Investigation on the Changes of Main Compositions and Extractives of Rice Straw Pretreated with Sodium Hydroxide for Biogas Production

This study was conducted to investigate the changes of main compositions and extractives and their effects on biogas yield enhancement. Four NaOH doses (4%, 6%, 8%, and 10%) and four loading rates (35, 50, 65, and 80 g/L) were used. The rice straw was first pretreated by NaOH in solid-state conditions and anaerobically digested. The main compositions and extractives were then analyzed. The results showed that, compared to the untreated rice straw, 3.2%-58.1% more biogas yields were obtained with 4%-10% NaOH-treated rice straws. Hemicellulose, cellulose, and lignin were decomposed by 35.2%-54.2%, 14.2%-16.4%, and 8.0%-44.5%, respectively, for 4%, 6%, 8%, and 10% NaOH-treated rice straws. Considerable fractions of them were converted to relatively readily biodegradable substances, as indicated by increases of 80.3%-173.6% cold-water extractives and 80.4%-152.8% hot-water extractives. Some irresistible substances were removed, as represented by a 30.9%-51.8% decrease of benzene-ethanol extractives. The chemical structures of hot-water and benzene-ethanol extractives were also changed obviously. It was also found that the soluble sugar contents in the 6% NaOH-treated rice straw were twice that of the untreated one. The results specified that NaOH pretreatment was one of efficient methods to enhance biogas production of rice straw, and the changes of main compositions and extractives made important contributions to the enhancement.