Tests of ethylene and propylene mixture adsorption and desorption

Unsaturated hydrocarbons—ethylene and propylene—are components of the gas mixture that is released from self-heating coal hot spots together with the increase in their temperature. The concentrations of these hydrocarbons in mine air are used to determine fire hazard indices that allow assessing the development of the coal self-heating process. During the migration of the gas mixture through goafs, the hydrocarbons undergo adsorption on the coal, which influences their concentrations in collected mine air samples and may be the cause of incorrect fire hazard assessment. This paper presents the results of adsorption capacity and adsorption kinetics testing as well as desorption testing of ethylene and propylene mixtures with methane as dominant mine gas component. The experiments were conducted on five samples of Polish hard coals. The quantities of the adsorbed mixtures rise together with the increase in pore volume and internal coal structure availability as well as with the decrease in the coal metamorphism. In most cases, the tested coals adsorb larger amounts of propylene–methane than of ethylene–methane mixtures. The higher amounts of adsorbed mixtures with propylene resulted from the greater reactivity and stronger interactions of propylene molecules with the reactive oxygen groups present on the coal surfaces. The propylene mixture adsorption is also a longer process. Ethylene mixtures exhibit greater desorption from coals than propylene mixtures. Propylene is more difficult to release from coals due to the strength of the bonding, as well as the size and shape of its molecule.