(578h) Platform Chemicals from Hardwood Black Liquor Via Hydrothermal Liquefaction: Influence of Process Conditions on Product Yields and Quality

Black liquor is a complex aqueous by-product from the paper and pulp industry comprising of 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 the black liquor in boilers to generate bioproducts and biochemicals. In this work, HTL of hardwood black liquor from Eucalyptus is carried out, and the influence of process variables such as temperature (250 – 400 ^oC), biomass loading (7.5 – 30 wt.%), and residence time (15 – 60 min) on yield and quality of the products is studied. Out of the various operating parameters, temperature and solid loading governed the yield of different product phases, while only temperature had a major role on the product quality.Increasing the operating temperature from 250 ^oC to 400 ^oC decreased the yield of 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. The lignin structure depolymerizes at 250 ^oC to form methoxy phenols, which gets then hydrolysed at temperatures above 300 ^oC to give dihydroxybenzenes such as catechol and its derivatives. The dihydroxybenzenes further undergo dehydration and dealkylation at subcritical conditions to produce phenol and alkyl phenols. Increasing the operating temperature led to an increase in the selectivities to phenol, cresols and xylenols. Maximum selectivity to phenol (23.7%) was obtained at 350 ^oC, 30 min and 15 wt.% solid loading. Na⁺ and K⁺ were the 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. The optimum operating conditions for maximum selectivity towards phenol in liquid product is shown to be 350 ^oC, 45 min and 30 wt.% solid loading. The optimum temperature for maximum energy recovery from hydrochar is 400 ^oC, 30 min, 15 wt.% solids. The findings suggest that HTL is a promising treatment method to recover energy through hydrochars as well as produce platform chemicals using waste black liquor from the pulp mills.