Purification of technical hydrogen

Other papers describing the use of sodium borohydride have appeared in the literature. Thus, Kojima et al. (2004) suggested that the reaction be run in a closed pressure vessel, to collect hydrogen under higher pressure. 

Natural gas, liquefied petroleum gas (LPG), and carbon materials serving as the raw materials for cheap technical hydrogen usually contain marked amounts of sulfur compounds. In addition, sulfur-containing odorants (tetrahydrothiophene or ethylmercaptan) often are purposely added to the gas distribution network so that gas leaks may be detected in a timely manner, because natural gas is usually odorless. 

Even in small amounts, sulfur compounds are extremely harmful to the activity of most catalysts used in fuel cells. The admissible amounts of these impurities are in the ppb (parts per billion) range. In larger amounts, sulfur contaminants will also be harmful to hydrocarbon fuel reforming. 

In a review paper, Cayan et al. (2008) summarized study results on the identification of impurities in coal syngas on the performance of SOFCs. 

On an industrial scale, sulfur compounds are eliminated from natural gas by their hydrogenation to H2S and subsequent absorption by zinc oxide (ZnO). For small fuel cell power plants, this method is too unwieldy and is not used. De Wild et al. (2006) suggested using commercial absorbents of the NGDM-1 type (doped, although details have not been reported) for desulfurization in small power plants. This is a rather simple but very effective method, leaving less than 20 ppb of residual sulfur after the treatment. 

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