Energy Fuels 2011, 25, 154–158 Published on Web 12/14/2010
: DOI:10.1021/ef101100y
Oxidative Adsorption of Elemental Mercury by Activated Carbon in Simulated Coal-Fired Flue Gas Changxing Hu,*,† Jinsong Zhou,‡ Zhongyang Luo,‡ and Kefa Cen‡ ‡
† Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China, and State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
Received August 17, 2010. Revised Manuscript Received November 29, 2010
Bench-scale adsorption experiments were conducted to explore and verify the change of mercury speciation during elemental mercury (Hg0) adsorption by activated carbon (AC) impregnated with chlorine using a continuous mercury monitor. Results uniquely demonstrated that Hg0 was completely oxidized to divalent mercury (Hg2þ) during the adsorption process of Hg0 by AC whether in N2 gas or in simulated coal-fired flue gas. Without the presence of a chemical oxidizing element, such as chlorine, on the surface of the sorbent, AC would not adsorb Hg0 in N2 gas. When the oxidizing element was consumed, the breakthrough of Hg0 adsorption by AC occurred. Hg0 was detected as the only Hg species at the outlet of the adsorption bed. However, the adsorption of Hg0 by AC did not reach its full capacity. When the adsorption atmosphere was switched from N2 gas to simulated flue gas, AC started to absorb Hg0 again. When the AC reached its full capacity for Hg0, oxidation of Hg0 continued, because when breakthrough occurred, Hg2þ was detected at the reactor outlet. However, the oxidation of Hg0 was not due to chlorine on the surface of AC but the components in simulated flue gas, such as NO, NO2, HCl, etc., which catalyzed oxidation of Hg0 on the AC surface.
of mercury by AC. For examples, Dunham et al. and Olson et al. have proposed a mechanism to explain the effects of NO2 and SO2 on adsorption of Hg by AC.5,6 In the presence of NO2, Hg0 is catalytically oxidized on the surface to form the nonvolatile nitrate Hg(NO3)2, which is bound to basic sites on the carbon. Previous studies showed that carbon sorbents have a small capacity for elemental mercury capture from a nitrogen gas stream.7-10 These results are in contrast to the real or realistic simulated flue gas streams, where carbons typically have much larger mercury adsorption capacities. Carbons, such as AC, unburned carbon in fly ash, and thief carbons, are catalysts for the oxidation of mercury in flue gas.11,12 These possible steps are introduced in the capture of Hg0 on a sorbent.1 Among these steps, the physical adsorption of Hg0 on the surface of the carbon by the van der Waals force is assumed to be an intermediate step, leading to subsequent
1. Introduction Mercury pollution control technologies for coal combustion systems can be roughly divided in three ways, corresponding to three stages, viz. before burning, during burning, and after burning. The control technology after burning mainly refers to mercury removal from the coal-fired flue gas. It is feasible to use the existing flue gas pollution control devices of coal-fired power plants to reduce mercury emissions. At present, the existing flue gas pollution control devices mainly include particle-control equipment [electrostatic precipitator (ESP) or fabric filter (FF)], NOx-control equipment [selective catalytic reduction (SCR), etc.], and flue gas desulfurization (FGD) equipment. Simultaneous removal of mercury, particulate, SO2, and NOx pollutants can be achieved using existing pollutant control devices with new mercury pollution control technologies, such as adsorption technology, corona discharge plasma technology, and electrocatalytic oxidization combined treatment technology.1-4 Injection of activated carbon (AC) upstream of particulate control devices is a retrofit control technology, with the widest potential application for controlling mercury emissions from coal-fired power plants.1 In this technology, the capture of elemental mercury (Hg0) is generally considered to be more difficult than that of divalent mercury (Hg2þ). Many studies have been performed to understand the adsorption mechanism
(5) Dunham, G. E.; Olson, E. S.; Miller, S. J. Impact of flue gas constituents on carbon sorbents. Proceedings of the Air Quality II: Mercury, Trace Elements, and Particulate Matter Conference; McLean, VA, Sept 19-21, 2000; Paper A4-3. (6) Olson, E. S.; Sharma, R. K.; Miller, S. J.; Dunham, G. E. Mercury in the environment. Proceedings of the Specialty Conference on Mercury in the Environment; Minneapolis, MN, Sept 15, 1999; pp 121-126. (7) Karatza, D.; Lancia, A.; Musmarra, D.; Pepe, F. Adsorption of metallic mercury on activated carbon. Proceedings of the 26th International Symposium on Combustion; The Combustion Institute, Pittsburgh, PA, 1996; pp 2439-2455. (8) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Ind. Eng. Chem. Res. 2000, 39, 1020–1029. (9) Granite, E. J.; Freeman, M. C.; O’Dowd, W. J.; Hargis, R. A.; Pennline, H. W. J. Environ. Manage. 2007, 84, 628–634. (10) O’Dowd, W. J.; Pennline, H. W.; Freeman, M. C.; Granite, E. J.; Karash, A.; Lacher, C.; Hargis, R. Fuel Process. Technol. 2006, 87, 1071– 1084. (11) Presto, A. A.; Granite, E. J.; Karash, A.; Hargis, R. A.; O’Dowd, W. J.; Pennline, H. W. Energy Fuels 2006, 20, 1941–1945. (12) Presto, A. A.; Granite, E. J. Environ. Sci. Technol. 2006, 40, 5601– 5609.
*To whom correspondence should be addressed. Fax: þ86-57488130123. E-mail:
[email protected]. (1) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreth, K. C. Fuel Process. Technol. 2003, 82, 89–165. (2) Chen, Z. Y.; Mannava, D. P.; Mathur, V. K. Eng. Chem. Res. 2006, 45 (17), 6050–6055. (3) Ko, K. B.; Byun, Y.; Cho, M.; Namkung, W.; Shin, D. N.; Koh, D. J. Chemosphere 2008, 71 (9), 1674–1682. (4) Wang, M. Y.; Zhu, T. L.; Luo, H. J.; Tang, P.; Li, H. J. Environ. Sci. 2009, 21, 1652–1657. r 2010 American Chemical Society
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Energy Fuels 2011, 25, 154–158
: DOI:10.1021/ef101100y
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A continuous emission monitor (CEM) DM/6A/MS/1A, which was made in Japan by the Nippon Instrument Corporation, was employed in the experiment. The CEM had the additional capability of distinguishing mercury species in the sample gas stream. The gaseous Hg0 and Hg2þ were first separated by capturing Hg2þ with a potassium chloride (KCl) solution from the sample gas. Then, Hg0 and Hg2þ were detected by two detectors, respectively, in the following two ways. One way was that the washed sample gas with Hg0 passed through a electronic cooler with a potassium hydroxide (KOH) scrubber to remove interfering gases and moisture and a detector [cold vapor atomic absorption spectrometry (CVAAS)] to make a measurement of Hg0 in the sampling gas. The other way was that KCl solution containing Hg2þ reacted with a tin chloride (SnCl2) solution to deoxidize Hg2þ to Hg0. Hg0 was carried by the cleaning air through the other electronic cooler to remove interfering gases and moisture and the other detector to make a measurement of Hg2þ in the sampling gas. The CEM with an automatic zero adjustment had a nominal range of 0.1-1000 μg/m3 and was calibrated by an internal permeation device, with a response time of less than 1 min and with a sensitivity of 0.1 μg/m3. In addition, the surface chemical compositions of AC were analyzed using a JEOL JEM/2010 electron microscope with the INCA energy-dispersive X-ray microanalysis from Japan. 2.2. Experimental Procedure. Before Hg0 was adsorbed, the AC was placed on the fixed-bed reactor and the sample gas was adjusted to a desired flux in the bypass. Each time, the AC sorbent was controlled to be about 0.05 g (the height of the adsorption column was about 2 mm); the heating temperature of the gas channel and the operating temperature of the fixed adsorption bed were all kept at 130 ( 1 °C. The Hg0 adsorption experiment by AC was divided into four steps, which were conducted continuously. Detailed information about the concentration of each component gas during the process of the experiment is shown in Table 2. The time used for the whole experiment was about 60 h. It took 16 min in total for step 1. AC was first placed in the fixed-bed reactor, and the concentration of Hg0 was controlled at 14.2 μg/Nm3 with the total flow of 1.3 L/min in the bypass. Through this step, a consistent and stable initial condition was established. In step 2, the adsorption Hg0 by AC was conducted in N2 gas first to study the adsorbing between the surface of AC and Hg0. This step was maintained for about 16 h and 33 min. After step 2 was finished, the simulated flue gas components were introduced into the gas flow one by one, as shown in step 3 of Table 2. The total flow was always kept at the same value by adjusting the N2 balance gas. The effect of different flue gas components on Hg0 adsorption was evaluated in this step. After all gas components were added, the adsorbing of Hg0 by AC in the complete simulated flue gas started. The adsorption time in step 4 was quite long, at about 41 h and 22 min.
Figure 1. Bench-scale experiment system of mercury adsorption on adsorbents. Table 1. Flow of All Simulated Flue Gas Components simulated flue gas components
part A
part B
O2 CO2 SO2 NO NO2 HCl H2O N2 (carrier gas) N2 (balance gas) total of A þ B
flow (mL/min) 0 or 78 0 or 156 0 or 130 0 or 65 0 or 26 0 or 52 0 or 130 300 total part A 1300
oxidation and chemisorptions.13,14 However, the process of Hg0 adsorption by AC is very complicated. No experimental results have been reported to directly provide a better understanding of the oxidization of Hg0 to divalent mercury (Hg2þ) during the adsorption process. This study aimed to explore and verify the change of mercury species during Hg0 adsorption by AC, which was the most direct method to discover the adsorption mechanism of Hg0 by AC. 2. Experimental Section 2.1. Experimental Material and Analysis. All of the adsorption experiments were carried out on the bench-scale experiment system with a fixed adsorption bed in N2 or simulated flue gas, as shown in Figure 1. In the experiment, Hg0 was generated by a mercury permeation tube. The Hg0 concentration was changed by altering the temperature of the permeation tube or the flow of carrier gas. Simulated flue gas components, such as SO2, NO, NO2, CO2, and O2, were adjusted by controlling the flows of these components from the standard gas sources. These conventional gases were continuously measured by NGA 2000 MLT series. The concentrations of HCl and H2O were determined according to the concentration and flow of the corresponding standard gases. H2O was added to the simulated flue gas using a peristaltic pump, which transferred water into the glass tube wrapped with a heating line. The flows of all simulated flue gas components are shown in Table 1. The AC was generated from wood by a chemical activation technique using zinc chloride (ZnCl2) by Shanghai Activated Carbon Co., Ltd. The AC had a specific surface area of ca. 1450 m2/g and a micropore volume of ca. 0.59 cm3/g.
3. Results and Discussion 3.1. Hg0 Adsorption in N2. Figure 2 shows the Hg concentration at the outlet of the fixed adsorption bed. In the beginning, the Hg0 adsorption efficiency by AC was very high. Hg0 was detected in the outlet gas 4 h later, and the Hg0 concentration reached 14.2 μg/Nm3 at the 16th hour. Hg2þ was never observed in the outlet during the whole process. The above Hg0 adsorption phenomena seemed to indicate that the Hg0 adsorption by AC in N2 might be physical adsorption. If the Hg0 adsorption by AC was physical adsorption, the saturated adsorption of the AC was reached at the 16th hour, as shown in Figure 2. Then, no more Hg0 would be absorbed by AC because all of the pores in the surface of AC were filled by Hg0. However, the following adsorption in simulated coal-fired flue gas provided interesting and important results, which showed that Hg0 adsorption by AC in N2 was not due to a physical adsorption process.
(13) Olson, E. S.; Sharma, R. K. Novel catalytic carbons for mercury sorption in air. Proceedings of the Air and Waste Management Association 93rd Annual Meeting; Salt Lake City, UT, June 18-22, 2000; Paper AE113 805. (14) 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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Table 2. Concentrations of General Simulated Flue Gases during the Process of the Experiment concentrations of general simulated flue gases (average) step 1 2
3
4
process stabilized Hg0 adsorbed in N2 þCO2 þO2 þSO2 þNO2 þNO þHCl þH2O
adsorbed in simulated gas
time (h:min:s) 0:0:0-0:16:00 0:16:10-15:59:40 15:59:50-16:13:10 16:13:20-16:26:40 16:26:50-16:44:00 16:44:10-17:08:10 17:08:20-18:18:30 18:18:40-18:59:10 18:59:20-19:29:50 19:30:00-19:46:30 19:46:40-48:06:20
N2
balance balance balance balance balance balance balance balance balance balance balance balance balance 48:06:30-48:37:10 balance 48:37:20-57:39:20 balance 57:39:30-60:52:00 balance balance balance
CO2 (%)
O2 (%)
SO2 (ppm)
NO2 (ppm)
3.66 3.58 2.93 2.97 3.02 3.24 3.25 3.39 6.35 6.33 6.28 6.48 6.86 6.18 6.10 6.08
6.91 6.14 6.20 6.17 6.56 6.59 6.50 6.65 6.52 6.59 6.25 7.09 6.48 6.59 6.61
2966 3202 3673 3477 3372 3362 2054 1980 2004 742.5 0 1304 1357 1363
0.97 15.63 20.19 22.01 28.23 36.09 33.61 32.86 35.23 48.08 36.2 37.8 38.2
NO (ppm) HCl (ppm)
0.7 291.9 310.2 309.7 320.5 321.3 326.7 323.6 327.2 336.9 314.3 323.4 326.7
∼50 ∼50 ∼50 ∼50 ∼50 ∼50 ∼50 ∼50 ∼50 ∼50 ∼50
H2O (%)
∼10 ∼10 ∼10 ∼10 ∼10 ∼10 ∼10 ∼10 ∼10 ∼10
Figure 2. Hg0 adsorption by AC in N2 and in simulated flue gas.
drop. There was no rebound of the Hg0 concentration during the rest of the adsorption experiment, while the Hg2þ concentration at the outlet slowly increased. Finally, the Hg2þ concentration at the outlet reached the same level as the initial concentration of Hg0. It is believed that the conversion of Hg0 to Hg2þ occurred during the adsorption in the presence of the simulated coal-fired flue gas. As shown in Figure 2, there was a relatively stable concentration of Hg2þ from 48:37:20 to 57:39:20 after SO2 flow to the simulated flue gas was stopped. The concentration of Hg2þ increased rapidly when SO2 was added again at 57:39:20. It suggested that there was a competitive adsorption between SO2 and Hg2þ on AC and SO2 could inhibit the adsorption of Hg2þ on AC. Olson and co-workers had the similar opinion that SO2 would cause adsorbed Hg to desorb from AC.6 The Hg2þ concentration measured at the outlet of the bench-scale system was even higher than that at the inlet (14.2 μg/Nm3), because some adsorbed Hg2þ desorbed at the temperature of 130 °C. 3.3. Discussion of Physical or Chemical Adsorption. It was clear from Figure 2 that Hg0 was detected at the outlet of the fixed bed after AC reached saturation in Hg0 adsorption in
3.2. Hg0 Adsorption in Simulated Coal-Fired Flue Gas. When the gas flow into the reactor was switched from N2 to simulated gas components one by one, the concentration of Hg0 at the outlet dropped, as shown in Figure 2 (step 3). The influences of different gas components on Hg0 adsorption were observed during the switch from 15:59:50 to 19:29:50, as shown in Figure 3. Almost all other gas components, except CO2, enhanced Hg0 adsorption by AC, especially NO. According to the research proposed by Yan et al., there was a competitive adsorption between CO2 and Hg0 on AC.15 Accordingly, the Hg0 concentration increased because surface conditions of AC favored CO2 adsorption when CO2 was added at 15:59:50. There was a peak of Hg2þ when SO2 was added into the simulated flue gas at 16:26:50, as showed in Figure 3. Olson et al. had observed that SO2 would cause adsorbed Hg to desorb from AC when SO2 was added to a simulated flue gas mixture.6 However, this peak disappeared very quickly. During the adsorption of Hg0 by AC in simulated flue gas (in step 4), the Hg0 concentration at the outlet continued to (15) Yan, R.; Ng, Y. L.; Liang, D. T.; Lim, C. S.; Tay, J. H. Energy Fuels 2003, 17, 1528–1535.
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Figure 3. Concentrations of Hg0 and Hg2þ during switching from N2 gas to simulated flue gas.
the presence of N2. However, when the AC reached saturation in the presence of the simulated flue gas, Hg2þ was found but no Hg0 appeared at the outlet of the fixed bed. Such observations suggested that the conversion of Hg0 to Hg2þ occurred during adsorption in the presence of the simulated coal-fired flue gas. Furthermore, the conversion rate of Hg0 to Hg2þ was quite high and up to nearly 100% according to the results after saturation of the AC in simulated flue gas, as shown in Figure 2. Results of Hg0 adsorption by the chlorine-impregnated AC showed that this AC could not cause such high Hg0 conversion in N2 gas. Flue gas components could convert only part of Hg0 to Hg2þ without AC, as accepted by previous researchers.16-18 Our results suggested that oxidation of adsorbed Hg0 occurred only in the presence of AC and simulated flue gas. Ghorishi et al. also found that Cl-containing ACs were very effective in the removal of Hg0 in a combustion flue gas and were rather insensitive to the adsorption temperature in the range of 100-200 °C.19 Huggins et al. looked at undoped lignite-based AC and iodine-impregnated AC, as well as fly ash, etc. They observed that the bonding of Hg to the surface was often consistent with the formation of a bond between Hg and halogen or sulfur. They did not observe elemental Hg adsorbed to carbon.20 It seemed that physisorption was extremely impossible in the adsorption Hg0 by AC in simulated flue gas. However, Karatza et al. measured the equilibrium capacities of Hg0 on AC in nitrogen.7 These capacities were quite low, as compared to the capacities for Hg0 in simulated flue gas. It seemed that physisorption of Hg0 is not impossible but the capacities are low.
Table 3. Percentage of Elements on AC and Treated AC sorbent
AC treated AC
element
peak value
weight percent
atom percent
C O S Cl C
33474 1179 195 248 1759
96.66 2.79 0.24 0.31 100
97.68 2.12 0.09 0.11 100
According to the previous experiences on saturated adsorption, the adsorption of Hg0 by chlorine-impregnated AC in N2 might be physical adsorption. However, our further research suggested that adsorption of Hg0 by AC in N2 gas occurred through the chemical adsorption process as well. The AC used in this study was produced by a chemical activation method using ZnCl2, which might reside on the surface of AC.17 Table 3 shows the results of AC surface chemical element analysis with the Cl element concentration of 0.31% in weight on the AC surface, which was greater than that of Hg0 in N2 gas. Chlorine-impregnated of AC is favorable for Hg0 adsorption, strongly suggested by previous studies.19,21-23 To prove the effect of the key chemical element (such as Cl, etc.) in the Hg0 adsorption by AC, a high-temperature desorption system was used to remove the elements on the surface of AC as reported in another study.24 After heat treatment at 1200 °C in argon gas, the heat-treated AC almost had the same physical characteristics as before but the chemical elements were all removed by decomposition, as shown in Table 3. The more information about heattreated AC could be found in ref 24. The Hg0 adsorption curve of the heat-treated AC in N2 gas is shown in Figure 4. When the gas flow was switched to the
(16) Gao, H. L.; Zhou, J. S.; Luo, Z. Y.; Wu, X. J.; Hu, C. X.; Ni, M. J.; Cen, K. F. J. Eng. Thermophys. 2004, 52, 1057–1060. (17) Hu, C. X. Mercury emission from coal-fired power plant in China and stability adsorption mechanism of mercury on activated carbon. Ph.D. Thesis, Zhejiang University, Hangzhou, China, 2007. (18) Laudal, D. L.; Brown, T. D.; Nott, B. R. Fuel Process. Technol. 2000, 65-66, 157–165. (19) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K.; Jozewicz, W. S. Environ. Sci. Technol. 2002, 36 (20), 4454–4459. (20) Huggins, F. E.; Yap, N.; Huffman, G. P.; Senior, C. L. Fuel Process. Technol. 2003, 82, 167–196.
(21) Mamani-Paco, R. M.; Helble, J. J. Bench-scale examination of mercury oxidation under non-isothermal, post-combustion conditions. Proceedings of the Air and Waste Management Association 93rd Annual Meeting, Salt Lake City, UT, June 18-22, 2000; Paper AE1A 584. (22) Senior, C. L.; Sarofim, A. F.; Fang, T. Z.; Helble, J. J.; MamaniPaco, R. Fuel Process. Technol. 2000, 63, 197–213. (23) Niksa, S.; Helble, J. J.; Fujiwara, N. Environ. Sci. Technol. 2001, 35, 3701–3706. (24) Hu, C. X.; Zhou, J. S.; He, S.; Luo, Z. Y.; Cen, K. F. Fuel Process. Technol. 2009, 90, 812–817.
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method showed almost no capacity of Hg adsorption in N2 gas. However, those ACs were of high Hg0 adsorption capacities in simulated flue gas. 4. Conclusions Bench-scale experimental study of adsorption of Hg0 by chlorine-impregnated AC in N2 and in simulated flue gas produced some important results, which contributed to the understanding of the Hg adsorption mechanism by AC. Our results suggested that the adsorption of Hg0 by AC was a complete chemical adsorption process in N2 gas or in simulated flue gas. Hg0 was oxidized to Hg2þ by chlorine on the carbon surface and absorbed by AC in N2 gas. The oxidizing elements were consumed during the adsorption process. After AC lost the ability of absorbing Hg0 through chemical oxidation, the breakthrough occurred as Hg0 was detected at the outlet of the reactor. However, the adsorption of Hg0 by AC did not reach its full capacity. When the gas flow was switched from N2 to the simulated gas compounds one by one, AC started absorbing Hg0 again. However, the cause of oxidation of Hg0 was not the elements on the surface of AC, but the components in simulated flue gas, such as NO, NO2, HCl, etc., catalyzed by carbon on the AC surface. When the AC reached its full capacity of Hg0 sorption, oxidation of Hg0 continued, and when breakthrough occurred, Hg2þ was detected at the reactor outlet.
Figure 4. Hg0 adsorption by treated AC in N2 gas.
adsorption bed at the 5th minute, the drop of Hg0 concentrations was about 30% of the initial Hg0 concentration. The adsorption curve rose back quickly and reached the initial level of the Hg0 concentration about 5 min later. It was completely different from the adsorption phenomena of the original, chlorine-impregnated AC in N2, as shown in Figure 2. It could be inferred from Figure 4 that the heat-treated AC had only a limited adsorption capacity of Hg0 in N2 gas. In other words, after the loss of surface chemical elements (such as Cl, etc.), AC would have very limited ability of adsorbing Hg0 in N2 gas. Hg0 adsorption capacities of ACs produced by a physical activation method were also compared.24 It was found that three ACs produced by a physical activation
Acknowledgment. This research was supported by the Chinese National Natural Science Foundation (50476056) and the Open Fund of the State Key Laboratory (ZJUCEU2007006).
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