BIOMASS ENERGY
Plants are the most common source of biomass. They have been used in the form of wood, peat and straw for thousands of years. Today the western world is far less reliant on this high energy fuel. This is because of the general acceptance that coal, oil and electricity are cleaner, more efficient and more in keeping with modernisation and technology. However this is not really the right impression. Plants can either be specially grown for energy production, or they can be harvested from the natural environment. Plantations tend to use breeds of plant that are to produce a lot of biomass quickly in a sustainable fashion. These could be trees (e.g. willows or Eucalyptus) or other high growth rate plants (such as sugar cane or maize or soybean).

WOOD RESIDUES
Wood can be, and usually is, removed sustainably from existing forests world-wide by using methods such as coppicing. It is difficult to estimate the mean annual increment  (growth) of the world’s forests. One rough estimate is 12,5x109 m3/yr with an content of 182 EJ equivalent to 1,3 times the total world coal consumption. The estimated global average annual wood harvests in the period 1985-1987 were 3,4x 109 m3/yr (equivalent to 40 EJ/yr.), so some of the unused increment could be recovered for energy purposes while maintaining or possibly even enhancing the productivity of forests.

Operations such as thinning of plantations and trimming of felled trees generate large volumes of forestry residues. At present these are often left to rot on site - even in countries with fuelwood shortages. They can be collected, dried and used as fuel by nearby rural industry and domestic consumers, but their bulk and high water content makes transporting them for wider use uneconomic. In developing countries where charcoal is an important fuel, on-site kilns can reduce transport costs. Mechanical harvesters and chippers have been developed in Europe and North America over the last 15 years to produce uniform 30-40 mm wood chips which can be handled, dried and burned easily in chip-fired boilers.

The use of forest residues to produce steam for heating and/or power generation is now a growing business in many countries. American electricity utilities have more than 9 000 MW (output of 9 nuclear power plants) of biomass-fired generating plant on line, much of it constructed in the last ten years. Austria has about 1250 MW of wood-fired heating capacity in the form of domestic stoves and district heating plant, burning waste wood, bark and wood chips. Most of these district heating systems are of 1-2 MW capacity, with a few larger units (around15 MW) and a number of small-scale CHP systems.

Timber processing is a further source of wood residues. Dry sawdust and waste produced during the processing of cut timber make very good fuel. The British furniture industry is estimated to use 35 000 tonnes of such residues a year, one third of its production, providing 0,5 PJ of space and water heating and process heat (FOE, 1991). In Sweden, where biomass already provides nearly 15% of primary energy, forestry residues and wood industries contribute over 200 PJ/yr., mainly as fuel for CHP plant.

AGRICULTURAL RESIDUES
Agricultural waste is a potentially huge source of biomass. Crop and animal wastes provide significant amounts of energy coming second only to wood as the dominant biomass fuel world-wide. Waste from agriculture includes: the portions of crop plants discarded like straw, whether damaged or surplus supplies, and animal dung. It was estimated, for example, that 110 Mt of dung and crop residues were used as fuel in India in 1985, compared with 133 Mt of wood, and in China the mass of available agricultural residues has been estimated at 2.2 times the mass of wood fuel.
Every year, millions tonnes of straw are produced world-wide with usually half of it surplus to need. In many countries this is still being burned in the field or ploughed back into the soil, but in some developed countries environmental legislation which restrict field burning has drawn attention to its potential as an energy resource
Effort to remove crop residues from soils and to use them for energy purposes leads to a central question:  how much residue should be left and recycled into soil to sustain production of biomass ? According to the experience from developed countries around 35% of crop residues can be removed from soil without adverse effects on future plant production.
Industrial waste that contains biomass may be used to produce energy. For example the sludge left after alcohol production (known as vinasse) can produce flammable gas. Other useful waste products include, waste from food processing and fluff from the cotton and textiles industry.

SHORT ROTATION PLANTS
Biomass can be also be produced by so-called short-rotation plantation of trees and other plants like grasses (sorghum, sugarcane, switchgrass). All these plants can be used as fuels like wood with the main advantage of their short span between plantation and harvesting – typically between three and eight years. For some grasses harvesting is taking place every six to 12 months. Recently there are about 100 million hectares of land utilised for tree plantation world-wide. Most of these trees are used for forest products markets.
Parameters which are important in evaluating species for short rotation plants include availability of planting stock, ease of propagation, survival ability under adverse conditions and the yield potential measured as dry matter production per hectare per year (t/ha/y). Yield is a measure of a plant’s ability to utilize the site resources. It is the most important factor when considering biomass production due to the need to optimize/maximize yield from a given area of land within a given time frame at the least possible cost. High yielding species are therefore preferred for biomass energy systems.
Some plant communities have shown superiority in dry matter production compared to others grown under similar conditions. Although reported dry matter production of different tree species varies over a wide range depending on soil types and climate, certain species stand out. For Eucalyptus species, yields of up to 65 t/ha/y have been reported, compared to 30 and 43 t/ha/y in Salix and Populus species respectively.
Despite the fact that biomass plantation can be of great importance for most developed countries experience has shown it is unlikely to be established on a large scale in many developing countries, especially in poor rural areas, so long as biofuels (particularly wood) can be obtained at zero or near zero cost.