[Stoves] Research topic: Increasing turbulence at the level of combustion of TLUD gases.

Crispin Pemberton-Pigott crispinpigott at gmail.com
Sat Oct 5 23:38:26 CDT 2013


Dear Paul

 

For what it is worth (saving time for others who try) I have looked at both
types of solutions. Do you remember me showing you a conical version of this
in Matsapha in about 2003 on the Vesto stove converted to burn coal with a
chimney and a sunken pot? That was a drop-in concentrator though it was not
fixed. The idea was that it could be dropped into place after fuelling or
refuelling. That resolved the problem of access and still brought the gases
together with the flames, if any, to burn everything.

 

Design principle: the flames need to meet the smoke. This is best
accomplished by not creating problems in the first place. 

 

Don't spread the burning fuel out over a large area like a Jiko which is
very difficult to get to burn clean at anything other than one power level.

Don't accomplish with hardware things that can be done with air and flames.
For example don't use a catalytic converter on the exhaust gases to react
unburned gases, burn them properly in the first place.

Don't feed air into a place where there is no space to burn. After the
secondary air enters, there has to be a place for the reaction to take
place.

Don't let flames touch pots - it kills the combustion and creates a lot of
soot. Finish the burn first.

All the TLUD's with natural draft, well, nearly all of them, introduce
secondary air too late and put in too much. The result is often a central
yellow flame (glowing particles) emerging from the burner surrounded by
copious excess air doing nothing but robbing heat.  

If you have a central diffusion flame gasping for air emerging from the
centre of your TLUD, you have failed to bring the secondary air to the
middle, and it has no chance of ever burning cleanly. It is already too
late.

If you choose to bring in secondary air high in the stove (all pyrolysers
need separately provided secondary air) you are going to have to deal with a
flame that is outside the burner, chilling in the open air, subject to the
cold surface of the nearby pot.

If you choose to bring in secondary air well below the top of the stove you
face the possibility that it will get the materials so hot they will not
last. This is a problem with a concentrator disk. If they work well if the
air is brought in low enough to burn the gases well, the plate gets so hot
it disintegrates. Cooling it by using the heat/disc as a preheater for
secondary air works.

That is why I settled eventually for using the air jets themselves as the
concentrator. 

Energy imparted to the air flow is driven by convention pulling from above
and to a lesser extent from below if it is a vertical concentric sleeve. 

The hole size and shape are important. A sharp edged hole is not as
effective as imparting velocity (kinetic energy) to the air as a shaped one.
There is only so much energy available.  Fans are a substitute for this
draft power.  The exercise is aimed at getting the air jets to penetrate the
rising gas stream far enough to reach the middle. Here is a picture of a
successful result:

 



 

Note: this is not a great flame, but it is a demonstration of the principle
that the secondary air should reach the middle. This was burning pistachio
nut shells with too much primary air.

 

Here is the same stove (showing the design is right) burning switchgrass
pellets with the correct amount of primary air:

 



 

You can see the burning deck of blue at the level of air entry, you can see
the air entering and cooling (initially) the hot clear gas stream creating
particles that glow, then they burn in the centre as there is enough time
and temperature to complete the reaction.  

This could have been accomplished with a concentrator disk, but that might
not have created the disrupting secondary air mix needed to break up the
vertical flow of gases from below into combustible portions.

 

The ring of air idea (instead of jets) entering the space was implemented on
the POCA with some good success but remember that is a charcoal stove and
the primary air requirement is much higher (5-6x), meaning there was a lot
less need to disrupt the gases to penetrate the flow with additional air.
My guess is that all pyrolysers that make char will burn cleaner and shorter
using air than using metal, and will last longer as well.  A conical device
might with well. A conical (pyramidal) structure was used on the GTZ-7 coal
stove with a very small amount of secondary air. It relied on the ceramic
(chamotte) to last at high temperature. The combustion chamber below was
shoe-box shaped, a square exit and an opening pyramidal flame space above.
As has been reported before, the PM was, most of the time, net negative
after ignition was fully established. So it was a 'concentrator ring'
approach but using ceramic which could take the heat and might even have
offered a catalytic function (because it was rough).

 

So, there are some considerations for researchers.

 

Regards

Crispin

 

 

-----Original Message-----
From: Paul Anderson [mailto:psanders at ilstu.edu] 
Sent: Sunday, October 06, 2013 10:19 AM
To: Jock Gill
Cc: Discussion of biomass cooking stoves; crispinpigott at gmail.com
Pemberton-Pigott; Ron Larson; Paul Olivier; Tom Miles; Paul Olivier; Sarah
Uof IL Haiti stove; James S. Schoner
Subject: Research topic: Increasing turbulence at the level of combustion of
TLUD gases.

 

Jock, Crispin, and all Stovers, (James, for my website listed/tagged as R&D
work to be done.)

 

As far as I know, there has not been much study of the impact of variations
and options for increasing turbulence (using natural draft, 

not force air) at the level of the top of the TLUD fuel cylinder.   This 

is certainly a topic that merits attention and could have significant impact
on improving stove performance.

 

The same TLUD stove (for purposes of need to control variables) to be tested
with only changes to the combustion area of the pyrolysis 

gases.   And what is found to be better, then also needs to be tested on 

other TLUD designs.

 

However, even with rigorous observations and controls, ultimately emissions
data will be required, and for that, equipment will be needed.

 

This would be a fine topic for some student group to undertake, or for a 

master's thesis, or any of the many Stovers.   I and probably several 

other Stovers would gladly facilitate/assist anyone undertaking this very
interesting task.

 

Paul

 

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD

Email:   <mailto:psanders at ilstu.edu> psanders at ilstu.edu

Skype: paultlud      Phone: +1-309-452-7072

Website:   <http://www.drtlud.com> www.drtlud.com

 

On 10/5/2013 7:37 AM, Jock Gill wrote:

> Crispin,

> 

> When I read your "In the time-temperature-turbulence equation" lights went
off.  I expect this is why my iCans with three washers as deflectors, in
contra distinction to a "concentrator ring", burn cleaner.  The deflectors
increase both turbulence and residence time in the combustion zone.
Secondary air slots also contribute to the turbulence.  At least my iCans
with secondary air slots burn much cleaner than those with various holes
used for secondary air.  My current system burns cleanly enough to deposit
very little soot on cooking utensils or food.  They are essentially soot
free.  Perhaps "soot free" should be a goal?

> 

> Cheers,

> 

> Jock

> 

> Jock Gill

> P.O. Box 3

> Peacham,  VT 05862

> 

> Cell: (617) 449-8111

> 

> 

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