**4. Conclusions**

Based on whether the endothermic and/or exothermic reactions occurred (or not) in DSC tests, the TGA curves of upstream oily sludge could be sectionalized as four weight loss stages. No endothermic or exothermic reaction was detected, but a 10% weight loss ratio was obtained in Stage 1. The first and second exothermic reaction occurred in stages two (weight loss = 10%) and three (weight loss = 20%), and the ending temperatures were 614.62–664.91 K and 719.15–822.69 K, respectively. The temperature region of Stage 4 was between 635.968–900.00 ◦C, and the weight loss was 5%.

Five types of thermal reactions existed in the four incineration stages, i.e., dehydration and volatilization (Stage 1), the combustion of light components (Stage 2), the combustion of heavy components (Stage 3), and the decomposition of inorganic carbonate (Stage 4). The two combustion reactions caused the exothermic phenomenon, while the endothermic phenomenon was attributed to the decomposition reaction.

The integral methods (the FWO method and the KAS method) were efficient to obtain the activation energy, while the differential method (the Friedman method) was more suitable to solve the reaction order and pre-exponential factors. The average activation energies of stages one, two, three, and four were 60.87 KJ/mol, 78.11 KJ/mol, 98.82 KJ/mol, and 15.96 KJ/mol, respectively. The reaction order and pre-exponential factors of stages one, two, and three were 0.82, 3.50, and 2.50, and e13.32min−1, e19.69min−1, and e21.00min−1, respectively. Due to the significant errors and relatively low R<sup>2</sup> values that appeared in the linear fitting curves of Stage 4, the reaction order and the pre-exponential factor could not be obtained under the Friedman method.

The *Ep* values of Stage 2 (or Stage 3, as shown in Table 1) obtained under the Kissinger approach were lower than the *E*0 values from the KAS method, which means that the peak operating temperature was more suitable as the incineration temperature in engineering use.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-4601/16/3/384/s1, Figure S1: The solution of *<sup>P</sup>*(*μ*). Figure S2: The linear fitting results for reaction orders and pre-exponential factors of stage 1.

**Author Contributions:** Conceptualization, Y.Z. and Y.Q.; Data curation, X.W.; Formal analysis, Y.Z. and Y.Q.; Investigation, X.W.; Methodology, Y.Q.; Resources, X.W.; Visualization, F.X.; Writing—original draft, Y.Q.; Writing—review & editing, F.X.

**Funding:** This research received no external funding.

**Acknowledgments:** This study was supported by the Zhejiang Heze environmental Tech Shares Co.,LTD. (support number:B220170264). Furthermore, all the authors would like to thank Ding Zhinong, the CEO of Zhejiang Heze environmental Tech Shares Co., LTD, and thankful to Gu Lifeng for the constructive suggestions to this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
