Biomass wastes can be divided into four categories: urban, agricultural, livestock and wood wastes. Urban biomass waste is a special category, available in relatively small quantities. It usually comprises timber waste from construction sites, some organic household refuse, and wood and other material from urban gardens. Most of this is cycled through an urban refuse collection and processing infrastructure where the biomass waste must be separated from the other refuse if it is to be burned as fuel. While separation is an expensive process there is often a fee available for disposing of the waste and this helps keep fuel costs low.
Agricultural wastes are available throughout the world and they include a number of very important biomass resources. Across Europe and North America there are enormous quantities of wheat and maize straw produced each year. These farming residues are seasonal and require storing if they are to supply a year-round fuel. Sugar cane processing produces a waste called bagasse at the processing plant (the waste left in the field is called trash) where it can easily be utilised to generate electricity. Rice produces straw in the fields and husks during processing. The shells and husks from coconuts can be used to generate electricity. Indeed wherever crops are grown and harvested there is normally some residual material which can be used as a source of energy.
There is one important caveat. From the perspective of sustainability it is important that some biomass material is returned to the soil after a crop has been harvested if the soil is to retain its fertility. If all the biomass material is removed, artificial fertilisers must then be used and this will normally prove to be unsatisfactory both environmentally and from an energy balance perspective.
Livestock residues are another special category of biomass. While there is probably the equivalent of around 20-40 EJ of livestock residue generated each year, most of this is in the form of dung which has a very low energy content and is not a cost-effective fuel for power generation. It is only where livestock is farmed intensively that it becomes economical to utilise the waste and then only when the operation is being carried out on a sufficiently large scale.
Dairy and pig farms fall into this category and it can be cost effective to use a biomass digester to convert the animal effluent into a biogas, mostly methane, which can be burned in a gas engine to generate power. A similar process occurs naturally in the landfill sites used to dispose of urban waste, and this gas can also be collected and burned. Sewage farms which treat human waste are another source of methane-rich gas.
Wood waste comprises material that can beneficially be removed from natural and managed forests to improve the health of the plantation, residues left in a forest after trees have been logged and the waste produced during the actual processing of wood in sawmills and paper manufacturing plants.
Process plant waste is the cheapest and most economical to utilise. Many sawmills and most modern paper plants burn their waste, producing heat and electricity for use in the facility. Any surplus power may be sold. Residues left after logging are generally expensive to collect and transport but they have been utilised in situations where the demand for biomass fuel is high. Similarly the removal of dead trees and undergrowth from natural forest, while improving their health and reducing the risk of fire, is an expensive process that only becomes cost effective if the value of the fuel is high.
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