Abstract: The oxygen deficient pre-oxidation technology can effectively solve the problems of different ignition, low burnout rate and massive NO_x emission in high rank coal based fuel combustion.Based on the co-activation behavior of O₂ and H₂O on the surface of coal based fuel, the evolution mechanism of nitrogen element in the process of O₂/H₂O pre-oxidation of high rank coal was studied.The isothermal fixed-bed reactor was employed as the experimental system, and the samples were loaded in a micro fluidization device.The tests were performed at 1 173 K under O₂/H₂O atmosphere.The composition of downstream gas was measured on-line by a high resolution infrared flue gas analyzer.Subsequently, the proximate analysis, ultimate analysis and XPS analysis were carried out for determining the migration pathway and precipitation mechanism of nitrogen element and carbon element in the reaction process.The results illustrated that the content of residual nitrogen in semi-char particles decreased obviously, and the proportion of N₂,HCN and NH₃ in the gaseous products increased apparently, and the conversion ratio of fuel-N to NO decreased significantly.Furthermore, O₂ and H₂O molecules could accelerate the conversion of condensed aromatic ring structure into small ones and the inside nitrogen migrated to the edge of the structures, promoting the conversion of thermal stable pyridine(N-6) and quaternary nitrogen(N-Q) into pyrrole(N-5) which was the precursor of HCN.In the reaction process, the enhancement of O₂ and the addition of H₂O could promote the rapid release of structural nitrogen and reduce the loss of fixed carbon in the process of nitrogen stripping.Therefore, the O₂/H₂O pre-oxidation could effectively optimize the release pathway of nitrogen in the process of pre-oxidation, regulate the occurrence of residual nitrogen in semi-char, and gradually optimize the fuel quality of high rank coal.