**4. Conclusions**

This study investigated the performance of a 500 MWe tangential-firing coal boiler for optimization of the air distribution with an installation of SOFA. Using CFD, the reference case was established in a good agreemen<sup>t</sup> with the design data and different air distributions were evaluated for key performance parameters. Increasing the SOFA ratio led to lower NO emission because the NO reduction reactions were more active within the burner zone. However, too large SOFA ratios caused negative impacts on the boiler performance and increased the propensity of slagging and corrosion. With a moderate level of air staging, the NO reduction was also active between the CCOFA and SOFA levels and, therefore, the OFA distribution could be optimized to achieve good boiler performance as well as a low NO emission. For total OFA ratios of 25% and 30% (the burner zone stoichiometric ratio of 0.847 and 0.791, respectively), increasing the SOFA ratio to 15% and 20%, respectively, was ideal for significant reduction in NOx, unburned carbon in fly ash, and furnace exit gas temperature, compared to the case with CCOFA alone before the retrofit. Too high SOFA or CCOFA ratios at the fixed OFA ratios rapidly increased the unburned carbon, because of insufficient mixing between the strong air jets penetrating deep into the center and char particles concentrated near the wall. The heat absorption pattern and boiler efficiency were not noticeably influenced by the distribution between CCOFA and SOFA.

**Author Contributions:** Mesh generation and tests, H.J. and H.A.; CFD simulations, H.J., K.K., and J.P.; boiler design data and validation of reference case, H.A. and Y.G.; writing—original draft preparation, H.J.; writing—review and editing, C.R.; funding acquisition and project administration, C.R.

**Funding:** This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) affiliated with the Ministry of Trade, Industry and Energy of the Korean Government (Contract No. 20181110200190).

**Acknowledgments:** The authors would like to thank Doosan Heavy Industries and Construction for their help and technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
