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

Doctoral thesis, 2013

Fluidized bed technology has been commercially applied over several decades. However there is still a lack of knowledge that can provide a detailed understanding of the combustion process in fluidized-bed furnaces. Understanding of mixing of fuel particles is crucial in order to achieve efficient combustion, while optimizing the number of fuel feeding ports. Thus, it is important to develop tools for reliable design and scale up of fluidized-bed boilers, including modeling from first principles by Computational Fluid Dynamics (CFD). In fluidized-bed boilers, there is typically a low mass fraction of large fuel particles in an inert bed of finer solids. The extreme size disparity between the two types of particles makes themodeling of the fuel mixing so complex that the conventional Eulerian-Eulerian (E-E) and Eulerian-Lagrangian (E-L) techniques are not able to correctly handle the particulate mixture. Therefore, the main objective of the present work is to develop new numerical strategies, within the E-E and E-L frameworks, so that they would be able to deal with the fuel mixing process. As for the E-L, our treatment for the problem of fuel mixing includes applying a three-grid method, consisting of a fine, a coarse and a moving grid. The fine grid is employed to resolve the flow of the carrier phase and to treat the small (inert) particles, whereas the coarse grid is used to calculate the drag forces acting on the fuel particles. Furthermore, the moving grid is used in order to correctly calculate the pressure gradient force on the fuel particles. In an alternative approach, we also propose a tracking technique that is a combination of the E-E and the E-L. The gas and the inert phase are treated as interpenetrating continua and resolved within the E-E framework, whereas the fuel particles are regarded as a discrete phase. The forces acting on a fuel particle are calculated by using the velocity and pressure fields of the inert solid and gas phases. To investigate the performance of the methodology, several numerical cases are simulated. Using a statistical analysis, preferential positions and the dispersion coefficient of the fuel particles are obtained under different operating conditions. The detailed information on the motion of the fuel particles obtained from simulations is compared with that from experiments. It is observed that numerical results are in good agreement with the experimental results. Besides the numerical work, detailed information on the dynamics of the inert particles in bed is obtained using the Particle Image Velocimetry (PIV) technique. Furthermore, a digital image analysis technique is applied to track an illuminated tracer particle in the bed, in an attempt to reproduce the behavior of the fuel particles. The results of the experimental work are presented in the form of the average velocity vectors of the inert and tracer particles. The slip velocity, defined as the velocity difference between the inert and tracer particles, is also presented. Such measurements have not been reported so far in the literature