Investigation of adsorption and desorption of acetylene on hard coal samples from Polish mines

Abstract

Acetylene is a mine air component in addition to other hydrocarbons: ethylene, propylene, propane and ethane. Increased acetylene concentrations in the mine atmosphere are associated with its emission from a coal deposit at elevated temperatures caused by the self-heating of coal. This study investigates the sorption capacity of hard coals with respect to acetylene, and the acetylene volume is related to the proportion between carbon and oxygen as well as to moisture and porosity levels. The largest amounts of acetylene are adsorbed by low-rank coals with high oxygen content, highly porous and characterized by large specific surface areas and high moisture content. Higher rank coals displaying lower porosity tend to adsorb significantly smaller amounts of acetylene. These coals have a compact structure, and their behavior is non-polar due to the presence of a layer of chain hydrocarbons surrounding the aromatic coal polymer. The analysis of desorption isotherms indicates that the process of acetylene adsorption is irreversible. Desorption isotherms do not coincide with adsorption isotherms, forming an open hysteresis loop. Certain amounts of acetylene held more firmly to the coal structure are likely to remain there, their actual quantities depending on the coal type. The adsorption of acetylene is a polar process, involving the electron-donor and electron-acceptor centers' interactions with dipolar molecules of acetylene. The heat of the adsorption of acetylene based on the Clausius–Clapeyron equation ranges from 15 to 27kJ/mol, depending on the type of coal and the degree of coverage.In the consequence of acetylene adsorption, its concentration in mine air is reduced, which undermines the reliability of the self-heating hazard assessments.