20 Minimization and Use of Coal Combustion By-Products (CCBs): Concepts and Applications Harold W. Walker, Panuwat Taerakul, Tarunjit Singh Butalia, and William E. Wolfe The Ohio State University, Columbus, Ohio Warren A. Dick The Ohio State University, Wooster, Ohio 1 INTRODUCTION AND BACKGROUND During coal-fired electric power production, four main types of coal combustion by-products (CCBs) are produced: fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) material (1,2). In 1998, 97.7 million metric tons of CCBs were produced in the United States (see Figure 1). Fly ash was generated in the largest quantity (57.1 million metric tons), with FGD material the second most abundant CCB (22.7 million metric tons). Roughly 15.1 million metric tons of bottom ash were generated and 2.7 million metric tons of boiler slag were produced. Although the majority of CCBs produced currently enters landfills and surface impoundments, there is great potential for the effective and environmen- tally sound utilization of these materials. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. Currently, the amount of CCBs entering landfills and surface impound- ments is greater than half of the total municipal solid waste (MSW) disposed of in the United States (see Table 1). Of the 97.7 million metric tons of CCBs generated in 1998, 69.4 million metric tons of CCBs (or 70%) were disposed of in landfills or surface impoundments (1). In 1997, the most current year for which data are available, the total MSW disposed of in landfills was 119.6 million tons (3). The amount of CCBs disposed each year is greater than the amount of paper (37.4 million metric tons), plastic (15.5 million metric tons), wood (8.4 million metric tons), and glass (6.9 million metric tons) discarded. 57.1 15.1 2.7 22.7 97.7 0 20 40 60 80 100 120 Fly Ash Bottom Ash Boiler Slag FGD Total CCPs Million Metric Tons FIGURE 1 CCB production in million metric tons in the United States in 1998 (1). TABLE 1 Amount of CCBs Disposed of in Landfills in the United States in 1998 Compared to Disposal of Municipal Solid Waste (MSW) a Material Metric tons × 10 6 Reference Total MSW 119.6 (3) CCBs 69.4 (1) Paper 37.4 (3) Plastic 15.5 (3) Wood 8.4 (3) Glass 6.9 (3) a Data for disposal of MSW are for 1997, the most current year for which data are available. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. Recently, the American Coal Ash Association (ACAA) proposed that CCBs be considered a product, and therefore they recommend that these materials be referred to as coal combustion products (CCPs). Considered as a commodity, CCBs are ranked as the third largest nonfuel mineral commodity produced in the United States (1,4). As shown in Table 2, the amount of CCBs generated every year exceeds the amount of Portland cement generated in the United States, is significantly greater than the production of iron ore, and falls behind the produc- tion of crushed stone, sand, and gravel. The purpose of this chapter is to review the current state of the art in technology for minimizing CCB generation, maximizing CCB use, and reducing the disposal of CCBs in landfills and surface impoundments. This chapter will first present a review of important federal regulations influencing the generation and utilization of CCBs in the United States. Next, the physical, chemical, and engineering properties of CCBs will be discussed, and the operational factors affecting CCB generation will be presented. The chapter will conclude with a discussion regarding strategies for minimizing CCB production and maximizing the utilization of CCBs. Potential barriers to utilization and minimization in the future will also be discussed. 2 FEDERAL REGULATIONS INFLUENCING CCB GENERATION AND USE Governmental regulations of emissions from electric power plants combined with efforts to improve air quality have had a profound effect on the amount and type of CCBs produced in the United States over the past 25 years. The Clean Air Act of 1967 was the first legislation to establish the authority of the federal govern- ment to promulgate air quality criteria (5). It set the groundwork for future “technology-forcing legislation,” i.e., legislation that sets standards unattainable TABLE 2 Amount of CCBs Produced in the United States in 1998 Compared to Traditional Nonfuel Mineral Commodities Commodity Metric tons × 10 6 Reference Crushed stone 1,500 a (4) Sand and gravel 1,020 a (4) CCBs 97.7 (1) Cement 85.5 a (4) Iron ore 62 a (4) a Estimated. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. utilizing existing technology. This regulatory approach required industry and utilities to develop new technologies to meet promulgated standards. The Clean Air Act Amendments of 1970 established Natural Ambient Air Quality Standards (NAAQS) and set specific pollutant removal requirements (New Source Performance Standards or NSPS) for both stationary and mobile sources (5). NSPS, which are applicable to coal-fired utilities, were written in part 60, subpart D, Da, Db, and Dc of 40 CFR (Code of Federal Regulation) (6). NSPS in 40 CFR, part 60, subpart D, set air pollutant levels for coal-fired steam generators with heat input rates over 73 megawatts (MW), constructed or sub- stantially modified after August 17, 1971. Amendments to the Clean Air Act in 1990 added new provisions to reduce the formation of acid rain by decreasing sulfur and nitrogen oxide emissions. Key to these provisions was the requirement to reduce annual SO 2 emissions by 10 million tons below 1980 levels, and to reduce NO x emissions by 2 million tons below 1980 levels. To achieve these emission reductions, the Clean Air Act Amendments of 1990 promulgated NO x and SO 2 performance standards and set up an innovative emission trading system for SO 2 reduction. In phase I of the SO 2 -reduction program, the legislation required 110 identified utilities to reduce SO 2 emissions to 2.5 lb/mmBTU by January 1995. Phase II mandated further reductions in emissions to 1.2 lb/mmBTU for all utilities generating at least 25 MW of electricity. It is estimated that phase II requirements will affect 2128 utilities in the United States (7). The NO x reduction program was also separated into two phases. In phase I, Group 1 boilers (dry-bottom wall and tangentially fired boilers) were required to meet NO x performance standards by January 1996 (8). Phase II set lower NO x emission limits for Group 1 boilers and established initial NO x emission limitations for Group 2 boilers (cell burner technology, cyclone boilers, wet bottom boilers, and other types of coal-fired boilers) (7). To meet these federal regulations, coal-fired utilities have switched to alternative fossil fuels or installed air pollution control technologies such as electrostatic precipitators, baghouses, and wet or dry SO 2 scrubbing systems. Currently, CCBs generated as a result of air pollution control processes are regulated under subtitle D of the Resource Conservation and Recovery Act (RCRA), which pertains to nonhazardous solid wastes (9). In 1988, and then again in 1999, the U.S. Environmental Protection Agency (EPA) issued a Report to Congress examining the environmental impacts associated with CCB use and disposal (10,11). Reports in both 1988 and 1999 concluded that CCBs were nonhazardous and nontoxic materials. In early 2000, based on its own findings in the Report to Congress as well as input from environmental groups, the EPA maintained its previous ruling that CCBs will continue to be regulated under subtitle D of the RCRA. As a result, the use and/or disposal of CCBs is regulated at the state level. For example, regulations in the state of Ohio consider fly ash, Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. bottom ash, boiler slag, and FGD generated from coal or other fuel combustion sources to be exempt from regulation as hazardous waste (12). 3 PHYSICAL, CHEMICAL, AND ENGINEERING PROPERTIES OF CCBS Information regarding the physical, chemical, and engineering properties of CCBs is required before these materials can be safely and effectively utilized. The physical and engineering properties, in particular, are important parameters affecting the behavior of CCBs in various engineering applications. Information regarding the chemical composition is important for addressing potential environ- mental impacts associated with CCB utilization and disposal. Chemical data are also useful for explaining physical properties when pozzolanic or cementitious reactions take place. As mentioned above, the four main types of CCBs are fly ash, bottom ash, boiler slag, and FGD material. Fly ash is a powdery material removed from electrostatic precipitation (ESP) or baghouse operations, while bottom ash is a granular material removed from the bottom of dry-bottom boilers. Boiler slag is a granular material that settles to the bottom of wet-bottom and cyclone boilers. It forms when the operating temperature in the boiler exceeds the ash fusion temperature. Boiler slag exists in a molten state until it is drained from the boiler. The majority of FGD material is a mixture of fly ash and dewatered scrubber sludge. Scrubber sludge is produced when flue gases are exposed to an aqueous solution of lime or limestone. The wet scrubber sludge is dewatered and stabilized with fly ash and extra lime. Alternatively, the scrubber sludge can be oxidized to calcium sulfate (CaSO 4 ) to produce synthetic FGD gypsum. Dry FGD processes are widely used, in which limestone is injected directly into the boiler or flue gas stream. Dry FGD by-products are removed from the flue gas by electrostatic precipitation or baghouse operations. 3.1 Physical and Engineering Properties of CCBs A number of the physical and engineering properties of fly ash, bottom ash, boiler slag, and FGD material are summarized in Table 3 (10,11,13–16,18,19). Fly ash is usually spherical, with a diameter ranging from 1 to 100 µm. Fly ash has the appearance of a gray cohesive silt and has low permeability when compacted. Bottom ash and boiler slag are granular in shape, with sizes ranging from 0.1 to 10.0 mm. Boiler slag has a glassy appearance. Bottom ash has a permeability higher than fly ash, while boiler slag has a permeability similar to that of course sand. Fly ash, bottom ash, and boiler slag have dry densities that range between 40 and 100 lb/ft 3 (10,11,15,16). Fly ash has lower shear strength than both bottom ash and boiler slag. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. The physical characteristics of FGD material depend on the type of FGD system used: wet or dry (see Table 3). Wet FGD systems generate by-products with diameters ranging from 0.001 to 0.05 mm. Dry FGD systems produce by-products with diameters ranging from 0.002 to 0.074 mm. FGD material generally has low permeability, ranging from 10 –4 to 10 –7 cm/s. The unconfined compressive strength is affected by the moisture content of FGD, and the percentages of fly ash and lime. For example, wet FGD scrubber sludge is similar to toothpaste in consistency and has little unconfined compressive strength. However, the strength of wet FGD is greatly improved when FGD sludge is stabilizing by mixing with lime and fly ash. 3.2 Chemical Properties of CCBs The chemical characteristics of fly ash, bottom ash, and boiler slag depend greatly on the type of coal used and the operating conditions of the boiler (10,11). Over 95% of fly ash consists of oxides of silicon, aluminum, iron, and calcium, with the remaining 5% consisting of various trace elements (10,11). The chemical composition of fly ash is affected by the operating temperature of the boiler, because the operating temperature influences the volatility of certain elements. For example, sulfur may be completely volatilized at high temperature and removed during lime scrubbing, thus reducing the amount in the fly ash, bottom ash, and boiler slag (10,11). Table 4 shows the trace-element content of fly ash, bottom ash, boiler slag, and FGD material (10,11,20). The elemental composition of fly ash from two TABLE 3 Summary of Physical Characteristics and Engineering Properties of Fly Ash, Bottom Ash, Boiler Slag, and FGD Material (10,11,13–16,18,19) Physical characteristics Fly ash Bottom ash/ boiler slag FGD material Wet Dry Particle size (mm) 0.001–0.1 0.1–10.0 0.001–0.05 0.002–0.074 Compressibility (%) 1.8 1.4 Dry density (lb/ft 3 ) 40–90 40–100 56–106 64–87 Permeability (cm/s) 10 –6 –10 –4 10 –3 –10 –1 10 –6 –10 –4 10 –7 –10 –6 Shear strength Cohesion (psi) 0–170 0 Angle of internal friction (deg) 24–45 24–45 Unconfined compres- sive strength (psi) 0–1600 41–2250 Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. TABLE 4 Trace Element Composition of Fly Ash, Bottom Ash, Boiler Slag (10), and FGD Material (20) Element (mg/kg) Fly ash Bottom ash/boiler slag Dry FGD materialMechanical ESP/baghouse Range Median Range Median Range Median Range Median Arsenic 3.3–160 25.2 2.3–279 56.7 0.50–168 4.45 44.1–186 86.5 Boron 205–714 258 10–1300 371 41.9–513 161 145–418 318 Barium 52–1152 872 110–5400 991 300–5789 1600 100–300 235 Cadmium 0.40–14.3 4.27 0.10–18.0 1.60 0.1–4.7 0.86 1.7–4.9 2.9 Cobalt 6.22–76.9 48.3 4.90–79.0 35.9 7.1–60.4 24 8.9–45.6 26.7 Chromium 83.3–305 172 3.6–437 136 3.4–350 120 16.9–76.6 43.2 Copper 42.0–326 130 33.0–349 116 3.7–250 68.1 30.8–251 80.8 Fluorine 2.50–83.3 41.8 0.4–320 29.0 2.5–104 50.0 — — Mercury 0.008–3.0 0.073 0.005–2.5 0.10 0.005–4.2 0.023 — — Manganese 123–430 191 24.5–750 250 56.7–769 297 127–207 167 Lead 5.2–101 13.0 3.10–252 66.5 0.4–90.6 7.1 11.3–59.2 36.9 Selenium 0.13–11.8 5.52 0.6–19.0 9.97 0.08–14 0.601 3.6–15.2 10.0 Silver 0.08–4.0 0.70 0.04–8.0 0.501 0.1–0.51 0.20 — — Strontium 396–2430 931 30–3855 775 170–1800 800 308–565 432 Vanadium 100–377 251 11.9–570 248 12.0–377 141 — — Zinc 56.7–215 155 14–2300 210 4.0–798 99.6 108–208 141 Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. types of collection methods is shown. Mechanical collection methods generally collect larger particles from the flue gas, while finer ash particles are collected by electrostatic precipitators (ESPs) or baghouses. However, similar ranges of most trace elements are found in both types of collection methods. Some exceptions to this are arsenic, boron, lead, and selenium, which may be found at slightly higher fractions in fly ash collected by ESPs or baghouses. Cadmium and fluorine may be present at higher levels in ash collected by mechanical methods. The chemical characteristics of FGD by-products depend on the type of absorbent used and the sulfur content of the coal. In the United States, approxi- mately 90% of FGD systems use lime or limestone as a sorbent (17). In lime-based FGD processes, the absorbent reacts with sulfur in the flue gas and forms a calcium compound, either calcium sulfite or calcium sulfate, or a calcium sulfite–sulfate mixture (10,11). In systems that use dual-alkali scrubber technol- ogy, sodium hydroxide, sodium sulfite, or lime is used as absorbent solution. These types of systems generate calcium sulfite and sodium salts (10,11). In spray-drying scrubber systems, sodium sulfate and sodium sulfite are produced with sodium-based reagents. When fly ash is added to FGD, the quantity and characteristics of the fly ash will also affect FGD chemical characteristics. The most significant components in FGD include calcium and sulfur, with lesser amounts of silica, aluminum, iron, and magnesium if fly ash is added. The elemental composition of dry FGD materials has been determined based on data from a variety of dry-scrubber technologies, including spray dryer systems, duct injection, lime injection multistage burner (LIMB) processes, and a number of fluidized bed combustion (FBC) processes (i.e., bed-ash process and cyclone ash process) (18,19). The calcium content of dry FGD material varies in the range from 10% to 30% depending on the particular scrubber technology. The sulfur content of dry FGD material typically varies between 4% and 11%. The silicon content of dry FGD may range from 2% to 11%, while the aluminum content can vary from 1% to 7%. Table 4 shows the trace-element content of dry FGD materials (20). Although detectable amounts of arsenic, cadmium, chromium, copper, lead, molybdenum, nickel, selenium, and zinc are present in dry FGD materials, levels of these constituents are typically lower than EPA land applica- tion guidelines for sewage sludge. For many CCB applications, it is important to understand the leaching behavior of these materials. The EPA’s Toxicity Characteristic Leaching Proce- dure (TCLP) is a commonly used method for characterizing the leaching potential of organics, metals, and other inorganic constituents from CCB matrices (21). Table 5 shows the results of TCLP analyses of dry FGD materials and ash produced from various air pollution control technologies. Typically, very low levels of organic materials are found in CCBs, and therefore, TCLP tests focus on examining the leaching behavior of inorganic constituents. The TCLP values for FGD shown in Table 5 were determined for a variety of dry scrubber Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. TABLE 5 Range of Values Observed for TCLP Analysis of Dry FGD Materials (19,20) and Ash (14) Chemical constituent (mg/liter) FGD Ash pH 9.58–12.01 — TDS 11,840–13,790 — Ag <0.024 0.0–0.05 Al 0.12–0.20 — As <0.005 0.026–0.4 B 0.543–2.17 0.5–92 Ba <0.002 0.30–2.0 Be 0.141–0.348 <0.0001–0.015 Ca 1,380–3,860 — Cd <0.003 0.0–0.3 Co <0.014–0.026 0.0–0.22 Cr <0.005–0.028 0.023–1.4 Cu <0.013 0.0–0.43 Fe <0.029 0.0–10.0 Hg <0.0002 0.0–0.003 K 1.3–22.1 — Li 0.04–0.18 — Mg <0.04–1,360 — Mn <0.001 0.0–1.9 Mo 0.025–0.088 0.19–0.23 Na 1.32–9.82 — Ni <0.01 0.0–0.12 P <0.12 — Pb <0.001–0.017 0.0–0.15 S 132–979 — Sb <0.24 0.03–0.28 Se <0.001–0.005 0.011–0.869 Si 0.10–0.33 — Sr 0.83–3.38 — V <0.019–0.024 — Zn <0.006 0.045–3.21 Cl – 19.6–67.8 — SO 3 2– <1.0–43.2 — SO 4 2– 236–2,800 — Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. technologies. TCLP leachate typically meets most primary and secondary drink- ing water standards. Levels of silver, arsenic, barium, cadmium, copper, iron, mercury, manganese, nickel, phosphorus, antimony, and zinc in leachate are typically below the limit of detection. For all FGD materials shown in Table 5, high pH values are observed, thus making FGD an attractive product for applica- tions requiring alkaline materials. Typically, with the exception of sulfur and calcium, higher levels of most inorganic elements are found for TCLP tests carried out with ash than for FGD. It should be noted that the acidic conditions and high liquid-to-solids ratio of the TCLP test are perhaps more favorable for leaching than conditions typically observed in field applications. 4 FACTORS AFFECTING CCB GENERATION The physical and chemical properties of CCBs and the quantity of CCBs produced will depend on the mechanical design and operation of the combustion process, the type of air pollution control equipment utilized, as well as the characteristics of the coal used in the combustion process (11). In order to minimize CCB generation, it is important to understand how these factors affect the type and amount of solid by-product produced. In all cases, however, efficient energy production and low-pollutant air emissions must be maintained. 4.1 Boiler Technology The boiler used in an electric power plant is a closed vessel that is heated from the combustion of coal to produce hot water or stream. There are four major types of boiler technologies in current commercial application: pulverized coal (PC) boilers, stokers, cyclones, and fluidized bed combustion systems. Figure 2 shows the approximate distribution of ash and slag produced by different kinds of boiler technology. The most widely used boiler technology is the PC boiler. The coal used in PC boilers is finely ground prior to combustion. The large effective surface area of finely ground coal used in PC boilers increases combustion efficiency. The greater efficiency of combustion reduces the total volume of ash by-products produced. There are two types of pulverized coal boilers; wet-bottom and dry-bottom boilers. The larger-sized ash that falls to the bottom in a dry-bottom process remains dry and becomes bottom ash. For the wet-bottom process, ash is removed as a flowing slag. Large ash particles fall to the bottom of the furnace and flow out of the furnace in a molten state which later solidifies as slag (10,11). As seen in Figure 2, dry-bottom PC boilers produce 80% fly ash and 20% bottom ash. PC boilers with a wet-bottom design produce 50% fly ash and 50% slag. The predominance of fly ash in these two types of boilers is primarily a result of the small particle size of ground coal used in the combustion process. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. [...]... Agency, Wastes from the Combustion of Fossil Fuels, Volumes 1 and 2—Methods, Findings, and Recommendations, Report to Congress, EPA 530-S-9 9-0 10, March 1999 12 Ohio Administrative Code, Chapter 374 5-5 1-0 4 (B) (4), Exclusion: Identification and Listing of Hazardous Waste, June 25, 1998 13 American Coal Ash Association, Fly Ash Facts for Highway Engineers, FHWA-SA9 4-0 81 Washington, DC: Federal Highway... concrete strength development and so is restricted by industry standards In the United State, the specification of LOI of fly ash is between 3% and 5% for use in ready-mix concrete (37,38) The use of low-NOx technology may increase the LOI above 5% and thus reduce the utilization of fly ash in concrete applications (39) For example, in 1998, the ACAA reported that 19 out of 20 coal-fired utilities in Ohio... tons of soil and could save up to 11.2 million metric tons of soil if 100% of all CCBs were used at the current utilization rate for structural fill (13.2%) In 1998, 2.2 million metric tons of FGD were used, which represented only 8% of all FGD produced Of this amount, 1.6 million metric tons of FGD were used in the production of wallboard This represented the single largest use of FGD and 73% of all... FGD processes also have wet and dry systems Examples of recovery FGD systems include Wellman-Lord and magnesium oxide systems and aluminum sorbent and activated-carbon sorbent systems (11) 4.3 Types of Coal Different types of coal have different heating values and also different ash contents The highest-ranked coal with respect to heating value is anthracite, Copyright 200 2 by Marcel Dekker, Inc All... Office of Federal Register, July 1, 1999 9 United States Code, Title 42, Section 6921 (b)(3)(A)(i), Identification and Listing of Hazardous Waste Washington DC: U.S Government Printing Office, 1995 10 U.S Environmental Protection Agency, Wastes from the Combustion of Coal by Electric Utility Power Plants, Report to Congress, EPA/530-SW-8 8-0 02, February 1988 11 U.S Environmental Protection Agency, Wastes... lower total volume of CCBs produced, and enhance utilization 5.2 Use of Coal Combustion By-products A number of applications of CCBs have been developed and demonstrated in order to reduce the amount of CCBs disposed of in landfills The first column in Table 6 shows demonstrated applications of CCBs In 1998, the American Coal Ash Association (ACAA) reported that 28 million metric tons of CCBs were used... compressive strength of 1200 psi or less at 28 days (40) Flowable fill is also known as control density fill (CDF), controlled low-strength material (CLSM), Copyright 200 2 by Marcel Dekker, Inc All Rights Reserved unshrinkable fill, flowable mortar, plastic-soil cement slurry, K-Krete, and/ or Flash Fill (40) Flowable fill may contain a mixture of fly ash, bottom ash, water, and Portland cement This application... variations in soil and fly-ash leaching behavior 7 BARRIERS TO CCB UTILIZATION Currently, the cost of disposing and managing CCBs in landfills is relatively low, and this reduces the incentive to utilize CCBs For example, landfill costs for coal-fired utilities in Ohio range from $2 to $40 per ton (18,39) A lack of standards for using CCBs is also a barrier for CCB utilization For example, the use of CCBs in... boiler slag and FGD material are by-products from the combustion of coal and are considered to be solid wastes from a federal regulatory perspective Currently, 100 million tons of CCBs are produced in the United States every year Approximately 70 million tons of CCBs are disposed of in landfills and surface impoundments The two primary strategies for minimizing CCBs include reduction at source and effective... generation and disposal A number of applications have been developed for using CCBs, including the use of fly ash as a substitute for cement in concrete and grout applications, the use of fly ash in flowable and structural fill, the use of calcium sulfate-rich FGD scrubber sludge as a replacement for natural gypsum in wallboard manufacturing, and a variety of mine reclamation applications The utilization of . effective and environmen- tally sound utilization of these materials. Copyright 200 2 by Marcel Dekker, Inc. All Rights Reserved. Currently, the amount of CCBs entering landfills and surface impound- ments. the type of coal used and the operating conditions of the boiler (10,11). Over 95% of fly ash consists of oxides of silicon, aluminum, iron, and calcium, with the remaining 5% consisting of various. generation and characteristics of CCBs. There are two main categories of air pollution control technologies that generate CCBs during coal combustion: particulate control and gaseous emission control technologies. Particulate