Dear Crispin,
Message: 2
Date: Mon, 10 Sep 2012 17:58:01 -0400
From: "Crispin Pemberton-Pigott" <[email protected]>
To: "Stoves" <[email protected]>
Subject: Re: [Stoves] Wood Stove Demo - Video
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
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.
We want the grate to burn as much charcoal as possible and in that process
keep the combustion chamber as hot as possible - to get an overall cleaner
combustion.
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.
True. The flame seen has around 2 KW fire-power. This is the maximum rating
for this stove. For the average "South Indian Domestic Cooking", the average
fire-power used is around 1.5 KW. At around 1.5 KW fire-power, the flame is
slightly shorter and also looks better.
The normal LPG stoves used here have a maximum fire-power rating of around 2
KW per burner, and people do not complain.
But with a wood stove, people are likely to push in more wood to get a
higher fire-power initially ( while boiling water for cooking rice, etc. ).
Once water boils, the fire-power is immediately brought down.
The height of the stove is kept low for a particular reason. In India at
many places, ladies sit close to the ground for cooking. Here the stove has
to be essentially short in height. It is a matter of user-friendliness.
In this stove, the diameter of the "flame-concentrator hole" also decides
the fire-power.
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.
True in normal conditions. Here the design of the fuelwood stand ( sheet
metal design ) helps a little bit. The fuelwood does not easily burn until
pushed nearer to the grate.
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.
This is a very important point. In a well-designed FD stove, the ratio
between primary and secondary air is comparatively well-managed. In ND
stove, it is slightly difficult.
But we feel something can be done here - as we progress. The fuelwood stand
is already height-adjustable, and probably this is the first step in this
direction. Although drop-down door is a simple idea, we do not know whether
this will be user-friendly. People are highly conscious about convenience.
Regards,
Rajan
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