Experimental Study On Methane Separation From Low Concentration Coalbed Via Hydrate Formation In L-tryptophan +THF System

Coalbed methane is an unconventional source of natural gas stored in coal seams.The main combustible component is CH₄.In the case of the same heat,the amount of CO₂produced by coal-bed methane incineration is lower than that of coal and crude oil.The main combustible products are carbon dioxide and water,which is a kind of high-quality and clean energy.China has abundant reserves of coalbed methane,which can be used as an effective substitute for natural gas.Although the resources are abundant,the progress of commercial development and utilization of coalbed methane is still slow,especially the low-concentration coalbed methane discharged from coal mining can’t be effectively utilized.This is not only a huge waste of resources,but also increases the greenhouse effect and damages the environment.Therefore,the rational use of low concentration coalbed methane is of great significance to solve the current gas shortage,reduce the greenhouse effect and achieve the goal of"double carbon".Hydrate separation,purification,storage and transportation of methane from low concentration coalbed methane is a new gas separation technology emerging in recent years,which has broad application prospects.However,at present,the hydrate separation method of low concentration coalbed methane is still in the laboratory research stage due to its slow hydrate formation rate and low methane separation efficiency.In view of these problems and the above problems,this paper conducted relevant experimental studies on the separation and purification of low-concentration coalbed methane by gas hydrate method on the independently established test platform.The main research contents include:（1）30 mol%CH₄/70 mol%N₂was used to simulate low-concentration CBM,and l-tryptophan,a new environment-friendly biological promoter,was used to measure the effect of L-tryptophan on hydrate formation of low-concentration CBM at different concentrations.The results show that l-tryptophan plays an important role in hydrate production at a lower concentration,while it prevents hydrate production at a higher concentration.Meanwhile,the effect of L-tryptophan is more obvious in terms of kinetics,and it has no obvious influence on phase equilibrium conditions of hydrate generated at a lower concentration of coalbed methane.（2）In view of the harsh phase equilibrium conditions for hydrate formation from low-concentration coalbed methane,tetrahydrofuran（THF）was introduced as a thermodynamic accelerator to calculate the phase equilibrium conditions for hydrate formation from low-concentration coalbed methane in aqueous solutions with molar concentrations of 1 mol%,3.0mol%and 5.56 mol%THF.The results show that,THF aqueous solution can significantly reduce the phase equilibrium pressure of hydrate generated from low concentration coalbed methane,and THF molar concentration is its stoichiometric concentration 5.56 mol%,which has the best promoting effect.（3）The promotion effect of L-tryptophan and THF on low concentration CBM hydrate was studied.The results show that the addition of L-tryptophan in THF solution can increase the hydrate formation rate,further shorten the reaction time,and increase the hydrate production to a certain extent.Under the initial temperature of 274.15 K and initial pressure of10 MPa,the reaction t₉₀of the mixed system with L-tryptophan was reduced from 159 min to95 min,which was 40.25%shorter than that of the single THF system.（4）Subsequently,experiments on the formation of low-concentration CBM hydrate were carried out at different initial gas-liquid ratios,initial pressures and initial temperatures,and their kinetic characteristics were studied.It is found that the initial gas-liquid ratio has obvious influence on the dynamic characteristics of low concentration coalbed methane generation.With the decrease of the initial gas-liquid ratio,the hydrate production decreases,the separation factor and CH₄concentration in the hydrate also decrease.The initial gas-liquid ratio has the best effect on CH₄separation when it is near the theoretical stoichiometric water mixture number.The recovery of CH₄and the concentration of CH₄in hydrate increase with the increase of initial pressure,but the separation factor is not linearly correlated with pressure,and different pressures have different effects on hydrate formation of CH₄and N₂.With the increase of temperature,the production of low concentration CBM hydrate decreases gradually,but its influence is not as great as that of pressure.There is no significant difference in CH₄concentration in hydrate formed by low concentration coalbed methane under four temperature gradients.Comprehensive experimental results show that under the initial pressure of 10MPa,temperature of 274.15 K,300 ppm L-tryptophan+5.56 mol%THF system,the concentration of CH₄in the raw gas increases from 30 mol%to 67.5 mol%,and the recovery rate of CH₄reaches 62.04%.