Study On Biogas Upgrading Technology Of Corn Straw By Anaerobic Digestion

Nowadays,China’s demand for natural gas is rising sharply with the rapid development of social economy.How to effectively purify biogas to make it reach the standard of biological natural gas as an ideal alternative natural gas energy was a hotspot of current research.As a clean renewable energy,biogas is an important alternative to fossil energy.However,the high carbon dioxide content of biogas limits its value as a fuel.Biogas upgrading convert biogas to biomethane while removing carbon dioxide.Compared with raw biogas,biomethane has higher calorific value and wider application.In addition,biomethane can be directly injected into the existing natural gas network,which solved the transportation and storage problems of biogas.In addition,biomethane could be used to replace fossil fuels as vehicle fuel,further reducing carbon dioxide emissions.Using corn straw as digestion substrate,CO₂ was converted into methane by using exogenous H₂ in in-situ upgrading and hybrid upgrading system.The effects of total solid（TS=5%,10%and 15%）and the amount of exogenous hydrogen（H₂:CO₂=4:1,5:1 and 6:1）on biogas production and volatile fatty acids（VFAs）characteristics of anaerobic digestion（AD）systems were investigated to determine the appropriate reaction conditions.According to the reactor performance of H₂ and CO₂ by hydrotrophic methanogens under different conditions,the variation rule of each component in biogas was analyzed.High-throughput sequencing was used to explore the effect of exogenous hydrogen on microbial community structure in AD system.The main research contents and conclusions are as follows:（1）Both the in-situ upgrading and hybrid upgrading system showed excellent hydrogen consumption and methane production capacity.The methane content of the biogas produced in the whole reaction process can reach more than 90%,when the system fermentation concentration is low（TS=5%）.However,under the same working conditions,methane content in biogas produced by hybrid upgrading system was higher than that of the in-situ upgrading system.The maximum value of methane in the in-situ upgrading system were 78.9%and 83.6%,while the maximum value of methane content in the biogas produced by the mixed purification system is 95.7%and 96.5%,when H₂:CO₂=4:1and TS were 10%and 15%.At the same time,the content of CO₂ and H₂ in biogas were less than 5%,and the consumption rate of hydrogen was more than 98%.（2）The introduction of exogenous hydrogen can significantly improve methane production in AD system.In the in-situ upgrading system,the AD system with different TS were most obvious when H₂:CO₂=5:1.Compared with the control group,the cumulative and yield of hydrogenation stage can be increased by more than 20%,and can reach 41.2%,when TS=5%.However,when H₂:CO₂=5:1 in hybrid upgrading system,the cumulative methane production in AD system was the most significant.Compared with the control group,the cumulative methane production in the experimental groups with different fermentation concentrations increased by 40.2%,36.4%and 38.1%,respectively.With the increase of TS in AD system,the methane production in AD system was improved more significantly by hybrid upgrading system compared with the in-situ upgrading system.（3）The p H value of AD system has increased with exogenous hydrogen bubbled into the reactor,and with the increase of TS of exogenous hydrogen bubbled into the AD system gradually reduce the influence of p H,and hybrid upgrading system compared with in-situ upgrading system,the hybrid upgrading system under the condition of same phase of p H changes was more stable.（4）There was no significant difference in VFAs concentration between the in-situ upgrading system and the biogas generation phase of hybrid upgrading system during the whole operation.Compared with the control group,the introduction of exogenous hydrogen had no significant effect on the concentration of acetate in AD system,while the concentration of propionate and butyrate were temporarily accumulated due to the introduction of exogenous hydrogen,and it did not affect AD process.The concentration of total volatile fatty acids increased with the introduction of exogenous hydrogen,which was mainly caused by the accumulation of propionate and butyrate.（5）With exogenous hydrogen bubbled into the reactor,the in-situ upgrading system and the biogas generation phase of hybrid upgrading system can reduce the richness of bacteria and archaea communities and improve the diversity of bacteria and archaea communities.Due to the lack of nutrients and the single reaction,the richness and diversity of the bacterial community decreased compared with the initial reaction,while the richness and diversity of the archaea community increased with the progress of the reaction in the biogas upgrading phase in hybrid upgrading system.Firmicutes,Bacteroidetes,Chloroflexi,Proteobacteria,Actinobacteria,Synergistetes and Acidobacteria were the main dominant phylum in all samples.Firmicutes and Bacteroidetes were the dominant phylum in the first two,and their relative abundance accounted for 61.93%-78.49%in all samples.Clostridium＿sensu＿stricto＿1 and Terrisporobacter were the dominant bacteria in AD systems at the initial stage of reaction,and Clostridium＿sensu＿stricto＿1 class continued to be the dominant bacterium among the experimental groups of biogas generation phases as the reaction progressed.In the biogas upgrading phase of hybrid upgrading system,Firmicutes and Bacteroidetes,as the first two dominant phylum,showed little change in relative abundance with the progress of the experiment,while Proteobacteria and Chloroflexi,as the third and fourth dominant phylum level,showed opposite changes in relative abundance.The relative abundance of Proteobacteria increased slightly with the addition of exogenous hydrogen,while the relative abundance of Chloroflexi decreased.Clostridium＿sensu＿stricto＿1 and vadin HA17 were the main dominant genus of bacterial communities at the genus level.（6）The richness of the archaea community increased and the diversity of archaea community decreased,with the introduction of exogenous hydrogen into AD system in in-situ upgrading system and the biogas generation phase of hybrid upgrading system.The richness and diversity of the archaea community in the biogas upgrading phase of hybrid upgrading system increased with the progress of the reaction.Euryarchaeota and Crenarchaeota are the main dominant bacteria at archaea level,that the relative abundance,in all samples,were more than 99%,among which Euryarchaeota was the absolute dominant bacteria,accounting for81.17-92.02%relative abundance.In t the in-situ upgrading system and the biogas generation phase of hybrid upgrading system,hydrotrophic methanogens were enriched with exogenous hydrogen,and the relative abundance of Methanobacterium increased and became the first dominant bacterium.The sum of the relative abundance of hydrotrophic methanogens in each experimental group was 48.89%,56.70%and 47.82%,respectively.In the biogas upgrading phase of hybrid upgrading system,the relative abundance of hydrotrophic methanogens in each experimental group was 66.00%and 65.32%,with the passing of exogenous hydrogen,among which methanobacteria was the absolute dominant species with the relative abundance exceeding 40%.