A transparent scale model of a two-stage fluidized bed coal dryer with a small diameter lower stage and a large diameter upper stage, separated by a conical transition zone with secondary air injection ports, has been constructed to study the effects of secondary air injection upon the fluidization characteristics of the lower riser stage. The superficial velocity of the lower stage of the riser was held constant within the turbulent fluidization regime while the superficial gas velocity in the upper riser stage was varied by changing the volumetric flow rates of air introduced between the upper and lower riser stages. Through examination of time series pressure data via standard deviation, autocorrelation, spectral density plots and visual observation of dense bed height, it becomes apparent that secondary air injection has a dominant effect upon the fluidization characteristics below the injection location, leading to a transition from a dense to a dilute bed.
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