## 3 Modeling Methodology
### Influence of OFA Arrangement  
The OFA arrangement possibly generated influences on the NO\({}_{x}\) emissions and boiler co-combustion performance. Figure 13 depicts the average temperature and species concentrations along the furnace height, with the blending ratio of semi-coke being 45%. It can be seen in Figure 13 that the average temperature and mole fraction of CO and NO in the main combustion zone exhibited unapparent association with the OFA arrangement. However, above the main combustion zone, each condition shows its own characteristics because of the different positions of OFA nozzles. The injection of OFA with low temperature brought about a decrease in the average temperature within the furnace, and the downward trend slowed down with the increase of the OFA position. The injected OFA diluted the CO and NO concentrations, and promoted the combustion of unburned pulverized coal, leading to a decrease of CO and an increase of NO. The uptrend of the NO\({}_{x}\) production was mitigated by deferring the injection of OFA. Table 5 depicts some indicators at the furnace outlet under various OFA arrangements. The NO\({}_{x}\) concentration at the furnace outlet showed a downward trend and there was a tendency for the decrease to flatten out while the height of OFA increased. The increase of the height of OFA caused the extension of the reduction zone. The deficiency of oxygen in the reduction zone prolonged the residence time of the NO\({}_{x}\) generated in the furnace. Hence, the conversion of NO\({}_{x}\) to N\({}_{2}\) was facilitated, which suppressed the formation of NO\({}_{x}\) and reduced the average NO\({}_{x}\) concentration at the furnace outlet. [PERSON] et al. [PERSON] et al. (2019) indicated that the NO\({}_{x}\) emissions decreased as the OFA position gradually moved away from the main combustion zone, which was consistent with the present simulation results. The yield of the carbon content in fly ash has a substantial increase as the height of OFA rises. As the distance between the uppermost burners and the OFA burners was enlarged, the time that coke spends was shortened. Compared with the case of the original OFA position, the NO\({}_{x}\) concentration was decreased by 82 mg/m\({}^{3}\) and the carbon content in fly ash was increased more than double when OFA was injected in the height of S2. In addition, if the carbon content in fly ash was relatively high, it showed a negative impact on the application of ash. As the height of OFA injected rose, the CO and unburned pulverized coal consume in the burnout zone and the burnout zone was closer to the furnace outlet, which caused the average outlet temperature to increase from 1307.7 K to 1335.2 K. The burnout ratio under various OFA arrangements showed slight difference because the burnout ratio was affected by the combustion in primary combustion zone. Hence, the carbon content in fly ash and NO\({}_{x}\) emission were taken into account comprehensively to select an appropriate height of OFA to ensure the NO\({}_{x}\) emission can be reduced without obvious impact on combustion efficiency.  
Figure 12: Some indicators at the furnace outlet under various \(\alpha_{1}\); (**a**) the NO\({}_{x}\) concentration; (**b**) the carbon content in fly ash; (**c**) The outlet temperature; (**d**) The burnout ratio