**5. Conclusions**

This paper proposes three energy-saving strategies for a polluted-soil thermal remediation system—variable-condition mode (VCM), heat-returning mode and air-preheating mode—and their thermal performance and efficiency were discussed by energy analysis and exergy analysis. The mathematical models of a polluted-soil thermal remediation system including burner, pipe, well nd soil for energy and exergy analysis are built. The following main conclusions are reached:


• The comprehensive energy-saving strategies are mainly to compare air-preheating mode combined with variable-condition mode (VCM) and heat-returning mode combined with variable-condition mode (VCM). The results indicate that the Case 3.4 has the best energy utilization ratio in all three stages by combining the advantages of variable-condition mode (VCM) and heat-returning mode, and the results of Case 4.4 have the same implications. That is to say, combination of two energy-saving strategies can bring the improvement of fuel saving and energy efficiency at the same time and it is recommendable.

Research on methods and effects of energy-saving strategies is very beneficial for the full use of energy, for the reason of reduced natural gas consumption and higher energy utilization. Energy analysis and exergy analysis are also intuitive for the presentation of results. Further to say, resource problem can be solved better through the research of efficient use of energy.

**Author Contributions:** Conceptualization, Y.-Z.L.; methodology, T.-T.L.; software, T.-T.L.; validation, Y.-Z.L.; formal analysis, Z.-Z.Z.; investigation, E.-H.L.; writing—original draft preparation, T.-T.L.; writing—review and editing, T.L.; visualization, E.-H.L.; supervision, Y.-Z.L.; project administration, Y.-Z.L.

**Funding:** The project was funded by the Open Research Fund of Key Laboratory of Space Utilization, Chinese Academy of Sciences through gran<sup>t</sup> no.LSU-JCJS-2017-1.

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