Effect of Sulfur Impregnation Method on Activated Carbon Uptake of

The effect of the sulfur impregnation method on mercury removal efficiency was examined through experi ments conducted on commercially available sulfu...
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Environ. Sci. Technol. 1997, 31, 2319-2325

Effect of Sulfur Impregnation Method on Activated Carbon Uptake of Gas-Phase Mercury JOHN A. KORPIEL AND RADISAV D. VIDIC* Department of Civil and Environmental Engineering, 943 Benedum Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15261-2294

The dynamics of granular activated carbon (GAC) adsorbers for the uptake of gas-phase mercury was evaluated as a function of temperature, influent mercury concentration, and empty bed contact time. Sulfur-impregnated carbons exhibited enhanced mercury removal efficiency over virgin carbon due to the formation of mercuric sulfide on the carbon surface. The effect of the sulfur impregnation method on mercury removal efficiency was examined through experiments conducted on commercially available sulfurimpregnated carbon (HGR) and carbon impregnated with sulfur in our laboratory (BPL-S). Although HGR and BPL-S possess similar sulfur contents, BPL-S is impregnated at a higher temperature, which promotes a more uniform distribution of sulfur in the GAC pore structure. At low influent mercury concentrations and low temperatures, HGR and BPL-S performed similarly in the removal of mercury gas. However, as the temperature was increased above the melting point of sulfur, the performance of HGR deteriorated significantly, while the performance of BPL-S slightly improved. At high influent mercury concentrations, HGR performed better than BPL-S, regardless of temperature. For both HGR and BPL-S, the observed dynamic mercury adsorptive capacities were far below the capacities predicted by the stoichiometry of mercuric sulfide formation. In HGR carbon the sulfur is very accessible, but agglomeration that occurs at high temperatures causes the sulfur to be relatively unreactive. In BPL-S carbon, on the other hand, the sulfur remains in a highly reactive form, but its location deep in the internal pores makes it relatively inaccessible and prone to blockage by HgS formation.

Introduction In the past couple of decades, environmental control agencies have expressed increasing concern about the release of mercury to the environment. Two types of combustion processes that are major sources of mercury emissions are coal-fired power plants (CFPPs) and municipal waste combustors (MWCs). It is estimated that world-wide fossil fuel combustion produces 1500 t of mercury annually from electricity generation and 1210 t from other industrial uses, with CFPPs accounting for 10-15% of the U.S. total mercury emissions (1). Mercury emissions from CFPPs exist in various valence states: elemental Hg0 and oxidized Hg2+ forms, such as HgCl2 and HgO. Elemental mercury gas, however, is the dominant form of mercury in the plume of a CFPP, ranging from 92% to 99% of the total mercury concentration in the * Corresponding author phone: 412-6124-1307; fax: 412-624-0135; e-mail: [email protected].

S0013-936X(96)00926-1 CCC: $14.00

 1997 American Chemical Society

air (2, 3). Several studies (4-6) have indicated that electrostatic precipitators are ineffective in removing volatile elements such as mercury, allowing at least 90% of the mercury to be discharged into the atmosphere. Control of Mercury Emissions. Currently there are no regulations on the control of mercury emissions from the electric utilities. However, the Clean Air Act Amendments of 1990 (Title III, Section 112[b][1]) required the major sources to use maximum available control technology and mandated the U.S. EPA to perform a study on the significance of mercury emissions from various sources. Air pollution control processes capable of controlling mercury emissions include activated carbon adsorption (712) and wet scrubbing (13, 14). Activated carbon adsorption can be accomplished in two different processes: powdered activated carbon (PAC) injection and fixed-bed granular activated carbon (GAC) adsorption. PAC injection involves the injection of PAC directly into the plant’s flue gas stream where it adsorbs gas-phase mercury and is collected in downstream particulate control devices, such as fabric filters or ESPs. In situations in which fixed-bed GAC adsorption may be used, the adsorber should be placed downstream of the flue gas desulfurization (FGD) units and particulate collectors, serving as the final treatment process before the flue gas is discharged into the atmosphere. Although wet scrubbing is primarily intended for the removal of SO2 from the flue gas, it can provide significant removal of mercury under certain conditions (15, 16). Sinha and Walker (9) investigated the effect of sulfurization of activated carbon on the mercury adsorptive capacity at 25 and 150 °C. At 25 °C, they found that the adsorptive capacity was greatest for the virgin carbon and decreased with increasing sulfur loading. However, at 150 °C, the adsorption of mercury by the virgin carbon was negligible compared to that of the sulfur-impregnated carbons, due to the mercury reacting with sulfur on the carbon surface to form HgS. Otani et al. (12) discovered that increasing the sulfur content of activated carbon from 0% to 13.1% by weight causes an increase in mercury adsorptive capacity at 36 °C. They also observed that the amount of mercury adsorbed by sulfurimpregnated activated carbon is much less than that predicted by the stoichiometry of the reaction Hg + S f HgS, which led to a conclusion that some fraction of the impregnated sulfur does not participate in the reaction with mercury gas. The unreacted sulfur is considered to be chemically adsorbed and stable. Krishnan et al. (15) observed that temperature had an insignificant effect on Hg0 sorption by HGR, a commercially available sulfur-impregnated carbon, within the first 4 h of the experiment. Beyond 5 h, HGR at 140 °C captured more Hg0 than HGR at 23 °C. Heat treatment of virgin activated carbon samples at 140 °C for 4 h resulted in a significant reduction in mercury capture. The authors explained that this was due to the deactivation or depletion of surface active sites. Exposing HGR to heat treatment, however, did not affect its dynamic adsorptive capacity, because the primary sorption site (S) was not deactivated. Livengood et al. (16) found that chemically pretreating virgin activated carbon with sulfur can greatly increase the removal capacity. The relative superiority of sulfur-impregnated carbon with respect to virgin carbons was enhanced at lower temperatures. The authors found that the dynamic mercury adsorptive capacity of activated carbon decreased with increasing temperature, decreasing inlet mercury concentration, and increasing particle size. Vidic and McLaughlin (17) investigated the effects of temperature, mercury concentration, and method of sulfur

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FIGURE 1. Schematic representation of the experimental setup. impregnation on mercury removal performance using three different carbons: F-400 (virgin carbon), HGR (commercial sulfur-impregnated carbon), and F-400S (F-400 impregnated with sulfur at 600 °C under inert atmosphere). The authors observed that the sulfur-impregnated GACs showed much greater adsorptive capacity than virgin activated carbon, especially in the temperature range of 25-90 °C. The enhanced performance of sulfur-impregnated carbon was due to the chemisorption of mercury facilitated by the formation of HgS on the carbon surface. The mercury removal efficiency by HGR was found to be highly temperature specific, exhibiting poor performance at 140 °C. At the low mercury concentration of 55 µg/m3, F-400S performed better than HGR while containing 21% less sulfur. This was explained by the improved method of sulfur impregnation at 600 °C, which promotes more uniform distribution of the sulfur in the GAC pore structure. Sulfur analysis revealed that the chemically bonded sulfur found on F-400S is less likely to be volatilized at higher temperatures, while the condensed sulfur found on HGR is easily removed at temperatures greater than 140 °C. The main objective of this study was to further understand the role of different impregnation methods on the availability of sulfur to bond gas-phase mercury and the corresponding impact on the dynamics of a fixed-bed adsorption systems under various process and operational conditions.

Materials and Methods Two types of activated carbon were used in this study: HGR and BPL-S. HGR, a commercially available sulfur-impregnated carbon, was supplied by the manufacturer (Calgon Carbon Corporation, Pittsburgh, PA) in 12 × 30 and 4 × 10 U.S. Mesh sizes. BPL-S was produced by impregnating a bituminous coal-based virgin activated carbon, BPL (Calgon Carbon Corporation, Pittsburgh, PA), with sulfur in a pure nitrogen atmosphere at 600 °C. The particle sizes used in this study were 60 × 80 and 4 × 10 U.S. Mesh size, having geometric mean particle diameters of 0.021 and 0.49 cm, respectively. Prior to their use in the experiments, the carbon was dried in an oven at 105 °C for 24 h and stored in a desiccator. The sulfur contents of HGR and BPL-S are 9.7% and 10.0%, respectively. A mercury permeation device manufactured by VICI Metronics (Santa Clara, CA) was used as the source of elemental mercury (Hg0) throughout the study. The per-

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meation device was designed to provide a uniform and stable release of mercury gas into the carrier gas stream at a given temperature. The permeation device was sealed in a glass permeation tube holder with inlet and outlet ports and was immersed in a temperature-controlled oil bath. Industrial grade nitrogen was used in the experiments as a carrier gas to transport the mercury gas out of the permeation tube holder. The flow rate of the carrier gas stream was controlled by Model FC-280S mass flow controllers (MFCs) connected to a Model RO-28 readout/control box (Tylan General, Torrance, CA). Two three-way valves, placed immediately upstream and downstream of the glass permeation tube holder, were used to bypass the permeation device when flushing the system with clean carrier gas. By varying the oil bath temperature and the carrier gas flow rate, a wide range of mercury concentrations could be generated. A schematic representation of the experimental setup is shown in Figure 1. A mercury trapping impinger solution was used to collect batch mode mercury samples in order to calibrate the atomic absorption spectrophotometer and to determine the total mercury concentration in the effluent stream from a fixedbed carbon adsorber. The impinger solution used for absorbing the gas-phase mercury was prepared with 1.5% potassium permanganate in 10% (3.6 N) sulfuric acid as described by Shendrikar et al. (18). All glassware was covered with aluminum foil due to the instability of the impinger solution in the presence of light, and the solution was always used within 12 h from the time it was prepared. Analytical Methods. The concentration of elemental mercury in the gas stream was measured continuously by a Perkin-Elmer Model 403 atomic absorption spectrophotometer (AAS) (Perkin-Elmer, Norwalk, CT) fitted with an 18-cm hollow quartz gas cell (Varian Australia Pty. Ltd., Mulgrave, Victoria, Australia). The wavelength of the mercury hollow cathode lamp (Fisher Scientific, Pittsburgh, PA) was adjusted to 253.7 nm. Calibration of the AAS was performed by passing a mercury laden gas at a flow rate of 1.0 L/min through the quartz cell while capturing the effluent gas in an impinger train consisting of two 250-mL gas impingers with coarse fritted cylinders (Corning, Inc., Horseheads, NY) connected in series. The mercury-laden stream was collected for 1 h, with AAS absorbance and flow rate measurements recorded at 5-min intervals. After the impinger train was taken offline, the solutions were transferred into PTFE bottles, and 10

mL of concentrated nitrohydrochloric acid (aqua regia) was used to rinse off any mercury that might have condensed on the glass walls. Liquid-phase mercury standards were prepared by spiking clean impinger solution with a known volume of 1000 µg/L mercury atomic absorption solution (Aldrich Chemical Company, Milwaukee, WI). A 15-mL solution containing 12% sodium chloride and 12% hydroxylamine sulfate in DI water was added to each PTFE bottle to reduce the excess permanganate. The bottles were vigorously shaken, vented to release the pressure generated by the reaction, and stored for liquid-phase analysis. All liquid-phase mercury analyses were performed using a Varian Model VGA-76 gas generation accessory (Varian Australia Pty. Ltd., Mulgrave, Victoria, Australia) according to EPA Method 7470 (19). The gas-phase mercury detection limit for the analytical system used in this study was 2.4 µg/m3. The sulfur content of activated carbons was measured using a Leco Model SC 132 sulfur determinator (Leco Corporation, St. Joseph, MI). Activated carbon surface areas were determined using an Orr surface-area pore-volume analyzer Model 2100 (Micromeritics Instrument Corporation, Atlanta, GA) and calculated using the nitrogen BET isotherm method. Adsorber Experiments. For short empty-bed contact time (EBCT) experiments, the experimental adsorber consisted of a 0.64 cm inner diameter stainless steel column containing 100 or 200 mg of 60 × 80 U.S. Mesh size carbon. Two 200 U.S. Mesh size stainless steel screens were used to secure the GAC bed, and the walls of the column were threaded in order to accommodate various bed depths. For long EBCT experiments conducted at elevated temperatures, the experimental adsorber consisted of a 2.2 cm inner diameter stainless steel column containing 10 g of 4 × 10 U.S. Mesh size carbon. For long EBCT experiments conducted at room temperature, an experimental adsorber consisted of a 2.4 cm inner diameter acrylic column that was charged with 10 g of 4 × 10 U.S. Mesh size carbon to yield a GAC bed depth of 3.7 cm. One 80 U.S. Mesh size stainless steel screen was used to secure the GAC bed in both cases. Throughout the study, the adsorbers were operated in downflow mode to minimize the potential for fluidization of the packed bed. The adsorbers were placed in a temperaturecontrolled oven in order to regulate the temperature of the influent gas and the GAC bed. To ensure that the temperature of the influent gas is the same as the temperature of the bed by the time the gas reached the bed, 3.5 m of coiled Teflon tubing was placed in the oven upstream of the adsorber inlet. Two three-way valves, one immediately upstream and the other immediately downstream of the oven, allowed the adsorber to be bypassed until the onset of the adsorption experiment. Prior to the start of each experiment, both the mercury permeation device and the adsorber were bypassed, while the clean carrier gas was passed through the AAS cell at the desired flow rate for a 1 h warm-up period. After the energy level of the lamp was stabilized, the absorbance reading of the AAS was zeroed. At this time the mercury permeation device was placed on-line, and the system was allowed to stabilize for a period of 2 h, after which time the absorbance reading was recorded. The adsorber was placed on-line, and the time was recorded as time zero. The effluent mercury concentration was continuously monitored, and the dynamic adsorptive capacity of the GAC was computed by integrating the area above the breakthrough curve. Quality Assurance/Quality Control Experiments. The following conclusions summarize the findings of QA/QC experiments performed on the experimental system: (a) Gas-phase mercury calibration curve for the AAS is independent of the flow rate of mercury-laden gas through the hollow quartz cell.

TABLE 1. Comparison of GAC Types GAC type

sulfur content (% by wt)

BET Asp (m2/g)

BPL HGR BPL-S

0.7 9.7 10.0

1026 482 824

(b) Adsorption of mercury onto the tubing and experimental adsorbers was found to be negligible. (c) Temperature of the influent mercury stream was in equilibrium with the oven temperature at the inlet to the adsorber. (d) Combustion of the spent carbon in an oxygen atmosphere at 900 °C, with subsequent collection of the emitted gases into an impinger train, revealed that the mass of mercury adsorbed by the carbon was within 1% of the mercury mass computed by integration of the area above the breakthrough curve. (d) Interference in the AAS measurement of elemental mercury due to possible HgS formation in the gas phase downstream of the adsorber containing sulfur-impregnated carbon is negligible.

Results and Discussion Effect of Impregnation Method on Sulfur Distribution. HGR is manufactured by condensing sulfur gas onto BPL carbon at relatively low temperatures, while BPL-S carbon was produced by impregnating sulfur onto BPL carbon at a temperature of 600 °C. Although the different impregnation methods yielded similar sulfur contents (Table 1), sulfur impregnation of HGR and BPL-S carbons caused a 53% and 20% decrease in the initial BET specific surface area of 4 × 10 U.S. Mesh size BPL carbon, respectively (Table 1). Due to the higher impregnation temperature, the sulfur in BPL-S carbon is suspected to be more evenly distributed in the pore structure, occupying the deeper, narrower pores. The sulfur in HGR carbon, on the other hand, is most likely condensed at the external surface of the carbon, blocking the access to the narrower high energy pores. Thermogravimetric analysis (TGA) was conducted by heating samples of BPL, HGR, and BPL-S carbons up to 400 °C in an argon atmosphere. Figure 2 shows that BPL and BPL-S carbons underwent negligible decreases in weight, while the weight of the HGR carbon sample decreased by 8.5%. Since both HGR and BPL-S carbons are manufactured by impregnating BPL carbon with sulfur, this outcome implies that BPL-S carbon lost a negligible amount of its impregnated sulfur, while HGR carbon lost 88% of its sulfur content. This shows that the bonding of sulfur molecules to the carbon matrix is much stronger in BPL-S than in HGR carbon. Sulfur exists in several allotropes, including Sλ (S8 rings), Sπ (S8 chains), and Sµ (chains of variable length), with S8 rings as the only form at room temperature (20-22). Since the sulfur gas at 200 °C is in the form of S8 (76.5%) and S6 (23.5%) rings (20, 22), it is reasonable to assume that HGR carbon contains sulfur predominantly in the form of voluminous S8 rings. At 600 °C, sulfur gas possesses a significant fraction of S6 (58.8%) and S2 (16.4%) molecules (22), which are less voluminous and more reactive because they possess a greater fraction of sulfur terminal atoms (23). Therefore, the smaller S2 and S6 chains can more easily migrate into the narrower pores of the carbon matrix and, as the carbon cools down to room temperature at the completion of the impregnation process, steric hindrance impedes reformation of the more voluminous allotropic Sλ from the other two allotropes (2325). Thus, BPL-S carbon possesses greater proportions of sulfur in the more reactive S2 form than the normal proportion indicated by the corresponding equilibrium constant.

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FIGURE 2. Thermogravimetric analyses of activated carbons in argon atmosphere.

FIGURE 3. Effect of flow rate on mercury uptake by HGR carbon. Based on the discussion presented above, it is believed that the sulfur in HGR carbon is predominantly in the form of S8 rings and is weakly bonded to the carbon surface in the macroporous region of the carbon particle. On the other hand, the sulfur in BPL-S carbon is strongly bonded to the carbon surface in the microporous region of the carbon particle, requiring mercury to diffuse a longer distance for chemisorption to occur. A much larger proportion of the sulfur in BPL-S carbon is believed to be in the form of S2 and S6 chains, which are more reactive than the S8 rings present in HGR carbon. GAC Performance at Short EBCT. All adsorber experiments conducted at short EBCT (e0.11 s) utilized 60 × 80 U.S. Mesh size GAC. Experiments were conducted at 140 °C using 100 mg of HGR carbon and an influent mercury concentration of 55 µg/m3. As shown in Figure 3, the performance of HGR carbon improved as the flow rate increased from 0.1 to 5.0 L/min. Although the dynamic mercury adsorptive capacity was observed to increase with increasing flow rate, it was in all cases negligible as compared to the capacity predicted by the stoichiometry of HgS formation (0.607 g of Hg/g of HGR).

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If the external mass transfer is the rate-limiting step in the overall process of mercury uptake from the gas phase, then an increase in the flow rate, which results in a decrease of the concentration boundary layer thickness, should have caused an improved rate of mercury uptake from the beginning of the experiment. Since this was not observed experimentally, it can be concluded that the external mass transfer resistance is negligible under these experimental conditions. Similarly, internal mass transfer resistance is typically not considered a rate-limiting step for gas-phase applications of activated carbon. In addition, it has already been suggested that sulfur in HGR carbon is located in a macroporous region of a GAC particle, making it easily accessible for mercury molecules. The only other possibility is that the rate of mercuric sulfide formation and subsequent diffusion into the sulfur bulk phase is the rate-limiting step in the adsorption dynamics of HGR carbon. As the flow rate is increased at a given mercury concentration, the mercury loading rate (µg/min) increases, causing an increase in the amount of mercury molecules that contact the sulfur surface in a given time period. Higher mercury loadings establish a surface excess of mercury, which increases the rate of HgS formation and drives the HgS

FIGURE 4. Effect of temperature on mercury breakthrough from HGR and BPL-S adsorbers.

FIGURE 5. Mercury breakthrough from HGR adsorber in on-line/off-line operation. molecules to diffuse more rapidly into the sulfur bulk phase. The increasing mercury loading rates accelerate the exothermic reaction of HgS formation, resulting in thermal agitation, which breaks up the sulfur chains more rapidly. The resulting increase in the number of sulfur terminal atoms causes the reactivity of the impregnated sulfur to increase. This hypothesis is further discussed later in the text. In order to compare the effect of temperature on the performance of HGR and BPL-S carbons, adsorber experiments using 200 mg of 60 × 80 U.S. Mesh carbon were conducted for a duration of 10 h at the low influent mercury concentration of 55 µg/m3, a flow rate of 1.0 L/min, and at temperatures of 25, 90, and 140 °C. The breakthrough curves illustrated in Figure 4 show that HGR and BPL-S carbons performed similarly in the uptake of mercury at 25 and 90 °C. However, when the temperature was increased to 140 °C, the performance of BPL-S carbon improved slightly while

HGR carbon exhibited significant deterioration in the ability to remove mercury from the feed stream. This may be due to the fact that 140 °C is above the melting point of sulfur (115.2 °C), which induces the sulfur that is weakly bonded to the surface of HGR carbon to melt and agglomerate as a liquid in the form of long polymer chains (20) and decreases the sulfur surface area available for contact with the incoming mercury molecules. Thus, the performance of HGR carbon at 140 °C may be limited by the slow diffusion of mercury through the liquid-state sulfur. The stronger bonding and more uniform distribution of sulfur in BPL-S carbon prevented the sulfur from agglomerating, which ensured that the performance of BPL-S carbon did not deteriorate at higher temperature. An adsorber experiment in which the mercury-laden gas was periodically diverted around the adsorber (while the temperature of the adsorber was maintained constant) was

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FIGURE 6. Effect of mercury concentration on mercury uptake by HGR and BPL-S carbons.

FIGURE 7. Effect of temperature on mercury uptake by HGR and BPL-S carbons at 1.0 s EBCT. conducted at 140 °C using the influent mercury concentration of 55 µg/m3, a flow rate of 1.0 L/min, and 100 mg of 60 × 80 U.S. Mesh HGR carbon. As shown by the breakthrough curve (Figure 5), four mercury loading steps were performed over a period of 8 days. Note that in the first two loading steps, after 100% breakthrough was reached and the adsorber was bypassed for over 1 day, the adsorber exhibited additional capacity after it was placed back on-line. The performance of HGR in each mercury loading step is summarized in Table 2. The following is a possible explanation for the behavior illustrated in Figure 5. By the time 100% breakthrough is reached in the first loading step, the available surface area of the sulfur agglomerates is completely covered by a monolayer of HgS. Since the diffusion of HgS through liquid-

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TABLE 2. Summary of HGR Performance in Each Mercury Loading mercury loading Step

duration (h)

Hg uptake (µg of Hg/g)

cumulative Hg uptake (µg of Hg/g)

1 2 3 4

4.1 5.6 13.5 21.1

34.9 45.4 155.0 263.0

34.9 80.3 235.3 498.3

state bulk sulfur is slow, the HgS monolayer blocks the incoming Hg molecules from reacting with the bulk sulfur that is still available in the carbon pores. During the period that the adsorber was bypassed, the HgS molecules had sufficient time to diffuse into the sulfur liquid phase, breaking

up sulfur chains to provide additional sulfur terminal atoms to react with mercury molecules in a subsequent loading step. The observation that the mercury removal performance actually improved in the third and fourth loading step may be due to the fact that the sulfur polymer chains began to break apart into smaller chains as more and more mercury reacted with sulfur, thereby increasing the fraction of sulfur terminal atoms, which led to an increase in the rate of HgS in the later part of the column experiment. Figure 6 illustrates the effect of the influent mercury concentration on the performance of HGR and BPL-S carbons at 140 °C. As discussed earlier, BPL-S carbon performed significantly better than HGR carbon at 140 °C and low influent mercury concentration of 55 µg/m3. Figure 6 shows that as the influent mercury concentration was increased from 55 to 684 µg/m3, the performance of HGR carbon improved while the performance of BPL-S carbon deteriorated to the extent that it performed worse than HGR carbon. The deterioration in the performance of BPL-S carbon at high concentrations may be due to the rapid formation of large HgS molecules that can block the access to the narrower pores and prevent the mercury molecules from accessing the highly reactive S2 molecules that are present in the microporous region of the BPL-S carbon. The inaccessibility of sulfur in BPL-S carbon may also explain observations in other experiments that the dynamic mercury adsorptive capacity was less than 1% of the capacity predicted by the stoichiometry of HgS formation (i.e., less than 1% of the impregnated sulfur was consumed by the time 100% breakthrough was reached). GAC Performance at Long EBCT. Column experiments at long EBCT (1.0 s) were performed at high mercury concentration (684 µg/m3) in order to accelerate mercury breakthrough for such high carbon masses in the adsorber. Figure 7 shows the mercury uptake by HGR and BPL-S carbons as a function of time at temperatures of 25 and 140 °C. Similar to the results observed at the short EBCT, HGR carbon performed better than BPL-S carbon under the condition of high influent mercury concentration and high temperature. HGR carbon also performed better than BPL-S carbon at room temperature, although the performance of both carbons improved with temperature decrease.

(3) Prestbo, E. M.; Bloom N. S. Presented at the Pittsburgh Coal Conference, Pittsburgh, PA, Sep 1994. (4) Kaakinen, J. W.; Jorden, R. M.; Lawasani, M. H.; West, R. E. Environ. Sci. Technol. 1975, 9, 862. (5) Germani, M. S.; Zoller, W. H. Environ. Sci. Technol. 1988, 22, 1079. (6) Billings, C. E.; Sacco, A. M.; Matson, W. R.; Griffin, R. M.; Coniglio, W. R.; Harley, R. A. J. Air Pollut. Control Assoc. 1973, 23, 773777. (7) Chang, R.; Offen, G. R. Power Eng. 1995, 99, 51-57. (8) Young, B. C.; Miller, S. J.; Laudal, D. L. Presented at the 1994 Pittsburgh Coal Conference, Pittsburgh, PA, Sep 1994. (9) Sinha, R. K.; Walker, P. L. Carbon 1972, 10, 754-756. (10) Matsumura, Y. Atmos. Environ. 1974, 8, 1321-1327. (11) Otani, Y.; Kanaoka, C.; Usui, C.; Matsui, S.; Emi, H. Environ. Sci. Technol. 1986, 20, 735. (12) Otani, Y.; Emi, H.; Kanaoka, C.; Uchijima, I.; Nishino, H. Environ. Sci. Technol. 1988, 22, 708. (13) Meij, R. Water, Air, Soil Pollut. 1991, 56, 21. (14) Chang, R.; Owens, D. EPRI J. 1994, July/Aug, 46. (15) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. Environ. Sci. Technol. 1994, 28, 1506. (16) Livengood, C. D.; Huang, H. S.; Wu J. M. Presented at the 87th Annual Meeting and Exhibition of the Air and Waste Management Association, Cincinnati, OH, June 1994. (17) Vidic, R. D.; McLaughlin, J. B. J. Air Waste Manage. Assoc. 1996, 46, 241. (18) Shendrikar, A. D.; Damie, A.; Gutknect, W. F. Technical Report EPA-600/7-84-089; U.S. EPA: Triangle Park, NC, 1984. (19) Technical Report EPA-600/7-84-089; U.S. EPA: Washington, DC, 1984. (20) Hampel, C. A., Ed. The Encyclopedia of the Chemical Elements; Reinhold Book Corporation: New York, 1968. (21) Pryor, W. A. Mechanisms of Sulfur Reactions; McGraw-Hill Book Company, Inc.: New York, 1962. (22) Tuller, W. N., Ed. The Sulfur Data Book; McGraw-Hill Book Company, Inc.: New York, 1954. (23) Daza, L.; Mendioroz, S.; Pajares, J. A. Clays Clay Miner. 1991, 39, 14. (24) Daza, L.; Mendioroz, S.; Pajares, J. A. Solid State Ionics 1990, 42, 167. (25) Daza, L.; Mendioroz, S.; Pajares, J. A. Appl. Catal. B: Environ. 1993, 2, 277.

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Received for review November 1, 1996. Revised manuscript received February 21, 1997. Accepted April 3, 1997.X

X

Abstract published in Advance ACS Abstracts, June 1, 1997.

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