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Energy & Fuels 1999, 13, 374-378
Oxidative Fixation of SO2 into Aqueous H2SO4 over a Pitch-Based Active Carbon Fiber above Room Temperature Isao Mochida,* Shuji Miyamoto, Keiichi Kuroda, Shizuo Kawano, Kinya Sakanishi, and Yozo Korai Institute of Advanced Material Study, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816, Japan
Akinori Yasutake Chemical Research Laboratory, Technical Headquarters Nagasaki Research and Development Center, Mitsubishi Heavy Industries Ltimited, 5-717-1 Fukahori-machi, Nagasaki 851-03, Japan
Masaaki Yoshikawa Research and Development Center, Osaka Gas Company, Limited, 6-19-9 Torishima, Konohana-ku, Osaka 554, Japan Received April 3, 1998. Revised Manuscript Received December 8, 1998
Oxidative fixation of SO2 into aqueous H2SO4 was examined at 50-70 °C over a pitch-based active carbon fiber (ACF) of large surface area. A higher temperature was found to require higher humidity and a longer contact time for the complete removal of SO2 at the stationary state. Smaller adsorption of SO2 and less condensed water on the ACF make it difficult to fix SO2 on the ACF and to elute aqueous H2SO4 continuously from the ACF, as revealed by the adsorbed SO2 on ACF and the eluted aqueous H2SO4 from the bed. Among the ACFs of different calcination temperatures, a particular ACF of the largest surface area calcined at 1100 °C showed the largest activity regardless of the reaction temperatures and humidity, indicating a major influence of the number of active sites. The hydrophilicity of the ACF has little influence on condensing water on the ACF above room temperature.
Introduction More efficient SOx removal from the atmosphere as well as from flue gas is still required because the consumption of fossil resources is steadily increasing worldwide, so a large amount of SO2 is still exhausted. Active carbon has been reported to adsorb SO2 and convert it to H2SO4 in the presence of oxygen and humidity.1-3 The reductive recovery of SO2 from the carbon surface consumes carbon and is probably not economical.4 We have reported that H2SO4 adsorbed on active carbon fiber is continuously recoverable as aqueous H2SO4 at room temperature by the humidity within the reactant gas to regenerate the active surface continuously.5,6 (1) Kisamori, S.; Kawano, S.; Mochida, I. Chem. Lett. 1993, 1899. (2) Kisamori, S.; Kuroda, K.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1337. (3) Mochida, I.; Kuroda, K.; Miyamoto, S.; Sotowa, C.; Korai, Y.; Sakanishi, K.; Yasutake, A.; Yoshikawa, M. Energy Fuels 1997, 11, 272. (4) Juntgen, H.; Kull, H.; Walker, P. L., Jr., Eds.; Chemistry and Physics of Carbon; Marcel Dekker: New York, 1989; p 145. (5) Mochida, I.; Kisamori, S. Langmuir 1994, 10, 1241. (6) Mochida, I.; Kuroda, K.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Chem. Lett. 1996, 541.
A temperature higher than room temperature is preferable for the removal of SO2 in flue gas because the treated gas can be dispersed to the atmosphere through a chimney. In the present study, oxidative fixation of SO2 in the flue gas was examined over a pitch-based ACF of large surface area at 50-70 °C to enlarge the applicability of the dry desulfurization process. The influences of the reaction temperature, humidity, contact time, and calcination temperature of ACF were studied to find key factors for oxidative SO2 removal on the ACF and its elution from the bed. The amount of adsorbed SO2 and aqueous H2SO4 over ACF during the SO2 removal was quantified through temperature-programmed desorption (TPD) of SO2 to clarify the influences of the above factors on the desulfurization efficiency. Experimental Section OG series of pitch-based ACFs were supplied by Osaka Gas Co. They were heat-treated in argon gas at several temperatures. Their properties are summarized in Table 1. SO2 removal was carried out at 30-70 °C using a fixed-bed flow reactor. ACF (0.1-0.75 g) was packed densely in the reactor U tube (8 mm in diameter). The length of the ACF
10.1021/ef9800718 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/02/1999
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Table 1. Properties of Pitch-ACFs elemental analysis ACFs
C
H
N
O (diff)
ash
S.A. (m2/g)
OG-5A-as -H900 OG-10A-as -H1100 OG-15A-as -H1100 OG-20A-as -H1000 -H1100 -H1200
89.6 92.3 91.6 96.2 93.9 96.8 95.5 96.7 97.5 98.0
1.1 0.9 0.9 0.1 0.9 0.1 0.9 0.6 0.1 0
0.7 0.7 0.6 0.6 0.7 0.4 0.3 0.3 0.2 0.2
8.2 5.6 7.4 2.9 4.1 2.2 2.8 1.9 1.6 1.2
0.3 0.5 0.2 0.2 0.4 0.5 0.5 0.5 0.6 0.6
563 539 977 793 1162 879 1894 1531 1460 1429
bed was ca. 30 mm for a ACF loading of 0.25 g. The flow rate was fixed at 100 mL/min, and W/F was varied from 1.0 × 10-3 to 7.5 × 10-3 g min mL-1. A model flue gas containing 1000 ppm SO2, 5 vol % O2, and 10-20 vol % H2O in nitrogen was used. Eluted aqueous H2SO4 was trapped at the bottom of the U tube. Therefore, the concentrations of SO2 at the inlet and outlet gases were observed continuously by a flame photometric detector (R268Y; Hamamatsu Photonics). SO2 adsorption was performed by flowing SO2, O2, and H2O vapor in a helium stream at 150 mL/min over the ACF (200 mg) suspended in a quartz basket in a microbalance (Cahn 1000). Temperature-programmed desorption analysis (TPD) of the ACFs as-received, heat-treated, and after SO2 removal and elution of adsorbed H2SO4 was carried out using a quartz-glass apparatus equipped with a mass spectrometer (AQA-200; Nichiden-Anelva Inc.). A 0.1 g sample was heated in flowing helium up to 400 °C at a heating rate of 10 °C/min to quantify desorbed SO2.
Figure 1. Breakthrough profiles of SO2 at different reaction temperatures over OG-20A-H1100: 1000 ppm of SO2, 10 vol % H2O, W/F ) 1.0 × 10-3 g min mL-1. Reaction temperature: (1) 30 °C (15 h) (relative humidity (rh) 463%), (2) 40 °C (rh 266%), (3) 50 °C (rh 159%), (4) 60 °C (rh 99%), (5) 70 °C (rh 63%).
Results SO2 Removal at 30-70 °C. Figure 1 illustrates breakthrough profiles of a stream containing 1000 ppm SO2 over OG-20A-H1100 at different reaction temperatures under the conditions of 10 vol % H2O and 1.0 × 10-3 g min/mL W/F. At 30 °C, complete removal of SO2 continued at least 15 h, as reported in a previous paper.3 A higher reaction temperature shortened the time before breakthrough and lowered the level of SO2 removal. At 70 °C, SO2 broke through by 2 h and no stationary removal was observed. Thus, the temperature strongly influenced the extent of oxidative SO2 removal over ACF. Figure 2 shows SO2 removal at 30-70 °C at a higher W/F, 2.5 × 10-3 g min/mL. A strong influence of the reaction temperature can be also seen, although complete removal was achieved at 30 and 40 °C for this contact time. Figure 3 illustrates the activity of OG-20A calcined at 1000-1200 °C for removal at 50 °C. The largest activity was observed with ACF calcined at 1100 °C, as observed for 30 °C. The largest activity appears to reflect the largest number of active sites and hydrophobicity induced by calcination at 1100 °C, as postulated in previous papers,3,6 while hydrophilicity, which is expected with ACF of low-temperature calcination, appears to have little contribution at this temperature. Influences of Reaction Conditions on the Removal at 50 °C. W/F. Figure 4 illustrates breakthrough profiles of SO2 over OG-20A-H1100 at 50 °C and 10 vol % H2O for varying W/F. The larger W/F prolonged the breakthrough time and enhanced the level of
Figure 2. Breakthrough profiles of SO2 at different reaction temperatures over OG-20A-H1100: 1000 ppm of SO2, 10 vol % H2O, W/F ) 2.5 × 10-3 g min mL-1. Reaction temperature: (1) 30 °C (60 h), (2) 40 °C (40 h), (3) 50 °C (4) 60 °C, (5) 70 °C.
Figure 3. Breakthrough profiles of SO2 over OG-20A heat treated at different temperatures: 1000 ppm of SO2, 5 vol % O2, 10 vol % H2O, W/F ) 2.5 × 10-3 g min mL-1, reaction temperature ) 50 °C. Sample: (1) OG-20A-H1000, (2) OG20A-H1100, (3) OG-20A-H1200.
stationary removal, thus W/F of 7.5 × 10-3 g min/mL completely removed SO2 at 50 °C for 40 h. Humidity. Humidity in the reaction gas also influenced SO2 removal over OG-20A-H1100 as shown in
376 Energy & Fuels, Vol. 13, No. 2, 1999
Figure 4. Breakthrough profiles of SO2 at different W/Fs over OG-20A-H1100: 1000 ppm of SO2, 5 vol % O2, 10 vol % H2O, reaction temperature ) 50 °C. Contact time: (1) W/F ) 1.0 × 10-3 g min mL-1, (2) W/F ) 2.5 × 10-3 g min mL-1, (3) W/F ) 5.0 × 10-3 g min mL-1, (4) W/F ) 7.5 × 10-3 g min mL-1 (40 h).
Figure 5. Breakthrough profiles of SO2 at some H2O concentrations over OG-20A-H1100: 1000 ppm of SO2, 5 vol % O2, W/F ) 2.5 × 10-3 g min mL-1, reaction temperature ) 50 °C. H2O: (1) 10 vol %, (2) 15 vol %, (3) 20 vol % (40 h).
Figure 5, where the reaction temperature and W/F were 50 °C and 2.5 × 10-3 g min/mL, respectively. A humidity of 10 vol % allowed the breakthrough of SO2 at 7 h and 40% leak under the stationary conditions, whereas 20 vol % humidity completely removed SO2 at the same temperature for at least 40 h. ACFs of Variable Surface Area. Figure 6 illustrates the SO2 removal at 50 °C over pitch-based ACFs of variable surface area calcined at their optimum temperature. OG-20A-H1100 showed the highest activity. ACFs of smaller surface area exhibited lower activity. The number of active sites, which was largest for OG-20A-H1100, may be lower over the ACFs of smaller surface area. Adsorption of SO2. Figure 7 illustrates the adsorption of SO2 from a He stream containing 1000 ppm SO2 over 200 mg of OG-20A-H1100 at 30, 50, and 70 °C, as measured by a Cahn thermobalance. Adsorption of SO2 alone increased monotonically up to saturation at 2 mg by 40 min at 30 °C, while it was saturated at 1 mg by 30 min at 50 °C and at 0.5 mg by 1 h at 70 °C. Oxygen at 5 vol % in the flue gas markedly increased the adsorption at any temperature, allowing adsorption to reach 10 and 4.5 mg, respectively, at 30 and 70 °C.
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Figure 6. Breakthrough profiles of SO2 over Pitch-ACFs of various activation: 1000 ppm of SO2, 5 vol % O2 10 vol % H2O, W/F ) 2.5 × 10-3 g min mL-1, reaction temperature ) 50 °C. Sample: (1) OG-5A-H900, (2) OG-10A-H1100, (3) OG-15AH1100, (4) OG-20A-H1100.
Figure 7. Adsorption profiles of SO2 over OG-20A-H1100: flow rate ) 150 mL/min (He balance), sample amount 200 mg, adsorption temperature ) 30, 50, 70 °C. (1) SO2 ) 1000 ppma, 30 °Cb; (S) SO2 ) 1000 ppma, 50 °Cb; (3) SO2 ) 1000 ppma, 70 °Cb; (4) SO2 ) 1000 ppm, O2 ) 5 vol %a, 30 °Cb; (5) SO2 ) 1000 ppm, O2 ) 5 vol %a, 50 °Cb; (6) SO2 ) 1000 ppm, O2 ) 5 vol %a, 70 °Cb; (7) O2 ) 5 vol %a, 30 °Cb; (8) O2 ) 5 vol %a, 50 °Cb; (9) O2 ) 5 vol %a, 70 °Cb (a. gas composition, b. adsorption temperature).
Strong influences of temperature were seen on both simple and oxidative adsorption of SO2 over the ACF. TPD Analysis of Adsorbed SO2. Figure 8 (a and b) illustrates TPD profiles of SO2 adsorbed with 5 vol % O2 for 1 and 15 h, respectively, over OG-20A-H1100 at 30-70 °C. Evolution of SO2 was detected up to 400 °C. SO2 evolved as illustrated by TPD profiles at two temperature ranges of 40-100 and 150-350 °C regardless of the adsorption time. The SO2 evolved in a higher temperature range was believed to originate from adsorbed SO3 or H2SO4, as discussed in a previous paper.7 The amount of evolved SO2 at a lower temperature range originating from physically adsorbed SO2 was always smaller than that at a higher temperature range. SO3 or H2SO4 was the major adsorbed species on the ACF when O2 was present. A longer adsorption time markedly reduced SO2 desorption but increased the H2SO4 desorption. A higher adsorption temperature reduced the amounts of desorbed SO2 of both species, the species desorbed at a higher temperature being reduced more markedly.
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Energy & Fuels, Vol. 13, No. 2, 1999 377
Figure 8. (a) TPD profile of adsorbed SO2 species over OG-20A-H1100 after 1 h of adsorption: flow rate ) 100 mL/min, analysis of initially adsorbed SOx in dry air 2000 ppm SO2 + 5 vol % O2 1 h followed by purge with He for 1 h. Adsorption temperature of SO2: (1) 30, (2) 50, (3) 70 °C. (b) TPD profile of adsorbed SO2 species over OG-20A-H1100 at som temperature after 15 h of adsorption: flow rate ) 100 mL/min, analysis of adsorbed SOx in dry air 2000 ppm SO2 + 5 vol % O2 for 15 h followed by purge with He for 1 h. Adsorption temperature of SO2: (1) 30, (2) 50, (3) 70 °C.
Figure 9. (a) TPD profile of SOx species adsorbed over OG-20A-H1100 after 1 h of reaction: ACF ) OG-20A-H1100 (100 mg), 1000 ppm SO2, 5 vol % O2, 10 vol % H2O, W/F ) 1.0 × 10-3 g min mL-1. Reaction temperature: (1) 30, (2) 50, (3) 70 °C. (b) TPD profile of SOx species adsorbed over OG-20A-H1100 after 15 h of reaction: ACF ) OG-20A-H1100 (100 mg), 1000 ppm SO2, 5 vol % O2, 5 vol % H2O, W/F ) 1.0 × 10-3 g min mL-1. Reaction temperature: (1) 30, (2) 50, (3) 70 °C.
Figure 9 (a and b) illustrates TPD profiles of SO2 adsorbed in the presence of both O2 and H2O at 30-70 °C. In this figure, a single broad profile of SO2 desorption was observed regardless of reaction temperatures. The desorption occurred between 150 and 370 °C, showing a maximum around 300 °C for adsorption at 30, 50, and 70 °C. H2O significantly increased the amount of desorbed SO2 which appeared to originate from adsorbed H2SO4 on ACF. It must be noted that more SO2 remained on ACF regardless of the adsorption time when adsorption was carried out at lower temperatures. Hence, higher activity at a lower temperature is ascribed to as more adsorption of SO2 and more elution of adsorbed H2SO4. Discussion
midity and a larger W/F for complete and continuous removal of SO2. Under the conditions examined in the present study, ACF with the largest surface area exhibited the largest activity at 50-70 °C as well as 30 °C after the optimum calcination at 1100 °C. A higher or lower calcination temperature reduced the activity, as shown in Figure 3. Calcination at 1100 °C removes almost all oxygen functional groups from the surface. The largest number of active sites for SO2 adsorption and oxidation is produced on the surface, as discussed in previous papers.3,6,7 The mechanism of SO2 removal consists of adsorption and oxidation of SO2 into SO3 which is hydrated into H2SO4. H2SO4 is eluted out as aqueous H2SO4 from the ACF-surface by a sufficient supply of water.8-11 TPD of adsorbed SO2 on ACF identified SO3 or H2SO4 as the
The present study reveals that pitch-based active carbon fibers of large surface area can oxidatively adsorb SO2 in the form of SO3 and hydrate into H2SO4, which is continuously recovered as aqueous H2SO4 even at temperatures above 50 °C when sufficient humidity is supplied to condense enough water at the respective temperature. A higher temperature requires more hu-
(7) Mochida, I.; Miyamoto, S.; Kuroda, K.; Kawano, S.; Yatsunami, S.; Korai, Y.; Yasutake, A.; Yoshikawa, M. Energy Fuels 1999, 13, 369. (8) Daley, M. A.; Mangun, C. L.; Debarr, J. A.; Lizzio, A. A. Carbon 1997, 35, 411. (9) Martyniuk, H.; Wieckowska, J. Fuel 1997, 76, 563. (10) Mochida, I.; Kuroda, K.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Fuel 1997, 76, 533. (11) Mochida, I.; Kuroda, K.; Kawano, S.; Matsumura, Y.; Yoshikawa, M.; Grulke, E.; Andrews, R. Fuel 1997, 76, 537.
378 Energy & Fuels, Vol. 13, No. 2, 1999
Figure 10. Stationary removal of SO2 vs RDH: sample OG20A-H1100, 1000 ppm SO2, 1000 ppm O2. Reaction temperature: (0) 30, (]) 40, (O), 50, (4) 60, (3) 70 °C. (1) H2O ) 10 vol %, W/F ) 1.0 × 10-3 g min mL-1. (2) H2O ) 10 vol %, W/F ) 2.5 × 10-3 g min mL-1. (3) W/F ) 1.0 × 10-3 g min mL-1: (9) 30 °, H2O 5 vol %; (b) 50 °C, H2O 9.1 vol %; (1) 70 °C, H2O 18.6 vol %. RDH + (PH2O,T - Psat,T/Psat,T) × 100 (relative humidity at the reaction temperature).
major adsorbed species in the presence of O2 and O2 plus H2O, respectively, especially under the stationary conditions.7 Both O2 and H2O increased the amount of adsorbed SO2 by oxidizing and hydrating the adsorbed SO2. Raising the reaction temperature makes it difficult to adsorb SO2 and condense water onto the ACF surface from the gas phase, requiring higher humidity and a larger W/F for complete removal. A higher temperature was found to reduce the saturated adsorption of SO2 in the form of aqueous H2SO4. The elution of H2SO4 also becomes slower at higher reaction temperatures under a fixed absolute humidity because of a small condensation of water over on ACF. Both factors are concluded to make it difficult to achieve complete SO2 removal at higher temperatures. The levels of stationary SO2 removal are plotted against relative humidity at the respective temperature in Figure 10 when the absolute humidity was fixed at 10%. A correlation between SO2
Mochida et al.
removal and relative humidity is evident, being dependent on W/F but independent of the temperature. The calcination at 1100 °C provided the maximum activity regardless of reaction temperature among the as-received and calcined OG-20As. Calcination is believed to induce active sites and hydrophobicity as the results of elimination of oxygen functional groups. Both enhance the removal of SO2 through acceleration of SO2 adsorption and elution of aqueous H2SO4.3,7 At a higher temperature, the relative humidity becomes smaller when the absolute humidity is fired, as observed in a real flue gas, less condensation water tending to take place over the ACF. Thus, elution of aqueous H2SO4 from the ACF bed becomes slow. A lower calcination temperature leaves more oxygen functional groups on the ACF surface, more condensation of water being expected to increase the activity. However, this is not the case. A lower number of actives site over the ACF of lower temperature calcination lost activity very sharply. A calcination temperature that is too high also reduces the activity sharply, the partial graphitization removes the active site and reduces the surface area. This conclusion suggests that it is difficult to prepare an ACF active for the continuous removal of SO2 at temperatures above 50 °C by controlling the oxygen functional groups. Other functional groups, such as basic nitrogen groups, may be useful. Introduction of more oxygen functional groups of the activation of ACF and their elimination by calcination can provide more active sites to be more active at higher reaction temperatures. Another approach is to design a process where the removal of SO2 at a higher temperature and recovery of aqueous H2SO4 at a lower temperature are alternatively performed at two different temperatures. Enhanced H2SO4 recovery at a lower temperature can regenerate the active site on the ACF to adsorb SO2 at a higher temperatures. Such a switching operation is under examination. EF9800718
