Physicochemical and Sorption Properties of Natural Coal Samples with Various Degrees of Metamorphism

Porosity of eight samples of natural coals was measured in which, with increasing degree of metamorphism, the content of carbon in the organic mass grows from 80 to 93%, and that of oxygen decreases from 14 to 2%. The methods of low-temperature sorption of nitrogen and mercury porosimetry were used, and the isotherm of water sorption was examined (+27 degrees C). Each method was used to calculate the pore volume and the monolayer capacity in sorption of water and sorption of nitrogen, and the pore surface area. The Dent sorption equation was used to calculate the amounts of firmly and weakly bound water in each sample. It was shown that that the amount of strongly bound water monotonically depends on the amount of oxygen in the organic mass. For the natural coal samples under study, the volume of sorbed water is close to the total pore volume determined by the method of mercury porosimetry and is an order of magnitude larger than the pore volume measured by the method of low-temperature nitrogen sorption. Reasons for this discrepancy are discussed.