Some aspects of influence of the composition of volatile products and extracted material on grain swelling processes and volume changes of commercial coals of different rank

Abstract The course of pyrolysis process of the three industrial samples of lower, middle, and higher rank coals was studied using the TG/FT-IR technique and a laboratory unit for investigation of carbonization process by X-raying. The coal samples from the zones of plastic layer were studied by scanning electron microscopy and next extracted by a mixture of chloroform and methanol. The obtained extracts were analyzed by FT-IR and UV spectroscopies and STEM-HAADF technique. It was stated that the composition of volatile products of a lower rank coal (LRC) is characterized by the greatest relative content of saturated and unsaturated hydrocarbons in comparison with a middle rank coal (MRC) and a higher rank coal (HRC). However, a higher contribution ratio of water vapour is observed in the composition of volatile matter of HRC. It is suggested that low molecular products of destruction formed inside swelling grains of MRC can react with each other and form new compounds with a greater molecular mass. These can be macromolecules of heteroorganic compounds. The STEM-HAADF images showed that there are topological structures of various types in the material of extracts from the gas-saturated zone of coals of different rank. The presence of inorganic elements in the extracted material changes depending on coal rank. Heterorganic compounds of different shape and composition can catalyze the reactions taking place during thermal destruction of coals in a selective way. In MRC, these reactions will cause a decrease in pressure inside its swelling grains and without their substantial growth in volume. Such reactions do not take place in grains of HRC. Moreover, an increased contribution of water vapour formed during destruction can cause its adsorption inside grains and an increase in volume of swelling grains. This can be the cause for an increase in generation of dangerous coking pressure in the charge of studied HRC.