Biomass is the sum total of all the Earth's living matter within the biosphere. It is continually regenerated by the sun through the process of photosynthesis. The energy reaching the planet is equivalent to about seven times its primary energy consumption. If biomass is converted to a fuel when it is at its peak as a store of chemical energy the process is carbon neutral. This means that the carbon emitted when it is burnt is equal to the carbon absorbed during growth. It is not a complete zero sum since there is a carbon component in the energy used in accumulating, processing and transporting the biomass. Biomass conversion falls into two categories:
• cultivated short rotation crops
Biomass produces energy from a variety of sources:
• fast-growing trees and shrubs, for example, willow, poplar, miscanthus
• residues from agricultural crops and forest thinning and felling
• animal waste, liquid and solid, such as poultry litter
• industrial residues, for example from saw mills
• municipal solid waste.
Biofuels fall into two categories:
• dedicated sources such as short rotation crops, e.g. coppiced willow
• dependent sources, which are the remains of a primary crop such as wheat and maize -the stubble or wheat straw and the stems and stocks of corn remaining after the crop has been harvested are significant sources of bioenergy.
There are three main systems for converting biomass to energy:
• direct combustion
• anaerobic digestion from organic waste producing biogas
• landfill gas conversion.
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