Biofuels: Thermodynamic sense and nonsense

摘要

Much of the current enthusiasm for biofuels appears to ignore basic thermodynamic and other constraints.rnThe fundamental problem with growing fuel is that combustible plant matter is almost invariably solid, while the major demand for energy at present is in the form of gas or liquid fuels. All current conversion processes are of low efficiency even for the convertible parts of the plant. For example the energy which could be obtained from burning a kilogram of wheat grain is about twice that available from the ethanol into which it can be converted by fermentation. Furthermore, all current liquid fuel processes can use only part of the plant.rnThis paper highlights biofuel technologies which make sense, such as co-firing straw with coal in power stations, and those which because of thermodynamic considerations are nonsense, such as making ethanol from grain in Europe or from maize in the USA.rnSince arable land is a scarce resource in most of Europe, locally grown biofuels are unlikely to become a major replacement for fossil fuels. Strategies which can help to maximise this contribution are suggested, and promising, emerging technologies are highlighted.