*Article* **Operational Flexibility of a CFB Furnace during Fast Load Change—Experimental Measurements and Dynamic Model**

#### **Jens Peters \*, Falah Alobaid and Bernd Epple**

Institute for Energy Systems and Technology, Technical University of Darmstadt, Otto-Berndt-Straße 2, 64287 Darmstadt, Germany; falah.alobaid@est.tu-darmstadt.de (F.A.); bernd.epple@est.tu-darmstadt.de (B.E.) **\*** Correspondence: jens.peters@est.tu-darmstadt.de; Tel.: +49-(0)-6151-16-22689; Fax: +49-(0)-6151-16-22690

Received: 24 July 2020; Accepted: 24 August 2020; Published: 28 August 2020

**Abstract:** The share of power from fluctuating renewable energies such as wind and solar is increasing due to the ongoing climate change. It is therefore essential to use technologies that can compensate for these fluctuations. Experiments at 1 MWth scale were carried out to evaluate the operational flexibility of a circulating fluidized bed (CFB) combustor during transient operation from 60% to 100% load. A typical load following sequence for fluctuating electricity generation/demand was reproduced experimentally by performing 4 load changes. The hydrodynamic condition after a load change depends on if the load change was in positive or negative direction due to the heat stored in the refractory/bed material at high loads and released when the load decreases. A 1.5D-process simulation model was created in the software APROS (Advanced Process Simulation) with the target of showing the specific characteristics of a CFB furnace during load following operation. The model was tuned with experimental data of a steady-state test point and validated with the load cycling tests. The simulation results show the key characteristics of CFB combustion with reasonable accuracy. Detailed experimental data is presented and a core-annulus approach for the modeling of the CFB furnace is used.

**Keywords:** CFB combustion; operational flexibility; load transients; fluctuating electricity generation; lignite; renewables
