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It is a widely-held opinion that POX leads to lower efficiency than steam reforming due to the POX reaction being exothermic. However, a thorough examination of the thermodynamics shows that POX and ATR have higher reforming efficiencies than steam reformers. This raises the question why there is a need to use steam reforming or an ATR if the POX's efficiency is higher. The minimum allowable oxygen to carbon (O/C) ratio is 1 for the POX process. This generates high heat that leads to undesirable high temperatures (low H2, CO2 selectivity, materials of construction constraints, etc.). The steam reformer and ATR allow lower O/C ratios, keep the temperature down, and result in higher CO2 and H2 selectivity (more H2 yield per mole of fuel). Autothermal Reforming: The coupling of SR with POX is termed autothermal reforming (ATR). Some define ATR as a SR reaction and a POX reaction that take place over microscopic distances at the same catalytic site, thus avoiding complex heat exchange (21). Others have the less restrictive definition that ATR occurs when there is no wall between a combined SR reaction and catalytic POX reaction. ATR is carried out in the presence of a catalyst that controls the reaction pathways and thereby determines the relative extents of the POX and SR reactions. The SR reaction absorbs part of the heat generated by the POX reaction, limiting the maximum temperature in the reactor. The net result can be a slightly exothermic process. Autothermal reforming provides a fuel processor compromise that operates at a lower O/C and lower temperature than the POX; is smaller, quicker starting, and quicker responding than the SR, and results in high H2 concentration. A catalytic POX reaction must be used to reduce the temperature to a value compatible with the SR temperature. Other Reforming Combinations: There have been fuel processor configurations where a non- catalytic POX is placed in series with a steam reformer. Without catalyst, the POX reaction must be at a higher temperature than the steam reformer reaction. These reactions must take place in separate compartments with heat exchange and a wall between them (18). This configuration is not considered within the definition of autothermal reforming. State-of-the-Art Components Developers have brought fuel processing technology to the point where conversion of all fuels of interest to fuel cells have been demonstrated to a degree. Natural gas steam reforming is used in commercial fuel cell units. There has been equal success with steam reforming light distillates, although these fuels are not commonly used. Tests have been performed on reactors and complete small fuel processors using methanol, gasoline, and diesel, all suitable for vehicle use. These tests have not advanced to operation over prolonged periods. However, there have been tests that indicate these fuels can be processed in POX and ATR reactors with high levels of sulfur. Water-gas shift and methods to lower CO even to a few ppm have been developed, but the final CO cleanup processes are in an early stage of development. All fuel processors need additional engineering development to reduce volume, weight, and cost to allow widespread fuel cell power unit use. The state-of-the-art information below is based primarily on U.S. or closely- related fuel cell programs. 8-14PDF Image | Fuel Cell Handbook (Seventh Edition)
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