[Stoves] Distributing Secondary Air from the Outside or Center of TLUD gas burner

Crispin Pemberton-Pigott crispinpigott at outlook.com
Wed Dec 17 13:31:22 CST 2014


Dear Julien

 

The flame looks great.

 

If you put a ‘gear shaped’ metal plate on top instead of a round one it will divide the secondary air into discrete jets and give you better mixing. When the sheet of air comes out of the slot it has to be broken up by turbulence before it can mix with the gas. Jets provide that mixing right from the emergence point. 

 

Kirk, in general avoid providing air in a ‘sheet’ because it takes time (distance) and energy (turbulence) to break up the sheet and get it mixed into the gases.

 

If you were to take this <http://stoves.bioenergylists.org/files/styles/large/public/harris-figd.jpg?itok=UkbfYFHp>  burner and squeeze it into a rectangle with the pipe across the long dimension, you would have a typical German coal furnace burner. They have the luxury of vertical height to complete the mixing. Cooking stoves don’t. 

 

For both of you: Operating the burner without anything on top strongly affects the flows inside. You can simulate a pot by placing a perforated metal disk (big holes) like a shower floor drain or something on the pot rests. This allows the development of the internal gas flow patterns and you will still be able to see what is happening. In the case of the ‘glowing red’ picture there will be significant amounts of cold air entering the fire from above down the central hole. It won’t do that when a pot is in place. 

 

Everyone: Don’t develop a burner without a pot in place (or a substitute). For all practical purposes, the pot is part of the stove and forms the upper side of the combustion chamber.

 

Regards
Crispin

 

 

 

Hello All;

 

I have been doing a bit of testing on TLUD pyrogas burners over the past 9 months.

 

I have been working on two approaches.  (1) bringing secondary in from the circumference, and (2) bringing air in from the middle (with Kirk Harris)

 

1) AIR FROM THE CIRCUMFEREANCE

 

A robust design that I have come up with was illustrated in the recent "experimental vignette" I posted on the effect of changing riser height, Increasing ND-TLUD riser height accelerates gasification and increases bed temperature" 

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-December/009596.html

In this example, preheating secondary air was not done, because it was not necessary for the experiment.

 

Prototypes of this burner were reported in a posting to this list "Trials on TLUD Gas Burners - Burner Diameter"  with and without preheating secondary air.

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-July/009043.html




The burner is robust in the sense that it works over a very wide range of gasification rates without producing visible smoke.  The burner has not yet been tested for CO and particulate emission over a power curve or turndown curve.  That needs to be done.

 

The features of this burner are:

 

a)  The diameter of the burner's outer cylinder is 1.3 x the diameter of the TLUD reactor.  This was demonstrated in the experimental prototypes to increase the horizontal space for gas flame expansion.  The result was the bulk gas flame doesn't form a central conical peak, by has lower multiple peaks instead.

 

b) The holes for secondary air are not near the top of the reactor, but elevated.  This allows space for rising pyrogas to become entrained in the jets of secondary air as they enter through the air holes.  In earlier experiments, I observed that pyrogas does not rise straight up for the fuel bed, but moves toward the side walls of the reactor.  This means that nothing is needed to force pyrogas to the sides.

 

c) There is a "deflector ring" above the secondary air holes.  I found that this ring was necessary to prevent vertical flamelets arising from the secondary air holes.  These vertical flamelets produce smoke, because there is not enough turbulence, and the flame is cooled as it passes up the riser walls.  The "deflector ring" is not quite the same as a "concentrator ring".  The deflector is only wide enough to prevent vertical flamelets.  The aperture of the deflector ring is kept wide so that it doesn't restrict the gas flame.  What we what are horizontal flamelets, and it is interesting to see that you can get both a horizontal and vertical flamelets arising from the same air hole.  

 

d) There are no obstructions over the mouth of the TLUD reactor.   It is important that secondary air can settle toward the fuel bed when gasification rate is low.  When the gasification rate is high, the bottom of the gas flame is around the level of the secondary air holes.  When the stove is turned down, the secondary air settles down to the middle of the fuel bed, and the bottom of the gas flame is in contact with the fuel bed.

 

e) Secondary air holes are relatively large.  The size of secondary air holes was tested in an experiment that has not yet been reported.  If the secondary air holes were small, the gas flame was smaller and the gasification rate was lower than with larger air holes.  If the secondary air holes are two large, then become ineffective at controlling and shaping the jets of secondary air.  When the secondary air holes are the right size, they impart sufficient structure to the jets of secondary air, projecting them inwards, whilst not imposing a high frictional/turbulent resistance to secondary air entry.   The size of secondary air holes is important.  The best size needs to be determined for each stove design.

 

Round secondary air holes are better than a continuous secondary air gap, or rectangular holes.   Buoyance within the gas burner is not uniformly distributed, but varies according to flame activity.  I find that round holes are better able to self-regulate air flow according to the localized differences in draft.  That is because they impose a discrete and symmetrical resistance to air entry.  If you have a continuous gap, then the secondary air flow can't compartmentalize, so at low gasification rates you can get too much secondary air, the gas/air mixture is too lean, and you get smoke and flameouts.

 

One of the key assumptions guiding the design of this burner is to allow space for the gas flame to go where it wants to, and provide no interference.   That is an assumption that is open to be challenged by other designs.

 

 

2) AIR FROM THE CENTER

 

A while back, Kirk Harris posted the design for his new Wonderwerk TLUD stove  ... and it got no comments!!!  I suspect that many people didn't realize the significance of what he has done.  He introduced secondary air to the center of the burner through slits in a horizontal pipe.  Take a look at his stove, because he has achieve a large turn down capacity and very low CO emissions.  

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-October/009408.html

 

Kirk was kind enough to send me a description of his design before it was publically released.   Kirk's burner is linear.  Following his lead, I have done a very similar think, only circular.  We have been bantering back and forth by email since.

 

Some pictures of the central burner are attached in a pdf.

 

The first picture is of the burner running by itself.  The second picture is of the central burner mounted in a peripheral burner similar to the one described above.

 

This is a prototype under development, and we are not yet sure if it will work.  The geometries have to be adjusted to that there is enough space for the flame to expand.

 

One of the problems with the central burner on its own is that flame is consistently symmetrical as with Kirk's linear burner nor as with burner described above.  That means that you can get a taller flame in one quadrant than the others.

 

The central distributer is being combined with peripheral secondary air to see if the flame can be made consistently symmetrical, and to see if there is better mixing of pyrogas with secondary air, if the pyrogas passes through two opposing streams of air.  The soot around the underside of the burner in the second picture shows that this is a work in progress.  If secondary air is also added from the outside, then less air is drawn from the middle, because the draft in the burner has to be shared between the two modes of feeding secondary air.

 

This burner has to be tested on a full range of gasification rates.

 

The final test will be to see if very low CO and particulate emissions can be achieved.  The burner in Kirk's Wonderwerk TLUD stove is a very practical design that is easy to build.  This new burner is trying to take a good idea and make it complicated.  If you can take something simple and make it complicated, then that must be an improvement :-}

 

Food for thought,

Cheers,

Julien

 


-- 

Julien Winter
Cobourg, ON, CANADA

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