There is nothing magical about microwaves - they are just heating the wood to a high temperature where the carbon chars. You can do the same using any form of heat - the low tech and cheap solution is to use some of the waste wood to generate the heat.
Except they heat by increasing molecular dipole rotation which, while not magic, is different to conduction. Which means the heat can penetrate more efficiently.
Is heat penetration really an issue with making char from waste wood?
I have seen some very clever designs where the syngas generated by the carbonisation process is used to provide the heat to carbonize the wood. With this approach you can drive the whole process cheaply with a very low carbon loss.
In practice it is not that different to just rapid heating. The organic chemists have been arguing about this for some time [1] and any non-heating effect is very minor.
