1606
Energy & Fuels 2005, 19, 1606-1610
Study on Alkali-Metal Vapor Removal for High-Temperature Cleaning of Coal Gas Yili Li,* Jian Li,* Yuquan Jin,* Youqing Wu,# and Jinsheng Gao# College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, People’s Republic of China Received July 5, 2004. Revised Manuscript Received March 9, 2005
Six natural mineral sorbents and a series of self-made sorbents were used to remove alkalimetal vapor from high-temperature coal gas. Results indicate that the self-made sorbent GM6 is the best sorbent with the highest sodium compound content, under the conditions of 1173 K, a gas velocity of 16 L/min, a reaction time of 3 h, and a NaCl concentration of 0.13 mg/L. It is shown that the sodium compound content is dependent on the chemical constituents and the microporous structure of sorbent, based on the surface area data. Results of X-ray diffraction (XRD) analysis and the kinetics indicate that the adsorption process of GM6 is physical adsorption in an N2 atmosphere and it is controlled by the diffusion of a sodium resultant layer. The adsorption process in a N2/H2O(2%) atmosphere is mainly a chemical adsorption that is controlled by both the chemical control and the product-layer diffusion control.
1. Introduction Coal contains alkali metal (sodium and potassium) in various chemical and physical forms. Of the many alkali compounds released into the gas phase, the chloride form (HCl, NaCl, and KCl) has been identified as the major component that is present after the combustion or gasification of coal. The release of alkali species into the gas phase is a natural consequence of coal combustion or gasification. The released alkalimetal vapor is a precursor of hot condensate that causes the corrosion of various parts of the combustor, gasifier, and the downstream system for secondary energy recovery. To protect the gas turbine from erosion and hot corrosion, alkalis such as chlorides and sulfates of sodium and potassium in the PFBC flue gas must be reduced to acceptable levels. The current industrial gasturbine specification limit for total alkalis in the combustion gas entering a gas turbine is equivalent to 24 ppb at an air/fuel (petroleum distillate) ratio of 50.1 However, the concentration of alkali-metal vapor in high-temperature coal gas is many times greater than the allowable alkali limit of 24 ppb. Therefore, a method to control alkali-metal vapor is required. One of the promising techniques involves the use of nonvolatile inorganic sorbents to remove alkali-metal vapor from the hot coal gases.2,3 This approach can be * Author to whom correspondence should be addressed. Telephone: +86-10-67392080. Fax: +86-10-67391983. E-mail:
[email protected]. # Present address: Department of Chemical Engineering for Energy Resources, East China University of Science and Technology, Shanghai 200237, PRC. (1) Lee, S. H. D.; Teats, F. G.; Swift, W. M. Alkali-vapour emission from PFBC of Illinois coals. Combust. Sci. Technol. 1992, 86, 327336. (2) Tran, K.-Q.; Steenari, B.-M.; Lindqvist, O.; Hagstrom, M.; Pettersson, J. B. C.; Lisa, K. M. Capture of alkali metals by kaolin. In Proceedings of the International Conference on Fluidized Bed Combustion, 2003; pp 403-409.
implemented through the use of a granular bed filter that is composed of sorbent or by injection ahead of the final filter of dry sorbent powder into the hot gas stream. Various studies have considered the feasibility of passing the flue gases through a fixed-bed filter of appropriate sorbents. This requires a sorbent with high efficiency and also requires an understanding of the fundamental kinetics of adsorption. Most of the previous studies have concentrated on the selection of sorbents and the overall feasibility of the process. However, fundamental mechanisms and kinetics of alkali-sorbent interactions are not consistent. Lee and Johnson4 studied the adsorption of NaCl, KCl, and K2SO4 on several substrates, including bauxite, silica, and diatomaceous earth. They proposed a chemical fixation on silica and a physical adsorption on bauxite. In another study, Lee et al.5 reported that the adsorption of alkali on activated bauxite relied on physical adsorption and chemical fixation, the latter being dominant in the presence of sufficient water vapor. Luthra et al.6 compared many additives, including emathlite, which was observed to be a very suitable sorbent at temperatures of
