Energy Fuels 2011, 25, 144–153 Published on Web 12/30/2010
: DOI:10.1021/ef1009499
Fundamental Study on Decomposition Characteristics of Mercury Compounds over Solid Powder by Temperature-Programmed Decomposition Desorption Mass Spectrometry Shengji Wu,† Md. Azhar Uddin,*,‡ Saori Nagano,‡ Masaki Ozaki,‡ and Eiji Sasaoka‡ † Faculty of Environmental Science and Technology, School of Materials and Environmental Engineering, Hangzhou Dianzi University, Xiasha Higher Education Zone, Hangzhou 310018, China, and ‡Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushima Naka, Kita-ku, Okayama 700-8530, Japan
Received July 26, 2010. Revised Manuscript Received December 2, 2010
Stabilities and/or decomposition characteristics of mercury compounds are important for the design of solid sorbents for mercury vapor removal from coal-derived flue gas and fuel gas. However, available data on the stability and decomposition behavior of mercury compounds are inadequate. The stability and/or decomposition behavior of mercury compounds, such as HgS (metacinnabar and cinnabar), HgO, HgSO4, HgCl2, and Hg2Cl2, were investigated by the temperature-programmed decomposition and desorption technique using a mass spectrometer (TPDD-mass) method. The effects of solid diluents, such as quartz, SiC, Al2O3, TiO2, or activated carbon (AC), on the decomposition characteristics were also studied by the TPDD-mass method. In particular, the stability and reactivity of mercury chloride (HgCl2) in coal combustion flue gas and coal-derived fuel gas conditions were examined. The following results were obtained: (1) the order of the main peak temperature of mercury evolution from the decomposition of the mercury compound diluted with quartz sand in He flow was as follows: HgS (metacinnabar) = HgO < HgS (cinnabar) < HgSO4; (2) HgSO4 was hydrolyzed with H2O; (3) HgO was reduced by SO2 in the presence of H2O and O2; (4) HgCl2 and Hg2Cl2 over SiO2 were more easily decomposed than the other mercury compounds; (5) Among the diluents of HgCl2, SiO2, SiC, Al2O3, TiO2, and AC, HgCl2 was most easily decomposed to Hg0 over SiO2; (6) AC as a diluent apparently stabilizes HgCl2; and (7) HgCl2 gas could be converted to Hg0 over quartz wool, Pyrex wool, ceramic (SiO2-Al2O3) wool, carbon fiber, and AC at high temperatures (>ca. 200 °C).
in 1995 to 257 tons in 2003, an average annual increase of 3.0%.1,2 Adsorption of mercury vapor using activated carbon (AC) or AC impregnated with sulfur (S),3-6 chlorine (Cl),7-9 iodine (I),5,7,10,11 and bromine (Br)12 is the technology most widely used for the removal of mercury from incineration flue gas.13,14 However, the major drawbacks of AC are high cost, low capacity, narrow operating temperature range, and slow regeneration and adsorption rates. In recent years, the Thief process has been presented by the National Energy Technology Laboratory (NETL) of the U.S. Department of Energy (DOE).15 In this process, an unburned partially combusted
1. Introduction The atmospheric emission of elemental mercury from the flue gas of coal-fired power plants remains a major environmental issue. It has been estimated that mercury emissions from coal combustion in China increased from 202 tons *To whom correspondence should be addressed. Telephone: þ81-86251-8897. Fax: þ81-86-251-8897. E-mail:
[email protected]. (1) Wu, Y.; Wang, S. X.; Streets, D. G.; Hao, J. M.; Chao, M.; Jiang, J. K. Trends in anthropogenic mercury emissions in china from 1995 to 2003. Environ. Sci. Technol. 2006, 40, 5312–5318. (2) Jiang, G. B.; Shi, J. B.; Feng, X. B. Mercury pollution in China. Environ. Sci. Technol. 2006, 40, 3673–3678. (3) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. Sorption of elemental mercury by activated carbons. Environ. Sci. Technol. 1994, 28, 1506– 1512. (4) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel sorbents for mercury removal from flue gas. Ind. Eng. Chem. Res. 2000, 39, 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. (7) Lee, S. J.; Seo, Y.-C.; Jurng, J.; Lee, T. G. Removal of gas-phase elemental mercury by iodine- and chlorine-impregnated activated carbons. Atmos. Environ. 2004, 38, 4887–4893. (8) 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, 4454– 4459. r 2010 American Chemical Society
(9) 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. (10) United States Department of Energy (U.S. DOE). Sorbents for mercury removal from flue gas. DOE/FETC/TR-98-01; U.S. DOE: Washington, D.C., 1998. (11) Lee, S. J.; Seo, Y.-C.; Jurng, J.; Lee, T. G. Atmos. Environ. 2004, 38, 4887. (12) 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. (13) Lee, S.-H.; Park, Y.-O. Gas-phase mercury removal by carbonbased sorbents. Fuel Process. Technol. 2003, 84, 197–206. (14) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Flue gas effects on a carbon-based mercury sorbent. Fuel Process. Technol. 2003, 65-66, 343–363. (15) O’Dowd, W. J.; Pennline, H. W.; Freeman, M. C.; Granite, E. J.; Hargis, R. A.; Lacher, C. J.; Karash, A. A technique to control mercury from flue gas: The Thief process. Fuel Process. Technol. 2006, 87, 1071–1084.
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also reported the effects of HCl on the mercury removal with iron-based sorbents, because chlorine in coal is released as HCl in the coal-derived fuel gas.21-26 In the case of coal combustion flue gas, the presence of HCl considerably accelerates mercury removal by an AC.27 Consequently, we were interested to investigate whether mercury removal from coal-derived fuel gas by AC would be affected as well when HCl and H2S both exist. This study suggested that HCl contributed to the mercury removal and the mercury species captured on the AC in the presence of HCl and H2S was more stable than that in the presence of the H2S-only system.28 However, the mercury species over the sorbents could not be identified because the amount of mercury species was too little to analyze. Meanwhile, we investigated mercury sorption mechanisms and the thermal stability of the mercury species formed on the AC in the coal combustion flue gases using the temperatureprogrammed decomposition and desorption (TPDD) technique. We suggested that elemental mercury is removed as mercury chloride species in the presence of HCl and the absence of SO2 and mercury species containing both SO22and Cl- might be formed in the presence of a high concentration of HCl (g100 ppm) and SO2 (500 ppm) in the simulated flue gas.29,30 However, the surface mercury species over the AC could not be clarified. For the development of a highly active sorbent for mercury removal from coal-derived flue gas and fuel gas and the development of a mercury recovery process from a spent AC, it is important to know the surface mercury species formed over the sorbents and the characteristics of the mercury species. Lopez-Anton et al. reported that the temperature appearance range of the main mercury species using a TPDD technique is HgCl2 < HgS < HgO < HgSO4.31 Feng et al. identified and quantified different Hg species (Hg0, HgCl2, HgO, and HgS) in airborne particulate matter by virtue of inductively coupled plasma-mass spectrometry (ICPMS).32 The TPDD technique is a useful method for the clarification of the characteristics of surface mercury species and mercury compounds (reagents).7,28,30,31,33 The purpose of this study is to evaluate the decomposition characteristics of
coal particle is used as a sorbent, much like an inexpensive activated charcoal. However, the mechanism of the mercury removal by AC is not yet well-known. Huggins et al. studied the mercury-adsorbed species on various carbonaceous sorbent materials.16 Their data from S and Cl X-ray absorption near edge structure (XANES) spectra as well as the Hg X-ray absorption fine structure (XAFS) data strongly support the hypothesis that the interaction of acidic species (HCl, HNO3, H2SO4, etc.) in the flue gas with the sorbent surface is an important mechanistic process responsible for the creation of active sites for mercury capture by chemisorption.16 Fuel gas generated from coal gasification also contains elemental mercury. However, little attention has been paid to the capture of elemental mercury from coal-derived fuel gas. The retention of Hg compounds on both sulfur-impregnated and unimpregnated carbon has been demonstrated.17 It has been reported that chemically reactive solid sorbents can remove Hg0 from coal-derived synthesis gas at elevated temperatures (260-315 °C).18 Studies of the effect of various synthesis gas constituents on mercury speciation suggest that a reducing environment is not favorable for Hg oxidation via gas-phase reactions alone and that elemental mercury is expected to remain in the synthesis gas from coal gasification.19 It has also 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.20 Previously, we have reported that elemental mercury can be removed from the coal-derived fuel gas containing H2S with iron oxides (bulk and unsupported) and iron sulfides 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. We have (16) Huggins, F. E.; Yapa, N.; Huffman, G. P.; Senior, C. L. XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures. Fuel Process. Technol. 2003, 82, 167–196. (17) Lopez-Anton, M. A.; Tascon., J. M. D.; Martinez-Tarazona, M. R. Retention of mercury in activated carbons in coal combustion and gasification flue gas. Fuel Process. Technol. 2002, 77-78, 353–358. (18) 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. (19) 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. (20) 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. (21) Nakajima, W.; Tougaki, N.; Wu, S.; Nagamine, S.; Sasaoka, E. Removal of gas phase mercury by solid sorbent. Proceedings of the Trace Element Workshop; Yokohama, Japan, July 18-19, 2002. (22) Togaki, N.; Uddin, M. A.; Nakasima, W.; Wu, S.; Nagamine, S.; Sasaoka, E. Activity of adsorbents for removal of mercury vapor with H2S. Proceedings of the 20th Pittsburgh Coal Conference; Pittsburgh, PA, Sept 15-19, 2003. (23) Wu, S.; Morimoto, T.; Togaki, 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. Proceedings of the 227th American Chemical Society (ACS) National Meeting; Anaheim, CA, March 28-April 1, 2004. (24) 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. (25) 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. Proceedings of the 22nd Pittsburgh Coal Conference; Pittsburgh, PA, Sept 12-15, 2005.
(26) 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. Proceedings of the 23rd Pittsburgh Coal Conference; Pittsburgh, PA, Sept 25-28, 2006. (27) 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. (28) Uddin, M. A.; Ozaki, M.; Sasaoka, E.; Wu, S. Temperatureprogrammed decomposition desorption of mercury species over activated carbon sorbents for mercury removal from coal-derived fuel gas. Energy Fuels 2009, 23, 4710–4716. (29) Uddin, M. A.; Yamada, T.; Ochiai, R.; Sasaoka, E.; Wu, S. Role of SO2 for elemental mercury removal from coal combustion flue gas. Energy Fuels 2008, 22, 2284–2289. (30) Ochiai, R.; Uddin, M. A.; Sasaoka, E; Wu, S. Effects of HCl and SO2 concentration on mercury removal by activated carbon sorbents in coal-derived flue gas. Energy Fuels 2009, 23, 4734–4739. (31) Lopez-Anton, M. A.; Yang, Y.; Ron, P.; Maroto-Valer, M. M. Analysis of mercury species present during coal combustion by thermal desorption. Fuel 2010, 89, 629–634. (32) Feng, X.; Lu, J.; Gregoire, D. C.; Hao, Y.; Banic, C. M.; Schroeder, W. H. Analysis of inorganic mercury species associated with airborne particulate matter/aerosols: Method development. Anal. Bioanal. Chem. 2004, 380, 683–689. (33) Ozaki, M.; Nagano, S.; Uddin, M. A.; Sasaoka, E.; Wu, S. Evaluation of decomposition characteristics of mercury compounds using mass spectrometer. Proceedings of the 25th Pittsburgh Coal Conference; Pittsburgh, PA, Sept 29-Oct 2, 2008.
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Figure 1. Experimental setup for the TPDD or TPR experiment with a mass spectrometer.
reagent mercury compounds, that is, the effect of different diluents and inlet gases on the decomposition using a temperature-programmed desorption technique combined with a mass spectrometer (TPDD-mass). We have reported part of the data on decomposition characteristics of reagent mercury compounds by TPDD-mass previously.33 In this paper, detailed reaction pathways with additional experimental results on the decomposition character of the reagent mercury compounds under various reaction conditions are reported. Additionally, research concerning the impact of supporting materials (quartz wool, Pyrex wool, ceramic wool, carbon fiber, and AC) on the adsorption and stabilization of HgCl2 was also carried out. The data obtained can be applied to the identification of mercury species captured on the sorbents, which can also be used to speciate mercury in contaminated soil and sediment samples. This method can also be used for the regeneration of used sorbents and the recovery of absorbed mercury.
Figure 2. TPDD-mass spectra of HgS (cinnbar and metacinnabar), HgO, and HgSO4 in He flow.
spectrometer (Shimadzu QP2010). Mass numbers (m/z) 202 and 200 for Hg0, 272 and 270 for HgCl2, 36 for HCl, 70 for Cl2, and 64 for SO2 were mainly monitored. 2.3. Apparatus and Procedure of the Temperature-Programmed Reaction (TPR). The reaction of gas-phase HgCl2 over the solid surface was also examined by the TPR method, using quartz wool, Pyrex wool, ceramic (SiO2-Al2O3) wool, carbon fiber, and AC as a sample. The partially modified experimental setup shown in Figure 1 was used: a small vaporized of HgCl2 with He carrier gas feeder was added, and HgCl2 gas was obtained by heating solid HgCl2 powder under a He flow. The inlet gas was a mixture of HgCl2 (ca. 1800 ppb), SO2 (500 ppm), O2 (5%), H2O (4%), and He (balance gas). The inlet gas was fed into the reactor at 60 cm3 STP/min. 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 continuously with the mass spectrometer. In these experiments, mass numbers (m/z) 270 and 200 were selected for monitoring HgCl2 and Hg0, respectively.
2. Experimental Section 2.1. Reagent-Grade Mercury Compounds and Test Sample Preparation Method. The reagent-grade mercury compound powders, including HgS (cinnabar), HgO (yellow), HgSO4, HgCl2, and Hg2Cl2, were purchased from Wako Pure Chemicals. HgS (metacinnabar) was purchased from Johnson Matthey Company. These powders were diluted with solid powders. Commercial SiO2, TiO2, Al2O3, and coconut shell AC were used as a solid powder (ca. 300 °C). Figure 15 shows the formation of Hg0 from HgCl2 over the glass wools; Pyrex glass wool for the decomposition of HgCl2 to Hg0 was more active than quartz glass wool. From this experiment, it was concluded that, if the glass wool was used as a packed bed material in a packed bed reactor for the decomposition of HgCl2, the glass wool contributed to HgCl2 decomposition above ca. 150 °C. 151
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Figure 14. Blank test using only U-tube in He flow.
Figure 17. Decomposition of HgCl2 to Hg0 over AC under 4% H2O-He flow.
Figure 15. Comparison of decomposition of HgCl2 to Hg0 over Pyrex glass and quartz wool in 500 ppm SO2-4% H2O-5% O2-He flow.
Figure 18. Decomposition of HgCl2 to Hg0 over AC under 500 ppm SO2 -4% H2O-5% O2-He flow.
supposed that carbon material is suitable for the adsorption of Hg0. This result may be reasonable because ACs are wellknown as sorbents for mercury removal in combustion flue gas. Therefore, an AC sample (150 mg) was used as a solid. A mixture of H2O-He was chosen as the carrier gas for the first stage experiment of HgCl2 decomposition over the AC. The detail of TPDD-mass data is shown in Figure 17. A Hg0 desorption peak was observed at almost the same temperature as in the case of the carbon fiber (Figure 18). The peak position was similar to that of HgCl2 diluted with AC powder shown in Figure 12. Furthermore, above 90% of HgCl2 in the feed was reduced during the TPDD runs, as shown in the upper part of Figure 17. From these results, it was confirmed that carbon is both a good HgCl2 adsorbent at low temperatures and a good catalyst for the decomposition of HgCl2 to Hg0 at high temperatures. In the lower part of Figure 17, the evolution of HCl was not observed along with the Hg0 peak around 300 °C. This result indicates that Hg0 (gas) and HCl (gas) were not formed by one-step decomposition of HgCl2. That is, after the HgCl2 decomposition, Hg0 desorbed from the AC but the precursor of HCl still remained on the AC.
Figure 16. Decomposition of HgCl2 to Hg0 over the carbon fiber and ceramic wool in 500 ppm SO2-4% H2O-5% O2-He flow.
Figure 16 shows the formation of Hg0 over the ceramic wool and carbon fiber. The formation profile of Hg0 over the ceramic wool was similar to that of the quartz glass wool. However, over the carbon fiber, the Hg0 formation profile was quite different from the other solids. A Hg0 formation peak was observed at around 250 °C. This result suggests that HgCl2 adsorbed over the carbon fiber and some adsorbed HgCl2 decomposed to Hg0 and desorbed near the peak temperature. 3.4. TPR of HgCl2 over AC. Because the Hg0 desorption peak was only observed in the case of the carbon fiber, it was 152
Energy Fuels 2011, 25, 144–153
: DOI:10.1021/ef1009499
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Because it was confirmed that AC is a good HgCl2 adsorbent at low temperatures, a TPDD experiment was carried out using the AC under a mixed gas simulated coal combustion flue gas assuming that HgCl2 is present in the gas. As shown in Figure 18, HgCl2 was also removed by the AC and Hg0 evolution started around 200 °C. At high temperatures (above 300 °C), HgCl2 in the feed seemed to be catalytically converted to Hg0 on AC. The HCl evolution profile in Figure 18 differed from that in Figure 17. The evolution profile of HCl was consistent with that of Hg0. This result indicates that Hg0 (gas) and HCl (gas) were simultaneously produced by one-step decomposition or very quick serial decomposition. Although the details of the serial decomposition need to be clarified by further research, the experimental results may be explained by the following set of reactions:
4. Conclusions In this study, the decomposition of mercury compounds was investigated using a TPDD-mass method. The following results were obtained: (1) the order of the main peak temperature for metallic mercury desorption from decomposition of mercury compounds in He flow was as follows: HgS ðmetacinnabarÞ ¼ HgO < HgS ðcinnabarÞ < HgSO4
HgCl2 þ 1=2O2 þ C f Hg0 þ COCl2
ð14Þ
COCl2 þ H2 O f CO2 þ 2HCl
ð15Þ
(2) HgSO4 was hydrolyzed by H2O; (3) HgO was reduced by SO2 in the presence of H2O and O2; (3) HgCl2 and Hg2Cl2 over SiO2 were more easily decomposed than the other mercury compounds; (4) HgCl2 diluted with SiO2 evaporated at a lower temperature than when mixed with the other diluents (SiC, Al2O3, TiO2, and AC); (5) During TPDD of HgCl2, HgCl2 was evaporated, came into contact and interacted with the diluents, and decomposed to Hg0; (6) Stabilization of adsorbed HgCl2 depends upon the types of diluents (AC surface strongly stabilized adsorbed HgCl2); and (7) HgCl2 gas could be converted to Hg0 over quartz wool, Pyrex wool, ceramic (SiO2Al2O3) wool, carbon fiber, and AC at high temperatures.
From the above results, it was suggested that, if HgCl2 is present in the coal combustion flue gas at low temperatures, then HgCl2 could be captured by some fly ash containing unburned carbon. At high temperatures, the captured HgCl2 could also be converted to Hg0 by coal fly ash particles because coal fly ash is usually composed of unburned carbon, silica, and silica-alumina. A further study is required to confirm this hypothesis.
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), the National Natural Science Foundation of China (20707004), the Qianjiang Project (2008R10038), and the project sponsored by the Scientific Research Foundation (SRF) for the Returned Overseas Chinese Scholars (ROCS), State Education Ministry (SEM).
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