Movement behavior of char particles in a bubbling fluidized bed at high temperature – The influence of particle shape

Abstract

The movement of fuel particles has a strong effect on the spatial distribution in fluidized bed boilers, not only due to the resulting fuel mixing patterns but also to the impact on fuel conversion rate (through varied bed-fuel mass and heat transfer fuel). Thus, understanding the movement of char particles becomes critical in order to control the temperature and combustion characteristics of fluidized bed boilers. Further, the preferred orientations exhibited by non-spherical fuel particles are known to influence their motion. In this study, we developed (including experimental validation) a CFD-DEM framework to investigate the movement behavior (both translational and rotational) of a single char particle under combustion conditions (i.e. high temperature) in a fluidized bed. The work studies how the char movement is influenced by the fluidization velocity and the char particle shape: spherical (as typically adopted in modeling studies) vs. non-spherical. For non-spherical char particles, the impact of applying different degrees of particle homogeneity (glue index) in the modeling was also studied. The metrics applied to evaluate the char particle movement behavior were spatial distribution and linear and rotational velocity of the particle.The results show that the movement of spherical char particles is generally more vigorous than that of non-spherical ones. An increase in fluidization velocity yields a higher probability of populating the splash zone instead of the bed surface, due to the enhanced bubble flow. Increased fluidization velocity yields also higher rotational velocity (especially for non-spherical particles, for which the orientation angle distribution becomes eventually evenly distributed).