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Microwave Plasma Enhanced Reduction of SO2 to Sulfur with Carbon Xuehai Wang,†,‡ Aiqin Wang,† Xiaodong Wang,† and Tao Zhang*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Science, P.O. Box 110, Dalian 116023, China, and Graduate School of the Chinese Academy of Sciences, Beijing, China ReceiVed October 12, 2006. ReVised Manuscript ReceiVed January 23, 2007
The emission of SO2 from industrial flue gas contributes to a large part of atmospheric pollution. With the more stringent regulations for SO2 emission, more efficient technologies for SO2 removal have to be developed. In the present work, we for the first time employed microwave plasma to reduce SO2 with three carbons (activated carbon, charcoal, and coke). At a microwave power of 110 W, SO2 can be completely reduced to sulfur by the activated carbon. The reactivity of the three carbons followed the order of activated carbon > charcoal > coke, which was consistent with that under conventional heating. However, much lower temperatures were required to obtain the same percentage conversion of SO2 under microwave plasma than that under conventional heating. Furthermore, in the presence of O2 in the feed gas, SO2 can also be efficiently reduced with carbons under the microwave plasma.
Introduction Sulfur dioxide is one of the major components of acid rain and other forms of atmospheric pollution. With the more stringent regulations for the emission of SO2, the treatment of SO2 in flue gas or in oxygen-free tail gas has become a significant problem. Currently, commercial flue gas desulfurization processes, based on SO2 scrubbing with lime or limestone, are mostly of the throw-away type, requiring a large space with complicated facilities and disposal of the used sorbents.1 Direct catalytic reduction of SO2 to elemental sulfur provides another promising way to remove sulfur dioxide from flue gas. This process can be potentially applied to flue gases containing a small amount of oxygen or to the case where SO2 in the flue gas is isolated or concentrated using a proper adsorption/regeneration system. The recovery of sulfur from this type of gas in a single-stage catalytic converter, avoiding the multistage Claus plant, would decrease the cost and facilitate the commercialization of dry regenerative flue gas cleanup processes. Various reducing agents, including carbon monoxide,2-5 hydrogen,6,7 and methane,8 as well as carbon materials,9-11 have * Corresponding author tel.: +86-411-84379015; fax: +86-41184691570; e-mail:
[email protected]. † Chinese Academy of Science. ‡ Graduate School of the Chinese Academy of Sciences. (1) Zhu, T.; Kundakovic, L.; Dreher, A.; Flytzani-Stephanopoulos, M. Catal. Today 1999, 50, 381-397. (2) Ma, J. X.; Fang, M.; Lau, N. T. Appl. Catal., A 1997, 150, 253268. (3) Kim, H.; Park, D. W.; Woo, H. C.; Chung, J. S. Appl. Catal., B 1998, 19, 233-243. (4) Liu, W.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Appl. Catal., B 1994, 4, 167-186. (5) Wang, X. H.; Wang, A. Q.; Li, N.; Wang, X. D.; Liu, Z.; Zhang, T. Ind. Eng. Chem. Res. 2006, 45, 4582-4588. (6) Chung, J. S.; Paik, S. C.; Lee, H. S.; Nam, I. S. Catal. Today 1997, 38, 193-198. (7) Ishiguro, A.; Liu, Y.; Nakajima, T.; Wakatsuki, Y. J. Catal. 2002, 206, 159-176. (8) Yu, J. J.; Yu, Q.; Jin, Y.; Chang, S. G. Ind. Eng. Chem. Res. 1997, 36, 2128-2133.
been investigated for this reaction. Compared with those gaseous reducing agents, carbon materials have the advantage of cheap and rich sources without the problem of production and transportation. Therefore, utilization of carbon materials for reducing SO2 to sulfur will be environmentally benign and costeffective. However, a high temperature is often required to induce the reaction between sulfur dioxide and carbon under conventional heating. For example, Bejarano et al. employed oil-sand fluid coke as a reducing agent for SO2 reduction and found that effective reduction of sulfur dioxide was only achieved in the range of 700-1000 °C.10 For the regenerator off-gas, the stream temperature is much lower than that of the flue gas. Therefore, lowering the reaction temperature between carbon and SO2 is desirable for recovery of sulfur from this type of tail gas in a single-stage catalytic converter. To overcome the high-temperature problem, Cha et al. have recently developed a new process by utilizing microwave heating for removing and destroying SO2 and NOx from combustion flue gases.12-14 Their work showed that microwave heating, as a promising energy-efficient power, could largely decrease the reaction temperature. Compared to microwave heating, microwave plasma is more energy-efficient, since a large amount of energy is consumed to produce free radicals, which have a much greater reactivity than atoms and molecules in the ground state. Therefore, employing plasma techniques to remove SO2 is receiving more and more attention.15-18 Considering that microwave-absorptive carbon was effective to initiate and (9) Lepose, R. Ind. Eng. Chem. Res. 1940, 32, 910-918. (10) Bejarano, C.; Jia, C. Q.; Chung, K. H. EnViron. Sci. Technol. 2001, 35, 800-804. (11) Bejarano, C.; Jia, C. Q.; Chung, K. H. Ind. Eng. Chem. Res. 2003, 42, 3731-3739. (12) Cha, C. Y. U.S. Patent 5,256,265, 1993. (13) Cha, C. Y. U.S. Patent 5,269,892, 1993. (14) Cha, C. Y.; Kim, D. S. Carbon 2001, 39, 1159-1166. (15) Veldhuizen, E. M.; Zhou, L. M.; Rutgers, W. R. Plasma Chem. Plasma Process. 1998, 18, 91-111. (16) Chang, M. B.; Balbach, J. H.; Rood, M. J.; Kushner, M. J. J. Appl. Phys. 1991, 69, 4409-4417.
10.1021/ef0605091 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
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Wang et al.
maintain stable microwave plasma at atmospheric pressure,19,20 we employed different types of carbon in the present work to reduce SO2 under microwave plasma. To the best of our knowledge, this is the first report that SO2 can be reduced to elemental sulfur by carbon with high efficiency under microwave plasma. Experimental Section 1. Materials. Three types of carbon, activated carbon (Jiangshu Changzhou Carbon Company, China), charcoal (Shenyang Charchoal Company), and coke (Shenyang Dongling Coke Company), were used as reducing agents. The carbons were all pelletized and sieved to 40-60 mesh before use. 2. Activity Test. The reaction between SO2 and carbon was carried out in a fixed-bed flow reactor system under microwave plasma or conventional furnace-heating at atmospheric pressure. The microwave plasma system employed in this work was described earlier.21 A quartz tube reactor of 6 mm i.d. was aligned vertically at the center of the microwave cavity so that the plasma region was seated in the microwave field of maximum intensity. The reactor was loaded with 0.2 g of carbon. The feed gas containing 5000 ppm SO2 in He passed through the carbon bed at a flow rate of 60 mL/min, corresponding to a residence time of 0.42 s. The effluent gas passed through an ice-water trap, where elemental sulfur was condensed. SO2 and COS in the effluent gas were separated by a Gaspro capillary column and detected by flame photometric detection, while CO and CO2 were separated by a Porapack Q column and detected by thermal conductivity detection. In our experiments, no other sulfur-containing compounds were detected except for SO2 and COS. An infrared pyrometer with a temperature range of 150∼1000 °C was used to measure the temperature of the carbon bed. The temperature was also checked by a thermal couple, which was inserted into the carbon bed immediately after the microwave generator was turned off to measure the temperature of the carbon bed. The temperatures measured by the two different methods were basically consistent with each other, which was also observed by Chang et al.22 and Tang et al.23 The percent conversion of SO2 (X) and the selectivity (S) to elemental sulfur are defined as follows: X)
[SO2]in - [SO2]out [SO2]in
× 100%
where [SO2]in is the inlet concentration of SO2 and [SO2]out and S)
[SO2]in - [SO2]out - [COS]out [SO2]in - [SO2]out
× 100%
[COS]out are the outlet concentrations of SO2 and COS, respectively. The sulfur yield Y ) XS. The energy efficiency E (g-SO2/kW h g-carbon) is defined as follows: E)
V × [SO2] × X% × 60 × 1000 × 64 Pin × m × 24 500
where V is the total gas flow rate (mL/min), [SO2] is the (17) Ma, H.; Chen, P.; Zhang, M.; Lin, X.; Ruger, R. Plasma Chem. Plasma Process. 2002, 22, 239-254. (18) Mok, Y. S.; Nam, I. Chem. Eng. J. 2002, 85, 87-97. (19) Heintze, M.; Magureanu, M. J. Catal. 2002, 206, 91-97. (20) Tsuji, M.; Nakano, K.; Kumagae, J.; Tsuji, T.; Yoon, S. H.; Korai, Y. Chem. Lett. 2002, 338-339. (21) Tang, J.; Zhang, T.; Liang, D.; Xu, C. C.; Sun, X.; Lin, L. Chem. Lett. 2000, 916-917. (22) Chang, Y.; Sanjurjo, A.; McCarty, J. G.; Krishnan, G.; Woods, B.; Wachsman, E. Catal. Lett. 1999, 57, 187-191. (23) Tang, J.; Zhang, T.; Liang, D.; Yang, H.; Li, N.; Lin, L. Appl. Catal., B 2002, 36, 1-7.
Figure 1. Effect of microwave power on SO2 conversion (solid symbols) and sulfur yield (open symbols) over activated carbon (squares), charcoal (circles), and coke (triangles).
concentration of SO2 in the reactants, X% is the SO2 conversion, Pin is the incident power of the microwave (W), m is the weight of loaded carbon, 24 500 is the volume (mL) of 1 mole of gas at room temperature, and 64 is the molecular weight of SO2.
Results and Discussion 1. Microwave Plasma Induced by Carbons. In this work, carbon acted as both a reductant and a plasma trigger. Prior to plasma generation, the carbon was treated in a He flow (flow rate: 60 mL/min) under 60 W microwave irradiation for a few minutes; then, the tuning plunger was adjusted to minimize the reflected power. As the reflected microwave power dropped rapidly to the smallest value, stable and blue-colored plasma was obtained over the whole carbon bed. After 40 min, the He flow was switched to the feed gas, and the color of the plasma changed to yellow. A value of 50 W was found to be the lowest power for obtaining stable microwave plasma. 2. SO2 Reduction with Carbons under Microwave Plasma. The effect of microwave power on the reduction efficiency of SO2 over the three carbons is illustrated in Figure 1. Both the SO2 conversion and the sulfur yield increased with the incident microwave power. The reactivity of the carbons varied with the carbon type, following the order activated carbon > charcoal > coke. Table 1 lists the surface areas of the three carbons and their reactivities toward SO2 reduction at a fixed power of 110 W. The activated carbon has the largest surface area, 737 m2/g, while the coke has the smallest area of only 78 m2/g. Humeres et al.24 suggested that both the structure and composition of carbons have imposed influences on the intrinsic reactivity of carbon toward SO2 reduction. Besides, when the rate-limiting step is a solid-gas reaction, the surface area will become an important factor affecting the reaction rate. Considering the three types of carbon materials we used in the present work are different both in structure and composition and in surface area, it is not surprising that they presented different efficiencies for removing SO2. On the other hand, in the microwave plasma system, the carbons can also absorb part of the microwave energy and result in an increase in the carbon-bed temperature. As shown in Table 1, the carbon-bed temperature at 110 W also follows the order activated carbon (351 °C) > charcoal (314 °C) > coke (283 °C), indicating that activated carbon has the best microwave-absorption capability among the three carbons. However, presently, it is not clear whether there is a correlation between the microwave-absorption capabilities and the reactivities toward SO2 reduction. From Table 1, it can also be seen that SO2 conversions at 110 W over the three carbons (activated carbon, charcoal, and (24) Humeres, E.; Moreira, R. F. P. M.; Peruch, M. G. B. Carbon 2002, 40, 751-760.
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Table 1. SO2 Reduction by Different Carbon under Microwave Plasmaa carbon types activated carbon charcoal coke a
sulfur SO2 surface area conversion yield temperature E (g-SO2/ (m2 g-1) (%) (%) (°C) kW h g-carbon) 737
99.6
99.4
351
2.13
214 78
85.5 77.3
82.2 73.8
270 234
1.82 1.65
Reaction conditions: SV ) 18 000 mL/g h; microwave power ) 110
W.
Figure 3. Effect of O2 content in the feed gas on SO2 conversion, sulfur yield, and COS selectivity at a microwave of 100 W over activated carbon.
Figure 2. Effect of temperature on SO2 conversion (solid symbols) and sulfur yield (open symbols) under conventional heating over activated carbon (squares), charcoal (circles), and coke (triangles).
coke) were 99.6%, 85.5%, and 77.3%, respectively. Especially with the activated carbon as the reductant, both the SO2 conversion and the sulfur yield attained almost 100%. The energy efficiency over the activated carbon can be as high as 2.13 g-SO2/kW h g-carbon, which is about 25 times greater than that under the microwave heating mode, as reported by Cha and Kim.14 Though a low SO2 concentration (1200 ppm) and a large amount (30-50 g) of carbon were used in Cha et al.’s work, such a great difference in energy efficiency, we believe, is mainly caused by the different energy conversion modes and the different intrinsic nature of the carbon materials employed. For microwave heating, since nonthermal microwave-specific effects have been testified to be implausible theoretically and experimentally,25 the reactant molecules are activated by thermal conversion. Therefore, in Cha et al.’s work, SO2 was completely converted to sulfur at input power greater than 400 W, which was much higher than that needed in our work. In this respect, microwave plasma is more energy-efficient than microwave heating. On the other hand, Cha et al. used FMC char and Korean anthracite coal as the carbon sources, which might be intrinsically less reactive than the activated carbon we used in the present work. For comparison, the reactivities of the three carbons for the reduction of SO2 under conventional heating are shown in Figure 2. It is evident that a high reaction temperature was required to achieve a substantial amount of SO2 conversion. Below 550 °C, the reaction between SO2 and carbon hardly occurred due to hindered kinetics.10 Only at temperatures higher than 650 °C was SO2 reduction by carbon prominent. Comparing the results obtained under the two reaction modes, we can see that microwave plasma has a unique activation effect on the reactant molecules, leading to a significant decrease in the reaction (25) Zhang, X. L.; Hayward, D. O.; Mingos, M. P. Catal. Lett. 2003, 88, 33-38.
temperature. Moreover, the reactivity of the three carbons under the conventional mode also follows the order activated carbon > charcoal > coke, which is consistent with that under the microwave plasma. In our previous work,26 we have shown that SO2 can be efficiently reduced with CO over CoO/γ-Al2O3 at a relatively low temperature under microwave plasma, which has been interpreted as a coupling effect between the microwave plasma and the catalytic function. However, in the present work, we got a similar high efficiency of SO2 removal with carbon, without the employment of any catalyst. 3. Effect of O2 Content. Since the activated carbon exhibited the highest reactivity toward SO2 reduction, we subsequently investigated the effect of O2 in the feed gas on the efficiency of SO2 removal under the microwave plasma. Figure 3 shows the SO2 conversion and sulfur yield as a function of the O2 content in the feed gas. It can be found that the addition of O2 caused an increase in the SO2 conversion. Meanwhile, the sulfur yield followed a volcano curve with the O2 concentration, with the maximum sulfur yield obtained at 0.5% O2 in the feed gas. This result is consistent with the observation by Feng and Jia when they investigated the effect of O2 and H2O on the carbothermal reduction of SO2 under conventional heating.27 It also can be seen that COS selectivity (COS is the only detectable byproduct in this case) increased linearly with an increase in O2 concentration. The formation of COS is due to a side reaction between sulfur and CO. It is noted that, under the presence of oxygen, the formation of both CO and CO2 was largely promoted, thus resulting in an increased formation rate of COS, accompanied by a significant consumption of carbon. Conclusions We developed a new route for SO2 reduction with carbons, which is significantly promoted by microwave plasma. In this reaction, carbons acted as both a reducing agent and a microwave-plasma trigger. Such a microwave plasma route was effective even in the presence of O2 in the feed gas. Compared with traditional heating, the microwave plasma presents great advantages due to its high efficiency for SO2 removal by various carbons, in particular by the activated carbon. EF0605091 (26) Wang, X. H.; Wang, A. Q.; Li, N.; Wang, X. D.; Liu, Z.; Zhang, T. Catal. Lett. 2006, 109, 109-113. (27) Feng, W.; Jia, C. Q. EnViron. Sci. Technol. 2005, 39, 9710-9714.
