[Stoves] Trials on TLUD Gas Burners - Counter Current Flow

[Ronal W. Larson]

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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