Sub- and Supercritical Water Liquefaction of Kraft Lignin and Black Liquor Derived Lignin

• Main Authors: Jukka Lappalainen, David Baudouin, Ursel Hornung, Julia Schuler, Kristian Melin, Saša Bjelić, Frédéric Vogel, Jukka Konttinen, Tero Joronen
• Format: Article
• Language: English
• Published: MDPI AG 2020-06-01
• Series: Energies
• Subjects: hydrothermal liquefaction; black liquor; lignin from black liquor
• Online Access: https://www.mdpi.com/1996-1073/13/13/3309

To mitigate global warming, humankind has been forced to develop new efficient energy solutions based on renewable energy sources. Hydrothermal liquefaction (HTL) is a promising technology that can efficiently produce bio-oil from several biomass sources. The HTL process uses sub- or supercritical water for producing bio-oil, water-soluble organics, gaseous products and char. Black liquor mainly contains cooking chemicals (mainly alkali salts) lignin and the hemicellulose parts of the wood chips used for cellulose digestion. This review explores the effects of different process parameters, solvents and catalysts for the HTL of black liquor or black liquor-derived lignin. Using short residence times under near- or supercritical water conditions may improve both the quality and the quantity of the bio-oil yield. The quality and yield of bio-oil can be further improved by using solvents (e.g., phenol) and catalysts (e.g., alkali salts, zirconia). However, the solubility of alkali salts present in black liquor can lead to clogging problem in the HTL reactor and process tubes when approaching supercritical water conditions.