Biorefineries in Kraft pulp mills for biofuels production: A critical review

Kraft pulp production generates residues and by-products of significant importance to the mill. Solid residues from forestry activities are commonly used to generate steam in power boilers. In the recovery cycle, black liquor generates steam (and subsequently energy) by burning in the Tomlinson boiler, while white liquor is regenerated. Well-developed alternative technologies can use these residues and by-products to generate different types of biofuels. This review addresses the use of such technologies integrated with Kraft mills, in the concept of biorefineries, showing advantages, disadvantages, and successful examples. Solid residues from forestry can be used to produce bio-oil through processes such as fast pyrolysis and hydrothermal liquefaction. Bio-oils are currently used for heating through combustion in commercial/industrial boilers, but greater appreciation occurs if used as biofuels, which is done through catalytic upgrading processes. Black liquor gasification generates synthesis gas, which can be burned for energy co-generation, used to produce synthetic fuels, or as a hydrogenating agent for bio-oil or crude tall oil catalytic upgrading. Kraft biorefineries are gradually being implemented, justifying efforts to improve existing and new biomass conversion technologies.