The role of H2O in structural nitrogen migration during coal devolatilization under oxy-steam combustion conditions

Oxy-steam combustion is a promising technology for reducing CO 2 emissions. During coal devolatilization under oxy-steam condition, the high steam concentration has a considerable influence on the migration of structural nitrogen. In this paper, the transformation of nitrogenous functionalities and the release of nitrogen-containing precursors in N 2 and N 2 /H 2 O atmospheres at 1173–1373 K were measured experimentally. Using density functional theory calculation, the reaction networks of nitrogen migration under N 2 and N 2 /H 2 O conditions were established, with the corresponding thermodynamics data. On this basis, a detailed kinetics analysis was conducted. Results showed that compared to a N 2 atmosphere, the char prepared in a N 2 /H 2 O atmosphere exhibits a lower content of pyridinic N and higher contents of quaternary and pyrrolic N. This is because high OH radical concentrations inhibit the transformation of N-Q to N-6. Moreover, the adsorption of OH on pyridinic N leads to the formation of 2-pyridone, thereby making the conversion of N-6 to N-5 feasible. Furthermore, the decomposition of both N-6 and N-5 leads to the formation of HCN, while N-5 is the major source of NH 3 . In this process, the presence of hydroxyl groups inhibit the release of HCN and NH 3 , leading to lower emissions under N 2 /H 2 O than N 2 conditions. • Migration pathways of structural nitrogen during coal devolatilization were illustrated. • Effect of high steam concentration (79%) on nitrogen migration behaviors was revealed. • Kinetics mechanisms of nitrogen migration in the presence of H 2 O were improved. • The major sources of HCN and NH 3 during devolatilization were identified.