Black liquor

Industrial by product

In industrial chemistry, black liquor is the by product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers. The equivalent material in the sulfite process is usually called brown liquor, but the terms red liquor, thick liquor and sulfite liquor are also used.

Approximately 7 tonnes of black liquor are produced in the manufacture of one tonne of pulp. The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. The black liquor comprises 15% solids by weight of which two thirds are organic chemicals and the remainder are inorganic. Normally the organics in black liquor are 40 45% soaps, 35 45% lignin and 10 15% other organics. The organic matter in the black liquor is made up of water/alkali soluble degradation components from the wood. By the 1990s, most kraft mills were consuming nearly all of their black liquor byproduct, It is normally concentrated to 65 80% by multi effect evaporators and burned in a recovery boiler to produce energy and recover the cooking chemicals. The viscosity increases as the concentration goes up. At about 50 55% solids the salt solubility limit is reached. Tall oil is an important byproduct separated from the black liquor with skimming before it goes to the evaporators or after the first evaporator stage.

Pulp mills have used black liquor as an energy source since at least the 1930s. As a result, the forest products industry has become one of the United States' leading generators of carbon neutral renewable energy, producing approximately 28.5 terawatt hours of electricity annually.

New waste to energy methods to recover and utilize the energy in the black liquor have been developed. The use of black liquor gasification has the potential to achieve higher overall energy efficiency than the conventional recovery boiler, while generating an energy rich syngas from the liquor. The syngas can be burnt in a gas turbine combined cycle to produce electricity (usually called BLGCC for Black Liquor Gasification Combined Cycle; similar to IGCC) or converted through catalytic processes into chemicals or fuels such as methanol, dimethyl ether (DME), or F T diesel (usually called BLGMF for Black Liquor Gasification for Motor Fuels). This gasification technology is currently under operation in a 3 MW pilot plant at Chemrec's test facility in Piteå, Sweden. The DME synthesis step will be added in 2011 in the "BioDME" project, supported by the European Commission's Seventh Framework Programme (FP7) and the Swedish Energy Agency. Used for biofuels production, the black liquor gasification route has been shown to have very high conversion efficiency and greenhouse gas reduction potential.

Hydrothermal liquefaction is suitable for converting black liquor to advanced biofuels due to the process's ability to handle high moisture inputs.

Where recovery boiler capacity is limited and a bottleneck in the pulp mill, the lignin in the black liquor may be extracted, then exported or used as fuel in the mill's lime kiln. This often replaces fossil based fuel with biofuel.

A tax credit created by the U. S. Congress in 2005 as part of the 2005 Highway Bill to reward and support the use of liquid alternative fuel derived from hydrocarbons in the transportation sector was expanded in 2007 to include non mobile uses of liquid alternative fuel derived from biomass. This change meant that, in addition to fish processors, animal renderers and meat packers, kraft pulp producers became eligible for the tax credit as a result of their generation and use of black liquor to make energy. For one large company (International Paper) this could amount to as much as $3.7 billion in benefits. Weyerhaeuser announced in May 2009 that it was also pursuing the tax credit. Some paper industry analysts criticized the paper industry's eligibility for the alternative fuel mix tax credit on the grounds that it increased fossil fuel use, but the industry countered that adding a fossil fuel was a requirement of the law and that, regardless, this did not result in a net increase in fossil fuel use since companies were merely replacing the existing fossil fuel they already mixed with black liquor—natural gas—with one of the three fuels specified by the law: gasoline, kerosene or diesel. The bio fuel credit for black liquor ended on January 1, 2010.