Research On Key Technologies Of Liquefaction Plant For CMM Containing Nitrogen And Oxygen

In China, it is an urgent task to increase resource utilization efficiency, optimize the energy structure to maintain the sustainable energy development. The rational utilization of coal mine methane (CMM) containing nitrogen and oxygen has already become the focus of current research.For Hegang Nanshan CMM gas source, the paper proposes the methods of deeply removing acid gas from CMM by alcohol amine solution chemical absorption, deeply removing H2O from CMM by 4A molecular sieve adsorption, liquefy CMM by mixed refrigerant liquefaction cycle, and removing nitrogen and oxygen from CMM by low temperature distillation to produce LNG product as high calorific value. Then, the paper developed a liquefaction plant for CMM containing nitrogen and oxygen. Based on the above CMM liquefaction plant, the paper does a lot of basic research and experimental analysis to promote domestic production and application of liquefaction plant for CMM containing nitrogen and oxygen in China.According to the basic principle of combustion and explosion, based on Le Chatelier formula and the principle of full-scale distribution and combination, the paper presents the explosion limit formulas of multivariate mixed CMM, which contains nitrogen, oxygen, and carbon dioxide and so on. Based on the CMM liquefaction system, it analyzes the influences of the CH₄ mole fraction in CMM containing nitrogen and oxygen on the CH₄ recovery rate, nitrogen and oxygen mole fraction in LNG at different liquefaction pressure. It also analyzes the influences of the operating pressure on the explosion limit of CMM containing nitrogen and oxygen at different temperatures. Then, it presents a process flow of the liquefaction system for CMM containing nitrogen and oxygen, which contain CH₄ mole fraction of 30%⁸7%. By analyzing the relationship between the gas component and its explosion limit at each theoretical plate in the low-temperature distillation tower, it presents the distillation tower structure with a low-temperature distillation stripping section and a reboiler, and without a rectifying section and a condenser. By economic analyzing of the liquefaction plant for CMM containing nitrogen and oxygen, it presents that LNG5 liquefaction plant for CMM containing nitrogen and oxygen is only suitable for CMM containing nitrogen and oxygen, which contain CH₄ mole fraction of 50% to 87%.According to the double-film theory model, it establishes the diffusion-rate equation of the acid gases in alcohol amine solution and determines the enhancement factor formulas of the chemical absorption in different chemical reaction types to achieve the linkages between the physical absorption and the chemical absorption. Based on the different ion balance formulas of the acid gas in the alcohol amine solution, and the experimental data of quasi-equilibrium constant, it established the solution model of acid gases in alcohol amine solution to determine the explicit analytic formula of acid gas partial pressures, ps and pc, which is convenient for the engineering applications. It adopts the numerical simulation method to optimization analyze and adaptability analyze on the acid gas removal process by the alcohol amine solution chemical absorption. As for CO₂ removal process in LNG5 liquefaction plant for CMM containing nitrogen and oxygen, when the CO₂ mole fraction is 3%⁰.129%, the circulating amount of MEA solution should be 30kmol/h²0kmol/h and the reboiler heat load should be 76kW⁴4kW. It builds the acid gas removal system in LNG5 liquefaction plant to carry out the experimental study on the removing CO₂ from CMM by MEA alcohol amine solution. The CO₂ mole fraction in the purified CMM is only 20ppm to meet the design valve of CO₂≤50ppm. The operating cost of the acid gas removal system using MEA alcohol amine solution is 14kW/(kg·h).Based on the gas-solid diffusion theory, it establishes the mathematical model of the mass transfer to discuss the penetration properties of the molecular sieve bed. According to the engineering experience, it presents the calculating formulas for the dehydration process. For the dehydration system in LNG5 liquefaction plant for CMM containing nitrogen and oxygen,it analyzes the effect of the CMM inlet temperature and flow rate on the breakthrough time of the molecular sieve bed. It also analyzes the effect of the adsorption cycle time and the heating time of the molecular sieve in the non-normal conditions, on the molecular sieve regeneration gas flux to discuss the adaptability of the molecular sieve dehydration system. It builds the dehydration system in LNG5 liquefaction plant to carry out the expiration study on the removing H2O from CMM by 4A molecular sieve. The dew point temperature of the dried CMM is -75℃, namely the H₂O mole fraction being less than 1ppm, to meet the dehydration system design specifications.For the single-stage mixed refrigerant liquefaction process for LNG5 liquefaction plant for CMM containing nitrogen and oxygen, it establishes the steady-state mathematical model of these key equipments and numerical analyzes the influences of the different mixed refrigerants on the mixed refrigerant compressor power, the temperature distribution and the temperature differences between the hot fluid and the cold fluid in the heat exchangers to define the mixed refrigerant components for LNG5 liquefaction system. It analyzes the effect of the mixed refrigerant components on the smallest temperature difference in the heat exchanger and the vapor fraction in the key stream to define the optimization composition of the mixed refrigerant. It builds the dynamic mathematical model to analyze the dynamic response of LNG5 liquefaction system when the LCMM pressure, temperature, flow rate and mixed refrigerant flux are changing. It builds the liquefaction system in LNG5 liquefaction plant for CMM containing nitrogen and oxygen to carry out the experimental study on the cooling process in the cold box. When the mixed refrigerant is the mixture of CH₄, C₂H₄, C2H₆, n-C4H10 and N₂, the refrigerant power is 0.508 kW·h/Nm³ per LNG, which is higher than the theoretical value of 0.48 kW·h/Nm³ per LNG by only 5.8%. Because the mixed refrigerant compressor outlet pressure of 2.16 MPa is less than the theoretical value of 2.5 MPa, which resulting in the increase of the mixed refrigerant and the mixed refrigerant compressor power.Based on the theoretical plate theory, it builds the dynamic equilibrium mathematical model for the distillations column. It cites Bain-Hougen formula and S-B-F model to define the packing layer pressure drop, P, to solve the MESH equations. It analyzes the influences of the packed column theoretical plate number and the boiling ratio on the nitrogen and oxygen mole fraction in LNG produce and CH₄ mole fraction in the reflux CMM, and analyzes the temperature and pressure trends in the distillations column. It analyzes the effect of the CH₄ mole fraction in CMM on the LNG production, and the LNG component. The dynamic response of the liquefied CMM flux, pressure, temperature and component changes on the distillation tower pressure, the tower bottom liquid level, and the oxygen mole fraction in LNG product. It provides technical guidance for the operation of the automatic control system in the distillations tower. It builds the low temperature distillation system in LNG5 liquefaction plant to carry out the experimental study on the removing nitrogen and oxygen from CMM by low temperature distillation. The N₂ mole fraction in LNG can be distillated from 7.12% to 0.19% using the stripping section of the distillation tower, which is better than the theoretical value of 0.545%.The successful operation of LNG5 liquefaction plant for CMM containing nitrogen and oxygen are verify the reasonableness of the process of the liquefaction plant for CMM containing nitrogen and oxygen. These also provide the technical and experimental guidelines for the application and commissioning of the liquefaction plant for CMM containing nitrogen and oxygen in China.