This is a rocket stove type design that uses wood as the insulation in the chamber around the chimney (riser/stack pipe). This wood is pyrolized by the heat of the stove and the produced gases are fed into the chimney and burnt, As air does not flow through this the wood it releases gasses and turns into biochar/charcoal rather than burning to an ash.
The stove design was based upon a design made by Edward Revill from Swansea Biochar and developed with his assistance.
See a video of Ed explaining this stove.
3D models were made using Art of Illusion http://www.artofillusion.org/ STL CAD file for top of forest stove