[Stoves] Heat / cook stove - proposed design

Sat Nov 26 06:58:44 CST 2011

Dear Darren

All those ideas are good.

Don’t make any changes now re the grate because as we discussed some months
ago – you do not really know yet what will happen with wood so there is no
point is stressing. You are the first to make this style and put wood only
in it. We cut the wood very small (finger and thumb I call it) just for
ignition. It has to pack pretty densely though. You will tell us what you
think when you see it running.

Another option is out a re-bar grate on top of the grate which will have the
same effect: limiting the amount of fuel that can fall/slide/shove under the
ceramic bridge.

Note to Paul A: this approach (with a finer grate) will burn pellets, I am
sure, with editing of the secondary air supply size and maybe position
(though I don’t worry about that position – it’s pretty good).

Regards

Crispin

+++++++++

I am weighing up whether to cut away some more material from the bricks at
either side of the throat to give space to modify the grate angle and/or
height once the stove is complete or if I should modify the grate (quite
possibly make a new one from bar as you suggested) so that it would lift up
between the bricks.  I'm thinking it would be best not to cut the bricks. 

I was just looking at pricing for refractory board to make the sloping sides
in the upper part of the combustion chamber.  I'm planning to have three
boards on all sides except where the door is.  

I'm looking at 300mm x 200mm x 10mm (soldering) boards - prices on ebay,
about £10 each).  My father rang me and I told him what I'm up to and he
said he thought he had a sheet of fireproof roofing sheet (asbestos
replacement), which his brother had given him, in his shed which should be
big enough for what I need....

I've decided to add a second secondary air inlet on the front of the burner,
if for no other reason than to have it inputting some hot air up by the
window in the door to keep it clean.  Also means I can play about adjusting
primary air, flue damper, both secondary air inlets and see what effect each
has on the burn in the combustion chamber.

I've got some fire cement I'm planning to use between the bricks to try to
stop air leaking in through the gaps.  Everything does fit together tight
but I'm trying to seal the whole burner up so there are no air leaks where
they are not wanted.

Best

Darren

On 25/11/2011 22:20, Crispin Pemberton-Pigott wrote: 

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

_______________________________________________
Stoves mailing list

to Send a Message to the list, use the email address
stoves at lists.bioenergylists.org

to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists
.org

for more Biomass Cooking Stoves,  News and Information see our web site:
http://www.bioenergylists.org/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20111126/4d85ec0a/attachment.html>