Experimental investigation on the NO formation of pulverized coal combustion under high-temperature and low-oxygen environments simulating MILD oxy-fuel combustion conditions

The NO formation experiments simulating moderate and intense low-oxygen dilution (MILD) oxy-coal combustion conditions were conducted on a laminar diffusion flame burner with the coflow temperatures of 1473-1873 K and the oxygen volume fractions of 5 %-20 % in O2/CO2, O2/Ar and O2/N2 atmospheres. The flame images of pulverized coal combustion were captured to obtain the ignition delay distances, and the axial species concentrations were measured to obtain the variation of NO formation and reduction. The NO yield in O2/Ar atmosphere decreased by nearly 0.2 when the oxygen volume fraction decreased from 20 % to 5 % and by about 0.05 when the coflow temperature decreased from 1873 K to 1473 K. The NO yield in O2/CO2 atmosphere was 0.1-0.15 lower than that in O2/Ar atmosphere. The optimal kinetic parameters of thermal NO and fuel NO formation rate were obtained by a nonlinear fit of nth-order Arrhenius expression. Finally, the relative contribution rates of thermal NO to total NO (Rth) and NO reduction to fuel NO (Rre) were quantitatively separated. Rth decreases with the increase of oxygen volume fraction, below 6 % at 1800 K, 25 % at 2000 K. Rre is almost unaffected by the coflow temperature and affected by the oxygen volume fraction, reaching 30 % at 5 % O2.