[Stoves] Trials on TLUD Gas Burners - Counter Current Flow
Dean Still
deankstill at gmail.com
Tue Aug 5 00:41:32 CDT 2014
Hi Ron,
Kirk has returned to Aprovecho and we are doing a complete test of his
stove for free as part of his recent Stove Camp award.
We'll publish the report when finished. We're trying to get 7 full WBT
tests done before Friday. Great to finally see a TLUD with turndown!
Best,
Dean
On Mon, Aug 4, 2014 at 8:45 PM, Ronal W. Larson <rongretlarson at comcast.net>
wrote:
> Julien, cc list and adding Kirk Harris
>
> See few added notes below.
>
>
> On Aug 4, 2014, at 7:19 AM, Julien Winter <winter.julien at gmail.com> wrote:
>
> Hi Kirk and Stovers;
>
> Thanks for your comments, Kirk. They are will be very useful as I, and
> anyone else continues to tinker with counter current flow for secondary
> air, especially when the primary air is turned down and fuel is moistened.
>
> *[RWL: Seems Kirk probably sent you an off-list message. Not surprising,
> as he is that type person - not blowing his own horn. This is to ask Kirk
> to also send a more recent message that I saw a draft of about a month ago.*
>
> * I hope he will also update it and he (and others at Aprovecho) report on
> what happened at the stove camp. I believe Kirk won a major award - as his
> most recent stove was able to achieve a TDR of about 6 (per Aprovecho, not
> Kirk). It seems to me his stove would likely get all "4’s" if a little
> larger and employed a skirt.*
>
>
> For readers who may not have see it, Kirk contributed an insightful study
> on TLUD operation at low primary air. His posting was on May 15, 2014 and
> can be found here:
> http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-May/008751.html
>
> Kirk's "Method 1" and "Method 4" have a component of counter current air.
> Kirk's insight was that getting secondary air down to the fuel bed can play
> an important role in keeping the gas flame alight when the primary air has
> been turned down to as low as possible. I think this is also important for
> fuel that is a bit damp, which would be common in humid climates.
>
> The main difference between what Kirk did, and the burner I have mooted
> ("the cone") is the velocity of secondary air. The cone attempts to create
> a straight path, and the full strength of draft to propel the secondary on
> downward trajectory.
>
> *[RWL: I think there are now other differences. What Kirk took to stove
> camp was a quite different model, with secondary air probably closer to the
> char. A main beautiful new feature was a very cheap “fan-blade” shaped
> unit that achieved a nice swirl.*
>
>
> If a cowling was added about the cone burner, it could be possible to
> preheat secondary air.
>
> There are few assumptions (not necessarily correct) that are guiding my
> design choices. Basically, this is the "Add Secondary Air and Let Her Rip"
> school of thought.
>
> *[RWL: I have thought that your earlier designs appeared to have an
> excess of secondary air. I hope the most recent with the (very nice 60
> degree) cone has less. But Kirk’s is also with controllable secondary air
> - and I think is needed to avoid too much secondary air.*
>
>
> 1) A broad horizontal flame is better than a tall narrow one (such as the
> tall conical flames that you can get with narrow aperture concentrator
> disks),
>
> *[RWL: Agreed, but a swirl does the same - and involves less material.
> Same for next. For sure, we want the shortest possible stove.*
>
>
> 2) so a wide burner allows horizontal space for the gas flame to develop.
>
> 3) It is better not to put objects in the path of the flame, such as
> concentrator rings. Instead, try to manipulate mixing of reactants by how
> the secondary and pyrogas are introduced into the burner. Once reactants
> have been introduced, let the flame (i.e. the reaction) go where it wants
> to go.
>
> *[RWL: Not sure I agree with all of this this. If the “object” is
> carrying the secondary air, then one gets both pre-heating and a nice body
> to hold a flame. The last part (on “going”) is influenced by
> the “fan-blade” shape and swirl.*
>
>
> 4) Secondary air should be added only once (except for small pilot holes)
> so you have only one chance to get it right. Trying to add substantive
> amounts of secondary air higher up the burner may not incorporate properly
> into the flame. It may also arrive too late for burning larger
> hydrocarbons if the exothermal heat from oxidizing more reactive gases was
> released lower in burner. (I have noticed that an air leak below a
> secondary air hole can cause incomplete combustion at that hole.) It is
> best to supply stoichiometric oxygen so that reactive and less reactive
> gases burn together.
>
> *[RWL Agree with most. But I think one will get a considerable amount of
> unburnt gases if one tries for a stoichiometric mix. Even a gigawatt scale
> coal plant doesn’t try for that. Crispin has argued for 50% excess air I
> believe.*
>
>
> I have some experimental results that I have not yet shared. I found that
> how the secondary air was introduced (size and shape of holes or
> slots) affected the rate of gasification in the TLUD.
>
> *[RWL: I look forward to seeing more. You have been doing excellent
> work.*
>
>
> 5) Starting the gas fire low over the fuel bet is good. Radiant heat
> keeps the char pyrogas hot, and not much cooler than the flaming
> pyrolysis. As Kirk as pointed out, some burning char at the top of a
> turned-down TLUD maintains both gasification and the gas flame.
>
> *[RWL: I may be wrong, but think Kirk is not now doing the last. But if
> you mean by “top”, the top layer of the main char bed, then I agree on that
> helping to maintain a flame. But not the “gasification” part (if you mean
> the downward moving pyrolysis front zone). That front movement is dictated
> only (I think) by the amount of primary air. Yes, part of the reason for
> upward flowing gases is the existence of the flame above, but the turndown
> can be accomplished only with a control of primary air (and all TLUDS
> should have that turn down feature - demanded by all cooks.*
>
>
>
> As I said above, these are assumptions. They are speculations that are
> underdetermined by available evidence. They shouldn't go unchallenged.
> Some people have gone a completely different way to insert objects into the
> burner to create turbulence --- that is also a worthwhile inquiry. Point
> #4 is worth challenging, because you may want one level of secondary air
> for a turned-down TLUD, and a large stream of secondary air for a TLUD
> running full blast, which is what works for Kirk.
>
> *[RWL: My preference is for a more or less constant ratio of secondary
> to primary air. If about 6 is stoichiometric with an exiting Oxygen stream
> of zero, then 10-11% exiting oxygen (which hopefully can be lowered to keep
> temperatures high), should mean a ratio of 9 or so?*
>
>
> The next problem will be: what would a TLUD look like from a 3-D printer?
> TLUD builders are going to be forced to start "thinking out of the can”.
>
> *[RWL: We need a computer model to “print" the TLUD but we need a
> different model to know what it should look like. I think we are now about
> half way to a good TLUD design. Nat Mulcahy at World Stove is probably a
> lot closer. *
> * A friend sent me a 2011 article by Carter and Shockley on TLUD design
> that listed all the drawbacks. I recommend it (see *
> http://www.biochar.ac.uk/abstract.php?id=41&pr=a)*, to see what we have
> still to work on. Getting the char more quickly, safely, and easily is a
> must. As Dean Still keeps arguing, we need to get down to a $5 design. I
> think that is possible. We need to show that batch operation is an
> acceptable tradeoff for getting the time and cost-saving benefits of TLUDs.
> One other aspect of Kirk’s work is that he uses insulation throughout
> - common with most rocket’s, but for some reason not used by many TLUD
> designers. Avoiding that energy loss mechanism will drop operating costs.
> But mainly I think we have to design with a specific pot and skirt in
> mind. Then we will easily get to level 4’s- with that extra 15% (?) in
> efficiency.*
>
>
> * Again, Julien - thanks for your good work. And Kirk.*
>
>
> Keep tinkering.
>
> Cheers,
> Julien.
>
>
> --
> Julien Winter
> Cobourg, ON, CANADA
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
>
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves, News and Information see our web site:
> http://stoves.bioenergylists.org/
>
>
>
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
>
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves, News and Information see our web site:
> http://stoves.bioenergylists.org/
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20140804/44c6a692/attachment.html>
More information about the Stoves
mailing list