Dilute acid pretreatment of starch-containing rice hulls for ethanol production

Rice hulls are potential low-cost feedstocks for fuel ethanol production in many countries. Most of the relevant publications are devoted to homogenous (high quality) hulls generated in industrial mills. On the other hand, small-scale mills give rise to hulls containing grain fragments and bran. In the present work, the dilute-acid pretreatment of such heterogeneous rice hulls was investigated based on a central composite rotatable experimental design. The variables were: temperature (140–210°C), biomass load (5–20%), and sulfuric acid concentration (0.5–1.5% based on reaction mixture). A total of 16 experimental runs were carried out, including a 23-plan, two replicates at the central point and six star points. Low temperatures were found to be favorable for the hydrolysis of xylan and of the easily hydrolyzable glucan fraction. High glucose formation (up to 15.3% of the raw material dry weight), attributable to starch hydrolysis, was detected in the hydrolysates obtained under the least severe pretreatment conditions. Several models were developed for predicting the effect of the operational conditions on the yield of pretreated solids, xylan and glucan conversion upon pretreatment, and on enzymatic convertibility of cellulose. The pretreatment temperature exerted the most significant effect on the conversion of the polysaccharides. Optimum results were predicted for the conversion of easily-hydrolyzable glucan in the material pretreated at 140.7°C, and for the enzymatic saccharification of cellulose in the material pretreated at 169°C. These results are interpreted that a two-step acid hydrolysis may be the best pretreatment strategy for heterogeneous rice hulls produced in small mills.