Evolution characteristics of structural nitrogen and the microstructure of anthracite particles in the process of O2/Ar and O2/H2O pre-oxidation

[Display omitted] •More nitrogen was stripped from condensed aromatic structures in O2/H2O pre-oxidation;•O2/H2O pre-oxidation strengthened the conversion of anthracite-N to HCN and NH3;••O and •OH accelerated the decomposition of condensed structures into small ones;•N-5 became dominant structure at expense of N-Q and N-6 after O2/H2O pre-oxidation. Pre-oxidation is considered a promising technology for improving combustion efficiency and inhibiting the emission of NOx in high rank coal use. This investigation concentrated on the evolution characteristics of anthracite-N during pre-oxidation processes under O2/H2O atmospheres. A typical high rank coal was used in this study. Isothermal pre-oxidation tests were performed under different reaction atmospheres (O2: 3, 6, 10 vol%; H2O: 0, 10 vol%), and the results illustrated that the enhancement of the O2 volumetric fraction and the addition of H2O expressed positive effects on promoting the release of anthracite-N (16.7% → 31.7%) rapidly, and more structural nitrogen tended to be removed as HCN and NH3 (0.85% → 5.48%) during the O2/H2O pre-oxidation process. The XPS results demonstrated that the increasing O2 volumetric fraction and addition of H2O in the pre-oxidation atmosphere led to the enrichment of pyrrole (N-5), which was the precursor of HCN, at the expense of pyridine (N-6) and quaternary nitrogen (N-Q). Furthermore, the increasing pre-oxidation degree accelerated the attachment of oxygen atoms on the anthracite surface, generating massive amounts of reactive functional groups. The Raman results illustrated that the variations in the chemical structure of nitrogen-containing complexes was caused by the cracking of condensed aromatic structures into reactive fragments or defect structures due to carbon-O2/H2O reactions during the pre-oxidation process, leading to massive amounts of internal structural nitrogen exposed to the edge of aromatic structures (N-Q/N-6 → N-5). When the pre-oxidation occurred in an atmosphere consisting of 10 vol% O2 and 10 vol% H2O, the conversion ratio of anthracite-N to NO during the whole pre-oxidation process reached the minimum value of 5.31%, and the reactivity of the semi-char was the highest.