Here is one by Doug Williams from Fluidyne. It is supposed to produce little or no tar. This was posted as part of their 25th anniversary a few years back.
It depends on what the purpose is. For 40kw shaft-power I agree with Kim.
For heat only requirements a much simpler system is the stratified-downdraft, or Top-lighted updraft.
These types can be configured open top, for continuous operation.