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

Coalbed methane is mainly in an adsorption state. Coalbed methane is a disastrous gas that threatens the safety of coal mine production and a greenhouse gas that causes climate warming and is also an efficient and clean alternative to natural gas. Thermodynamics is mainly the study of the thermal properties of matter from the point of view of energy conversion. The study of coalbed methane adsorption with thermodynamics is an excellent cooperation between formal science and natural science. For many years, the Langmuir isothermal adsorption model has been used to study the influence of internal and external conditions on the adsorption characteristics of coal. Based on statistical thermodynamics, a temperature-pressure adsorption equation (TPAE) has been derived. The four parameters to be determined in TPAE can be obtained by regression of the measured adsorption experimental data, the isothermal adsorption that meets the national standard, and the temperature and pressure adsorption data that does not meet the national standard, including a temperature, a pressure, and an adsorption data defining as thermobaric adsorption. The TPAE has a minimum mean error and a minimum STDEV for the regression of this thermobaric adsorption data compared with Langmuir parameters. During adsorption, the enthalpy change can be calculated by Clausius-Clapeyron equation with a conclusion that adsorption is exothermic and can be carried out spontaneously. The unit isosteric enthalpy of adsorption (UIEA) decreases with the increase of adsorption capacity. The thermobaric adsorption data of lean coal has been used to illustrate the negative effect of temperature and the positive effect of pressure. The critical buried depth of the lean coal is 951m. The critical temperature at the depth is 316.52K. The critical pressure at the depth is 9.51MPa. The thermobaric adsorption data of coking coal has been used to show that only six sets of thermobaric adsorption data are needed to accurately represent the coal seam temperature of this lean coal from 18 to 72oC, the pressure from one megapascal to 19 megapascals, and the buried depth of 100 meters to 1,900 meters. The Langmuir parameters from the normal coal and deformed coal of Pingdingshan Coal Mine have been used to explain that, at a buried depth of 1,000 meters, because the UIAE of normal coal is -1.558 KJmol/cm3/g higher than that of deformed coal, it is adsorbed first. Because desorption is endothermic and cannot be carried out spontaneously, energy needs to be obtained from the outside, and the unit isosteric desorption enthalpy (UIDE) value of deformed coal is 0.64 KJmol/cm3/g, and priority desorption is preferred. Defining a new comprehensive index OT (outburst threshold) for coal and gas outburst thresholds by timing the unit isosteric desorption enthalpy (UIDE) with the square of solidity coefficient of coal (f), and describing that the UIDE of deformed coal is 0.64 KJmol/cm3/g and the solidity coefficient f of deformed coal is 0.5, so its comprehensive index OT is 0.16, close to zero, which means that the coal and gas outburst will occur very likely. The thermobaric adsorption data of long-flame coal and anthracite has been used to compare the difference in adsorption behavior between two kinds of coal. For long-flame coal, the partial differential of temperature has always been relatively positive and there has been no negative value since the beginning. Therefore, as the burial depth increases, the temperature rises, and the adsorption capacity of long-flame coal also increases. Two fundamental difficulties of the field test method are the non-direct measurability of the lost gas and the non-repeatability of the field test. Using the coal sample after the residual gas volume test as the coal sample of the adsorption method and completing a reverse cycle can ensure that the geological factors of the target coal seam remain unchanged. Under isobaric conditions, the temperature has affected the adsorption amount negatively. In other words, the adsorption amount will reduce with temperature increasing. Under isothermal conditions, pressure has affected the adsorption amount positively. In other words, the adsorption amount will increase with temperature increasing. Under isosteric condition, if T2 > T1, then P2 > P1.