Insights into the HCl formation and volatilization mechanism from organochlorine in coal: A DFT study

The elimination of hydrogen chloride (HCl) in flue gas is crucial for pollutants control and equipment safe operation in coal-fired power plant. However, the evolution and volatilization mechanism of organochlorine (Clorg) in coal to flue gas HCl during coal combustion is currently ambiguous. To solve this issue, organochlorine existing forms in various possible circumstances upon coal molecule are constructed using cluster models. Based on these models, the reaction paths, the thermodynamic and kinetic parameters of HCl generation reactions are explored deeply by DFT. To explore the HCl volatilization mechanism, the natural binding of generated HCl with distinct chemical groups in coal is studied with the examination of binding energy, electron density difference, and interaction region indicator, etc. The volatilization order of HCl at different active site is discussed by the ab initio molecular dynamics. The results show that methyl chloride is the most stable occurrence form of Clorg, and most Clorg in coal could be converted to HCl with moderate energy barriers within 100 kJ/mol. Different active sites inside coal possess various affinity for HCl that arises from their diverse bonding types of van der Waals interaction, hydrogen bond, and chemical bond. The combination of binding analysis and AIMD analysis indicates that the chemical groups on the coal surface such as oxygen-containing groups, thiophene sulfur, and pyridine nitrogen enhance the affinity of coal to HCl, causing an increase of the HCl volatilization temperature from 300 to 400 K to 700–800 K, thus impelling the HCl release characteristics complex in coal combustion process.