Desulfurization, for fuel-cell systems

With regard to the application of desulfurization in fuel-cell systems, there are still challenges. It seems that SOFCs and HT-PEFCs offer some advantages over MCFCs and PEFCs. Further research is required on the long-term effects and the reasons for the sudden death of materials. 

Ultradeep desulfurization of fuel oils is used for producing not only clean fuels but also sulfur-free hydrogen used in fuel-cell systems, in which the hydrogen can be produced potentially through the reforming of fuel oils. Fuel-cell systems must be run with little-to-no sulfur content, because sulfur can irreversibly poison the precious metal catalysts and electrodes used [12]. 

As desulfurization of gasoline for the fuel cell applications prefers a desulfurization process on-board or on-site at low temperatures without using H2 gas, considerable attention has been paid to developing an adsorptive process under ambient conditions without using H2 gas for the gasoline-based fuel cell systems. 

The selective ODS has shown many potential advantages for deep desulfurization of the fuels for fuel cell applications, because the process usually has higher desulfurization capacity than the adsorption desulfurizaton, and also can run at mild operating conditions without the use of H2. For ODS of liquid hydrocarbons fuels, direct use of oil-soluble peroxides or 02 as oxidants in an ODS process is greatly attractive, as the process does not involve a complicated biphasic oil-aqueous solution system. The key in ODS is how to increase the oxidation selectivity for the sulfur compounds. 

It is therefore obvious that a desulfurization unit upstream of the autothermal reformer is essential if sulfur-containing fuels are used. As mentioned previously, the upper value for the sulfur mass fraction of the fuel entering the fuel-cell system is 10 ppm [1-3]. In the following sections, different approaches will be presented for the desulfurization of middle distillates in the gas and liquid phases. 

The US company Mesoscopic Devices patented a method for an adsorptive desulfurization process with integrated regeneration in mobile fuel-cell systems... 

Assuming that the residue stream can be used for another purpose in the overall system, distillative separation becomes a relevant process in mobile fuel-cell systems for rough desulfurization, provided that it considerably simplifies subsequent desulfurization with further processes. 

Nearly all fuels used to operate carbonate fuel cell power plants contain sulfur. Sulfur compounds deactivate nickel-based catalysts used in the carbonate fuel cell anode. Sulfur has a tendency to be chemisorbed on active nickel, forming nickel sulfide (as shown in Reactions 4-6). The catalyst deactivation causes loss of reforming activity and hence limits the catalyst life. For stable long-term carbonate fuel cell operation, the sulfur concentration in fuel needs to be reduced to a lower level prior to introduction to the anode by utilizing an efficient fuel desulfurization system. As a rule-of-thumb, sulfur should be removed to bring the concentration down to the sub-ppm level. 

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