The advantages to the approach are several. One is that we can control to a significant degree the temperature of the pyrolysis reaction because of the closed retort. You cannot do that in a system where combustion and pyrolysis are combined in the same vessel. Another is that the firebrick enclosure retains and refracts a significant amount of heat, enabling the use of much less wood to drive the reaction. Ideally, several batches per day are run so that the batches that follow the first need even less fuel, as the thermal mass of the firebrick retains significant heat.