Subcritical water hydrolysis of rice straw in a semi-continuous mode

Abstract Rice straw is an agricultural residue rich in cellulose and hemicelluloses. Therefore, the objective of this study was to obtain fermentable sugars and bioproducts from this biomass by subcritical water hydrolysis (SWH) in a scaleable semi-continuous mode. Temperatures of 180 °C, 220 °C and 260 °C, and liquid/solid mass ratios of 7.5 g water/g straw and 15 g water/g straw were evaluated on the reducing sugar yield (Y RS ), efficiency of conversion (E), kinetic profiles, composition of inhibitors and bioproducts, and physicochemical characteristics of remaining solid material. The pressure was set at 25 MPa and the reaction time was evaluated up to 15 min. The thermogravimetric analysis and Fourier Transform Infrared Spectroscopy provided additional information on the bulk and surface modification of the residual biomass. The highest Y RS (33.4 ± 4.3 g/100 g dry straw) and E (72.1 ± 4.6 g sugar equivalents in glucose per 100 g of carbohydrates) were obtained at 220 °C and liquid/solid mass ratio of 7.5 g water/g straw. The analysis of the hydrolyzed solution indicated xylose (0.29–7.81 wt.%), arabinose (0.35–4.85 wt.%), cellobiose (0.04–3.41 wt.%), glucose (0.05–1.54 wt.%), and levulinic acid (0.04–0.77 wt.%). The fermentable sugars and bioproducts can be applied in several industrial fields, especially for the production of bioethanol and other higher value-added chemical compounds. Furthermore, SWH performed in semi-continuous mode is a green technology within the biorefinery concept that can be operated at industrial scale for processing lignocellulosic biomass.