Study of Mercury Speciation from Simulated Coal Gasification

5400

Ind. Eng. Chem. Res. 2004, 43, 5400-5404

Study of Mercury Speciation from Simulated Coal Gasification Dennis Y. Lu,*,† David L. Granatstein,† and Donald J. Rose‡ CANMET Energy Technology Centre, Natural Resources Canada, Building #1, 1 Haanel Drive, Ottawa, ON K1A 1M1 Canada, and Electricity and Industrial Combustion Branch, Environment Canada, 351 St. Joseph Boulevard, 10th Floor, Gatineau, Quebec, QC K1A 0H3 Canada

The identification and quantification of individual physicochemical forms of mercury (Hg) emissions from coal-fired systems is imperative for addressing questions concerning atmospheric fate and emission control. The flue gas composition and ash characteristics can have a significant impact on Hg speciation. Unfortunately, there is a lack of information available on mercury behavior under gasification conditions, which are different from combustion conditions. A benchscale test apparatus was designed and built to simulate the synthesis gas conditions. The primary goal of this bench-scale work was to determine which gas constituents typical of gasification (CO, CO2, HCl, Cl2, NH3, HCN, COS, and H2S) affect the oxidation of elemental mercury, which was delivered to the system via temperature-controlled permeation tubes. There appear to be a number of interactions between various synthesis gas constituents that affect mercury speciation. The results indicated that the reducing environment is not favorable for Hg oxidation via gasphase reactions alone and that more elemental mercury is expected to remain in the syngas from coal gasification. However, depending on the temperature and concentration, there is clearly an interaction between fly ash and gas combinations to promote the mercury oxidation rate. Bench-scale tests also indicated that the chemistry of mercury is very complex. Further verification of this study has been planned at a pilot-scale entrained-flow gasifier. Introduction There are typically three forms of mercury present in coal combustion flue gas: elemental mercury, Hg(0) or Hg0; oxidized mercury, Hg(II) or Hg2+; and particlebound mercury, Hg(p).1 The majority of mercury will end up in the vapor phase as either Hg(0), which is not water-soluble and is consequently difficult to remove with typical air pollution control devices, such as electrostatics precipitators, baghouses, and sulfur wet scrubbers, or Hg(II), which is more reactive and more water-soluble. The global atmospheric background level of mercury increases if the mercury is emitted in an elemental form, whereas oxidized species will be deposited near the source because of their high solubility in water and, thus, will be removed by local atmospheric processes. Mercury speciation is, therefore, important because it affects both control strategies and the environmental impact.2 The understanding of Hg speciation from coal-fired systems was started with thermodynamic modeling predictions and in-house experimental investigations of mercury reactions in simulated flue gases. In the furnace zone, where the temperature is high (>1200 °C) and the reaction rates can be high enough to reach equilibrium, it has been found that all of the coalcontained Hg will be released in elemental form. The Hg is believed to be speciated in the period of flue gas transport from the furnace exit to the stack, and therefore, certain knowledge about the cooling process of flue gases is necessary to understand the phenomenon of Hg transport through the system. A complex chem* To whom correspondence should be addressed. Mailing address: Bldg. #1, 1 Haanel Drive, Ottawa, ON, K1A1M1 Canada. Tel.: (613) 996-2760. Fax: (613) 992-9335. E-mail: [email protected]. † Natural Resources Canada. ‡ Environment Canada.

istry of Hg speciation is expected because of the involvement of numerous homogeneous and heterogeneous reactions, which are mostly kinetically controlled. Recently, there has been a considerable amount of kinetic modeling of gas-phase Hg chemistry, focusing on the region downstream of the furnace to the stack,3-6 but based on the elementary reaction kinetics. The extent to which such reactions occur in coal-fired power plants is not known. The flue gas composition and ash characteristics can have a significant impact on Hg speciation. The concentrations of common gaseous components of combustion and gasification processes are listed in Table 1. Unfortunately, there is a lack of information available on mercury behavior under gasification conditions, which are different from combustion conditions. In typical synthesis gas (syngas) from an O2-blown coal gasifier, thermodynamic calculations predict that only Hg(0) is stable rather than HgO, which is the precursor to form other stable Hg(II) species, such as HgCl2 or HgSO4, in the downstream flue gas under standard combustion oxidizing conditions. Therefore, Hg(0) is expected to be dominant in such a reducing environment.7,8 Galbreath and Zygarlicke9 explained this prediction by the observations that gasification conditions generally reduce the ferric iron (Fe3+) in minerals, so the reduction of Hg(II) is therefore expected as well. However, the chemical and physical processes governing the interactions of mercury forms with syngas components are poorly understood; in particular, the results of heterogeneous reactions occurring in gasification syngas are lacking. Reed et al.10 studied the effect of gasifier operating parameters and hot-gas filter temperature on the emissions of trace elements, including Hg, in a 2-MWt gasification pilot plant. The Hg balance closures were poor, implying that the amount of Hg in the synthesis gas was underestimated. On the other hand, the investigation done by Meij11 indicated that

10.1021/ie034121u CCC: $27.50 Published 2004 by the American Chemical Society Published on Web 07/08/2004

Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5401 Table 1. Typical Flue Gas Concentrations in Coal Combustion and Gasification combustion flue gas

synthesis gas

T (°C)

O2 (%)

CO (ppm)

HCl (ppm)

SO2 (ppm)

NOx (ppm)

100-600

2-6

10-300

1-100

100-2000

300-1000

T (°C)

H2 (%)

CO (%)

HCl (ppm)

H2S (ppm)

NH3 (ppm)

200-600

10-20

5-35

10-200

200-10000

100-1000

Figure 1. Experimental schematic.

there was no evidence of any significant Hg removal at a hot-gas filter temperature of 580 °C. However, data on Hg emissions from gasification systems have not been sufficiently reliable, and the mass balance closures have high associated error ranges because of problems with sampling and analysis, which are making the understanding mercury behavior under gasification conditions difficult.8 Recently, the Tampa Electric Polk County (FL) Power Plant was selected for the studies on mercury removal and measurement technology specifically applicable to integrated gasification combined cycle (IGCC) processes were demonstrated in the plant, which is based on ChevronTexaco gasifier technology.12 This is the first time that low concentrations of mercury have been successfully measured in high-pressure synthesis gas containing hydrogen sulfide. To achieve effective mercury emissions control, it is necessary to investigate the chemical interactions between Hg and other constituents present under gasification reducing conditions, including both homogeneous and heterogeneous processes occurring in syngas. The approach of this study is to identify the chemical transformation of mercury associated with syngas under various combinations of gas compositions, temperatures, and the presence or absence of fly ash, considered as effective factors in Hg transformation. Experimental Section Experiments were carried out in an apparatus consisting of a Hg vapor generator, static gas mixer, tubular reactor with a temperature control system, and impinger train. A schematic of the experimental setup is shown in Figure 1. Most components of this bench-scale testing setup are Teflon, Teflon-lined, or glass, and the entire system, including the gas manifold, is heat-traced to maintain a constant temperature before delivery into the impinger train. Hg Vapor Generator. Elemental Hg was introduced using a laboratory-scale mercury vapor generator, calibrated and certified by VICKI Co. (0.21 µg/min), by the

permeation tube method. The mercury generator was operated in vent mode, supplied with inert carrier gas at a flow rate adjusted to produce the desired amount of mercury. In this work, the Hg generator was heated to 30 °C, and the Hg was entrained in an Ar stream at a constant total flow rate of 10 L/min. When the temperature reached 30 °C, the generator outlet was connected to the impinger inlet or to the in-line gas mixer when other gases were introduced. Static Gas Mixer. The static gas mixer is a 12.7mm-i.d. by 432-mm-long stainless steel tube that has multiple baffles inside that create turbulence to promote mixing. The flow rate of individual gases is adjusted by the control valve prior to the gas mixer, where Hg vapor from the generator is introduced and mixed. An Ar carrier gas delivers the Hg/gas mixture to the tube reactor and to the impinger train. Tubular Reactor with Temperature Control System. The reactor is a 38-mm-i.d. by 457-mm-long quartz wool tube that has a wide inlet opening of 50mm-i.d. and a 12.7-mm-i.d. outlet. The reactor is enclosed in, but not touching, an electric heater that controls the interior temperature by radiant heat transfer. Temperatures in the experiments ranged from 250 to 750 °C. The temperature profile was prechecked to keep the temperature steady for each experiment. Impinger Train. Gas exiting the gas mixer passes into EPA Method-5 trains using the Ontario Hydro Method (OHM) for mercury speciation. The method is a modified version of the standard OHM approach with the H2O2/HNO3 solution excluded. The solutions that are used in the impingers to collect mercury are 50 mL of KCl for oxidized Hg and 50 mL of a mixture of 50% KMnO4/50% H2SO4 for elemental Hg. The gas flow from the tubular reactor is already cooled when it reaches the impingers, and ice water was not required. The experimental variables include the gas constituents in typical gasification syngas; the reaction temperature; and the introduction of fly ash to simulate the heterogeneous reactions when syngas goes through a hot-gas cleanup unit (ceramic filter), for example. Gas Compositions. Gas compositions used in these experiments are based on the typical gasification process as described in Table 2. N2, CO2, and CH4 were not used because Hg is known not to react with these gases.5 Blended gases are composed of mixtures of Ar, H2, CO, CO2, NH3, COS, H2S, HCN, and Hg. Reaction Temperature. The temperatures investigated in this study covered a wide range of 25-750 °C. The main temperature for the reaction of Hg and single gases was set at 400 °C, with multiple gases at 750 °C, and with ash initially at 250 °C. The retention times for the temperatures of 250, 400, and 750 °C were 6, 4, and 2 s, respectively. To maintain the desired temperature in the reaction zone in the tubular reactor, the tube furnace set points applied were 255, 415, and 775 °C, respectively. Fly Ash. Coal fly ash (99% -300 mesh) from the baghouse of CETC’s vertical combustor was used to ascertain the effect of both the gaseous and solid phases

5402 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 2. Gases in These Experiments and Typical Gasification Composition 1 2 3 4 5 6 7 8 9 10 11 12 13 14

H2 CO CO2 NH3 CH4 Cl2 HCN COS HCl H2S Hg H 2O Ar N2

Table 5. Effect of Temperature on Hg(0) Oxidation in Hg(0) + Cl2

typical gasification

experimental range

run

gas

temp (°C)

ash (g)

Hg2+ (%)

Hg rec (%)

1-10% 5-50% 0.5-5% 100-10000 ppm 0.1-10% 5-50 ppm 50-500 ppm 20-200 ppm 0.01-0.1% 0.05-5% 0.1-10% 1-5% 10-20%

5-10% 10-30% 500 ppm 50 ppm 50 ppm 50 ppm 100-500 ppm 200-1000 ppm 2-12 ppm balance -

8 9 4 12 35 34 37 38

Cl2 Cl2 Cl2 Cl2 Cl2 Cl2 Cl2 Cl2

25 250 400 750 25 250 400 750

0 0 0 0 15 15 15 15

5.1 14.3 23.6 nda 68.5 38.5 16.3 17.6

98.9 91.1 103 97.3 16.8 44.6 37.0 7.59

Table 3. Fly Ash Analysis (Baghouse Sample from Vertical Combustor) major oxides

amount (wt %)

constituent

amount

SiO2 Al2O3 Fe2O3 CaO CuO LOI

51 33.8 5.2 1.4 0.05 3.1

C S Hg Cl F

0.22 wt % 0.51 wt % 0.116 µg/g 606 µg/g 249 µg/g

Table 4. Properties of Original Coal and Combustion Parameters operating conditions constituent

amount (wt %)

parameter

value

C H N S O ash

76.97 5.00 1.47 0.94 5.97 8.76

peak temp coal feed P/S air ratio O2 CO2 CO

1354 °C 27.4 kg/h 23/77 3.47% 17.38% 14 ppm

on the speciation of Hg in gasification flue gas. The size of the fly ash is smaller than 50 µm. Characteristic analysis data of the fly ash are listed in Table 3. The original coal is an eastern bituminous coal, and its ultimate analysis and combustion conditions are reported in Table 4. Fifteen grams of fly ash was placed in the tubular reactor between layers of quartz wool fiber. It takes less than 0.5 s for the gas to flow through the fly ash layer. After a number of runs, the used ash was analyzed with the solid Hg analyzer and the tube was refilled with fresh fly ash. When the gas flow rates reached steady state, elemental Hg was generated and passed through the gas mixer and gas reactor before entering the impingers. The experiment was deemed to begin when Hg was released into the impingers. The total flow rates of the gas mixtures were varied depending on the number of gases, the required gas concentrations, and the gas concentration in the cylinders. The reaction time was 10 min for each run. As soon as the experiments were completed, each impinger was plugged and moved to the digestion/extraction stage. The samples with high pH (higher than pH 2) were treated with 0.3 mL of 0.1 N HNO3 to preserve the solution under acidic conditions. The collected samples were analyzed by CVAAS (cold vapor atomic absorption spectroscopy). At the beginning of the tests, a blank sample was extracted from the prepared KCl and KMnO4/H2SO4 solutions, and the Hg content in the blank was analyzed. Because mercury in the gas stream exists in either the elemental form [captured as Hg(II) in KMnO4/H2SO4

a

nd ) value is lower than the detectable limit.

solution] or the oxidized form [captured as Hg(0) in KCl solution], the percent oxidation was determined by

% oxidation ) 100 ×

Hg(II) [Hg(0) + Hg(II)]

Results and Discussion Oxidation with Single Gases. These experiments were designed to study the potential gas-phase oxidation of Hg(0) in the post-gasification region of coal gasification processes via the reaction with individual gas constituents. This oxidation (if any) can occur in the duct regions downstream of the gasifier, typically where the temperature is lower than 700 °C.4 Most tests involving single-gas oxidation were carried out at 400 °C. The homogeneous reactions with HCl and Cl2 were also studied at other temperatures: 25, 250, and 750 °C. Table 5 shows the effect of temperature on Hg(0) oxidation by Cl2. There is little reaction at room temperature, and the oxidation rate appears to increase with increasing temperature to a point. The maximum oxidation rate of Hg with Cl2 (in the absence of fly ash) observed in this study was 23.6% at a reaction temperature of 400 °C. However, oxidation of Hg was found to be insignificant at 750 °C, and the Hg(II) formed at this temperature was below the detectable limit. Despite these experimental results, equilibrium calculations predict complete oxidation of Hg(0) to HgCl2 in the presence of 50 ppm Cl2 at the temperatures in this study. Other than the effects of Cl2, no gas-phase oxidation of Hg(0) by simulated single gases was observed at various temperatures. It should be noted that the predicted levels of oxidized mercury in this study might be lower than the actual levels because some forms of Hg(II) might be more easily adsorbed onto the cold stainless steel surfaces of the gas mixer. This might be the reason the total amount of mercury collected in the impingers was lower than that collected in the baseline test when the Hg generator was working under the same conditions, especially when Cl2 was present in the gas mixture. Oxidations with Gas Combinations. The oxidation processes are complicated and probably include many unknown elementary reactions between Hg(0) and compounds of Cl, O, and S. A chlorinating agent, such as Cl2 or chlorine free radical, is most likely involved in the potential Hg(0) oxidation mechanisms in gasphase reactions. The objective of this part of the study was to verify the influence on the oxidation of Hg(0) with either HCl or Cl2, together with other syngas constituents, such as H2, CO, CO2, H2S, and/or COS. It was observed that the reaction of Hg(0) with Cl2 was depressed at temperatures of 250 and 400 °C but promoted at the higher temperature of 750 °C in the

Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5403 Table 6. Effect of Gas Combinations on Hg(0) Oxidation in Hg(0) + Cl2 run

gas

17 18 22 27 29 30 31 32 26 33

HCl + Cl2 + CO HCl + Cl2 + CO + H2S HCl + Cl2 + CO Cl2 + CO HCl + Cl2 + CO HCl + Cl2 + H2S HCl + Cl2 + CO + H2S HCl + Cl2 + CO + CO2 HCl + COS + H2S HCl + H2 + CO + COS + H2S

temp ash Hg2+ Hg rec (°C) (g) (%) (%) 250 250 400 750 750 750 750 750 750 750

0 0 0 0 0 0 0 0 0 0

8.7 6.2 11.1 5.1 9.6 5.7 5.2 27.4 7.4 16.4

54.6 60.4 71.6 66.8 48.6 83.4 64.5 113 80.9 27.3

presence of other syngas constituents, such as HCl, CO, and H2S. The effects of gas mixtures on the reaction of Hg(0) + Cl2 are shown in Table 6. At 250 °C, the addition of 20% CO and 300 ppm HCl inhibited the Hg(0) oxidation from 14.3% [base reaction of Hg(0) + Cl2] to a level of 8.7%. A similar reduction from 23.6 to 11.1% upon addition of CO and HCl was observed at 400 °C. The inhibition effect of syngas constituents is probably related to the impact of chlorine free radical scavenging caused by these species. In all experiments at 750 °C, however, oxidation of Hg(0) with Cl2 was enhanced by the addition of other gases (Table 6). In the presence of 20% CO, oxidation of Hg(0) was promoted from an unclear level (less than the detectable limit in Table 5) to a notable level of 5%. This oxidation reaction was more pronounced (to almost 10%) when 300 ppm HCl was also added. The maximum oxidation of Hg(0) (27%) was observed when a combination of C species and Cl species was present in the gasification syngas. In contrast to CO, H2S appeared to be an inhibiting reagent in the reaction of Hg(0) + Cl2 regardless of the presence of CO or HCl. The effects of syngas constituents on Hg speciation via the reaction of Hg(0) + HCl were also studied at temperatures of 250, 400, and 750 °C. The gas constituents included 20% CO, 9% H2, 20% CO2, 0.1% H2S, and 50 ppm COS. It was interesting to note that significant Hg(0) oxidation with HCl occurred under certain combinations of syngas constituents at 750 °C (Table 6), although no combination of simulated syngas produced a significant effect on the reactions of Hg(0) with HCl at lower temperatures (250 and 400 °C). A possible explanation of this is that there are no obvious reaction pathways for the gas-phase oxidation of Hg(0) by HCl; however, Hg(0) oxidation from the reaction of Hg(0) + HCl + COS is probably attributed to the formation of a chlorine radical via the reaction between HCl and COS. Nevertheless, the result observed here was far below that predicted from equilibrium calculations, which give a greater than 50% conversion of Hg(0) in this simulated condition. Heterogeneous Catalytic Oxidation of Hg(0). These experiments were designed to simulate conditions in hot-gas cleaning of the coal-fired gasification process. In hot-gas cleaning devices, syngas penetrates fly ash on the surfaces of the filter candles, and oxidation of Hg(0) can occur on the surface of fly ash through heterogeneous catalytic reactions, with the oxidized Hg(II) at least partially remaining on the filter. In this study, the fixed-bed reactor was packed with the model fly ash, and the extent of Hg(0) oxidation and removal across the reactor was determined. Results showed that fly ash has a very significant effect not only on mercury speciation, but also on

Figure 2. Effect of fly ash on Hg(0) oxidation at various temperatures.

mercury removal. Depending on the reaction temperature and the gas stream composition, Hg captures of 50-95% were observed in these tests. Hg capture increased with increasing temperature: about 55% of the total Hg was collected in the ash fixed bed at a temperature of 250 °C, 70% at 400 °C, and 92% at 750 °C. Analysis of the ash samples after the tests confirmed that a relative proportion of Hg had been retained in the ash. In addition to the removal of mercury, the fly ash bed affected the Hg speciation as well, depending on the gas stream constituents. Generally, higher values of Hg in oxidized form were observed in the gas stream after passing through the fly ash, and the results are listed in Figure 2. The first vertically striped column shows the effects of fly ash on gas mercury speciation at 250 °C. Compared to the situations without fly ash, oxidation of Hg(0) was found to double when only Cl2 was present in the syngas. Pronounced oxidation was observed even when HCl was present, as well as a notable level of Hg(II) in the presence of either CO or H2S via heterogeneous catalytic reactions. However, at this lower temperature (250 °C), CO and H2S led to the inhibition of Hg(0) oxidation similar to indications from the gas-phase work. The inhibitory effect of CO and H2S on Hg(0) oxidation can be related to scavenging of the catalytically generated chlorinating agents, Cl2 for instance. Compared to the gas-phase-only reaction of Hg(0) + Cl2 at 400 °C (Table 5), there were no significant changes to gas-stream Hg speciation in the presence of fly ash except that much less total mercury was collected in the impingers. On the other hand, Hg(0) oxidation via reaction with HCl was promoted when fly ash was added to the reactor (Figure 2). A remarkable increase in Hg(0) oxidation efficiency was noticed in the presence of CO and/or H2S at these higher temperatures. The maximum oxidation rate (97%) in this study was obtained with the gaseous combination of 20% CO, 0.1% H2S, 300 ppm HCl, and 50 ppm Cl2 at 400 °C. These results might imply that operating conditions for hotgas cleanup can be optimized with respect to Hg capture in coal gasification systems. The results obtained at 750 °C are mostly similar to those obtained at a temperature of 400 °C. With the involvement of fly ash, Hg(0) oxidation was more pronounced and gas-phase Hg(II) appeared in all combinations of syngas constituents. Ghorishi and Lee6 performed a test where the fly ash was first exposed to HCl in the absence of Hg(0) and then exposed to the

5404 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004

Hg(0) stream without the presence of HCl. No oxidation of Hg(0) was observed. The calculations of Reed et al.,13 using a thermodynamic equilibrium model, predicted that Hg capture in a gasifier hot-gas filter was attributable to chemisorption of HgS. These two studies indicate that either HCl or H2S is needed during the oxidation of Hg(0) in the presence of fly ash. The heterogeneous mechanisms of Hg(0) oxidation, including the fly ash catalytic process and activation by HCl or H2S, are not well understood. It appears that Hg speciation might be changed dramatically in the hotgas filtration system in coal gasification processes via heterogeneous catalytic reactions. Possibly, some metallic oxides, such as Fe2O3, CuO, and CaO, in the fly ash produce an active chlorinating agent from gas-phase HCl or an active site from gas-phase H2S. These active sites and agents can be reacted with the fly ash on which a surface chlorine or sulfur radical is attached, for example, a gas-phase Cl free radical or a gas-phase chlorine molecule (Cl2). Subsequently, Hg(0) molecules are attacked by the radicals, resulting in the most probable oxidation state, HgCl2 or HgS, which either remains on the fly ash surface or is released back into the gas stream and then collected in the KCl impinger as Hg(II).6 The process can be described in terms of a detailed kinetic mechanism consisting of elementary gas-phase and gas-solid reactions with a better understanding of the mechanisms involved in the catalytic oxidation of Hg(0). This can be done either by adding individual oxides to a base ash or by selecting specific fly ashes with desired contents of metallic oxides. Conclusions Hg(0) oxidation via gas-phase reactions with individual syngas constituents is very slow, and only in the presence of Cl2 were significant amounts of oxidized mercury produced at elevated temperatures. In this study (T ) 25-750 °C), a maximum Hg(0) oxidation efficiency of 24% was observed in the presence of 50 ppm Cl2 at 400 °C. Despite this result, equilibrium calculations predict complete oxidation of Hg(0) to HgCl2 under these conditions, indicating that the assumption of gasphase equilibrium for mercury species in gasification flue gases is not valid. In tests of Hg speciation in simulated syngas conditions, Hg(0) conversion to Hg(II) in gas-phase reactions occurred only when either Cl2 or HCl + COS was involved in the mixed gas stream. The presence of high concentrations of CO or H2S inhibited Hg(0) oxidation from the system of Hg(0) + Cl2 depending on the reaction temperature. Considering the low concentrations of Cl2 and COS in the syngas stream in postgasification regions, together with typical reducing conditions, the conditions in coal gasification processes are not favorable for mercury oxidation via gas-phase reactions alone and coal-contained Hg will remain in the elemental form in the duct downstream of the gasifier. It should be noted that these simulated conditions might differ from those of the real gasifier associated with the time-temperature profile, resulting in a reduced concentration of Cl radicals, which are supposed to be important in the Hg chemistry from coal combustion systems. The most significant influence on Hg speciation in this investigation was found in the presence of fly ash, simulating the process of hot-gas filtration during gasification. The focus of the study was on the effect of

fly ash on Hg speciation in the gas phase via various heterogeneous catalytic oxidation mechanisms. The observations indicated that fly ash cake is an effective material for promoting oxidation of Hg(0) with typical Cl and S species in syngas; as well it plays a remarkable role in mercury removal. Depending on the reactor temperature and the gas stream composition, Hg captures of 50-95% were observed in these tests. In summary, it appears that Hg speciation might be changed dramatically in gasification flue gas in the presence of fly ash. Acknowledgment Funding by Natural Resources Canada and Environment Canada is gratefully acknowledged. The authors also acknowledge the contributions of V. Ko, R. Dureau, and C. Salvador for assisting with the experimental work and for many helpful suggestions and discussions during the experimental work. Literature Cited (1) Sloss, L. L. Mercury Emissions and EffectssThe Role of Coal; IEAPER/19; IEA Coal Research: London, Aug 1995. (2) Brown, T. D.; Smith, D. N.; Hargis, R. A.; O’Dowd, W. J. Mercury measurement and its control: what we know, have learned, and need to further investigate. J. Air Waste Manage. Assoc. 1999, 49 (6), 1. (3) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. A study of gas-phase mercury speciation using detailed chemical kinetics. J. Air Waste Manage. Assoc. 2001, 51, 869. (4) Sliger, R. N.; Kramlich, J. C.; Marinov, N. M. Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury. Fuel Process. Technol. 2000, 65-66, 423. (5) Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. J.; MamaniPaco, R. Gas-phase transformations of mercury in coal-fired power plants. Fuel Process. Technol. 2000, 63, 197. (6) Ghorishi, S. B.; Lee, C. W. Fundamentals of Mercury Speciation and Control in Coal-Fired Boilers; U.S. Environmental Protection Agency; Final Report EPA-600/R-98-014; U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1998. (7) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Trace elements from combustion and gasification of coalsAn equilibrium approach. Prog. Energy Combust. Sci. 1994, 20, 115. (8) Sloss, L. L. MercurysEmissions and Control; CCC/58; IEA Coal Research: London, Feb 2002. (9) Galbreath, K. C.; Zygarlicke, C. J. Mercury speciation in coal combustion and gasification flue gases. Environ. Sci. Technol. 1996, 30 (8), 2421. (10) Reed, G. P.; Ergudenler, A.; Grace, J. R.; Watkinson, A. P.; Herod, A. A.; Dugwell, D.; Kandiyoti, R. Control of gasifier mercury emissions in a hot gas filter: The effect of temperature. Fuel 2001, 80, 623. (11) Meij, R. A sampling method based on activated carbon for gaseous mercury in ambient air and flue gases. Water Air Soil Pollut. 1991, 56, 117. (12) Ghani, J. K.; Steele, R. D.; McDaniel, J. E. Monitoring and removal of mercury in a Texaco IGCC gasifier system. In Proceedings of the 4th Combined Power Plant Air Pollutant Control MEGA Symposium; Air & Waste Management Association: New York, 2003; CD-Paper 091. (13) Reed, G. P.; Brain, S.; Cahill, P.; Fantom, I. R. Control of trace elements in gasification: A measurement methodology to validate thermodynamic predictions. In Proceedings of the 16th International Conference on Fluidized Bed Combustion; Preto, F. D. S., Ed.; ASME Press: New York, 1997; Vol. 2, p 1285.

Received for review September 11, 2003 Revised manuscript received March 8, 2004 Accepted May 25, 2004 IE034121U