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

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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