Hydrogen from reformed hydrocarbons

Cationic contaminants may emanate from many sources. Metals, such as iron and copper, in system components may ionize due to corrosion exchange with protons in the membrane. Metallic salts, such as sodium and calcium, may enter the fuel cell from coastal water or from deicing agents. The most likely source of cationic contaminants is from the fuel line. Hydrogen from reformed hydrocarbons usually contain parts per million (ppm) of ammonia. This ammonia can be oxidized to ammonium ions and enter the polymer electrolyte. 

Liu, D. et al., Characterization of kilowatt-scale autothermal reformer for production of hydrogen from heavy hydrocarbons, Int. ]. Hydrogen Energ., 29,1035, 2004. 

The reforming process (as applied to a hydrocarbon or alcohol) yields a product stream that consists predominantly of hydrogen, carbon monoxide, carbon dioxide, water, unconverted feedstock, and trace by-products. This product stream mixture, called reformate, is unsuitable for direct use in low-temperature PEMFC and AFC, and some trace by-products (notably organosulfur compounds) will poison both high-temperature fuel cells and low-temperature fuel cells. A membrane for separating and purifying hydrogen from reformate must also be chemically compatible with the compounds in the reformate stream. 

The decomposition may occur either uni- or bimolecularly to form alkoxy and peroxy radicals. These oxy radicals abstract a labile hydrogen from the hydrocarbon to produce either an alcohol or a hydroperoxide. The alkyl radical thus formed readily adds oxygen to reform a peroxy radical, and the process continues. When the autoxidation occurs In the absence of an antioxidant, the termination of the kinetic chain occurs chiefly by the combination of two peroxy radicals. This termination Is a source of alkoxy radicals which can undergo chain scission and give rise to volatile products and carbonyl groups. 

Although PEFC are best fuelled with neat hydrogen, hydrogen containing gasses resulting from reformed hydrocarbons or methanol can be used as well, however, at the expense of power density due to dilution of the fuel by CO2 and potentially N2 as well as trace amounts of residual CO. 

Currently, there is a lot of interest in developing power sources in the 10-100 W range for portable power applications (Kundu et al, 2007 Service, 2002). One promising approach involves the use of a reformer that produces hydrogen from a hydrocarbon source, coupled with a fuel cell stack that utilizes the hydrogen to produce power. Such a device can be used to power radios, computers, electronic displays, and small unmanned air vehicles (National Research Council, 2004 Office of the Secretary of Defense, 2005). 

Fuel reforming is described in detail in Chapter 8. It will suffice to say at this stage that the production of hydrogen from a hydrocarbon usually involves steam reforming . In the case of methane, the steam reforming reaction may be written as... 

Natural gas is by far the preferred source of hydrogen. It has been cheap, and its use is more energy efficient than that of other hydrocarbons. The reforming process that is used to produce hydrogen from natural gas is highly developed, environmental controls are simple, and the capital investment is lower than that for any other method. Comparisons of the total energy consumption (fuel and synthesis gas), based on advanced technologies, have been discussed elsewhere (102). 

Cortright, R. D. Davda, R. R. Dumesic, J. A., Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002,418,964. 

RKN A process for making hydrogen from hydrocarbon gases (from natural gas to naphtha) by steam reforming. Developed by Haldor Topsoe in the 1960s as of 1975, 24 plants were operating. 

Fig. 8.1 A schematic diagram of hydrogen from hydrocarbon feed by steam reforming...

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