Wet torrefaction of biomass waste into value-added liquid product (5-HMF) and high quality solid fuel (hydrochar) in a nitrogen atmosphere

Wet torrefaction (WT) offers distinct advantages over other pretreatment methods for producing hydrochar, making it also a promising technology for converting biomass waste into value-added platform chemicals. In this research, we conducted a comprehensive investigation into the influence of reaction conditions on the WT process, evaluating its effects on the surface morphology and elemental composition of the resulting hydrochar, as well as on the formation of value-added liquid products, such as 5-hydroxymethylfurfural (5-HMF). During the course of our study, we utilized wood cellulose pulp residue (WCPR) as the feedstock and subjected it to WT in a nitrogen atmosphere. This process encompassed a temperature range of 180–260 °C, H2O/WCPR ratios ranging from 10 to 25, and reaction durations spanning from 15 to 60 min. Our findings unequivocally revealed that the reaction conditions during the WT of WCPR significantly influence the properties of the resulting hydrochar and the distribution of liquid products. Elemental and proximate analyses showed that as the reaction temperature and time increased during the WT of WCPR, the hydrochar composition experienced significant changes, including an increase in carbon content and a reduction in oxygen content. At the same time, the distribution of the liquid product revealed that 220 °C was the optimal temperature for producing 5-HMF, achieving an impressive selectivity of 73.3 % without the need for a catalyst. In summary, our research has established the optimal conditions for WT of WCPR as follows: a temperature of 220 °C, a reaction time of 30 min, and an H2O/WCPR ratio of 10. Various properties of the obtained hydrochar were thoroughly assessed, including the higher heating value (HHV), decarbonization, dehydrogenation, deoxygenation, enhancement factor, surface area, pore diameter, as well as solid, carbon, hydrogen, and energy yields. The highest carbon content, reaching 68.3 %, was achieved at 260 °C after 30 min of treatment, resulting in an HHV of 27,340 kJ/kg and an enhancement factor of 1.43. Finally, we have proposed a comprehensive reaction pathway to elucidate the WT of WCPR under these optimized conditions.