Bench-Scale Experimental Study on the Effect of Flue Gas

Sep 4, 2003 - This paper outlines the results of a systematic study on the capture of trace mercury vapor from simulated flue gases, using activated c...
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Bench-Scale Experimental Study on the Effect of Flue Gas Composition on Mercury Removal by Activated Carbon Adsorption Rong Yan,* Yuen Ling Ng, David Tee Liang, Chun Siong Lim, and Joo Hwa Tay Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Centre, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723 Received February 25, 2003. Revised Manuscript Received July 28, 2003

This paper outlines the results of a systematic study on the capture of trace mercury vapor from simulated flue gases, using activated carbons. The experiments were conducted on a benchscale fixed-bed test rig with intensive focus on the variable flue gas components and compositions. To understand the interaction and competitive adsorption of different gas components well, these gases (O2, CO2, SO2, and moisture) were introduced, one by one, into the simulated flue gas system, which basically contained only nitrogen and elemental mercury vapor. The performances of five commercially available activated carbons and one prepared H2S-exhausted activated carbon were evaluated under different flue gas compositions. The experimental data suggested that the adsorption of mercury is greatly dependent upon the flue gas compositions. For sulfur-impregnated carbon, adsorption capacity is more constant than virgin carbon over a wide range of humidities and CO2 and SO2 concentrations. The H2S-exhausted activated carbon demonstrated an even better performance than the virgin carbon. Furthermore, the chemistry and related potential carbon surface reactions were discussed in-depth for a better understanding of the impact of variable flue gas components on the capacity of activated carbons for mercury removal in the simulated coal-fired flue gases.

1. Introduction Mercury emission in flue gases is of significant environmental concern, because of its toxicity and high volatility. Various environmental regulations have addressed mercury emission levels from hazardous and municipal waste incinerations. On December 14, 2000, the United States Environmental Protection Agency (USEPA) announced that it would regulate mercury emissions from coal-fired boilers under Title III of the Clean Air Act Amendments (CAAA) of 1990. Hence, interests have increased in regard to the assessment and exploration of cost-effective technologies for the removal of mercury from fossil-fuel flue gases. Mercury in fuels, although bound mostly with inorganic pyrite fractions, is always volatilized and converted to vapor (Hg0) in the high-temperature regions of both incinerators and coal-fired combustors. As the flue gas cools, Hg0 can be oxidized to mercury chloride (HgCl2) and other mercury compounds (HgO and HgSO4) in vapor and solid phases.1,2 Most of the existing air pollution control technologies (such as scrubber, electrostatic precipitators (ESPs), and baghouse) and some novel technologies are only effective at removing certain mercury species. For example, oxidized mercury species are generally water-soluble and can, therefore, * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Hall, B.; Schager, P.; Lindqvist, O. Water, Air, Soil Pollut. 1991, 56, 3-14. (2) Brown, T. D.; Smith, D. N.; Hargis, R. A.; O’Dowd, W. J. J. Air Waste Manage. Assoc. 1999, 49, 628-640.

be effectively captured by wet scrubber technologies. Conversely, Hg0 gas is difficult to capture and must be either converted to an oxidized form or adsorbed by sorbents injection upstream of an ESP or baghouse.3,4 Activated carbon adsorption is a technology that offers a great potential for control of gas-phase mercury emissions, including elemental mercury. Several results from bench-scale tests indicate that the activated carbon capacity for mercury removal is highly dependent on temperature, the mercury species, and the concentration of mercury and other flue gas constituents.5-12 Researchers have generally performed bench-scale mercury adsorption tests in nitrogen;5-8 it is a logical starting point to perform sorption experi(3) Biswas, P.; Wu, Y. C. J. Air Waste Manage. Assoc. 1998, 48, 113127. (4) Meserole, F. B.; Chang, R.; Carey, T. R.; Machac, J.; Richardson, C. F. J. Air Waste Manage. Assoc. 1999, 49, 694-704. (5) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. Environ. Prog. 1997, 16, 47-53. (6) Vidic, R. D.; McLaughlin, J. B. J. Air Waste Manage. Assoc. 1996, 46, 241-250. (7) Vidic, R. D.; Chang, M. T.; Thurnau, R. C. J. Air Waste Manage. Assoc. 1998, 48, 247-255. (8) Lancia, A.; Musmarra, D.; Pepe, F.; Volpicelli, G. Adsorption of Mercuric Chloride Vapours from Incinerator Flue Gases on Calcium Hydroxide Particles. In Combustion Technologies for a Clean Environment; Carvalho, M. D. G., Lockwood, F. C., Fiveland, W. A., Papadopoulos, C., Eds.; Gordon and Breach Publishers: 1991; pp 619-631. (9) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Fuel Process. Technol. 2000, 65-66, 343-363. (10) Li, Y. H.; Lee, C. W.; Gullett, B. K. Carbon 2002, 40, 65-72. (11) Lee, T. G.; Biswas, P.; Hedrick, H. AIChE J. 2001, 47, (4), 954961. (12) Carey, T. R.; Hargrove, O. W., Jr.; Richardson, C. F.; Chang, R.; Meserole, F. B. J. Air Waste Manage. Assoc. 1998, 48, 1166-1174.

10.1021/ef030041r CCC: $25.00 © 2003 American Chemical Society Published on Web 09/04/2003

Effect of Flue Gas Composition on Mercury Removal

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Figure 1. Experimental setup for the adsorption of mercury by activated carbon.

ments under simple conditions, to provide insight into the carbon-mercury chemistry. However, with reactive gases present in real processing condition, complex interaction between mercury and various flue gas components can occur at the carbon surface, which causes variable carbon performances. A full factorial design experiment in determining the relative effects of SO2, HCl, NO, and NO2 on the elemental mercury capture ability of a commercial activated carbon was recently conducted at the Energy and Environmental Research Center (EERC) at the University of North Dakota;9 the results indicated a major interaction between SO2 and NO2, which controls the mechanism responsible for poor carbon performance. The effect of activated carbon surface moisture on low-temperature mercury adsorption was investigated,10 whereas the influence of SO2 on mercury (Hg0) capture by in situgenerated sorbents was also evaluated.11 In addition, Carey et al.12 studied the flue gas composition (SO2, HCl, NOx, and water), together with other factors that affect mercury control in simulated flue gases using activated carbon, and different impacts of SO2 were observed on the Hg0 capture by two calcium-based sorbents.13 The effects of flue gas composition on carbon adsorption capacity suggest that the mercury adsorption mechanism is not purely physical. Interactions between mercury and flue gas components on the carbon surface could be important. Although research has been performed to study the adsorption of mercury by activated carbon, current knowledge in regard to the nature of the adsorption is limited. Much of the carbon capacity data in the literature was generated with greater mercury concentrations than those in the coal-fired flue gas. The kinetics and mechanisms that are involved in the mercury adsorption process, using activated carbon, particularly at low mercury concentration (∼10 µg (N m3)-1) and short exposure time, have not been fully understood. Our previous study evaluated the perfor(13) Ghorishi, S. B.; Sedman, C. B. J. Air Waste Manage. Assoc. 1998, 48, 1191-1198.

mance and properties of several commercial activated carbons in regard to the removal of mercury vapor from simulated flue gases where, nevertheless, only N2 and elemental mercury vapor were involved.14 Yet, it provides a good start and a basis to understand the potential influence of additional gas components in the real flue gas. In this phase of work, the attention was focused on the interaction and competitive adsorption of typical flue gas species (O2, moisture, CO2, SO2, and a mixture of these gases) with elemental mercury vapor by introducing other gas species into the simulated flue gas system progressively. The effects of these components on mercury capture by selected activated carbons were investigated, and the related carbon surface reactions are fundamentally discussed. 2. Experimental Section 2.1. Experimental Setup. The apparatus used in this study is illustrated in Figure 1. The mercury permeation device was designed to release mercury vapor over a range of temperatures. Nitrogen, which serves as a balance and carrier gas, was purged into the U-shaped tube that contained the mercury source, which was heated mostly at 30 °C (Ts) through a water bath. Glass beads were packed in the inlet branch of the glass tube, to encourage a good mixing of gases and to equilibrate the gas temperature to the bath-controlled temperature before arrival at the mercury source. The nitrogen gas was passed through the mercury source, and when it emerged from the outlet, the nitrogen gas carried a trace amount of elemental mercury vapor (Hg0). Additional streams of humidified N2, O2, CO2, and SO2 were introduced progressively, to obtain a simulated flue gas with variable components and compositions in the range of a typical combustion flue gas. A mixing chamber was connected between the mercury source and the carbon adsorption system. The HCl gas shown in Figure 1 is for further studies that involve the introduction of speciated mercury (HgCl2 and Hg0) into the simulated flue gas. An additional carbon bed was used to clean the gas from the carbon bed outlet before the gas was exhausted. (14) Yan, R.; Liang, D. T.; Tsen, L.; Wong, Y. P.; Lee, Y. K. Mercury Speciation in Combustion Flue Gases and Its Capture Using Activated Carbon Adsorption. Proceedings of the 95th Annual Conference and Exhibition, Air and Waste Management Association, Baltimore, MD, June 23-27, 2002.

1530 Energy & Fuels, Vol. 17, No. 6, 2003 The selected activated carbons used in adsorbing mercury tests were packed in another U-shaped tube that was heated to a desired reaction temperature (Tr). Adsorption in this study primarily was conducted at 90 °C, which is similar to the duct temperature of flue gases in a typical combustion plant. Similarly, glass beads were packed in the inlet branch of the U-shaped tube. The mercury concentrations at the inlet and outlet of the carbon column were measured using a mercury analyzer (Nippon model AM-2 Mercury Monitor for Gas). The activated carbon adsorption efficiency (Ef), quantified by a comparison between the mercury content of the gas at the inlet and outlet of the adsorption column, was used to evaluate the carbon performance under different parametric conditions, such as the mass loading of various carbons, the flow rate, and the concentration of different components. The simulated flue gas was prepared with a low level of mercury vapor (∼10 µg (N m3)-1), and the capacity of the activated carbon samples used was high; therefore, the amount of carbon selected for the experiments was small, to demonstrate the initial breakthrough phenomenon within a reasonable experimental time frame. The mercury Ef value at the initial stage of the adsorption process was used to represent the adsorption kinetics and carbon capacity in the mercury uptake. 2.2. Mercury Analyzer and Calibration. The Nippon AM-2 Mercury Monitor for Gas consisted of the mercury analyzer and a standard gas box. In the analyzer, the gas sample that was drawn in by an internal pump was first scrubbed with deionized water (to remove water-soluble gas species) and dehumidified before elemental mercury vapor in the sample was collected onto the mercury collector. After collection, the sample gas in the collection unit was purged and replaced by precleaned air. The mercury collector was then heated, to release the adsorbed mercury from the collector, and the released mercury vapor was quantified using the cold vapor atomic absorption spectroscopy (CVAAS) process. The following parameters were used primarily for the experiments: flow rate of the gas sample through the gas monitor, 500 mL/min; nitrogen flow rate through the mercury source, 300 mL/min; flow rate of precleaned air used as makeup gas, 200 mL/min; sampling time, 1 min; inlet mercury concentration, Ci ≈ 10 µg (N m3)-1 at a water bath temperature of Ts ) 30 °C and a reaction temperature of Tr ) 90 °C; and weight of activated carbon sample, 1 g. Additional notes on parameters found in the subsequent text were addressed for certain exceptional cases. Because only a trace amount of mercury was used in the experiment, it was important to calibrate the testing rig and confirm the precision of the analysis on a regular basis. The standard gas box, which is designed to generate controlled samples of elemental mercury vapor over a range of temperatures, was used for these purposes. Gas was extracted from the standard gas box, using a syringe, and injected into the mercury monitor. A comparison between the result given by the monitor and the known value of the gas sample served to confirm the accuracy of the test. The calibration curves at different mercury concentration ranges were prepared, and the monitor was determined to be capable of measuring a wide range of mercury concentrations, down to 3 ng/m3, which is applicable for the removal of trace mercury in natural gas (which is 93

n/a 800-1600 coconut shell 4×8 pellet 850-1650 0.48 4 4 61 10.7 99.1

Masda GAC n/a >900 n/a n/a powder n/a n/a 1.5 g. Evidently, in the latter case, the carbon capacity in the larger sample was greatly exceeded by the mercury input into the adsorption system, and, hence, any decrease in the mercury uptake by the carbon was not observable within the short experimental time. Therefore, an activated carbon sample size of 1 g was chosen for most of the following experiments. The mercury removal efficiencies in adsorption tests with different carbons at a usage of 1 g are given in Table 3, showing a comparison of the two cases: with and without the presence of O2. The presence of oxygen generally enhanced mercury vapor adsorption for all five commercial carbons, particularly for non-sulfur-impregnated IndoGerman, from 29.8% to 44.2%. Oxygenforming reactive carbon-oxygen complexes at the carbon surface have been reported by Lizzio et al.;16 oxygen on carbon surfaces is believed to encourage the formation of bonds with elemental mercury, thus enhancing mercury adsorption.10 Mercury adsorption by activated carbon in a gas stream that contained oxygen was improved, compared to that with just nitrogen, as found previously.17 This observation suggested that the oxygencontaining species on the activated carbon surface were not sufficient for the improvement of mercury capture: An additional oxygen supply in the gas stream was responsible for the oxidation of elemental mercury and, hence, the improved capture of mercury. (16) Lizzio, A. A.; De Barr, J. A. Fuel 1996, 75, (13), 1515-1522. (17) Olson, E. S.; Miller, S. J.; Sharma, R. K.; Dunham, G. E.; Benson, S. A. J. Hazard. Mater. 2000, 74, 61-79.

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Figure 2. Effect of oxygen on the efficiency of Waterlink carbon (Ts ) 30 °C, Tr ) 90 °C). Table 3. Effect of Oxygen on Carbon Adsorption Efficiency (Ts ) 30 °C, Tr ) 90 °C, 1 g of Carbon) adsorption efficiency (%) carbon

0% O2

6% O2

Waterlink CBII Calgon Norit IndoGerman Masda H2S-exhausted carbon fresh carbon before H2S adsorption

98.7 97.4 93.3 29.8 97.5 86.5 73.4

99.6 98.0 95.4 44.2 99.7 63.1 73.4

In the absence of oxygen, the fresh KOH-impregnated carbon performed even better (73.4%) than commercial IndoGerman carbon (29.8%), in regard to its capacity for elemental mercury vapor uptake. After H2S adsorption, sulfur deposition on the KOH-impregnated carbon led to an enhanced capacity for mercury uptake (86.5%), because of the formation of mercuric sulfide (HgS), which is a relatively stable form of mercury. This observation correlates well with previous works18-20 in which elemental sulfur was observed to form on caustic impregnated carbon when a stream of H2S gas was passed through the carbon and elemental sulfur on carbons reacted with mercury to form HgS. The addition of oxygen in the gas stream to fresh KOH-impregnated carbon did not increase the mercury adsorption efficiency of this carbon. Contrary to what was observed for the other five commercial carbons, the mercury adsorption efficiency of the H2S-exhausted carbon actually was reduced, from 86.5% to 63.1% in the presence of oxygen. So far, the reason for this phenomenon is unclear, and further study is needed for a better understanding. Based on the previous experimental observations, the oxygen concentration was maintained at 6% in the baseline simulated flue gas of N2, O2, and elemental mercury for subsequent experiments. 3.3. Effect of Moisture on Mercury Adsorption Efficiency. The five commercial carbons were used for this series of experiments. Moisture was introduced into the baseline flue gas; the content was expressed as the relative humidity (RH), which was in the range of 0%80%. The presence of moisture in the flue gas generally (18) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 19801986. (19) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 214, 407-415. (20) Vitolo, S.; Pini, R. Geothermics 1999, 28, 341-354.

Figure 3. Effect of moisture on the efficiency of activated carbons (Ts ) 30 °C, Tr ) 90 °C, 6% O2, 1 g of carbon).

decreased the mercury adsorption efficiency of the carbons (see Figure 3). Waterlink and Masda carbons were able to maintain their efficiencies at >99%, despite the RH attaining a value of 80%, although a slight decrease of their efficiencies was still observed. Norit carbon maintained an efficiency of ∼96%, whereas a decrease of adsorption efficiency from 98% to 96% was observed for Calgon carbon. The effect of moisture was more prominent for IndoGerman carbon; its efficiency decreased from 44% at RH ) 0% to 6% at RH ) 60%. Water moisture could have condensed on the carbon surface, forming a water film and depleting the reaction and adsorption sites that are available for mercury reaction and uptake. On the contrary, Li et al.10 reported a significant positive effect of carbon surface moisture on Hg0 adsorption at room temperature (27 °C), where the carbon capacities were observed to be drastically reduced after a low-temperature (110 °C) treatment to remove moisture. It was suggested that Hg0 bonding on the carbon surfaces was associated with oxygen and the presence of surface moisture promoted mercury bonding through interaction between H2O and carbon-oxygen complexes to create surface conditions that favor Hg0 adsorption.10 3.4. Effect of CO2 on Mercury Adsorption Efficiency. Three carbons were involved in this series of tests: Waterlink and IndoGerman carbons, as the representatives of sulfur-impregnated carbon and nonimpregnated carbon, respectively, and the H2S-exhausted carbon. The presence of CO2 in the baseline flue gas, in the range of 0%-20%, generally did not cause a significant decrease in the mercury adsorption efficiency of Waterlink carbon, with the efficiency remaining high (>97%; see Figure 4). However, the mercury adsorption efficiency of the non-sulfur-impregnated activated carbon (IndoGerman) showed a decrease, from 44% to 25%, with increasing CO2 concentration from 0% to 20%. The H2S-exhausted carbon performed well in mercury uptake in the presence of CO2, with efficiencies in the range of 60%-80%. The variation of the data could be due to the slight difference in surface sulfur concentration of the individual carbon pellet during the sorption of H2S gas. The decrease of IndoGerman capacity in the presence of CO2 was probably due to the reduction in the active sites that resulted from the competitive

Effect of Flue Gas Composition on Mercury Removal

Figure 4. Effect of CO2 on the efficiency of activated carbons (Ts ) 30 °C, Tr ) 90 °C), 6% O2, 1 g of carbon). Legend is as follows: (O) Waterlink carbon, (4) IndoGerman carbon, and (0) dried H2S-exhausted KOH-impregnated carbon.

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2000 ppm SO2. Drying this carbon did not cause a significant change in the efficiency, except at a SO2 concentration of 1000 ppm, where the mercury capturing efficiency was increased from 59% to 73%. It was previously reported13 that the capture of mercury by various limes increased as the SO2 concentration increased; the authors attributed this observation to the reaction of calcium sorbents with SO2 to create active sulfur sites for the chemisorption of Hg0 species. On the other hand, negative effects of the SO2 presence on mercury capturing by various sorbents (Darco FGD carbon, CaO and TiO2 powders) were also observed.11,12 In this study, the mechanism for the improved adsorption of IndoGerman carbon in the presence of SO2 was likely to be a sulfurization process that occurred at carbon surfaces. The oxygen (at a concentration of 6% in the gas phase) could react with activated carbon surfaces to form C-O or C(O) complexes (reaction 1) that oxidize the adsorbed SO2 to form SO3 (reaction 2), as previously reported by RaymundoPin˜ero et al.21 With the possible oxidation of SO2 to SO3, the mechanism behind the improved mercury adsorption performance of IndoGerman carbon could be the formation of HgSO4 in the presence of moisture through reactions 3 and 4.

O2(ads) + 2C T 2C-(O)

(1a)

C-O T C(O)

(1b)

2SO2(ads) + O2(g) T 2SO3(ads)

(2)

SO3(ads) + H2O(l) T H2SO4(l)

(3)

Hg(ads) + C(O) + H2SO4(l) T HgSO4(l) + H2O + C (4) Figure 5. Effect of SO2 on the efficiency of activated carbons (Ts ) 30 °C, Tr ) 90 °C, 6% O2, 1 g of carbon). Legend is as follows: (O) Waterlink carbon, (4) IndoGerman carbon, (b) H2S-exhausted KOH-impregnated carbon, and (0) dried H2Sexhausted KOH-impregnated carbon.

adsorption of CO2 and Hg0 on this carbon, which demonstrated surface conditions that favor CO2 adsorption. A few works have been published so far, in regard to the effect of CO2 presence on the capture of Hg0 using sorbents. Recently, Miller et al.9 reported the complete ineffectiveness of Norit FGD (LAC) carbon, in regard to capturing elemental mercury upon exposure to a baseline gas mixture of O2, CO2, N2, and H2O; the carbon provided only 10%-20% of Hg0 capture at 107 °C. 3.5. Effect of SO2 on Mercury Adsorption Efficiency. Figure 5 presents the results of Waterlink, IndoGerman, and H2S-exhausted carbon with the introduction of SO2 (0-2000 ppm) into the baseline flue gas system. Again, the presence of SO2 did not cause a significant decrease in the mercury adsorption efficiency of Waterlink carbon. However, it has a dramatic, positive impact on the performance of IndoGerman carbon, with the mercury adsorption efficiency increasing from 44% to 64% at 500 ppm SO2 and reaching a high value of 85% at 2000 ppm SO2. On the other hand, the effect of SO2 on the performance of the H2Sexhausted carbon was slightly negative; the adsorption efficiency of this carbon decreased from 63% to 52% at

where SO2(ads) represents adsorbed SO2 at the carbon surface, C-O are stable complexes, and C(O) are reactive complexes. Further study is needed to understand the mechanisms involved fully. On the other hand, increasing the SO2 concentration demonstrated a negative effect on the H2S-exhausted carbon performance in mercury uptake, probably because of the high sulfur content (42%) found in this carbon. In the experiment conducted by Vitolo et al.,20 an optimum sulfur concentration (12.5% and 27.5% for two different carbon samples used) was observed for mercury capture beyond which the mercury removal by activated carbon decreased tremendously. Passing a gas stream of SO2 could have resulted in the oversaturation of sulfur on the surface of the H2S-exhausted carbon, causing it to lose active sites and/or catalytic properties for mercury capture, which, thus, causes a decline from the optimum sulfur concentration. 3.6. Effect of Mixture of CO2, H2O, O2, and SO2. The performance of Waterlink, IndoGerman, and H2Sexhausted carbon (all fixed at 1 g of carbon usage), in regard to the uptake of mercury in a complex flue gas (Hg + N2 + 6% O2 + 12% CO2 + 40% RH + 1000 ppm SO2), was evaluated to simulate the real flue gas situation. The results obtained under the complex flue gas are outlined in Table 4 and compared with those found under baseline flue gas cases. Altogether, seven (21) Raymundo-Pin˜ero, E.; Cazorla-Amoro¨s, D.; Linares-Solano, A. Carbon 2001, 39, 231-242.

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Table 4. Effect of Mixtures of O2, Water, CO2, and SO2 on Mercury Adsorption Efficiency (Ts ) 30 °C, Tr ) 90 °C, 1 g of Carbon) mercury adsorption efficiency (%) flue gas composition

Waterlink

IndoGerman

H2S-exhausted carbon

case 1, elemental Hg + N2 case 2, Hg + N2+ 6% O2 case 3, Hg + N2+ 6% O2 + 40% RH case 4, Hg + N2+ 6% O2 + 12% CO2 case 5, Hg + N2+ 6% O2 + 1000 ppm SO2 case 6, Hg + N2+ 6% O2 + 12% CO2 + 40% RH case 7, Hg + N2+ 6% O2 + 12% CO2 + 40% RH + 1000 ppm SO2

98.7 99.6 99.0 99.2 99.5 98.9 99.9

29.8 44.2 9.6 41.6 66.4 36.3 50.2

86.5 63.1 75.7 61.9 58.6 55.0 59.8

cases of flue gas compositions are involved (Table 4). In general, the performance of sulfur-impregnated Waterlink carbon remained high (at least 98.7%) in the presence of O2, H2O, CO2, SO2, or combinations of these. However, obvious variations were observed, relative to the other two carbons, which reflected the significant influences of flue gas compositions on the carbon performance in Hg0 uptake. From Case 1 (the simplest flue gas previously studied) to Case 2 (with the presence of 6% O2), the performance of IndoGerman carbon was significantly increased whereas the H2S-exhausted carbon actually reduced the mercury uptake, as mentioned previously. From Case 2 to Cases 3, 4, and 5, the CO2, moisture, and SO2 were respectively introduced into the baseline flue gas. Negative effects were observed, relative to IndoGerman carbon, with the presence of CO2 and particularly water, whereas SO2 enhanced the performance of this carbon, from 44.2% to 66.4%. As for the H2S-exhausted carbon, in contrast, both CO2 and SO2 demonstrated slightly negative effects, whereas the presence of moisture enhanced the performance of this carbon from 63.1% to 75.7%. The opposite effects of SO2 and water on the two carbons accounted for their different natures of adsorption. Chemisorption of the Hg0 species dominated the process that involved the H2S-exhausted carbon, where water generally enhanced the carbon performance, whereas carbon capacities decreased with the presence of SO2, because of the oversaturation of sulfur on the carbon surface. As for IndoGerman carbon, which is a nonimpregnated carbon, physisorption was generally the major related mechanism, and, hence, the water film might block the active sites available for mercury uptake. Nevertheless, with increasing SO2 concentration in the gas phase, the sulfurization of this carbon improved the mercury uptake, probably by chemisorption. Case 6 showed, by comparison with Cases 3 and 4, the effects of combining CO2 and water in the flue gas. An even-worse performance was observed with the H2Sexhausted carbon (55.0%) after the combination of CO2 and moisture in Case 6. The introduction of CO2 to Case 3 enhanced the performance of IndoGerman carbon from 9.6% to 36.3%, the results of which were similar to the data observed in Case 4 (41.6%), which implied a stronger competitive adsorption in this carbon due to CO2 than that due to water. Also, it indicated the importance of the presence of CO2 in the moist flue gas when IndoGerman carbon is used for mercury capture. Finally, Case 7 indicated the full combination of the components studied (Hg + N2 + 6% O2 + 12% CO2 + 40% RH + 1000 ppm SO2) in the simulated flue gas. Improved performances were observed, relative to all

three carbons studied, compared to Case 6. Waterlink carbon has a slight increase in efficiency of 1%, to 99.9%. IndoGerman carbon demonstrated a much better performance (50.2%) with the introduction of SO2 in Case 7, compared to an efficiency of 36.3% in Case 6. The positive effect observed was probably due to the presence of SO2 in this carbon in Case 7, as was also observed previously in Case 5. The performance of the H2S-exhausted carbon was slightly improved as well, to 59.8% in Case 7, compared to 55.0% in Case 6, although the negative effects of the presence of SO2 were previously observed, with respect to Case 5 (compared to Case 2), probably because of the coexistence of SO2 with water in Case 7. As mentioned previously, the amount of carbons used was maintained at 1 g, to amplify the effect of any reaction and interaction of gas species within a reasonably short experimental time, because the adsorption capacity was high for all the activated carbon samples and high sample loading might be far excessive, relative to the trace level of mercury input into the simulated flue gas (∼10 µg (N m3)-1). Increasing the sample weight would increase the gas residence time and, hence, increase adsorption efficiency. This phenomenon was evidenced with tests (Cases 6 and 7) that were conducted using various carbon sample weights (1-3 g, with incremental steps of 1 g). The results are shown in Figure 6. As estimated, increasing the carbon weight generally increased the mercury capture for all the three carbons. Nevertheless, a significantly higher increase was observed in both IndoGerman and H2S-exhausted carbons in Case 7, where the full combination of the gas components concerned was involved. 4. Conclusions Generally, with variable flue gas compositions, carbon from Waterlink, which is representative of commercial sulfur-impregnated carbons, maintained high performances, with efficiencies of >98.7% in all seven cases. IndoGerman carbon, which is a virgin carbon, demonstrated significant variation, with efficiencies in the range of 9.6%-66.4%. The H2S-exhausted carbon performed even better than the commercial IndoGerman carbon, with efficiencies in the range of 55.0%-86.5%. For the H2S-exhausted carbon, the results showed a promising option for the reuse of waste material. Further studies are required to optimize the carbon performance by creating a more uniform sulfur distribution on the carbon surface. The studied carbons displayed a wide margin in regard to their ability to adsorb mercury vapor from the simulated flue gas with variable gas compositions.

Effect of Flue Gas Composition on Mercury Removal

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Figure 6. Mercury adsorption efficiency, relative to varied carbon weights at two gas compositions (for (a) case 6 and (b) case 7). Legend is as follows: (O) Waterlink carbon, (4) IndoGerman carbon, and (0) H2S-exhausted KOH-impregnated carbon.

Based on the nature of adsorption, the interactions of these coexisting components that occur at carbon surfaces are the key issue relative to the different carbon capacities observed in this study, which, so far, have not been fully explored yet. The findings in this work suggest that a balance likely exists between the chemisorption and physisorption that are involved in the carbon capacity in mercury capture. Sulfurization and humidification of a carbon both reduce the active sites at the carbon surface, to a certain extent, yet they enhance chemisorption through reaction between mercury and sulfur and the catalytic oxidation of SO2 to

SO3 in the presence of water. Competitive adsorption between CO2 and water was observed relative to IndoGerman carbon. So far, different sorbents have demonstrated variable performances with the various flue gas compositions, which accounts for the different nature of sorbents involved and surface reactions related. It is crucial to test the sorbent performance in the mercury uptake under realistic conditions that include the major gas components. EF030041R