Experimental Study On Methane Separation From Coalbed Methane Via Gas Hydrate Formation

Coalbed methane is a kind of high quality clean energy,and its main component is CH4.The resource of coalbed methane is greatly abundant in China.But the low utilization rate of the coalbed methane,especially the low concentration coalbed methane,not only caused a lot of waste of energy,but also exacerbated the greenhouse effect.Therefore,it is great significative to use the low concentration coalbed methane effectively for slowing down the contradiction between energy supply and demand and for speeding up the adjustment of energy structure.A method,CH4 seperation from low concentration coalbed methane by hydrate formation is a important way to use coalbed gas efficiently,which has broad application prospects.However,this method is still in the experimental stage.The slow reaction rate,low recovery rate of CH4 and low separation factor are the key problems to restrict this technology applying to industry.In addition,whether or not the method has economic benefits is the key factor whether the technology can be applied to practice.In this work,aiming at the key problems,the experiment of the hydrate formed with low concentration coalbed methane is studied and economical efficiency of this method is analysed.The main research works are summarized as follows:(1)In this work,aiming at the severe phase equilibrium of the coalbed methane gas hydrate,phase equilibrium data of TBAC semiclathrate hydrate formed with the coalbed methane were measured at three TBAC concentrations(0.49 mol%,1.0 mol%,and 3.3 mol%)and were reported.The result shows that TBAC can obviously rise the phase equilibrium temperature and reduce the phase equilibrium pressure of the coalbed methane hydrate formation.And with the increase of TBAC concentration,the phase equilibrium temperature increased at the same pressure.This can save the energy consumption of low temperature environment.(2)The high pressure-micro differential scanning calorimeter(HP-MicroDSC)was employed to characterize the dissociation and to measure the dissociation enthalpies of TBAC semiclathrate hydrate formed at 3.3 mol% TBAC in the presence of coal mine gas.It was found that,in the 3.3 mol% TBAC solution,the water was involved in the formation of hydrate,which is proved that 3.3 mol% is the stoichiometric concentration of pure TBAC semiclathrate hydrate.(3)The purification characteristics of low concentration coalbed methane at different TBAC concentrations(0.49 mol%,1.0 mol%,3.3 mol%)were studied.The result shows that the best effects of the purification of low concentration coalbed methane were in the 0.49 mol% TBAC solution.The concentration of CH4 increased from 30% to 42.8%,and the recovery of CH4 was about 16.5%.Then,the purification characteristics of low concentration coalbed methane at different subcooling degree(7 K,8 K and 9 K)in the 0.49 mol% TBAC solution were studied.It was found that 8 K is the optimum subcooling degree for purifying low concentration coalbed methane.Under the condition of optimum subcooling degree,the effects of the concentration of surfactant(sodium dodecyl sulfate,SDS)on the purification performance of low concentration coalbed methane were studied.It was found that,compared with other SDS concentrations(300 ppm,500 ppm),900 ppm SDS had the best effect on the purification of low concentration coalbed methane.The concentration of CH4 increased from 30% to 49.8%,and the recovery of CH4 was about 25.6%.The result also shows that CH4 concentration of the gas mixture released from the TBAC hydrate(49.8 mol%)was greatly increased as compared to that obtained at 0.29 mol% TBAB(42.5 mol%).(4)A model of economics of exergy has been built and been used for a study of an experimental system of purification of low concentration coalbed methane by hydrate formation.The result indicated that,on the best effect of two levels of CH4 seperation from the low concentration coalbed methane,the chemical exergy of coalbed methane is increased by 140%,but the exergic efficiency for purpose is only 21%.The loss factor of exergy decrease with increasing of CH4 recovery,while the exergic efficiency for purpose present an accelerated growth tendency.According to the highest CH4 recovery obtained from experiment,the cost price of the product gas is higher than its profit price nearly three times,which results in the method having no economic value.When CH4 recovery reaches 78%,it will present a certain economic value by this method.