[Stoves] Strata combustor general principles

Sun Sep 20 13:48:29 CDT 2015

Dear Kirk

>I don't quite understand Crispins flame tube.  

Unfortunately it is a very difficult thing to photograph. It works in the
same way as a concentrating cone (which I prefer for radiant reasons) or a
ring - to bring the gases and air and flames into a small space to ensure
burnout of the PM. The CO burn is a byproduct. It is primarily intended to
burn the smoke. CO is not a major issue. The rules were not to increase the
CO level, i.e. 'do no harm' as the ADB put it. But CO was never a problem.
The target was PM2.5 (coal stoves don't make PM10 unless they have a fan).

>The stove appears to be loaded with charcoal, so there should not be any
smoke.  

There can be plenty of smoke from charcoal. It depends a great deal on how
it is ignited. The fuel is in fact 'wet' lignite from Nalaikh which is a
small town perhaps 25 km from Ulaanbaatar. There is a huge coal mine there
that was closed about 1992. It has been worked informally since then by the
people who lost their jobs. They pull out about 1.2m tons a year from it. By
hand mostly.

>If the clay is insulating clay than the tube would provide a hot
environment for the CO to burn, if there is a source of secondary air.  

There is nothing 'insulating' about the clay. It is just clay from the
river. I don't think there is clay that is insulating. Clay is a poor
conductor of heat, however, if that helps. The benefits of making a clay
'insulative' component are temporary and affect the short term performance,
providing nearly nothing in the long run. For a space heating stove it
provides the slightest flame temperature benefit under constant conditions.
You can calculate it or measure it. It doesn't take much thickness to ensure
a high combustion temperature - maybe 15mm. A guide is provided by the work
in South Africa on charcoal gasifiers which run at really high temperatures.
Fired clay is almost always a poor conductor of heat. 

There is no provision of secondary air because there is sufficient air in
the combustion chamber. The battle with these stoves is to seal them. They
usually have far too much air at all times everywhere. The total air supply
hole needed by a 10 kW fire is perhaps 30 x 80mm if the chimney is about 10
feet tall. 

>If the stove burns wood, by what principles does the tube work?  Is air
injected inside the tube? 

The fuel is irrelevant - this is a design principle: burning poorly in a
chamber (not that I advocate it) produces PIC and CO which can most easily
be burned by funneling all of it together in a small space. The tube is
about 12" long. For the BLDD6 downdraft coal stove (Univ of JHB) the tube is
significantly longer and has a strong induced vortex which turns the outer
region into an insulator and the inner region into a very strongly mixed,
turbulent flame.

>This would make use of the Venturi effect like Roberto's stove.  

It doesn't do that. It is a straight tube. Flames some out the far end. The
heat exchanger is too small but this was designed as a retro-fit that could
reduce emissions 80%, not a de novo design. There is a pair of stoves at the
SEET lab in storage that were created specifically to balance the heat
exchanger size with the flame tube - in that case 65mm ID. They look like
traditional stoves but longer.

>The flame gasses accellerating into the tube would drop in pressure and the
pressure difference would help the atmosphere push secondary air into the
flame to equalize the pressure, providing good mixing.  

This is not needed as the air-fuel ratio is already good. 

>This seems to me like it might work well if that is how it works.  Is there
something else that I don't see?

Well, it works very well, but not that way. I suppose you could call what
you describe is staged mixing. This has staged combustion and pre-mixing. 

The thing that is hard to see is the way a crossdraft fire burns in a
channel like that. With the lid on (or a pot) the fire works its way to
towards the source of air which is on the other end from the tub. All smoke
and gases generated by the pyrolysing coal have to pass over the very hot
coke burning region just before the whole lot goes into the tube. This
simple change from bottom lighting to 'end lighting' is the major source of
fuel saving, and the tube is the major source of the PM reduction.

I can try to explain using this photo:

This is immediately after TLUD ignition of the kindling. So imagine that
continuing throughout the burn. The emissions and burn rate profile are
completely different from the standard urban users'. I was told in January
the ignition method is spreading in the country without anyone pushing it so
it must have obvious benefits.

You might be surprised to know that this invention was strongly criticised
by one of the major stove program managers because, he said, "You cannot
solve this problem for $1."

Here is another $1 solution.

Standard $20 Mongolian traditional ger stove with an added flame tube that
exits inside the heat exchanger.

Refuelling is accomplished by [pushing the burning coke to the back (through
the door in the foreground) and pushing fresh coal towards it until they
overlap. The fire will then burn slowly through the coal towards the door
(because that is where the air comes from).

ELCD burn without refuelling.

Regards

Crispin

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150920/fd889d38/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.jpg
Type: image/jpeg
Size: 10227 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150920/fd889d38/attachment.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image006.jpg
Type: image/jpeg
Size: 24312 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150920/fd889d38/attachment-0001.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image007.jpg
Type: image/jpeg
Size: 9346 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150920/fd889d38/attachment-0002.jpg>