Influences of secondary gas injection pattern on fluidized bed combustion process: A CFD-DEM study

Computational fluid dynamics coupled with the discrete element method (CFD-DEM) is applied to study coal combustion in a lab-scale bubbling fluidized bed. The predictions in terms of outlet gas compositions and temperature profiles are compared with the experimental measurements in the literature, and a reasonable agreement is achieved. The temporal and spatial properties of the coal combustion process are then investigated from the particle level, and the influences of the secondary gas injection parameters, including the secondary gas ratio, the jetting velocity, and the injection height are demonstrated. The results indicate that each coal particle has different entrainment behaviors, heating process, and chemical reactions, and the secondary gas injection pattern significantly influences the mixing between the oxygen and coal particles. The increase of the secondary gas ratio enhances the gas temperature fluctuation, resulting in severe hot spots which are undesirable in fluidized bed combustors. The typical formation process of a hot spot is further captured and analyzed. It is revealed that the hot spot is mainly caused by the violent combustion of volatile gas in the gas bubbles. These results provide valuable insights regarding the influence of the secondary gas injection and the bubbles on chemical reactions.