[Stoves] Down drafting primary & secondary air

Wed Mar 12 12:21:13 CDT 2014

Dear Jock

>I went out and got a can to use around the reactor and draft cans to create
a down draft system for the primary and secondary air.  

Looking at the down draft name, it is actually the right term. It is not
downdraft combustion, but it is definitely a downdrafting preheater.

>On the left, you see the reactor can and draft can sitting on the corner
brackets allowing primary air into the bottom of the can.  Note that the 1/4
inch secondary air gap is not too far below the top of the can on the right.
Note that the right hand can has NO apertures for air around its bottom.
All air entering the system must be down drafted from the top.

Yes. It was not exactly what I had in mind but your experiment was
interesting nonetheless. Let's look at it first then I will return to the
secondary air preheating concept.

>This shows the center ring and the corner brackets after the first run in
down draft mode. The run was so hot it burned them just about clean.  

That is due to the great reduction in the supply of secondary air - probably
to the right level as you did not report black smoke from a lack of air
sufficient to burn the generated gases.

>.Clearly, the hottest gasses flow under them towards the center, which is
so hot it is essentially soot free.  

It is not so much that it is hot (which it is) but that the combustion
conditions are correct and the carbon is burned out. Good combustion
conditions result in a clean burn. 

>This would appear to keep the highest temperatures away from the sides of
the reactor can.

You have achieved this using the reflectors which happen to have more space
in the centre so the combustion is drawn there. The same thing can be
achieved without metal - just air entry, but the air supply has to be done
carefully.

>The first 12 or so minutes were dominated by blue flames with very little
yellow developing.

Which means few particles.

>In the body of the run, there was clearly much too much primary air as the
pyrolysis was releasing so much gas so rapidly it was overwhelming the
secondary air.  Result:  soot carried out of the system by out of control
flames.

Aha - you did mention there wasn't enough secondary air. Good. That is
caused by excessive production of gases, and in the case you built, it was
because you created a self-heating retort. The preheating of the primary air
was so great that rather than having a primary-air-fed fire dominating the
production of gas, you had a heated retort with thermal runaway. This is
good because it is 'the other side' of where you want to be. Before, you
were 'on this side' of where you should be.

>Fire self extinguished abruptly about minute 47 with no smoke!  

Because it had self-roasted everything out of the fuel.

>There was simply nothing left in the charcoal to give off smoke.  

Yup.

>Of course the yield of charcoal was reduced by this exuberant heat.  

Let's not be too hasty. The resulting loss of char may have been because of
having too much secondary air available part of the time, or the secondary
air may have reacted with the char on top and started the gas combustion.
The char produced may have a much higher carbon content than before which
also means a lower total mass because it is more 'dried out'.

>This run time was a good 15 minutes or so faster than runs in the outer can
with the air supply holes in the bottom.  The longer run times, over 60
minutes, with the exact same reactor, self extinguish but give off a light
gray smoke when they do.

So this shows that the charring temperature was higher with the preheating
of primary air. That can't be too surprising.

>Net net, the down draft approach to primary and secondary air creates a
VERY different experience.  

Yes.

Now I return to an alternative construction.

Leave the primary air as it was with air entering from the cold outdoors.
Elevate the secondary air preheater and close the gap between it and the
combustion chamber. Air can enter the upper opening but not enter the
bottom, and the bottom is above the primary air entrance holes.  No hot air
can get through the preheater to the primary air holes, OK?

This will give you a demonstration of the combination of cold primary air
and preheated secondary air, with an element of secondary air preheating and
self-balancing (which will show up as useful when you control the primary
air supply to vary the burn rate).

>Comments?  Suggestions?  Are you happy now ;-)

I am very happy because you are experimenting with two new design
attributes. 

Further trials: the primary air can be preheated a bit, but not a lot, and
you can avoid creating a self-heating retort with thermal runaway. How much
depends on a lot of things but the heat is near the top and the primary air
holes are near the bottom and if you over-heat the air, it will oppose the
upward flow inside the burner to the point little will go down. So.after
creating the secondary air preheater with it sealed to the inner can just
below the secondary air entrance, you can put yet another can around the
entire system. This on should be larger than the pineapple can and will
serve as a primary air preheater that is heated by the secondary air
preheating can.  That will give you some preheating but not too much. In a
Vesto, this entrance has a sliding controller, used to control the primary
air supply.

The balance you are creating is having sufficient secondary air, controlled
partly by the height of the combustion chamber, the height of its preheater
and the hole or gap sizes. The same applies to the primary air supply. If
you overheat the fuel it will gas without having any air at all. That is too
much. If you don't have enough heat the char will smoke when the flame goes
out.

It would not be good to assume that not having visible smoke at flame-out
means there is no PM and CO being produced. Get the glowing char into a
lidded clay pot the way they do in Cambodia whenever they adjust the fire or
finish cooking. 

Regards

Crispin

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