Further Fuel Processing - Carbon Monoxide Removal

9 Further fuel processing - carbon monoxide removal 

We have seen that the water-gas shift reaction 

For PEM fuel cells, further carbon monoxide removal is essential after the shift reactors. This is usually done in one of three ways  

This method has the obvious disadvantage that hydrogen is being consumed, and so the efficiency is reduced. However, the quantities involved are small - we are reducing the carbon monoxide content from about 0.25%. The methane does not poison the fuel cell, but simply acts as a diluent. Catalysts are available, which will promote this reaction so that at about 200°C the carbon monoxide levels will be less than 10 ppm. The catalysts will also ensure that any unconverted methanol is reacted to methane, hydrogen, or carbon dioxide. 

Note that recently workers at the National Renewable Energy Laboratory in the United States have been studying promotion of the shift reaction by bacteria. If this approach is successful, there may be less need for the final CO clean-up stage. Biological systems are referred to in Section 8.8. Another method of CO removal, which has been investigated by Honeywell and others, is the electrochemical oxidation of CO. Honeywell claims that this technique can reduce CO levels from 500 ppm down to ppb levels, but these ideas are at an early stage of development. 

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Due to the operating requirements of PEFC stack technology, shift reactors and a carbon monoxide removal step are required to produce reformate of sufficient quality. Similarly, the stack operating temperature and its humidity requirements require a water management system as well as radiators for heat rejection. Some developers use pressurized systems to benefit from higher reactant partial pressures on both anode and cathode. Fuel processing for PEFC APU systems is identical to that needed in residential power or propulsion applications. The additional issue for PEFC is the minimization of steam needed for the fuel processor system. Since an APU is a mobile and/or remote unit, the need for external sources of water should be minimized. The reformate stream is further diluted by additional steam, if that water is not removed prior to the fuel cell stack. 

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