Fuel Mixing in Gas-Solid Fluidized Beds: A Computational and Experimental Study

Fluidized bed technology has been commercially applied over severaldecades. However there is still a lack of knowledge that can provide adetailed understanding of the combustion process in fluidized-bed furnaces.Understanding of mixing of fuel particles is crucial in order toachieve efficient combustion, while optimizing the number of fuel feedingports. Thus, it is important to develop tools for reliable design andscale up of fluidized-bed boilers, including modeling from first principlesby Computational Fluid Dynamics (CFD).In fluidized-bed boilers, there is typically a low mass fraction oflarge fuel particles in an inert bed of finer solids. The extreme size disparitybetween the two types of particles makes themodeling of the fuelmixing so complex that the conventional Eulerian-Eulerian (E-E) andEulerian-Lagrangian (E-L) techniques are not able to correctly handlethe particulate mixture. Therefore, the main objective of the presentwork is to develop new numerical strategies, within the E-E and E-Lframeworks, so that they would be able to deal with the fuel mixingprocess.As for the E-L, our treatment for the problem of fuel mixing includesapplying a three-grid method, consisting of a fine, a coarse and a movinggrid. The fine grid is employed to resolve the flow of the carrierphase and to treat the small (inert) particles, whereas the coarse gridis used to calculate the drag forces acting on the fuel particles. Furthermore,the moving grid is used in order to correctly calculate thepressure gradient force on the fuel particles.In an alternative approach, we also propose a tracking techniquethat is a combination of the E-E and the E-L. The gas and the inertphase are treated as interpenetrating continua and resolved within theE-E framework, whereas the fuel particles are regarded as a discretephase. The forces acting on a fuel particle are calculated by using thevelocity and pressure fields of the inert solid and gas phases.To investigate the performance of the methodology, several numericalcases are simulated. Using a statistical analysis, preferential positions and the dispersion coefficient of the fuel particles are obtainedunder different operating conditions. The detailed information on themotion of the fuel particles obtained from simulations is compared withthat from experiments. It is observed that numerical results are ingood agreement with the experimental results.Besides the numerical work, detailed information on the dynamicsof the inert particles in bed is obtained using the Particle Image Velocimetry(PIV) technique. Furthermore, a digital image analysis techniqueis applied to track an illuminated tracer particle in the bed, inan attempt to reproduce the behavior of the fuel particles. The resultsof the experimental work are presented in the form of the average velocityvectors of the inert and tracer particles. The slip velocity, definedas the velocity difference between the inert and tracer particles, is alsopresented. Such measurements have not been reported so far in theliteratur