Comprehensive and Quantitative Separation of Different NO Routes during Moderate and Intense Low-Oxygen Dilution Oxy-fuel Combustion of Pulverized Coal Particles

Moderate and intense low-oxygen dilution oxy-coal combustion (MILD-OCC) technology is prospective as a result of its high thermal efficiency, low NOx emission, and applications for carbon capture and sequestration. MILD-OCC experiments were carried out on the flat flame burner combustion facility where the diffusion flamelet can provide the stable hot coflow of the fixed temperatures and oxygen volume fractions. The flue gas species concentrations at different axial heights above the burner tip were measured by the flue gas analyzer. First, pulverized coal particle combustion experiments under CO2, N2 and Ar dilution with different coflow temperatures and oxygen volume fractions were carried out to quantitatively separate thermal and fuel NO. The NO formation kinetics were obtained through nth-order Arrhenius nonlinear fitting to calculate the relative contributions of thermal and fuel NO. Second, the comparison between the experiments of coal and char combustion was made to quantitatively separate the formation and reduction of volatile NO and char NO. Finally, different concentrations of initial NO were added to the oxidizer to simulate recycled NO, and the experiments on the influence of different concentrations of recycled NO were conducted to quantitatively separate the recycled NO reburning. Therefore, the NO formation and reduction mechanisms during the whole process of MILD-OCC were quantitatively separated, and their relative contributions were identified. With the increase of the coflow temperature from 1473 to 1873 K under the condition of 10% O2, the relative contribution rate of volatile NO reduction to fuel NO decreases from 25 to 17%, with the relative contribution rate of char NO reduction to fuel NO increasing from 2 to 9% and the relative contribution rate of recycled NO reburning to fuel NO increasing from 7 to 19%. The results can provide a comprehensive and quantitative perspective on the NOx routes and the control method of low NOx emissions for MILD-OCC.