## 5 Conclusions  
The conclusions are as follows:  
1. The average temperature in the riser is significantly higher at descending load compared to ascending loads, which is due to the heat that is stored in the refractory/bed material at high loads and released when the load decreases. The process model shows this specific characteristic of CFBs with high accuracy, especially for the bed temperature.
2. The model predicts the duration of the positive load changes correctly. A statement for negative load changes cannot be given with certainty, but the response of the furnace directly after the load change seems to be reproduced well also at negative load transients. A high level of detail is necessary when modeling the material properties of the refractory and the bed material to achieve a high agreement between experiment and simulation for the dynamic behavior.  
3. In the pilot, the freeboard temperature decreases sharply at lower loads, because the combustion reactions shift to the high-density bed. The model can reproduce this behavior well, although it overestimates the temperature in the freeboard at low loads.
4. The CO concentration increases at lower loads due to decreasing temperatures in the freeboard and an imperfect mixing of air and burnable components in the riser. The model neglected this imperfect mixing and was not able to show the incomplete combustion at low loads/temperatures.
5. As expected, the entrainment increases with increasing load, while the overall pressure difference from the bottom to the top only slightly changes. The APROS model with the annulus core approach overestimates the entrainment but reproduces the total pressure drop well. Improvements are necessary for future studies to match the pressure profile of the experiment with higher accuracy.  
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 different 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.  
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.  
This research was funded by the Research Fund for Coal and Steel (RFCS) grant number 754032 and the APC was funded by the German Research Foundation and the Open Access Publishing Fund of Technical University of Darmstadt.  
Financial support is acknowledged from the RFCS project of the European Commission under grant agreement 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.  
The authors declare no conflict of interest.