[Stoves] Excess air

Wed Aug 21 15:04:02 CDT 2013

Thanks, Ron

In essence I think it comes down to what you are trying to do.  Do you want
a crude measure of stove performance or an accurate one?  In running a
really big stove (and I'm thinking of one that burns around 90kg of fuel per
second) it is critically important to get the combustion optimal, with a
balance between CO going up the stack, C leaving in the ash and minimal
N2/Ar needing unnecessary heating. You have to get the excess air just
right.  You play around, altering it about 0.1% each time, until you get it
right for the stove and the fuel.  Near the optimum, 0.1% shift in the
excess air can cost you 100g of carbon per second, or 0.3% in efficiency as
you get excess nitrogen - that's how closely you have to monitor the excess
air.  So if the big stovers can use such a measure, why can't we?

Kind regards

Philip

From: Ronal W. Larson [mailto:rongretlarson at comcast.net] 
Sent: 21 August 2013 06:16
To: Discussion of biomass cooking stoves; Philip Lloyd
Subject: Re: [Stoves] Excess air

Prof.Lloyd:

   I agree that combustion efficiency is hugely important.  I would love to
see it reported separately  (and could be theoretically, I think as CO is
already measured and reported). 

   I would love to see excess air reported (and I think that possible also).

   I would love to see more on the oxygen content of various woods (air to
fuel ratios for combustion are given as 4-7. If you are making char, the
range is probably wider, as lignin, cellulose and hemicellulose all have
rather different O2 content, and contribute differently to char.  

   Why not separately also report on the H2 content of all the fuels, and it
also has a small content in chars.  Then we can move to sulfur.

   Why not report the lost radiant energy?

   If you had all this in an expanded WBT procedure  (with all Crispin and
you want about each and every fuel), what would the average user of Jim
Jetter's test do differently than they are now doing?  I just continue to
see these as useful academic exercises that don't advance stove development.

  I nit pick below a  bit more.

On Aug 21, 2013, at 12:06 AM, "Philip Lloyd" <plloyd at mweb.co.za> wrote:

Dear List

In one of his responses to Crispin, Ron said "RWL:  In summary, I think you
are raising issues that are hopelessly complicated for the world of stove
testing and comparisons.  I see insufficient reason so far to explore your
metric words "possible" and "to check" and "Obviously".   I hope you will
try again to convince this list (with citations), if you disagree."

I think stove testing needs to be comparable between both stoves and fuels.

     [RWL:  I think this is now being done.  Each fuel seems to have enough
known about it and it is reported.  The unused CO is reported.   I don't
know this for sure,  but think that the same stove with different fuels
gives very comparable results - and especially in a tier-ranking sense.
(Anyone have data on this last point?)

What Crispin has done is bring standard combustion theory to bear on the
question so as to allow this. 

     [RWL:  Jetter's work is about improving the performance via
measurement.  Details about each fuel doesn't seem as important as reporting
the fuel combustion efficiency - which is (sort of) in the data.  (meaning
high CO means low combustion efficiency).

    I am not sure that any important aspect of combustion theory is missing.
If so - exactly what?

I need, for instance, to be able to compare a
single stove burning either wood or charcoal.  

    [RWL:  I hope not too often. Rarely will the same stove be the right
thing to use for both fuels.

I have to be able to take the
oxygen present in the cellulose and other constituents of the wood into
account in calculating the excess air, because it contributes to the
combustion, whereas with charcoal there is essentially no oxygen present in
the fuel.  

   [RWL:  Maybe this is important for something you are doing, but why
impose this on all stove testing?  I think we should concentrate first on
getting excess air measured accurately, not on the percentage coming from
the fuel (which can mostly be determined from the literature - no need I see
to encumber each test with that level of detail).  The operation and
performance of charcoal-making and charcoal-using stoves is so different
(even ignoring how the char in a char-using stove was produced), that I
think the O2 content of the fuel is in the noise.  Can you give a counter
example?

The difference is real, measurable, and has an impact on the
efficiency of combustion.

   [RWL:  Yes to all - but I don't think it needed as a new adjunct to the
WBT.  On the best stoves  (tiny CO emitted) , the information will change
the second or third significant efficiency digit, and I am still worried
about the first digit.

I, for one, am convinced.

    [RWL:  I will be when I see a written justification for holding up
progress on getting an agreed ISO standard.   How about helping me get char
production ( a first digit issue) as an accepted part of the standard?

   Need to repeat, some of the above might have been covered in yesterday's
webinar which had  to be cancelled due to an equipment glitch.   Ron

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