Effects of different combined pretreatments on biogenic methane production by anaerobic digestion of lignite

It is crucial to choose efficient and practical pretreatment for increasing methane production. However, the treatment effect is often unsatisfactory through a single pretreatment method. In particular, it has an uncertain influence on the methane production of lignite with complex composition. In order to explore the effect of fermenting methane from lignite through the different joint pretreatment, with 1.00% HCl + 5.00% H₂O₂(group 1), 6.00% NaOH + 5.00% H₂O₂(group 2), 1.00% HCl + 10.00 g(group 3), 6.00% NaOH + 10.00 g(group 4), and 5.00% H₂O₂ + 10.00 g ligninase(group 5) as the experimental groups, and with the unpretreated coal samples (group 6) as the control group, the experiments of methane production through anaerobic fermentation were carried out under suitable strain sources and environmental conditions. The colorimetry, the gas chromatography-mass spectrometry and the scanning election microscopy were used to analyze the saccharide, the volatile fatty acid content and the degradation characteristics n the process of joint pretreatment of methane so as to reveal its influence mechanism. The results showed the following: (1) Different joint pretreatment increased methane yield similarly. The treatment of groups 4 and 5 with the cumulative methane production up to 20.36 mL/g and 8.83 mL/g respectively, proved to be more effective for the degradation of coal. Compared with the group 6, the methane production of the two groups has increased by 24.24 and 10.51 times respectively. (2) The COD removal rate of each experimental group was higher than that of the control group, and the pH variation was also relatively small. (3) At the beginning of the reaction, the lowest polysaccharide content belonged to group 3(0.37 μg/mL), whereas the highest number fell into group 6(2.15 μg/mL). The polysaccharide content of the two groups showed the same variation tendency(first decreasing and then increasing). (4) The reducing sugar contents of experimental groups 2, 3, and 5 remained at relatively high level on the whole. At the end of the reaction, the carbohydrate content of each gas-producing group was not zero. (5) All the joint pretreatments promoted the degradation of acetate and butyrate, and improved the carbon conversion rate. This research demonstrated the effectiveness of enhancing biogenic methane from coal through the joint pretreatment. It is hoped that the methods and the findings of this study may shed light on the industrialization applications of biogas production from coal.