Influences of Reactant Injection Velocities on Moderate or Intense Low-Oxygen Dilution Coal Combustion

The present work numerically investigates the effects of injection velocities or momenta per unit mass of the primary and secondary reactant streams on the moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal. When the injection-nozzle diameters are changed, the inlet velocity of the primary stream with the inlet temperature of 313 K is varied between 26 and 99 m/s, while that of the secondary air (highly preheated) with the temperature of 1623 K is increased from 16 to 102 m/s. The modeling is verified by the measurements of Weber et al. in the International Flame Research Foundation (IFRF) furnace [Weber, R.; Smart, J. P.; van der Kamp, W. Proc. Combust. Inst. 2005, 30 (2), 2623-2629]. Results reveal that the primary reactant velocity exerts a stronger influence on the flame temperature and NO emission than the secondary air velocity. Considerable reductions of the maximal temperature (about 180 K) and NO emission (200 ppm) are obtained by raising the primary reactant velocity from 26 to 67 m/s. Under the MILD combustion, the sum of the contributions from the thermal-NO, prompt-NO, and N2O-intermediate routes only accounts for less than 2.5% to the total NO emissions. Although it is well-known that the fuel-NO route dominates the NOx emissions from coal combustion, our prediction shows that up to 10% is reduced through the NO-reburning mechanism. Moreover, only about 15% of fuel N is found to convert to fuel NO from the MILD combustion, which is much less than that from the conventional combustion.