Bioenergy is energy generated by burning biomass, which most frequently means plants or plant residue. In Finland, the most important form of bioenergy is the black liquor, or spent or waste liquor, a liquid waste generated during pulp production, which consists of substances derived from the wood itself and of chemicals used in pulp production. The black liquor is burned in a recovery boiler, which allows the recovery and re-use of the chemicals. Normally, pulp mills cannot use all the energy generated in the recovery process, and the surplus is often led to the district heating networks of nearby housing estates or converted to electricity and led into the to electric networks. In fact, many Finnish forest industry towns are heated by a district heating system based on the use of a by-product from the forest industry. In pulp-making, the yield is generally about 50 percent, which means that half of the raw material used ends up as pulp, while the rest is used in energy production.
Forest energy usually means energy from logging residue and stumps; these are called energy wood. Forest energy is only a part of wood-based energy.
Using renewable resources is important, as greenhouse gases are produced especially by the burning of fossil fuels, such as oil and coal.
Burning wood is considered as neutral with regard to carbon dioxide emission, as the carbon released into the atmosphere with the burning of wood is considered to be absorbed into new, growing trees.