Structure-reactivity insights in combusting rice straw hydrochar: Role of hydrothermal temperature

Hydrothermal carbonization (HTC) combined with combustion is promising for the energy use of crop straw. To guide the design and optimization of the integrated process, the structure-reactivity relationship for combusting the hydrochar needs to be explored. This study evaluated the impact of HTC temperature on the chemical composition, fuel properties, surface functional groups, crystalline structure, and combustion behavior of the hydrochar derived from rice straw. Moreover, the structure-reactivity relationship for combustion was built by performing Pearson correlation analysis and Linear regression. During HTC, cellulose and hemi-cellulose were degraded to form aromatic rings, thereby decreasing the H/C and O/C atomic ratios but increasing the aromaticity of the hydrochars. The combustion reactivity was reduced at elevated HTC temperatures. Aromaticity and H/C and O/C atomic ratios are significantly correlated with the comprehensive combustibility index (S) and activation energy (Ea). The structure-reactivity relationship for combustion was revealed by performing linear regression between S and H/C atomic ratio and between Ea and aromaticity. The S increased monotonically with the H/C atomic ratio, whereas the Ea decreased monotonically with the aromaticity. The findings provide guidance for predicting the combustion reactivity and activation energy of the hydrochar derived from rice straw at varying HTC temperatures.