Biomass wastes can be divided into four categories: urban waste, agricultural waste, livestock waste, and wood waste. Urban biomass waste is a special category, available in relatively small quantities. It usually comprises timber waste from construction sites and some organic household refuse together with 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. (However, more urban waste can be burned in a specialized power-from-waste plant and this may make separation of biomass waste from household refuse uneconomical.) While separation is an expensive process there is often a fee available for dis- posing of the waste and this helps keep fuel costs low.
Some organic urban waste ends up in landfill sites, although the use of landfills is being reduced in most developed regions because of the growing need to recycle. When organic waste is buried in this way it can generate meth- ane through anaerobic decomposition underground. This methane is both an extremely potent greenhouse gas and a potential hazard, but it can be collected relatively easily and then either flared or used in a power generation plant to provide electricity.
Agricultural wastes, often referred to as agricultural residues, are one of the most important sources of biomass today. These 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 valuable fuels but they are seasonal and therefore require storing if they are to provide a year-round supply for a power station. Sugarcane processing produces a waste called bagasse at the processing plant where it can easily be utilized to generate electricity. Further waste, called trash, is left in the field but this can be collected and used too. Rice produces straw in the fields and husks during processing—both potential fuels. The shells and husks from coconuts can be used to generate electricity as can waste from oil palms, while the periodic recycling of oil palms and rubber trees (plantation trees have a life of 20–30 years) can provide wood waste for power generation. Many other crops from all parts of the world produce stalk waste that can be utilized. Indeed, wherever crops are grown and harvested there is normally some residual material that can be used as a source of energy.
There is one important caveat to the use of agricultural wastes as power plant fuel. 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 fertilizers must then be used. This is likely to be considered an unsatisfactory trade-off 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 (though it is used for heat- ing in some parts of the world). It is only where livestock is farmed intensively that it becomes economical to utilize the waste, and then only when the oper- ation 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 containing methane that can be burned in a gas engine to generate power. It is often not cost effective for small farms to install their own digesters, but in Denmark there are schemes where the waste from a number of small farms is collected and then processed centrally. Sewage farms that treat human waste are another potential source of methane-rich gas. Meanwhile, poultry farm residues have been used in combustion plants in the United Kingdom and United States.
Wood waste comes from three sources: 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 utilize. Many sawmills and most modern paper plants burn their waste, pro- ducing 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 utilized in situations where the demand for biomass fuel is high. Similarly, the removal of dead trees and undergrowth from natural forests, 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.