Investigation on the impact of water on the competitive adsorption characteristics of CH₄/CO₂ in coal

In order to study the effect of water on the competitive adsorption of CH₄/CO₂ and mixed gases in coal, the self-developed multi-gas competitive adsorption experimental system of coal containing gas was used to conduct the research on the single component adsorption of CH₄ and CO₂ by coal with different water content. The experimental results show that: Water does not change the basic law of the isothermal adsorption curve of CH₄ and CO₂, and under the same conditions, water does not affect the ordering of coal's adsorption capacity for a single component of gas, and the adsorption capacity of coal for CO₂ is always greater than that of CH₄. Under the same conditions, the inhibition rate of water on CH₄ is greater than that of CO₂ adsorption capacity, indicating that the inhibition degree of water on coal's adsorption of weakly adsorbent gases is greater. The research on the effect of water on the competitive adsorption of mixed gas shows that: when the proportion of gas injected into the coal remains unchanged, under the same adsorption equilibrium pressure, the total adsorption of mixed gas decreases with the increase of water content, and the adsorption amount of CH₄ and CO₂ after adsorption equilibrium decreases with the increase of water content in the coal. When the water content is unchanged, the greater the gas adsorption equilibrium pressure under the same gas injection ratio, the greater the adsorption amount of CH₄ and CO₂ after adsorption equilibrium. The volume fraction of CO₂ in the free phase is always lower than that of the gas source, while the volume fraction of CH₄ is always higher than that of the gas source. Under different conditions, the values of CO₂/CH₄ selection coefficients are greater than 1, ranging from 4.8 to 5.4, and the adsorption affinity of coal for CO₂ is greater than that of CH₄. The selection coefficients of CO₂/CH₄ decrease with the increase of water content of coal samples under the same gas adsorption conditions. This study further improves the theoretical analysis of factors affecting coal gas adsorption, and provides a theoretical basis for engineering practice for underground gas injection and gas extraction technology.