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Energy & Fuels 2003, 17, 1330-1337
Study of the Influence of Calcium Sulfate on Fly Ash/ Ca(OH)2 Sorbents for Flue Gas Desulfurization J. Ferna´ndez* and M. J. Renedo Departamento de Ingenierı´a Quı´mica y Quı´mica Inorga´ nica, E.T.S.I.I. y T., Universidad de Cantabria, Avda. de los Castros s/n, 39005 Santander, Spain Received December 13, 2002. Revised Manuscript Received May 21, 2003
The influence of calcium sulfate (CaSO4) on the physicochemical properties and desulfurization behavior of mixtures of fly ash and calcium hydroxide (fly ash/Ca(OH)2) at different initial ratios and hydration times was investigated. CaSO4 decreases the hydration reaction rate, and its influence is less pronounced after 15 h of hydration. Sorbents with smaller particle sizes, higher porosities and specific surface areas, and different compositions are obtained at high hydration times when CaSO4 is present. In relation to the sorbents obtained, CaSO4 has a positive or negative effect on the desulfurization ability, depending on the preparation conditions. The sorbent that is prepared at a higher fly ash/Ca(OH)2 ratio, hydration time, and gypsum amount (fly ash/ Ca(OH)2/gypsum ratio of 5/3/1.5 at 37 h) presents the best calcium utilization in the desulfurization reaction. From the results obtained, it seems that the positive effect on the desulfurization ability for sorbents prepared when CaSO4 is present is dependent on the different silicates with higher surface area formed. This effect is found at shorter hydration times as both the fly ash/Ca(OH)2 ratio and the CaSO4 amount increase. High fly ash/Ca(OH)2 ratios and high gypsum amounts will be the experimental conditions used to optimize the preparation of the sorbents.
Introduction Dry processes for the removal of SO2 from flue gas using calcium-based sorbents injected directly into the duct are not as common as wet processes; however, they do offer an attractive alternative with a simple technology, as a retrofit option for existing coal-fired power plants. It is also a complementary process to increase the SO2 capture when a previous process of desulfurization at high temperature is used in newly installed power plants. Some of these combined processes are LIMB/COOLSIDE or LIMB/ADVACATE.1,2 In-duct injection systems use a dry, powdered sorbent, typically calcium hydroxide (Ca(OH)2), which reacts with the humidified flue gas. Sorbents that are obtained via the hydration of lime or hydrated lime with different sources of silica lead to significantly higher conversion of calcium (in terms of the number of moles of SO2 per mole of calcium in the sorbent), compared to the conversion obtained using hydrated lime.3-13 When fly ash is used as a silica * Author to whom correspondence should be sent. E-mail: fernandj@ unican.es. (1) Sedman, C. B.; Maxwell, M. A.; Hall, B. Pilot Plant Support for ADVACATE/MDI Commercialization. Presented at the SO2 Control Symposium, 1991. (2) Clean Coal Technology Demonstration Program. U.S. Department of Energy, Washington, D.C., 1993. (Program updated 1992.) (3) Jozewicz, W.; Jorgensen, W.; Chang, J. Proceedings of the 10th Symposium on Flue Gas Desulfurization; U. S. EPA, Government Printing Office: Washington, DC, 1986. (Paper No. EPA-600/9-87004a.) (4) Jozewicz, W.; Jorgensen, W.; Chang, J.; Sedman, C.; Brna, T. J. Air Pollut. Control Assoc. 1988, 38, 796. (5) Jozewicz, W.; Chang, J.; Sedman, C.; Brna, T. J. Air Pollut. Control Assoc. 1988, 38, 1027. (6) Davini, P. Fuel 1996, 75, 6.
source, the pozzolanic reaction of silica with Ca(OH)2 was considered to be responsible for the improvement of solid utilization.3,14-18 Kind and Rochelle19 proposed a mechanism for the reaction between the fly ash and Ca(OH)2, the rate-limiting step of which was the dissolution of the silica from the fly ash when a sufficient amount of Ca(OH)2 was present. Calcium sulfate (CaSO4) is the product of the reaction between the calcium-based sorbent and SO2 at high temperature, and it is also present with calcium sulfite (CaSO3) in the process at low temperature. The effect of CaSO4 on the structure and the use of fly ash/Ca(OH)2 sorbents for SO2 removal have been widely investigated, because when a dry desulfurization process using fly ash/Ca(OH)2 sorbents is being applied in a power plant (the ADVACATE process or a combined (7) Renedo, M. J.; Ferna´ndez, J.; Garea, A.; Ayerbe, A.; Irabien, J. A. Ind. Eng. Chem. Res. 1999, 38, 1384. (8) Renedo, M. J.; Ferna´ndez, J. Ind. Eng. Chem. Res. 2002, 41, 2412. (9) Jung, G.-H.; Kim, H.; Kim, S.-G. Ind. Eng. Chem. Res. 2000, 39, 1264. (10) Jung, G.-H.; Kim, H.; Kim, S.-G. Ind. Eng. Chem. Res. 2000, 39, 5012. (11) Garea, A.; Ortiz, M. I.; Viguri, J. R.; Renedo, M. J.; Ferna´ndez, J.; Irabien, J. A. Thermochim. Acta 1996, 286, 173. (12) Garea, A.; Ferna´ndez, I.; Viguri, J. R.; Ortiz, M. I.; Ferna´ndez, J.; Renedo, M. J.; Irabien, J. A. Chem. Eng. J. 1997, 66, 171. (13) Ferna´ndez, J.; Renedo, J.; Garea, A.; Viguri, J.; Irabien, J. A. Powder Technol. 1997, 94, 133. (14) Jozewicz, W.; Rochelle, G. J. Environ. Prog. 1986, 5, 218. (15) Peterson, J.; Rochelle, G. Environ. Sci. Technol. 1988, 22, 1299. (16) Hall, B.; Singer, C.; Jozewicz, W.; Sedman, C.; Maxwell, M. J. Waste Manage. 1992, 42, 103. (17) Ho, C.; Shih, S. Ind. Eng. Chem. Res. 1992, 31, 1130. (18) Sanders, J.; Keener, T.; Wang, J. Ind. Eng. Chem. Res. 1995, 34, 302. (19) Kind, K.; Rochelle, G. T. J. Air Waste Manag. Assoc. 1994, 44, 869.
10.1021/ef020291s CCC: $25.00 © 2003 American Chemical Society Published on Web 08/14/2003
Influence of CaSO4 on Fly Ash/Ca(OH)2 Sorbents
process such as LIMB/COOLSIDE or LIMB/ADVACATE), CaSO4 is also present in the pozzolanic system. Much research on the effect of CaSO3 and/or CaSO4 on mixtures of fly ash (or other silica sources) and lime have been conducted; however, no general conclusions have been obtained. When CaSO4 or CaSO3/CaSO4 mixtures were used in the hydration system at 65°C for 12 h, Ho and Shih20 found solids that were more reactive toward SO2 than when these compounds were not present. Tsuchiai and co-workers21-23 studied the hydration of fly ash/lime/ CaSO4 but only made reference to a higher calcium utilization efficiency and Brunauer-Emmett-Teller (BET) specific surface area (SSA), compared to that of hydrated lime. Isukiza et al.,24 continuing with the study of the same system, concluded that the activity of the sorbent was enhanced by the presence of CaSO4 when it was added in the hydrothermal treatment of mixtures of CaO and fly ash. Kind, Wasserman, and Rochelle25 studied the fly ash/ Ca(OH)2/CaSO3/CaSO4 system, investigating mainly the effect of CaSO3/CaSO4 in the solution chemistry. They found that the role of CaSO4 in the formation of the solids was to buffer the solution, maintaining a high level of calcium in solution for longer hydration times. This would produce an increase in the rate of reaction between dissolved silica and calcium to yield reactive calcium silicates. Brodnax and Rochelle26 and Arthur and Rochelle,27 while preparing calcium silicates from iron blast furnace slag or recycled glass and Ca(OH)2, found an increased Ca2+ concentration and a higher rate of surface formation in the presence of gypsum. In a previous work,28 the properties of products that were formed by adding two different CaSO4 amounts to fly ash/Ca(OH)2 mixtures with a ratio of 5/3 that had been hydrated for 3, 7, and 15 h have been studied. The purpose of the present work is to make a deeper study of the influence of CaSO4 on the sorbent structure and desulfurization behavior, using a wider range of CaSO4/ fly ash/Ca(OH)2 ratios and hydration times to clarify the conditions necessary to optimize the preparation of the desulfurant sorbents. Experimental Section Preparation of Sorbents. Samples were prepared using commercial calcium hydroxide (Ca(OH)2, supplied by Calcinor S. A.), ASTM Class F coal fly ash (from a pulverized coal boiler and collected in an electrostatic precipitator of Pasajes (Guipuzcoa, Spain)), a bituminous coal-fired power plant, and CaSO4‚ 2H2O (supplied by Merck, with a purity of 99% and a BET SSA of 26.08 m2/g). The chemical composition and physical properties of Ca(OH)2 and fly ash are shown in Table 1. (20) Ho, C.; Shih, S. Can. J. Chem. Eng. 1993, 71, 934. (21) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Ind. Eng. Chem. Res. 1995, 34, 1404. (22) Tsuchiai, H.; Ishizuka, T.; Nakamura, H. G.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 851. (23) Tsuchiai, H.; Ishizuka, T.; Nakamura, H. G.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 2322. (24) Ishizuka, T.; Tsuchiai, H.; Murayama, T.; Tanaka, T.; Hattori, H. Ind. Eng. Chem. Res. 2000, 39, 1390. (25) Kind, K.; Wasserman, P.; Rochelle, G. Environ. Sci. Technol. 1994, 28, 277. (26) Brodnax, L. F.; Rochelle, G. T. J. Air Waste Manage. Assoc. 2000, 50, 1655. (27) Arthur, L. F.; Rochelle, G. T. Environ. Prog. 1998, 17, (2), 86. (28) Ferna´ndez, J.; Renedo, M. J.; Pesquera, A.; Irabien, J. A. Powder Technol. 2001, 119, 201.
Energy & Fuels, Vol. 17, No. 5, 2003 1331 Table 1. Physical Properties and Chemical Composition of Ca(OH)2 and Fly Ash hydrated lime fly ash Physical Properties adsorption-desorption of N2 BET specific surface area (m2/g) intrusion-extrusion of mercury pore volume (cm3/g) macropore volume, >50 nm (cm3/g) mesopore volume, 6.7-50 nm (cm3/g) skeletal density (g/cm3) laser diffraction mean volume diameter (D 4,3)
16.20
2.98
1.395 1.319 0.076 1.830
0.680 0.658 0.022 1.889
6.59
27.64
Chemical Properties composition (wt %) SiO2 Al2O3 Fe2O3 MgO CaO Ca(OH)2 CaCO3 Na2O K2O insoluble loss by heat impurities
49.26 30.04 6.92 0.85 1.91 84.32 11.98 0.64 2.44 0.17 6.31 3.70
Solids were prepared by slurrying fly ash/Ca(OH)2/CaSO4‚ 2H2O mixtures at different weight ratioss5/3/0, 5/3/0.5, 5/3/ 1.5, 5/3/3, 12/11/0, and 12/11/1.2sin a 2 L, discontinuous stirred tank reactor that was maintained at a constant temperature of 90 °C; the tank has three mouths with a reflux refrigerator to maintain a constant liquid/solid ratio. The amount of water was 1.5 L, and the amount of fly ash and Ca(OH)2 was kept constant at 150 g. Samples of the reaction mixture were taken out at different reaction times (over a range of 3-37 h), with the weight of the total solids extracted being
