[Stoves] Magh Utham Woodgas Burner

[Crispin Pemberton-Pigott]

Dear Dr N

 

You have made nearly ideal changes to the stove.

 

Attached for others to see is your top view of the flame when things are
going well.

 

Notice that at '3 o'clock' there is a extra flame that is poking out between
the regularly spaced 7 others.

 

For those who did not see the other pictures, it is vertically below the
line of 7 and between them. It gives more coverage.

 

It is important to see that all the flames have reached the outer wall of
the combustion chamber. This is not so good. The flame will chill against
the outer shell and increase the CO level in that region.

 

In addition, there is still a triangular region between each of the flames
through which it is possible to pass at least some smoke that remains
unburned.

 

So.I suggest that without changing anything else, increase the number of
holes, completing a ring of holes in the same vertical position as the '8th
hole', but recalculating the hole diameter to the total area remains the
same.

 

There will then be 14 flames radiating from the centre, giving what may be
100% coverage of the combustion chamber, but with the total area of the
secondary air holes being equal to the present 8 holes. If there are any
holes through to top plate (it appears there are) leave them closed.

 

I notice that at 12 o'clock there is a very small blue flame around an
external wall hole. This is not a good sign as there is a flame nearby that
is not 'getting everything'. It means there is still unburned CO in the
region against the wall. This reinforces the idea of introducing a higher
number of holes from the centre.

 

Two things should be the result: the flames seen coming from the centre
(which are actually flames surrounding the preheated secondary air) should
be reduced in length until they do not quite reach the outside wall; the
coverage of the whole diameter should improve, burning as much of the CO as
possible on what are now the dark zones.

 

If you make an adjustable primary air controller (by any means) you will
find that opening it just as the flames start to die, will turn it into a
charcoal burning stove. The reason for doing this is that you can put in
chopped up fuel that would normally burn in a great whoosh! and use it in a
controlled fashion. The burning of the woodgas first and the char later
allows the same rather worthless fuel to be consumed over a longer period
with a clean flame. If the air is well controlled, it is possible to refuel
the stove without emptying it first as is often thought necessary with a
woodgas stove. Not so. In many cases it is possible to add large fuel,
provided the surface area is large enough. At the start, open the primary
air until the wood is burning well, then shut it down to create a gas-making
chamber as you have now. When it is almost entirely charcoaled, open it
again.

 

Maintaining a continuous flame in the later stage is what I have been
working on in SeTAR lately, though using coal/coke as a fuel with at least
some success. Keeping it hot and limiting the air flow is the main criterion
for success.

 

Dr N, you may find that a series of horizontal, rectangular holes in two or
three layers will give better coverage. The complete alternative is to do
what is in a Vesto which is to have external holes pointing in instead of
internal holes pointing out. The objective is both cases is to cover all the
burning area with preheated plumes of secondary air. In both cases the
result can be a natural draft, refuelable stove capable of burning a variety
of low quality fuels.

 

Keep going, you are doing really well!

Crispin

 

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