[Stoves] Request for help on TLUD operating data
Ronal W. Larson
rongretlarson at comcast.net
Mon Aug 18 19:00:45 CDT 2014
Jaakko, cc list etal
1. This is to also acknowledge, with thanks, a list of 68 papers citing your own work, that you sent not long after this below. I will start trying to get into those as soon as possible.
2. I replied also a bit earlier to a third paper from you today - so this can be abbreviated.
More below. You replied with “Answer”. I will answer with bold numbered “RWLx”’s.
On Aug 18, 2014, at 3:55 AM, Saastamoinen Jaakko <Jaakko.Saastamoinen at vtt.fi> wrote:
> From: Ronal W. Larson [mailto:rongretlarson at comcast.net]
> Sent: 15. elokuuta 2014 0:15
> To: Discussion of biomass; Saastamoinen Jaakko
> Cc: Julien Winter; Crispin Pemberton-Pigott
> Subject: Re: [Stoves] Request for help on TLUD operating data
>
> Jaakko, cc list, Julien, Crispin
>
> 1. Apologies to Julien and Crispin for putting off direct responses to their helpful replies - as time is short today.
>
> 2. I vaguely recall half a dozen earlier messages from Dr. Saastamoinen (hereafter Jaakko), but I had not realized that we have had a real world-class combustion, gasification and pyrolysis expert contributing on our efforts to do a better job with simple third-world cookstoves. Extreme thanks to Jaakko for his response below, but also for all his papers and expertise on our list topic.
>
> 3. As I looked up Jaakko’s background, employment, etc - I was fortunate to find one no-fee paper that was highly appropriate to my yesterday question. It is downloadable from Jim Mason’s (rapidly improving!!) GEKgasifier website: I urge all interested in TLUD’s to read this 14-year old paper at
>
> http://gekgasifier.pbworks.com/f/ignition+front+saasta.pdf
>
> 4. Your figure 2, performed in a wonderfully instrumented (essentially a TLUD) laboratory experiment, showed the straight line for weight loss drop that I was hoping would exist. I have still only skimmed the paper, but your experimental evidence for a straight-line (making “C” and “C/B” small) was very encouraging for the possible TLUD design change I am working on.
>
> 5. A few questions:
>
> a. Are there later papers that may have extended your above paper (which, I repeat, is more informative on TLUDs in a highly scientific sense, than any I can recall). By you or others?
>
> Answer: There are several papers that have extended the paper. You can find from Scopus 68 papers published after this paper(Saastamoinen et al., Propagation of the ignition front in beds of wood particles. Combustion and Flame, Vol. 123, pp. 214-226, 2000) that are referring to this paper. However, these are more related to grate combustion. There are also many good papers on the subject that do not refer to my paper. One can find from COPUS links to these papers and then look references of the newer papers. This way one can get a broad figure about the subject. After this paper I have co-authored some other papers (Horttanainen, M.V.A., Saastamoinen, J.J., and Sarkomaa, P.J., Ignition front propagation in packed beds of wood particles. IFRF Combustion Journal, Article Number 200003, May 2000, ISSN 1562-479X, 21 p.,http://www.journal.ifrf.net/articles.html, electronic journal; Saastamoinen, J.J., Horttanainen, M., and Sarkomaa, P., Ignition wave propagation and release of volatiles in beds of wood particles. Combustion Science and Technology, Vol. 165, pp. 41-60, 2001; Saastamoinen, J.J., and Taipale, R., NOx formation in grate combustion of wood. Clean Air: International Journal on Energy for a Clean Environment, Vol. 4 (3), 30 p., 2003; Horttanainen, M., Saastamoinen, J., and Sarkomaa, P., Operational limits of ignition front propagation against airflow in packed beds of different fuels. Energy & Fuels, Vol. 16, pp. 676-686, 2002).
[RWL1: Thanks for these good leads. I believe I can see most of these at the NREL library - but not for a few days.
> .
> b. Because of my background, I think of the upward flow of primary air and pyrolysis gases as three resistances and a “current” (gas) generator. The lowest chip/pellet region has a resistance RL that continually gets smaller as its volume decreases. The upper (char) region keeps gaining in height, but is losing weight rapidly as well; for height (and other) reasons it (RU) presumably is increasing (at the end of a run, RL is zero). The middle pyrolysis zone resistance would not seem to change much during a run. And the “current” source also would not seem to change much during a run (But maybe it does.)
> So my first (electrical analog) observation is that the only way that we can have a straight line is if the sum of RL and RU is a constant (call RC)
> Second - If I had to guess that the change in either would “exactly” balance the other, I would have said no way. But for us, it is decidedly serendipitous/fortuitous.
> The reason for RU increasing must include viscosity changes.
>
> The second question is - your explanation for this nice linear behavior) and what different data might exist for fuels other than the thin chips used in the test results shown in your Figure ? (Pellets, vertical “straws”, small “blocks”, etc) Is there numerical viscosity data we can use for this pyrolysis gas combination?
>
> Answer: The references, especially this paper (Horttanainen et al., Operational limits of ignition front propagation against airflow in packed beds of different fuels. Energy & Fuels, Vol. 16, pp. 676-686, 2002) presents results for other fuels with different shapes (saw dust, woodchips, pellet etc..). You could ask Prof. Horttainen (mika.horttanainen at lut.fi , Lappeenranta University of Technology) his thesis on this subject. Danish researchers have studied combustion of a bed of straw. We made a experiments with different fuels (wood chips, wood blocks). I think is it is public because funding was from EU-project so I can send it (Saastamoinen et. al, Experiments with a pot furnace and a prototype horizontal stoker burner, Research Report ENE3/T0059/99, 34 p.+app. 2 pp. VTT Energy) on request (but next week at the earliest).
[RWL2: This is to ask Prof. Horttanainen for information on how to find his thesis.
I was unable to find your report “ENE3……” at the VTT site, so would appreciate your sending this. It sounds appropriate.
>
> For what purpose you need the viscosity (for pressure loss in the bed)? Gas viscosity can be calculated (methods of Bomley & Wilke, 1951), if the combination of the gas is known. However, tar may give some problem in the calculations.
[RWL3: Yes for pressure loss. I am postulating that the higher and increasing total viscosity in the char is approximately balancing the reduced pressure drop in the depleting lower fuel supply. If we had some theory (or data) on this, we might find changes with moisture or fuel shape or temperature (or something) that would help to keep the term “C” small - and allow secondary air to be controlled “ exactly” as one controls primary air.
I have seen some viscosity data for some gases at different temperatures - but none for what I think we have in the top part of a (Natural Draft) TLUD.
>
> c. Finland being such a “hotbed” (pun intended) for forestry, science, and bioenergy, is there any organized Finnish work about stoves - and especially char-making stoves for developing countries? Or char-making saunas? (I have visited Helsinki twice - and hugely impressed by the society I have seen - but know very little about VTT or other bioenergy groups.)
>
> Answer: There is some research on wood stoves for heating purposes and saunas at VTT (see for example: Paloposki, T., Saastamoinen, J., Klobut, K., and Tuomaala, P., Analysis of wood firing in stoves by the oxygen consumption method and the carbon dioxide generation method. Biomass and Bioenergy, Vol. 61, 1-24, 2014) and at University of Eastern Finland but practically no research on stoves on cooking. One of my research area is solid fuel combustion. Cooking with TLUD is a hobby. It also gives an insight on the different combustion processes occurring also in big power plants for solid fuels. There is some research on production of char (for later combustion in coal plants and as by-product in production of bio-oil.) and terrified wood (by mild pyrolysis) but I am not aware of research on biochar for agriculture.
Saastamoinen et. al, Experiments with a pot furnace and a prototype horizontal stoker burner, Research Report ENE3/T0059/99, 34 p.+app. 2 pp. VTT Energy) on request (but next week at the earliest).
[RWL4: Yes. This would be of interest. Anything on char production.
>
> Question 3. Should we be promoting VTT or any specific Finnish groups with GACC or similar? I am suggesting, based on the little research I have now done, that Finland has a lot to offer in our small charcoal-making world.
> Answer: University of Eastern Finland (its aerosol group) is specialized in measuring and studying emissions (particles, etc.) from stoves.
[RWL5: I found their site - but nothing on charcoal making. See more further below.
> A few inserts below.
>
>
> On Aug 14, 2014, at 6:31 AM, Saastamoinen Jaakko <Jaakko.Saastamoinen at vtt.fi> wrote:
>
>
> Dear Ron,
>
> I have studied propagation of the ignition front in a fuel layer (considering grate combustion) and published some papers on it.
>
> There are different stages.
>
> 1. Ignition at the top. It takes some time that the ignition front propagation reaches a steady velocity.
> [RWL: Yes, but with controllable primary air, which most TLUDS allow, there can be large early primary and then a cut back. And usually operated exactly that way.]
>
>
> 2. After that the ignition front propagates approximately at constant velocity. There may be some increase in the velocity at the initial stage due to the accumulation of hot char above the pyrolysis front giving more radiation to the non-ignited fuel. The velocity depends on the primary air rate. If the primary air rate is too small, the front does not propagate.
> [RWL: We have been calling this TDR=turn down ratio. Any theory on how large this can be? I think there is experimental evidence of at least 5.
>
> Aswer: The papers of Horttainen et al. studies the operational limits.
[RWL6: I look forward to reading these. Low TDR is considered a problem for TLUDs - so we need to find ways to improve this parameter.
>
> Also, if the air rate is too high, the burning is quenched due to cooling by the air so that the flame temperature goes down giving less radiation to preheat the non-ignited fuel and also keeping the non-ignited fuel cool.
> [RWL: This is the first time I have seen this. I can see a problem with a fan/blower, but also natural draft?
>
> Answer: Not on natural draft but forced flow.
[RWL7: Yes I missed this constraint at first We need to better understand both - and when to recommend one over another.
>
>
> The velocity of the ignition front has a maxim at certain air rate. There is accumulation of char above the pyrolysis front. The maximum amount char is obtained with low primary air rate (but high enough to keep the front moving). If the air rate is high, also some char is burned above the pyrolysis front due to excess air especially if the fuel is moist.
> [RWL: I am sure you can help us with fuel moisture issues. Should we be “curing/drying” all fuel (maybe above the cookstove)?
>
> Answer: Fuel drying before is beneficial to get high yield of char. The ignition is propagated by the heat from the ignited fuel. So more heat is required for heating the fuel to ignition. In addition, water vapour the effective heating value of the mixture of pyrolysis gasesand water vapour. If the fuel is moist, the heat in the pyrolysis combustion is not enough, but also char is burned. For moist fuels only a thin char layer above the front is formed due to combustion of char.
[RWL8: Agreed to all. Can we claim that one should always try to pyrolyze the driest possible feedstock?
>
> The excess oxygen that is not consumed in the pyrolysis front reacts with char giving less char. Even the ignition velocity is quite constant, the burning rate of the whole batch including the char may increase during the burning with high primary air rate, since the amount and thickness of char layer accumulating above the pyrolysis front is increasing and can react with excess oxygen. Then you would have a positive value for C in your formula (considering the whole weight loss of the batch), if there is much excess air.
> [RWL: The design mod I have in mind requires small C, but keeping below a certain primary air flow rate should not be a major constraint. By “excess air” in the last sentence, I presume you mean excess primary air? (we have been using “excess” with secondary air)
>
> Answwer: Yes, I mean excess primary air.
[RWL9: I should have recognized this earlier.
>
> 3. In the end, the pyrolysis front reaches the bottom and this is may also be accounted by the term C*t^2 in your model. In this stage the amount of pyrolysing particles at the bottom decrease leaving more excess air to react with the char. It seems that sign of C depends on the air rate. With high air rate, the sign goes to more positive direction, since the rate of flame propagation is low in the stage 2, but the rate of char combustion becomes high at the stage 3.
> [RWL: I need help with the terms “stage 2” and “stage 3”. I think that with controllable primary air, that we can avoid the "high rate of char combustion” at the end of a run - assuming (as I do) that we want to maximize char production. I presume stage 3 is this final stage as the pyrolysis front reaches the bottom. And (a question) stage 3 is the 95% of the time period with a “constant” power level (constant rate of fuel conversion)? So “stage 1” is the short start up period as the pyrolysis from moves.
>
> Answer: Yes.
[RWL10: Thanks. To repeat correction of a typo of mine: The 95% sentence should refer to stage 2, not stage 3.
>
> Again. Thanks very much for a very complete description of TLUD operations - obviously informed by the above cited reference, available from Jim Mason and the “GEK” folks.
>
> Ron (on 14. elokuuta 2014)
>
>
Again thanks.
Ron
>
> Jaakko
>
>
> -----Original Message-----
> From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Ronal W. Larson
> Sent: 14. elokuuta 2014 0:39
> To: Discussion of biomass
> Subject: [Stoves] Request for help on TLUD operating data
>
> List:
>
> I am trying to better understand TLUDs - for purposes of improving their performance. This is to ask all char-making stove developers to report back, either on-list or off-list, on a fundamental characteristic of all TLUDs: the way performance changes over time.
>
> I have seen plots showing the weight of the stove-fuel combination; basically dropping relatively linearly as the pyrolysis front moves from top to bottom. My question is on the word "relatively". Is that weight loss at the end dropping more (concave down) or less (concave up) rapidly than linearly?
>
> Stated as an equation, for a test run with a single fuel and a single primary air center the average linear equation could be:
>
> W(t)= Wo-A*t, with A = (W(t=tf)-W(t=o))/tf
>
> but what I need is a quadratic form:
>
> W(t) = Wo - B*t + C*t^2
>
> where B is certainly positive, but equal to A only if C is zero.
>
> C can be either positive (concave up) or negative (concave downward). The sign of C is the most important question I am asking - but even an approximate ratio C/B would be a big help.
>
> I only need a rough plot of the weight - then I can come up with the A,B, C constants. I for sure need the starting and ending weights, and then at least a few other weights (and their times) near the end. Knowing more on the stove would be helpful - especially the fuel can diameter. The fuel (sticks, chips, pellets, etc) is needed also.
>
> Who can help? (and leads to published material would be just as good.)
>
> I will summarize and report - and can do this without using any names.
>
> Ron
>
>
>
>
>
>
>
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