Effects of reaction condition on NO emission characteristic, surface behavior and microstructure of demineralized char during O2/H2O combustion process

Surface behavior and structural characteristics of char particles generally play a vital role in affecting char NO emission characteristics during combustion process. A typical bituminous Shenhua from northwest China was employed for demineralization in this research. The demineralized coal without catalytic interference of alkali metal salts was employed as the experimental sample. Moreover, by means of the utilization of isothermal combustion test, Temperature Programmed Desorption (TPD) and Raman spectrometer, the conversion ratio of nitrogen content in char to NO, as well as the evolution of char surface behavior and micro-structure in the process of O-2/H2O combustion under different conditions were determined. With the increase of reaction temperature (T-r) and O-2 concentration, more surface active sites (C-f), oxygen containing functional groups (C(O)) and small aromatic ring structures generated on char particle surface inhibiting the emission of NO during O-2/H2O combustion process. Under identical reaction conditions (Tr, O-2 and H2O concentration), the partially oxidized chars with moderate burnout degree (X-c approximate to 0.3) had the largest amount of C-f on particle surface. On the other hand, the amount of C-f and small aromatic ring structures first increased and then decreased as the H2O concentration enhanced. When the C-f amount and the value of I(Gr+VL+Vr)/I-D reached the maximum at the H2O concentration of approximately 8.5 vol%, the conversion ratio of char-N to NO reached the minimum value. Meanwhile, the increase of H2O concentration initially accelerated the NO reduction rate and this positive effects gradually diminished when the H2O concentration exceeded the critical value (8.5 vol%). Thus, the optimal combustion condition for char combustion with the lowest NO conversion ratio in this study was: 1473 K, 30% O-2 and 8.5 vol% H2O.