Performed tests showed that at 298 K hard coals sorb relatively small amounts of hydrogen. Those amounts depend on carbon and oxygen content in tested coals. The most considerable amounts of hydrogen are sorbed by coals characterized by strong surface hydrophobicity and high content of aliphatic hydrocarbons. The hydrophilic nature of coal surface does not lead to higher sorption of hydrogen. It was found that the change in amount of sorbed hydrogen is closely related to the moisture. For high moisture coal a significant decrease in hydrogen sorption is observed. Also tests on hydrogen desorption on hard coals were carried out using method of lowering hydrogen pressure above the sorbent. Obtained results showed that tested coals desorb various amounts of hydrogen. Process of sorption is reversible only for some coals, while for the others the desorption isotherm partially lies beneath the sorption isotherm, which indicates that in addition to hydrogen some other chemical substances are emitted from coal.
Propylene is the unsaturated hydrocarbon emitted to the atmosphere of a coal mine in the process of coal selfheating. Its concentration in mine air is one of the indicators for the assessment of the development of the process of coal selfheating. Coal sorption capacities with respect to propylene may be the reason for the decrease in its concentration in mine air. This may affect the accuracy of the assessment of the self-heating process development, in particular, in the case of coals of considerable sorption capacities. The paper presents the results of coal sorption capacities with respect to propylene performed on 10 samples of bituminous coals acquired from operated coal seams. The sorption capacities of tested coal samples varied depending upon level of metamorphism, porosity, and chemical characteristics of a coal surface. The highest sorption capacities were reported for high-porosity coals of easily accessible pore structure, high specific surface area values, low level of metamorphism, and high oxygen content. The interactions between the electron-donor (and electron-acceptor) centers and π bond between the carbon atoms of the gaseous reactant probably play the most important role in the sorption of propylene on coals. Coals of low oxygen content, nonpolar surface structure, low porosity, and compact internal structure are characterized by low propylene sorption capacity. The results of the experimental study also indicate the partially irreversible character of the sorption process reflected in the open isotherm hysteresis. Some amount of propylene molecules remains in the coal structure, depending upon the coal properties and the experimental conditions applied.
Propylene and acetylene are released to mine air with the increase in the temperature of self-heating coal. Concentrations of these gases in mine air are applied as indicators of the progress of the self-heating process. Hydrocarbons emitted from the self-ignition center are sorbed on coal, while migrating through the mine workings. Coal crushed during the mining process is characterized by a high sorption capacity, which facilitates the sorption phenomena. This results in the decrease in hydrocarbons content in mine air, and in the subsequent incorrect assessment of the development of the self-heating process. The results of the experimental study on propylene and acetylene sorption on Polish coals acquired from operating coal mines are presented in this paper. Bituminous coal is characterized by a high sorption capacity with respect to unsaturated hydrocarbons, like propylene and acetylene. The sorbed volumes depend on the grade of metamorphism, porosity, and chemical characteristics of coal. Low level of metamorphism, increased porosity, and oxygen content result in higher sorption capacity of coals. The reduction in grain size of coals also results in the increased sorption capacity with respect to hydrocarbons. The most significant increase in the volumes of sorbed propylene and acetylene with the decrease in grain class was observed for coals of low porosity, high grade of metamorphism, and low to medium sorption capacities. The 10-fold decrease in coal grain size resulted in the 3 to 6-fold increase in the volume of sorbed propylene, and 2-fold increase for acetylene. The decrease in grain size results in higher accessibility of pore structure, increased pore volume and area, and higher number of active centers interacting with hydrocarbons of dipole characteristics. For coals with low grade metamorphism, high porosity, and high sorption capacity the volumes of sorbed propylene and acetylene increased only slightly with the decrease in coal grain size.
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