Study on integrated technology of large scale straw biogas plant and its operation effect

Abstract: In China, the production of corn straw annually has become a problem, because of lack of resource utilization. Producing biogas with corn straw is one of the available ways to solve this problem. However, this way still not be promoted widely. The main reason is the low rate of return with the traditional craft. Integrating advanced technology of straw biogas plant is the key for sustainable development of straw biogas industry, focusing on the improvement of engineering economy. Our study team had conducted many researches on the key steps of straw biogas plant, such as straw pretreatment, straw anaerobic fermentation, agitation optimization, biogas slurry recirculation, and developed several key technologies. The corn stalk has a compact structure, thus it was not able to be degraded easily in the anaerobic fermenter. The mixture of Saccharomycetales, Sordariales, Hypocrees, Lactobacillales and other microbial as the pretreatment agent were applied in our studies study. And the pretreatment was carried out in a continuous mixed reactor with continuous aeration. Under these conditions, the degradation rate of straw lignocellulose could reach above 48%. After pretreatment, the gas production rate of straw was increased by 20%, and the peak-time of gas production was shortened by more than 3 days. The CSTR and vertical plug flow (VPF) fermentation reactor were compared in this study. The results showed that the CSTR fermentation technology was more common and mature. The reactor structure, the structure of the stirring device, the stirring style and the baffle plate all had a significant influence on the mixing effect of the CSTR. Our study team had developed an optimization method using a CFD simulation. The anaerobic fermentation only with corn straw, the anaerobic process was not stable because of easy acidification and nutritional imbalance. In our research, we applied the recirculation of biogas slurry to the anaerobic fermentation process. The results showed that recirculation of biogas slurry could help keeping the fermentation process stable. Under the conditions with recirculation of 80% biogas slurry, the biogas production rate of maize silage was increased by 97%-180%. Based on these single technologies, we proposed an integrated technology of "stalk silage-micro aerobic hydrolysis pretreatment-CSTR medium temperature & stirring optimization-biogas slurry near fully recirculation". The integrated technology had been employed in the Tianlong biogas plant in Sanhe, Hebei Province. The operation results showed that the biogas production rate of straw reached 555.78 m3/t with a relatively high CH4 concentration of 56% and low H2S concentration of 162×10(6, which proved that the integrated technology could be stable and efficient t in the actual operation of a large-scale biogas plant.