Interactions between organic nitrogen and inorganic matter in the pyrolysis zone of underground coal gasification: Insights from controlled pyrolysis experiments

In recent years, a variety of N-heterocyclic compounds and inorganic nitrogen species were identified during the field-scale experiment on underground coal gasification (UCG), leading to concerns about the environmental impact. In an attempt to unravel the kinetics responsible for the generation of various nitrogen compounds (NCs), carbazole is selected in this paper as a model organic nitrogen compound in coal matrix. Hydrous pyrolysis experiments on carbazole were conducted using an autoclave in the presence and absence of minerals or their main active components in coal. Effects of inorganic matter, reaction mechanism and chemical kinetics were investigated on the basis of the experimental data. The results demonstrate negligible influence of water on thermal destruction of carbazole. However, the combination of water and aluminium oxide may play a role in the N - O atom exchange, resulting in a new ammonia formation mechanism. Pyrite exhibits extremely higher reactivity during carbazole cracking than other minerals. The calculated apparent activation energies for carbazole co-pyrolysis with inorganic matter range from 202 to 305 kJ/mol. According to the influence of inorganic matter on carbazole cracking, the pyrolysis zone of UCG could be further divided into positive effect subzone, transition subzone and negative effect subzone.