[Stoves] Saving the WBT
Ronal W. Larson
rongretlarson at comcast.net
Mon Aug 19 13:17:03 CDT 2013
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://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" <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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