This project investigated the use of fluidized bed technology for drying wood chips.  The excellent heat and mass transfer in fluidized beds allows smaller, cheaper and more controllable drying.  The problem of fluidizing irregularly shaped particles like wood-chips was largely overcome.  It is expected that portable dryers using this technology can be located near the chipping operation and significantly reduce transportation costs by eliminating transport of water to the final destination. It also allows air to be used as the drying medium because the beds are cool (<120F) and the beds act as flame-arrestors if any burning material enters them. The dryer gas is used efficiently by varying the dryer gas rate and temperature to each of several dryer beds, and the chips never see temperatures that cause “blue-haze“.

The economics of forest thinning to reduce the risk of fires is heavily dependent on transportation costs to move the chips to market.  Living trees are about 50% water and the final use for the chips for either pellets or burning is 10-20%.  Reducing the water content near the forest dramatically reduces the weight of the chips to be transported.  In some cases the wet chips are so heavy that the trucks reach maximum weight before they are full. With reduced government subsidies for forest thinning these improved economics may be significant in allowing more thinning and reduce the risk of forest fires.

Conventional rotary driers are common in the US but are expensive to make portable, and have maintenance and control issues. The belt dryers commonly used in Europe are quite large and expensive.

Although fluidized beds have a good reputation in drying applications they have not been used with wood chip drying because of the chips tendency to form “rat-holes”; a phenomenon where chips lock together in tube-like structures and let the gas pass through the passages with no fluidization or drying.  Traditional fluidized beds with flat floors require far too high a gas velocity to prevent “rat-holes” to be practical. In addition to preventing rat-holing, the design of a chip drier should prevent the formation of “blue haze” and to use dryer gas efficiently by having the off-gas as close to water saturation or 100% Relative Humidity (RH) as possible.

A batch bench scale test unit was built and tested and indicates that wood chips can be successfully fluidized with some special techniques to eliminate “rat-holing”.  The test unit mimics one stage in a multistage dryer and includes the mechanism to move the chips from bed to bed. Mathematical modeling of the system is simple and accurate and allows evaluations of test units and design projections for scale up to commercial systems.