Ind. Eng. Chem. Res. 2003, 42, 3413-3419
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H2S Removal by Fine Limestone Particles in a Powder-Particle Fluidized Bed Tsutomu Nakazato,* Yan-Bai Lin, Masayuki Kusumoto, Nobuyoshi Nakagawa, and Kunio Kato Department of Biological and Chemical Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
An efficient dry desulfurization process for hot coal gas was proposed. The process uses a powderparticle fluidized bed as the reactor, in which coarse particles of several hundred micrometers are fluidized via hot gas consisting of H2S, H2, and N2 while fine limestone particles are fed continuously into the bed. Fine limestone particles of less than 30 µm were used as H2S sorbent. The effects of operating parameters on the H2S removal efficiency were investigated. Desulfurization via 5-µm limestone was very effective, demonstrating over 90% H2S removal at 1073 K for the case of Ca/S ) 2, even under the conditions of a very short residence time (0.3 s) of gas in the bed. Investigation of the sorbent pore structure before and after calcination and desulfurization revealed the correspondence of higher specific surface area of CaO to higher H2S removal efficiency. Introduction The critical situation regarding the environment has shed light on the need for more efficient power generation from fossil fuels. Most of the world’s power plants drive turbines by burning coal. As a result, every year tens of thousands of tons of sulfur are emitted into the atmosphere from the chimneys of the power plants. Improvement of the overall efficiency of power plants would contribute greatly to the reduction of pollutant emissions. Much attention has been focused on the integrated gasification combined cycle (IGCC),1 which combines both steam and gas turbines to give higher thermal efficiency and lower pollutant emissions. In the IGCC system, hot fuel gas is produced by the gasification of coal by using steam and either air or oxygen as the gasification agents at high temperature and pressure. Hot fuel gas is then purified in a gas cleanup system and sent to a gas turbine where it is burned. Due to the high reduction conditions of the gasifier, sulfur in the coal is converted to, for the most part, H2S. Hydrogen sulfide must be removed from the hot fuel gas in order to protect the equipment in the later stages of the process from its corrosive effects and also in order to meet strict government regulations for sulfur emissions. Desulfurization can occur either after the gas has been cooled (typically below 300 °C), which reduces overall plant efficiency and increases capital costs, or under high pressure and temperature, which has higher efficiency. Therefore, high-temperature desulfurization from hot fuel gas is a technology that is key to the success of IGCC systems. External and regenerable H2S sorbent bed processes based on iron oxides,2 zinc ferrite,3,4 zinc titanate,5 and other novel mixed oxides6 can be used at temperatures of up to 650 °C. However, desulfurization at temperatures higher than 650 °C would contribute substantially to improving the overall thermal efficiency of IGCC systems. * To whom correspondence should be addressed. Tel.: +81-277-30-1459. Fax: +81-277-30-1457. E-mail: nakazato@ bce.gunma-u.ac.jp.
Among the possible sorbents for H2S removal at such high temperatures, natural calcium-based materials, such as limestone, have the advantages of being low cost and abundantly available. However, most of the studies on the H2S sulfidation reactivity of limestone have been performed in thermobalance using small differential reactors with particle sizes of over 100 µm at long reaction times and low heating rates.7-12 These conditions are very different from those used in practical systems. Moreover, particle size may be the most important parameter in gas-solid reactions, as well as in the sulfidation process, because particle size affects H2S diffusion into the particle and, as a result, the overall reaction rate. Ada´nez et al.13 performed H2S removal tests using small-size calcium-based sorbents of 9-43 µm in an entrained bed reactor. Approximately 20-55% H2S removal was obtained at Ca/S ) 2, where the reaction temperature was 1000 °C and the residence time of particles was 0.8 s. However, if the particle diameter is less than 40 µm, the formation of aggregates should occur predominantly, which affects the efficient gassolid contact. This type of fine particle belongs to group C in Geldart’s classification14 and is characterized as being very difficult to fluidize. Kato et al.15 invented an efficient dry desulfurization process for SO2 removal using a powder-particle fluidized bed (PPFB). In this process, fine sorbent particles (group C particles) are fed continuously into the fluidized bed of coarse inert particles. The fluidizing coarse particles prevent the fine particles from agglomerating, and the fine particles have very long residence times in the bed, which can be hundreds to thousands times longer than that of the feed gas.15-18 Previous studies15,19 using very fine calcium-based sorbents demonstrated that PPFB yielded high SO2 removal efficiency, demonstrating over 80-90% SO2 removal at 1073-1123 K, where Ca/S ) 2 and the apparent mean residence time of gas was only 0.1 s. On the other hand, the reaction of H2S with limestone-derived CaO proceeded nearly as quickly as that of SO2 with CaO.20 Therefore,
10.1021/ie020805a CCC: $25.00 © 2003 American Chemical Society Published on Web 06/12/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003
Figure 1. Schematic diagram of experimental apparatus. Table 1. Properties Coarse Particles and Fine Particles material dp (µm) Fp (kg/m3) S0 (m2/g) Umf (m/s)c Ut (m/s)c
coarse particles
fine particles
silica sanda 270 (40-80 mesh) 2550
