Comparative study on co-firing characteristics of normal and superfine pulverized coal blended with NH3 under the MILD combustion mode

Coal-fired power plants remain a significant source of CO2 emission, NH3/coal co-firing in existing coal-fired boilers can rapidly and effectively reduce CO2 emissions. However, the unique physicochemical properties of NH3 result in unstable combustion and high NOx emissions. Superfine pulverized coal combustion and moderate or intense low-oxygen dilution (MILD) combustion are two advanced technologies with stable combustion and low NOx emissions. This study systematically investigated the coupling of these two technologies in NH3/coal co-combustion. Results show that the synergistic effect of superfine pulverized coal and MILD combustion ensures stable NH3/coal flame propagation, with a shorter lift-off distance and a higher peak temperature (70 K–100 K increase) under higher NH3 blending ratio (NBR) conditions. As the NBR increases from 0 to 40 cal%, the maximum Das-O2 for the superfine particles (18 μm) decreases from 0.55 to 0.49, while for normal particles (125 μm), it decreases from 3.9 to 3.4. The combustion state of superfine particles more easily approaches the MILD combustion state. The value of Rchar-H2O/Rchar-CO2 increases from 0.64 to 1.93, indicating the CChar-H2O reaction dominates the char gasification behavior. Compared to that for normal particles, the NO emission concentrations for superfine particles combustion at 0, 10 cal% and 20 cal% NBR were reduced by 48 %, 64 % and 47 %, respectively. These findings provide new insights and theoretical guidance for stable combustion and low NOx emissions of NH3/coal co-firing.