Black liquor is a by-product generated during the paper manufacturing process. In the paper making process, wood chips are dissolved in an aqueous solution or white liquor. This causes the chemical lignin, which binds the cellulose fiber in wood, and other organic matter to dissolve, forming the black liquor. The remaining cellulose pulp is harvested to make paper.
When released into the environment without being treated, this liquid waste can be extremely hazardous. To prevent environmental hazards, paper mills attempt a recovery process to recycle the waste.
Conventionally, Tomlinson recovery boilers were used to recycle the black liquor. In its original form, black liquor has only 15% solid biomass and is not readily combustible. The recovery process begins by concentrating it. Concentrated Black liquor has about 70% solids, which are burnt in the recovery boiler. This process results in the formation of some gases and smelt. The smelt is further processed and reused as pulping chemicals while the gases are burnt to generate steam, which can be used to power the mills or move turbines to generate electricity.