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CH4 + 1⁄2O2 → CO + 2H2 (8-10) When natural gas fuels are used in a PAFC or a PEFC, the reformate must be water-gas shifted because of the high CO levels in the reformate gas. A PAFC stack can tolerate about 1 percent CO in the cell before having an adverse effect on cell performance due to catalyst poisoning. The allowable CO level in the fuel gas for a PEFC is considerably lower. The shift conversion is often performed in two or more stages when CO levels are high. A first high-temperature stage allows high reaction rates, while a low-temperature converter allows for a higher conversion. Excess steam is used to enhance the CO conversion. A single-stage shift reactor is capable of converting 80 to 95 percent of the CO (24). The water gas shift reaction is mildly exothermic, so multiple stage systems must have interstage heat exchangers. Feed temperatures of high- and low-temperature shift converters range from approximately 260 to 370 °C (500 to 700 °F) and 200 to 260 °C (400 to 500 °F), respectively. Hydrogen formation is enhanced by low temperature, but is unaffected by pressure. When used in a PEFC, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer: CO + 1⁄2O2 → CO2 H2 + 1⁄2O2 → H2O The selectivity of these competing reactions depends upon the catalyst and determines the quantity of required oxygen (25). (8-11) (8-12) Liquid Fuel Processing: Liquid fuels such as distillate, naphtha, diesel oils, and heavy fuel oil can be reformed in partial oxidation reformers. All commercial POX reactors employ noncatalytic POX of the feed stream by oxygen in the presence of steam with reaction temperatures of approximately 1,300 to 1,500 °C (2,370 to 2,730 °F) (24). For illustration, the overall POX reaction for pentane is C5H12 + 5/2O2 → 5CO + 6H2 (8-13) The overall reaction is exothermic, and largely independent of pressure. The process is usually performed at 20 to 40 atmospheres to yield smaller equipment (24). A typical fuel composition for a fuel oil fed POX reformer is presented in Table 8-3. The CO contained in this reformate may need to be converted with a shift converter or selective catalytic converter, depending upon the specific fuel cell being fed. 8-18PDF Image | Fuel Cell Handbook (Seventh Edition)
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