Fuel Processing for Fuel Cells - Institut für Technische Chemie und ...

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Author's personal copy Fuel Processing for Fuel Cells 19 Natural gas Hydrocarbon Compressor SOFC 800–1000°C MCFC 600–700°C Desulfurization S < 1 ppm Pre-reformer (hydrocracking) Reformer (SR, ATR, CPOX) 600–900°C No CO limit for operation CO: > 1 Vol% H 2 - enrichment (membrane filters) CO-polishing (PrOX, SelOx, SMET) 120–200°C Shift reactor (LT/HT-WGS) 180–500°C CO: < 10 ppm CO: » 1–3 Vol% DMFC 25–90°C Direct MeOH feed LT/HT-PEMFC 60–180°C CO: < 10–300 ppm PAFC 190–200°C CO: < 5 Vol% Figure 3 Scheme of the requirements of the fuel processing in order to meet the specific requirements of various fuel cells; adapted from Duisberg et al. (2008). Blackened parts are not necessary in general; their usage depends on the process demands due to fuel source and hydrogen quality. Sulfur content in mg/kg 22,500 15,000 7500 1000 875 750 625 500 375 250 125 0 1985 1990 1995 2000 2005 2010 2015 Year Gasoline Non-road-diesel Diesel Jet-fuel / kerosene Figure 4 Sulfur levels in liquid fuels in relation to the directives of the EU and EPA, taken from van Rheinberg et al. (2011).
Author's personal copy 20 Torsten Kaltschmitt and Olaf Deutschmann et al., 2008). Several technologies are available to provide ultra-clean hydrocarbon primary fuels, but it has become apparent that adsorptive desulfurization technologies are the best in terms of cost (van Rheinberg et al., 2011). Adsorptive desulfurization is a widely used technology for the removal of sulfur from fuels to a level of ultra-clean quality. Simplicity and adaptability make this technology flexible and easy to integrate into the process due to the fuel processor and fuel cell type. A broad variety of sorbent materials are available on the market, ranging from zeolites and activated carbon over catalysts containing transition and precious metals to metal oxides, for example, ZnO. Actual research focuses on the development of more sulfur-tolerant catalyst systems for fuel processors and anode materials for fuel cells, requiring less precleaning of the fuel and thereby minimizing the balance of plant (BOP) and cost reduction. Pre-reforming is performed to crack higher hydrocarbon species into hydrocarbons ranging from C 1 to C 6 as they have better reforming characteristics in ATR compared to long-chain and aromatic compounds (Borup et al., 2005). Muradov et al. have reported about a novel combined pre-reforming–desulfurization process for the conversion of high-content sulfur diesel fuel to hydrogen-rich reformat suitable for fuel cells (Muradov et al., 2010). The pre-reformer uses hydrogen recycled from the fuel processor/purification unit to convert sulfur-enriched diesel fuel (3180–5240 ppmw) into short-chain hydrocarbons, mainly propane, and H 2 S. Muradov et al. assume that 13–16% of the hydrogen fraction needs to be used for recycling in their pre-reformer unit in order to convert diesel fuel to propane and butane. As pre-reforming catalyst, they use a mixture of zeolite and Ni-Mo/alumina catalyst at 400–500 C with pressures around 14 bars. The desulfurization of the pre-reformer effluent gas was performed with adsorption technology using an aqueous ferric sulfate solution, which led to an overall desulfurization yield of 96–98%, with 100 h, and coke formation on the catalytic surface as well as deactivation of the pre-reformer was observed, depending on the process parameters. Since primary fuels are directly usable for reformer applications such as SR, ATR, or catalytic partial oxidation (CPOX), only desulfurization is necessary to protect the sulfur-sensitive active sites in most reforming catalyst. Pre-reforming appends another reactor unit to the complete fuel processor system and their application is doubtful due to coking/ regeneration, volume, and weight increase in mobile applications. 4.2 Conversion of hydrocarbon fuels to hydrogen The reactants in fuel processing, that is, the fuel C x H y O z , steam, H 2 O, and the oxidizer, O 2 , contain carbon (C), hydrogen (H), and oxygen (O) atoms. From the point of thermodynamic equilibrium, a convenient way for
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