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Fuel Processing Techniques The generic term most often applied to the process of converting liquid or gaseous light hydrocarbon fuels to hydrogen and carbon monoxide is “reforming”. There are a number of methods to reform fuel. The three most commercially developed and popular methods are 1) steam reforming, 2) partial-oxidation reforming, and, 3) autothermal reforming. Steam reforming (SR) provides the highest concentration of hydrogen and can obtain a conver- sion efficiency. Partial oxidation (POX) is a fast process, good for starting, fast response, and a small reactor size. Non-catalytic POX operates at temperatures of appro ximately 1,400 °C, but adding a catalyst (catalytic POX or CPOX) can reduce this temperature to as low as 870 °C. Combining steam reforming closely with CPOX is termed autothermal reforming (ATR). Steam Reforming: Historically, steam reforming has been the most popular method of converting light hydrocarbons to hydrogen. The fuel is heated and vaporized, then injected with superheated steam into the reaction vessel. The steam-to-carbon molar ratio is usually in the neighborhood of 2.5:1 but developers strive for lower ratios to improve cycle efficiency. Excess steam is used to force the reaction to completion as well as to inhibit soot formation. Like most light hydrocarbons, heavier fuels can be reformed through high temperature reaction with steam. Steam reforming is usually carried out using nickel-based catalysts. Cobalt and noble metals are also active, but more expensive. The catalytic activity depends on metal surface area. For nickel, the crystals sinter quickly above the so-called Tamman temperature (590 °C), approaching a maximum size related to the pore diameter of the support. The crystal growth results in loss of surface area and activity (17). The steam reformer can operate with or without a catalyst. Most commercial applications of steam reforming use a catalyst to enhance reaction rates at decreased temperatures. Lower temperatures favor high CO and hydrogen concentration. The reforming catalyst also promotes the water-gas shift reaction. Steam reforming is endothermic, thus favored by high temperatures. But it is a slow reaction and requires a large reactor (4). As a result, rapid start and transients cannot be achieved by steam reforming due to its inherently slower indirect heating (18). Steam reforming suits pipeline gas and light distillate stationary fuel cell power generation well. The exothermic water-gas shift reaction occurs in the steam reformer reactor. The combined reaction, steam reforming and water gas shift, is endothermic. As such, an indirect high temperature heat source is needed to operate the reactor. This heat source usually takes the form of an adjacent, high-temperature furnace that combusts a small portion of the fuel or the fuel effluent from the fuel cell. Efficiency improves by using rejected heat from other parts of the system. Note that the intrinsic water-gas shift in the reactor may not lower the CO content to the fuel cell requirement, and additional shifting will be needed for lower temperature fuel cells. Steam reforming of higher hydrocarbons can be used to produce methane suitable for use in high temperature internal reforming fuel cells. Steam pre-reforming of hydrocarbons, as a process step in the manufacture of hydrogen, ammonia, methanol, carbon monoxide, and syngas, is an established technology. All higher hydrocarbons are converted over a nickel-based catalyst into a gas mixture containing hydrogen, methane, and carbon oxides. Establishment of methanation and shift reaction equilibria at the process conditions determines the composition of 8-12PDF Image | Fuel Cell Handbook (Seventh Edition)
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