Research On CO₂Purification Index And Liquefaction And Methane Enrichment Processes For Nitrogen-containing Cbm

As a kind of coal mine associated gas mainly consisted of methane, thecoalbed methane (CBM) is a potential energy source that is clean and highefficiency. Rational development and utilization of CBM can not onlysupplement the natural gas resources to a certain extent, and thus alleviate theshortage of oil and gas resources, but also reduce greenhouse gas emissions, andtherefore has important economic and social benefits. CBM is usually exploitedin remote mountain areas, away from the natural gas pipeline network, and itsqualities are different from ordinary natural gas, thus it is inconvenience orinappropriate for CBM to access into the existing pipeline network. As a result,transportation and storage of CBM is the primary problem, and liquefaction is agood alternative for CBM transportation and storage. The majority of China’sCBM resources are in the form of mine gas, which contains large amounts ofnitrogen and oxygen as a result of mixing with air. Therefore methaneenrichment has to be considered in order to produce qualified LNG (liquefied natural gas) products. After separation of oxygen by some safe methods, CBMbecomes mainly a mixture of nitrogen and methane. High concentration ofnitrogen brings out new issues for liquefaction technology.This paper focuses on the nitrogen-containing CBM which has beendeoxidized, emphasized on the effects of nitrogen content on the purificationand liquefaction technologies, and from point of system energy conservation,studied the efficient integrated liquefaction and methane enrichment processes.The main content can be divided into three areas.(1) For the purification, the presence of high content of nitrogen may leadto a lower solubility of CO2in the nitrogen-containing CBM liquefactionprocess than in the conventional natural gas liquefaction process, and thus theconventional CO2purification index may be no longer applicable. As a result,this paper studied the solubility of CO2in the nitrogen-containing CBMliquefaction conditions, and so provided basis for determining the CO2purification index.1) A solid-liquid equilibrium experimental apparatus using a static-analyticmethod with gas chromatographic analyses was designed, constructed and tested.Data for CO2solubility in CH4/N2mixtures at conditions of nitrogen-containingCBM liquefaction process were measured. It revealed that CO2solubility inCH4/N2mixtures are different from that in the pure methane: it changes slightly with nitrogen content at the same temperature, increasing firstly and thendecreasing when nitrogen content increases, and decreases rapidly along withtemperature. Liquefaction temperature decreases rapidly when nitrogen contentin CBM increases, making CO2solubility reduce to a very low value whenLCBM stored at atmospheric pressure, which asks for an improve for CO2purification standard. As a result, traditional CO2purification index can bedirectly referred for nitrogen-containing CBM only when obtaining LCBM at ahigher pressure.2) PR and SRK equations of state methods were used to calculate thesolubility of CO2in CH4/N2mixtures, and the results were compared withexperimental data. It is revealed that, due to the lack of accurate binaryinteraction coefficients for N2/CH4and N2/CO2, the deviation betweencalculation results and experimental results are very large when nitrogen contentis high.(2) For the liquefaction process, effects of nitrogen content on processselection, parameter setting, and system performance were investigated.1) Firstly, four classic processes were studied respectively, includingnitrogen expansion cycle with or without propane precooling (C3-NEC/NEC),and mixed refrigerant cycle with or without propane precooling(C3-MRC/MRC). The results revealed that: with a certain liquefaction rate fixed, when nitrogen content increases, unit power consumption firstly increasesrapidly and then slows down or even tends to decline. With a certain methanerecovery rate fixed, unit power consumption always increases along withnitrogen content. On the other hand, with a certain nitrogen content, there areoptimal values for both the liquefaction rate and the methane recovery ratewhich make the unit power consumption minimum. It is also found out that theliquefaction process itself can realize some certain of methane enrichment, butstill cannot reach the LNG product requirements.2) Based on the analysis results of each process, energy and exergyefficiencies were analyzed and compared for these four liquefaction processes.The results indicated that: liquefaction processes with mixed refrigerant havebetter energy efficiency as well as exergy efficiency than that with nitrogenrefrigerant. However, when nitrogen content of CBM is very high, mixedrefrigerant processes cannot achieve high liquefaction rate. The exergy loss inthe heat exchanger system is higher for the mixed refrigerant than for thenitrogen refrigerant. That means the mixed refrigerant processes have moreroom for optimization in the heat exchanger system. On the other hand, addingpropane pre-cooling in high-temperature regions can effectively reduce the heattransfer exergy loss, and thus improve the energy efficiency as well as theexergy efficiency. In addition, for nitrogen expansion liquefaction processes, when the liquefaction temperature is low, it is better for the nitrogen to befurther cooled before the second stage of expansion.(3) For the methane enrichment process, from the view of energyconservation, novel processes which integrate the two parts of liquefaction andmethane enrichment were proposed, and the energy conservation effect wasinvestigated.1) for the liquefaction-distillation route, by three aspects of energyintegration of the liquefaction and the distillation, the overall process can bereduced by20%in energy consumption; with the increase of nitrogen content,energy consumption growth for the overall process is still obvious; by reducingthe separated nitrogen purity, the overall energy consumption can be decreasedand still ensure a high methane recovery rate.2) For the adsorption-liquefaction route, novel processes which integratethe two parts of adsorption and liquefaction by utilizing the released nitrogenwith some residue pressure were proposed. Two utilization approaches weredesigned, and the effects of nitrogen content as well as residual pressure on theenergy conservation were investigated. One approach uses the released nitrogenwith residual pressure to directly expand and consequently produce cold energy,and to precool the concentrated CBM. This method can be used for a variety ofliquefaction processes. For CBM with high nitrogen content, the energy conservation effect is very significant even if the residual pressure of adsorptionis not high enough. Furthermore, the liquefaction process structure can besimplified for mixed refrigerant cycles. Another approach is designed fornitrogen expansion processes, which uses the released nitrogen with residualpressure as part of the refrigerant and performs a semi-open nitrogen expansionrefrigeration cycle. This approach can also achieve a significant energyconservation effect for CBM with high nitrogen content.