Effects of Process Water Recycling and Particle Sizes on Hydrothermal Carbonization of Biomass

Hydrothermal carbonization (HTC) is a promising thermochemical process for the conversion of biomass with high moisture content, and therefore, interest in the development of continuous technology is growing. However, the development of continuous HTC systems requires the in-depth understanding of reaction kinetics, heat transfer mechanisms, exothermic or endothermic nature of the reaction, effect of operational parameters, recyclability of process water, and variability in biomass feedstock. The current paper discusses the effects of recyclability of process water and particle size on the product yield. To investigate the water recycling effect, HTC experiments were performed on sawdust using recycled water in a batch reactor. Moreover, to study the particle size effect, 3 small reactors containing specific particle sizes of biomass were placed in a larger reactor simultaneously to ensure consistency in the process condition. The results showed that the process water recycling increases the mass yield and higher heating value (HHV) of the solid product by 12% and 2%, respectively, after the first recycle. Physicochemical properties of hydrochar were studied by carrying out the ultimate and proximate, HHV, thermogravimetric coupled with Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller analysis, and results are discussed. Regarding the particle size study, higher mass yield, a decrease in heating value, and an increase in total organic carbon in the process water were observed by increasing the particle size.