Microwave-assisted hydrothermal carbonization of corn stalk for solid biofuel production: Optimization of process parameters and characterization of hydrochar

Abstract

The optimization of microwave-assisted hydrothermal carbonization for corn stalk was conducted to study the effects of reaction temperature (122.7–257.3 °C), residence time (4.8–55.2 min), and biomass loading (0.98–6.02 g/50 mL H2O) using the response surface methodology. The hydrochars produced from hydrothermal carbonization of cork stalk under different reaction conditions were characterized to understand their physical, chemical, and structural properties. Statistical analysis shows that the carbonization temperature is the dominant parameter determining the product yield as well as heating value and quality of hydrochar. Due to the interactive effects amongst the process parameters, hydrochar mass yield and heating value cannot be maximized simultaneously. The highest energy yield under the predicted optimum conditions of 181.9 °C, 39.7 min and 3.8 g/50 mL H2O reached 80.55%. The higher heating value of 22.82 MJ/kg was observed at 230 °C, 45 min and 2 g/50 mL H2O, which is 41% higher than that of the raw corn stalk. The characterization results reveal that the microwave-assisted hydrothermal carbonization is a viable process for producing hydrochar, which can be used as a direct solid fuel or auxiliary fuel.