There are five categories of solid biofuels, depending on their source. The yield and energy content of some of the biomass types are given in Table 4.2.

1. Wood from forests and forest residues.

2. Crop residues.

3. Crop specifically grown for energy generation.

4. Animal waste.

5. Municipal waste.

Wood biomass

Materials, such as wood, have always been used by humans as a source of energy, but depending on the development of a country the amount of biomass used can differ greatly (Table 4.1).

The wood from forests consists of felled wood, thinnings, logging residues, wood-processing residues and waste from clearing. The wood available will be that surplus to the need for construction and industrial wood products from forests, plant­ations and trees outside forests.

Crop residues

Crop residues include haulms of legumes, stalks of sorghum, maize and millet and straw from rice, wheat and barley. Although there is a considerable quantity of bio­mass as residues, it is widely distributed and seasonal in its availability. The yields can be up to 20 t/ha with an energy content of 14 GJ/t giving a yield of 280 GJ/ha.