[Stoves] Saving the WBT

Mon Aug 19 11:00:05 CDT 2013

Dear Ron

Very good question and correctly put.

Excess air is air above and beyond the need of the fire to have
stoichiometric combustion. Typically for good heat transfer efficiency and a
good CO burnout it should be between 50% and 150% with 80% being typical. It
is rare to get a <25% EA value and get low CO and H2.

So, what happens if the excess air drops to 0%? Then it means there is air
getting in, but only enough to run the combustion theoretically. In practise
there is a lot of CO and H2 that does not get burned because those molecules
can't find that last remaining O2molecules available. 

The normal calculation of EA is actually not correct from a chemically
balanced point of view. That is why I developed the chemically balanced EA
calculation embedded in the HTP spreadsheet. Very briefly, think of it this
way. A rocket engine using solid fuel burns in the emptiness of space
without any air at all. How? The O2 is embedded in the chemicals that make
up the fuel. It is quite possible to have excess air (equivalent) in a space
rocket. So.what is the difference between that and a fuel that has a lot of
oxygen in it? Only scale. Biomass has a lot of oxygen in it. Cold damp fires
can also generate O2 from water using the water gas shift reaction.

All these things are taking place in a stove so we need a way to calculate
what the available oxygen is and turn that available O2 into a figure that
means something - a metric for available 'air' which is actually oxygen
treated as if it was the amount of air that would contain that much oxygen.
If I roast wood in a retort at low pressure I can extract quite a lot of
oxygen. It might react with H2, it might not.

Back to the EA being zero. When the EA is 0%, the O2 that is available might
come from the fuel (burning biomass) or from some air (kerosene fire). Where
it comes from is not so important, we just need a metrics for correcting
calculating how much of it there is so we have a clue what to do to improve
combustion.

Standard EA is

    O2-(CO/2)     .

21- [O2-(CO/2)]

It gives the wrong answer for real fire burning any fuel that contains O2,
like wood or coal but gives a pretty good answer for fuels like kerosene or
pure charcoal.

When the fire (pyrolysis) is dependent mostly on O2 from within the fuel,
the amount that has to be added in the form of air is far less than the
stoichiometric necessity. So, how to quantify that amount?

The analysis that says the air supply was 1/3 of that needed by the fuel is
determined by measuring the chemistry in gas produced. Depending on how the
analysis is done, it may also give a consistently wrong answer - I did not
check yet but all the indications of over-simplification are there.

Whether the answer is 'right' or not, there is usefulness in having a metric
for the amount of air that was theoretically the minimum to burn the fuel.

With a gas-maker, the amount will be expressed as a fraction or % of the
stoichiometric air demand. The result is a gas with some O2 in it (in the
form of CO and CO2 and maybe HO and so on) but not nearly enough, all taken
together, to burn the gas. That is the point of a gasifier - to make
unburned fuel that can be burned in a gas stove.

So in order to discuss how much air was supplied, we need a way to talk
about it. It is expressed using a metric that is based on the elemental
analysis of the fuel. Therefore if you change the fuel, it changes the need
for air in absolute terms but maybe not in relative terms. We can live with
that - it at least put the discussion on the page in a manner that other can
follow.

Because the metric is based on the elements in the fuel, it is possible it
is, unlike the standard EA calculation, correct 'chemically'.  We will have
to check. That is the sort of check we should be doing for all metrics
before we run off to make them international standards. Obviously.

Regards
Crispin

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Ronal W. Larson
Sent: Monday, August 19, 2013 11:15 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Saving the WBT

Crispin and list:

   Sorry.  Still not understanding.  Who in the stove business has a problem
with excess air that is too small?  I read about EA ratios of 3, 4, 5.., not
0.3, 0.4, 0.5 ...

Ron

On Aug 19, 2013, at 9:06 AM, "Crispin Pemberton-Pigott"
<crispinpigott at gmail.com <mailto:crispinpigott at gmail.com> > wrote:

Dear Ron

>I am going to stay away from equivalency ratio until I see some way to use
it.

It is used to talk about the air supply when there is no excess air
available. Once EA goes to 0%, how do you describe a further reduction in
the air supply?

So that is the use for it.

Regards

Crispin

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