On 08/02/2021 13:20, Jeff Layman wrote:
why the wood is supposed to burn better on a bed of ash.


My thought on this is a bit fuzzy but in general supplying one unit of
oxygen and 4 units of nitrogen will release about the same amount of
heat whether the fuel is wood or coal. Too much primary air will gasify
more char and require more secondary air to burn the CO. so you could
end up with too much heat and insufficient oxidation of the products .

Only varying the downwash air means that all the volatiles get burnt
first and then any air left gets to the char.


This can get quite pronounced with dry wood burned at high power, like
now, a char bed builds up a few inches thick and then burns away before
more wood is added. In fact I can produce about 1kg of charcoal a day if
I wanted by smothering it once the flame goes out.

If you consider oven dry wood is about 50% carbon and the remainder is
hydrogen and oxygen in near the same proportion as water and has about
15-18MJ/kg you will see this is close to burning the same 0.5kg of coke
with 30MJ/kg.