One possible gas injection system is a series of transverse “slot valves” along the length of the bed.  Injecting the gas through narrow slots provides the necessary high velocity shearing action to disrupt the natural tendency of irregular shaped particles to self-organize into rat-holes.

In most of the scientific literature fluidized-beds with slots are known as “slot rectangular spouted beds”, and the design has received a lot of attention. In the biomass dryer application the slots must be capable of variable gas flow, both for control and to close on gas flow failure so that chips do not drop through the floor into the hot gas below.

The studies show that sharp edged holes or slots are undesirable, and that deep-throated nozzles are much better.  This presumably organizes the air into narrow jets that can penetrate deeper into the bed.  These deep nozzles are easy to incorporate into the “slot valve”, but the exact nature of the optimum shape has not been established.

All of the literature reviewed studied “slot rectangular spouted beds” in a batch configuration.  No studies were seen where flow across the slots was generated by a continuous flow of material. Most of the studies find that sloping sides on either side of the slot greatly improve the ability to get a stable fluidized bed.  Our testing with a pseudo-continuous process (recirculation of chips) also showed the need for steep sloped floors.  This is needed to force the falling chips into the region of the slot so they can be quickly picked up and re-fluidized.  This design results in a saw-tooth floor pattern.

The slot valve does not need to be located only at the base of sloped bed.  It can also be embedded across the middle of a sloped bed.  This fluidizes the chips as they flow down the sloped floor.

The potential length of each sloped section is unclear, and depends significantly on how high the jet from each slot valve can throw the chips.  In one test configuration a baffle was added above the slot valve at the bottom of the bed.  This created a higher velocity pneumatic conveyance “riser” that could carry the chips many feet vertically onto the top of the next downstream bed.  This is the basis for ongoing patent dispute with Northern Arizona University which unilaterally claimed the inventions as an extension previous work we had done at the University. The riser technique allows very long beds with multiple slot valves, but wastes some air in the high velocity risers and wastes a lot of space in the dryer container.

Later tests indicated that shorter beds worked well without the riser configuration.

Since there is a need to transport the chips from one bed to the next it makes sense to put the slot valve at the bottom of a sloped section so it can both fluidize the chips and provide the energy to transport them to the next dryer section.  The length of these sloping sections is not well established, but it appears that sections of 1 ft long or greater are possible.  One feature that is easy to add to this design is a permanent or variable deflector above the slot valves to help direct the chips to the next downstream section, or under certain circumstances to prevent the flow to the next section.

The studies have exposed one potential downside of the slot valves, and that is instabilities can occur with long slots, and that interactions between successive slots can also disrupt the fluidization.  Some of the findings included rules that indicate maximum slot lengths that would require breaks along the width of the bed to prevent instabilities. This needs to be checked experimentally.  The flow of new chips into the jet may prevent these instabilities.