Recycled Materials and Byproducts in Highway Applications—Summary Report, Volume 1 (2013)

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13 background Coal combustion byproducts are generated from the fossil fuel used in electric power generation, which produce about 50% of the electricity demand in the United States. At this time, there are a number of coal combustion byproducts that are marketed for various uses. These include bottom ash, fly ash, boiler slag, and flue gas desulfurization (FGD) materi- als (Kalyoncu 2000; EPA 2005). Each type of byproduct is obtained from a different location in the typical steam gener- ating system (Figure 1). There are various options for storage of these byproducts (Figure 2). Coal combustion byproducts have traditionally included: • Boiler slag: Obtained from molten ash collected in wet- bottom boilers where the molten ash is water cooled. The molten ash shatters into black angular pieces that range in size from coarse sand to fine gravel and have a smooth appearance. The material is collected in wet- bottom boilers or cyclone units. The major components are silica, aluminum, iron, and calcium (Butalia and Wolfe 2000; EPA 2008). • Bottom ash: Collected from the bottom of dry-bottom boilers and range in size from fine gravel to fine sand. The material is heavier than fly ash. The major compo- nents are similar to the boiler slag (Butalia and Wolfe 2000; EPA 2008). • Fly ash: Entrained particles in the exhaust gases leav- ing the combustion chamber. This consists of the finest particles collected from coal burning processes. The major components are also similar to those found in boiler slag and bottom ash. • Flue Gas Desulfurization (FGD): FGD is a mixture of gypsum (CaSO4), calcium sulfite (CaSO3), fly ash, and unreacted lime or limestone that results from the removal of sulfur dioxide (SO2) from the exhaust (Kalyoncu 2000). This is also referred to as synthetic gypsum. The major components are calcium, sulfur, silica, iron, and aluminum. FBC ash is a new byproduct, the result of new boiler tech- nology within the conventional coal burning power plant. Figure 3 shows where the new technology fits within the coal combustion process. FBC is a process of burning coal in which the coal is inserted in a bed of particles that are sus- pended in the air and that react with the coal to more cleanly heat the furnace. With the FBC technology, coal is burned at a slightly lower temperature, which helps prevent some nitrogen oxide gases from forming (ACAA 2009). Coal combustion is accomplished by combining the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process so that complete combustion can be accomplished along with the removal of sulfur gases. FBC materials are a combination of unburned coal, ash, and spent bed material used for sulfur control. The spent bed material, removed as bottom ash, contains reaction products from the absorption of gaseous sulfur oxides such as SO2 and SO3 (EERC 2009). FBC can also contain some amount of free lime. Atmospheric FBC (AFBC) systems may be either bubbling (BFBC) or cir- culating (CFBC). Pressurized FBC (PFBC) is a new combus- tion technology. The chemistry of types of coal combustion byproducts is dependent on components in the raw coal fuel source and technological changes. There are four types, or ranks, of coal that can be used as a fuel source in power plants: anthracite, bituminous, sub-bituminous, and lignite. A limited amount of anthracite coal is burned; therefore, the primary compo- sition of coal combustion byproducts is controlled by the differences between bituminous, sub-bituminous, and lignite coal. The common components found in all types of coal are silica, alumina, iron oxide, and calcium oxide with varying amounts of unburned carbon. Sub-bituminous and lignite coals have higher concentrations of calcium and magnesium oxide, but reduced percentages of silica and iron oxide, and a lower unburned carbon content (i.e., loss on ignition) com- pared with bituminous coal. Changes in power plant equip- ment, such as the new FBC systems, not only produces a fifth coal combustion byproduct but can change the chemistry of the other byproducts that are collected after passing through the FBC process. The influence of new processes on tradi- tional properties of fly ashes should be monitored for chem- istry changes that may affect highway application placement or performance. Literature review Summary A total of 144 documents were located, reviewed, and summa- rized to provide information about material preparation, han- dling practices, construction processes, and costs. A summary chapter three coaL combuStion byproductS

14 Condenser Turbine Cooling Water Bottom Ash SCR Air Heater Pulverized Coal Precipitator Or Baghouse FGD Fly Ash FGD Byproduct Gypsum FGD Waste Treatment & Dewatering Stack Flue Gas SCR: Selective Catalytic reduction DeNOx system FGD: Flue Gas Desulfurization System Schematic after Using Coal Ash in Highway Construction: A Guide to Benefits and Impacts. EPA April 2005 (See Next Figure) Burner Boiler Slag FIGURE 1 Typical coal burning power plant schematic (after EPA 2005). Precipitator Or Baghouse Transfer System FlyAsh Silo Dry Storage Conditioned Fly Ash to Utilization or Disposal Dry Fly Ash to Utilization Pond Ponded Ash Excavated and Stockpiles Utilization Dry Wet Fly Ash Storage Options FIGURE 2 Fly ash storage options (continued from previous figure) (after FHWA 2005). of material preparation and quality control considerations found in the literature included: • Increased variability in the physical and chemical prop- erties of the byproducts elevated the need for additional quality control testing. • Few byproduct quality control procedures were un- covered, although the need for verification of phy s- i cal and chemical properties was found throughout the literature because of the dependency of the by - products on coal source and power plant equipment configurations. • There was a lack of byproduct specifications for pur- chasing material. This limited the ability of the agency to evaluate QC/QA programs. A summary of materials handling practices for coal com- bustion byproducts included:

15 • HMA QC/QA likely to deal with a larger variability in in-place density when using these byproducts. • Extra testing and monitoring of the optimum moisture content when using byproducts in stabilized soils. No specific failures were identified in either the litera- ture review or the survey responses; however, several com- ments were made that indicated difficulty in testing and material properties: • Additional testing was needed to account for byprod- uct variability in such specified properties as HMA density and achieving needed support from stabil- ized soils as a result of inconsistent optimum mois- ture contents. • Nonuniformity of the physical and chemical byprod- uct properties was a problem when achieving the desired application properties. • Type C fly ash in PCC applications resulted in problems with durability of the concrete (Estakhri et al. 2006). Only limited cost information was found in the literature, and this included: • Power plant owners approached lower value byprod- ucts with a “cost avoidance” philosophy. The lower the market value of the byproduct, the less likely the plant owner was to spend money on improving quality and consistency. • Power plants without their own landfills had a higher economic incentive to find markets for byproducts. Typi- cal plant landfill costs range from $3 to $15/ton for plants with their own landfills. Landfilling using another com- pany increased costs from $10 to $35/ton. • Byproducts from different sources needed to be kept separate, because the physical and chemical properties are dependent on the coal source and technology used by each power plant. • Leaching was a concern and the byproducts needed to be stockpiled so that ground-water contamination was prevented. • FBC solidified when water was added. This will be a material stockpiling concern that needs to be addressed if this byproduct is to be used in highway applications. It is possible that some highway applications would require a covered storage area. • Depending on the highway application, an extra storage silo was necessary for the byproduct. • Fugitive dust control needs to be considered when han- dling the byproducts. The main focus of research and applications to date has been to use these byproducts as substitutes for virgin materials, using the existing materials, design, and construction specifications. Alternatively, options that may be less restrictive were used with the byproducts. For example, bottom ash was more likely to be used in cold mix emulsifiers, which have less restric- tive requirements for gradation and durability than conven- tional HMA (Kalyoncu 2000). When using coal combustion byproducts in embankments, the slope stability of blends met requirements when heights were less than 20 m with a hori- zontal to vertical ratio of 2:1 or flatter with a factor of safety higher than 1.3. Compaction was important to achieving the design requirements (Kim et al. 2005). Construction differences that might be considered included: • Monitoring wells for water quality when using byprod- ucts in fill applications. Coal Limestone Steam out Air Distributor Water in Steam out Coal and Limestone Feed Exhaust Intake Air Dust Collector Water in Fly Ash Recycle Device Air Preheater Fluidized Air Flue Gas FIGURE 3 Schematic of a fluidized bed combustion (FBC) boiler (EERC 2009).

16 • Recycled Materials Resource Center: www.rmrc.unh. edu/ • Turner–Fairbank Highway Research Center: http:// www.fhwa.dot.gov/research/tfhrc/ agency Survey reSuLtS for coaL combuStion byproductS The results showed the main use for Types C and F fly ash is in PCC applications, followed by flowable fill and soil stabilization (Table 1). A limited number of states used coal ash, boiler slag, and combustion ash (unknown type). No states were using FGD in highway applications. Table 2 and Figure 4 indicate the states that reported using coal combus- tion ash byproducts in multiple highway applications. • Transportation costs limited the use of byproducts to local projects. • Agencies and contractors had increased testing costs because of the extra efforts needed for QC/QA. Limited laboratory research investigated transforming by products into more acceptable highway applications. Researchers used a combination of fly ash and FGD (syn- thetic gypsum), combined by disk pelletization using moderate temperatures to cure the resulting pellets, to form aggregates. Additional background and research information can be found at the following websites: • American Coal Ash Association trade organization: www.acaa-usa.org • Energy and Environmental Research Center, University of North Dakota: http://www.undeerc.org/ Byproducts Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embankments Flowable Fill HMA Pavement Surface Treatments (non- structural) PCC Soil Stabilization Coal Bottom Ash 1 1 1 7 7 3 2 4 1 Boiler Slag 0 1 1 4 1 8 5 2 0 Type C Fly Ash 0 0 1 5 19 5 0 33 15 Type F Fly Ash 0 0 0 3 19 4 0 41 7 FGD Scrubber Ash 0 0 0 0 0 0 0 0 0 Combustion Ash, Unknown Type 4 2 1 4 2 2 3 1 3 TABLE 1 2009 AGENCy SURvEy RESPONSES FOR USE OF COAL COMBUSTION ByPRODUCTS IN HIGHWAy APPLICATIONS Number of Applications States Coal Bottom Ash Boiler Slag Type C Fly Ash Type F Fly Ash FGD Combination or Unknown 8 — — — — — ID 7 VA — — — — — 6 — — — — — — 5 — — — — — — 4 — — IA, KS, KY, MO, MS, VA IA, MS, VA — — 3 MD, MO, NC IL, WV CO, DE, OH, OR, TX, WA CO, DE, KY, ND, WA — FL, NC, UT 2 VT KY, MO AL, DC, FL, GA, LA, MN, ND, NY, SC, UT, WV AL, CT, DC, MN, NY, OH, PA, SC, TX, UT, VT, WV — KS 1 AL, GA, IN, NH, NJ, NY, OH, PA, WI AL, FL, IN, KS, MA, MS, NJ, OH, TX, VT, WI AK, AR, AZ, CT, IL, IN, NE, NJ, NM, OK, WI AR, AZ, GA, ID, IL, IN, KS, LA, MA, ME, MO, NC, NE, NH, NJ, NM, NV, OR, OK, WI — AL, PA TABLE 2 STATES USING COAL COMBUSTION ByPRODUCTS IN HIGHWAy APPLICATIONS FROM SURvEy

17 Coal Combustion Products 2009 Coal Bottom Ash 3 1 1 1 1 MD-3 NJ - 11 1 0 0 3 3 1 7 VT-2 NH- 1 2009 Boiler Slag 2 1 2 1 1 1 1 1 MA-1 NJ - 11 1 0 3 3 VT - 1 2009 Fly Ash, Type C 4 4 4 4 4 3 DE - 3 3 3 3 2 2 2 2 2 2 2 2 1 1 1 NJ - 1 1 1 3 CT - 1 1 1 1 1 4 2 DC - 2 2 2009 Fly Ash, Type F 4 1 1 3 4 2 DE - 3 3 2 3 2 2 1 3 2 2 1 2 1 1 1 NJ - 1 1 1 1 CT - 21 1 1 1 1 2 1 2 4 2 DC - 2MA- 1 NH-1 VT-1 FIGURE 4 Agency survey results for coal combustion byproducts (numbers indicate the number of applications that use the byproduct).