**5. Conclusions**

The conclusions are as follows:


This study is relevant for future research on the flexible operation of CFB combustion regarding load change operation. The presented CFB model is highly suitable to examine concepts for accelerated load ramps. The work gives valuable concepts for, and insight into the process modeling of the CFB combustion technology. In ongoing work, the developed model is further optimized for di fferent kinds of fuels such as refuse-derived fuel (RDF) and biomass and will be validated with experimental data of co-combustion tests with lignite and straw/RDF.

**Author Contributions:** Conceptualization, J.P. and F.A.; Methodology, J.P. and F.A.; Validation, J.P. and F.A.; Investigation, J.P.; Resources, B.E.; Writing—Original Draft Preparation, J.P.; Writing—Review & Editing, J.P. and F.A.; Visualization, J.P.; Supervision, B.E. and F.A.; Project Administration, B.E.; Funding Acquisition, B.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Research Fund for Coal and Steel (RFCS) gran<sup>t</sup> number 754032 and the APC was funded by the German Research Foundation and the Open Access Publishing Fund of Technical University of Darmstadt.

**Acknowledgments:** Financial support is acknowledged from the RFCS project of the European Commission under gran<sup>t</sup> agreemen<sup>t</sup> n. 754032 (FLEXible operation of FB plants co-Firing LOw rank coal with renewable fuels compensating vRES—FLEX FLORES). We acknowledge support by the German Research Foundation and the Open Access Publishing Fund of Technical University of Darmstadt.

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