Biomass to biofuels, a chemical perspective

Abstract

Biomass is an interesting starting material for transportation fuels because of its renewable nature. The main constituents of biomass are the carbohydrate polymers cellulose and hemi-cellulose, and lignin. Current transportation fuels are mixtures of mainly hydrocarbons derived from crude oil. Biomass derived biofuels are preferably used as blends with fossil fuels to avoid the need for adaptations in the existing car fleet. The total blend then needs to meet the set fuel specifications for fossil fuels. The conversion of biomass to biofuels therefore centres on the removal of oxygen from carbohydrates to obtain hydrocarbons. Oxygen is preferably eliminated in the form of CO₂ or H₂O, because the heat of combustion of these molecules is zero and all energy is concentrated in the remaining products. Based on gross chemical formulae, the conversion options from biomass to biofuels have been investigated, and interesting intermediates have been identified. Conversion routes from carbohydrates to hydrocarbons tend to proceed via C₁-intermediates (CO, CH₄ and carbon). Gasification and subsequent Fischer Tropsch synthesis seems the logical processing route to obtain hydrocarbons from biomass via these intermediates. If oxygen containing products are allowed, conversion can proceed via larger intermediates. For blending with gasoline, ethanol emerges as an interesting component. Longer carbon chain molecules are required for diesel. These can only be obtained by at least two process steps, involving the combination of carbon chains (e.g. by etherification or esterification), and reduction of polarity (by hydrogenation or oligomerisation). The lignin present in biomass has a complex, highly cross-linked structure, and can probably best be converted to syngas via gasification or applied directly as solid fuel.