Subcritical water hydrolysis of rice straw for reducing sugar production with focus on degradation by-products and kinetic analysis

The competitive reactions of reducing sugar production and degradation in the subcritical water hydrolysis of rice straw were investigated to optimise reducing sugar yield. The optimised conditions (280 °C, 20 MPa, rice straw concentration of 5 wt.% and agitation speed of 200 rpm) resulted in a reducing sugar yield of 0.346 g/g rice straw because of the enhanced reducing sugar production and decreased sugar degradation. The sugar yield increased when the temperature increased from 250 °C to 280 °C, but it decreased when the temperature further increased to 300 °C because of the degradation of monosaccharides (e.g. glucose and xylose) into by-products (e.g. 2-methyltetrahydrofuran and acetic acid). A first-order reaction model was developed to elucidate the competitive reaction kinetics of sugar production and degradation at various temperatures. The highest reducing sugar yield based on the model was achieved at 280 °C with the highest production and lowest degradation rates.