Alkali Pretreatment of Rice Straw for Increasing the Biodegradability

Abstract

Biofuel production from lignocellulosic biomass such as rice straw becomes increasingly important in light of the needs for alternative fuels, sustainable economy, and environmental mitigation. This research is aimed at the development of new pretreatment technologies that will enable faster and more efficient degradation and conversion of lignocellulosic biomass into biofuel and other valuable products. Laboratory experiments were performed to investigate the pretreatment of rice straw with three alkali chemicals, including lime (Ca(OH)2), sodium hydroxide (NaOH), and potassium hydroxide (KOH). Orthogonal experimental design was used to determine the effects of three key factors (alkali chemical, alkali loading and temperature) that influence the pretreatment efficiencies. Alkali loading was tested at 2%, 6%, and 10% levels (based on dry biomass), and temperature was 20, 35, and 50C. The treatment time was 24 hours. The lignin removal, solids and sugar loss from the rice straw as a result of alkali pretreatment were measured. The pretreated straw was evaluated for biodegradability using enzymatic hydrolysis and anaerobic digestion. The results showed that NaOH treatment at 20C and with 10% chemical loading achieved the highest lignin removal of 35%. Statistical analysis showed that NaOH was the most effective chemical for lignin removal. Temperature ranging from 20C to 50C had no significant effect on lignin removal. With 23 day anaerobic digestion at mesophilic temperature of 35C, rice straw pretreated with 10% NaOH at 20C for 24 hours had the biogas yield of 0.6 L /g VS, 50% higher than the biogas yield from untreated straw. After enzymatic hydrolysis using cellulase from Trichoderma reesei ATCC 26921 and cellobiase from Aspergillus niger, the pretreated straw had the reducing sugar yield of 298 mg glucose/g VS, 185% higher than the untreated straw.