Energy & Fuels 1997, 11, 267-271
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Adsorption of SO2 on Bituminous Coal Char and Activated Carbon Fiber Joseph A. DeBarr* and Anthony A. Lizzio Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820
Michael A. Daley† Materials Science and Engineering Department, University of Illinois, Urbana, Illinois 61801 Received November 8, 1996. Revised Manuscript Received January 20, 1997X
The SO2 adsorption behaviors of activated carbons produced from Illinois coal and of commercially prepared activated carbon fibers (ACFs) were compared. There was no relation between surface area of coal-based carbons and SO2 adsorption, whereas adsorption of SO2 on the series of ACFs was inversely proportional to N2 BET surface area. Higher surface area ACFs had wider pores and adsorbed less SO2; thus, pore size distribution is thought to play a significant role in SO2 adsorption for these materials. Oxidation with HNO3 and/or H2SO4, followed by heat treatment at 700-925 °C to remove carbon-oxygen complexes, resulted in increased SO2 adsorption for both coal chars and ACFs. This behavior was explained by an increase in the available number of free sites, previously occupied by oxygen and now available for SO2 adsorption. The use of nitrogen-containing functional groups on ACFs of proper pore size shows promise for further increasing SO2 adsorption capacities. Knowledge of the relationship among the number of free sites, pore size, and surface chemistry on corresponding SO2 adsorption should lead to the development of more efficient adsorbents prepared from either coal or ACFs.
Introduction Anthropogenic air pollutants from localized regions impact not only extended geographical areas but the global atmosphere as well. One of the key precursors to acid rain, sulfur dioxide (SO2), is a major pollutant produced both by combustion of fossil fuels and by incineration of solid waste. Carbon-based flue gas desulfurization (FGD) systems have achieved commercial success and can remove up to 100% of the SO2 from combustion flue gas streams.1-3 One of the unique advantages of an activated carbon-based FGD process is that, in addition to SO2, it removes nearly every impurity found in combustion flue gas, including particulate material, heavy metals, organic materials, and other air toxics. No other existing FGD process has that capability. There are several research groups presently involved in the development of novel carbon-based processes and materials for flue gas cleanup.4-18 In this †
Present address: Kimberly-Clark Corp., 1400 Holcomb Bridge Rd., Roswell, GA 30076. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Richter, E.; Knoblauch, K.; Ju¨ngten, H. Gas Sep. Purif. 1987, 1, 35-44. (2) Tsuji, K.; Shiraishi, I. In EPRI SO2 Control Symposium [Proceedings]; Washington, DC, 1991; pp 307-324. (3) Brueggendick, H.; Pohl, F. G. In Tenth Annual International Pittsburgh Coal Conference [Proceedings]; 1993; pp 787-794. (4) Gangwal, S. K.; Howe, G. B.; Spivey, J. J.; Silveston, P. L.; Hudgins, R. R.; Metzinger, J. G. Environ. Prog. 1993, 12, 128-136. (5) Livengood, C. D.; Markussen, J. M. Power Eng. 1994, Jan, 3847. (6) Davini, P. Fuel 1989, 68 (2), 145-148. (7) Davini, P. Carbon 1990, 28 (4), 565-571. (8) Davini, P. Carbon 1993, 31 (1), 47-51. (9) Lu, G. Q.; Do, D. D. Sep. Technol. 1992, 2 (1), 19-28. (10) Lu, G. Q.; Do, D. D. Environ. Prog. 1996, 15, 12-18. (11) Cha, C. Y.; Vaillancourt, M. B.; Kim, S. S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1992, 37 (1), 1453-1458. (12) Kisamori, S.; Kawano, S.; Mochida, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 421-426.
S0887-0624(96)00199-5 CCC: $14.00
study, we look at the possibility of improving the physical properties and surface chemistry of a carbon to enhance its catalytic activity toward SO2 adsorption. This may lead to improvements in efficiency of activated carbon-based FGD systems and help reduce operating costs associated with their use. Historically, materials used in carbon-based FGD systems have consisted of granular activated carbons prepared from lignite or bituminous coals.1-3 Although activated carbon fibers (ACFs) were developed in the early 1970s, relatively little research has been done to examine their SO2-removal properties. Recent studies have reported that ACFs have relatively high SO2 adsorption capacities.16-19 In this paper, we present a comparison of SO2 adsorption for both coal-based carbons and ACFs, as well as ideas on carbon properties that may influence SO2 adsorption. Experimental Methods Sample Preparation. Figure 1 presents an overview of the experimental plan for production of activated char. The chars used in this work were prepared from an Illinois No. 2 (13) Rubio, B.; Izquierdo, M. T.; Mastral, A. M.; Mayoral, C. In International Conference on Carbon [Proceedings]; Granada, Spain, 1994; pp 402-403. (14) DeBarr, J. A.; Lizzio, A. A. In International Conference on Carbon [Proceedings], Granada, Spain; 1994; pp 268-269. (15) Lizzio, A. A.; DeBarr, J. A.; Donnals, G. L.; Kruse, C. W.; Rood, M. J.; Gangwal, S. K. Production and Use of Activated Char for Combined SO2/NOx Removal. Final Technical Report, Illinois Clean Coal Institute, Carterville, IL, 1995. (16) Fei, Y.; Sun, Y. N.; Givens, E.; Derbyshire, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (4), 1051-1055. (17) Mochida, I.; Hirayama, T.; Kisamori, S.; Kawano, S.; Fujitsu, H. Langmuir 1992, 8, 2290-2294. (18) Kisamori, S.; Mochida, I.; Fujitsu, H. Langmuir 1994, 10, 12411245. (19) Kim, J. Y.; Hong, I.; Lee, J. G. In 22nd Biennial Conference on Carbon [Proceedings], San Diego, CA; 1995; pp 534-535.
© 1997 American Chemical Society
268 Energy & Fuels, Vol. 11, No. 2, 1997
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Figure 2. SO2 adsorption for ACFs.
Figure 1. Production of activated char from Illinois coal. hvCb coal, sample IBC-102 of the Illinois Basin Coal Sample Program.20 A physically cleaned 48 × 100 mesh sample having 3.6% mineral content was prepared from the parent coal and used throughout as the feedstock for activated char. Chars were prepared in a 5 cm i.d. batch fluidized bed reactor (FBR). In each run, 200 g of IBC-102 coal was fluidized in flowing N2 (6 L/min), heated to 900 °C, held for 0.5 h at 900 °C, and then cooled to room temperature under flowing N2. A multistep heating procedure was used to minimize agglomeration of coal particles in the FBR. This procedure included the following steps: (1) increase sample temperature to 350 °C, (2) hold for 45 min at 350 °C, (3) increase sample temperature to 375 °C, (4) hold for 45 min at 375 °C, (5) increase sample temperature to 400 °C, (6) hold for 20 min at 400 °C, and (7) heat to final temperature (900 °C). In each case, the sample temperature was increased at a rate of 10 °C/min. Steam activation was done to develop microporosity and increase the surface area of the char. To activate the char previously prepared at 900 °C, a 50 g sample was placed in the FBR and heated to 860 °C in flowing N2. The N2 flow was replaced by 50% H2O/50% N2 (6 L/min) for 0.75 h to achieve 30% carbon conversion. The steam-activated char was subjected to a nitric acid (HNO3) treatment (oxygen deposition). Typically, 10 g of the char was added to 0.2 L of 10 M HNO3 solution and refluxed at 80 °C for 1 h. The HNO3-treated carbon was washed with distilled H2O to remove excess acid and vacuum-dried overnight at 25 °C. In some cases, the HNO3-treated char was heated in N2 to 525, 725, or 925 °C and held for 1 h to desorb carbon-oxygen (C-O) complexes formed by the HNO3 treatment. For this study, ACFs were obtained from Nippon Kynol in Japan. These fibers are made from a phenol-formaldehyde resin that has been melt-spun and then acid-cured to form a fully cross-linked, amorphous polymer structure. This precursor fiber, known as Kynol, is then carbonized and activated using the combustion byproducts of liquefied petroleum (mostly CO2 and H2O). The fibers can be produced in a range of surface areas between 600 and 2500 m2/g depending on the time and temperature of activation. The initial oxygen content of 4-8 wt % can be dramatically increased through a treatment with strong acid. These highly oxidized ACFs were prepared by treating them with equal volumes of nitric/sulfuric acid (no water) for a period of 12 h. The samples were then (20) Harvey, R. D.; Kruse, C. W. J. Coal Qual. 1988, 7, 109-114.
washed with fresh distilled water a total of five times for periods of 3 min with continuous stirring. The samples were air-dried at 125 °C to remove adsorbed water. ACFs with basic surface chemistry were prepared using methods described elsewhere.21 Sample Characterization. The kinetics of SO2 adsorption on samples was determined using a Cahn TG-131 thermogravimetric (TGA) analyzer system. In a typical run, a 3050-mg sample was placed in a platinum pan and the sample temperature increased at 20 °C/min to 120 °C in N2 flowing at 200 cm3/min. After the weight and temperature stabilized, the N2 was replaced by a mixture of gases containing 5% O2, 7% H2O, and the balance N2. Once there was no further weight gain due to adsorption of O2 and H2O, SO2 was added so that the simulated flue gas was comprised of 5% O2, 7% H2O, 2500 ppmv of SO2, and balance N2. The weight gain was recorded versus time (up to 10 h) by a computerized data acquisition system. The amount of SO2 adsorbed was calculated from the weight gain observed in the TGA, assuming all of the SO2 adsorbed is converted to H2SO4. Temperature programmed desorption (TPD) experiments were done in a flow-through, 2.5 cm i.d. stainless steel fixed bed reactor system. In a typical run, 0.5 g of sample was heated in flowing nitrogen (0.5 L/min) at 5 °C/min to a final temperature of 1000 °C and held for 1 h to achieve nearly complete desorption of CO and CO2 from the carbon surface. Nondispersive infrared analyzers (Rosemount Model 880) were used to monitor the concentrations of CO and CO2 in the effluent gas continuously. N2 BET surface areas of chars and ACFs were determined using the standard Brunnauer, Emmett, and Teller (BET) method applied to an experimental adsorption isotherm over a relative pressure range of 0.01-0.20. The adsorption isotherms were measured with a Coulter Omnisorb using nitrogen at 77 K. The average micropore size and micropore volume were calculated for ACFs and chars by applying the Dubinin-Radushkevich (DR) equation to the experimental nitrogen adsorption isotherm measured at 77 K.
Results and Discussion SO2 Adsorption vs Surface Area and Pore Size. Figure 2 shows that the amount of SO2 adsorbed varies inversely with surface area and micropore volume for the ACFs studied. During preparation of ACFs, the degree of burnoff increased in order for ACF-10, ACF15, ACF-20, and ACF-25, as reaction time increased at a fixed activation temperature. Results in Table 1 show that the surface area and micropore volume also in(21) Economy, J.; Foster, K.; Andreopoulos, A.; June, H. CHEMTECH 1992, 597-605.
Adsorption of SO2 on Coal Char and ACF
Energy & Fuels, Vol. 11, No. 2, 1997 269
Figure 3. Micropore size distributions for ACFs as calculated using the DRS equation. Table 1. Physical Properties of ACFs
sample ACF-25 ACF-20 ACF-15 ACF-10 ACF-10; oxid ACF-10; oxid; 400 °C ACF-10; oxid; 700 °C a
BET wt loss surface av during area micropore micropore heating (m2/g) vol (mL/g) width (Å) (%) 1900 1600 1400 520 310 540 633
0.800 0.674 0.590 0.223 0.132 0.234 0.270
18.0 13.2 12.6 5.3 5.3 5.0 5.2
-a 10.0 18.4
Not determined.
creased in order for ACF-10, ACF-15, ACF-20, and ACF25.22 Others have also observed a decrease in adsorption of SO2 with increasing surface area and micropore volume of ACFs (polyacrylonitrile fibers).17,19 The oxygen and nitrogen (
