Molecular Simulation Study on the Effect of Coal Rank and Moisture on CO₂/CH₄Competitive Adsorption

Abstract

The CO 2 -enhanced coalbed methane recovery (CO 2 -ECBM) technique is based on competitive adsorption. In this study, three models of different coal ranks were established using the molecular dynamics (MD) method. A combination of MD and grand canonical Monte Carlo (GCMC) simulations was used to investigate the competitive adsorption of CO 2 /CH 4 on dry and moist coal. The effects of coal rank and moisture content on pore structure, chemical structure, mixed gas adsorption capacity, and adsorption selectivity are discussed in detail. Simulation results show that from low-to high-rank coals, the total pore volume, porosity, and proportion of effective pores increase, which leads to an increase in their adsorption capacity. In addition, the oxygen-containing functional groups on the pore surface of coal enhance the displacement effect of CO 2 on CH 4 , and with an increase in coal rank the adsorption selectivity of CO 2 /CH 4 decreases. Moreover, the adsorption capacity of CO 2 and CH 4 will decrease owing to moisture. Water molecules will preferentially occupy the high-energy adsorption sites on the pore surface of coal, and then hydrogen bonding and capillary condensation will form water clusters. Therefore, in the case of moist coal, the adsorption selectivity of CO 2 to CH 4 fluctuates and shows different patterns of variation according to the different effective pore volumes of different coal ranks. From the perspective of CO 2 -ECBM, achieving a certain moisture content can have a beneficial effect, and the optimal moisture content of medium-and high-rank coal should be higher than that of low-rank coal. Under low-pressure conditions, the adsorption selectivity of CO 2 /CH 4 of dry and moist coal is larger than that of high-pressure conditions. In our work, we advance the understanding of the microscopic mechanism of competitive adsorption of CO 2 /CH 4 , which provides a theoretical basis for improving CO 2 -ECBM technology.

Effect of the Pyrrolic Nitrogen Functional Group in the Selective Adsorption of CO2: GCMC, MD, and DFT Studies

“…One of the common and efficient tools to study the molecular system’s interactions is the radial distribution function (RDF). , To be more specific, this method is used to investigate the possibility of the presence of a chemical type (α) around the other chemical moiety (β) at a specific distance ( r ) from each other. The mathematical definition for RDF is as follows …”

“…In another study, the selective adsorption and selective diffusion of the CO 2 −CH 4 binary mixture in bituminous coal were evaluated by Zhao et al 15 Using GCMC and MD methods, they studied the impacts of temperature, pressure, and bulk mole fraction and suggested an optimal injection depth for CO 2 -ECBM. Li et al 18 used a combination of the MD method and GCMC simulation to investigate the effects of coal rank and moisture on the competitive adsorption of the CO 2 −CH 4 mixture in three different coals. They reported that oxygen-containing functional groups positively impact the selective adsorption of CO 2 , while the presence of moisture has a negative effect on the adsorption capacity.…”

Computational Study on the Microscopic Adsorption Characteristics of Linear Alkylbenzene Sulfonates with Different Chain Lengths on Anthracite Surface

“…With the development of science and technology, it is becoming a reality to use computer simulations to explain the macroscopic properties from a microscopic point of view [51][52][53][54][55][56][57][58][59]. In particular, MD simulation has proven to be a useful tool for analyzing coal interface properties at the molecular level [60][61][62][63][64][65][66]. Traditional experiments screen flotation agents and dust suppression agents by trial-and-error method.…”

A dynamic evolution model of coal permeability during enhanced coalbed methane recovery by N2 injection: experimental observations and numerical simulation

“…That is due to the adsorption capacity of the coal matrix on N 2 is weaker than that on CH 4 , thereby CH 4 cannot be displaced through competitive adsorption. 27 Instead, CH 4 desorption is promoted through reducing the CH 4 partial pressure. Previous studies were mainly focused on the effect of CH 4 displacement, and there is a lack of investigation on the dynamic evolution of coal permeability during CH 4 displacement by N 2 injection.…”