Thermodynamics Study on the Coal Bed Methane Adsorption Characteristic, Edition 1

Coalbed methane is an important unconventional natural gas resource. China is a big country in terms of coal resources, and coalbed methane resources are also very abundant. The development and utilization of coalbed methane are of great significance to making full use of clean energy, improving the safe production conditions of coal mines, and protecting the atmospheric environment on which human beings depend. Since the 80s of the 20th century, China has carried out experimental research and development of modern coalbed methane technology. The adsorption characteristics of coalbed methane and the mechanism of coal gas storage are extremely important for the scientific development and utilization of coalbed methane. Most of the previous studies were carried out by isothermal adsorption according to the Langmuir equation. Starting from statistical thermodynamics, this book derives a four-parameter temperature-pressure adsorption equation, or TPAE for short, so that temperature becomes a variable in the adsorption equation. These four parameters can be obtained by regression of the measured temperature and pressure adsorption data. Published measured temperature and pressure adsorption data confirm that TPAE is more practical, simple and accurate than Langmuir’s equation. Using the Clausius-Clapeyron equation, the TPAE calculation results confirm that the isosteric adsorption enthalpy of coalbed methane is negative, that is, the adsorption process is an exothermic process that can be carried out spontaneously. The isosteric enthalpy of desorption of coalbed methane is positive, that is, the desorption process is an endothermic process that cannot be carried out spontaneously. The full differential of TPAE was used to accurately calculate the burial depth and adsorption amount of coal adsorption maximums under the dual action of temperature and pressure. It was confirmed that the unit isosteric adsorption enthalpy of normal coal and deformed coal decreased with the increase of adsorption amount. However, at the same temperature and pressure, normal coal is preferentially adsorbed, and deformed coal is preferentially desorbed. From the perspective of thermodynamics, the calculation of coal and gas outburst thresholds combining thermodynamics and coal firmness coefficient is proposed. Through the calculation of the thermobaric adsorption data of long-flame coal, it is concluded that the increase of burial depth, temperature and pressure only has a positive effect on long-flame coal, that is, there is no adsorption maximum value of long-flame coal. At the same time, a suggestion of using thermobaric adsorption experiments to treat the adsorption test samples of coal core desorption experiments in coal boreholes in geological exploration is proposed.