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 CO2 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/H2O 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/H2O 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/H2O 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 NH3. In this process, the presence of hydroxyl groups inhibit the release of HCN and NH3, leading to lower emissions under N-2/H2O than N-2 conditions.