Calculation methods on methane adsorption phase density in coal: A critical review and new insights

Adsorption phase density is a bridge to accurately reflect the absolute adsorption amount of adsorption characteristics. Although the calculation method of adsorption phase density has been widely studied, its accuracy has not yet reached a consensus. This paper systematically reviewed and summarized the previous calculation methods of adsorption phase density, including empirical methods, indirect methods, and direct methods. The indirect methods consist of fitting method and graphical method, while the direct methods include nuclear magnetic resonance (NMR) method and adsorption phase volume method. Through the analysis of the results of these methods, it is found that the calculation data basis for the fitting and graphical methods is the high-pressure adsorption isotherm data, and the measurement method, data processing, and theoretical model selection all affect the accuracy of the calculation results. In the direct methods, the key aspect of the NMR method is the determination of absolute adsorption quantity. Accurately determining the adsorption space is a challenge in the adsorption phase volume method. Considering the complexity of the pore system in coal, this paper provided a detailed description of a direct calculation method for the adsorption phase density that takes into account multistage adsorption behavior. This method was based on microporous filling and surface coverage theory, combined with fluid intrusion pore measurement experiments to obtain pore volume information in the range of 0.38-1.5 nm and surface area information for pores larger than 1.5 nm. This information was used to determine both an adsorption space distribution model and an adsorption behavior distribution model, which allowed the adsorption phase density distribution model to be obtained according to the defining equation of adsorption phase density. And the results of high-pressure adsorption experiments were used as the verification basis. The adsorption phase density obtained by this method is 0.36 g/cm3, which can be used as a theoretical upper limit standard for evaluating the adsorption phase density. Therefore, this paper recommends the use of Van der Waals density as the adsorption phase density in the empirical method. Finally, this paper outlined the research ideas on the influence of temperature and pressure on the adsorption phase density.