Chemical looping gasification is an innovative process for chemical recycling of residues. Two circulating fluidized bed reactors are coupled with a metal oxide as oxygen carrier that circulates between the reactors. In the fuel reactor, the feedstock is converted to syngas. Heat and oxygen for the gasification are provided by the oxygen carrier. In the air reactor, the oxygen carrier is re-oxidized using air. Thus, the production of an almost nitrogen free syngas is possible without the need for a costly air separation unit.
In this work, a CFD model was developed for the fuel reactor in ANSYS Fluent 21R2. The multiphase flow is modeled based on Euler-Lagrange methodology using the discrete element method (DEM). To the authors knowledge, this is the first model for biomass chemical looping gasification using CFD-DEM. Drag forces acting on the particles from the gas are modeled with an EMMS model. Biomass gasification is a complex process including many species and parallel and consecutive reactions. To make a simulation of the fuel reactor feasible, a reduced reaction network was defined. The model consists of 14 species and 9 reactions including pyrolysis of feedstock particles, gasification, oxygen carrier reactions as well as tar decomposition by steam reforming and water gas shift. Custom kinetics specific to the feedstock and oxygen carrier used in this study were implemented.
In 2022, the first demonstration of autothermal chemical looping gasification at MW scale was achieved at TU Darmstadt using biogenic residues as feedstock and ilmenite, a natural titanium-iron-oxide as oxygen carrier. The CFD model was applied to selected operating points from the pilot operation and validated with measurement data. The reactor hydrodynamics and thermodynamics could be predicted accurately. The pressure profile was in good agreement with the experimental measurements in both, the dense bed at the bottom of the reactor and the upper lean zone. Oxidation degree of the oxygen carrier at reactor outlet corresponded well with experimental samples indicating a correct representation of oxygen transfer to the reactor. Syngas composition correlated reasonably well with the experimental measurements.