Reaction kinetics for the hydrothermal carbonisation of cellulose in a two-phase pathway

Abstract

Hydrothermal carbonisation (HTC) of biomass has gained attention in recent years due to its potential applications in functional sustainable materials. Cellulose as the most abundant organic polymer on earth and a key component of multiple biomass sources has been extensively studied, and it is widely accepted that glucose is the major product of the polysaccharide’s hydrolysis above 200 °C. However, differences observed in the hydrochar produced from the HTC of cellulose and glucose have suggested that a direct reaction takes place in the polysaccharide's conversion. In the present study, a solid-state reaction pathway for the HTC of cellulose is elucidated using reaction kinetics modelling and optimisation in a progressive reaction mechanistic and the effect of temperature on reaction selectivity is analysed. The experiments were conducted with glucose and cellulose in a batch reactor with 20% mass feedstock at 220 °C, 240 °C and 260 °C, over a residence time distribution. And the kinetic parameters were established by fitting the experimental data with a series of reaction mechanism and kinetic models via multivariable optimisation. Whilst the activation energy for the hydrochar formation reaction from glucose was 46.6 kJ/mol, the same reaction from cellulose, assuming complete hydrolysis, produced a value three times bigger. Therefore, a composite cellulose model, in which hydrochar through soluble intermediates, is restricted to the glucose reaction constant and balanced with a direct solid-state reaction pathway, produced activation energies of 210.8 kJ/mol for hydrolysis of cellulose and 207.3 kJ/mol for the solid-state reaction.