[Stoves] Smokeless transition

Crispin Pemberton-Pigott crispinpigott at outlook.com
Mon Dec 15 12:42:11 CST 2014


Dear Marc

 

Thank you for reporting your progress. Not everyone does that.

 

>I achieved that just increasing the chimney height (about 175% of the
reactor's height) and, following Crispin's notes: opening more secondary air
holes below the surface of the fuel bed. I suppose this creates a some type
of "pilot flame", as say Kirk, keeping high temperatures in the charcoal's
surface, and thus, maintaining the flame in the transition phase.

 

It has the advantage of lighting faster as well. When the flame lives close
to the fuel it heats the fuel more rapidly by radiation. 

 

I still don't know if this result is enough reliable and replicable, as I
used the fuel itself as the main element of the control of the air flow
(Crispin said) and the used fuel (almond husks) is far from being homogenic
and predictable (particle size varies from 5mm to 5cm)

 

I suggest you rattle the container well before each burn. Then it will
settle as much as possible.

 

Note to Kirk : it didn't work for me the "basket method" proposed by Julien.
Maybe it can work for others fuels and other drafts, as I only tried it once
with a shorter chimney (about 100% of the reactor's height)

 

Personally I don’t think a ratio is a good design approach. You need a
certain vertical space to burn the gases before you attempt heat transfer. A
total of 125-150mm is normally OK assuming you have the fuel deck covered
with secondary air and it is burning hot near the fuel bed. If you bleed in
secondary air through a lot of small holes, there will be no secondary air
in the middle. Some try to compensate for this by putting in a vertical
tube. That tube is a sign that he secondary air entrance has not been
handled correctly. It is a common ‘solution’ to a problem that should not be
there in the first place. Try to avoid it.

 

What I want in this case is:  to heat an light-mass earth oven, with mininum
attend to the fire, fast temperature gain at the begining, keeping constant
temperature at about 200ºC, and maximum time of use (and, of course,
smokeless and clean!). In this very specific case, I choose to burn the
charcoal, as I gain about 50% more time to cook and don't worry about
refuelling the reactor.

 

In that case you should continue with the current layout, meaning where the
secondary air holes. I have an extra 3 x 8mm holes low on the side of the
fuel chamber to maintain a pilot flame no matter what happens with the fuel.


 

Some bad smells

Although no visible smoke was produced in the process, it did some bad
smell. 

 

The smell is most likely condensed volatiles that did not get burned. Before
you do anything else, get the combustion looking good. See the message today
to Saloop. It all applies to your stove as well. After you have the flame
correctly places, then we will discuss smell. There may be none at all
because the cause is poor mixing or too much secondary air.

 

Last smoke to remove

At the very end of the process, when the char has been already pirolised and
very little lefts, I had smoke. It was a different smoke, very light and
odorless, and lasts about 2 minutes. 

 

It is still smoke and is most likely high in carbon. It means the flame went
out or went out over a portion of the surface. I need to know more about the
times involved – when the char started to burn and so on – to say more.

 

I suspect it is the last remains of CO gases, which are not enough to
mantain the flame. 

 

Excessive air from all sides can put out the fire by cooling it. It
definitely had high CO in that condition.

 

Maybe the solution is to open a third layer of secondary air entrance very
near to the bottom, as Crispin suggested?

 

That can help. I suggest about 60mm above the bottom. Try it. You can always
close it with a short bolt.

 

Regards
Crispin

 

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