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with boiling end points below 205 °C), is less difficult and lower in cost so should be used where possible, certainly with low temperature fuel cells. Sulfur species in the fuel are converted to H2S, if necessary, then the H2S is trapped on zinc oxide. A minimum bed volume of the zinc oxide reactor is achieved at temperatures of 350 to 400 °C. Thermodynamic and economic analyses show that it is appropriate to use high temperature cleanup with high temperature fuel cells. There is a vast difference between removing sulfur from a gaseous fuel and a liquid fuel. The sulfur in a liquid fuel is usually removed after it is converted to a gas. This by removing the sulfur in the reforming reactor at high temperature, or by incorporating sulfur resistant catalysts. Sulfur resistant catalysts are being developed, but none are mature enough for present use. ANL is developing catalysts to reform gasoline, and have demonstrated that their catalyst can tolerate sulfur. The ANL catalyst has been shown to tolerate (100s of hours) sulfur present in natural gas in an engineering scale reformer. At least one developer has a liquid-phase fuel desulfurizer cartridge that will be used to remove sulfur prior to fuel vaporization. Other developers remove the sulfur immediately after vaporization and prior to reforming. Hydrogen must be recirculated to the removal device to convert the sulfur species to H2S so that it can be entrapped on zinc oxide. Zinc oxide beds are limited to operation at temperatures below 430 °C to minimize thermal cracking of hydrocarbons that can lead to coke formation. Thermodynamics also favor lower temperatures. At higher temperatures, the H2S cannot be reduced to levels low enough for shift catalyst or to reach fuel cell limits. For sulfur removal in the reformer, the presence of significant concentrations of steam in the fuel gas has a negative impact on the reaction equilibrium, leading to a higher concentration of H2S than could be achieved with a dry fuel gas. Carbon Monoxide Reduction: The use of CO as a fuel in high temperature cells and water-gas shift reactions to lower carbon monoxide to conditions suitable for a PAFC or a PEFC have been previously described. Fuel gas reformate contains 0.5 to 1 percent by volume of CO even after the shift reactions. Present PEFCs operate below 100 °C. At these temperatures, even small amounts of CO are preferentially adsorbed on the anode platinum (Pt) catalysts. This blocks access of H2 to the surface of the catalyst, degrading cell performance (29). Reformate for PEFC stacks must contain very low (<50 ppm) CO to minimize Pt absorption to a reasonable value to maintain sufficient active sites for the oxidation of H2. This can be achieved in two ways, by air injection into the anode at up to about 4 percent of the reformate feed rate or by reducing CO concentration prior to the cell: even at 50 ppm, catalyst poisoning by CO must be mitigated by the injection of some air at the anode. For the latter approach, a preferential oxidizer (PROX) is used to reduce CO concentration prior to the cell. It has highly dispersed supported Pt or Pt-Ru (ruthenium) catalyst. Such catalysts act on the principle of selective adsorption of CO onto the active Pt or Pt-Ru (relative to H2), leading to CO being selectively oxidized by stoichiometric amounts of air co-fed to the catalyst bed. As the CO is oxidized, the gas temperature rises, which decreases the selectivity of CO adsorption on the catalyst and also increases the kinetics of the reverse water-gas shift reaction. In practice, the PROX process is carried out in stages to permit cooling between stages. The PROX is a relatively large unit that operates at 100 to 180 °C (22). Preferential gas cleanup by selective oxidation results in 0.1 to 2 percent H2 lost (30). 8-22PDF Image | Fuel Cell Handbook (Seventh Edition)
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