Inert solids mixing in an indirect bubbling fluidized bed gasifier

Abstract

ABSTRACT This work aims at developing and testing a method to obtain quantitative evaluations of the lateral mixing of bed material in a bubbling fluidized bed. The method combines experiments and mathematical modeling and represents a useful tool for the investigation of the complex solids mixing phenomenon in fluidized bed units. In this work, the method developed is used to evaluate the lateral solids mixing in the Chalmers biomass gasifier. For this, a fluid dynamically downscaled model of the cited gasifier has been calculated, designed and built according to Glicksman’s scaling laws. The experimental method used is based on indirect measurements of a tracer agent in the downscaled model. The mathematical model is based on solving a partial differential equation describing the random transient dispersion of particles. Experimental results show good reproducibility of the method developed. The good qualitative agreement between experimental results and the fitted simulation shows how the macroscopic lateral solids mixing behavior in the studied geometry is predictable by numerically solving a partial differential equation. The results from the cold model are scaled up to conditions valid for the large-scale gasifier, yielding a value of the dispersion coefficient of 0.028 m2/s. Timescales for the lateral solids mixing due to the bubbling bed dispersion and to the flow of bed material flowing across the large-scale gasifier are compared. Under typical operational conditions for the Chalmers gasifier, this comparison indicates a slightly higher contribution from the original bubbling solids mixing than from the solids flow across the unit.