Physicochemical characteristics and FTIR-derived structural parameters of hydrochar produced by hydrothermal carbonisation of pea pod ( Pisum sativum Linn.) waste

Conversion of pea pod (Pisum sativum Linn.) waste into energy and chemical products is a sustainable waste treatment option. In this work, hydrothermal carbonisation (HTC) of pea pod (Pisum sativum Linn.) waste was performed at 210–270 °C for 30–480 min. The effects of reaction severity (temperature and time) on the physicochemical and structural properties of hydrochar, including CHNO content, CHO recovery, higher heating value (HHV), structural parameters determined by Fourier-transform infrared spectroscopy (FTIR) and O/C and N/C atomic ratios determined by X-ray photoelectron spectroscopy (XPS), were investigated. The relationships among the properties of hydrochar were revealed. The results showed that the maximum variation rate of CHO recovery exhibited the trend O > H > C. The N recovery seemed to be independent on severity. With decrease in hydrochar yield, the O/C atomic ratio determined by elemental analysis decreased linearly and approached that of lignite and sub-bituminous coal at high severity, while the C recovery and HHV increased linearly. The O/C atomic ratio calculated by XPS approached that determined by elemental analysis as severity was equal to or greater than that at 230 °C for 480 min, while the N/C atomic ratio calculated by XPS is lower than that determined by elemental analysis at different severities. The structural parameters determined by FTIR (CH2/CH3, ‘C’ factor, C=O/Car and C–O–R/Car) trended down with rise in reaction severity. The results will provide more accurate information on the physicochemical characteristics and structural parameters of pea pod hydrochar to control its properties for use in diverse applications.