Improving The Production Of CH₄ During Anaerobic Digestion Via Exogenous Hydrogen

With the increasing demand for natural gas in China,there is no doubt that the storage of natural gas which is kind of un-renewable energy will decline rapidly due to the huge exploitation quantity.In order to improve this situation,looking for a clean,renewable alternative energy sources is inevitable.Biogas as one kind of renewable energy is rich in resources and environment friendly,which is considered as an ideal alternative to natural gas.However,biogas usually contains about 50%CO₂,which makes biogas with lower CH₄ content and can not be used directly.How to purify and upgrade biogas is one of the key researches around the world.It is known that hydrogenotrophic methanogens can convert CO₂ to CH₄ with H₂ during anaerobic digestion?AD?,so it is feasible to fix endogenous CO₂ with exogenous H₂ to achieve the in situ-upgrading of biogas.This process can significantly increase CH₄ content and decrease CO₂ content to achieve a win-win situation.This research used H₂ and CO₂ as feeding gas to investigate the influences of H₂injection and operating conditions on AD with the enriched culture of hydrogenotrophic methanogens.The abundance and structure of microorganism were also analyzed during the whole process.The main research results and conclusions are as follows:?1?Both thermophilic and mesophilic enrichments showed obvious biomethanation potential from H₂ and CO₂,especially under the thermophilic contidion.The maximum methane content in the whole process was 97.03%,which can be obtained at 55?,when the hydrogen injection was 800mL/d and the mixing speed was 350rpm.There was no significant accumulation of VFA during this process means that no obvious inhibition occurred due to the exogenous H₂.The slight liquid-gas mass transfer limitation only occurred under mesophilic condition and was weakened with the enhancement of mixing.It should be noticed that the pH increased rapidly with the addition of H₂,which may affect the growth of methanogens and have negative impact on AD.Microbial community analysis showed that the diversity and abundance of archaeal species had significant change during different periods.of mesophilic and thermophilic reactors.The abundance of Methanothermobacter and Thermotogae were much higher than others in the theromphilic reactor.?2?Compared with pure H₂,using H₂ and CO₂ as feeding gas can make the reactor have better recovery stability.When the feeding gas was 900mL/d and the ratio of H₂ and CO₂ was 4:1,the maximum CH₄ content was 92.03%with the mixing speed was 300rpm.While maintaining the mixing speed,increasing the ratio of H₂ and CO₂ resulted in the decrease of CH₄ content.Almost 10%CO₂ can be observed during the whole process,which may be produced by other metabolic pathways of microorganisms.There were no obvious inhibition and liquid-gas mass transfer limitation observed during the whole AD.The influence of H₂ on pH was reduced due to the exist of exogenous CO₂.Microbial community analysis showed that the diversity of archaea decreased during the process.Methanothermobacter was the dominant specie after the reaction with the abundance was95.97%.?3?During the hydrogenotrophic methanogenesis,the high consumption of H₂always occured after 8-hour reaction,and the formation of methane was later than the dissolution of hydrogen.Appropriate increasing in hydrogen partial pressure has significant effect on easing gas-liquid mass transfer limitation of hydrogen,but it will bring negative impacts if increasing too much.The high agitation strength intensified the gas-liquid transfer of hydrogen and increased the methane generation rate at high hydrogen partial pressures.