Dong-Liang Zhong

Efficient purification of low-concentration coalbed methane (LCCBM) is critical for energy utilization and greenhouse gas emission reduction. Hydrate-based gas separation is a promising approach for this application, but its practical deployment is constrained by sluggish formation kinetics and limited CH4 selectivity. This study reports the synthesis of copper oxide-decorated graphite nanoparticles (CuO@GNP) via chemical coprecipitation and develops a ternary combined system integrating CuO@GNP as a composite promoter, rhamnolipid (Rha) as a biosurfactant, and tetrahydrofuran (THF) as a thermodynamic promoter for hydrate-based CH4 separation from LCCBM. Among the prepared materials, 20 wt % CuO@GNP exhibited the most favorable properties, with well-dispersed CuO nanoparticles on the GNP surface and the highest specific surface area (24.90 m2·g–1), a 72.6% increase over pristine GNP. Under the tested conditions of 277.15 K and 3.6 MPa, the 20 wt % CuO@GNP nanofluids enhanced normalized gas consumption from 9.4 to 11.3 mmol/mol water and improved the separation factor from 6.80 to 7.40 relative to the GNP system. Liquid-phase temperature profiling and in situ microscopic observations indicated that CuO@GNP promotes multisite nucleation, accelerates hydrate growth kinetics, and facilitates heat dissipation, thereby sustaining continuous hydrate formation. The CuO@GNP-Rha-THF ternary system provides an effective strategy for hydrate-based CH4 recovery and offers insights into the design of surface-engineered nanofluids for gas separation applications.

Adsorption-based gas separation is recognized as a promising and efficient method for the purification of low-concentration coalbed methane (LCCBM). However, the performance of activated carbons (AC) is limited by their strong surface hydrophobicity and the micropore blockage of mineral ashes. Conventional acid modification exhibits a low ash-removal efficiency due to the poor penetration of acid solutions. In this work, a stepwise modification method using sodium dodecyl sulfate (SDS) and H2SO4 was proposed to treat AC. Experiments including low-temperature liquid nitrogen adsorption, Fourier transform infrared spectroscopy (FT-IR), and dynamic breakthrough adsorption were carried out to investigate the effects of the stepwise modification on the pore structure, surface functional groups, and CH4/N2 adsorption-separation performance of the modified AC. The results indicate that SDS reduced the contact angle of the acid solution on the AC surface and promoted the penetration of H2SO4 into deep micropores, thereby enhancing the ash removal rate. At an H2SO4 concentration of 0.05 mol/L, the stepwise modification increased the pore volume and specific surface area, thereby providing more active sites for CH4 adsorption. The CH4 saturation adsorption capacity of the modified AC showed a 22.6% increase compared to that of the raw AC, while the N2 adsorption capacity decreased. Compared with the AC treated by pure water, SDS solution, or H2SO4 solution, the AC modified by the stepwise method exhibited a higher CH4/N2 separation factor and a longer CH4 breakthrough time. However, as the H2SO4 concentration was further increased, the CH4/N2 separation performance of the modified AC gradually declined. This study provides a theoretical reference for the efficient LCCBM purification using the SDS and H2SO4 stepwise modification method.

The solidified natural gas (SNG) technology presents a prospective strategy for CH₄ storage and transportation. Low gas storage capacity and slow formation rate remain the key challenges for its field applications. This study suggested a compound system of cyclopentane (CP) + graphite nanoparticle (GNP) nanofluid to enhance the formation kinetics of CH₄ hydrate. Results indicated that both gas consumption and hydrate formation rate were higher at a higher CP concentration, peaking at 14 wt%, where t₉₀ (the time to reach 90% of the final gas uptake) was 65.7 min, and the gas uptake reached 0.1346 mol/mol. However, an excessive CP (21 wt%) negatively affected CH₄ hydrate generation kinetics due to the excessive cage occupancy of CP in 5¹²6⁴ cavities. A lower temperature was determined to be more favorable for CH₄ hydrate formation within nanofluids, which was visually demonstrated by the denser hydrate crystals formed at 275.15 K. Moreover, storage stability analysis revealed that CH₄ hydrate formed in CP + GNP nanofluids can be preserved at atmospheric pressure and 268.15 K without significant decomposition. This work provides a superior scheme for hydrate-based CH₄ storage, offering great contributions to SNG technology advancement.

Coalbed methane (CBM) is an unconventional natural gas primarily stored in coal seams. The efficient recovery of CBM mainly depends on the desorption and diffusion process. In this study, a stepwise wetting–corrosion method employing a combination of surfactant (cocamidopropyl betaine) and acid (hydrochloric acid) was proposed to promote the desorption and diffusion of CBM. The microstructure and CH₄ desorption–diffusion characteristics of the coal samples treated with the stepwise wetting–corrosion method were evaluated at varying concentrations of cocamidopropyl betaine (CAB) and hydrochloric acid (HCl). The relationship between wettability, specific surface area, and CH₄ adsorption–desorption was identified, and the effect of pore connectivity on CH₄ diffusion was investigated. The results indicate that the stepwise wetting–corrosion treatment eliminated mineral blockages within the coal matrix, thereby clearing the microporous pathways and improving the overall pore connectivity for methane transport enhancement. By preventing the contact between the surfactant and the acid, the breakdown of surfactant molecules was inhibited. This enabled homogeneous acidizing throughout the coal matrix, which reduced the specific surface area and increased the methane desorption rate by 13.99%. In addition, a significant reduction in the mass transfer Biot number and a notable enhancement in methane diffusivity were obtained. Therefore, the stepwise wetting–corrosion method combining CAB and HCl shows a potential to increase gas production and will provide an alternative to traditional high-energy fracturing techniques, contributing to efficient and sustainable CBM extraction.