Re: Down draft stoves are grate!

["Ron Larson" <ronallarson@qwest.net>]

Dean:

 

    1.  The most important part of this message relates to your last sentence.  I was just about to send a note reminding you about the "science toys".  (For others, Dean described these during our aborted visit to the Shell Foundation meeting in October - and I ordered up a set then.  I'll bet that Dean can sell a bunch if he brings more than one set for me.  One is a set of sticks to build a stronger "Bucky Fuller Geometric Dome" - using a very clever modification developed by our list's own Richard Boyt.

    There are many other important sides to Dean that I have been meaning to mention - one being that he used to run a whale-viewing operation from a small remote village in Mexico.  Anyone else care to add more on Dean (who is a very pleasant bright fellow.)?

 

2.  As with my message to Piet Verhaart, my apologies to you and Larry Winiarski - for not noting your good past work on down-draft stoves.  I am pleased to hear that they have worked well - but sorry to hear that users have not taken to them.

 

3.  You are right that my main point was about down-draft, air-controlled stoves - which I still have not seen in print.  I believe that pyrolysis and gasification processes have great promise when we worry about clean combustion - and that promise seems like it may go up with a down-draft stove (which needs a chimney).

 

Thanks for your message.  See you in a couple of weeks.    Ron

 

 

----- Original Message -----

From: Dean Still

To: stoves@crest.org ; Tom Miles ; Tempra Board ; Tami Bond ; Stuart Conway ; Ron Larson ; Rogerio Miranda ; richard njagu ; Richard Boyt ; Piet Visser ; Peter Verhaart ; peter scott ; Patrick Flynn ; Marian Grebanier ; Lisa Buttner ; Laurie Childers ; Larry Winiarski ; Lanny Henson ; Kirk R. Smith ; kevin hallinan ; kenneth mark bryden ; Kenmore, Peter (AGPP) ; Ken Goyer ; Karissa Ansell-Bell ; Jonathan E. Sinton ; Jeff Conant ; Grant Ballard-Tremeer ; George Rudy ; Frank Vignola ; Elizabeth Bates ; Don O'Neal ; Derick Calderon ; Delacie Barney ; David Pennise ; Daniel M. Kammen ; Cousins, Ananda ; brad ; horizon

Sent: Saturday, December 29, 2001 12:28 AM

Subject: Re: Down draft stoves are grate!

Dear Ron,

 

When you mention downdraft stoves, I see that you are thinking more about charcoal making stoves but I wanted to mention that Larry Winiarski has been playing with downdraft for quite a while. Aprovecho still uses the downdraft pattern for the fuel magazine where people will use it because it is cleaner burning and the sticks of wood are somewhat self feeding. Instead of feeding sticks of wood into a Rocket elbow in the shape of the letter L (sidefeed), downdraft Rocket elbows look more like the letter U, with one shorter vertical side. Sticks are fed vertically down the shorter side of the elbow and (in our case) are lit at the bottom. The fire is drawn up the taller side of the U which is very well insulated. The pot sits on top of the insulated part of the apparatus. The feed magazine is shorter than the combustion chamber and only insulated at the bottom, which helps the flame to travel in the right direction, toward the pot.

 

Rocket stoves started out using the downfeed/downdraft feed magazine pattern because it is more efficient. Why? Because 1.) a easily controlled amount of air is appreciably warmed as it is sucked down into the fire and then 2.) gases pass right over the hot bed of coals cleaning up emissions. Coals fall in front of the sticks into the flame path. Larry published a description of downdraft/downfeed in Boiling Point 21. But we've seen downdraft patterns in older heating stoves, etc.

 

We don't see downdraft/downfeed very much in recent Aprovecho stoves because folks don't often take to this pattern. It requires learning a new way of lighting the stove and the fire is down in the bottom of a tube. So, Larry uses sidefeed although the coals fall into the wrong place, under the fire, where they don't do as much good. I like downdraft/downfeed myself : you don't have to bend over to place the sticks in the fire or check its condition. And you can use really long sticks. You can see it in three of the stoves we currently use: the huge incinerator in the dump in Managua, and in a heating stove and a bread oven at the Research Center.

 

It's an important feature of the downdraft/downfeed pattern that is is easily adapted to burning materials that fall from a hopper, sawdust, husks, etc. The loose stuff falls on an inclined ladder in the insulated combustion chamber that makes for cleaner burning. Great for incinerators where you're having to deal with weird stuff thrown down a shute. Larry has used this design for cooking stoves, too, for burning Guatemalan coffee husks, etc.

 

See you in Seattle! I'm bringing you a set of my science toys.

 

Best,

 

Dean

-----Original Message-----
From: Ron Larson <ronallarson@qwest.net>
To: stoves@crest.org <stoves@crest.org>
Date: Thursday, December 27, 2001 9:37 PM
Subject: Down draft charcoal making stoves?

   I.  Intro and background.   This is to explore the world of down draft stoves.  So far on this list, I remember no positive remarks about a down draft stove design.  ( I do remember down-draft recommended as an incinerator - but this is not the air-controlled design I talk about below.  Also Elsen Karstad has employed down-draft for charcoal making of sawdust - but he is not controlling air flow directly and is not using the waste heat.)  The downdraft gasifiers used for coupling to IC engines that I have seen developed by Tom Reed, Das and others also seem to not control primary air separately - mainly because (I surmise) they desire to operate continuously from a level-controlled fuel hopper.   I also came to some of the following conclusions from my "book report" on wood-burning pottery kilns - where air control is very important (but fuel supply also is)

 

    2.  Eindhoven work.  Piet Verhaart did some work on down draft stoves when at Eindhoven.  His work and the report he recommended by Hassan Khan were not encouraging.  Apparently no down draft stoves of any type are now in production or study anywhere in the world (anyone know anything different?)

 

    Also, Prasad said on Nov. 30.   "The question is whether (the down draft stove) can be used as a charcoal producer as
well: without really thinking through, an off-hand answer would be - No."

    I have concluded that Prasad was thinking of a different design - and I hope he will conclude after reading this that a down-draft design with direct air control can produce charcoal.  

 

    Alex recommended the Khan paper because it had documented a "sweet spot" where emissions were lowest.  The sweet spot was the "magic" combination of fuel feed rate and chimney height (indirect control of total air flow, not direct control of two air flows) that gave lowest emissions.  I am not sure, but think that a different set of dynamics will hold in this stove, but a "sweet spot" could well also occur here ( a best power output level).

 

C.  Relationship to Stove categorization.    As I was preparing my last message on categorizing different types of stoves, I was wondering whether I should be dismissing the charcoal-making down draft.  I have concluded not - I now think it very possible. I have had limited agreement from Ron West, Das, and Alex English. This note is to see how others feel - and hopefully someone will give it a try.  I will eventually, but can't see any free time for quite a while.  It would be wonderful that someone has already done the desired test or has the equipment lying around to do some quick testing.

 

D.  Direct Control of air flow..   I think the reason that Verhaart, Khan, and Prasad did not produce charcoal was that they were only indirectly controlling air flow. (Actually, Khan does record some circumstances with charcoal production - but it was clearly not near the optimum operating conditions).   The Khan design was rather like an inverted Apprevecho "Rocket" stove - power control via fuel feed rate.  There was no separate mechanism to control air flow.  All secondary air had to go through the same fuel path as the primary or pyrolysis air - there was no separate valving of either primary or secondary air flow.  So pyrolysis gases exiting the fuel chamber had a large excess oxygen content, unlike the pyrolysis/gasification stoves this list has also been discussing. 

 

E.  Reference the "Dasifier".    To make a true inverse of the updraft charcoal making stoves, one must have a tight-fitting lid on the fuel supply, and one must keep the primary air flow through the fuel supply very low.  Then, separate secondary air must be introduced and the pyrolysis gases thereafter combusted - possibly with premixing, or possibly with a diffusion flame.  The nearest figure I have for this was provided by Tom Reed in introducing the "Dasifier" on November 5 (useful comments also by Andrew Heggie on Nov. 6).  You will see that Das was producing charcoal in the upper left part of the diagram - but without the controllable top that I am advocating.  However, he was able to produce charcoal because of the depth and density of the pellets .  The exiting gas from the top container presumably has zero or a very low oxygen content.  His required secondary air is coming from the bottom.  Das agrees (I think) that secondary air coming horizontally from the side would preclude the need for the lower chamber if one wanted to produce charcoal - and (again, I think) would not require the gasifying action of the lower chamber if one wished to consume the charcoal.  The difference is how much of the charcoal is exposed to the secondary air.

 

F.  A stove geometry.  In Tom Reed's figure of Das' gasifier, if you think of the narrow tube as a broad flat disk shape, and ignore the lower left gasifier portion and the injector, then you have a natural draft, charcoal making cook (and heating) stove.   I am thinking of a snake-like flame path that returns to a chimney near the start.  Alternatively, perhaps the chimney and fuel container should be well separated with a large temperature difference as in the Eindhoven work.

 

 

G.   Advantages.    The advantages that I see are:

    1.  Downdraft fits more easily with a chimney than do typical updraft designs - in fact, won't work without a sizeable chimney.  This may prove to be necessary in all applications where the stove is used for room heating and where Indoor Air Quality is a key decision criterion

    2.  Can control the excess air ratio - and thereby achieve high temperatures and hopefully greater efficiency.

    3.  Fits in well with applications requiring a large metal cooking surface of the type Rogerio Miranda has been producing in Nicaragua.  Power output changes (turndown ratio) should be very large and rapid (3:1 in updraft designs)

    4.  Could be designed with one or more convertible drop-in pot elements (as noted by Verhaart et al) for convective rather than conductive heat transfer to the pot.

    5.  The fuel supply can be replenished when the pyrolysis front has reached the top.of the (bottom lit) fuel container (unlike the up-draft charcoal-making designs).  The fuel supply is perhaps more handy being above the cooking surface (but maybe somewhat more dangerous there.)

    6.   Can be designed for charcoal consumption as well as production without modifying position of charcoal or cookpot.

    7.  Presumably cleaner, more controllable, and less labor intensive than fuel-controlled stoves (and has or could have saleable charcoal co-product).

    8.  Cook surface can possibly be manufactured locally from surplus barrel ends (can conceive of staying less than $25 with locally made chimneys.).

    9.  An oven is an easy add-on (with temperature control dependent on both placement and by primary air flow).

    10.  Should work well with range of fuels - not dependent on high density fuels.

    10.  No particular advantage obvious for use of forced draft.  A chimney that gets outside the house looks like it should have sufficient draft.  It is not obvious that extra draft is needed for charcoal combustion.

    11.  Might be able to use with insulated cookpots that have lower side and top losses.

    12.  Can be coupled with an auxiliary, heat-capturing water heater designed something like a Samovar.

 

H.  Disadvantages

    1.  This looks inherently more expensive than the UD version - as it requires a chimney of probably more than one meter height and a large metal stove top (but no more expensive than other stoves with those features, and I guess the whole thing could be made from ceramics and tight fitting cook pots.)

    2.  Probably less efficient as exit gas temperatures may be higher and the exposed metal surface areas larger (although the cook surface can perhaps be covered when not needed by insulating bricks.)  Large exposed cooking surfaces are common (and maybe even considered desirable) in US woodburning cook-stoves.

    3.  Will require user education on handling separate primary and secondary air supplies.

    4.  Conceptual stage only - problems are certain to develop.  But this DD seems to be the complete inverse analog of workable updraft controllable-air pyrolysis and gasifier models - and is not the down draft on which others have discontinued work . (I urge that we not call this the Inverted Up Draft or IUD design; for non English speakers, you should know that the term "IUD" has already been taken - it already has a well known different meaning that you should use caution in determining.)

 

I welcome thoughts of all stovers.  Is this DD-AC-CM (down draft- air controlled - charcoal making) stove design worth further exploration?  Any references around to build on?

 

Ron