[Stoves] Excess air

[Andrew Heggie]

On 21 August 2013 19:07, Crispin Pemberton-Pigott
<crispinpigott at gmail.com> wrote:

>
> Interestingly, for the same level of O2 in the stack, the EA is different
> under different chemical conditions. Complex pyrolysing conditions provoke
> all sorts of strange chemistry so the chemically balanced approach is
> required, actually. For a given device, the best combustion tends to take
> place under a certain EA level, but this varies (a lot) between devices. The
> BLDD6 coal stove works best with 30% EA which is a very low value and really
> surprised me. I would have guessed that was not possible.

OK we're talking at slightly cross purposes. I think where you are
coming from is to do more with the batch nature of most cook stoves.

With a premixed fuel:air system, like a gas flame, the reactions are
for all intents and purposes instantaneous, the oxygen disociates in
the flame and immediately grabs a couple of  hydrogen and then soon
after another oxygen reacts with the left over carbon.

When you burn wood the sequence of necessary events is longer and
unle,s the wood is very small, like sawdust in a cyclonic burner, the
processes all overlap. So a stick may be completely burning at its
surface whilst the middle is heating up. As wood heats up and
pyrolyses it evolves an offgas whose constituents change, early
species emitted are things like acetic acid and this has a high oxygen
content, later the major constituents are things like carbon monoxide,
hydrogen and methane.At the end when just char is burning there is no
contribution of oxygen from the fuel in species in the exhaust.
Clearly these changes in offgas require differeing amounts of oxygen
to burn out, so stoichiometric air changes with time.

The thing about burning coal is that it contains barely any oxygen and
hence this does not have any contribution to to the ultimate analysis
of the exhaust.

In fact burning coal with very little excess air is necessary because
coal burns hotter than wood and hot enough to form nitrogen oxides if
oxygen is present. This is why air supply is cascaded into the
reaction after cooling the initial combustion to keep temperatures
below the point at which NOx would form (1500C??). This quenching is
the very thing we wish to avoid with a flame in a cook stove because
we need to maintain high flame temperature to allow carbon particles
to burn out.

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.

AJH