1774
Energy & Fuels 2005, 19, 1774-1782
Desulfurization of Gaseous Fuels Using Activated Carbons as Catalysts for the Selective Oxidation of Hydrogen Sulfide Xianxian Wu,* Viviane Schwartz, Steven H. Overbury, and Timothy R. Armstrong Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37831 Received April 1, 2005. Revised Manuscript Received June 14, 2005
The removal of low concentrations of hydrogen sulfide (H2S) from hydrogen-rich gaseous fuels by selective catalytic oxidation, using activated carbon as the catalyst, was studied. The capacities of four activated carbons for reducing the H2S concentration down to the parts per billion (ppb) level were determined to be strongly related to their microstructures and impurities, even though their activity and selectivity were strongly dependent on the test conditions, such as reaction temperature, O2:H2S ratio, space velocity, and length-to-diameter (L/D) ratio of the catalyst bed. Because the side reactions that form COS and SO2 are sometimes unavoidable under real fuel processing conditions, the complete and exclusive conversion of H2S to elemental sulfur (S) requires that activated carbon has catalytic activities not only for the oxidation of H2S, but also for the oxidation of COS and the reaction between H2S and SO2. Because one of the activated carbons (sample W-22) had such catalytic functions and a relatively large capacity of trapping sulfur in its micropores, this carbon showed a combination of excellent activity and selectivity on removing H2S from both hydrogen and simulated reformate streams at a reaction temperature of ∼150 °C.
1. Introduction The presence of sulfur is a persistent problem associated with fossil fuels, and it must be removed before the fuels can be used as energy sources or chemical feedstocks. Through a typical desulfurization process, the sulfur can be recovered via the Claus process1,2 and the fuels may still contain a small amount of sulfur compounds. For example, gasoline or diesel fuels contain from a few parts per million (ppm) to several hundred ppm sulfur, pipeline natural gas contains up to 20 ppm sulfur, and the sulfur content in coal-derived gaseous fuels may vary from several hundred ppm to more than 1.0%, depending on the type of coal, the sulfur content in the feedstock, and the process.3 When these readily available fuels are used to produce hydrogen for protonexchange membrane (PEM) fuel cells, the sulfur content must be further reduced prior to entering the fuel cell, because sulfur severely poisons fuel processing catalysts and platinum electrocatalysts.4 Commonly, sulfur in fuels is present in the form of hydrogen sulfide (H2S) or is chemically combined with hydrocarbons and can be converted to H2S during fuel processing. Several approaches can be applied to remove * Author to whom correspondence should be addressed. Telephone: 1-865-576-6690. Fax: 1-865-576-8424. E-mail:
[email protected]. (1) Pie´plu, A.; Saur, O.; Lavalley, J. C.; Legendre, O.; Ne´dez, C. Catal. Rev.sSci. Eng. 1998, 40, 409. (2) Smet, E.; Lens, P.; Van Langenhove, H. Crit. Rev. Environ. Sci. Technol. 1998, 28, 89. (3) Kohl, A. L.; Nielsen, R. Gas Purification, 5th Edition; Gulf Professional Publishing Company: Houston, TX, 1997. (4) Hidalgo-Vivas, A.; Cooper, B. H. Chapter 15: Sulfur Removal Methods. In Handbook of Fuel CellssFundamentals, Technology and Applications, Vol. 3: Fuel Cell Technology and Applications; Vielstich, W., Gasteiger, H. A., Arnold, L., Eds.; Wiley: New York, 2003; p 177.
low concentrations of H2S from those hydrogen-rich gaseous fuels.3 “Liquid-phase absorption” processes absorb H2S via slurries of metal oxides or solutions of basic compounds such as ammonia, alkanolamine, or alkaline salts. “Dry sorption” processes scavenge small quantities of sulfur in a nonregenerative manner, using a dry sorbent. The main disadvantages of these methods are their relatively high cost and the production of spent sorbent (liquid or solid), which needs further chemical transformation to be recycled or disposed of. In comparison, direct partial oxidation of H2S to sulfur via the reaction
1 1 H2S + O2 f Sn + H2O 2 n
(1)
in the presence of a catalyst is more attractive for several reasons. First, the reaction has the thermodynamic potential to remove H2S to the parts per billion (ppb) level at temperatures of
