[Stoves] Wood Stove Demo - Video

[Crispin Pemberton-Pigott]

Dear Rajan

 

I watched the demonstration of the stove in use and it looks very nice in
terms of materials used and execution.

 

I have some comments which may help you and others wanting to build this
layout of stick-burning stove.

 

The grate is similar to the Rocket Stove 'fuel shelf' but has the advantage
of limiting to some extent the amount of primary air getting to the fuel.
This is supplemented by the fact that the grate keeps the hot coals that
inevitably fall off the end of the sticks up in the air, not letting them
drop to the bottom of the combustion chamber.

 

The amount of waste charcoal produce is reduced with the grate. The Namibian
Tsotso stove uses a similar approach though the grate is not removable. 

 

All Rocket-type stoves can be improved by adding this type of grate. There
are two sources of air in these stoves: the air passing past (under) the far
end of the sticks that are burning and the air that enters through the hole
into which the fuel is passed. There is a balance needed to get a clean
burn.

 

When you have 3 sticks inserted, you are getting a reasonable burn and a
premised flame, basically. Air enters the grate holes vertically and
pyrolyses the fuel and the gases are burned above using secondary air that
enters with the fuel.

 

The point that some secondary air is passing through the grate is correct
and it is indeed preheated by the coals burning on it. As you can see, the
flames are not 'finished' by the time they hit the pot so there is some
improvement that can be made in terms of height. Giving more flame space
will increase the draft (which affects how much air enters the grate holes)
so balance is needed through experimentation. 

 

The interesting part was your demonstration that when you put in 5 sticks
the stove starts to smoke even without a pot (the pot already creates smoke
because the flames are touching it). Note, everyone looking at the video,
that the fire changes from having a premixed flame to a diffusion flame and
there is a change in colour as a result.

 

This is not the fault of the extra fuel. It is cause by a lack of secondary
air which is having to enter through the space where the fuel goes in, and
now that space is partially blocked by the additional fuel. The more fuel
you add, the more secondary air you need, and the less you get because it is
progressively blocked by the fuel. The video clearly shows the two different
flame types and the comment is that the 'flame changes colour'. It has gone
from a whole wood burning premixed flame to a wood gasifying diffusion flame
because the primary air supply is fixed and the secondary air supply has
been reduced.

 

Reducing the total amount of air in the chamber as also increased the
temperature in there! This further accelerates the production of wood gas
creating even worse combustion conditions. When the pot is on, the flame has
no chance to find oxygen around the top of the combustion chamber (which is
the problem of a diffusion flame - the gases are trying to diffuse into the
air and find O2). As a result the fire becomes smoky and the CO will rise
dramatically. It is a choked fire unable to breathe. The effect is quite
reversible as the operator shows.

 

So the lesson is that the secondary air supply has to be maintained while
balancing the burn rate by decreasing the primary air supply to compensate
for the increased gas production rate.  In general, it is wise to use a
grate (probably with fewer total holes) and a drop-down door to close as
much air as possible entering with the fuel, then provide secondary air
through holes inside the chamber. It is possible to get a balanced
combustion at a range of power levels without getting high excess air in the
low power condition.

 

Thanks so much Rajan for sharing the video. It is helpful to show a stove
misbehaving as well as working properly.

 

Regards

Crispin

 

From: rajan_jiby at dataone.in [mailto:rajan_jiby at dataone.in] 
Sent: Sunday, September 09, 2012 7:24 AM
To: stoves at lists.bioenergylists.org
Cc: Paul S. Anderson; Crispin Pemberton-Pigott; mukundan; Sujatha Srini;
sudhakar mukundan; vijayaganapathi
Subject: Wood Stove Demo - Video

 

Dear All,

 

Here is a demo video of the Servals wood stove attached to the blog.


http://servalsgroup.blogspot.in/2012/01/servals-modified-natural-draft-wood.
html

 

Regards,

 

Rajan

 

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