| Due to the shortage of fossil energy, rising prices and the resultant environmental pollution problems, the use of biogas energy has become a research hotspot in recent years. Biogas liquefaction can improve the energy density and combustion characteristics of biogas, so it can be used as clean energy to replace natural gas used in various fields of production and living. At the same time, biogas liquefaction system can solve the problem of biogas storage, transportation difficulties, and it's also a technology means to achieve a comprehensive, large-scale use of biogas. Liquefied biogas was referred to as biomass LNG (Liquefied Natural Gas). However, in comparison with natural gas, biogas has several problems, such as low composition of methane, high content of carbon dioxide, imperfect in the purification process and liquefaction system, and high-energy consumption and so on. These problems have seriously restricted the development of liquefaction biogas technology.Based on the analysis of LNG liquefaction and purification, the key technologies of preparing biomass LNG was studied. Based on the anaerobic fermentation theory, the effect of the pretreatment method of corn straw mixing with cow dung and mud on the content of methane in biogas was researched, respectively. Based on LNG cold energy utilization technology and obvious difference in critical temperature and pressure between CH4and CO2, the separating efficiency of separating CO2from biogas was calculated by PR equation and phase equilibrium theory. According to thermodynamic law. the effect of process parameters on exergy loss of the biogas liquefaction system was analyzed. Based on the research of liquefied natural gas system, the effect of process parameters on system energy consume was analyzed. Skid-mounted package and modulization were introduced into the biogas liquefaction system, and the coupling biogas liquefaction system with purification and liquefaction was established.In this paper, the pretreatment method of storing corn straw in obturation environment was proposed to improve cellulose degradation rate, based on the anaerobic fermentation and microbial fermentation theory. CMC enzyme, xylanase, filter paper enzyme and exonuclease activity were tested for the pretreatment stalk. The results showed that the enzyme activity of middle corn stalks reached the highest level. The mixed fermentation experiment including two parts was conducted. One part was mixing corn straw with cow dung, and the other part was mixing corn straw with the sludge. The results showed that the methane content of the former was about20%above the latter. Corn straw closed cultivation of microorganisms can culture microorganisms and reduce pretreatment costs, which provides an effective pretreatment method for large-scale dry fermentation biogas industrialization.A purifying system separating CO2from biogas was constructed based on LNG cold energy utilization technology. The effects of the different temperature and pressure on separating efficiency were analyzed. The CO2separation system by liquefaction in low tempreture was constructed, and the thermodynamic parameters of this system were calculated by PR equation. The simulation results indicated that the volume fraction of CO2reduced to less than9%. The system is ideal for separating CO2in the range of20%to50%from biogas. The optimum technological conditions was obtained, with the pressure of3.7MPa, the temperature of-78.5℃and the flow rate of0.8g/s. Under this condition, the volume fraction of CH4increased from58.2%to82.8%, and the volume fraction of CO2decreased from36.4%to8.8%.According to thermodynamic law. the exergy loss of the biogas liquefaction system was analyzed. A small mixed refrigerant gas liquefaction process was designed and the exergy loss of the equipment was computed and simulated. The results showed that the compressor was subjected the most impairment from the exergy loss of the system, followed by the heat exchanger and cooler. The mixed refrigerant biogas liquefaction system was studied in a small experimental platform. The experiment conclusions indicated-that the exergy loss of the compressor was reduced by12.2%, and the cooler was reduced by27.2%, by adopting the cooling capacity of flash gas to cool the outlet gas of compressor, which proved the validity of the method.According to the thermodynamic theory of the core principle, two liquefaction systems were designed and the parameters of compressors and coolers were calculated. The key parameters of the two processes were compared. The distribution of heat load-tempreture in heat exchangers was also analyzed. And then the performance curves of multi-component mixed refrigerant existing phase changes coupling in the heat exchanger were determined. Effect of the number of stages of mixed refrigerant cycle on biogas liquefaction system was analyzed. The results showed that the energy consumption of the mixed refrigerant process with precooling was lower than that of the single-stage one. The effect of parameters on the energy of liquefaction system was simulated. The laws on effect of the change of temperature, pressure and component fraction at the same time on the energy consumption of liquefaction were obtained.Based on the research of small-scale liquefied natural gas system in skid-mounted package, skid-mounted package and modulization were introduced into the biogas liquefaction system. Temperature, pressure, enthalpy, entropy, molar flow and vapors fraction in liquid and gas phase fraction of every node in the whole process were obtained by using the coupling precooling and refrigerant purification system. The results showed that the liquefaction rate of biogas liquefaction system was0.9242and the power consumption of the equipment was0.455kWh/Nm3...
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