Improving methane production performance via NaOH pretreatment of sugarcane leaves and its mechanism analysis

Abstract: China is a country with a vast agricultural base and a large amount of tropical agricultural wastes including sugarcane leaves (SL). The annual yield of SL in China was as high as 36 million tons in 2017, while the utilization ratio was less than 20%. Significant quantity of SL remained unused and was burned in open fields causing serious environmental problems. Recently, the interest for the use of SL is mainly based on its chemical compositions, which have high carbohydrate content, in the form of cellulose and hemicelluloses (>70%), that can be used for bioenergy production. Anaerobic digestion (AD) of SL to produce biogas may offer a promising approach to SL utilization and mitigate air pollution. However, due to the cellulose and hemicellulose are densely packed by layers of lignin and they are protected from enzymatic hydrolysis, it is necessary to have a pretreatment step prior to biogas production in order to break lignin mesh and expose cellulose and hemicellulose for enzymatic action. NaOH pretreatment method was used to pretreat the crushed SL aimed to improve the methane production performance of SL in this paper. The methane production rate of AD (anaerobic digestion) of sugarcane leaves under different NaOH concentration and different pretreatment time was obtained. The change of micro-physical structure, chemical composition and chemical structure of SL before and after pretreatment were studied and compared, and the internal mechanism of NaOH pretreatment SL was revealed by means of modern analysis and testing, such as scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and so on. The results of the tests indicated that compared with unmodified SL, the cumulative methane production the NaOH modified SL increased by 22.02%-89.94%, and the anaerobic digestion time T80 shortened by 2-4 days. Among all these treatments, SL of 6% NaOH-5d had the best methane production performance. The maximum daily methane yield was 328.50 mL/d, and the methane yield reached 154.08 mL/g. After the NaOH pretreatment, the epicuticular wax and cell walls structure of SL were destroyed, the decomposition of silica and lignin in the surface of SL were promoted, and the bondage of cellulose was broken. The lignocellulose structure of NaOH modified SL changed obviously. Some functional groups of lignin, such as hydroxyl, methoxy and carbonyl groups, were broken down in varying degrees, the compact macromolecular structure was decomposed, the crystallinity of cellulose was reduced, parts of hydrogen bonds of cellulose were destroyed, and the intermolecular and intramolecular degradation of the hemicellulose took place. The lignocellulose content of NaOH modified SL decreased in varying degrees, and the amount of organic matter that could be decomposed and utilized by microorganisms increased. After AD, the degradation rates of lignin, cellulose and hemicellulose of 6% NaOH-5d increased by 9.27%, 25.14% and 21.52%, respectively. Therefore, NaOH pretreatment is an effective way to improve the methane production performance of SL.