Dear All,
We started on this "small adventure" in the hope of arriving at a "low cost
improved wood stove" - and not a technical achievement.
Kindly find my comments below - for all the questions.
Best Regards,
Rajan
Today's Topics:
1. Re: A low cost modified traditional woodstove (Johan Wessels)
2. Re: A low cost modified traditional woodstove(Preheated air
and Charcoal) (Clement Aigbogun)
3. Re: A low cost modified traditional woodstove(Preheated air
and Charcoal) (Crispin Pemberton-Pigott)
4. Re: A low cost modified traditional woodstove ([email protected])
5. Re: A low cost modified traditional woodstove(Preheated air
and Char... ([email protected])
6. Re: A low cost modified traditional woodstove(Preheated air
and Char... (Crispin Pemberton-Pigott)
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Message: 1
Date: Thu, 2 Dec 2010 04:56:44 +0200
From: "Johan Wessels" <[email protected]>
To: <[email protected]>, "Discussion of biomass cooking stoves"
<[email protected]>
Subject: Re: [Stoves] A low cost modified traditional woodstove
Message-ID: <003201cb91cc$8f37b620$6b0f1...@userfdbcf83518>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
Dear Rajan
Have you got a drawing available for this stove?
We have not made any drawing. It is an extremely simple stove - as I have
already explained in detail.
We are thinking of field trials straightaway. We will proceed further
depending on the feedback we receive from actual users.
Once into regular production, the photograph will definitely come on the
internet.
Please try the "air pipe" with a three stone fire or any traditional stove
to see the improvement in the flame. The horizontal air pipe ( with some
perforations at the hot tip ) should deliver the pre-heated air right at the
centre of the stove at the very bottom - where charcoal normally gets
deposited during the burning process. Our aim is to keep this area intensely
hot ( by burning a small quantity of the charcoal with pre-heated air - and
not the entire charcoal ). This gives a very stable flame without extra
efforts like blowing in air, etc.
Your observations will be very valuable to all of us.
Regards
Johan Wessels
South Africa
----- Original Message -----
From: <[email protected]>
To: "stoves" <[email protected]>
Sent: Wednesday, December 01, 2010 4:00 PM
Subject: [Stoves] A low cost modified traditional woodstove
Dear All,
I spent some time recently with Servals team in developing a low cost
improved traditional woodstove - and the result was good.
Basically a traditional looking stove ( a 4 litre metallic container
converted ) - clay lining ( can be changed over to refractory or
stainless
steel ), flame concentrator at the top ( similar to TLUD stove ), air
pipe
( metallic, around 12" long ) at the bottom.
The flame concentrator has two functions ( as we all know ) :
1. Better mixing of combustion air with the woodgas - for a more
complete
burn.
2. Brings the flame to the centre of the cooking pot bottom.
The air pipe ( horizontally placed ) brings a small quantity of air below
the burning wood mass at the centre. This air burns a little quantity of
charcoal, keeping this area intensely hot - this also keeps a stable
flame
throughout. Since the pipe is metallic, this air gets preheated ( inside
the pipe ) before it reacts with charcoal. We can imagine what pre-heated
( upto around 300 C ) air can do to an already burning charcoal mass.
We also tried a grate at the stove bottom ( instead of the air pipe ).
But
the result was not good. Air pipe has the advantage of pre-heating the
air
and also allowing only a controlled quantity of air in. Some perforations
near the hot tip of the pipe adds to the performance.
It takes about 10 minutes for the air pipe to start functioning - ie.
when
the stove bottom ( inside ) gets heated up sufficiently to create a
suction through the pipe.
The cross-sectional area of the air pipe can be around 10 % of the
cross-sectional area of the fuel-window ( through which the fuel is fed
in ). For an average domestic stove, 1 1/2" dia pipe should be ideal.
The flame concentrator and the air pipe can be an add-on to any
traditional woodstove - including a three stone fire. Especially the air
pipe is a valuable add-on with negligible cost involved.
The small stove we tested had a fire-power of around 3 KW ( average
around
2 KW ). The flame looked almost as good as a TLUD stove ( ND ) flame. The
fire-power can be easily upgraded for larger versions of the stove. Flame
control is like in any trditional stove ( withdrawing the burning wood ).
With Best Regards,
Rajan
Message: 2
Date: Thu, 2 Dec 2010 03:17:47 -0800 (PST)
From: Clement Aigbogun <[email protected]>
To: Discussion of biomass cooking stoves <[email protected]>
Subject: Re: [Stoves] A low cost modified traditional
woodstove(Preheated air and Charcoal)
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hello Sir,''We can imagine what pre-heated ( upto around 300 C ) air can
do to an already burning charcoal mass''.
Question: What does preheated air do to an already burning charcoal
mass?Clem
This question is already answered in a most beautiful way by Mr. Crispin
below.
Message: 3
Date: Thu, 2 Dec 2010 22:44:58 +0800
From: "Crispin Pemberton-Pigott" <[email protected]>
To: "'Discussion of biomass cooking stoves'"
<[email protected]>
Subject: Re: [Stoves] A low cost modified traditional
woodstove(Preheated air and Charcoal)
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Dear Clem
Question: What does preheated air do to an already burning charcoal mass?
Clem
Good question.
When burning a high carbon fuel like charcoal it is important to keep the
temperature of the gases coming off the fuel as hot as possible in order
to
burn as much of the CO as possible. CO is difficult to light if it is
below
850 degrees. The concentrating ring or cone used in many stove designs
tries
to bring any available flame together with all smoke (which includes CO)
do
one lights the other.
This is easier to accomplish if the general environment is hot. It is
important that, if adding secondary air, it be preheated. What matters is
that the resulting mix have a minimum temperature, not that the heat come
from a particular source.
Preheating primary can badly affect some fuels unless the air supply is
restricted. It tends to over-produce gases from light biomass briquettes
unless it is very restricted. High carbon and dense fuels benefit from
having the primary air preheated because it harder to get them to make
combustible gases.
Choked, hot high carbon fuels make rich CO and burn very cleanly if the
combustion region is kept hot. The worst case is to make a good high CO
gas
which is vented into a heat exchanger which never gets hot. That pretty
much
guarantees high CO in the final mix.
Many stoves that appear to be burning with low smoke have too small a
region
above the primary combustion for the flame to finish burning properly. It
is
typically the chilling of the half-finished burn that looks clean but is
high in CO. If the gas flame from charcoal of a gasifier does not have at
least 50mm (usually 100 is needed) for flame space, it will not complete
the
burn and have high CO. By flame space I mean the distance from the
beginning
of the flame and the bottom of the pot or heat exchanger. If you see
flames
running along the bottom of a pot, or exiting into the air, you will know
it
has high CO.
Regards
Crispin
Message: 4
Date: Thu, 02 Dec 2010 08:54:36 -0600
From: [email protected]
To: Discussion of biomass cooking stoves <[email protected]>,
Johan Wessels <[email protected]>
Cc: [email protected], Discussion of biomass cooking stoves
<[email protected]>
Subject: Re: [Stoves] A low cost modified traditional woodstove
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; DelSp="Yes";
format="flowed"
Rajan,
Very interesting. Some questions and comments:
1. Is your stove something like a Rocket stove but with an air pipe
to confine and also preheat the incoming air?
This stove has comparatively lesser height. The combination of flame
concentrator and air pipe has helped in reducing the height to some extent.
2. Is the incoming air (via the pipe) so reduced that it will be
(mostly) primary air, meaning that no (or very little) of the O2 that
enters can get through the hot char and burning wood zone?
The idea of air pipe is only to keep the charcoal bed intensely hot, so that
we get a stable flame at the stove top. So, the quantity of pre-heated air
introduced via the air pipe is quite small - just enough to burn a small
quantity of charcoal ( which is naturally forming above the air pipe from
the burning of wood ).
We first tried a grate below the fuel bed. We found that there was no
control on the air coming up through the grate. Sometimes too much of air
coming up cooled down the whole process rather than giving that extra heat
by burning the charcoal. Looks like the air went around the charcoal up.
But the air pipe never gave any such problem. It always helped in
maintaining intense heat in the charcoal bed.
The air pipe starts functioning only after around 10 minutes ( from the time
of starting the fire ) after enough heat is developed in that area to create
enough suction for the air ( natural draft ) through the pipe. By this time
the charcoal also starts forming there.
3. Is there any vertical distance between the top of the air pipe and
the bottom of the passage through which the stick-wood fuel enters?
(meaning, have you separated those two points of entry? They are
essentially not separated in a standard Rocket Stove.)
No. It is exactly like a rocket stove. There is a gap at the top of fuelwood
through which extra air enters. The flame concentrator takes care of this
extra air.
4. If 2 and 3 are correct, then estimate the amount of additional air
(mainly to be secondary???) that is able to enter via the passage
through which the stick-wood fuel is entering.
5. The addition of a concentrator (lid or disk or whatever) seems to
be important to you.
Yes. This idea of flame concentrator, I have learned ( taken from the
Champion TLUD Stove ) from you. So, you are my teacher regarding this.
The air pipe is my concept.
What difference is there in performance if the
concentrator is not used? I think ( but Aprovecho people can confirm)
that a concentrator has been tried on standard Rocket stoves and found
to not deliver sufficient enhancement to justify adding a concentrator
to standard Rocket stove technology. Therefore, IF the "Rajan stove"
does have advantage from the concentrator, that indicates that there
is some entry of secondary air into the stream of combustible gases.
Is that before, or at, or after the hole in the concentrator?
There is always a gap between the top of fuelwood and the top of the
fuelwood window ( through which fuelwood is fed into the stove ). Extra air
enters through this gap.
Rocket stove takes advantage of height to complete the combustion of gases.
Flame concentrator reduces this requirement of extra height. Of course,
certain minimum height cannot be avoided - as Mr. Crispin has clearly
described.
The flame concentrator has made a lot of difference in this stove.
6. Please describe the flame (is there one?) that comes above the
concentrator.
In this design, the flame concentrator is a trough facing upward ( In
Champion TLUD Stove, it is facing downward ). The flame is inside this
trough ( also touches the cooking pot ). The depth of the trough is
approximately 40 mm. There is another approximately 40 mm gap between the
bottom of the trough and top of firewood. Another gap of around 20 mm at the
vessel support. So, totally around 100 mm height for the flame.
The amount of soot getting on to the vessel surface ( side ) is very much
reduced - compared to ordinary chullahs.
After reading the answers to the above, (and having some photos too,
please) then all of us can better comment on the nature of the
combustion and stove.
The photos will take a little bit of time.
Regarding thermal efficiency tests and emission tests, Servals can always
send a stove over to the testing laboratory - once the "marketed model" has
been finalised.
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