## 3 Results and Discussion  
### Combustion Characteristics of Raw Materials  
Figure 1 displays the TG-DTG plots for OS, SC, HTB, KOL and BA. The combustion characterization parameters of the three raw materials are listed in Table 2. As can be seen from the plots, the only significant weight loss temperature interval occurs in the SC combustion. A peak is only visible on the DTG curve at 487.0 \({}^{\circ}\)C. The combustion process of SC can be roughly divided into three stages. Before 310 \({}^{\circ}\)C, the water of crystallization and volatiles in the coal precipitate. From 310.0 to 666.4 \({}^{\circ}\)C, a significant weight loss occurs due to the coal's low content of volatile matter. The burning of the fixed carbon dominates. It can also be seen from the DSC curve that SC releases a large amount of heat during the second stage. The heat release in the weight loss process is up to 20.0 w g\({}^{-1}\) at 551.2 \({}^{\circ}\)C. After 666.4 \({}^{\circ}\)C, the weight has not changed significantly. The result is similar to the weight loss curve of bituminous coal reported by [PERSON] et al. [18].  
The three varieties of wood pellets' TG, DTG and DSC curves show the same pattern. The DTG curves of HTB, KOL and BA all show two distinct peaks; thus, their combustion process could be separated into four parts. The first high point of the DSC curve is near 325.2 \({}^{\circ}\)C, the second is between 220.0 and 400.0 \({}^{\circ}\)C. It could be that some of the carbon in the wood pellets occurs in the form of compounds, which are thermally decomposed at the higher temperature to form the gaseous products and precipitate carbon. The carbon combustion reaction happens and the weight decreases. In Stage 3, the DTG curve's peak arrives again. This peak has a smaller value than SC's peak and the temperature interval is narrower because of the lower fixed carbon content of wood pellets. Therefore, the combustion is not concentrated at this stage. Additionally, BA is highest at the DSC second peak. The higher weight loss resulting from its high fixed carbon content supports this. As shown in Table 2, the \(C_{\rm i}\) of wood pellets is higher than that of SC and OS, and they provide the best combustion performance and the lowest ignition temperature. Among the three types of wood pellets, HTB has the lowest \(T_{\rm i}\) and the highest \(T_{\rm h}\). And BA has the best combustion performance. Generally, there is little difference in the combustion characteristics of the three types of wood pellets.  
The TG curve of OS also shows a multistage decline. The DTG curve appears as two distinct peaks, which is consistent with previous studies [35; 36]. Unlike wood pellets, the second peak of the DTG curve for OS is higher than the first peak. However, the peak values are lower than those of HTB, KOL, BA and SC, which results from its own low carbon content (shown in Table 1). As soon as the weightlessness begins, the heat release of OS gradually increases to 12 w g\({}^{-1}\). The mass loss of OS is concentrated at 190.0-590.0 \({}^{\circ}\)C, while the (_dw/dt_)\({\rm max}\) is 1.9% min\({}^{-1}\). Only 44% of OS's weight is lost. The thermal decomposition of the compound components occurs firstly during the warming process, followed by the combustion of the fixed carbon. The \(C_{\rm i}\) of OS is only 1.68, with a poor overall combustion performance.  
Figure 1: TG, DTG and DSC curves of coal (**A**), wood pellets (**B**) and oil sludge (**C**).