Pilot-scale study of the effect of selective catalytic reduction catalyst on mercury speciation in Illinois and Powder River Basin coal combustion flue gases

A study was conducted to investigate the effect of selective catalytic reduction (SCR) catalyst on mercury (Hg) speciation in bituminous and subbituminous coal combustion flue gases. Three different Illinois Basin bituminous coals (from high to low sulfur [S] and chlorine [Cl]) and one Powder River Basin (PRB) subbituminous coal with very low S and very low Cl were tested in a pilot-scale combustor equipped with an SCR reactor for controlling nitrogen oxides (NOx) emissions. The SCR catalyst induced high oxidation of elemental Hg (Hg0), decreasing the percentage of Hg0 at the outlet of the SCR to values <12% for the three Illinois coal tests. The PRB coal test indicated a low oxidation of Hg0 by the SCR catalyst, with the percentage of Hg0 decreasing from approximately 96% at the inlet of the reactor to approximately 80% at the outlet. The low Cl content of the PRB coal and corresponding low level of available flue gas Cl species were believed to be responsible for low SCR Hg oxidation for this coal type. The test results indicated a strong effect of coal type on the extent of Hg oxidation.     

Pilot-Scale Study of the Effect of Selective Catalytic Reduction Catalyst on Mercury Speciation in Illinois and Powder River Basin Coal Combustion Flue Gases

Abstract

A study was conducted to investigate the effect of selective catalytic reduction (SCR) catalyst on mercury (Hg) speciation in bituminous and subbituminous coal combustion flue gases. Three different Illinois Basin bituminous coals (from high to low sulfur [S] and chlorine [Cl]) and one Powder River Basin (PRB) subbituminous coal with very low S and very low Cl were tested in a pilot-scale combustor equipped with an SCR reactor for controlling nitrogen oxides (NO_x) emissions. The SCR catalyst induced high oxidation of elemental Hg (Hg⁰), decreasing the percentage of Hg⁰ at the outlet of the SCR to values <12% for the three Illinois coal tests. The PRB coal test indicated a low oxidation of Hg⁰ by the SCR catalyst, with the percentage of Hg⁰ decreasing from ～96% at the inlet of the reactor to ～80% at the outlet. The low Cl content of the PRB coal and corresponding low level of available flue gas Cl species were believed to be responsible for low SCR Hg oxidation for this coal type. The test results indicated a strong effect of coal type on the extent of Hg oxidation.     

Investigation of selective catalytic reduction impact on mercury speciation under simulated NOx emission control conditions

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NOx) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NOx reduction, promote the formation of oxidized mercury (Hg2+). The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg0) oxidation. Bench-scale experiments were conducted to investigate Hg0 oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO2) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO2 in the flue gases on Hg0 oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg0 to Hg2+ under SCR reaction conditions. The importance of HCl for Hg0 oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg0 oxidation occurring across the SCR reactors in the field.     

Effect of NOx control processes on mercury speciation in utility flue gas

The speciation of Hg in coal-fired flue gas can be important in determining the ultimate Hg emissions as well as potential control options for the utility. The effects of NOx control processes, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), on Hg speciation are not well understood but may impact emissions of Hg. EPRI has investigated the reactions of Hg in flue gas at conditions expected for some NOx control processes. This paper describes the methodology used to investigate these reactions in actual flue gas at several power plants. Results have indicated that some commercial SCR catalysts are capable of oxidizing elemental Hg in flue gas obtained from the inlets of SCR or air heater units. Results are affected by various flue gas and operating parameters. The effect of flue gas composition, including the presence of NH3, has been evaluated. The influence of NH3 on fly ash Hg reactions also is being investigated.     

Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NOx Emission Control Conditions

Abstract

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NO_x) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NO_x reduction, promote the formation of oxidized mercury (Hg²⁺).

The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg⁰) oxidation. Bench-scale experiments were conducted to investigate Hg⁰ oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO₂) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO₂ in the flue gases on Hg⁰ oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg⁰ to Hg²⁺ under SCR reaction conditions. The importance of HCl for Hg⁰ oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg⁰ oxidation occurring across the SCR reactors in the field.     

A predictive mechanism for mercury oxidation on selective catalytic reduction catalysts under coal-derived flue gas

This paper introduces a predictive mechanism for elemental mercury (Hg(o)) oxidation on selective catalytic reduction (SCR) catalysts in coal-fired utility gas cleaning systems, given the ammonia (NH3)/nitric oxide (NO) ratio and concentrations of Hg(o) and HCl at the monolith inlet, the monolith pitch and channel shape, and the SCR temperature and space velocity. A simple premise connects the established mechanism for catalytic NO reduction to the Hg(o) oxidation behavior on SCRs: that hydrochloric acid (HCl) competes for surface sites with NH3 and that Hg(o) contacts these chlorinated sites either from the gas phase or as a weakly adsorbed species. This mechanism explicitly accounts for the inhibition of Hg(o) oxidation by NH3, so that the monolith sustains two chemically distinct regions. In the inlet region, strong NH3 adsorption minimizes the coverage of chlorinated surface sites, so NO reduction inhibits Hg(o) oxidation. But once NH3 has been consumed, the Hg(o) oxidation rate rapidly accelerates, even while the HCl concentration in the gas phase is uniform. Factors that shorten the length of the NO reduction region, such as smaller channel pitches and converting from square to circular channels, and factors that enhance surface chlorination, such as higher inlet HCl concentrations and lower NH3/NO ratios, promote Hg(o) oxidation. This mechanism accurately interprets the reported tendencies for greater extents of Hg(o) oxidation on honeycomb monoliths with smaller channel pitches and hotter temperatures and the tendency for lower extents of Hg(o) oxidation for hotter temperatures on plate monoliths. The mechanism also depicts the inhibition of Hg(o) oxidation by NH3 for NH3/NO ratios from zero to 0.9. Perhaps most important for practical applications, the mechanism reproduces the reported extents of Hg(o) oxidation on a single catalyst for four coals that generated HCl concentrations from 8 to 241 ppm, which covers the entire range encountered in the U.S. utility industry. Similar performance is also demonstrated for full-scale SCRs with diverse coal types and operating conditions.     

Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants

A kinetic model for predicting the amount of mercury (Hg) oxidation across selective catalytic reduction (SCR) systems in coal-fired power plants was developed and tested. The model incorporated the effects of diffusion within the porous SCR catalyst and the competition between ammonia and Hg for active sites on the catalyst. Laboratory data on Hg oxidation in simulated flue gas and slipstream data on Hg oxidation in flue gas from power plants were modeled. The model provided good fits to the data for eight different catalysts, both plate and monolith, across a temperature range of 280-420 degrees C, with space velocities varying from 1900 to 5000 hr(-1). Space velocity, temperature, hydrochloric acid content of the flue gas, ratio of ammonia to nitric oxide, and catalyst design all affected Hg oxidation across the SCR catalyst. The model can be used to predict the impact of coal properties, catalyst design, and operating conditions on Hg oxidation across SCRs.     

Effect of NOx Control Processes on Mercury Speciation in Utility Flue Gas

Abstract

The speciation of Hg in coal-fired flue gas can be important in determining the ultimate Hg emissions as well as potential control options for the utility. The effects of NO_x control processes, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), on Hg speciation are not well understood but may impact emissions of Hg. EPRI has investigated the reactions of Hg in flue gas at conditions expected for some NO_x control processes. This paper describes the methodology used to investigate these reactions in actual flue gas at several power plants. Results have indicated that some commercial SCR catalysts are capable of oxidizing elemental Hg in flue gas obtained from the inlets of SCR or air heater units. Results are affected by various flue gas and operating parameters. The effect of flue gas composition, including the presence of NH₃, has been evaluated. The influence of NH₃ on fly ash Hg reactions also is being investigated.     

Surface compositions of carbon sorbents exposed to simulated low-rank coal flue gases

Bench-scale testing of elemental mercury (Hg0) sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg0. The results of the fixed-bed testing under simulated lignite combustion flue gas composition for activated carbons showed some initial breakthrough followed by increased mercury (Hg) capture for up to approximately 4.8 hr. After breakthrough, the Hg in the effluent stream was primarily in an oxidized form (>90%). Aliquots of selected activated carbons were exposed to simulated flue gas containing Hg0 vapor for varying time intervals to explore surface chemistry changes as the initial breakthrough, Hg capture, and oxidation occurred. The samples were analyzed by X-ray photoelectron spectroscopy to determine changes in the abundance and forms of sulfur, chlorine, oxygen, and nitrogen moieties as a result of interactions of flue gas components on the activated carbon surface during the sorption process. The data are best explained by a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site. Because loss of HCl is also coincident with Hg breakthrough or loss of the divalent Hg ion (Hg2+), the competition of Hg2+ with S(VI) on the basic carbon site is also implied. Thus, the role of the acid gases in Hg capture and release can be explained.     

Predicting extents of mercury oxidation in coal-derived flue gases

The proposed mercury (Hg) oxidation mechanism consists of a 168-step gas phase mechanism that accounts for interaction among all important flue gas species and a heterogeneous oxidation mechanism on unburned carbon (UBC) particles, similar to established chemistry for dioxin production under comparable conditions. The mechanism was incorporated into a gas cleaning system simulator to predict the proportions of elemental and oxidized Hg species in the flue gases, given relevant coal properties (C/H/O/N/S/Cl/Hg), flue gas composition (O2, H2O, HCl), emissions (NO(X), SO(X), CO), the recovery of fly ash, fly ash loss-on-ignition (LOI), and a thermal history. Predictions are validated without parameter adjustments against datasets from lab-scale and from pilot-scale coal furnaces at 1 and 29 MWt. Collectively, the evaluations cover 16 coals representing ranks from sub-bituminous through high-volatile bituminous, including cases with Cl2 and CaCl2 injection. The predictions are, therefore, validated over virtually the entire domain of Cl-species concentrations and UBC levels of commercial interest. Additional predictions identify the most important operating conditions in the furnace and gas cleaning system, including stoichiometric ratio, NO(X), LOI, and residence time, as well as the most important coal properties, including coal-Cl.     

Distribution of mercury in the combustion products from coal-fired power plants in Guizhou,southwest China

Method 30B and the Ontario Hydro Method (OHM) were used to sample the mercury in the flue gas discharged from the seven power plants in Guizhou Province, southwest China. In order to investigate the mercury migration and transformation during coal combustion and pollution control process, the contents of mercury in coal samples, bottom ash, fly ash, and gypsum were measured. The mercury in the flue gas released into the atmosphere mainly existed in the form of Hg°. The precipitator shows a superior ability to remove Hg^p (particulate mercury) from flue gas. The removal efficiency of Hg²⁺ by wet flue gas desulfurization (WFGD) was significantly higher than that for the other two forms of mercury. The synergistic removal efficiency of mercury by the air pollution control devices (APCDs) installed in the studied power plants is 66.69–97.56%. The Hg mass balance for the tested seven coal-fired power plants varied from 72.87% to 109.67% during the sampling time. After flue gas flowing through APCDs, most of the mercury in coal was enriched in fly ash and gypsum, with only a small portion released into the atmosphere with the flue gas. The maximum discharge source of Hg for power plants was fly ash and gypsum instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.

Implications: Method 30B and the Ontario Hydro Method (OHM) were used to test the mercury concentration in the flue gas discharged from seven power plants in Guizhou Province, China. The concentrations of mercury in coal samples, bottom ash, fly ash, and gypsum were also measured. By comparison of the mercury content of different products, we found that the maximum discharge source of Hg for power plants was fly ash and gypsum, instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.     

The impact of wet flue gas desulfurization scrubbing on mercury emissions from coal-fired power stations

This article introduces a predictive capability for Hg retention in any Ca-based wet flue gas desulfurization (FGD) scrubber, given mercury (Hg) speciation at the FGD inlet, the flue gas composition, and the sulphur dioxide (SO2) capture efficiency. A preliminary statistical analysis of data from 17 full-scale wet FGDs connects flue gas compositions, the extents of Hg oxidation at FGD inlets, and Hg retention efficiencies. These connections clearly signal that solution chemistry within the FGD determines Hg retention. A more thorough analysis based on thermochemical equilibrium yields highly accurate predictions for total Hg retention with no parameter adjustments. For the most reliable data, the predictions were within measurement uncertainties for both limestone and Mg/lime systems operating in both forced and natural oxidation mode. With the U.S. Environmental Protection Agency's (EPA) Information Collection Request (ICR) database, the quantitative performance was almost as good for the most modern FGDs, which probably conform to the very high SO2 absorption efficiencies assumed in the calculations. The large discrepancies for older FGDs are tentatively attributed to the unspecified SO2 capture efficiencies and operating temperatures and to the possible elimination of HCl in prescrubbers. The equilibrium calculations suggest that Hg retention is most sensitive to inlet HCl and O2 levels and the FGD temperature; weakly dependent on SO2 capture efficiency; and insensitive to HgCl2, NO, CA:S ratio, slurry dilution level in limestone FGDs, and MgSO3 levels in Mg/lime systems. Consequently, systems with prescrubbers to eliminate HCl probably retain less Hg than fully integrated FGDs. The analysis also predicts re-emission of Hg(O) but only for inlet O2 levels that are much lower than those in full-scale FGDs.     

Surface Compositions of Carbon Sorbents Exposed to Simulated Low-Rank Coal Flue Gases

Abstract

Bench-scale testing of elemental mercury (Hg⁰) sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg⁰. The results of the fixed-bed testing under simulated lignite combustion flue gas composition for activated carbons showed some initial breakthrough followed by increased mercury (Hg) capture for up to ～4.8 hr. After breakthrough, the Hg in the effluent stream was primarily in an oxidized form (>90%). Aliquots of selected activated carbons were exposed to simulated flue gas containing Hg⁰ vapor for varying time intervals to explore surface chemistry changes as the initial breakthrough, Hg capture, and oxidation occurred. The samples were analyzed by X-ray photoelectron spectroscopy to determine changes in the abundance and forms of sulfur, chlorine, oxygen, and nitrogen moieties as a result of interactions of flue gas components on the activated carbon surface during the sorption process. The data are best explained by a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site. Because loss of HCl is also coincident with Hg breakthrough or loss of the divalent Hg ion (Hg²⁺), the competition of Hg²⁺ with S(VI) on the basic carbon site is also implied. Thus, the role of the acid gases in Hg capture and release can be explained.     

Predicting Extents of Mercury Oxidation in Coal-Derived Flue Gases

Abstract

The proposed mercury (Hg) oxidation mechanism consists of a 168-step gas phase mechanism that accounts for interaction among all important flue gas species and a heterogeneous oxidation mechanism on unburned carbon (UBC) particles, similar to established chemistry for dioxin production under comparable conditions. The mechanism was incorporated into a gas cleaning system simulator to predict the proportions of elemental and oxidized Hg species in the flue gases, given relevant coal properties (C/H/O/N/S/Cl/Hg), flue gas composition (O₂, H₂O, HCl), emissions (NO_X, SO_X, CO), the recovery of fly ash, fly ash loss-on-ignition (LOI), and a thermal history. Predictions are validated without parameter adjustments against datasets from lab-scale and from pilot-scale coal furnaces at 1 and 29 MW_t. Collectively, the evaluations cover 16 coals representing ranks from sub-bituminous through high-volatile bituminous, including cases with Cl₂ and CaCl₂ injection. The predictions are, therefore, validated over virtually the entire domain of Cl-species concentrations and UBC levels of commercial interest. Additional predictions identify the most important operating conditions in the furnace and gas cleaning system, including stoichiometric ratio, NO_X, LOI, and residence time, as well as the most important coal properties, including coal-Cl.     

A study of gas-phase mercury speciation using detailed chemical kinetics.

Mercury speciation in combustion-generated flue gas was modeled using a detailed chemical mechanism consisting of 60 reactions and 21 species. This speciation model accounts for the chlorination and oxidation of key flue-gas components, including elemental mercury (Hg0). Results indicated that the performance of the model is very sensitive to temperature. Starting with pure HCl, for lower reactor temperatures (less than approximately 630 degrees C), the model produced only trace amounts of atomic and molecular chlorine (Cl and Cl2), leading to a drastic underprediction of Hg chlorination compared with experimental data. For higher reactor temperatures, model predictions were in good accord with experimental data. For conditions that produce an excess of Cl and Cl2 relative to Hg, chlorination of Hg is determined by the competing influences of the initiation step, Hg + Cl = HgCl, and the Cl recombination reaction, 2Cl = Cl2. If the Cl recombination reaction is faster, Hg chlorination will eventually be dictated by the slower pathway Hg + Cl2 = HgCl2.     

滤料负载粉尘层对气态汞脱除性能的实验研究

通过不同性能纤维滤料负载燃煤飞灰粉尘层,来模拟袋式除尘器滤袋表面粉尘附着层,进而研究袋滤器用不同性能纤维滤料和粉尘附着层对燃煤烟气中Hg0的联合脱除性能。在固定床实验系统上分别进行了不同纤维滤料和燃煤飞灰粉尘层,以及经实验优选得到的华博特滤料负载燃煤飞灰粉尘层脱除燃煤烟气中Hg0的实验研究。结果表明,燃煤飞灰粉尘层和华博特滤料对Hg0分别有一定的脱除作用,脱除效率可达35%和42.5%,它们对Hg0的脱除是物理吸附和化学吸附共同作用的结果;同时,华博特滤料负载燃煤飞灰粉尘层对Hg0的联合脱除效率受到吸附反应温度、入口汞浓度和烟气停留时间等因素的影响,最佳脱汞率可达64.4%;吸附反应温度越高,脱除效率越低;烟气停留时间越大,脱除效率越高;入口汞浓度的提高并不一定提高华博特滤料负载飞灰粉尘层的脱汞效果。     

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 approximately 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.     

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

Abstract

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.     

Mercury Balance on a Large Pulverized Coal-Fired Furnace

This paper presents results of a survey of mercury concentrations in coal, ash, water, fly ash, and flue gas discharges from a 5.5 × 10⁶ Ib/hr steam generator serving a 775 MW (net) turbine-generator set. Representative composite or grab samples were obtained for inlet coal and outlet ash and water. Stack samples were obtained for fly ash and mercury vapor emissions while the unit was operated at 660 MW (net) (85% of full load). Samples were analyzed by anodic stripping voltammetry, plasma emission spectroscopy, and neutron activation analysis to determine mercury concentration entering the furnace in the coal and leaving the furnace in the flue gas, fly ash, bottom and hopper ash, and water. Method inter-comparisons are discussed. A material balance for mercury has been calculated from fuel, ash, and stack gas flow rates. About 90% of the mercury in the coal is released and appears as vapor discharged in the stack gas while 10% remains in the residual ash. For a 700 MW (net) unit, about 5 lb/day of mercury vapor is released to the atmosphere.     

Release of mercury vapor from coal combustion ash

The long-term stability of Hg in coal combustion by-products (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 degrees 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 reevaluate results reported in this article.