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 (R_th) and NO reduction to fuel NO (R_re) were quantitatively separated. R_th decreases with the increase of oxygen volume fraction, below 6 % at 1800 K, 25 % at 2000 K. R_re is almost unaffected by the coflow temperature and affected by the oxygen volume fraction, reaching 30 % at 5 % O₂.