Hydrothermal carbonization of cola nut shell: Impact of deep eutectic pretreatment on the property of hydrochar

Hydrothermal carbonization of lignocellulosic biomass results in hydrochar with abundant surface functionality and low degree of carbonization. In this work, Cola Shell was subjected to a pretreatment with the Deep Eutectic Solvent (DES) prior to hydrothermal carbonization under different conditions (210 ºC and 260 ºC). The aim was to ascertain the impact of pretreatment with DES on the properties of hydrochars. The results show that pretreatment with DES at 110 ºC favors the disintegration of hemicellulose while promoting the cellulose hydrolysis and dehydration. The lignin removal formed a snow cloud of particles on hydrochar surface, which further undergoes polymerization at a high HTC temperature to form a cluster of fibrous carbonaceous material, contributing to increase the degree of carbonization and thermal stability of hydrochar. DES pretreatment also reduced structural defects while increasing the cellulose crystallinity. In addition, DES pretreatment increased the nitrogen content and energy yield without affecting the calorific value of hydrochar-derived Cola Nut Shell. • DES treatment enhances the energy yield of hydrochar. • DES treatment does not affect the calorific value of hydrochar. • Treatment at 110 ºC promotes cellulose hydrolysis, dehydration and crystallisation. • DES treatment promotes the removal of lignin at elevated temperatures (HTC). • DES treatment reduces hydrochar structural defects, increase carbonisation degree.