Euler-Euler Simulation of a Bubbling Fluidized Bed Gasifier: Development of an Intrinsic Particle Conversion Model

The efficiency of fluidized bed gasifiers is strongly linked to the conversion of the solid feedstock. The operating temperature of industrial fluidized bed reactors is limited below ash softening temperature leading to incomplete carbon conversion. To achieve a full carbon conversion, present fluidized bed reactor concepts need to be improved. Computational Fluid Dynamic (CFD) modeling is a time-efficient tool for advanced process development. However, the modeling of solid-gas reactions in fluidized beds is still at the state of current research activities. In the present work an intrinsic conversion model is developed to simulate the carbon conversion in a bubbling fluidized bed gasifier. Char as bed material is fluidized using a mixture of CO₂ and N₂. Due to the reaction of carbon with CO₂ the porosity of the particles increases resulting in a reduction of the particle density, while the particle diameter keeps nearly constant. For that reason the conversion model assumes that the particle size is constant, and the heterogeneous reactions increase the porosity of the particle and decrease its density. The numerical gasifier model combines the new conversion model with the Euler-Euler approach. Numerical results of the shrinking density model are compared against the commonly used shrinking core model for a bubbling fluidized bed gasifier. A parametric study is presented for different process conditions and the influence of the conversion model on the hydrodynamics is discussed.