Sample of black liquor. (Source: Wikipedia.org)

A liquid solution containing residues of lignin, hemicellulose and other inorganic substances which is produced as a waste product of papermaking. In the kraft process, wood chips are soaked in a hot mixture of water, sodium hydroxide, and sodium sulphide to remove lignin and hemicellulose and release fibrous cellulose pulp for papermaking. The production of one tonne of paper pulp produces approximately seven tonnes of black liquor. After thickening by evaporation, the black liquor can be used for energy purposes, either by direct combustion or by gasification to produce syngas.