Biomass is organic matter, mostly from plants such as wood, which can be used as a source of energy. The term also includes animal waste (digested plant food) such as cow or pig dung. The size of the biomass energy resource is enormous.
Every year, growing plants store enough energy to fuel the world for five years. The majority of this energy, about 90 percent of it, is stored in the wood of growing trees. Only 2 percent of the biomass generated each year is currently being used as fuel. Estimates suggest this could increase to 8 percent by the middle of the 21st century. .
What types of biomass are available? Biomass fuels are generally available to us as solids from a wide variety of sources but we can convert them into liquid and gaseous fuels.
Traditioal biomass fuels include wood, charcoal and farm waste.
Wood was the major source of energy from the earliest times until the Industrial Revolution when coal very largely replace it. In the developing world, wood often remains the major source of energy for heating and cooking. Dry fuels contain more energy per tonethan freshly cut material. Charcoal is another source, and is created by heating wood in the absence of air (called pyrolysis). Wood contains substances that are volatile, both gases and liquids, and these are lost during traditional charcoal burning. Since these volatiles in biofuels can account of 75 percent of their energy content, charcoal burning is a very wasteful and polluting way to make fuel. Farm wastes have been used as fuels since the earliest times. The wastes include dried animal dung, crop residues, husks from milling food crops, straw and forestry wastes from thinning and felling trees. People have also been cutting and drying turf of peat for thousand of years to make into fuel. In the modern world, large amounts of waste concentrated at one location. Such material can then be used as a fuel. Alternatives include: Agricultural waste, such as waste from processing sugar cane . This fibrous material is suitable for firing boilers to raise steam and to generate electricity. Other agricultural wastes include coconut waste and rice husks, both produced in large tonages but only available during particular seasons. Environmental concerns about straw stubble burning has meant that much more straw is now available as a potential fuel. Animal waste, including very wet wastes such an animal slurries from a cow shed. By processing them for a few days in large tanks called anaerobic digesters, we can obtain a useful fuel gas. The residue can be dried to give a fertilizer. Forestry waste was left to rot in the past. The development of machines to harvest the waste and convert it into wood chips has changed this. The chips can be transported and dried for use in firing boilers. Household waste represents a major source of fuel in industrial countrries, of where most municipal waste is buried in landfill sites. These can be disused clay pits or quarries. This is an increasingly expensive and wasteful option. There are two major ways to extract energy from household waste. One is to separate the combustible material before burial of the remainder. The other is to use the landfill site as a fuel gas producer. There, raw waste can be incinerated directly, reducing the volume of the waste and so lowering disposal costs. The energy recovered can be used for district heating through a combined heat and power scheme, (called CHP). If the raw waste is sorted first to remove things, which cannot burn, such as metals and glass, the combustion is more efficient. Alternatively, the materials which will burn can be processed to give fuel pellets, known as refuse-derived fuel (or RDF for short). When landfill sites are full they are capped to seal them. The absence of a supply of oxygen from the air creates anaerobic conditions. Under anaerobic conditions, natural micro-organisms convert wastes to a mixture of gases known as landfill gas. The two major components are carbon dioxide, which does not burn, and methane, the major component of natural gas. Originally the landfill gas was allowed to vent naturally to the atmosphere but this represented an explosion hazard. The next development was to collect and flare off the gas to reduce the danger. In some countries farmland has been taken out of use to reduce food production. This provides opportunities to grow energy crops specifically for their use as fuels. Fast-growing trees such as willow, poplar and eucalyptus, or certain varieties of perennial grass, can all be grown as energy crops. For trees, the technique of coppicing is used. New growth is cut every three or four years for use as an energy crop. The trees sprout again and replace the cut material with new growth. Most biomass fuels are used for heating and power, but liquid fuels can also be produced for use in transportation. Ethanol is an alcohol, which burns well and can be blended with conventional fuels such as petrol. Sugar cane and maize can both be used as the source of bio-ethanol. The crops are fermented to give a mixture of water and alcohol. This can be concentrated by distillation. In Brazil, millions of vehicles are powered by ethanol or ethanol-petrol mixtures. Vegetable oils can be used directly as fuels but they perform better after chemical treatment. We call this esterification. It involves chemically combining the oils with an alcohol, either methanol or ethanol. The resulting fuel can be blended with diesel for commercial use. The growing and combustion of biomass fuels simply cycles carbon through the environment. If we can operate on a sustainable basis it will be carbon dioxide neutral, the atmospheric concentration will remain stable. Biomass fuels currently cost more to produce than fossil fuels. Liquid biomass fuels produce some different combustion products to petrol or diesel. This does not mean, however, they are significantly "cleaner" fuels. The annual sustainable yield of liquid biofuels from good agricultural land is low. In reality, there is a limit to how much land could be allocated for this purpose. Every hectare set aside for biofuel production is a hectare less for growing food. The large scale growth of energy crops will change the landscape and can also have more subtle effects. Unless care is taken, the crops can cause changes to the local water table and increase the demand for nutrients from the soil. Biomass fuels are best suited for heat and power. Although production costs have not been falling as rapidly as other renewable energies such as photovoltaics, they seem certain to fall further in the future because of the economies of scale. If we use the same amount of land for energy crops as we use to grow softwood for paper making, we could provide up to 30 percent of worldwide electricity demand. Liquid biofuels are not cost-competitive with petrol or diesel. The production cost of biofuels is at least three times higher than the production cost of fuels derived from crude oil. Some countries do not have large oil resources of their own but can still produce biofuels. © 2000 Mena Report (www.menareport.com)
