Analyzing the fluidization of a gas-sand-biomass mixture using CFD techniques

Abstract

Fluidization taking into account the presence of the material to be gasified is a differential addressed in this study. Accordingly the solid phase was composed of a binary mixture of sand and biomass. This work deals with the numerical simulation using CFD of a gasifier bubbling fluidized bed for the system composed of gas - biomass – sand. In order to determine the best fluidization conditions, a factorial design 23 was carried out varying the biomass particle density and diameter and the biomass percentage in the solid phase. To perform the simulations, ANSYS CFX 15.0 was used, adopting an Eulerian approach coupled to the Kinetic Theory of Granular Flow. The k-ε turbulence model was adopted. Seventeen simulations were performed setting the gas superficial velocity to 0.38ms−1. Based on the results of the factorial design, it was possible to qualitatively identify the tests to which the system reached a bubbling fluidization. Segregation of the particulate medium occurred for assays where the ratio between the mass of each biomass particle and the mass of each sand particle was >1.0 coupled to a ratio between biomass and sand volume fractions in the bed ≤0.5. The variable with the highest significance in the model equation was the diameter of the biomass particle. Volumetric fraction profiles of gas, sand and biomass were obtained to the 17 factorial design conditions as well as a model that predicts the bed expansion. The assay that reached greater bed height (0.50m), staying on bubbling regime, was the one with 15% biomass volume fraction with 375μm diameter and 45% sand, indicating those are good conditions for fluidization.