Upgradation of coconut waste shell to value-added hydrochar via hydrothermal carbonization: Parametric optimization using response surface methodology

• CCD-RSM was used to determine the effects of parameters during HTC process. • A thorough evaluation of the optimize process was based on overall desirability. • 200 ℃-0 min was determined to be the optimal processing condition in HTC of CS. • Improved hydrochar properties showed high potential of HTC for upgrading biomass. The utilization of biomass energy is desirable to achieve carbon neutrality in the world. Hydrothermal carbonization of coconut shell was performed using center composite design with an aid of response surface methodology to determine the individual effects and combined effects of parameters on responses. The experimental design incorporates two variables and three responses. More specifically, the effects of temperature (180–220 °C) and hold time (0–60 min) on hydrochar yield, higher heating value (HHV) and energy yield were investigated. According to the results, hydrochar yield varies monotonically with temperature and hold time. With increasing temperature and hold time, hydrochar yield was dropped gradually. The highest hydrochar yield of 75.67 % was obtained at 180 °C-0 min and the lowest hydrochar yield of 63.13 % was achieved at 220 °C-60 min. HHV showed opposite trend to hydrochar yield, reaching a maximum value of 29.39 MJ/kg at 220 °C-60 min. The change in energy yield was influenced by the variation of hydrochar yield and HHV and does not change monotonically with temperature or time. It reaches the maximum value 90.83 % at 200 °C-0 min. Furthermore, to select best operating conditions, a comprehensive evaluation of the experiments was conducted based on the overall desirability. A series of characterization experiments were conducted on selected hydrochar samples. The results of functional group and pore structure changes showed that raw biomass has converted into value-added products with stable structure properties. In conclusion, hydrothermal carbonization as a pretreatment for upgrading coconut shells is a feasible process and can be used for biofuels production.