[Stoves] Excess air

[ajheggie at gmail.com]

[Default] On Wed, 21 Aug 2013 21:33:06 +0000,"Crispin
Pemberton-Pigott" <crispinpigott at gmail.com> wrote:

>
>I followed everything until the end. The last paragraph. 
"So yes stoichiometric air does vary as wood burns and excess air
mirrors this but it is not reasonable to mix this up with the fact
that oxygen from combustion air and from the fuel is conserved in the
exhaust gases."

I'll try and rephrase that: for a given dry lump of wood although the
surface may be simultaneously pyrolysing and the nascent char burning
to ash while the evolved offgas burns the centre is also heating up in
the absence of combustion air, the pyrolysis products are also
reacting within the mass, so pyrolysis offgas is diffusing out of the
lump. This offgas initially has species, like acetic acid that are
already moderately oxidised and react with little combustion air to
release little heat. Later, as the lump gets hotter, more fuel rich
gases like methane, hydrogen and carbon monoxide as well as tar
vapours are evolved and these require more oxygen for the secondary
combustion.

As excess air is always necessary to increase the chance of an oxygen
molecule reaching fuel molecules as stoich air varies so must excess
air to maintain the same level of free oxygen in the exhaust.

It is free oxygen which we are measuring with a wide band lambda
sensor (I think lambda being the accepted character in internal
combustion for equivalence ratio even though these sensors cannot
measure below zero oxygen as they are cells that generate a voltage
dependent on, but not directly, the proportion of oxygen in the flue
gas on one side and air on the other. e.g. they will still measure an
unvarying zero oxygen even if the fuel:air mixture is so rich CO is
being exhausted.)

>Re coal and Oxygen, there is not so much in the coal but wet coal fires can make O2 during ignition. 

What reaction do you surmise releases this oxygen?

AJH