A further development of biorefineries is the use of hybrid techniques com­bining biological conversion with catalytic downstream processing (Wester — mann P, J0rgensen B, Lange L, Ahring BK, Christensen CH (2007) Int J Hydro­gen Energy, accepted for publication). For instance, highly efficient autother­mal reformers capable of converting 1 mol ethanol to 5 mol hydrogen have recently been demonstrated [27]. Since 2 mol of ethanol can be achieved for each sugar molecule, the hydrogen yield of this two-step process is 83% of the theoretical maximum, compared to the 10-20% achieved by direct hydrogen fermentation. Hydrogen produced in the thermophilic ethanol fermentation process described above would add to this yield, approaching the theoretical maximum yield of 12 mol hydrogen/mol monosaccharide.

Hydrogen has been suggested as a future energy carrier to succeed the fossil fuel era [28]. The introduction of downstream catalytic conversion of biofuels leaves the possibility of combining a less complex fuel handling technology (ethanol instead of hydrogen) for transportation purposes with all the benefits of the fuel cell technology. In the transition period before a hydrogen-based energy economy has been realized, a gradual change to the use of renewable energy can be facilitated by the use of catalytically con­verted biofuels in existing internal combustion engines. Although ethanol in even high ethanol:gasoline mixtures can be used for ground transportation with few modifications of the engines, biogasoline produced by catalytic con­version of methane and bioethanol will have potential use as a high energy alternative for aviation and air transport. If these transportation means are sustained in the future, the availability of safe liquid fuels with high energy content storable under ambient conditions is a prerequisite.

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