Enhancement of methane yield from cotton stalks by mechanical pre-treatment

Abstract

Cotton stalks (CS) are lignocellulosic agricultural by-products, a potential source for biogas production, but pretreatment must be considered since hydrolysis is the rate-limiting stage for lignocellulosic biomass substrates. This study investigates the feasibility of mechanical pretreatment of CS to enhance methane production. Batch anaerobic digestion of CS samples with particle sizes ranging from 0.5 to 65 mm was carried out in 1 L eudiometer batch digesters for 48 days at 37°C. Results showed that methane yield was inversely proportional to particle size, and the quality of biogas was good (54.0–55.2% CH4). Significant increases in methane yield were observed with 20.3% and 26% for samples with a particle size of 3 mm and 0.5 mm, respectively, compared to untreated CS. The coefficient of anaerobic energy turnover was relatively low (20.2–25.5%). Reduction of the CS particle size to 3 mm or less is recommended to achieve effective methane conversion and decrease the retention time in an anaerobic digester from 31 to about 25 days. However, to offset the high energy demand required for grinding, further research should be conducted in combining size reduction with chemical and physicochemical pretreatment.