Flue Gas Desulfurization Materials Research Articles

Physicochemical and radiological characterization of flue gas desulfuration waste samples from Brazilian coal-fired power plants

Flue gas desulfurization (FGD) waste is an industrial by-product generated during the flue gas desulfurization process in coal-fired power plants. This by-product contain trace quantities of naturally occurring radionuclides and elements such as As, Ba, Co, Cr, Zn. The characteristics of FGD waste are important for its reuse and are mainly depend on the desulfurization process. In this work, two types of FGD materials collected from three coal-fired power plants using semi-dry and wet processes were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), X-ray fluorescence (XFR) and particle size analysis. The radioactive content of 238U, 232Th, 228Th, 226Ra, 228Ra, 210Pb and 40K and trace elements were also determined using Neutron activation analysis and Gamma-ray spectrometry. The major constituents for all samples were Ca, Si, S, Al and Fe. Wet FGD by-product presented also high magnesium content. The wastes contain mainly semi-hydrate calcium sulfite and calcium sulfate. The particle size of FGD from semi-dry process was lower than that from the wet process. The average activity concentration of 238U, 232Th, 226Ra, 210Pb, 228Ra, 228Th and 40K varied between were 50-71, 33-42, 28-52, 113-150, 26-33, 33-39, 161-390 Bq kg-1, respectively. According to the results of leaching and solubilization tests, FGD samples were classified as non-hazardous and non-inert. The obtained data are useful for evaluation of possible applications of FGD by-products.

A review on fly ash from coal-fired power plants: chemical composition, regulations, and health evidence.

Throughout the world, coal is responsible for generating approximately 38% of power. Coal ash, a waste product, generated from the combustion of coal, consists of fly ash, bottom ash, boiler slag, and flue gas desulfurization material. Fly ash, which is the main component of coal ash, is composed of spherical particulate matter with diameters that range from 0.1 μm to >100 μm. Fly ash is predominately composed of silica, aluminum, iron, calcium, and oxygen, but the particles may also contain heavy metals such as arsenic and lead at trace levels. Most nations throughout the world do not consider fly ash a hazardous waste and therefore regulations on its disposal and storage are lacking. Fly ash that is not beneficially reused in products such as concrete is stored in landfills and surface impoundments. Fugitive dust emissions and leaching of metals into groundwater from landfills and surface impoundments may put people at risk for exposure. There are limited epidemiological studies regarding the health effects of fly ash exposure. In this article, the authors provide an overview of fly ash, its chemical composition, the regulations from nations generating the greatest amount of fly ash, and epidemiological evidence regarding the health impacts associated with exposure to fly ash.

Ranking Coal Ash Materials for Their Potential to Leach Arsenic and Selenium: Relative Importance of Ash Chemistry and Site Biogeochemistry.

The chemical composition of coal ash is highly heterogeneous and dependent on the origin of the source coal, combustion parameters, and type and configuration of air pollution control devices. This heterogeneity results in uncertainty in the evaluation of leaching potential of contaminants from coal ash. The goal of this work was to identify whether a single leaching protocol could roughly group high-leaching potential coal ash from low-leaching potential coal ash, with respect to arsenic (As) and selenium (Se). We used four different leaching tests, including the Toxicity Characteristic Leaching Protocol (TCLP), natural pH, aerobic sediment microcosms, and anaerobic sediment microcosms on 10 different coal ash materials, including fly ash, lime-treated ash, and flue gas desulfurization materials. Leaching tests showed promise in categorizing high and low-leaching potential ash materials, indicating that a single point test could act as a first screening measure to identify high-risk ash materials. However, the amount of contaminant leached varied widely across tests, reflecting the importance of ambient conditions (pH, redox state) on leaching. These results demonstrate that on-site geochemical conditions play a critical role in As and Se mobilization from coal ash, underscoring the need to develop a situation-based risk assessment framework for contamination by coal ash pollutants.

Leaching characterization of dry flue gas desulfurization materials produced from different flue gas sources in China

By-products produced from dry flue gas desulfurization (DFGD) have been used in many engineering applications. Structural fill and/or mine reclamation applications utilize more than 74 and 22% of the annual DFGD production in Europe and the United States, respectively. In China, although there are reports showing the applications of DFGD materials in the production of bricks, precast concrete, highway construction, and agricultural applications, large-scale, high-volume uses have developed slowly. One of the major roadblocks to high-volume utilization is lack of knowledge regarding the associated utilization issues. In order to better understand the environmental response of Chinese DFGD materials, this study compared the leaching characteristics of three typical DFGD materials produced from different flue gas sources (i.e., coal-fired boiler, iron and steel sintering plant, and circulating fluidized bed (CFB) boiler). The goal of this study is to characterize the release of inorganic constituents of concern (COCs) under the leaching conditions (i.e., pH dependence, percolation column, and diffusion-controlled conditions) representing different potential application scenarios. Except for Se, the maximum concentrations of all selected COCs (Hg, B, Mo, As, Ba, Cd, Cr, Pb, Sb, Co, and Tl) under all tested leaching conditions were lower than the toxicity levels. The high release of Se observed under acidic conditions in the pH-dependent leaching tests might be adversely promoted by nitric acid induced oxidation of sparingly soluble selenite minerals to soluble selenate counterparts. Rapid initial releases of Mo, Se, B, and Tl were found during the early stages of the percolation column leaching tests. B, Mo and Se were found to be the most readily leachable COCs under the scenarios when the leaching is equilibrium-controlled. Under a diffusion-controlled leaching environment, Cr and Tl showed the highest cumulative mass release among the COCs.

Alloying effect of copper on the corrosion properties of low-alloy steel for flue gas desulfurization system

The alloying effect of Cu for a flue gas desulfurization materials was investigated using the electrochemical methods in the modified green death solution and the surface analyses. The test results demonstrated that the densely formed rust layer with high metallic Cu content improves the corrosion resistance of Cu-containing steel in the flue gas desulfurization (FGD) environment. The rust layer on the surface of the 0.02 wt% Cu steel, which has an insufficient Cu content, was less protective than others. The 0.05 wt% Cu steel represented the highest corrosion resistance due to the formation of the densely formed rust layer with optimum Cu content. Because the free standing Cu2S precipitates had the insoluble characteristic in highly acidic solution, it produced the relatively porous Cu-enriched layer on the 0.08 wt% Cu steel surface. From these phenomena, the corrosion resistance of specimen decreased as the Cu content of specimen increased from 0.05 wt% to 0.08 wt%.

Mercury emission and plant uptake of trace elements during early stage of soil amendment using flue gas desulfurization materials

A pilot-scale field study was carried out to investigate the distribution of Hg and other selected elements (i.e., As, B, and Se), i.e., emission to ambient air, uptake by surface vegetation, and/or rainfall infiltration, after flue gas desulfurization (FGD) material is applied to soil. Three FGD materials collected from two power plants were used. Our results show Hg released into the air and uptake in grass from all FGD material-treated soils were all higher (P < 0.1) than the amounts observed from untreated soil. Hg in the soil amended with the FGD material collected from a natural oxidation wet scrubber (i.e., SNO) was more readily released to air compared to the other two FGD materials collected from the synthetic gypsum dewatering vacuum belt (i.e., AFO-gypsum) and the waste water treatment plant (i.e., AFO-CPS) of a forced oxidation FGD system. No Hg was detected in the leachates collected during the only 3-hour, 1-inch rainfall event that occurred throughout the 4-week testing period. For every kilogram of FGD material applied to soil, AFO-CPS released the highest amount of Hg, B, and Se, followed by SNO, and AFO gypsum. Based on the same energy production rate, the land application of SNO FGD material from Plant S released higher amounts of Hg and B into ambient air and/or grass than the amounts released when AFO-gypsum from Plant A was used. Using FGD material with lower concentration levels of Hg and other elements of concern does not necessary post a lower environmental risk. In addition, this study demonstrates that considering only the amounts of trace elements uptake in surface vegetation may under estimate the overall release of the trace elements from FGD material-amended soils. It also shows, under the same soil amendment conditions, the mobility of trace elements varies when FGD materials produced from different processes are used. Implications This study evaluated environmental impact associated with land application of FGD material by considering the overall mass balance of an element in a life cycle starting from coal combustion process to the stage of land application. The approach allows the relative significance of trace element release potential among different FGD materials, as well as the distribution of trace elements in different release pathways (i.e., air emission, uptake in grass, and soil leachate), to be compared based on different environmental management schemes.

Stabilization of Flue Gas Desulfurization By-Products with Fly Ash, Cement, and Sialite

Flue gas desulfurization (FGD) sludge is a by-product generated by air pollution control systems used in coal-fired power plants. The objective of this work was to stabilize FGD gypsum and FGD filter cake with Class C fly ash. The effectiveness of other additives, such as cement, cement kiln dust, and sialite was also investigated. The chemical, physical, and mineralogical properties of FGD gypsum and FGD filter cake were analyzed. Compaction, California bearing ratio (CBR), and unconfined compressive strength (UCS) tests were conducted on FGD gypsum and FGD filter cake with fly ash. The UCS tests were also conducted on FGD materials stabilized separately with cement, cement kiln dust, and sialite. This study investigated the environmental implications of these two types of FGD by-products with different additives. The test results indicated that both CBR values and UCS for FGD gypsum and filter cake increased with an increase in fly ash contents. FGD gypsum has the potential to be used in geotechnical fill and embankment directly, while FGD filter cake cannot be used directly in pavements without stabilization.

Laboratory Investigation of Hg Release from Flue Gas Desulfurization Products

Flue gas desulfurization (FGD) is a process applied to remove acid deposition precursors from the coal combustion air stream. This process also removes mercury (Hg) resulting in accumulation of this element in FGD produced solids. This project investigated Hg release from FGD materials to the air and water. Hg concentrations in the synthetic leaching precipitation procedure extracts, designed to simulate rainwater pH conditions, were in general <10 ng L(-1). Unlike coal fly ash, which has been found to adsorb Hg from the air, FGD materials were found to release Hg to the air over time with the addition of water a dominant environmental factor promoting release. The chemistry of the atmosphere to which the FGD materials were exposed (i.e., air Hg concentration and presence of oxidants), as well as that of the material (i.e., salts removed), was found to influence the magnitude of emissions. Although this work showed a component of the Hg captured by the FGD process could be released to the air under laboratory conditions, the potential for release under disposal and beneficial use conditions needs to be determined.

TRASH OR TREASURE?: Putting Coal Combustion Waste to Work

Even as public debate rages over the question of whether coal should continue to provide the majority of U.S. electric power needs, the U.S. Energy Information Administration predicts in International Energy Outlook 2009 that, absent new policies to the contrary, the United States—along with China and India—is expected to account for 88% of the projected net increase in coal consumption between 2006 and 2030. Meanwhile, coal combustion waste (CCW)—the noncombustible remains from coal burning—continues to pile up at the rate of about 131 million tons per year in the United States alone, and electric utilities are looking to recycle a larger proportion of this material.

Investigation of a Mercury Speciation Technique for Flue Gas Desulfurization Materials

Most of the synthetic gypsum generated from wet flue gas desulfurization (FGD) scrubbers is currently being used for wallboard production. Because oxidized mercury is readily captured by the wet FGD scrubber, and coal-fired power plants equipped with wet scrubbers desire to benefit from the partial mercury control that these systems provide, some mercury is likely to be bound in with the FGD gypsum and wallboard. In this study, the feasibility of identifying mercury species in the FGD gypsum and wallboard samples was investigated using a large sample size thermal desorption method. Potential candidates of pure mercury standards including mercuric chloride (HgCl2), mercurous chloride (Hg2Cl2), mercury oxide (HgO), mercury sulfide (HgS), and mercuric sulfate (HgSO4) were analyzed to compare their results with those obtained from FGD gypsum and dry wallboard samples. Although any of the thermal evolutionary curves obtained from these pure mercury standards did not exactly match with those of the FGD gypsum and wallboard samples, it was identified that Hg2Cl2 and HgCl2 could be candidates. An additional chlorine analysis from the gypsum and wallboard samples indicated that the chlorine concentrations were approximately 2 orders of magnitude higher than the mercury concentrations, suggesting possible chlorine association with mercury.

Air-Substrate Mercury Exchange Associated with Landfill Disposal of Coal Combustion Products

Previous laboratory studies have shown that lignite-derived fly ash emitted mercury (Hg) to the atmosphere, whereas bituminous- and subbituminous-derived fly ash samples adsorbed Hg from the air. In addition, wet flue gas desulfurization (FGD) materials were found to have higher Hg emission rates than fly ash. This study investigated in situ Hg emissions at a blended bituminous-subbituminous ash land-fill in the Great Lakes area and a lignite-derived ash and FGD solids landfill in the Midwestern United States using a dynamic field chamber. Fly ash and saturated FGD materials emitted Hg to atmosphere at low rates (−0.1 to 1.2 ng/m2hr), whereas FGD material mixed with fly ash and pyrite exhibited higher emission rates (~10 ng/m2hr) but were still comparable with natural background soils (−0.3 to 13 ng/m2hr). Air temperature, solar radiation, and relative humidity were important factors correlated with measured Hg fluxes. Field study results were not consistent with corresponding laboratory observations in that fluxes measured in the latter were higher and more variable. This is hypothesized to be partially an artifact of the flux measurement methods.

Chemical and Physical Properties of Dry Flue Gas Desulfurization Products

Beneficial and environmentally safe recycling of flue gas desulfurization (FGD) products requires detailed knowledge of their chemical and physical properties. We analyzed 59 dry FGD samples collected from 13 locations representing four major FGD scrubbing technologies. The chemistry of all samples was dominated by Ca, S, Al, Fe, and Si and strong preferential partitioning into the acid insoluble residue (i.e., coal ash residue) was observed for Al, Ba, Be, Cr, Fe, Li, K, Pb, Si, and V. Sulfur, Ca, and Mg occurred primarily in water- or acid-soluble forms associated with the sorbents or scrubber reaction products. Deionized water leachates (American Society for Testing and Materials [ASTM] method) and dilute acetic acid leachates (toxicity characteristic leaching procedure [TCLP] method) had mean pH values of >11.2 and high mean concentrations of S primarily as SO(2-)4 and Ca. Concentrations of Ag, As, Ba, Cd, Cr, Hg, Pb, and Se (except for ASTM Se in two samples) were below drinking water standards in both ASTM and TCLP leachates. Total toxicity equivalents (TEQ) of dioxins, for two FGD products used for mine reclamation, were 0.48 and 0.53 ng kg(-1). This was similar to the background level of the mine spoil (0.57 ng kg(-1)). The FGD materials were mostly uniform in particle size. Specific surface area (m2 g(-1)) was related to particle size and varied from 1.3 for bed ash to 9.5 for spray dryer material. Many of the chemical and physical properties of these FGD samples were associated with the quality of the coal rather than the combustion and SO2 scrubbing processes used.

Placement of Coal Combustion By-Products at Surface Mining Control and Reclamation Act(SMCRA) Mines: A Short History of OSM Technical Efforts and Responses to Environmental Concerns

The use and disposal of Coal Combustion By-Products (CCBs) (i.e. fly ash, bottom ash, flue gas desulfurization material, and fluidized bed combustion material) at coalmines has become an area of intense interest, research, activity, and controversy during the last decade. Beginning in May of 1994, the Office of Surface Mining (OSM) has taken an active role in encouraging and promoting technological advances, research, and technology transfer related to the use and disposal of those material residues remaining after the combustion of coal to produce electric power. Currently, approximately 2 percent of the CCBs that are produced in the U.S. are placed back at about 2 percent of the mines sites where they originated. Research indicates that the placement of these materials on the mine site usually results in a beneficial impact to human health and the environment when it is used to mitigate other existing potential mining hazards. Beneficial uses include: (1) a seal to contain acid forming materials and prevent the formation of acid mine drainage; (2) an agricultural supplement to create productive artificial soils on abandoned mine lands where native soils are not available; (3) a flowable fill that seals and stabilizes abandoned underground mines to prevent subsidence and the production of acid mine drainage; (4) a construction material for dams or other earth like materials where such materials are needed as a compact and durable base; and (5) a non-toxic, earthlike fill material for final pits and within the spoil area. Concerning CCB placement at coal mines, some environmental groups believe, based on historic problems experienced at some power plants, that the use of these materials at coal mines places an unacceptable risk on public health and environmental quality. This paper will attempt to provide a response to criticism that SMCRA programs are not adequate to protect public health and the environment when CCBs are placed at a SMCRA permitted mine.

Long-Term Behavior of Fixated Flue Gas Desulfurization Material Grout in Mine Drainage Environments

In this research, we examine the long-term ( ;4 years! behavior of fixated flue gas desulfurization ~FGD! material grout following placement within the Roberts-Dawson underground coal mine. Surface water and groundwater samples were collected to examine the impact of grouting on water quality, and core samples were obtained to assess the geochemical stability of the grout material. Surface water samples collected from the main seep at the Roberts-Dawson mine indicated that 4 years after grout placement the long-term fluxes of acidity, iron, sulfur, and calcium were slightly elevated compared to pregrout conditions. The long-term discharge of these constituents was likely due to continued dissolution of grout material ~for Ca and S! as well as changes in flow paths and subsequent solubilization of metal salts accumulated within the mine voids ~for acidity, Fe, Al, and S!. Although the fluxes of these elements were elevated, no measurable deleterious impact was observed for the underlying groundwater or adjacent surface water reservoir. Groundwater samples collected from monitoring wells installed within the grout material indicated that acid mine drainage waters were neutralized by the grout material. Mineralogical analyses demonstrated minimal penetration of mine drainage water into the high strength fixated FGD material grout, and little weathering of the material was observed. These data indicate that the high strength fixated FGD material grout injected into the Roberts-Dawson mine was geochemically stable and could locally neutralize mine drainage waters. However, more complete grouting and more extensive mine flooding is likely needed in order to bring about significant improvements in seep water quality.

Flowable Fill Using Flue Gas Desulfurization Material

Abstract Flowable fills are an effective and practical alternative to commonly used compacted earth backfills. Flowable fill is a cementious material, commonly a blend of cement, fly ash, sand, and water, that does not require compaction, may be self-leveling at time of placement, may harden quickly within a few hours, and can be excavated in the future if need be. Many flue gas desulfurization (FGD) materials have low unit weight and good shear strength characteristics and thus hold promise for flowable fill applications. This paper focuses on the potential of using two types of FGD materials (spray dryer and wet fixated FGD material) in flowable fill as a replacement for conventional fly ash. Several design mixes were considered. The design mixes consisted of varying amounts of FGD material, cement, lime, and water. The mixes were tested in the laboratory for flowability, unit weight, moisture content, unconfined compressive strength, erodibility, set-time, penetration, and long-term strength characteristics. Tests were conducted for up to 90 d of curing. Without any additives, the FGD material was observed to be as good as a regular (normal set) flowable fill in terms of placeability, unconfined compressive strength, and diggability. FGD material flowable fill with additives and admixtures compares favorably with the characteristics of conventional quick set flowable fills.

A New Method to Assess Mercury Emissions: A Study of Three Coal-Fired Electric-Generating Power Station Configurations

U.S. Environmental Protection Agency (EPA) Method 7473 for the analysis of mercury (Hg) by thermal decomposition, amalgamation, and atomic absorption spectroscopy has proved successful for use in Hg assessment at coal-fired power stations. In an analysis time of ∼5 min per sample, this instrumental methodology can directly analyze total Hg—with no discrete sample preparation—in the solid matrices associated with a coal-fired power plant, including coal, fly ash, bottom ash, and flue gas desulfurization (FGD) material. This analysis technique was used to investigate Hg capture by coal combustion byproducts (CCBs) in three different coal-fired power plant configurations. Hg capture and associated emissions were estimated by partial mass balance. The station equipped with an FGD system demonstrated 68% capture on FGD material and an emissions estimate of 18% (11 kg/yr) of total Hg input. The power plant equipped with low oxides of nitrogen burners and an electrostatic precipitator (ESP) retained 43% on the fly ash and emitted 57% (51 kg/yr). The station equipped with conventional burners and an ESP retained less than 1% on the fly ash, emitting an estimated 99% (88 kg/yr) of Hg. Estimated Hg emissions demonstrate good agreement with EPA data for the power stations investigated.

Status review of mercury control options for coal-fired power plants

Status review of mercury control options for coal-fired power plants

Release of Mercury Vapor from Coal Combustion Ash

The long-term stability of Hg in coal combustion byproducts (CCBs) was evaluated at ambient and near-ambient temperatures. Six CCB samples with atypically high levels of total Hg were selected for study assuming a greater potential for release of measurable amounts of Hg vapor. The samples selected included two fly ash samples from U.S. eastern bituminous coal, two fly ash samples from South African low-rank coal, one fly ash from Powder River Basin (PRB) subbituminous coal blended with petroleum coke, and one PRB subbituminous coal fly ash incorporated with flue gas desulfurization material. Air scrubbed of Hg was passed through compacted 100-g aliquots of each sample at 1 mL/min and vented to a gold-coated quartz trap to collect released Hg vapor. The samples were maintained at ambient and near-ambient (37 °C) temperatures. All samples released low-picogram levels of Hg after 90 days. No pattern was evident to link the total Hg content to the rate of release of Hg vapor. An average of 0.030 pg Hg/g CCB/day was released from the samples, which equates to 2.2 x 10-8 lb Hg/ton CCB/year. If this were applied to a coal-fired power plant production of 200,000 tons of fly ash per year, there would be a maximum potential release of 0.0044 lb, or 2.00 g, of Hg per year. Experiments are continuing to determine long-term vapor release of Hg from CCBs. All samples have been set up in duplicate at ambient temperature with an improved apparatus to reevalu-ate results reported in this article.

Materials flow in the production and use of coal combustion products

One of the most versatile and therefore desirable forms of energy is electricity. Electricity accounts for more than one third of the total energy consumption in the United States, and more than half of the Nation’s electricity is produced by burning coal. During 1997, over 800 million metric tons of coal were burned by electric utilities. As a result, more than 95 million metric tons of coal combustion products (CCPs) were generated. In 1997 more than 26 million metric tons of CCPs were used in many applications. The largest use area for a specific CCP was the use of 8.6 million metric tons of fly ash in concrete and cement applications. The leading uses of bottom ash include use as structural fill (1.3 million metric tons) and in roadbase and subbase materials (1.2 million metric tons). Nearly 100% of the boiler slag was used in the blasting grit and roofing granule markets, while more than 1.5 million tons of flue gas desulfurization (FGD) material was used in the manufacture of wallboard. In 1966, the first year for which data are available, only 12.2% of the CCPs generated where used. This figure has increased and currently close to 28% of all CCPs are used. Fly ash and bottom ash production are expected to increase in the future and match projected increases in coal burned by the electric utilities. The production of FGD material is also projected to increase. The magnitude of this increase will depend upon a number of factors, some of which are regulatory. Growth in CCP is also expected to occur. Applying the principles of materials flow to CCPs, from source to ultimate disposition, can help manage our natural resources and protect the environment.

Evaluation of a dry FGD material as a flowable fill

Many flue gas desulfurization (FGD) materials have low unit weight and good shear strength characteristics and thus hold promise for flowable fill applications. This paper focuses on the potential of using spray dryer FGD ash in flowable fill as a replacement for conventional fly ash. Several design mixes were considered. The design mixes consisted of varying amounts of FGD, cement, lime, admixture, and water. The mixes were tested in the laboratory for flowability, unit weight, moisture content, unconfined compressive strength (UCS), erodibility, set time, penetration, and long-term strength characteristics. Tests were conducted for up to 90 days of curing. The FGD flowable fill without any additives was observed to be comparable to regular (normal set) flowable fill in terms of placeability, UCS, and ‘diggability’. FGD flowable fill with additives and admixtures compared favorably with the characteristics of conventional quick-set flowable fills.