[Stoves] Heat / cook stove - proposed design

[Agnes Klingshirn]

Sorry, but the photos were not attached. Crispin, please try again. Thanks,
Agnes

 

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Von: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] Im Auftrag von Crispin
Pemberton-Pigott
Gesendet: Freitag, 25. November 2011 23:21
An: 'Discussion of biomass cooking stoves'
Betreff: Re: [Stoves] Heat / cook stove - proposed design

 

Dear Darren

 

This is coming along well.

 

>Some photos are attached.

 

Got them, thanks.


>The door and hatch are made and have fire rope seals.  

 

Good move. The hopper should have no air passing through it. There is a
pyrolysis zone above and near the primary air entrance, then a burning zone
further to the left (back of the grate) then a gas burning zone above the
rear of the grate ascending into the vertical chimney.

 

>I've used a cast iron grate from an old coal stove that was the perfect
size.

 

It looks really. I think you should expect to make two changes once you get
going. One is to decrease the gap between the grate and the ceramic bridge
(we found 75mm is perfect for the fuel we are burning) and to increase the
10 degree angle as a means for achieving this. That means lifting up the
front or near end of the grate. What would make you want to try that would
be excessive smoke caused by too much fuel burning at once. 

 

We ran quite a number of tests with different spacings like 65, 70, 75 and
85 and 90mm but all having 10 degrees of slope. The 75 mm straight line
distance was perfect. It gave an excess air ratio of 80-120% and
extraordinarily low PM values for much of the time. Because your fuel is
different you are going to have to do some hit and miss too until you get a
really clean flame about 15 minutes after ignition and thereafter, no smoke.
Hopefully it will perform as expected.


*At the moment all the secondary air is going in through some square section
pipe with an internal diameter of 20x20mm that enters at the rear of the
combustion chamber above the ceramic blocks.  (shown as blue square in
stove2-7.jpg).  Is this going to supply enough air or should I put a similar
sized secondary air inlet on the front of the stove also?

This si going to be a very difficult question to answer without equipment.
Suppose you have enough air but your 100mm grate gap is too big. It might
incline you to provide more secondary air when you really needed to reduce
the burn rate. At some point, you have enough power, so the air to match it
is 'X' value. At a higher or lower burn rate, you need a different amount.
You are gonna tell us what you need! I think the 20 x 20 may be too much but
the fuel is different. The easy test is to open and close it when there is
only a little smoke. It should make very little as it transitions from one
firepower to another as it lights up. Somehow the feedback from you hand
closing it will tell you which way to take things.

 

If you can't see any need for it, close it totally. It just add air with no
effect other than to cool the output gases. If you need it, leave it open.
We have not found any need to adjust the secondary volume when it is
running. Most of the secondary air enters through the far end (back) of the
grate where there is less fuel.


*Crispins GIZ design has an area where the combustion chamber tapers wider
(an expansion chamber?).  I've drawn this in green in stove2-7.jpg . How
important is this?  

 

The expansion is important but not critical. If you can, do it. Professor
Lodoysamba found that filling the combustion chamber with wood and
top-lighting it provided immediate heat for cooking, very low emissions and
easy ignition (automatic) of the fuel under the hopper. Very clever.

 

When the fire reaches under the hopper, he had to shake the grate to move
fuel down onto the grate. That is all in terms of attention.

 

>What effect does it have on combustion? what would happen if I leave them
out?

The effect of the bricks/ceramic is to improve the combustion by having hot
walls. If you can't get real refractory bricks second hand, use the hardest
clay face bring you can find. It will last the longers in terms of off the
shelf material.


>.Hopefully I should finish and test fire it in the coming week.

Looking forward to more pictures and your comments.

 

>I've attached a diagram of how I'm now planning to build the grate and
throat area.  (I took the liberty of modifying the GIZ diagram Crispin
kindly sent to explain an appropriate layout) 

You construction matches the drawing.


As I'm only going to be burning wood and in my experience of wood stoves all
the wood burns to a light ash which would easily fall through the grate I am
planning to have a stationary grate.   Am I making the wrong decision here? 

The grate gaps look large which will create extra charcoal that might get
snuffed in the ashes. Other than that it is fine. If the grate turns out to
be too 'coarse' you can drop in a set of 12mm steel reinforcing bars welded
into a grid. Works fine.


>Hoping to get it finished soon.  Considering the number of people I've been
describing it to all year it would be good to also let them know how it
works!!!  I've never seen anything like this in action - I'm thoroughly
intrigued. 

You are going to have to try it with charcoal briquettes, wood pellets,
wood, chopped up wood, coal and switchgrass cubes if you want us all to be
happy! Good luck.

 

Regards

Crispin

 

 

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