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Energy & Fuels 2004, 18, 576-583
Low-Temperature H2S Removal from Steam-Containing Gas Mixtures with ZnO for Fuel Cell Application. 1. ZnO Particles and Extrudates Ivan I. Novochinskii,† Chunshan Song,*,† Xiaoliang Ma,† Xinsheng Liu,‡ Lawrence Shore,‡ Jordan Lampert,‡ and Robert J. Farrauto‡ Clean Fuels and Catalysis Program, The Energy Institute and the Department of Energy and Geo-Environmental Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802, and Corporate Research Center, Engelhard Corporation, 101 Wood Avenue, Iselin, New Jersey 08830 Received July 12, 2003
Sulfur removal is important for a fuel cell that uses a hydrocarbon fuel, such as natural gas, liquefied petroleum gas, and gasoline, to prevent the downstream sulfur poisoning of catalysts in the fuel processor and in the fuel cell anode. Although most sulfur species are removed prior to reforming, the reducing environment of the reforming stage (such as autothermal reforming) converts residual sulfur to hydrogen sulfide (H2S). H2S in the reformate must be removed to ensure longevity of the catalysts in downstream processing and in the anode chamber of fuel cell systems. A unique modified ZnO sample with a different morphology has been prepared and comparatively studied together with a commercially available ZnO sample under various conditions. Extremely low H2S outlet concentrationssas low as 20 parts per billion by volume (ppbv)shave been observed over the modified ZnO sample for extended periods of times. The sulfur-trap capacity (the amount of H2S trapped before breakthrough) also is dependent on space velocity, temperature, steam concentration, CO2 concentration, and particle size. Higher capacity is observed at higher H2S inlet concentration of 8 ppmv, compared to lower inlet concentrations of 1-4 ppmv. The trap capacity decreases monotonically as the temperature increases. Steam in the reformate inhibits the capture of H2S by ZnO; it seems to shift the equilibrium of the reaction ZnO(s) + H2S(g) S ZnS(s) + H2O(g) to the left, toward ZnO and H2S. The effect of steam seems to be reversible. Increasing the CO2 concentration in the feed up to 12 vol % decreases the capacity of ZnO for the capture of H2S.
Introduction In the past decade, the integrated gasification combined cycle (IGCC) and fuel cells (FCs) have been considered to be among the most-promising processes for advanced electric power generation. These emerging technologies will not only significantly improve the thermal efficiency but also will reduce or eliminate the environmentally harmful pollutants that are associated with the combustion of fossil fuels. Usually, after gasification of the fuels, sulfur exists as hydrogen sulfide (H2S). The deep removal of H2S from gasified fuel gases is a significant concern that stems from (i) the stringent environmental regulations, (ii) the need to prevent poisoning the catalysts downstream, (iii) the need to protect turbines from corrosion, and (iv) the need to protect the FC anode catalyst and also the fuel-processing catalyst in a proton exchange membrane (PEM) FC system that uses a hydrocarbon fuel. That is why an increasing amount of attention has been focused on the development of various materials and methods for sulfur removal for either IGCC or FC applications. * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Pennsylvania State University. ‡ Engelhard Corporation. (1) Westmoreland, P. R.; Harrison, D. P. Environ. Sci. Technol. 1976, 10, 659-661.
Westmoreland and Harrison1 and Hepworth et al.2 performed comparative studies on different absorbent systems that consisted almost entirely of different metal oxides. These studies can be divided into two groups, with respect to the operating temperatures: high temperature, >600 °C (Ba, Ca, Sr, Cu, Mn, Mo, W); and low temperature, 300-550 °C (V, Zn, Co, Fe). Candidate absorbents should meet several requirements to be considered for commercial gas desulfurization. The most important feature is that it should be theoretically capable of decreasing gas-phase sulfur concentrations to the low levels required by downstream processes (below 0.1 parts per million by volume (ppmv) for PEM FC and below 20 ppmv for IGCC) under the relevant conditions. In addition, the absorbent must have acceptable sulfur capacity (in terms of both theoretical value and achievable range), preferably be regenerable, and maintain activity and capacity through a large number of sulfidation/regeneration cycles. The absorbent should also be nonpyrophoric for FC applications.3 Finally, the cost of the absorbent must be reasonable. Not much attention has been given to the desulfurization of steam-containing gas mixtures with low H2S (2) Hepworth, M. T.; Ben-Slimane, R.; Zhjeng, S. Energy Fuels 1993, 7, 602-609.
10.1021/ef030137l CCC: $27.50 © 2004 American Chemical Society Published on Web 02/12/2004
Low-Temperature H2S Removal from Gas Mixtures. 1
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