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Carbon Monoxide Effects Carbon monoxide, a fuel in high temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low temperature cells (PAFC and PEFC) in proportion to the H2:CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC (13). Cell limits are: • PEFC – Consensus tolerance is <50 ppm into the anode. • PAFC – Major US manufacturer set tolerance limit as <1.0 percent into the anode. • MCFC – CO and H2O shift to H2 and CO2 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature and catalyst. • SOFC – CO can be a fuel. However, if the fuel gas contains H2O, the shift reaction (CO + H2O → H2 + CO2) is chemically favored. Carbon Deposition Effects The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur (7). Carbon deposition not only represents a loss of carbon for the reaction, but more importantly results in deactivation of catalysts in the processor and the fuel cell due to deposition at the active sites. Thermo- dynamic equilibrium provides a first approximation of the potential for coke formation. The governing equations are: C + CO2 ↔ 2CO C + 2H2 ↔ CH4 C + H2O ↔ CO + H2 (Boudouard) (carbon-hydrogen) (carbon-steam or gasification) (8-1) (8-2) (8-3) The possible formation of carbon using a particular fuel can be determined by the simultaneous solution of the above equations using their equilibrium coefficients.43 No solid graphitic carbon exists at low temperatures (~600 °C) in binary mixtures containing at least 2 atoms of oxygen or 4 atoms of hydrogen per atom of carbon (14). Fuel Cell Unit Size: The size of the fuel cell is a characteristic that impacts fuel processor selection. There is a lower level of power output at which it is no longer advantageous to incorporate a fuel processor. The decision is also application-specific. It is likely that releasing H2 by chemical reaction from a solid compound when mixed with water is economical for small portable units (below 100 W). An H2 storage cartridge can be replaced in seconds (15). Actually the power level at which the tradeoff is likely to occur changes as processing and storage technology advances. One fuel processor developer has produced a 100 W partial oxidation (POX) methane reactor the size of a coffee can. The unit includes a reforming zone, shift reactors, and all heat exchangers. H2 is 36 percent (assume dry) and the CO level can be reduced to 1 percent. The unit runs on methane, propane, and ethanol (16). Another research project is investigating methanol reformers for sub-watt fuel cell power sources for the Army. 43. Carbon is slightly less likely to be deposited than equilibrium coefficient calculations indicate, due to kinetics. 8-11PDF Image | Fuel Cell Handbook (Seventh Edition)
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