[Stoves] Saving the WBT
Frank Shields
frank at compostlab.com
Mon Aug 19 14:27:34 CDT 2013
Actually Ron it started with me and Saving the WBT : )
But you do have a very interesting discussion going on and should continue.
Perhaps changing the subject.
Regards
Frank
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Ronal W. Larson
Sent: Monday, August 19, 2013 11:17 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Saving the WBT
Crispin and list:
Sorry but I am even more confused than earlier.
This discussion started with Paul Olivier's term "air equivalency ratio"
and a number of 0.3 being good (I think only referring to a pyrolysis zone
(Paul uses the term gasification - which bothers me a good bit. Tom MIles
supports a sister list on gasification - where they are trying to get rid of
all char, not save it as some of us [especially Paul] want on this list).
You have not discussed this term (I will call AER) and so I repeat that I am
going to ignore AER until I see a site I can read about it. I still do not
know whether this AER term includes excess air (EA) - a term that you do
discuss. If they are not totally different "animals" I will be surprised.
The problem is that your description of excess air (EA) does not comport
with what I thought I knew. So I have read in the following, using the
Google search term: "definition of excess air ratio" (The added "ratio"
hopefully being OK). I read through these (without finding some of what you
have below)
http://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio
<http://en.wikipedia.org/wiki/Air-fuel_ratio>
http://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter11.html
http://www.engineeringtoolbox.com/boiler-combustion-efficiency-d_271.html
http://www.gilsoneng.com/reference/combustion.pdf
http://www.brighthubengineering.com/machine-design/15235-the-stoichiometric-
air-fuel-ratio/
see more below
On Aug 19, 2013, at 10:00 AM, Crispin Pemberton-Pigott
<crispinpigott at gmail.com> wrote:
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.
[RWL: agree totally]
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.
[RWL: agree totally]
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.
[RWL: Where do I go to see this HTP spread sheet? Why is the
"normal" approach not good enough? I don't find the comparison to a rocket
fuel (a lot of Aluminum) helpful.
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.
[RWL: I don't see why stove designers or testers should need to worry
about the O2 in the fuel. Nothing I have read gets to that level of
detail. You can measure the O2 in the fuel, in the incoming air, in the
resulting char, and in the exhaust stream. Every fuel is different. Every
stove is different. Why calculate anything - why not measure?
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).
[RWL: If EA = 0, there should be no "O2 that is available". What am I
missing in this 2nd sentence? Why introduce kerosene into this? I hope
stove testing doesn't have to report the O2 levels of starting fuels.
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)]
[RWL: I have found this formula nowhere. Cite? 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. 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.
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? I view O2 level measurements
important in knowing how to optimize efficiency. CO is important for health
reasons and knowing more about combustion efficiency. But since high CO
leads to low EA numbers in this formula, we should be careful of your (only
your?) definition.
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?
[RWL: We should be talking separately about "flaming pyrolysis" in
TLUDs (or maybe some gasification approaches) and retorts With the latter,
no O2 is added. 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. 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).
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.
[RWL: The chemistry is very different with the rate of temperature
increase (that is fast and slow pyrolysis give very different product
streams. I can't see how any form of computation is worth the effort. It
looks to me like simplification is badly needed. Doing zero computation
looks OK to me
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.
[RWL: Paul raised this issue in the context of pyrolysis. Not "burn
the fuel". Every char output is different, even with the same run for
different locations in the stove. And I am not understanding the term
"metric" here. And I think we should care about answers being "'right' or
not"
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.
[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. The TLUDs are
being promoted largely because they are not gasifiers - they are pyrolyzers
with an intent to produce char (the definition of pyrolysis). The original
discussions on this list about TLUDs only were on how to do better than the
10-20% conversion of most char-making in the bush. I'd like to see a
rewrite of this paragraph addressing TLUDs and retorts. 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. Not
so for pyrolyzers. Something theoretical on the production of particulates
and CO (and PAHs) could be very helpful for stove designers.
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.
[RWL: You have a big "maybe" there. In one cite I found, the air/fuel
stoichiometric ratio for wood combustion was given as 4-7. I'll bet the
equivalent "metric" for primary air in making char is wider. I wouldn't
even know how to define it, since it will vary with the magnitude of the
primary air flow (related directly to the power out). Maybe we are talking
about defining a term that, if too low, will not support pyrolysis, but if
too high will give soot on the cook pot (or similar). We are getting close
to a metric that will describe TDR = turn down ratio. EPA testing (webinar
tomorrow) could determine this perhaps (with a change in procedures). This
"metric" (if it exists) could involve fuel elemental analysis, but I guess
involves a lot more. Mostly, the high end is likely to be limited by the
amount of primary air, which is limited by the draft, fan speed, etc. The
low end influenced maybe by the intensity of the radiative transfer.
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. 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.
Ron
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:
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" <
<mailto:crispinpigott at gmail.com> 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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