[Stoves] Heat / cook stove - proposed design

Darren mail at vegburner.co.uk
Sat Nov 26 04:30:22 CST 2011


Tom,

Thanks for hunting down and posting the photos for me.

Crispin,

Thanks once more for the advice.  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 can be lifted higher.  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 - prices for soldering boards 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 
with the opening of primary air, flue damper, both secondary air inlets 
and see what effect each has on the burn in the combustion chamber.

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
>
>
>
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