Biological conversion of biomass to hydrogen either proceeds through photo­fermentation or dark fermentation. In dark fermentation the yield is only 10-20% of the potential hydrogen amount that theoretically can be de­rived from organic matter ([7] and Westermann P, J0rgensen B, Lange L, Ahring BK, Christensen CH (2007) Int J Hydrogen Energy (accepted for pub­lication)). Typical hydrogen yields are from 0.52 mol H2/mol hexose, when molasses was the substrate in a batch culture of Enterobacter aerogenes [8], to 2.3 mol when glucose was the substrate in continuous culture of Clostrid­ium butyricum [9]. Besides the low hydrogen yield, a major problem of fermentative hydrogen production is hydrogen-consuming microorganisms such as methanogens and acetogenic bacteria. In these processes, hydrogen is inevitably converted into methane or acetate, respectively, unless the re­sponsible microorganisms are excluded by sterilization of the biomass before fermentation and inoculation with specific hydrogen-producing microbes, or the process is carried out under conditions adverse to the hydrogen utilizers. A combination of biohydrogen production with fuel cell technology is, how­ever, rather straightforward since the fuel cell technology is available [10]. An upgrading of produced gases might be necessary before they are introduced into the fuel cells [11].

As a stand-alone process, fermentative hydrogen production from biomass is currently not feasible due to the low yield attained.

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