Hydrogen and its competitors, 2004

28Risø Energy Report 3Technologies for producing hydrogen5.1BiomassH 2 CO 2AgriculturalproductsPretreatmentGasseparationOrganicwasteFermentation(Metabolicallyengineeredorganisms)Figure 7: Hydrogen production based on fermentation [13].enzymes used have high energy demands, solar energyconversion efficiencies are low, and huge areas of photobioreactorsare needed [13].It is often argued that it is simpler and more efficient toextract the hydrogen from organic substrates using adark fermentation process.Dark fermentation under aerobic conditions usesbiomass such as plant materials and organic wastestreams (Figure 7). Most of the hydrogen is actuallyproduced during anaerobic metabolism of pyruvateformed during the catabolism of carbohydrates andother substrates. There is scope for metabolic engineeringof the micro-organisms to redirect electrons so asto increase the hydrogen yields [13].Thermochemical conversionBiomass consists of carbon, hydrogen, oxygen, andnutrients such as nitrogen and sulphur. Figure 8compares the composition of wood with that of hydrocarbonfuels. Wood consists mainly of cellulose, whichcontains 6.2% hydrogen. As a result, the total hydrogencontent of wood is close to 6% and biomass is a reasonablefeed for hydrogen production.A large number of thermochemical processes to producehydrogen from biomass are under development ([11,14];see also other references to reviews of processes andexperimental technologies).In thermal gasification, solid biomass is first convertedthermochemically into a gas. In the case of steam gasificationand other indirectly-heated gasification processes,the result is a synthesis gas which can then be convertedto hydrogen.The first step in gasification is pyrolysis, in which thedissociated and volatile components of the biomass (70-85% of the dry weight) are vaporised at temperatures oftypically around 600°C. The resulting gas, whichcontains H 2 , CO, CO 2 , tar and water vapour, is firstcleaned to remove impurities and then optionallyupgraded using the water gas shift reaction to yield CO 2and hydrogen. The final step is to purify the hydrogen.The residue from pyrolysis is a mixture of char (fixedcarbon) and ash. The char is subsequently gasified byreacting it with oxygen, steam and/or hydrogen, and thisprovides the heat needed to run the gasifier.Many gasification systems have been developed [11].Among the more unusual of these are the use of solarenergy to provide the heat for gasification [15] and theaqueous conversion of biomass to hydrogen undersupercritical conditions (374°C and 22 MPa) [16].Fast pyrolysis processes can convert biomass into twooxygenated oils. These can be stored as intermediates,and later steam reformed to yield hydrogen and CO.Biomass-derived methanol and ethanol may also besteam reformed in the presence of a catalyst to producehydrogen. Methane derived from anaerobic digestioncan yield hydrogen through pyrolysis as well asreforming [11].Research needsTechnologies must be developed and sized according tothe availability and quality of locally-available biomass.Biomass is assessed in terms of price (including transportcosts), distribution, mass, and physical and chemicalproperties. Future research should also assess whetherthermochemical and biological processes could becombined. Biological processes are in general less energyintensiveand more environment-friendly than thermochemicalprocesses.Biological routes• Development of crops with compositions optimised forbiological hydrogen production.