[Stoves] Space between biomass fuel and cookpot in an open-fire cookstove

Fri Jul 22 19:03:59 CDT 2022

Kevin:,  List and Crispin

	1.  I urge sending the list another video - one that emphasizes that this is not your normal 3-stone fire.  This stove intentionally makes char - and seems to do almost as good a job as a TLUD.    Almost, because no control of power during a run.  Or can we fix the “no control ”?
> 		 Crispin’s message shows that he wanted to consume the char.  (He wrote :  ”.That allows incoming air to burn the accumulating char, “.   Lots of reasons to save the char which you are nicely proving..

	2.   The stove world also needs to learn that the  air supplies of your innovation is unique - not at all like a TLUD. or rocket  What is called primary air in a TLUD here starts downward on the outside and then turns inward, with the syngas proceeding upward esiting at the top-most surface (much as with a TLUD).  There, the topmost char surface is minimally consumed, because the inner up-flow has minimum oxygen (as in TLUDs).

	3.  But the main issue relative to your stove is that the fuel height keeps decreasing as the fuel shrinks in diameter (would shrink more if the fuel was stacked vertically - which. might be preferred here, even if not preferred in a TLUD. ) I hope we can hear of some test with fuel stacked vertically.   The problem with falling embers may go away with no primary air entering at the bottom

> On Jul 22, 2022, at 12:11 PM, K McLean <kmclean56 at gmail.com> wrote:
> 
> Crispin,
> 
> Thanks for your always helpful advice.
> 
> For widespread adoption, we sometimes must sacrifice efficiency for simplicity.  With the cooking hole, that probably means using bricks without curves.
	[RWL1:   My perception is that a square hole is not sacrificing any efficiency - especially when using bricks (as in your sketch) rather than 3 stones.  And two cook pots.    If vertical sticks worked out, then a circular hole could be preferred.  But vertical sticks could mess up the needed inward part of the primary air.

> 
> The cooking hole is new and I hope others test variations.  Here is a video <https://drive.google.com/file/d/1Wn-wudQ_5InNPaKrSi6MtJS2rUvjeaKi/view?usp=sharing> of one cooking hole heating two cookpots.  We found it was smoky without the hole between the bottom bricks,

	RWL2. a). Your 10 second video is very helpful.  My perception is not much smoke.   I think this 10 secs are shortly after being lit.  Could we also see flames at later times and from differing angles?  (A clock or timer  in the video and an end-on shot before lighting - to document the heights - which is the subject here.
> 
	c:  Re “ test variations. I'll bet you are already seeing lots of variations - given the growing number and the rapidity with which this (zero cost, time-saving, money-making) stove is being adopted.  Trying to save my  time in drawing up ideas, here are a few verbal-only descriptions on what I’d like to see.  These partly need more bricks, so we need to determine how cost effectiveness is changed (in time and money both)

        d   The video is bricks for 2 sides, each 2 bricks high and 2 bricks wide - making 8 bricks total.  I wonder the boil time and char amount changes if  this is rearranged and enlarged.

		i.  set the pots on the four  lower bricks (and save 4 bricks and use less fuel and/or a deeper hole and or a larger home in al four (N-E-S-W) sides )

		ii.  Same as i) - but rotate (make taller) the removed 2 top bricks on each side and use as a partial “skirt”.
	, 
		iii.  Similar to i), but the two top bricks on each side go to the ends that had no  bricks.  Spacing of the brick to allow air to still come in from each end.  Can test various air opening widths.  Presently two large and two small widths.  Now could have 8 smaller openings between the 8 bricks - all now at the same level.

		iv.  Same as. II and iii) but add second layer of 8 or more (also upturned) bricks for “skirt”.   So this still looks somewhat like the present sketch, but twice as many bricks.- with the upper level all skirt functioning
> - 
> As a reminder, the cooking hole is a 22x22x24 hole in the ground.  The fuel is piled up so the top is 10 cm above ground.  Sticks and maize stalks are stacked in a criss cross manner to allow air to flow within the fuel pile.  The pile is lit on top.  It burns down without tending for 30-90 minutes without adding fuel.  Water in both pots boils in about 15 minutes and simmers for the rest of the 30-90 minutes.  I am told that the visible smoke is much less than from firewood in unmodified three stone cookstoves.

	[RWL3:  It would be very satisfying to know why there is less smoke.  Could be the size of the fuel?  (Rocket proponents say so).   Or is it because the gas leaving the top-most layer of char is essentially hot syngas - mostly CO, H2 and N2 (the tars have been converted already).  The entering upper  secondary air is not real hot, but some is entering between hot bricks and all could be.  Less true when using 3 stones.

> 
> <Youtube Photo.jpg>
> 
> I think custom bricks that are not fired in a kiln ("unburnt bricks") will harden while cooking meals.  We are testing this and I'm optimistic.  Ripple Africa has found that their unburnt bricks harden while cooking.

	RWL4:   Ripple Africa looks like a first rate charity.  Also making hand-built earthen low cost stoves, but not charcoal-making. (Malawi).   Anyone reading this have experience on the hardening of “unburnt bricks”?

> And our custom bricks that line the cooking hole are starting to harden.  Using custom, unburnt bricks to support cookpots will allow us to determine the best dimensions and just distribute molds.  Our networks make the distribution of molds in Africa and local production of the custom bricks easy.  It sounds like they should be about 10 cm high so that two bricks will elevate the cookpot 10 cm above the top of the fuel pile.

		[RWL5:   I think we are saying bricks are are about 5 x 10 x 20 cm (2 x 4 x 8 inches).  None are critical dimensions.   The hole dimensions can be modified for situations with bigger cook pots.

Kevin - congratulations on getting char-making stoves to well under $1 -  - and saving cook’s time - and making money - either by selling the char or putting in their fields.  Your users have reported some spectacular results - especially in drought areas.

Ron

> 
> Kevin
> 
> On Fri, Jul 22, 2022 at 6:16 PM Crispin Pemberton-Pigott <crispinpigott at outlook.com <mailto:crispinpigott at outlook.com>> wrote:
> Dear Kevin
> 
> I looked into this when developing the Vesto and there are two answers. One is the correct height (minimum?) Above the fire to get good draft to pull air into the fire at a high enough velocity to get good mixing, and the flame space needed to complete combustion. 
> 
> For wood, I found the vertical space needed is at least 100 mm, with a preference for 110 mm, and for charcoal 50 mm with  preference for 60 mm. 
> 
> You drawing shows two parallel rows of bricks. This should be two semi-circles in order to limit drawing in useless air. Cooling the flame with unnecessary excess air is a major cause of PM with wood stoves and CO in charcoal stoves. 
> 
> Having a fuel hole and an air hole is important, closing everything else if possible. Putting a stone/pebble grate as you have been, is a great help. That allows incoming air to burn the accumulating char, raising the combustion temperature. That of course reduces all emissions from incomplete combustion. 
> 
> Sheltering a fire on three sides is so-o much better than two parallel walls. So if they have bricks, even mud bricks as are common in the Eastern Cape, the result can be pretty good. 
> 
> As a general rule, put the air entrance at the back bottom, opposite the fuel entrance, not on the same side as per rocket stove. It will keep the flame centered and away from the walls. That plus >100 mm flame space will give a reasonable result. 
> 
> Keep up your good work!
> 
> Regards 
> Crispin 
> From: kmclean56 at gmail.com <mailto:kmclean56 at gmail.com>
> Sent: July 22, 2022 9:41 AM
> To: stoves at lists.bioenergylists.org <mailto:stoves at lists.bioenergylists.org>
> Reply to: stoves at lists.bioenergylists.org <mailto:stoves at lists.bioenergylists.org>
> Subject: [Stoves] Space between biomass fuel and cookpot in an open-fire cookstove
> 
> What is the minimum distance from the top of the fuel to the bottom of the cookpot?  I've heard that 8 cm is the minimum to get the best combustion.
> 
> We are training cooks to use common bricks to elevate two cookpots over one cooking hole.  I want to make sure the cookpots are high enough.
> 
> <2 burner drawing.jpg>
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