Research On Technologies Of In-situ Methane Enrichment Based On Two-phase Fermentation

As the worldwide fossil fuel is decreasing, the development of biomethane as substitute of natural gas has caused wide attention. Biomethane is high purity methane that was obtained by biogas purification removing impurities, such as CO2. Recently, industrial biogas purification methods mainly include pressure swing adsorption method (PSA), pressure washing method, chemical absorption method and membrane separation method. All of these methods above need separate biogas purification system besides methane fermentation equipment, which restricts economy of biomethane production with the problems as complex system and high energy consumption. In situ methane enrichment is a processthat produces biogas with high methane content,whose characteristics like simple system and low operation energy consumption make it have high development potential in the field of biogas technology.In order to promotethe development and application of in situ methane enrichment, this paper has carried out two aspects of research as below.First, in view of the importance of CO2removal result, the comparative studywas carried out and decarburization waywas innovated. Then process researches on in situ methane enrichment based on two-phase fermentation were carried out in this paper, focused on the anaerobic transformation utilization of lignocellulose materials as straw.(1) Research of in situ methane enrichment on the effect of decarburization wayThis paper compared the air rectification and vacuum decarburization, raised the ultrasonic-assisted air removal carbon method, conducted anaerobic fermentation and in situ methane enrichment experiments whose emphasis is the effect of circulation ratio of fermented liquid and rectification time on in situ methane enrichment in the condition of medium temperature (35℃) by using glucose artificial water distribution as anaerobic fermentation raw materials. Experiment results show that when the circulation ratio reached3/5, the average CH4content of biogas was up to89.99%, increased by27.4%compared with70.61%. With the extension of rectification time, the CH4content of biogas reduced, and the CH4wastage rate appeared increasing trend. The methane enrichment effect of vacuum decarburization is slightly worse than air rectification, but vacuum decarburization can reduce the N2content of biogas. Ultrasonic-assisted air rectification can improve the CO2rectification rate, reduce the concentration of free CO2in fermentation slurry,which is apparentespecially in the early stages of the air rectification decarburization.(2) Experiment on in situ methane enrichment based on straw two-phase fermentation The idea that removal of solubility CO2in methane-producing phase slurry was improved using high concentrations of volatile fatty acid (VFAs) producing in acid-producing phase was put forward, experiment system of in situ methane enrichment was built based on straw two-phase fermentation. The system operation experiments were conducted in the condition of fixed fermentation slurry circulation ratio, with regulating fermentation in acid-produced phase by adding acid, and changing fermentation slurry circulation ratio with regulating fermentation in acid-producing phase by pneumatic control production, respectively. Experiment results showed that this technology can significantly improve the methane content of biogas produced by straw anaerobic fermentation.with the highest CH4content up to86.9%. The key of operationcontinuity and stability of this process is the two phase separation which can be realized by controlling the dissolved oxygen content in acid-producted reactor. Controlling the pressure of the methane-producing phase is an important measure improving the CH4content in biogas. When the pressure in methane-producing phase increased by13.4kPa compared with atmospheric pressure, the methane content of biogas increased from71.49%to83.91%. In the system operation process, acid-producing phase is butyric-type fermentation which is good for methane-producing fermentation process.