## 5 Conclusions  
In the present study, the NO\({}_{x}\) generation and combustion characteristics of co-firing lean coal and semi-coke in a 600 MW supercritical wall-fired boiler were firstly studied by means of numerical simulation. Some strategies for the operation optimization were further discussed after the simulation validation by the in-situ data. The simulation results suggested that co-firing lean coal and semi-coke in the wall-fired boiler is a feasible measure. As the blending ratio of semi-coke increased, the NO\({}_{x}\) emission could be reduced without notable increase of the carbon content in fly ash when the nitrogen in semi-coke is lower than that in lean coal. The blending methods (in-furnace versus out-furnace) had certain impacts on the NO\({}_{x}\) emission and carbon content in fly ash, while the in-furnace blending method showed more flexibility in co-firing adjustment. The upper burners were recommended to inject semi-coke due to the difference of the coal characteristics in the \"in-furnace\" cases; the NO\({}_{x}\) emissions in the \"in-furnace-up\" case were even lower than that in the \"out-furnace\" case. However, the combustion efficiency in the \"out-furnace\" cases was higher than that in the \"in-furnace\" cases at the same blending ratio of semi-coke. The optimal \(\alpha_{1}\) was 0.75 to achieve a relatively low NO\({}_{x}\) concentration with good combustion performance. The operational performance of the boiler co-firing semi-coke was greatly affected by the arrangement of OFA as well. The NO\({}_{x}\) concentration under the S3 position of OFA was 80 mg/m\({}^{3}\) lower than that under the S1 OFA position, while the carbon content in fly ash under the S3 position of OFA was more than double of that in the original position. The appropriate increase of the height of OFA could generate a further decrease in the NO\({}_{x}\) emission with insignificant increase of the carbon content in fly ash when the \(\alpha_{1}\) and the blending method were optimal.  
Figure 13: The average temperature and species concentrations along the height of furnace (S1 refers to the original OFA nozzles of the boiler at the height of 31 m and S2 and S3 refer to the OFA nozzles at the height of 34 m and 39 m, respectively).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & **NO\({}_{x}\) Concentration** & **Carbon Content** & **Outlet** & **Burnout** \\  & **(mg/m\({}^{3}\), \% O\({}_{2}\))** & **in Fly Ash (\%)** & **Temperature (K)** & **Ratio (\%)** \\ \hline S1 & 612.01 & 2.48 & 1307.70 & 98.18 \\ S2 & 573.56 & 4.71 & 1319.40 & 97.63 \\ S3 & 530.00 & 5.37 & 1335.20 & 97.48 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Some indicators at the furnace outlet under various OFA arrangements.  
**Author Contributions:** Conceptualization, C.W.; Methodology, Q.F. and Q.L.; Validation, C.W. and Y.D.; Formal analysis, C.W. and P.W.; Investigation, Q.F., Q.L., L.Z., and J.Z.; Writing--original draft preparation, C.W. and Q.F.; Writing--review and editing, Q.W. and D.C.; Supervision, D.C.; Project administration, C.W.; Funding acquisition, C.W.  
**Funding:** This research was funded by the National Key R&D Program of China, grant number 2017 YFB0602003.  
**Acknowledgments:** The authors acknowledge financial support from the National Key R&D Program of China (2017 YFB0602003).  
**Conflicts of Interest:** The authors declare no conflict of interest.