According to different hydrogenation methods, biogas purification methods can be divided into in-situ hydrogenation purification and ectopic hydrogenation purification. The so-called in-situ hydrogenation purification method refers to the direct introduction of exogenous hydrogen gas into a real biogas fermentation system, which converts CO2 from biogas into CH4 in situ, achieving the purpose of purifying and upgrading biogas. Foreign scholars have conducted a series of exploratory studies on hydrogenation purification technology:
1. In situ hydrogenation purification
Taking the fermentation system of solid waste such as livestock and poultry manure and flocculent sludge as the research object, experiments have shown that introducing exogenous hydrogen gas can effectively improve the generation rate and methane concentration of biogas, while the carbon dioxide content in biogas is reduced from 38% in the comparative experiment to 12%, achieving preliminary biogas purification effect.
In addition, in order to solve the problem of pH increase in in-situ biogas purification, they conducted experimental research on co fermentation with acidic waste, effectively controlling the pH value of the fermentation system below 8.0; In response to the difficulty of low gas-liquid mass transfer rate, hollow fiber membrane (HFM) module aeration was used instead of traditional microporous aeration to improve the gas-liquid mass transfer of H2 at the fermentation liquid interface.
The results showed that under a fermentation temperature of 55 ℃ and a stirring intensity of 150r/min, when continuously aerated at a rate of 1440mL/(L? D), H2 was completely consumed, and the concentration of CH4 reached 90.2%. However, in a hydrogen free fermentation system, the generated biogas only contained 55.4% CH4, with a proportion of CO2 accounting for 44.6%.
2. Ectopic hydrogenation purification
In the experiment of heterotopic biogas purification, an anaerobic reactor enriched with hydrogen loving methane bacteria was used as a separate biogas purification unit. After purification treatment, the CH4 in biogas reached about 95%, successfully achieving biogas purification.
In terms of hydrogen sources, in addition to utilizing renewable electricity to electrolyze water for hydrogen production, coke oven gas can also be used to replace pure hydrogen, achieving the dual goals of biogas purification and coke oven gas resource utilization simultaneously.
3、 Development status of domestic hydrogenation purification technology
Recently, the Chinese Academy of Sciences Chengdu Institute of Biology has carried out the research on the preparation of biological natural gas by high-temperature in-situ hydro methanation. Hydrogen is directly introduced into the pig manure anaerobic digestion reactor, and the average methane yield has increased from 222L/kgVS in the hydrogenation stage to 292L/kgVS, and the average relative methane content has increased from 66% to 83%.
Through microbial diversity analysis, it was found that strictly hydrogenotrophic methanogens, such as Metanoculleus, Metanobrevibate, and Metanobacterium, are the main methanogens in the high-temperature in-situ hydrogenation methanation system. The addition of exogenous hydrogen gas weakens the synergistic relationship between homotrophic acetic acid producing bacteria and hydrogenotrophic methanogens, while the addition of sodium formate as an interspecific electron transfer carrier helps to degrade propionic acid and produce acetic acid, To ensure the stability of the system.