Hydrogen and its competitors, 2004

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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.
Risø Energy Report 3Technologies for producing hydrogen 295.1Wt %90Figure 8: Composition of common fuels.80706050403020100CoalOilFuelMethaneWood■ C ■ H ■ O• Pretreatment of feedstock to increase accessibility.• Systems for fermentable biomass, including light-darkfermentation systems for hydrogen.• New thermophilic micro-organisms for hydrogenproduction.• Effects of culture conditions on hydrogen production.• Co-production: for instance, the effluent from afermentation process for hydrogen can be used togenerate methane.Thermochemical routes• Biomass feedstock typically has a low bulk density andis fibrous in nature. These properties create challengesin developing feed transport and preparation systems,such as pelletising, that are suitable for reactors operatingat high temperatures and pressures.• Research to improve product selectivity so that onlyhydrogen, carbon dioxide and water are produced.• New technologies for gas separation, cleaning andprocessing so that hydrogen from biomass gasificationcan be used, for example, in fuel cells.• Discover new opportunities for technology developmentby understanding and optimising feedstockconversion technology and market opportunities.• Develop high-pressure aqueous processing technologyto produce hydrogen from biomass.• Technology for CO 2 capture (hydrogen production/decarbonisation accompanied by the capture andsequestration of carbon).• Biomass and micro-organisms have the potential toaccelerate the development of hydrogen into a majorfuel of the future. This will require significant R&D,however, since hydrogen made from biomass, evenwith reasonably efficient technologies, cannot yetcompete with hydrogen from natural gas.Hydrogen from electrolysisThough it has been known for 200 years, electrolysis isstill the only practical way to make hydrogen from water– and this situation is not likely to change in the foreseeablefuture. The largest source of electrolytichydrogen is in fact the chlor-alkali industry, which hasbeen operating for around 100 years. In chlor-alkalimanufacture the main products are chlorine and sodium(or potassium) hydroxide, and hydrogen is regarded as aby-product.Only a vanishingly small proportion (of the order of0.1%) of the world's hydrogen is produced directly bywater electrolysis. Even this small quantity has declinedin recent years as energy prices have fallen to the pointwhere hydrogen produced electrolytically for fertilisermanufacture can no longer compete with hydrogenfrom natural gas. Because electrolytic hydrogen iscreated indirectly, using electricity as an energy carrier, itis only economic if electricity is extremely cheap. Thismay be the case in countries that have large hydroelectricsystems, such as Egypt, Brazil, Iceland, Canada,Norway and Zaire, or a large nuclear component in thegenerating mix, such as France, Belgium and Switzerland[17].PrincipleThe decomposition of water by electrolysis involves twopartial reactions that take place at the two electrodes.The electron-conducting electrodes are separated by anelectrolyte that conduct ions but not electrons.Hydrogen is produced at the negative electrode(cathode) and oxygen at the positive electrode (anode).Energy to split the water molecules is supplied as electricityto the two electrodes, and the necessary exchangeof charge occurs as ions flow through the electrolyte.The following sections describe electrolysis processes asoptimised for hydrogen production: the well-tested lowpressureelectrolysis method, and two processes – highpressureand high-temperature electrolysis – that are stillunder development. Chlor-alkali electrolysis is notdiscussed further here.
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Page 72 and 73: 70Risø Energy Report 3References81
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Hydrogen and its competitors, 2004

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