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Energy & Fuels 2001, 15, 1396-1402
The Performance of Some ZnO-Based Regenerable Sorbents in Hot Coal Gas Desulfurization Long-Term Tests Using Graphite as a Pore-Modifier Additive L. Alonso,† J. M. Palacios,*,† and R. Moliner‡ Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Universidad Auto´ noma, Cantoblanco, 28049-Madrid, Spain, and Instituto de Carboquı´mica, CSIC, Marı´a de Luna 12, 50015-Zaragoza, Spain Received February 6, 2001. Revised Manuscript Received September 18, 2001
In a previous study it was shown that the overall reactivity in thermobalance tests and the efficiency at breakthrough in 2-cycle tests in a fixed bed reactor of a sorbent used for hot coal gas desulfurization increased substantially by the addition of a 5 wt % concentration of graphite during the preparation of sorbent ZT(0.8:1). In this paper, the study of the graphite effect as a pore forming additive is extended to other types of sorbents whose performance is investigated through the sulfidation breakthrough curves obtained in 20-cycle tests in a fixed bed reactor. Experiments show that the efficiency of the studied sorbents, ZFC, ZFT, and ZT, a zinc oxide, and a zinc ferrite doped with TiO2 or CuO, respectively, are dramatically increased by the addition of a 5 wt % graphite concentration as compared with the efficiency shown by the same sorbents prepared without graphite. The graphite effect is also evidenced by the achievement of a substantial delay of spalling appearance, a detrimental process consisting in the appearance and development of a crack network in the extrudate, decreasing the mechanical strength of the sorbent progressively down to excessively low values, after enough number of successive sulfidation-regeneration cycles. A first study trying to recover the reactivity of a sorbent, that became inactive after 20-cycles operation in a fixed bed reactor, by graphite addition is also included.
Introduction New technologies for electric power generation using coal gas efficiently, such as IGCC or MCFC, are demanding the development of regenerable sorbents for gas desulfurization taking place at high temperatures (>500 °C) achieving ppm H2S levels. Some sorbents, mainly based on ZnO or zinc ferrites doped with Ti or Cu oxides, have shown high reactivity in thermobalance tests and good short-term performances in different types of reactors.1-6 Unfortunately, in long-term tests in reactors these sorbents show a progressive efficiency loss at breakthrough and high degradation of their mechanical properties, as the number of successive sulfidation-regeneration cycles increases.7,8 * Author to whom all correspondence should be addressed. Tel: 34 91 585 47 87. Fax: 34 91 585 47 60. E-mail:
[email protected]. † Instituto de Cata ´ lisis y Petroleoquı´mica, CSIC. ‡ Instituto de Carboquı´mica, CSIC. (1) Gangwall, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J. Environ. Prog. 1989, 8, 265-269. (2) Lew, S.; Jothimurugesan, K.; Flytzani-Stepahanopoulos, M. Ind. Eng. Chem. Res. 1989, 28, 535-541. (3) Woods, M. C.; Gangwall, S. K.; Jothimurugesan, K.; Harrison, D. P. Ind. Eng. Chem. Res. 1990, 29, 1160-1167. (4) Ayala, R. E.; Marsh, D. W. Ind. Eng. Chem. Res. 1991, 30, 5560. (5) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1992, 31, 1890-1899. (6) Mojtahedi, W.; Salo, K.; Abbasian, J. Fuel Process. Technol. 1994, 37, 53-65. (7) Gupta, R.; Gangwall, S. K. Energy Fuels 1992, 6, 21-27. (8) Poston, J. A. Ind. Eng. Chem. Res. 1996, 35, 875-882.
Kinetic results in thermobalance together with the use of different theoretical models to simulate the involved gas-solid reaction show that the overall sulfidation rates of these sorbents are limited by intrinsic reactivity and by gas diffusion resistance both through intraparticle pores and the sulfide layer just formed.9-15 The diffusion controlled case is especially observed if sorbents are tested as cylindrical extrudates suitable for operation in fixed bed reactors. Consequently, a higher reactivity could be achieved if the sorbent porosity were increased by the application of suitable treatments during sorbent preparation. It is well-known that commercial catalysts are often prepared using different organic compounds acting as pore-forming additives, because they decompose into evolving gases during calcination at relatively low temperature (
