Using exergy to assess sustainability of biofuels

A wide range of biomass sources, such as traditional agricultural crops, residues from agriculture, and foresting, can be used to make biofuels and bioproducts. However, the use of biomass is accompanied by ecological drawbacks, particularly limitation of land or water and competition with food production. For biomass-based systems, a key challenge is to develop sustainable technologies based on criteria including thermodynamic efficiency in addition to environmental and economic aspects. Exergy-based thermodynamic efficiency is a convenient tool to select promising biomass-to-biofuels routes. This assessment relates to biomass gasification and subsequent biofuel synthesis, including Fischer-Tropsch hydrocarbons, methanol, and hydrogen. It is shown that all investigated routes have lower exergetic efficiency compared to fossil fuels. The conversion efficiency can be increased by improving the operation of biomass gasifier which shows the highest exergy losses.