Adsorption isotherms and kinetics of carbon dioxide on Chinese dry coal over a wide pressure range

Modeling of Supercritical CO2 Adsorption for Low-Permeability Coal Seam of Huainan-Huaibei Coalfield, China

Investigating the coal adsorption behavior on supercritical CO2 (ScCO2) is crucial for long-term CO2 geological storage. In this paper, low-permeability coal samples from the Huainan-Huaibei coalfields in China were selected. The high-pressure isothermal adsorption of CO2 was carried out at 36, 42, and 48 °C. The results of adsorption experiments were analyzed by fitting 9 types of modified adsorption models, including three different adsorption theories. Considering that different adsorption mechanisms may exist for CO2 in coal, 14 mixed adsorption models were established. The accuracy of the coefficient of determination (R2) and root-mean-square error (RMSE) for ScCO2 excess adsorption capacity was analyzed, mainly focusing on the accuracy of the key model parameters such as the adsorption phase density and the theoretical adsorption capacity. These parameters were discussed, combined with the predicted adsorption phase density of CO2 based on the intercept method. The results indicate that among the 9 types of modified adsorption considered, based on the adsorption phase density screening, the deviation of the predicted adsorption capacity from the experimental value was then considered. The Dubinin-Radushkevich (DR) model can effectively fit the adsorption behavior of CO2 at low pressure (<7.5 MPa). The Langmuir (L), Langmuir-Freundlich (LF), Extended-Langmuir (EL), and TOTH models can effectively fit the adsorption behavior of CO2 at high pressure (7.5-20 MPa), while the multimolecular layer models were unsuitable for fitting ScCO2 adsorption. The model fitting results showed that only the monomolecular layer and micropore-filled adsorption models were suitable for fitting the ScCO2 adsorption capacity. The DR-LF model best fits the adsorption data based on its key parameters of adsorption phase density and theoretical adsorption capacity. The established mixed model DR-LF fitting results showed that the CO2 in coal was dominated by microporous filling adsorption. The higher the temperature, the greater the contribution of microporous filling adsorption to the total adsorption. There still exists deviation in the adsorption phase density and theoretical adsorption capacity. The contribution percentage of different adsorption mechanisms of CO2 in coal needs to be further investigated.

A review of common practices in gravimetric and volumetric adsorption kinetic experiments

The availability of commercial gravimetric and volumetric systems for the measurement of adsorption equilibrium has seen also a growth of the use of these instruments to measure adsorption kinetics. A review of publications from the past 20 years has been used to assess common practice in 180 cases. There are worrying trends observed, such as lack of information on the actual conditions used in the experiment and the fact that the analysis of the data is often based on models that do not apply to the experimental systems used. To provide guidance to users of these techniques this contribution is divided into two parts: a discussion of the appropriate models to describe diffusion in porous materials is presented for different gravimetric and volumetric systems, followed by a structured discussion of the main trends in common practice uncovered reviewing a large number of recent publications. We conclude with recommendations for best practice to avoid incorrect interpretation of these experiments.

Experimental study of the moisture content influence on CH4 adsorption and deformation characteristics of cylindrical bituminous coal core

Moisture content in coal is an important factor affecting the coal seam gas extraction. It directly affects the storage and flow of gas in bituminous coal. In this paper, the cylindrical bituminous coal cores of Xutuan coal mine in Huaibei coal mine group were studied as experimental objects, using the laboratory self-designed experimental device Gas Adsorption and Strain Testing Apparatus system. The influence of the bituminous coal moisture content on gas adsorption characteristics was studied. Drying experiments of coal samples showed that they lose the original moisture content following the exponential decay function of time. At wetting, the saturated moisture content in coal samples increased following the Exponential Association function of time. The experimental results show that the average original moisture content and average saturated moisture content of raw coal samples are 1.3 and 2.4%, respectively. On this basis, the gas adsorption experiments on samples with different moisture contents under different gas pressures were carried out. With the moisture content increase, the gas adsorption capacity and saturation value decreased and the decrease rate gradually reduced. The single exponential decay function describes the gas adsorption capacity dependence on moisture content. Moisture content also affects the adsorption deformation of bituminous coal. At high moisture content, the adsorption deformation of bituminous coal is less.