That's perhaps the critical question. Where my chemistry falls apart is in figuring out the reaction chain for a complex hydrocarbon like PET. I assume that the coal bed in the primary chamber acts as a cracking tower, so that a good deal of the sequence happens before you get to the flame front. Is there anything that requires a longer or more complex intermediate stage than wood does? Are there any nasty intermediate hydrocarbon compounds that don't break down easily in the combustion environment? I would guess that there's a good deal less than 1.5 seconds between the end of the flame zone and the heat exchanger pipes that serve to drop the gas below reaction temperature. However, there's a good deal of time spent at elevated temperatures in the primary chamber.