Process analysis and kinetic modeling of coconut shell hydrothermal carbonization

Hydrothermal carbonization is a promising renewable technology to produce high quality solid biofuel with characters of sterilization, micronization, and homogeneous. In this work, coconut shell was used as feedstock to investigate the products distribution, reaction kinetics and the migration of potassium during HTC process. Experiments were carried out in 180–240 °C with residence time of 0–5 h. The results showed that the content of fixed carbon increased from 20.76 wt% to 49.80 wt%, and HHV increased from 20.75 MJ/kg to 31.77 MJ/kg at 240 °C with the holding time of 3 h. The ratio of H/C dropped from 1.78 to 0.86 and O/C dropped from 0.68 to 0.16 which all indicated the improvement of fuel performance. The effects of increasing temperature and prolonging residence time on the products were in the same direction but different in severity. The properties of the products changed little after the residence time was more than 1 h. In addition, chemical fractionation method was used to investigate migration behavior of potassium. The best removal temperature of potassium was 200 °C which decreased potassium concentration from 2.7 mg/g to 0.6 mg/g specifically. Due to the increase of specific surface and porosity caused by high temperature, the content of potassium rose slightly at 220 and 240 °C. A kinetic model was adopted to predict the products distribution and yield of solid phase and gas phase. The activation energies (Ea) of different reaction paths were calculated and the results showed that Ea of gas formation is greater than that of solid. Migration behavior of potassium and modeling of HTC process is of great significance for design, optimization of industrial HTC reactor and selecting the best time for discharging and feeding to achieve better performance.