Thermodynamic efficiency of biomass gasification and biofuels conversion

Biomass has great potential as a clean renewable feedstock for producing biofuels such as Fischer-Tropsch biodiesel, methanol, and hydrogen. The use of biomass is accompanied by possible ecological drawbacks, however, such as limitation of land or water and competition with food production. For biomass-based systems a key challenge is thus to develop efficient conversion technologies which can also compete with fossil fuels. The development of efficient technologies for biomass gasification and synthesis of biofuels requires a correct use of thermodynamics. Energy systems are traditionally analyzed by energetic analysis based on the first law of thermodynamics. However, this type of analysis shows only the mass and energy flows and does not take into account how the quality of the energy and material streams degrades through the process. In this review, the exergy analysis, which is based on the second law of thermodynamics, is used to analyze the biomass gasification and conversion of biomass to biofuels. The thermodynamic efficiency of biomass gasification is reviewed for air-blown as well as steam-blown gasifiers. Finally, the overall technological chains biomass-to-biofuels are evaluated, including methanol, Fischer-Tropsch hydrocarbons, and hydrogen. The efficiency of biofuels production is compared with that of fossil fuel