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To investigate the adsorption properties of methane in coal under low temperatures, the isothermal adsorption tests of the three coal samples with different metamorphic degrees were conducted at the ambient temperatures of 253.15−293.15 K, and the low‐temperature nitrogen adsorption (LNA) tests of coal were also performed. Then a relational expression of equilibrium pressure, temperature, and methane adsorption capacity (T−P model) was deduced to predict the adsorption isotherm at any other temperature based on the Polanyi adsorption theory. The results show that the gas adsorption capacity of coal can be significantly increased at low temperatures (below 273.15 K), and the adsorbed methane in anthracite is obviously more than that in lean coal and coking coal by the virtue of possessing a larger micropore/transition pore volume and specific surface area. The relations between adsorption potential (ε) and adsorption volume (ω) at different temperatures can be drawn on one single logarithmic curve, and a suitable pseudo‐saturation pressure can be obtained by the improved Amankwah's method. The predicted adsorbed capacities via the T−P model are in line with the measured results at other equilibrium conditions, indicating that the model can contribute to the deep coalbed methane resources estimation and the gas disaster prevention and control in coal mines. [ABSTRACT FROM AUTHOR]

This work focused on thermodynamic analysis of carbon dioxide (CO2) adsorption on a promising biochar as a CO2 adsorbent. The biochar was prepared by catalytic pyrolysis of cellulose material in ionic liquid at moderate temperature. The adsorption characteristics, such as adsorption capacity, interfacial potential, Gibbs free energy change, enthalpy change, entropy change, and internal energy change, influenced by adsorption temperature and gas pressure, were systematically investigated. The results indicated that CO2 adsorption on cellulose‐derived biochar was a spontaneous, physical, exothermic, and entropic decrement process, accompanied by adsorption capacity of 5.2 mmol/g and interfacial potential of −18.2 J/g at 273 K and 100 kPa. The process could be well described by adsorption potential theory. Then a quasi‐Gaussian distribution of site energy was verified for CO2 adsorption. The interfacial potential was found to be a monotropic function of the amount of CO2 adsorbed, and the latter was actually a differential of the former via adsorption potential. The positive temperature effect and negative pressure effect on negative Gibbs free energy change indicated that reducing adsorption temperature and increasing gas pressure were beneficial to CO2 uptake, accompanied by the increase of adsorption capacity and the reduction of interfacial energy, entropy, enthalpy, and internal energy. The strongest temperature effects on entropy change, enthalpy change, and internal energy change existed at given pressure or temperature. The pressure effect was stronger and more sensitive to pressure at lower adsorption pressure. More interestingly, the peak pressure or peak temperature with the strongest pressure effect possibly existed during CO2 adsorption. [ABSTRACT FROM AUTHOR]

A promising biochar as solid adsorbent for CO2 uptake was prepared by the catalytic pyrolysis of coconut shell in moderate‐temperature ionic liquid (IL). Then, it was characterized by means of SEM, EDS, BPEA, BET, NLDFT, FTIR, and TG‐DSC, and a mechanism interpretation of the porous biochar formation was conducted. In addition, the adsorption characteristics of CO2 on the as‐prepared biochar, such as adsorption capacity, adsorption potential, isosteric heat, and static selectivity at different adsorption temperatures and pressures, were systematically evaluated. The results indicated that the as‐prepared biochar exhibited an adequate CO2 adsorption with a capacity of 4.5 mmol/g at 273 K and 100 kPa. Then, a significant number of slit‐like pores were revealed to exist on the as‐prepared biochar with a peak pore size between a range of 0.6 nm‐2 nm. The porous structure formation was ascribed to the release of carbon‐, hydrogen‐, oxygen‐, sulphur‐, and nitrogen‐containing compounds during biochar preparation. Meanwhile, both the adsorption potential and isosteric heat of the CO2 uptake under the tested conditions decreased with an increase in the adsorption capacity, which ranged from 33 kJ/mol‐21 kJ/mol and 23 kJ/mol‐7 kJ/mol, respectively. Therefore, the isosteric heat could be considered as a piecewise function of adsorption capacity. In addition, the molar ratios of CO2 over N2 adsorbed under the tested conditions were above 11 and were accompanied by molar ratio peaks of 26 at 273 K and 19 at 298 K, respectively. Moreover, an interesting phenomenon occurred: the static adsorptive selectivity of CO2 over N2 first increased and then decreased and there was an increase in the adsorption pressure at the tested adsorption temperatures. [ABSTRACT FROM AUTHOR]