Methane and CO2 sorption hysteresis on coal: a critical review
Coalbed methane (CBM) recovery is a process of desorption rather than adsorption, and for CO2 sequestration, the sequestrated CO2 may desorb due to the reduction of gas pressure, it is necessary to understand the difference between adsorption and desorption processes, i.e., sorption hysteresis. On reviewing previous experimental studies on methane and CO2 sorption hysteresis, both methane and CO2 sorption hysteresis are observed extensively, which cannot be treated as causal phenomenon or experimental error. Only qualitative evaluations, such as 'no', 'weak', 'significant', have been suggested by other researchers, yet the mechanism is still an open question. In order to further understand this phenomenon, we propose a quantitative evaluation method and an improved hysteresis index. The improved hysteresis index (IHI) is calculated from the proportion between the hysteresis area of measured data and the hypothetical fully irreversible state. It can reflect the hysteresis tendency from the hypothetical fully reversible state to the irreversible state. Calculation results show that most IHI of subcritical CO2 sorption are from 0% to 40%, and the IHI of methane and supercritical CO2 sorption are relatively discrete compared with subcritical CO2 sorption. Subsequently, the controlling factors of hysteresis degree are examined by using the calculated IHI, and following relationships are observed: after drying the coal samples, the hysteresis degree of as-received coal decreases in most cases; the maximum pressure of CO2 has a strong positive controlling effect on the degree of hysteresis; the hysteresis degree of CO2 sorption is stronger than the hysteresis degree of methane sorption; the hysteresis degree is independent of individual coal properties, both for methane and CO2 sorption. By examining these results and previous experimental observations on sorption process, the mechanism of methane and CO2 sorption hysteresis is discussed and a possible explanation is suggested on the controlling effects of the observed factors, however further investigations are still needed. It is believed that methane sorption hysteresis is detrimental to CBM recovery because there are fewer minable reserves and lower permeability induced by hysteretic methane, whilst CO2 sorption hysteresis is beneficial for the long-term stability of CO2 sequestration. The critical desorption pressure may be overestimated by using adsorption isotherm, and both adsorption and desorption isotherms should be measured in future laboratory studies.