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

Abstract

Moderate and intense low-oxygen dilution oxy-coal combustion (MILD-OCC) technology is prospective as a result of its high thermal efficiency, low NO x 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 NO x routes and the control method of low NO x emissions for MILD-OCC.

Numerical Analysis on the Quantitative Separation of Different NOx Formation and Reduction Routes during MILD Oxy-Coal Combustion with the Improved Nitrogen Chemistry Model

“…The predicted coflow temperature is slightly lower than the experimental value during the heating stage of 1–2 cm above the burner outlet, likely due to the initial temperature setting of the burner wall boundary conditions. The measured ignition delay distance of pulverized coal is 2.0–3.5 cm, and the coflow temperature stabilizes at 2 cm above the burner outlet. Thus, the temperature deviation near the burner outlet has little influence on volatile and char combustion.…”

“…CO flowing out of the fuel capillaries diffusively burned with O 2 /CO 2 flowing out of the oxidizer channel, forming hundreds of diffused flamelets that provide a uniform and stable hot coflow. More detailed information about the Hencken burner can be found elsewhere. , Due to the symmetry of the Hencken burner structure, a quarter of the calculated grid was selected.…”

“…However, OCC has several drawbacks, including ignition delay, incomplete combustion loss, and low combustion stability. , Moderate and intense low-oxygen dilution (MILD) combustion, which extensively dilutes the oxidizer and preheats the fuel through intensively internal flue gas recycle (FGR), leads to a lower peak temperature, more uniform temperature distribution, extended reaction zone, and reduced reaction rate. , This technology not only improves thermal efficiency but also inhibits NO x emissions. By integrating the MILD combustion with the OCC technology, the MILD-OCC technology effectively mitigates the deficiencies of conventional OCC technology. , …”

“…This expanded the reaction zone, decreased the reaction rate, and reduced the NO x emissions by about 40%. Li et al , conducted the experiments on pulverized coal flame photography and NO x formation under O 2 /CO 2 , O 2 /N 2 , and O 2 /Ar atmospheres at 1473–1873 K and 5–20% O 2 using a flat diffusion flame burner. They studied the NO x formation characteristics of coal particles under different coflow temperatures, oxygen volume fractions, and atmospheres.…”