Hydrothermal carbonization of unwanted biomass materials: Effect of process temperature and retention time on hydrochar and liquid fraction

Hydrothermal carbonization (HTC) was applied to examine the feasibility in converting coconut husk (CH) and rice husk (RH) to renewable fuel resource and valuable dissolved organic chemicals. HTC was conducted with varying process temperature (140–200 °C) and retention time (1–4 h). CH was a better feedstock to produce hydrochar as solid fuel than RH because of its compositions was significantly different. An increase in process temperature from 140 to 200 °C resulted in a decrease in hydrochar yield of CH from 77.1 to 67.8%, and corresponding decreases in O/C and H/C from 0.6 and 1.4 to 0.4 and 1.2, respectively, and this was associated to dehydration and decarboxylation reactions. Fuel ratio and HHV were in the range of 0.66–0.86 and 20.7–23.9 MJ/kg, respectively. Liquid fractions (LF) from both RH and CH were found to be abundant in dissolved organic chemicals which were regarded as valuable intermediate chemicals, including furfural, furfuryl alcohol, hydroxymethylfurfural (HMF), and low molecular-weight carboxylic acids (lactic acid, formic acid, acetic acid, levulinic acid, and propionic acid).