[Stoves] is this new?

Sat Jan 19 16:38:52 CST 2013

Dear Marc

I think this is properly called Counter-flow secondary air. I have used it
in the Vesto with the addition of a second concentric ‘air tube’ between the
loose one you are using and the combusting gas. That innermost tube is the
combustion chamber and the air tube is the secondary air preheater. The
loose one is akin to the stove body which is used to create a negative
pressure in the sense that the air is drawn into the stove heating downwards
instead of upwards.

There is wisdom in this which is that the negative draft on the downward
flowing air is counter-balanced by the hotter gases rising in the chimney
with the hotter gasses ‘winning’ the draft contest.

If you get the downward path (and its temperature) right balancing (almost)
an upward hotter flow in the centre, you can get a low EA value (with
corresponding low CO and high heat transfer efficiency) at different power
levels – something notably missing from the cheap can-stoves.

One of the drawbacks of nearly all the current crop of gasifiers and batch
loaded stoves is they are not very controllable for power, and when they
are, there is little to no control over the secondary air volume unless
there is a fan involved.

By using the layout you have described, or a triple version as per a Vesto,
you can have self-regulating (or close to it) secondary air supply without
having to operate a second air controller. The variation in draft does it
automatically.

The position of the external air entry holes on the Vesto and the lower
chamber below the controller are at the height they are to create a
reasonable balance on the draft in the centre of the system that pulls in
primary and secondary air. The smaller holes through the air tube at the
level of the secondary entrance are to allow in additional secondary air if
the primary air is shut down rapidly (which would otherwise cause a very low
EA condition and smoke – which you seem to have experienced, although for a
different reason).

You can get the more common secondary air preheating by running the air up
the outside of the pyrolysing chamber with air entry at the bottom (see the
$1 Grasifier) but it is ‘unregulated’ by the draft inside – it operates
based on the heating that comes through the exterior wall. Stoves like this
include the original 1984 Tsotso Stove by David Hancock (the famous), the
Peko Pe and Paul’s gasifiers, the POCA charcoal stove and the metal+clay
Anglo SupraNova (though I plan to edit that last stove in a couple of months
to be more advanced).

Something you might try is to place the loose pipe on a ring that pretty
much = the inside diameter of the chamber, but loose enough to fall with its
own weight. The drill a bunch of holes at the bottom to allow in the
secondary air through the cylinder. I suggest 600mm2 per kW.  Ignite the
rice hull then place the pipe+ring on top with the ring on the bottom. As
the fuel drops in volume, the chimney will sink, always sitting on top of
the fuel and letting in the secondary air immediately above the fuel level.

The advantage of this is that it will definitely keep the flame going and
keep the top of the fuel bed really hot, hopefully burning some of the char
at all times, this preserving the ignition of the gases. As the gas is
already ‘gas’ by the time if emerges from the fuel, the secondary air holes
can be at or near the bottom – a few rows perhaps. The incoming gas must
reach the centre point (look inside to see the flames). A too-large diameter
tube is a common mistake in the design of these. A central air pipe is often
added to overcome a problem that should not have been there in the first
place. The inward distance travelled by the secondary air varies with the
draft applied and the hole diameter.

Obviously another concentric pipe fixed above the loose one can be the pot
support. 

What this whole apparatus does is recreate the combustion conditions that
are afforded by a downdraft combustor, without the downdraft combustor’s
ability to be refuelled while running. If an updraft batch process is OK for
the application, it is easier to apply the heat to a single pot directly
above. To vary the power of the stove, control the primary air.

Regards

Crispin

So, I was playing around with burners on a Belonio rice husk gasifier last
night. 

If you're not familiar, there are a bunch of photos of the basic design on
google image: batch stove images
<https://www.google.com/search?q=belonio+batch+stove&sugexp=chrome,mod%3D16&
um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=kh76UMnkBOiriAforYHQCw
&biw=1282&bih=717&sei=lB76UMTnOaSjigfp3YGgDw> 

I slid a metal cylinder into the opening of the top of the reactor, leaving
a gap along the sides. Here's a picture:

Inline image 1

Now, normally when you take the burner top off of these stoves, there's no
combustion inside because there is no secondary air available.

Well, I saw a roaring flame inside after sliding in the metal cylinder
(option #2 in the diagram)

As far as I can tell, the cylinder acts like a chimney, causing a pressure
drop which sucks producer gas from the bed, not allowing it to escape
through the gap on the sides. 

As a result, secondary air sinks through the gap and you get combustion at
the bottom of the cylinder.

Has anyone seen something like this before? I can't think of any examples. I
called it a "heat pump" in my field notes.

With the right dimensions is might be a good auto-regulating burner: more
producer gas producers more heat, pulling in more secondary air. 

I think it could be useful for charcoal stoves as well as TLUDs. 

I measured lower CO than usual with Belonio burners. Similar excess air
levels (though I only tested two sizings of the metal cylinder).

Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de Compiègne

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