Temperature-Programmed Decomposition Desorption of Mercury

Mar 30, 2009 - and improve mercury removal capacity.12. In contrast, very little ... of Hg0 (4.8 ppb), HCl (0, 1, 10, and 50 ppm), H2S (0 and 400 ppm)...
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Energy Fuels 2009, 23, 4710–4716 Published on Web 03/30/2009

: DOI:10.1021/ef801068z

Temperature-Programmed Decomposition Desorption of Mercury Species over Activated Carbon Sorbents for Mercury Removal from Coal-Derived Fuel Gas† Md. Azhar Uddin,*,‡ Masaki Ozaki,‡ Eiji Sasaoka,‡ and Shengji Wu§ Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan, and Department of Environmental Science and Technology, School of Mechanical Engineering, Hangzhou Dianzi University, Xiasha Higher Education Zone, Hangzhou 310018, China. ‡Okayama University. §Hangzhou Dianzi University. Received December 8, 2008. Revised Manuscript Received February 7, 2009

The mercury (Hg0) removal process for coal-derived fuel gas in the integrated gasification combined cycle (IGCC) process will be one of the important issues for the development of a clean and highly efficient coal power generation system. Recently, iron-based sorbents, such as iron oxide (Fe2O3), supported iron oxides on TiO2, and iron sulfides, were proposed as active mercury sorbents. The H2S is one of the main impurity compounds in coal-derived fuel gas; therefore, H2S injection is not necessary in this system. HCl is also another impurity in coal-derived fuel gas. In this study, the contribution of HCl to the mercury removal from coal-derived fuel gas by a commercial activated carbon (AC) was studied using a temperatureprogrammed decomposition desorption (TPDD) technique. The TPDD technique was applied to understand the decomposition characteristics of the mercury species on the sorbents. The Hg0-removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80-300 °C using simulated fuel gas and a commercial AC, and the TPDD experiments were carried out in a U-tube reactor in an inert carrier gas (He or N2) after mercury removal. The following results were obtained from this study: (1) HCl contributed to the mercury removal from the coal-derived fuel gas by the AC. (2) The mercury species captured on the AC in the HCl- and H2S-presence system was more stable than that of the H2S-presence system. (3) The stability of the mercury surface species formed on the AC in the H2S-absence and HClpresence system was similar to that of mercury chloride (HgClx) species.

ACs are often suggested to be employed for better mercury control, because Hg compounds react with impregnated chemicals, sulfur (S),1,4-6 chlorine (Cl),7,8 iodine (I),4,9,10 and bromine (Br).11 Although the mechanism of mercury capture with chemically modified ACs has not been fully understood yet, the beneficial role of Cl and S in the capture of mercury species is well-established. These carbons have shown enhanced sorption capacity as compared to virgin ACs because of the higher content of active S or halogen atoms. Huston et al. have reported on the X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) characterization of mercury binding on halogenated AC.11 They suggested that the virgin or chlorinated ACs appeared to contain mercury bound to chlorinated sites and possibly to sulfate species that have been incorporated onto the carbon from adsorbed SO2. The mercury containing brominated sorbents appear to contain mercury bound primarily at bromination sites. The mechanism of capture for the sorbents likely consists of surface-enhanced oxidation of the elemental

1. Introduction Integrated gasification combined cycle (IGCC) power generation from coal is receiving increased attention for its high efficiency. Fuel gas generated from coal gasification also contains elemental mercury. To date, most of the research activities, both practical and fundamental, have focused on the removal and conversion of elemental mercury from real or simulated coal combustion flue gas. Activated carbons (ACs) have been reported to be effective sorbents for elemental mercury removal.1-3 However, virgin ACs show poor removal of mercury species. For this reason, chemically treated † Progress in Coal-Based Energy and Fuel Production. * To whom correspondence should be addressed. Telephone and Fax: 81-86-251-8897. E-mail: [email protected]. (1) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. Sorption of elemental mercury by activated carbons. Environ. Sci. Technol. 1994, 28, 1506– 1512. (2) Vidic, R. D.; McLaughlin, J. B. Uptake of elemental mercury vapors by activated carbons. J. Air Waste Manage. Assoc. 1996, 46, 241– 250. (3) Ghorishi, S. B.; Gullett, B. K. Mercury behaviour in flue gases. Sorption of mercury species by activated carbons and calcium-based sorbents: effects of temperature, mercury concentration and acid gases. Waste Manage. Res. 1998, 16, 582–593. (4) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel sorbents for mercury removal from flue gas. Ind. Eng. Chem. Res. 2000, 39 (4), 1020– 1029. (5) Jurng, J.; Lee, T. G.; Lee, G. W.; Lee, S. J.; Kim, B. H.; Seier, J. Mercury removal form incineration flue gas by organic and inorganic adsorbents. Chemosphere 2002, 47, 907–913. (6) Liu, W.; Vidic, R. D.; Brown, T. D. Optimization of sulfur impregnation protocol for fixed-bed application of activated carbonbased sorbents for gas-phase mercury removal. Environ. Sci. Technol. 1998, 32, 531–538.

r 2009 American Chemical Society

(7) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K. Development of Cl-impregnated activated carbon for entrain-flow capture of elemental mercury. Environ. Sci. Technol. 2002, 36 (20), 4454–4459. (8) Vidic, R. D.; Siler, D. P. Vapor-phase elemental mercury adsorption by activated carbon impregnated with chloride and chelating agents. Carbon 2001, 39, 3–14. (9) United States Department of Energy (U.S. DOE). Sorbents for mercury removal from flue gas. 1998; DOE/FETC/TR-98-01. (10) Lee, S. J.; Seo, Y.-C.; Jurng, J.; Lee, T. G. Atmos. Environ. 2004, 38, 4887. (11) Hutson, N. D.; Attwood, B. C.; Scheckl, K. XAS and XPS characterization of mercury binding on brominated activated carbon. Environ. Sci. Technol. 2007, 41, 1747–1752.

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mercury vapor via interaction with surface-bound halide species with subsequent binding by surface halide or sulfate species. It has also been shown that a flue gas stream containing hydrogen chloride (HCl) vapor can impregnate AC in situ and improve mercury removal capacity.12 In contrast, very little attention has been paid to the capture of elemental mercury from coal-derived fuel gas. It has been reported that chemically reactive solid sorbents removed Hg0 from coal-derived synthesis gas at elevated temperature (260-3150 °C); however, the exact name or composition of the sorbents was not disclosed in the paper.13 Studies of the effect of various synthesis gas constituents on mercury speciation suggested that the reducing environment is not favorable for Hg oxidation via gas-phase reactions alone and that more elemental mercury is expected to remain in the synthesis gas from coal gasification.14 It has been reported that supported noble metals, such as palladium, platinum, iridium ruthenium, and silver are effective in capturing elemental Hg from simulated fuel gas at elevated temperatures. Fuel gas components, such as moisture, hydrogen sulfide, and carbon monoxide, have an affect on the adsorption of mercury from fuel gas at temperatures greater than 204 °C.15 Previously, we have reported that Hg0 reacts with H2S to form HgS over solid sorbents in the removal of elemental mercury from simulated coal-derived gases.16-18 The composition of the coal-derived gas before purification is favorable for this method because this process requires H2S, which is usually present in the unrefined gas derived from coal gasification. We have reported that elemental mercury can be removed from the coal-derived fuel gas containing H2S with iron oxides (bulk and unsupported) at temperatures ranging from 60 to 100 °C according to the following reactions: iron oxide reacts with H2S to form FeSx and some surface elemental sulfur (-S) species, which then react with elemental mercury (Hg0) to form HgS.19,20 It has been suggested that

surface elemental sulfur species may react with CO to form COS under some reaction conditions, depending upon the nature of the iron oxide sorbent.21 Iron sulfides (FeS and FeS2) also exhibited high activities for Hg0 removal.22 Chlorine in coal is released as HCl in the coal-derived fuel gas.14 Although a HCl capture device has to be installed in the gas cleanup process, some HCl may still escape the device. Because the Hg capture device may be located before or after the HCl capture unit, it is necessary to study the effect of HCl on the Hg0 removal activity of the sorbents. Previously, we have reported the effects of the presence of HCl on the mercury removal with iron-based sorbents.22 In this study, the contribution of HCl to the mercury removal by AC was studied using a temperature-programmed decomposition desorption (TPDD) technique. Already, it was reported that the presence of HCl considerably accelerated mercury removal from coal combustion flue gas by an AC.23 However, the effects of the presence of HCl in the presence of H2S on mercury removal fuel gas by an AC have not been clarified yet. 2. Experimental Section 2.1. Materials. The coconuts shell AC was purchased from Wako Pure Chemicals. The AC was crushed to particles (average diameter of 1.0 mm), washed with deionized water, and then dried at 110 °C for 25 h. The specific surface area and balk density of the AC was 1.20  103 m2 g-1 and 0.37 g cm-3, respectively. HgCl2 was also purchased from Wako Pure Chemical Industry Ltd. HgS and HgCl2 standard samples were prepared by mixing the HgS powder with either AC powder or silica sand (under a diameter of 0.15 mm). The weight ratio of HgS to AC or silica sand is controlled in the range from 1/3000 to 1/30. 2.2. Hg Removal and TPDD Experimental System. The reactivity of the samples for Hg0 removal was investigated using a flow-type packed-bed reactor under atmospheric pressure at 80-300 °C. The apparatus consisted of a Hg0 vaporizer, feed system, quartz glass reactor, furnace with temperature controller, and cold-vapor Hg0 analyzer. About 0.125 cm3 of the sorbent sample (particle diameter of 1 mm) was packed into a quartz tube reactor. The reaction is commenced when a mixture of Hg0 (4.8 ppb), HCl (0, 1, 10, and 50 ppm), H2S (0 and 400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) was fed into the reactor at 500 cm3 standard temperature and pressure (STP) min-1 (SV = 24.0  104 h-1). Measurements of the inlet and outlet concentrations of mercury were carried out using an atomic absorption spectrophotometer (Nippon Jarrell Ash, AA-855), which can analyze only elemental mercury up- and downstream of the adsorption bed. Elemental mercury is measured with the atomic absorption spectrophotometer in a coldvapor mercury technique. In this apparatus, the mercury concentration in the range of 0.1-500 ppb can be analyzed in the gas stream. However, the minimum detection limit with our apparatus is about 0.2 ppb. The inlet concentration of Hg (Hgin) was measured by passing the Hg-laden gas mixture through the reactor bypass to the mercury analyzer before and after the

(12) Ghorishi, S. B.; Gullett, B. K. Fixed-bed control of mercury; role of acid gases and a comparison between carbon-based, calcium-based, and coal fly ash sorbents. Presented at the 1st Electric Power Research Institute-Department of Energy/Environmental Protection Agency (EPRI-DOE/EPA) Combined Utility Air Pollutant Control Symposium, Washington, D.C., 1997. (13) Portzer, J. W.; Albritton, J. R.; Allen, C. C.; Gupta, R. P. Development of novel sorbents for mercury control at elevated temperature in coal-derived syngas: Results of initial screening of candidate materials. Fuel Process. Technol. 2004, 85, 621–630. (14) Lu, D. Y.; Granatstein, D. L.; Rose, D. J. Study of mercury speciation from simulated coal gasification. Ind. Eng. Chem. Res. 2004, 43, 5400–5404. (15) Granite, E. J.; Myers, C. R.; King, W. P.; Stanko, D. C.; Pennline, H. W. Sorbents for mercury capture from fuel gas with application to gasification systems. Ind. Eng. Chem. Res. 2006, 45, 4844–4848. (16) Nakashima, W.; Tougaki, N.; Wu, S.; Nagamine, S.; Sasaoka, E. Removal of gas phase mercury by solid sorbent. Trace Element Workshop, Yokohama, Japan, July 18-19, 2002. (17) Tougaki, N.; Uddin, M. A.; Nakashima, W.; Wu, S.; Nagamine, S.; Sasaoka, E. Activity of adsorbents for removal of mercury vapor with H2S. 20th Pittsburgh Coal Conference, Pittsburgh, PA, Sept 15-19, 2003. (18) Wu, S.; Morimoto, T.; Tougaki, N.; Nagamine, S.; Uddin, M. A.; Sasaoka, E. Characters of activated carbon for mercury removal of flue gas with hydrogen sulfide and iron oxide for mercury removal of coal derived fuel gas with hydrogen sulfide. 227th American Chemical Society (ACS) National Meeting, Anaheim, CA, March 28-April 1, 2004. (19) Wu, S.; Kawakami, J.; Oya, N.; Ozaki, M.; Uddin, M. A.; Sasaoka, E. Relation between sulfurization behavior and mercury vapor capture performance of iron oxide in coal derived fuel gas. 22nd Pittsburgh Coal Conference, Pittsburgh, PA, Sept 12-15, 2005. (20) Wu, S.; Uddin, M. A.; Sasaoka, E. Characteristic of the removal of mercury vapor in coal derived fuel gas over iron-based sorbents. Fuel 2006, 85, 213–218.

(21) Wu, S.; Oya, N.; Kawakami, J.; Ozaki, M.; Uddin, M. A.; Sasaoka, E. Development of iron oxide sorbents for Hg0 removal from coal derived fuel gas: Sulfidation characteristics of iron oxide sorbents and activity for COS formation during Hg0 removal. Fuel 2007, 86, 2857–2863. (22) Wu, S.; Ozaki, M.; Uddin, M. A.; Sasaoka, E. Mercury vapor capture from coal derived fuel gas in the presence of hydrogen chloride over iron-based sorbents. 23rd Pittsburgh Coal Conference, Pittsburgh, PA, Sept 25-28, 2006. (23) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Flue gas effects on a carbon-based mercury sorbent. Fuel Process. Technol. 2000, 65-66, 343–363.

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adsorption experiments. During the mercury contact period, the reactor exit concentration of mercury (Hgout) was continuously monitored. The instantaneous removal of Hg0 at any time was obtained by removal of Hg ð%Þ ¼ ½ðHgin - Hgout Þ=Hgin   100 The amount of Hg adsorption was determined by integrating and evaluating the area under the removal curve over the entire time of adsorption. After the Hg0-removal experiment, the desorption experiment was carried out using an atomic absorption spectrophotometer. In the TPDD experiments, about 0.125 cm3 of the mercury-adsorbed AC-based sample was packed in a quartz tube reactor and N2 was flowed in the reactor at a rate of 250 cm3 STP min-1. Then, the sample was heated from room temperature to 400 °C at a heating rate of 10 °C/min, and the reactor effluent was monitored for Hg continuously with an atomic absorption spectrophotometer. In the case of a standard sample, about 20 or 40 mg was used for the TPDD experiment. The TPD-mass apparatus consisted of a Hg0 vaporizer, a H2O vaporizer, a gas feed system, a quartz glass U-tube reactor, an electric furnace with a temperature controller, and a mass spectrometer (Shimadzu QP2010). The Hg0 vaporizer was used in the TPDD system to generate mercury vapor at a fixed concentration and quantify the amount of mercury produced during TPDD runs by the mass spectrometer. About 20 mg of the diluted reagent powder sample (diluents = either AC powder or silica sand less than 0.15 mm in size) was placed into a quartz glass U-tube reactor. Then, the sample was heated from room temperature to 550 or 880 °C with a heating rate of 10 °C/min in a helium (carrier gas) flow (60 cm3 STP min-1), and the reactor effluent was monitored continuously with the mass spectrometer.

Figure 1. Effect of the presence of HCl on the mercury removal by AC in the presence of H2S. Feed gas = Hg0 (4.8 ppb), HCl (0, 1, 10, and 50 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 balance at 500 cm3 STP min-1. SV = 24  104 h-1.

Figure 2. TPDD spectra of the used sample, as shown in Figure 1.

To understand the characteristics of mercury species over AC sorbent and also to clarify the reason for the acceleration in Hg0 removal performance of AC sorbent in the presence of HCl, the thermal stabilities of the mercury species on the used sorbents were examined by the TPDD technique. Figure 2 shows the TPDD spectra obtained from the used samples shown in Figure 1: the intensity of the atomic absorption spectrophotometer for Hg (rate of desorbed Hg) shown as a function of the desorption temperature in Figure 2. The peak temperature and shape for Hg desorption differed from each other. The mercury desorption amount (peak area) from the used AC sample increased with the HCl concentration. From Figure 1, if the all of adsorbed mercury from the used sample in the HCl-presence system desorbed during TPDD experiments, the mercury desorption peak area of the used AC samples should be the same for the 1, 10, and 50 ppm HCl-presence systems. However, the mercury desorption peak areas of the samples were not the same, but the range of difference was within the range of experimental error (the desorption amount was within (10%). The Hg desorption peak shape of the used AC sample changed with the concentration of HCl. For the sample used at the 50 ppm HCl-presence system, the peak temperature was the lowest, indicating that different kinds of mercury species were very formed on the AC surface. Table 1 summaries the amount of adsorption of mercury during mercury removal runs (for a 2 h time on stream) and the amount of mercury desorption during TPDD runs at various mercury removal conditions. It is evident that the amount of adsorption and desorption of Hg was matched within 100 ( 15%. In the absence of H2S and the presence of HCl, the activity of the AC for mercury removal was very high compared to the case when both H2S and HCl were present, as shown in

3. Results and Discussion 3.1. Mercury Removal Activity of AC Sorbents in the Absence and Presence of HCl. Previously, we have reported that mercury vapor can be captured with AC in the presence of H2S and in the absence of oxygen (or presence of a very low concentration of O2) from coal combustion flue gas.24 However, the performance of AC for Hg removal from coalderived fuel gas has not been studied very well. In this study, the effect of HCl on the Hg0 removal from a simulated coalderived fuel gas by an AC was examined. Furthermore, the Hg adsorption experiments were followed by the TPDD experiments as follows: The used AC in the fixed bed was heated in N2 flow for the TPDD study. In the abovementioned mercury removal experiments for 2 h, a model flue gas was used and the reaction temperature was set at 80 °C. Figure 1 shows the effect of HCl on the Hg0 removal activity of the AC sorbents. The Hg0 removal activity of AC increased significantly in the presence of 1 ppm HCl. Miller et al. already reported that the presence of HCl accelerated the Hg0 removal activity of AC from a coal combustion flue gas.23 From this experiment, it was confirmed that the presence of 1 ppm HCl accelerated the Hg0 removal activity of AC from a coal gasification fuel gas. In this experiment, the contribution of a very small amount of O2 as an impurity of this experimental system could not be neglected. The effect of the concentration of HCl on the rate of mercury removal was not apparently observed in the range of 1-50 ppm. (24) Morimoto, T.; Wu, S.; Uddin, M. A.; Sasaoka, E. Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2S. Fuel 2005, 84, 1968–1974.

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Table 1. Comparison of the Amount of Hg Adsorption and Desorption at Various Conditions Hg adsorption conditions run number 1 2 3 4 5

HCl concentration (ppm)

H2S concentration (ppm)

temperature (°C)

amount of Hg adsorptiona (μg)

amount of Hg desorption during TPDDb(μg)

0 1 10 50 50

400 400 400 400 0

80 80 80 80 80

0.8 1.8 2.2 2.0 2.5

0.9 (112%) 1.6 (88%) 1.9 (86%) 2.0 (100%) 2.2 (88%)

a Cumulative amount of Hg adsorption on the AC for a 2 h time-on-stream. b Cumulative amount of Hg desorption during the TPDD experiment after Hg adsorption from the AC.

Figure 3. Effect of the presence of H2S on the mercury removal by AC in the presence of HCl. Feed gas=Hg0 (4.8 ppb), HCl (1 ppm), H2S (0 and 400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 balance at 500 cm3 STP min-1. SV = 24  104 h-1.

Figure 5. Effect of the temperature on the mercury removal by AC in the presence of HCl. Feed gas=Hg0 (4.8 ppb), HCl (50 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 balance at 500 cm3 STP min-1. SV = 24  104 h-1.

Figure 4. TPDD spectra of the used sample, as shown in Figure 3.

Figure 3. This result indicates that H2S suppressed the mercury removal by the AC. It was also thought that Hg0 could directly react with HCl and produce a surface mercury species over the AC. Therefore, TPDD spectra of the AC used in the absence of H2S and the presence of HCl for Hg removal was examined and compared. As shown in Figure 4, the mercury desorption peak positions from the used samples were different from each other. This result indicates that the mercury species over the AC in the absence of H2S was quite different from that in the presence of H2S. 3.2. Effect of the Temperature on Mercury Removal by AC. Figure 5 shows the effect of the temperature on the Hg0 removal by AC. As shown in Figure 5, it was observed that the Hg removal activity of AC decreased with the increase of the temperature. The temperature range of 80-100 °C was preferable for the mercury removal in the presence of both H2S and HCl by the AC. Similar results, i.e., a decrease of the mercury removal rate with the increase of the temperature, have been reported by other researchers for S- or Cl-impregnated carbon, and they explained that the mercury reaction with chloride or elemental sulfur shows an exothermic behavior; for this reason, the highest Hg adsorption capacity is obtained at the lower temperature conditions.8,9 Moreover, it is noteworthy that Hg adsorption on a com-

Figure 6. TPDD spectra of the used sample, as shown in Figure 5.

mercial coconut shell in the absence of any acid gas was very low (results not shown). These results indicate that Hg adsorption even on virgin AC is not a physical adsorption of Hg0 or Hg0 filled in micropores of AC. We suggest that some sort of adsorbed surface mercury compounds related to chlorine and/or sulfur may be formed. Figure 6 shows the TPDD spectra of the samples after Hg0-removal experiments. From the TPDD study, it was found that the TPDD peak position of the samples was the same in all cases; i.e., the TPDD peak temperatures were not dependent upon the temperature of Hg0 adsorption. These results indicate that the mercury species formed on the sorbent in the presence of both HCl and H2S in the temperature range of 80-100 °C were similar. In the absence of H2S, mercury was more efficiently removed by the AC even at 150 °C, as shown in Figure 7. The TPDD peak shape and position of the sorbents after Hg0 removal at 150 and 200 °C were almost the same, as shown in Figure 8, although the mercury removal efficiency was considerably different (Figure 7). 4713

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Figure 7. Effect of the temperature on the mercury removal by AC in the absence of H2S. Feed gas=Hg0 (4.8 ppb), HCl (50 ppm), H2S (0 ppm), CO (30%), H2 (20%), H2O (8%), and N2 balance at 500 cm3 STP min-1. SV = 24  104 h-1. Figure 9. TPDD-mass spectra of the AC sample after Hg removal in the presence of both HCl and H2S.

Figure 8. TPDD spectra of the used sample, as shown in Figure 7. Figure 10. TPDD-mass spectra of the AC sample after Hg removal in the absence of H2S.

3.3. TPDD-Mass Spectra of the Used AC. To clarify the mercury species over the AC sample, TPDD-mass spectra of the used AC samples after Hg0 removal and standard sample (mercury compounds) were measured. Figure 9 shows the TPDD-mass spectra of AC sample after Hg0 removal. The intensity of the ion signals for Hg0 (m/z 202), HgCl2 (m/z 272), HCl (m/z 36), and SO2 (m/z 64) from the pretreated AC sample (after Hg0 removal) was plotted against the temperature. It was observed that HCl was evolved from the sample during TPDD at the lowest temperature followed by Hg0 and SO2. It is worthwhile mentioning that the Hg0 peak position in the TPDD-mass spectra was about 30 °C higher than that of the TPDD spectra measured in a packed-bed reactor using the atomic absorption spectrophotometer. This may be caused by the difference in reactor configuration and the discrepancy in temperature monitoring points, because a U-tube reactor was used in TPDD-mass experiments, whereas a straighttube reactor was used in TPDD experiments. The evolution of SO2 and Hg0 seemed to be correlated, because the temperature range of Hg and SO2 evolution was overlapped. However, the peak temperatures of Hg and SO2 evolution were not coincident. In the TPDD-mass experiments, helium was used as a carrier gas but some oxygen may still remain in the TPDD system. It was supposed that SO2 was produced by the reaction with oxygen as an impurity in the system. However, in the absence O in the system, AC itself can oxidize H2S to Sx and SO2; SO2 released during TPDD results from the decomposition of S-O groups and/or rearrangement of surface functional

groups and adsorbed H2S. Because the peak position of mercury evolution in the absence of H2S was different from that in the presence of H2S (Figures 2, 4, 6, and 8), the TPDD spectra of the AC samples used in the absence of H2S was measured using a mass spectrometer. The pretreatment conditions were as follows: inlet gas = Hg0 (4.8 ppb), HCl (1 ppm), H2S (0 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) at 500 cm3 STP min-1 (SV = 24  104 h-1); temperature and treatment time=80 °C for 2 h. As shown in Figure 10, the Hg0 evolution peak shape in TPDD-mass spectra was almost coincident with the Hg0 evolution peak shape in TPDD spectra of Figure 4. Evolution of HCl was also observed, and the amount was very large. To compare the TPDD spectra of the used AC and that of reagent HgCl2, the TPDD-mass spectra of the diluted reagent HgCl2 with the AC powder were measured. Figure 11 shows the TPDD-mass spectra of HgCl2 diluted with AC in different ratios (HgCl2/AC = from 1/30 to 1/3000). In all cases, evolution of Hg from HgCl2 was observed during the TPDD-mass experiments. However, the peak temperatures of Hg evolution were shifted to lower temperatures with the increase of the dilution of HgCl2; i.e., the peak shape of Hg0 evolution depended upon the amount of HgCl2 in the sample. The high-temperature peak in Figure 11 was coincident with that of the used AC sample, as shown in Figure 10. Furthermore, the effect of the type of diluents on the TPDD-mass spectra of HgCl2 was also examined. Figure 12 shows the TPDD-mass spectra of HgCl2 diluted 4714

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Figure 12. TPDD-mass spectra of reagent HgCl2 diluted with silica sand powder.

but the reaction rate is usually slow.25 For the formation of mercuric chloride on the carbon surface, similar reactions as 1, 1a, and 1b are reported by Lee et al.10 Furthermore, in the presence of a high concentration of H2, the reaction is thermodynamically unfavorable. If a small amount oxygen remained in our experimental setup, reaction 2 might occur. Reaction 2, consisting of reactions 2a and 2b, is more favorable to convert the mercury on the solid surface. In the presence of a small amount of O2, the following reactions may also occur: ð3Þ Hg þ 1=2O2 ¼ HgO

Figure 11. TPDD-mass spectra of reagent HgCl2 diluted with AC powder.

with silica sand (quartz sand) powder. It is evident from Figures 11 and 12 that the peak temperature of Hg0 evolved from HgCl2 diluted with silica sand is much lower than the peak temperature of Hg0 evolved from HgCl2 diluted with AC. From these results, it is understood that the evolution behavior of Hg0 from HgCl2 during TPDD depends upon some factors such as the degree of dilution and the type of diluents. These may happen because of the interaction of the Hg compounds or the products of decomposition of Hg compounds with the surface of the diluents. The vaporized HgCl2 in contact with the surface of the diluents might be converted to metallic mercury. A more detailed study will be needed for a deeper understanding of the TPDD behavior of Hg compounds. From the comparison of TPDD-mass spectra of AC after Hg removal and the spectra of HgCl2 diluted with AC and silica sand, it was not confirmed whether the mercury species captured over the AC surface is HgCl2 but the temperature range of the HgCl2 decomposition and desorption observed in HgCl2 diluted with AC was overlapped with that of the used sample. We suggest that the surface species formed AC during Hg removal in the presence of HCl may not be HgCl2 but an adsorbed mercury chloride species with unknown structure, such as HgClx. 3.4. Mechanism of the Mercury Removal by AC in the Presence of Both H2S and HCl. In the HCl-presence and H2Sabsence system, the following reactions can be proposed for mercury conversion:

HgO þ 2HCl ¼ HgCl2 þ H2 O

ð4Þ

It was very difficult to reduce the concentration of the remained O2 to parts per billion (ppb) order. Therefore, in our experiments, it is not known which one is the main reaction. In the presence of H2S, the following reactions could be proposed for mercury conversion: ð5Þ H2 S ¼ HSad þ Had HSad þ Hg ¼ HgSad þ Had

ð6Þ

H2 S þ 1=2O2 ¼ 1=xSx ad þ H2 O

ð7Þ

Sad þ Hg ¼ HgSad

ð8Þ

Although we have no direct evidence to support the formation HgSsd, we have suggested the above reactions in our previous paper.24 In the presence of both H2S and HCl, the following reaction may occur: HgCl2 þ 2H2 S ¼ HgS2 þ 2HCl ð9Þ

Hg þ 2HCl ¼ HgCl2 þ H2

ð1Þ

HCl ¼ Clad þ Had

ð10 Þ

Hg þ 2Clad ¼ HgCl2

ð100 Þ

2Hg þ 4HCl þ O2 ¼ 2HgCl2 þ 2H2 O

ð2Þ

4HCl þ O2 ¼ 2Cl2 þ 2H2 O

ð20 Þ

2Hg þ Cl2 ¼ 2HgCl2

ð200 Þ

From the results of this study, a mechanism of the mercury removal by AC in the presence of both H2S and HCl could be proposed. As shown in Figure 13, there could be two routes that exist for the conversion of Hg0 to HCl2. One of the routes is the direct conversion of Hg0 to HgCl2 or HgClx via reaction 2, and the other route is via HgO (reactions 3 and 4). The produced HgCl2 is converted to HgS by H2S according to reaction 9. The suppression of Hg removal by the presence of H2S may be explained by the following two reasons. One of them is the competition of adsorption of HCl and H2S

Reaction 1, consisting of reactions 1a and 1b, is thermodynamically favorable at low temperatures below 300 °C,

(25) Hall, B.; Schager, P.; Lindqvist, O. Chemical reactions of mercury in combustion flue gases. Water, Air, Soil Pollut. 1991, 56, 3–14.

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: DOI:10.1021/ef801068z

Uddin et al.

4. Conclusions In this study, the effect of HCl in the coal-derived fuel gas (mixture of H2, CO, H2O, N2, Hg0, H2S, and HCl) on the Hg0 removal performance of commercial AC was investigated with a packed-bed reactor and TPDD technique using an atomic absorption spectrophotometer or a mass spectrometer (TPDD-mass). It was found that the presence of HCl accelerated the mercury removal rate by AC. On the other hand, the presence of H2S suppressed the mercury removal in the presence of HCl by AC. It was observed in the TPDD study with AC samples after Hg removal experiments that the stabilities of the mercury species captured on the AC depended upon the presence of acid gases (HCl and H2S): the mercury species captured in the HCl-presence and H2S-absence system was more stable than that of the H2S-presence system. It was found that the stability of the mercury species captured on the AC in the H2S-absence and HCl-presence system was similar to that of mercury chloride (HgClx) species.

Figure 13. Mechanism of mercury removal by AC in the presence of HCl and H2S.

(reactions 1a and 5): H2S may be dissociatively adsorbed by reaction 5, and HCl may also be dissociatively adsorbed by reaction 1a over the same site of the AC. The other reason may be explained by the trace oxygen remaining inside of the experimental setup: if the trace amount of oxygen remaining in the reaction system contributed to the mercury removal via reactions 2a and 3 and if the surface oxygen is consumed by H2S via reaction 7, then the lack of oxygen for reactions 2a and 3 will result in suppression of Hg removal in the presence of H2S. The mercury removal rate would be slow if the reaction rate in the absence of HCl and presence of H2S, as shown in Figure 1, was controlled by reaction 8. At present, we do not have any direct experimental evidence to support these two routes suggested in Figure 13. Presently, we are conducting some experiments to clarify these reaction routes; however, adequate data have not been obtained yet. We are planning to report about these results in the future.

Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas (B) from the Ministry of Education, Science, Sports, and Culture, Japan (18310056) and the National Natural Science Foundation of China (20707004).

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