[Stoves] Saving the WBT

Thu Aug 22 14:46:05 CDT 2013

Dear Ron

Here is a global reaction – some of the chemistry that is active in biomass
burning:

That is taken from a paper by Authors+Tom Miles “Fuel properties of biomass”
1998.

It is really complicated. We do not need to go into all of it, but we should
remember that our simplifications hide information.

    O2-(CO/2)     .

21- [O2-(CO/2)]

    [RWL:  I have found this formula nowhere.  Cite?  

It is the standard way to calculate EA.

>What i have found is that EA can be found from either measuring O2 or CO2,
no need for bringing CO  into this one formula.  

There is a different formula for the calculation of EA based on CO2 instead
of CO. In both cases one is used to estimate the other. When both are
measured, the formula can be edited to be more accurate.

>I recognize that CO is important, but for a useful stove (not a charcoal
using stove), this has to be a very small modifier on a term that we aren't
even reporting in the tests  (but I would like to know).  Giving just O2
levels in the exhaust stream would be very illuminating.

The O2 level does not tell you how much extra air was put into the fire, it
only tells you what came out. What came out is important, but knowing if the
correct amount is being put in is important. Important relationships between
EA and CO(EF) are revealed by plotting them in real time.  Tuning a stove to
a fuel is done this way.

>>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.

     [RWL  A cite for this "wrong" statement?   

If the O2 supplied to a fire is 100% above that which is required to burn
the gases present at the time, and the formula reports that it is zero, or
300%, that is a wrong answer. The standard formula is a simplification of
the actual answer. For some conditions it is ‘good enough’. Others, not.

> We should be talking separately about "flaming pyrolysis" in TLUDs (or
maybe some gasification approaches) and retorts  With the latter, no O2 is
added.  

There is already O2 in the fuel if it is biomass.

>The available O2 is no doubt important in a retort operation, but since we
can't control it, I see little reason to quantify it, except in very general
terms  Retorts are not very attractive in stove designs because they are not
controllable.  

The World Stove is a self-heating retort.  It is controllable.

>I can't see any value in the term EA  (or AER) with operation of a retort
(and some aspects of a retort are probably occurring in TLUDs).

Fortunately designers see the matter differently.

>It looks to me like simplification is badly needed.  Doing zero computation
looks OK to me

You can directly measure mass and time and temperature and gas
concentrations but not much is learned from them directly. Metrics are
mostly products and ratios.

[RWL:   I guess we have a few promoting gasifier stoves, but why not simply
combust (as in a rocket) if you don't care about char.  

Because a Rocket stove is not all that successful a design and has
limitations inherent in its design rules. It is, for example, hard to burn
pellets in a Rocket stove. It also produces a relatively large amount of
char which is wasted (compared with other combustors). The system efficiency
is not as high as other inherently different designs. A lot of these
limitations are overcome with the institutional stove sizes. Other are not.
One size does not fit all nor should it. Not a problem.

>The TLUDs are being promoted largely because they are not gasifiers - they
are pyrolyzers with an intent to produce char (the definition of pyrolysis).

They being so promoted by some, other are promoting them because they burn
with low PM, not because they make char. Some TLUD’s make a lot of PM so the
‘layout’ is not universally successful. 

>
I don't see any difficulty in comparing stoves that operate on a total
combustion philosophy or a gasification (minimum char) philosophy.  One
operates on a parallel process philosophy and the other on series.  The
final results as a stove could differ a lot on pollutants, but the testing
should go similarly.  

Actually it is quite hard to write a protocol that is fair to both stoves if
the entire burn is not treated as a single event. The reason is that a
stick-burned pretty much burns whole fuel progressively, while a batch
loaded stove often has a different fuel composition as time passes. Thus
anything that is assessed over say, ½ the burn time can give very misleading
results for a metrics like CO per MJ because the number of MJ is difficult
to know without a real time method of analysis.

>Not so for pyrolyzers.  Something theoretical on the production of
particulates and CO (and PAHs) could be very helpful for stove designers.

Pyrolyzers face the same issue – what is burning and when? At the end of the
burn, for the last few minutes, the fuel composition changes quite a bit and
this is very easy to see on the real time gas charts and the kW plot. The
power level rises just at the end as the bottom char burns to CO2. In the
beginning I have seen very large amounts of fuel moisture coming out,
meaning the heat is being absorbed drying fuel, not heating the pot. Pellet
stoves are pretty consistent after ignition.

>
We are getting close to a metric that will describe TDR = turn down ratio.

The WBT turn down ratio is not appropriate, as it is merely the ratio of the
simmering to boiling power. Any gas of electric stove has an easily defined
TDR – move the lever. For manually adjusted fires it is difficult to say
exactly.

>>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.

    [RWL:  In summary, I think you are raising issues that are hopelessly
complicated for the world of stove testing and comparisons.  

Applying a different formula that gives the correct answer in as many
circumstances as possible is as easy as entering the formula in a
spreadsheet.

I hope these answer will clarify the issues around the correct calculation
of EA.

Regards

Crispin

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