Chemical pulp is manufactured through separating the fibres by boiling wood chips and chemicals together. After separating the fibres, the black liquor, or spent or waste liquor, is left. It 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. The amount of black liquor to be burned is decreasing because of removing for example some of the lignin before burning. Lignin is considered one of the most promising products of the new forest-based bioeconomy.