[Stoves] Excess air

Wed Aug 21 12:10:46 CDT 2013

Dear Andrew

You raise an important objection and it is not, in my experience, correct.
It is only correct when 'things go normally' such as in a constant state
furnace burning a low oxygen content fuel such as kerosene or coal.

" I disagree. Stoichiometric air is the amount of air needed to combine with
the fuel if combustion goes to completion and there is no fuel or oxygen
left over."

Actually is it not as simple as that for a fuel with any O2 in it. How do
you know if the O2 came from within the fuel or from air supplied to the
fuel? Thus the constant burning condition is different from the evolving
fuel composition condition - the latter being exactly what we have in a
pyrolyser.

"Excess air is that air over and above the stoichiometric air that is
supplied in order for all the fuel to be able to "meet" and combine with
oxygen from the air. 

Only by definition, not in practise. In practise conditions were being
regularly encountered when the calculated excess air ratio was -120,000% or
-185,000% (a negative value) when shortly before it had been at 2000%, a
positive value. The problem was traced to the formula itself - it does not
treat the combustion in a chemically balanced manner - that is, it makes
assumptions about the availability of O2 as if there was none in the fuel.
It is an incorrect assumption though it holds for gasoline and therefore
most vehicles.

That is why I developed a new way of calculating the 'effective EA' meaning
what the EA would have to be to get the conditions we are measuring, as if
there was no O2 in the fuel.

"It thus follows that there will be oxygen in the exhaust that is equal to
the excess air supplied."

Well that is the root of the problem. It is only true under certain sets of
conditions.  What happens when the reading for O2 plus the reading for CO/2
(which is to turn the O in the CO into O2 that supposedly came from the air
supplied) sums to a number larger than 21%? The denominator in the formula
turns negative.

That was the source of the impossibly high negative values for EA in a stove
that had a fire burning in it and obviously high excess air.

The 'correct' values for the place held by the number 21 in the standard
formula should actually be the total O2 in the emitted species, including
combustion moisture which id at present ignored completely.

The formula (HTP version) then replaces the CO/2 with a detected species
that still have oxidation potential so as to represent that potential
correctly. It is subtracted from the numerator's O2 measured value on top
and on the bottom.

   [(O2-(CO/2)]

[21-(O2-(CO/2)]  x 100 = EA%  is the standard formula, in case anyone forgot

Here is the alternative

   [(O2-(CO/2 plus all other oxidising potential)]

[All detected O-containing gases factored to O2)- [(O2-(CO/2 plus all other
oxidising potential)]  

With all values in %.

The result is the chemically balanced correct EA value (still pretending O2
is actually from real air) no matter what the conditions in the fire are and
no matter where the O2 came from. It works for rocket engines with excess
air (but consuming no air at all) and for stoves in which O2 is being
manufactured from the available water in the fuel. Because we know the O2 in
the fuel and the O2 in the air does not mean we know the O2 that will be
manufactured in the fire.

I measured stoves with a burning wood+coal fire that had O2 stack
measurements above 21%, even as high as 24%, while the fire was burning.
That O2 above the ambient level was coming from the fuel and the water in
the fire. So what then is the EA under such conditions?

"The fact that the fuel is already partially oxidised makes no difference to
the excess air value."

Actually it makes a measurable difference.

"It does make a difference to the total oxygen in the exhaust constituents
but that is an entirely different matter."

It is part of the same matter. EA is an expression of the availability of
O2, expressed in a way that assumes all O2 comes from the air. The
'standard' formula only works in certain cases. We have been using the
alternative in the HTP for at least 5 years for that reason. For those cases
for which the old version was intended, they give the same answer. Otherwise
they do not.

I know most people are not interested in this level of chemical detail which
is why Profs Lloyd and Annegarn and I talk about it at conferences and
dinner, not here. But the error is real and the effect on reported values is
real and there is a solution.

I believe that with the right equipment we can tell when the water
evaporates from the fuel which would lead us to the point where we can tell
in real time what the fuel composition is as it burns. That will give us
much more accurate firepower values in real time.

This is stuff fascinating.

Regards

Crispin

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