CO2 and CH4 adsorption on different rank coals: A thermodynamics study of surface potential, Gibbs free energy change and entropy loss

By Xidong Du, Yugang Cheng, Zhenjian Liu, Hong Yin, Tengfei Wu, Liang Huo, Couxian Shu. 2021

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Abstract In this paper, the pore structures of three different rank coals sampled from China (anthracite, bituminous coal and lignite) were characterized by CO2 and N2 adsorption. The isothermal adsorption curves of CO2 and CH4 on three samples were measured by gravimetric method and fitted by Langmuir model. The preferential selectivity (αCO2/CH4) was calculated using the Langmuir parameters of CO2 and CH4, and the Henry’s coefficient (KH) was obtained with the help of virial equation. More importantly, a comparative analysis of adsorption thermodynamics of CO2 and CH4 on three different rank coals, including surface potential (Ω), Gibbs free energy change (ΔG) and entropy loss (ΔS), was presented according to the adsorption data. It is found that the uptakes of CO2 and CH4 on anthracite are the largest, followed by lignite and bituminous coal in sequence. αCO2/CH4 increases with the increase of coal rank. Low temperature helps injected CO2 to displace pre-adsorbed CH4. The KH values on anthracite are the biggest, while KH values on bituminous coal are the smallest. Ω, ΔG and ΔS of CO2 and CH4 all exhibit a U-shaped function with maturity. Anthracite has the highest Ω, ΔG and ΔS, while bituminous coal has the lowest Ω, ΔG and ΔS. The thermodynamics parameters of Ω, ΔG and ΔS are affected by pore size distributions of three coals. Ω, ΔG and ΔS of CH4 are smaller than those of CO2. CO2 adsorption on coal is more favorable and spontaneous, and adsorbed CO2 molecules form a more efficient packing on coal.

Subfields: Global and Planetary Change, Mechanics of Materials, Ocean Engineering

View Paper: https://www.semanticscholar.org/paper/17b3d9e06a1f81ce6bcf05758703c12360d09e26