I am concerned about any retort system, it seems to me likely that not all the material will be subject to equal destructive temperatures. Look at what happens in making charcoal. That makes it likely that toxins will be produced.
The only system I would trust without pushing a lot of technology at it is a downdraft gasifier, that is guaranteed to subject everything to consistent destructive temperatures. Hopper residue and ventilation could still be a concern unless final burn is on pure biomass.