Encyclopedia of Renewable Energy. James G. Speight

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      Biofuels – From Synthesis Gas

      Biomass can be converted into fuels and chemicals indirectly (by gasification to syngas followed by catalytic conversion to liquid fuels) or directly to a liquid product by thermochemical means. The process yields synthesis gas (syngas) composed primarily of hydrogen and carbon monoxide, also called biosyngas.

      The production of high-quality syngas from biomass, which is later used as a feedstock for biomass-to-liquids (BTL) production, requires particular attention. This is due to the fact that the production of synthesis gas from biomass is indeed the novel component in the gas-to-liquids concept – obtaining syngas from fossil raw materials (natural gas and coal) is a relatively mature technology.

      Gasification is actually thermal degradation of the feedstock in the presence of an externally supplied oxidizing (oxygen-containing) agent e.g., air, steam, oxygen. Various gasification concepts have been developed over the years, mainly for the purposes of power generation. However, efficient biomass-to-liquids production imposes completely different requirements for the composition of the gas. The reason is that in power generation, the gas is used as a fuel, while in biomass-to-liquids processing, it is used as a chemical feedstock to obtain other products. This difference has implications with respect to the purity and composition of the gas.

In contrast, for biomass-to-liquids production, the amount of carbon monoxide and hydrogen is only important (the larger the amount, the better), while the calorific value is irrelevant. The presence of other hydrocarbon derivatives and inert components should be avoided or at least kept as low as possible. This can be achieved in the following ways: (i) by adjusting the amount of the various constituents of the gas stream, (ii) choice of the oxidizing agent.

      The amount of components other than carbon monoxide and hydrogen (primarily hydrocarbon derivatives) can be reduced via further transformation into carbon monoxide and hydrogen. This is, however, rather energy intensive and costly (two processes – gasification and transformation). As a result, the overall energy efficiency of syngas production and of biomass-to-liquids processing is also reduced, leading to higher production costs.

      The amount of various components can be minimized via a more complete decomposition of biomass, thereby preventing the formation of undesirable components at the gasification step. The minimization of the content of various hydrocarbon derivatives is achieved by increasing temperatures in the gasifier, along with shortening the residence time of feedstocks inside the reactor. Because of this short residence time, the particle size of feedstocks should be small enough (in any case – smaller than in gasification for power generation) in order that complete and efficient gasification can occur.

      In gasification for power generation, typically, air is employed as oxidizing agent, as it is indeed the cheapest among all possible oxidizing agents. However, the application of air results in large amounts of nitrogen in the product gas, since nitrogen is the main constituent of air. The presence of such large quantities of nitrogen in the product gas does not hamper (very much) power generation, but it does hamper biomass-to-liquids production. Removing this nitrogen via liquefaction under cryogenic temperatures is extremely energy intensive, reduces substantially the overall biomass-to-liquids energy efficiency and increases costs. Amongst other potential options (steam, carbon dioxide, oxygen), from a technical and economic point of view oxygen appears to be the most suitable oxidizing agent for biomass-to-liquids manufacturing. It is true that the oxygen-blown gasification implies additional costs compared to the air-blown gasification, because of the oxygen production. Nevertheless, the energy and financial cost of producing oxygen seems to be far lower than the renewable energy and financial cost of cleaning up the product gas from air-blown gasification from nitrogen. This is partly due to the fact that the production of high-purity oxygen (above 95% O2) is a mature technology.

      In principle, the larger the carbon and hydrogen content in raw materials, employed in gas-to-liquids processing, is, the easier and more efficient the carbon monoxide and hydrogen. Hence, the natural gas pathway is the most convenient one since natural gas is gaseous and contains virtually carbon and hydrogen only. Solid raw materials (biomass, coal) involve more processing, because first they have to be gasified and then the product gas should be cleaned up from other components such as nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter to the extent of getting as high as possible purity of syngas.

      Two basic types of biomass raw material are distinguished, viz, woody material and herbaceous material. Currently, woody material accounts for approximately 50% of total world bioenergy potential. Another 20% is straw-like feedstock, obtained as a by-product from agriculture. The dedicated cultivation of straw-like energy crops could increase the herbaceous share up to 40%.

      See also: Bioalcohols, Biodiesel, Biofuels – First Generation, Biofuels From Synthesis Gas, Biofuels – Second Generation, Biofuels – Third Generation, Biogas, Gasification, Methanol, Ethanol, Vegetable Oil.