills, or placement in coal mines where the CCRs are utilized in mine reclamation. The use of CCRs in mine reclamation reduces other environmental impacts, such as disturbance of new land areas required for landfilling such materials. Nevertheless, there is a poten- tial for other impacts to occur, which are explored in later chapters. Coal mines have a number of attributes that may support large volume place- ment of CCR in mines. Among these features are the following: · Existing Excavation. Surface coal mining creates large excavations that often require bulk materials for proper reclamation. Minefilling requires no new land disturbances, whereas there is often strong public opposition to the siting of new surface impoundments or landfills. · Infrastructure. Active mines generally have adequate existing infrastruc- ture, equipment, and know-how for the economical handling and engineered placement of bulk materials. · Geology. Coal is generally contained in a sedimentary rock sequence that includes low-permeability shales and clays (see Chapter 3). These materials may impede groundwater flow, including potential contaminants that might be associ- ated with such flow. It should be emphasized that not all prospective coal mine disposal sites have all of the favorable features noted above. As stressed throughout this report, each site should be evaluated on its own merits. Furthermore, the existence of the above beneficial features should not deter a full assessment of the potential environmental risks of disposing of CCRs in any site. Final site selection in- volves the due consideration of such risks, but it is appropriate also to include a consideration of benefits in the selection process. There are two different sources of the CCRs that are typically disposed of in mines. The first is the conventional coal-fired power plant that consumes virgin coal. The CCRs produced are typically hauled by truck back for disposal at the
COAL COMBUSTION RESIDUES 49 mine (or mines) that supplied the original coal, which may be many miles from and under different ownership than the power plant. The second major source of CCRs used in mines is the independent power producer that uses coal refuse from nearby abandoned mines (Sidebar 2.6). The refuse material typically has poor SIDEBAR 2.6 Pennsylvania's Program for Coal Mine Reclamation and Mine Drainage Remediation Pennsylvania's coal miners have extracted approximately 16.3 billion short tons of anthracite and bituminous coal from the state's mines since commercial mining began in 1800. While mines permitted under the Surface Mining Control and Rec- lamation Act (SMCRA) are required to be reclaimed after the coal is extracted, many pre-SMCRA mines were abandoned prior to reclamation. In Pennsylvania, there are more than 5,000 abandoned, unreclaimed mining areas covering ap- proximately 189,000 acres and more than 820 abandoned coal refuse piles. The coal refuse piles cover 8,500 acres, contain a volume of more than 200 million cubic yards of waste material, and can be substantial in size (see Figure 2.6). It is estimated that the acid leached from the coal refuse in these abandoned coal mines in Pennsylvania contributed to the degradation of more than 3,100 miles of streams. Pennsylvania's Bureau of Abandoned Mine Reclamation esti- mates the cost to eliminate these abandoned mine problems to be $14.6 billion. Pennsylvania receives an average of $30 million annually from the Office of Sur- face Mining (OSM) Abandoned Mine Lands (AML) fund; at this rate, it would take Pennsylvania nearly 500 years to complete the cleanup of its AML sites. One approach that Pennsylvania has taken to its AML problem is encouraging private funding for reclamation of abandoned coal refuse piles. The advent of FBC technology in the late 1980s enabled the once-useless coal refuse to be used as fuel. As of 2004, 15 independent power producers constructed plants near Penn- sylvania's coal refuse piles, using the refuse as fuel for their FBC boilers. Between 1987 and 2002, these plants used 88 million short tons of coal refuse for the gen- eration of electricity and process steam--energy that would otherwise have been derived from another virgin fuel source. The FBC CCRs generated by coal refuse- fired facilities are highly alkaline and have been used in mine reclamation and for treatment of acid mine drainage in areas near the plant. For example, the Mount Carmel co-generation plant consumed a total of 8 million short tons of coal refuse from 1990 through 2002 and produced 5 million short tons of CCR for mine recla- mation neighboring the plant during that period, reclaiming 209 acres. The FBC plants' ability to use the coal refuse as fuel, coupled with the potential to place the CCRs into nearby mines, makes the arrangement economically viable and has enabled privately funded reclamation of 3,400 acres of AML as of 2002. An example of this cost offset is the Big Gorilla Project (Sidebar 2.7), which was reclaimed by the Northeastern Power Company (the independent power producer operating the cogeneration plant at the site) at a total estimated cost of $3.4 mil- lion. That reclamation cost is less than or approximately equal to the estimated cost of conventional AML reclamation of the site with federal AML funds (National Mining Association, Washington, D.C., written communication, July 2005 and April 2006). SOURCE: PADEP, 2004.
50 MANAGING COAL COMBUSTION RESIDUES IN MINES FIGURE 2.6 Westwood FBC plant near Tremont in the southern anthracite field show- ing a coal refuse pile by the plant. NOTE: Photograph courtesy of Pennsylvania Department of Environmental Protection. thermal qualities and a large waste rock content such that it can only be fired in FBC boilers. Over the last decade, traditional utilities have increased their utilization of CCRs in mining applications. ACAA reports that CCR utilization in mines (including minefilling) increased from approximately 1 percent in 1995 to about 1.9 percent in 2003 (ACAA, 1995, 2005a).2 The data currently available on CCR use and disposal do not differentiate between the amount of CCRs being used in engineered products outside of coal mines, the amount being used in coal mines as minefill, and the amount being used in smaller engineering applica- tions (e.g., road aggregate) within the mine area. In total, ACAA reports that 2.3 million short tons of CCRs were used in mining applications in 2003. However, this total is known to be an underestimate of the use of CCRs in mines. In New 2 These numbers represent information obtained from ACAA's voluntary survey (Sidebar 2.5) and therefore may not include all utilization of CCRs in mining applications.
COAL COMBUSTION RESIDUES 51 SIDEBAR 2.7 The Big Gorilla Demonstration Project The Big Gorilla pit was an abandoned anthracite surface mine located near Hazelton, Pennsylvania, in the Silverbrook Basin. The pit was approximately 1,400 feet long by 400 feet wide and 90 feet deep. It was filled with about 120 million gallons of water that had been significantly affected by acid mine drainage (see Figure 2.7). The Silverbrook Basin is approximately five miles long and 1 mile wide. It is drained by the Silverbrook outfall, which forms the headwaters of the Little Schuylkill River. The demonstration project involved the dry-to-wet placement of approximate- ly three million tons of fluidized bed combustion (FBC) ash into standing mine water. Placement began in August 1997 and was completed in 2004 (see Figure 2.8). The ash was dumped onto two working platforms by 45 ton trucks and then dozed into the pool. As the mine pool was filled, compaction was accomplished using the trucks and dozers. The ash came from Northeastern Power Company's co-generation facility in McAdoo, Pennsylvania, which fires approximately 1,700 tons of coal refuse and 60 tons of limestone per day. Five monitoring wells and three test boring locations have been monitored con- tinuously. Numerous studies of the mineralogy of the ash and the evolution of the pit lake water chemistry have been conducted. The project used approximately three million tons of CCRs to eliminate the acidic mine pool. The results of the demonstration project include a possible reduction in the acid loading of the Silver- brook outfall, a decrease in concentrations of some metals, a slight increase in concentrations of some cations, and a test of the dry-to-wet placement method. SOURCE: Loop et al., 2004. Mexico alone, the two largest coal mines together place approximately 2.5 mil- lion short tons back into their mines annually (BHP Billiton, 2004). Although Pennsylvania's coal refuse-fired facilities consume a significantly smaller quan- tity of coal annually, they generate almost twice the amount of mine-placed CCRs as compared to that reported by traditional utilities in the United States. The placement of CCRs generated by coal refuse-fired facilities in Pennsylvania for mine reclamation rose steadily from 89,000 short tons in 1988 to the almost 5 million short tons in 2002 and is expected to continue to increase as more facilities are developed (PADEP, 2004). Common Mine-Specific CCR Applications There are a variety of disposal and use options for CCRs in mining opera- tions. This section highlights the CCR applications that are unique to surface and underground mines, such as minefilling, capping, mine sealing, and treating acid mine drainage (AMD). Because knowledge of the methods and geometries of
52 MANAGING COAL COMBUSTION RESIDUES IN MINES FIGURE 2.7 Big Gorilla pit prior to 1995 showing the 120 million gallons of water that had been significantly affected by acid mine drainage. NOTE: Photograph courtesy of Barry Scheetz, Pennsylvania State University. placement is needed to understand the behavior of CCRs in the environment (discussed in Chapter 3), this section also describes methods for emplacing CCRs in mines. In surface mines, minefilling generally involves the placement of CCRs as a monofill, a layered fill, or a blended mixture of coal refuse and CCR (Figure 2.9). Surface mine placement of CCRs is part of the reclamation process, which in- volves rehabilitation of the mine site for the purpose of reestablishing the prior use or creating the capability for an alternate land use (see also Chapter 7). In situations where surface mines lack sufficient spoil, CCRs have been used to achieve the approximate original contour of the land surface. In some cases, CCR material is used as a cover on the overburden or backfill in addition to soil. The FBC ash may also be used to form low-permeability caps when acid-producing spoil is present. Surface soils in the mine setting, often used for reclamation, may have ad- verse characteristics. Coal combustion residues have been used as soil amend- ments to ameliorate problems with infiltration rate, water retaining capacity, and soil acidity (Daniels et al., 2002; also see "Soil Amendments" above). Coal combustion residues may be used to abate or prevent subsidence of underground mines in conjunction with conventional materials or concrete. Cementitious fly ash is especially effective for such use, and FBC fly ashes have
COAL COMBUSTION RESIDUES 53 A B FIGURE 2.8 Big Gorilla pit showing the dry-to-wet placement of approximately three million tons of FBC ash into standing mine water. (A) Big Gorilla in the midst of the placement project. (B) Aerial shot of the filled Big Gorilla pit. NOTE: (A) Courtesy of Barry Scheetz, Pennsylvania State University; (B) Courtesy of Daniel Koury, Pennsylvania Department of Environmental Protection.
54 MANAGING COAL COMBUSTION RESIDUES IN MINES FIGURE 2.9 Methods of large-volume CCR emplacement in surface mines. been shown to have sufficient bearing capacity for most post-mining uses (Scheetz et al., 2004). For example, in Pennsylvania, CCRs have been used to fill cropfalls, which are long, narrow vertical surface openings that are created by subsidence in underground mines. The costs of using CCRs for subsidence control is substan- tially lower than using concrete; for example, costs may range between $2.50 and $4.50 per ton for CCR as compared to $60 to $70 per ton for concrete (Dwyer, 2004). Underground mines may be sealed off to decrease the possibility of AMD from polluting the surface waters, to reduce the occurrence mine fires, or for the overall safety of the general public. Mine sealing generally involves injecting a fly ash grout mixture into boreholes in the underground mines to seal off problem areas. Certain CCRs may also be used to treat pyritic spoils that result in acid mine drainage. Alkaline CCRs (especially FBC CCRs) can be used to neutralize exist- ing acidity in groundwater (see Chapter 3). Coal combustion residues can also act as a seal to reduce the oxidation of pyrite in the coal spoil, thus slowing the rate of generation of additional AMD. The FBC ash grout can be pressure-injected through drill-holes into subsurface voids in previously backfilled surface mines and in voids in abandoned underground mines to encapsulate the pyritic materials with the cementitious mixture (Sheetz et al., 2004). However, the long-term efficacy of this practice is still questionable because of lack of data. Methods for Placement of CCRs in Coal Mines As mentioned earlier, CCRs can be placed in mines for a variety of purposes. CCRs can be placed for both low-volume (e.g., paving pit floors, grouting frac- tured country rock, capping and encapsulating potential AMD-producing mate- rial) and high-volume applications (e.g., backfilling of pits and underground workings, alkaline addition for neutralization of AMD). In large-volume applica- tions, CCRs can be placed as distinct monofills, multiple layers, or blended mixtures of CCR and coal refuse materials (PADEP, 2004; Figure 2.9). CCR placement in mines currently occurs above or below the water table
COAL COMBUSTION RESIDUES 55 (see Chapter 3, Sidebar 3.1). Placement below the water table may involve the use of slurry methods or direct dumping of CCR into standing water (see Sidebar 2.7). The permeability of CCRs after placement will depend on the CCR proper- ties, with highly compacted cementitious fly ash having the lowest permeability and coarse bottom ash having higher permeability (see Chapter 3). Lime or cement can be added to CCR to increase its structural stability, to make it more cement-like, and to decrease its permeability. The method of emplacement of CCRs at the mine site is an important factor that will influence the structural stability and hydrogeological and geochemical processes taking place there. The flow of water through and around CCRs will depend on the geometry of emplaced zones and the hydraulic properties of the surrounding materials. Similarly, geochemical reactions taking place within CCR zones and between CCR and surrounding materials will depend on the relative surface area of the CCR zones and surrounding materials and the potential for transport of reactants between materials (see Chapter 3). For these reasons, it is important to consider the exact method and location of CCR placement in the design plan, and to accurately predict the structural, hydrological, and geochemi- cal processes that will occur after emplacement. There are three common methods of placing CCRs in mine settings: gravity, hydraulic, and pneumatic. These methods are described in detail below. Gravity. Gravity placement is by far the most common method of placing CCRs in or around surface mines. Typically, CCRs are brought to the mine and put in place by end-dumping off trucks, although occasionally belly-dump vehicles or conveyor belts may be used. Bulldozers or scrapers may be used for the final placement. Generally there is no formal compaction in any manner (e.g., rolling, vibrating) unless the layer is being used as a liner or final cover over a previously placed fill. However, the committee did visit minefills where the CCRs were placed in small lifts and then compacted using a traditional compaction method. More typically, trucks that bring the material drive over the previously placed CCR layers, resulting in some degree of compaction. This is not a systematic compacting procedure and is not an effective compaction method over sizable lift thicknesses. Systematic compaction can increase the strength of the fill material and produce a uniform fill (ASTM, 2002b). Pneumatic. Pneumatic placement is applicable primarily to underground mines and was used commonly in Europe and in non-coal mines in the United States two or three decades ago. However, pneumatic placement is no longer a common practice because of the hazards associated with the technique, such as the genera- tion of a considerable amount of static electricity, which could result in sparking. Sparking would be hazardous in both working and abandoned underground coal mines that may have accumulations of methane gas.
56 MANAGING COAL COMBUSTION RESIDUES IN MINES Hydraulic. The hydraulic method, applicable to CCR placement in both under- ground and surface mines, consists of making a slurry of the CCR with water and then pumping it to the location where it is to be placed. The process is straightfor- ward and is similar to grout placement. The CCRs are stored and may be mixed with other substances. The CCR mixture is then transferred to a mixer where water is introduced in the proper proportion. The designed mixture may require testing prior to use in order to ascertain the desired setting time and fluidity. The material is then pumped to the placement location through pipes. On exiting the pipe, the velocity of the slurry decreases and water separates from the solids. Fine particles from the CCR may remain suspended in water for quite some time; thus, discharge water may have to be decanted in a sludge pond for further settling. SUMMARY The combustion of coal generates large quantities of solid materials, collec- tively referred to as CCRs, which are grouped into two categories: the noncom- bustible portion of the coal itself (fly ash, bottom ash, boiler slag) and products from various air pollution control technologies installed at the combustion facil- ity (e.g., FGD materials). The physical and chemical characteristics of the CCRs produced are determined by several factors including the source coal, the com- bustion technology, the air pollution control equipment technology, and the resi- due handling equipment. The characteristics of CCRs vary greatly and are the major determinants of the possible uses of the residue. Thus, the committee recommends that regulatory agencies responsible for imposing pollution con- trol standards carefully consider the implications of air pollution control requirements for the marketability of CCRs to ensure that the full suite of environmental consequences is analyzed and understood. For the purpose of this study, data were sought on the amounts of CCRs generated and how these CCRs are subsequently disposed of or used, including how much is placed in coal mines. However, the committee found the available data regarding CCR generation and disposal or uses to be inadequate. The committee recommends expanding existing data gathering mechanisms to include comprehensive reporting of CCR generation quantities and classifi- cations, and clarifying those mechanisms to allow for a clear determination as to disposal or use. This chapter outlines the many alternatives available for CCR disposal and use, including applications in surface and underground coal mines. Many factors enter into the decision-making process when weighing the management options and economic impacts of CCR utilization or disposal. Such factors include the local possibilities for utilizing a particular CCR, the costs and demands of CCRs for alternate uses, the substitution of CCRs for unrecycled materials, the transpor- tation distance to industries able to use CCRs, the location and costs of CCR placement options (e.g., availability of CCR-receiving coal mines; availability of
COAL COMBUSTION RESIDUES 57 new land for landfills and surface impoundments), the local regulatory environment, and the potential effects on human health and the environment. The characteristics of a particular CCR stream, coupled with the aforementioned considerations, are key to determining the best options for disposal and use of CCRs. Therefore, the committee concludes that understanding both the characteristics of CCRs and the options avail- able for their disposal and use are critical to sound CCR management and that such characteristics and options are highly site specific.