## 3 Modeling Methodology
### Influence of Injection Strategies of Semi-coke  
For the \"in-furnace blending\" method, the semi-coke and coal are not mixed in advance before entering into the furnace but blended within the furnace instead. Hence, the injection strategies of the semi-coke can be varied and are very likely to generate significant effect on the NO\({}_{x}\) generation and combustion efficiency. Here, two options of the injection position of semi-coke were compared with the blending ratio of semi-coke kept unchanged as 45%. For one case, the semi-coke was injected into the furnace from all upper nozzles and the middle four nozzles in the middle layer, which was expressed as the \"in-furnace-up\" case. The other case was when the semi-coke was injected from all lower nozzles and the outer four nozzles in the middle layer, expressed as the \"in-furnace-down\" case. In addition, the \"out-furnace blending\" case with 45% semi-coke was further analyzed for the comparison to elucidate the impact of blending method on the combustion performance of blended fuels. It can be seen from Figure 8 that the average temperature along the height of the furnace was elevated as the height of semi-coke injection position was raised. Given that the semi-coke was injected at a higher position, it could be well-ignited and relieve the slagging at the bottom of the furnace. Consequently, the overall burnout situation of pulverized coal could be improved and the heat generated from combustion could also be enhanced to some extent, which induces an increase in the average furnace temperature. The mole fraction of CO at the furnace bottom shows a large difference, which is the biggest in the \"in-furnace-down\" case, but is the smallest in the \"in-furnace-up\" case. The temperature is relatively low, and carbon cannot be completely burned to generate carbon monoxide. The content of carbon in semi-coke is higher than that in lean coal, leading to more carbon monoxide generated in the \"in-furnace-down\" case than that in other cases. The mole fraction of CO in three different injection strategies of semi-coke tends to be consistent with the injection of the OFA. It can also be observed from Figure 8 that as the semi-coke is injected from the lower burners, the NO mole fraction increases most significantly beyond the position of the OFA burner in comparison with other two blending strategies. As displayed in Figure 9, there is a negative correlation between the CO concentration and the NO concentration at the injection position of semi-coke. Table 4 depicts several operation indices at the furnace outlet under various injection strategies of semi-coke. The NO\({}_{x}\) emission in the \"in-furnace-down\" case is evidently higher than that in the \"in-furnace-up\" case. When the semi-coke is injected from the upper burners, the NO\({}_{x}\) emission at the furnace outlet is reduced by 200 mg/m\({}^{3}\) compared with the case of \"in-furnace-down\", and is abated by 44 mg/m\({}^{3}\) in contrast to the condition of \"out-furnace blending\". The decrease of the semi-coke injection height can bring about an increase of the fuel-NO\({}_{x}\) production in the furnace because the nitrogen in semi-coke is lower than that in lean coal. The residence time of NO\({}_{x}\) is prolonged and the reduction of NO\({}_{x}\) increases. In addition, the unburned carbon in the middle and upper of the burner zone can be increased by the  
Figure 7: Some indicators at the furnace outlet for out-furnace blending combustion under various semi-coke blending ratios; (**a**) the NO\({}_{x}\) concentration and carbon content in fly ash; (**b**) the outlet temperature and burnout ratio. injection of semi-coke from the upper burners, which leads to that the amount of generation of NO\({}_{x}\) can be effectively reduced. The carbon content in fly ash rises with the increasing height of injecting semi-coke.