[Stoves] Servals Wood Stove

[rajan_jiby at dataone.in]

Dear Crispin,


>
> Message: 2
> Date: Mon, 10 Sep 2012 17:58:01 -0400
> From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> To: "Stoves" <stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] Wood Stove Demo  -  Video
> Message-ID: <156701cd8f9f$59d2c650$0d7852f0$@gmail.com>
> 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