A common alternative is a rotary kiln, similar to a giant cement mixer, where the entire heated vessel is rotated to evenly heat and mix the feedstock particles. The walls of the kiln must be very thick, to bear the entire weight of the biomass, steel vessel, and insulation (if it is insulated). The bearings of a rotary kiln are massive, and must be designed to withstand high temperatures. Because the entire kiln rotates, loading biomass and unloading char during continuous operation, injecting hot dry gas, and extracting vapors without allowing air to enter the vessel all become complex engineering challenges that are expensive to implement. And of course the energy needed turn the kiln plus tonnes of feedstock thousands of times per day will be substantial.
One of the main issues with this design is that the residence time of the feedstock cannot be regulated with any degree of precision. The internal vanes guide the biomass through the vessel, but some tends to fall back and some fall forward. So by the time the biomass traverses the entire kiln, there is typically a 50% variation in residence time. Hence the feed rate must be significantly slower than optimal to wait for the delayed particles to be fully processed, and that many particles will be over-processed.
To develop a staged heating process with this approach, to make it more energy efficient and productive, would require 3 separate vessels all with separate intake and outtake mechanisms for solids and gases, bearings and drives. This becomes even more expensive to build, operate and maintain than a single vessel design.
Repair and maintenance costs for rotary kilns tend to be high. Just taking it apart, or lifting it a bit to replace the bearings, can be a challenge because it is so heavy. And of course, all that steel is expensive to begin with. This approach to distribute heat might be fine for a clothes dryer, but when scaled up to tonnes of biomass, it is too primitive to have the versatility and financial performance we need for our product line.