About 80% of the world’s energy is currently supplied by fossil fuels such as coal, oil and gas. Present estimates suggest that, at current consumption rates, there are over 200-years’ worth of coal left, 60-years’of gas, and 40-years’ of oil. Fossil fuels are hydrocarbons, and their combustion releases carbon dioxide into the atmosphere, one of the main causes of the human-induced greenhouse effect.
Wood is still very widely used as a fuel in many parts of the ‘developing’ world. In some countries, other biofuels such as animal dung (ultimately also derived from the growth of plants) are also used. Such traditional biofuels are estimated to supply some 11% of world primary energy, though the data are somewhat uncertain.
If the forests that provide wood fuel are re-planted at the same rate as they are cut down, then such fuel use should in principle be sustainable. When forests are managed sustainably in this way, the CO2 absorbed in growing replacement trees should equal the CO2 given off when the original trees are burned. However, this is only true when complete combustion of the wood occurs and all the carbon in the wood is released as carbon dioxide. Although near-complete combustion can be achieved in the best available wood stoves and furnaces, most open fires and stoves are not so efficient. This means that not only is carbon dioxide released (albeit in somewhat smaller quantities if the combustion is incomplete) but other combustion products are also emitted, some of which are more powerful greenhouse gases than CO2. In particular, these can include methane, which on a molecule-for-molecule basis has 20 times the global warming potential of CO2 over a 20 year period. The incomplete combustion of wood can therefore release a mixture of greenhouse gases with a greater overall global warming effect than can be offset by the CO2 absorbed in growing replacement trees. This suggests an urgent need to improve the efficiency of traditional wood burning processes.
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