## 3 Modeling Methodology
### Influence of Semi-coke Blending Ratio  
Firstly, the influence of semi-coke blending ratio on the combustion efficiency and NO\({}_{x}\) emissions at the furnace outlet was investigated. Under the premise that the fuel heat into the furnace was kept unchanged, the ratio of semi-coke was set as 0%, 17%, 33%, 45%, 50%, and 70%, respectively. Here, the semi-coke and lean coal were blended before the coal mill and ground into powder, which was then sent into the furnace for co-combustion; so-called \"out-furnace blending\". The excess air coefficient in primary combustion zone was set to 0.75, and the total excess air coefficient was 1.14. The temperature distributions within the furnace at various blending ratios of semi-coke are illustrated in Figure 5. The average temperature first increased and then decreased with an increase in the furnace height; the temperature at the flame center was relative high. As shown in Figure 6, the changes of average temperature, CO, and NO mole fraction along the furnace height under various blending ratios of the semi-coke can be depicted more intuitively. The average temperature under various semi-coke blending ratios showed limited difference. This is mainly because the fuel was homogeneous and the net calorific value of semi-coke and coal was quite close (25.35 MJ/kg for semi-coke and 24.36 MJ/kg for coal). Due to the injection of the air and pulverized coal with low temperature, the average furnace temperature at the burner position dropped dramatically. However, the air could promote the pulverized coal combustion, and then the average furnace temperature increased once again. When the combustion reaction was nearly completed, the average temperature decreased with the heat transfer between the platen superheater and flue gas in the furnace. When the blending ratio of semi-coke was enhanced, the corresponding CO concentration on each cross-section was elevated. It is reasonable to believe that the combustion performance of semi-coke was inferior to that of lean coal mainly due to the lower content of volatile matter. Since the low-NO\({}_{x}\) burners were utilized in the boiler, the center of main combustion zone formed a rich-fuel and low-oxygen atmosphere through staged combustion, where the oxygen supplied for the pulverized coal combustion was deficient. The center of main combustion zone presents a reducing atmosphere and a large amount of CO was generated in the combustion of pulverized coal. From the comparison of CO mole fraction under different cases at the same furnace height, the higher the semi-coke blending ratio was, the higher the CO concentration in the primary combustion zone. Figure 5: The temperature contours within furnace at various semi-coke blending ratios. \begin{table}
\begin{tabular}{c c c} \hline
**Indicator** & **Experimental Value** & **Simulation Result** \\ \hline Carbon content in fly ash (\%) & 4.97 & 4.58 \\ NO\({}_{x}\) emission (mg/m\({}^{3}\) 6\% O\({}_{2}\)) & 508 & 505 \\ \hline \end{tabular}
\end{table} 
Table 3: Some indicators at the furnace outlet under the in-situ test and simulation results. Some indicators at the outlet of the furnace are compared in Figure 7. With an increase in the semi-coke blending ratio, the NO\({}_{x}\) emission at the furnace outlet decreased. The mole fraction of NO decreased with the blending ratio of semi-coke increased at the same furnace height as shown in Figure 6, which is consistent with the trend of NO\({}_{x}\) concentration. The lower content of nitrogen in semi-coke than that in coal might be a reason. With more semi-coke blended, the fuel contains less nitrogen and the generation of fuel NO\({}_{x}\) is reduced accordingly. Because the coal properties of semi-coke and lean coal are similar, the NO\({}_{x}\) emission of blended fuel combustion is positively related to the nitrogen content within fuel for the co-combustion of \"out-furnace blending\" method, but exhibits no-additive behaviors.