Hydrothermal treatment of black liquor for energy and phenolic platform molecules recovery in a pulp mill

The aim of this study was to study the conversion of black liquor under hydrothermal conditions and its integration in a pulp mill. Three sulfur-free black liquors produced from caustic soda cooking of prehydrolyzed softwood, prehydrolyzed hardwood and non prehydrolyzed hardwood chips were converted. Experiments were performed in a batch reactor, for temperature between 250 °C and 310 °C, and for holding time between 5 and 120 min. Three phases were formed: an aqueous phase containing monomeric phenolic compounds and the sodium cations, a biocrude containing most of the carbon, and a small amount of gas. The combustion of the biocrude could allow an energy recovery for the mill of up to 70%. The main monomeric compounds identified in aqueous phase were phenol, catechol, guaiacol and syringol, with a total yield up to 28 g kg−1 of dry BL, at 250 °C. Among them, guaiacol was the major product. Sodium recovery was 97 %, slightly better than typical kraft recovery value, and compatible with causticizing. Finally, results obtained with a kraft softwood lignin were compared to those obtained with softwood black liquor. Results show that biocrude yields were greater with black liquor, whereas platform molecules production was higher with lignin. Presence of carbohydrates derivatives in black liquor is identified as a major parameter for biocrude production as it would favor bonding between phenolic species.