Selective adsorption of CO₂/CH₄ separation performance of porous lignite char

The preparation of carbon-based adsorbent materials from lignite has garnered significant attention from researchers for its potential applications in gas adsorption and separation. This adsorption separation method is characterized by its ease of operation, relatively low energy consumption, and capability for continuous operation. However, it also presents limitations, as the adsorbent's selectivity, adsorption capacity, and operating cost can significantly influence its efficiency and economic viability in practical applications. Consequently, the development of green and efficient adsorbents with enhanced recyclability is of paramount importance preparation of adsorbent materials with high capacities for CO₂ and CH₄ separation and adsorption using lignite as a raw material not only facilitates effective CO₂ capture but also enhances the non-energy utilization of lignite. This study produced coke samples through pyrolysis at 600, 700, 800, and 900 ℃, along with thermal treatment durations of 1, 2, and 3 hours, using lignite sourced from Manglai in a vertical tube furnace. The dynamic penetration curves of the adsorption process were obtained by evaluating the adsorption and separation performances of the prepared samples for CO₂/CH₄ mixtures (simulating natural gas with a low concentration of CO₂) utilizing a multi-component competitive adsorption analyzer. The results indicated that the CO₂ and CH₄ adsorption capacities of the coke sample (ML-8002), treated at 800 ℃ for 2 hours under conditions of 25 ℃ and 0.1 MPa, reached 0.48 and 0.70 mmol/g per unit mass, respectively. The separation coefficient was 6.12, and the separation time per unit mass was as high as 418 s/g, demonstrating a favorable separation effect. Moreover, increasing the adsorption process pressure significantly enhanced the separation efficiency. Specifically, under conditions of 25 ℃ and 1 MPa, the sample's adsorption capacities for CO₂ and CH₄ reached 1.98 and 6.63 mmol/g per unit mass, respectively, indicating optimal selective adsorption separation. This material shows promise for applications in variable-pressure environments for the adsorption separation of CO₂ and CH₄ gases. The combined thermodynamic analysis and Langmuir-Freundlich model kinetic fitting results indicated that the heat of adsorption for each sample was less than 40 kJ/mol. Furthermore, the R² values of the CO₂ adsorption isotherms were all greater than 0.99, demonstrating a high degree of fit. This leads to the conclusion that the heat of adsorption for CO₂ is below the threshold for chemical adsorption, suggesting that the adsorption of CO₂/CH₄ is primarily governed by physical adsorption. The influence of coal coke surface functional groups on adsorption performance was investigated through the analysis of lignite coke surface functional groups. The nitrogen adsorption and desorption tests, along with scanning electron microscopy analysis of the ML-8002 coke samples, revealed that the formation of a more selective adsorption capacity for CO₂ is attributed to a microporous structure, which is predominantly characterized by pores measuring between 0.6 and 1 nm. This microporous structure accounts for more than 70% of the molecular diameter of CO₂, allowing the gas to enter the developed internal voids through the lignite coke’s macroporous structure. This configuration is more favorable for the selective adsorption and separation of CO₂ from CH₄. Consequently, lignite pyrolysis coke can be an effective material for CO₂/CH₄ adsorption and separation.