Solid bioenergy sources

The largest group of solid bioenergy sources is products made from wood.These are obtained when firewood is taken from forests and when waste is utilized from the industrial processing of wood products. In many places by-products from agriculture, such as straw, are also used for generating energy from biomass.
When forests are thinned, apart from the trunk of the tree, which is used for the furniture and construction industries, wood residues of a lesser quality are also collected. For each hectare of forest, between 0.4 and 0.8 tonnes of air dry firewood can be obtained from these forest wood residues.Together with the other quantities of wood residues that are produced during forest maintenance, this gives an annual fuel yield from a permanently used forest area of around 1.5 t/ha.
In modern wood harvesting, the trees are felled by harvester machines (Figure above). These cut down the tree using a gripper arm with a chainsaw mounted on it. In addition, harvester machines can automatically remove the branches from the trunk, strip off the dark wood bark, and cut the trunk into transportable lengths.This means that part of the value-increasing wood processing is carried out before the wood leaves the forest.

When the round trunks are machined into planks and beams, large amounts of residues are produced. However, for the most part these are utilized in the wood industry for other materials.Woodchips and shavings that are free of any bark, for example, are the base products for high-value chipboard sheets (Figure above ).


However, another part of these residues still has fragments of dark bark attached and is therefore unsuitable for utilization as a wood product (Figure above).These bark pieces are ideal for energy recycling. Because of the high ash content, these residues are utilized mainly in larger heat supply stations and combined heat and power plants as a co-firing substrate.
Other significant residues from agriculture include straw (Figure below) and other stem products such as hay.These post-harvest residues are often available locally in large utilizable quantities.

Mechanised straw harvest with  bale press   
Photo: Claas AG/www.claas.de

Straw is a natural residual product
Photo: creativ collection/www.sesolutions.de
The straw from 1 ha of cereal has an energy content of 73 GJ.This is roughly equivalent to 2000 litres of heating oil. However, straw and other stem products have combustion characteristics that are different from those of ligneous fuels.The ash melting point and emission behaviour of straw-type biomass mean that different technical approaches to utilization are required. To date, it has been possible to achieve large-scale energy recycling of stemproduct fuels in cogeneration plants, but not in decentralized installations.
Yet it is not only residue materials produced directly with the creation of biomass that can be considered for energy utilization. Products at the end of their lifecycle that are no longer useful as a material asset are ideal for energy recycling.The processing and combustion of old wood is one example of how this kind of bioenergy product can be obtained from secondary raw materials (Figure below).

Industrial wood waste processing
Photo: Dobelmann/www.sesolutions.de
Because of its previous use, this product can be contaminated with foreign matter such as chemicals, paints or similar. For this reason, many countries have restrictions on the energy recycling of old wood. Burning the wood in small combustion systems is often allowed only if the wood processing has been purely mechanical and the wood contains only insignificant contaminants (Figure below).

Mechanically prepared wood
Photo: Dobelmann/www.sesolutions.de
Another important category of residues that is not necessarily part of the old wood sector is wood residues from landscape management (Figure below).These occur during maintenance works by roads and waterways, and through work in parks.Wood residues from landscape management are usually a mix of wood, leaves and stem products. Only very rarely would the utilization of this mixture in a new product be considered.

Residues from landscape management
Photo: Dobelmann/www.sesolutions.de

Energy utilization suggests itself as a means of disposing of these materials.The combustible quality of these wood residue mixes can be classed as low, owing to the large number of impurities. Because of the soil that is generally still attached, these materials have a high ash content.The other visible impurities such as plastic wrappers, bags and other man-made garbage lead to high levels of toxic matter, with the result that the law demands the controlled disposal of the ash.
First published by James & James (Science Publishers) Ltd in the UK and USA in 2005
© The German Solar Energy Society (DGS), Ecofys 2005