Platform chemicals from hardwood black liquor via hydrothermal liquefaction: influence of process conditions on product yields and quality

Abstract

Black liquor is a complex aqueous by-product from the paper and pulp industry comprising lignin residues and inorganic chemicals such as NaOH and Na₂S. Valorization of black liquor through hydrothermal liquefaction (HTL) is a sustainable alternative to the current practice of burning it in boilers to generate bioproducts and biochemicals. In this study, systematic HTL experiments on hardwood black liquor from Eucalyptus were carried out and the effects of process variables such as temperature (250–400 °C), biomass loading (7.5–30 wt%), and residence time (15–60 min) on product yield, their quality and the distribution of various ions in the aqueous phase were thoroughly evaluated. Increasing the operating temperature from 250 °C to 400 °C decreased the yield of the liquid product from 79.8 wt% to 52.3 wt%, while increasing the residence time from 15 min to 60 min barely changed the yields by ±5 wt% without any prominent effect on the product yields. Increasing the solid loading from 7.5 wt% to 30 wt% increased the yield of hydrochar from 21.9 wt% to 48.6 wt%. Phenols, aliphatic hydrocarbons and ketones were the major organic compounds detected in the liquid product. Increasing the operating temperature led to enhanced selectivities to phenol, cresols and xylenols. Maximum selectivity to phenol (23.7%) was obtained at 350 °C, 45 min and 30 wt% solid loading. Na⁺ and K⁺ were abundant ions detected in the liquid phase. Increasing the reaction temperature led to an accumulation of water-soluble salt ions in the liquid product, while the ion concentration in hydrochar decreased with temperature.