[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
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