Experimental Study On The Enhancement Of Low-concentration Coalbed Methane Purification Using Graphene Oxide Nanofluids

Coalbed methane(CBM)is the associated energy generated during coal formation.The main component is CH₄,which is a high-quality clean energy.The reserves of coalbed methane resources in China are equivalent to those of conventional natural gas.The storage methods are mostly adsorbed in coal seams,and the mining methods are mainly extracted by drilling wells.However,the concentration of coalbed methane extracted underground is extremely unstable(15?60mol%).In order to ensure the safety of coal mining,a large amount of low-concentration coalbed methane cannot be directly used and can only be discharged into the atmosphere.This not only causes energy waste,but also aggravates the greenhouse effect.Therefore,improving the utilization rate of low-concentration coalbed methane is a key issue that needs to be solved urgently in the development of coalbed methane.The separation and purification of low-concentration coalbed methane by gas hydrate method is one of the effective ways to realize the efficient use of low-concentration coalbed methane.However,slow gas hydrate formation rate,low CH₄separation efficiency and high energy consumption are the key factors restricting the application of this technology.In response to these key issues,this paper studies the effects of different graphene oxide nanoparticles(GONs)concentration,cyclopentane(CP)concentration and different initial pressures on the formation and decomposition characteristics of coalbed methane hydrate,focusing on the analysis of coalbed methane hydrate formation in the process of gas consumption,CH₄ separation efficiency changes and coal bed methane purification mechanism,the main research work includes:(1)In order to improve CH₄ separation from low-concentration coalbed methane(LCCBM)via gas hydrate formation.The effect of GO nanofluids on low-concentration coalbed methane purification was evaluated by performing experiments at different concentrations of graphene oxide nanoparticles(GONs).It was found that both the rates of hydrate formation and gas consumption were enhanced when graphene oxide nanoparticles were added to the liquid phase.Moreover,CH₄ separation efficiency was greatly increased as compared to the systems without GONs.The CH₄ recovery ratio(R_CH4)reached a maximum value of 59.3% and the largest separation factor was gained at 500 ppm GONs among the three GONs concentrations tested in this work.This comparison implies that 500 ppm GONs is a preferable concentration for CH₄separation from LCCBM using gas hydrate formation in GO nanofluids.(2)Aiming at the problem of harsh hydrate formation conditions and high energy consumption,the effects of CP concentrations(7 wt%,14 wt%,21 wt%)on the purification characteristics of coalbed methane were studied in 500ppm GONs.The results of the study found that increasing the concentration of CP concentrations can accelerate the rates of hydrate formation and increase gas consumption,but the CH₄separation efficiency was reduced.The 7 wt%CP solution system has the best separation effect and can maintain a faster hydrate reaction rate,which is the best concentration among all tested CP concentrations.The effects of different initial pressures on the purification characteristics of coalbed methane were studied in500 ppm GONs and 7 wt%CP solutions.The study found that when the pressure was increased,the kinetics of hydrate formation was enhanced,the growth rate and gas consumption were greatly increased,and the induction time was effectively reduced,but the CH₄ separation efficiency drops sharply.Therefore,the experimental conditions of286.6 K,2.0 MPa,and 500 ppm GONs were preferred for CH₄separation from low-concentration coalbed methane.Based on the best one-stage hydrate-based separation conditions,two-stage hydrate-based separation were studied.The study found that the CH₄ concentration in low-concentration coalbed methane was increased from 30 mol%to 76 mol%after two-stage hydrate-based separation.Therefore,the GO nanofluids have a great potential to improve the kinetics of hydrate formation as well as the purification efficiency of low-concentration coalbed methane.(3)From the microscopic scale,the microscopic growth behavior and morphological characteristics of low concentration coalbed methane hydrate under different pressure conditions were studied.The study found that under 2.0 MPa,the hydrate mainly grows into the gas phase space after nucleation at the gas/liquid interface,the growth rate is relatively slow,and the hydrate form is fine sand-like;As the pressure increases,hydrates have multiple nucleation points at both the liquid phase and the gas/liquid interface.Once nucleated,it rapidly grows into the gas and liquid space.At 5.2 MPa,the hydrates in the liquid phase are converted into coarse sand due to a large amount of accumulation.This indicates that the higher the pressure,the more obvious the effect of promoting the formation of gas hydrates;destroying the hydrate film at the gas/liquid interface can ensure the continuous formation of hydrates.(4)In-situ Raman spectroscopy was used to characterize the structural changes during the formation of low-concentration coalbed methane hydrates.The study found that in the CP system of 3.6 MPa and 286.6 K,the low concentration coalbed methane hydrate is type II hydrate,CH₄molecules and N₂molecules compete to enter the hydrate cage,and CP molecules occupy the hydrate cage.When the hydrate reaction progresses from 5min to 30min,the characteristic peak intensity increase of CH₄ hydrate is much greater than that of N₂ hydrate,which indicated that CH₄ molecules entered the hydrate cage preferentially over N₂ and O₂ molecules.