MEASURING AND MODELLING SOLID MIXING AND PARTICLE RESIDENCE TIMES IN A GAS-SOLID FLUIDIZED BED REACTOR FOR BIOMASS FLASH PYROLYSIS

The purpose of this study was to simulate the behaviour of a relatively large and shallow fluidized bed for biomass flash pyrolysis (Waterloo Pyrolysis Process) with respect to the solid movement pattern and to experimentally measure the radial mixing behaviour of the solids.;Solid particle mixing was described as occurring by a dispersion mechanism. Axial and radial solid dispersion coefficients were experimentally evaluated as functions of operating parameters and local positions within the fluidized bed. A general correlation was derived based on the experimental results, able to predict the extent of solid lateral mixing as a function of most influencial parameters.;A further study was carried out to estimate the residence time distribution of biomass particles injected into the pyrolysis fluidized bed reactor. The experimental results demonstrated that the elutriation of low density particles (simulating the char) is a function of fluidizing velocity, particle size and particle density. Wood particles injected into the unit remained well mixed with the bed material (sand), regardless of the gas velocity used within the normal operating range of the pyrolysis process. Lighter particles simulating the solid pyrolysis product were observed to leave the reactor after a few seconds of residence time, entrained into the gaseous stream. A circulation model was used for interpreting the flow of the light particles through the fluid bed and was demonstrated to be a very realistic model able to predict very closely the real physical mechanism involved in the mixing-elutriation process occurring during the pyrolysis.;Practical charts were subsequently proposed, which are useful in indicating what are the physical characteristics of the particles which can be respectively elutriated from the fluid bed, or well mixed into the vessel.;A 0.27 m diameter fluidized bed reactor was designed. A unique method was developed which allowed the continuous determination of variations in solid tracer concentration with time at different radial and axial positions within the bed.;Solids distribution in a hypothetical large scale fluidized bed reactor was simulated, where solids concentration profiles around multiple feed points were calculated. Finally, design considerations were reviewed. (Abstract shortened with permission of author.)...