Optimization of process parameters and evaluation of fuel properties of agricultural waste-derived hydrochar catalyzed by citric acid based on response surface methodology

Hydrothermal carbonization (HTC) enables the valorization of agricultural waste-derived through resource recovery and the synthesis of alternative solid fuels. In this study, response surface methodology (RSM) was employed to evaluate the effects of temperature (200–240 ℃), time (20–60 min), and citric acid proportion (CA, 4–12 %) across 13 control and 17 experimental groups. The results indicated that, for the control group without catalyst, the maximum higher heating value (H-CS-HHV_Max) was 26.55 MJ/kg, and the maximum energy yield (H-CS-EY_Max) reached 65.49 %. For the experimental group catalyzed by citric acid, the maximum higher heating value (H-CA-HHV_Max) was 27.59 MJ/kg, and the maximum energy yield (H-CA-EY_Max) increased significantly to 91.90 %. Subsequently, the selected optimal hydrochar samples were characterized through scanning electron microscopy, brunauer-emmett-teller, fourier transform infrared spectroscopy, and X-ray diffraction to systematically investigate the evolution of structural properties. The catalytic mechanism of citric acid effectively suppressed both byproduct formation and excessive hydrolysis. Furthermore, thermogravimetric analysis demonstrated that citric acid significantly elevated both the ignition temperature (T_i) and the burnout temperature (T_f), thereby delaying the combustion process. Finally, kinetics analysis using the FWO and Friedman methods revealed that the activation energies of H-CA-EY_Max could be as low as 24.93 kJ/mol (FWO) and 36.76 kJ/mol (Friedman), while those of H-CA-HHV_Max reached 44.02 kJ/mol (FWO) and 52.66 kJ/mol (Friedman). Therefore, the superior combustion stability of citric acid-catalyzed hydrochar allows for its direct application as a clean and efficient fuel.