“Rat-holes” are stable passages that form in a bed of particles when attempting fluidization.  These are paths of least resistance that pass the gas through the bed without fluidization.  This bypassing prevent good gas-particle contact. Rat-holes occur most commonly when trying to fluidize irregular shaped particles (like wood chips) which tend to lock together. Once established they are difficult to disrupt, They LOOK like “rat-holes”. One way to prevent “rat-holes is to inject the dryer gas into the bed with a high velocity shearing action, rather than the even distribution through the floor common in most fluidized bed designs.