Pilot experiment on biogas production of dry fermentation of wheat straw and cow dung with composting pre-treatment

Abstract: As a big agricultural country, China produces a large number of crop stalks every year. However, most of them have not been effectively utilized, but are burned in the field. This not only is a waste of resource, but also causes a lot of environmental pollution problems. It is usually said that the garbage is misplaced resource, and then how can we use such a large number of crop stalks? One answer is anaerobic fermentation. Utilizing crop straw by anaerobic fermentation can not only reduce a lot of agricultural waste, but also produce a large amount of methane, which is renewable clean energy and can replace the traditional natural gas as energy, and the by-product of biogas slurry can also be used as high-quality organic fertilizer. According to the water content of the material, anaerobic fermentation can be divided into dry anaerobic fermentation and wet anaerobic fermentation. Generally speaking, dry fermentation refers to the fermentation material with total solid (TS) content of more than 20%. The content of TS less than 20% is wet anaerobic fermentation. At present, composting pre-treatment is mostly used in wet anaerobic fermentation. In order to expound the effect of composting pre-treatment on biogas production of wheat straw and fresh cow dung under dry fermentation, taking wheat straw and fresh cow dung as raw materials, using garage dry fermentation system, under the conditions of the ambient temperature of 26-35℃ and the TS concentration of 20%, the characteristics of biogas production under dry anaerobic fermentation after composting pre-treatment and the component changes of wheat straw and fresh cow dung were analyzed. Results showed that composting pre-treatment could improve the degradation rate of cellulose by 7%, but it couldn't increase the biogas production of wheat straw and fresh cow dung, on the contrary, this process of pre-treatment may reduce the total biogas production, because the process of composting would consume some part of hemicellulose, which may cause hemicellulose content to decrease by more than 13%, and hence reduce the total gas production. But the process of composting pre-treatment could significantly shorten the start-up time of dry anaerobic fermentation, and could improve the methane content and enhance the quality of biogas. From the engineering aspect, we collected gas only when the methane content in the gas is more than 30%, and composting pre-treatment had the higher initial value and increasing trend of methane content in the gas production. In this experiment, because of the process of composting pre-treatment, the target that the methane content in gas production reached 30% could be completed in advance of 4 d, which meant that gas supply time could be advanced and more gas could be collected. On the other hand, composting pre-treatment could increase compactness of fermentation materials, which could improve the feeding rate and processing load and further improve the volume loading rate of fermentation device, and at the same time, more crop stalks could be used in a fixed volume of the fermenting tank, which was beneficial to the mechanical feeding and the labor cost saving.