**4. Conclusions**

The comparative study on the behavior of alkali and heavy metals from waste sludge combustion under air and oxy-fuel conditions was conducted using a 30 kW CFB pilot test bed. Based on the experimental results, the major results are summarized as follows:


**Author Contributions:** Conceptualization, H.-N.J. and H.S.C.; methodology, H.-N.J.; software, H.- N.J.; validation, H.-N.J., H.-M.Y. and H.S.C.; formal analysis, H.-M.Y.; investigation, H.-M.Y.; resources, H.-M.Y.; data curation, H.-M.Y.; writing—original draft preparation, H.-N.J.; writing—review and editing, H.-N.J. and H.S.C.; visualization, H.-N.J.; supervision, H.S.C.; project administration, H.S.C.; funding acquisition, H.S.C. All authors have read and agreed to the published version of the manuscript.

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

**Acknowledgments:** This work was supported by the Korea Ministry of Environment as the Field of Green Financing on Green Integration Human Resource Development Project. Also, This study was carried out with the support of 'R&D Program for Forest Science Technology (Project No. 2021356A00- 2223-AC03)' provided by Korea Forest Service(Korea Forestry Promotion Institute).

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

#### **References**


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