Carbon monoxide oxidation reactor cell

Cutillo et al. also analysed the effect of introducing a carbon monoxide tolerant fuel cell into the system, which would make the overall system less complex [443]. Because such fuel cells were expected to be less efficient, about 3% lower efficiency was assumed. Another potential simplification was the removal of one of the water-gas shift reactors. The two stage water-gas shift reactors could be replaced by a medium temperature water-gas shift reactor with higher carbon monoxide outlet concentration in combination with the high carbon monoxide tolerant fuel cell. Alternatively, a water-gas shift reactor with heat-exchange capabilities, as discussed in Section 5.2.1, could be placed into such a system and combined with preferential oxidation and low temperature PEM fuel cell technology. 

The proper design of fuel reformer systems must pay careful attention to the minimization of carbon monoxide before the processed fuel stream enters the fuel cell stack. Many reformer systems use a secondary preferential oxidation reactor that selectively oxidizes the carbon monoxide present in reformate streams. In most transportation applications the steam reformer and the selective oxidation reactors do not operate under steady state conditions large transients may occur which produce relatively large amounts of carbon monoxide. It is highly desirable to have a low-cost real-time carbon monoxide measurement system that provides feedback control to the fuel processing system in order to protect the PEM fuel cells from performance degrading concentrations of carbon monoxide. 

An alternative method of approaching the poisoning effect of carbon monoxide is to clean up the reformed fuel stream prior to admission to the fuel cell. For instance, methanol is fed to a reforming processor which produces a gas stream containing approximately 55% H2, the required fuel mixed with 22% CO2, 21% N2 and 2% CO. The overall process is achieved by combining the exothermic partial oxidation reaction with an endothermic steam reforming reaction over the same catalyst particles. This achieves a very high rate of internal heat transfer and a very easily controlled reactor. In the last step the reformer output can then fed to a clean-up reactor where the fuel is reacted further with air to reduce the CO content from 2% to 10 ppm, a far more viable concentration for the electrocatalysts used in the low temperature fuel cell. 

The output of the reformer is typically fed to PEM fuel cells with operation temperatures of about 80 °C, hence the PEM feed needs special attention. The carbon monoxide content must be kept below 50 ppm so as not to adsorb on the PEM catalyst and lower the fuel cell efficiency. Therefore, a gas clean-up is needed at the exit of the fuel processor. Depending on the conversion route and the fuel - or in most cases the conversion temperature, which determines the water gas shift equilibrium between CO2 and CO - a complete fuel cell system needs a gas clean-up like a water gas shift reactor (reaction Equation 23.5) or a preferential oxidation reactor, i.e. selective oxidation of CO in the presence of H2 (reaction Equation 23.6) [21, 22]. 

V. Cominos, V. Hessel, C. Hofmann, et at Selective oxidation of carbon monoxide in a hydrogen-rich fuel cell feed using a catalyst coated microstructured reactor. Catal. Today 2005, 110, 140-153. 

Lenz et al. [73] described the development of a 3 kW monolithic steam-supported partial oxidation reactor for jet fuel, which was developed to supply a solid oxide fuel cell (SOFC). The prototype reactor was composed of a ceramic honeycomb monolith (400 cpsi) operated between 950 C at the reactor inlet and 700°C at the reactor outlet [74]. The radial temperature gradient amoimted to 50 K which was attributed to inhomogeneous mixing at the reactor inlet. The feed composition corresponded to S/C ratio of 1.75 and O/C ratio of 1.0 at 50 000 h GHSV. Under these conditions, about 12 vol.% of each carbon monoxide and carbon dioxide were detected in the reformate, while methane was below the detection limit. Later, Lenz et al. [74] described a combination of three monolithic reactors coated with platinum/rhodium catalyst switched in series for jet fuel autothermal reforming. An optimum S/C ratio of 1.5 and an optimum O/C ratio of 0.83 were determined. Under these conditions 78.5% efficiency at 50 000 h GHSV was achieved. The conversion did not exceed 92.5%. In the product of these... 

Formation of carbon monoxide over the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere is frequently observed especially under conditions of partial load, because most catalysts for preferential oxidation of carbon monoxide have some activity for WGS and its reverse reaction. Therefore oversizing the reactor bears the danger of impaired conversion and the same applies for partial load of the reactor unfortunately. Because the concentration of carbon monoxide that is tolerated by low-temperature fuel cells is usually in the range below 100 ppm or less, even low catalytic activity for reverse shift becomes an issue. 

Reforming of natural gas for solid oxide fuel cells is achieved either internally as described above or externally by a pre-reformer reactor [40,41]. Further processing of the fuel is not required, because of the unlimited tolerance of the fuel cell to carbon monoxide. The re-circulation of anode off-gas to the pre-reformer [42] is an interesting option for solid oxide fuel cells. Through these means, addition of water is omitted, which clearly decreases the complexity of the system and reduces cold start problems (see also Section 3.5). 

Delsman et al. analysed a 100-W methanol fuel processor/fuel cell system [4], It was composed of a microstmctured reformer/bumer heat-exchanger reactor for methanol steam reforming and anode off-gas combustion, several heat-exchangers, a cooled microstmctured heat-exchanger reactor for preferential oxidation of carbon monoxide, a heat-exchanger for feed evaporation and a low temperature fuel cell. 

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