## 3 Results and Discussion  
### Combustion Characteristics with Different Sludge Blending Ratios and Atmospheres  
Figure 2 shows the DTG and TG curves of sludge and coal in different blending ratios and atmospheres of 21% O\({}_{2}\)/79% N\({}_{2}\) and 21% O\({}_{2}\)/79% CO\({}_{2}\). The figure indicates that the process of co-firing the blending sludge and coal is divided into three stages: dehydration, precipitation of volatiles, and fixed carbon combustion. As shown in Figure 2, for blended samples, there are two obvious peaks on the DTG curve. With the increase of the sludge blending ratio, the ash content of the sludge is higher than that of coal, so the amount of soot after the blended sample is burned out. The peak value of the volatile matter peak increases significantly and the combustion reaction temperature range becomes larger. The fixed carbon peak-to-peak value decreases and the temperature range of the fixed carbon combustion phase becomes smaller. As the proportion of sludge increases, the weight loss rate of the volatile matter of the blended sample increases and the weight loss rate of the fixed carbon decreases. Both the volatile matter and the fixed carbon precipitate and burn in advance. Compared with the 21% O\({}_{2}\)/79% N\({}_{2}\) atmosphere, the burnout temperature and ignition temperature of the experimental sample under the 21% O\({}_{2}\)/79% CO\({}_{2}\) atmosphere are slightly higher. The burnout temperature addresses the lowest temperature at which the fuel is basically burned out, the main indicator of burnout performance. The lower the burnout temperature, the better the burnout performance. This is because N\({}_{2}\) and CO\({}_{2}\) have different thermo-physical properties and CO\({}_{2}\) has more active chemical properties [24,25]. As the specific heat capacity of CO\({}_{2}\) is higher than that of N\({}_{2}\), more heat is carried by the airflow, resulting in a lower temperature on the surface of the sludge or coal powder and a decrease in the burning rate. At the same time, the diffusion coefficient of O\({}_{2}\) is lower under an O\({}_{2}\)/CO\({}_{2}\) atmosphere and it is more difficult for oxygen to diffuse to the surface and interior of pulverized coal or sludge particles. In addition, when the CO\({}_{2}\) concentration is high, the reaction CO\({}_{2}\) + H\(\Leftrightarrow\)CO + OH competes with H + O\({}_{2}\)\(\Leftrightarrow\)H + O to consume H free radicals, thereby inhibiting the oxidation of CO. In addition, CO\({}_{2}\) inhibits H + O\({}_{2}\)\(\Leftrightarrow\)H + O by promoting the reaction H + O\({}_{2}\)(+M)\(\Leftrightarrow\)HO\({}_{2}\)(+M), because the latter high three-body collision efficiency reduces the reactivity of the system. Therefore, in the 21% O\({}_{2}\)/79% CO\({}_{2}\) atmosphere, the ignition temperature and burnout temperature of the experimental samples are higher and the CCI is slightly reduced. Reference [2] put forward that increasing the proportion of distillers' grains in coal results in a reduced ignition temperature, burnout temperature, and activation energy in the combustion stage and the Comprehensive Combustion Index (CCI) improves with the same tendency as SD and BC blending. Figure 3 shows the \(T_{\rm i}\), \(T_{\rm b}\), and CCI of the experimental cases under different atmospheres. The CCI rises first and reaches the maximum when the sludge blending ratio is 15%; that is, 41.13 \(\times\) 10  
Figure 2: Thermo-gravimetric curve of the samples heating in different atmospheres. (**a**) TG curve in 21% O\({}_{2}\)/79% N\({}_{2}\) conditions; (**b**) DTG curve in 21% O\({}_{2}\)/79% N\({}_{2}\) conditions; (**c**) TG curve in 21% O\({}_{2}\)/79% CO\({}_{2}\) conditions; (**d**) DTG curve in 21% O\({}_{2}\)/79% CO\({}_{2}\) conditions. under an air atmosphere and \(29.94\times 10^{-8}\) under an oxygen-rich atmosphere. The CCI value then decreases to \(24.52\times 10^{-8}\) and \(21.87\times 10^{-8}\) when the sludge blending ratio is 50%.