Mickenhans writes:
As it is important to get the highest possible temperature in the burning chamber, I would recommend to preheat ALSO the primary air flow, not only the air for secondary burning (= to burn the escaped wood gas).
Of course it is not relevant in a Finn oven where all the wood is burned, BEFORE the tube through which the primary air would flow, is heated up. But as you are trying to create a fire that has to burn for a longer time, it is possible to take benefits of this idea.
Probably my idea is not new but I haven't found any information on this subject.

Reinder/Demotech answers:
Indeed, in the prototype of the brickstove we build at Hans Baarslags's workshop (see preheating). The setup of this prototype was too rough and too leaky, also the tiles we used cracked immediately when heated. But the idea is clear: air could stream in under a thin stainless steel divider in a horizontal channel. Above this stainless steel sheet metal, the hot gas would stream out the fire box, in our design that is the place under the container where the wood could slowly sink and burn into the fire. Heat exchange between cold inflowing air and hot outflowing gas should be the result.

Mickenhans writes:
Does NO__x__ occur when burning wood at high temperatures?
In natural gas burners the temp. is kept below 650 degrees Centigrade, to minimize the production of NO__x__, and a second way to avoid this problem is to reclaim a small part of the exhaust gases and lead them with a vent back into the combustion chamber.

Reinder/Demotech answers:
I know nothing about this subject. We should ask the people of 'The 12 Ambachten'. They published about this subject. I'll post this question into their forum 'Verwarming, isolatie, huiselijk energieverbruik'