Model for attrition-enhanced char combustion in fluidized beds

• Processes taking place close to char particle surface are studied under fluidized bed conditions.
• Attrition affects the apparent constants for kinetics of char combustion.
• Attrition including internal gasification promotes char combustion.

Attrition in fluidized beds can be important due to particle collisions removing fines from the surface of a large char particle. A model for the attrition enhanced combustion of a char particle is presented. When oxygen penetrates the surface of a particle and reacts with the solid, the surface layer becomes more fragile. Fine particles are generated due to percolation and attrition contributing to the mass loss from the mother particle in addition to burning. A shrinking particle model including the effects of attrition and gasification has been developed. The oxidation is assumed to take place at the particle external surface in the classical shrinking particle model i.e. in a layer with zero thickness. The basis for the shrinking particle model is explained by studying the diffusion and reaction processes taking place in a narrow layer on the particle surface in more detail. The Arrhenius-type chemical reaction kinetics parameters (pre-exponential coefficient, activation energy, reaction order) are not purely of chemical nature but they are apparent in the classical shrinking particle model. A more realistic model, where oxidation takes place in a thin layer instead of taking place at the surface and gasification takes place in the core region, is discussed. The model predicts the thickness of this layer and oxygen concentration and char conversion (or density) profiles in this layer. The classical shrinking particle model is, however, applicable with modifications. Pore diffusivity, intrinsic reaction rate and attrition all contribute to the apparent parameters for kinetics calculated with respect to outer surface.