[Stoves] improving charcoal stoves

[Paul S. Anderson]

Dean and all, (especially Tom Reed who is asked for comments below)

Dean's comments, questions and concerns make a lot of sense.  Here are some
thoughts:

1.  From what I have seen of charcoal cooking in American backyard cooking and
around the world, flame (the burning of the CO and any lingering hydrocarbons)
is not sought, and seems to be intentionally avoided.  I doubt that devices or
methods to burn the CO from a typical charcoal cookfire will be found or, if
found, be acceptable for cooking practices.

2.  Successful clean burning of charcoal IS accomplished during the final
(post-pyrolysis) stage of using Reed's WoodGas CampStove, the first of the
T-LUD devices.  In that device, there is forced air both for primary and
secondary combustion.  Tom can get a nice blue flame in that situation where
there is only char fuel, and no raw biomass.  Reasons are (I believe) that the
forced primary air will assure a goodly supply of combustible gases (including
some H2 and maybe methane as well as CO), and the forced secondary air assures
mixing of new (with O2) air into those hot and highly combustible gases.

3.  The day I first met Tom Reed (about April or May 2001), he had a prototype
IDD (now called T-LUD) gasifier stove that had a feature that is not in the
commercially produced WoodGas CampStove.  That early stove had a little cover
or slider or "door-like" control so that Tom could direct a different 
amount of
the total air to the primary or to the secondary pathways, depending on 
whether
the stove was in the pyrolysis stage or in the char-burning stage.  Tom said
that the ratio of primary to secondary air was almost inverted between the two
stages.  Tom can give the exact numbers for the units of air, but I think it
was 1 primary to 5 secondary during pyrolysis stage and 4 primary to 1
secondary during the charcoal burning stage.  5 x 4 = 20, so there are some
serious changes going on.

And a question for Tom:  Is the primary to secondary air ratio during the char
burning stage different if (A) there is modest air flow yielding simple char
burning to make CO2 and H2O down near the level of the char bed, or if (B)
there is more forceful air flow resulting in active gasification that makes CO
and H2 come from the char bed and available for subsequent combustion?  
(See #5
below for more explanation about char combustion vs char gasification with
close-coupled combustion.)

4.  Lanny's experiments (central open core with charcoal around, right?)
reminded me of Richard Stanley's holey briquettes, of which I have made and
burned many.  I think a key to success is that an appropriate amount of 
air was
constrained to a small, fast rising column of air with "fanning impact" on the
char closest to the air.  That is how the holey briquettes do such a great job
of burning vigorously, but not all at once.  Main difference is that Lanny's
charcoal had some air spaces among the char pieces while Richard's briquettes
are solid (except for the central hole) and have raw biomass.   And I suspect
that Lanny's char could shift and fall some as the pieces get smaller, but
Richard's had to drop off the ash from the central hole while the rest of the
briquette stayed in place, but with an ever increasing diameter of the central
air shaft.

5.  Increased natural draft through a constricted channel acts like forced air
as in Reed's WoodGas unit.  And that is also similar to the use of a 
tin can to
hold the charcoal together when lighting a barbeque, and similar to a 
forge with
a bellows to push air onto the hot charcoal.  In all cases, a 
respectable amount
of air hits a relatively small area of charcoal, resulting in significantly
increased heat of the char, giving exothermic combustion to yield CO2 + 
H20, or
gasification (some endothermic reactions also) to yield CO and H2.

6.  The gasification process of char works best (perhaps even requires) when
there is a layer of char (carbon) above the hot zones, so that the CO2 can be
changed (reduced) into CO.  (It is more complicated than that, but I am 
not one
to quote the full equations.)  Therefore, a thin layer of large diameter
charcoal in the bottom of a typical charcoal grill is NOT going to 
generate the
quality combustible gases that are made in the Reed T-LUD gasifier (with many
layers of smaller diameter charcoal), or the Hanson stack of charcoal
lumps/briquettes.

7.  IMHO, a successful charcoal stove should have a "stack effect" of 
the char,
with the stack being sufficiently SMALL in cross-section so that the
appropriate amount of air (which will determine the combustion rate) is not
spread thinly, but is concentrated onto a relatively small amount of 
the char. HOWEVER, with this heat coming out of a relatively small 
exit, it is not
condusive to grilling a burger or skewers of anticuchos (Andean meat cooking
done on charcoal embers), and could give a hot spot on the bottom of a pot.

Personally, I would prefer to NOT have a better charcoal stove because that
would encourage people to use and therefore make more charcoal, which is very
wasteful of the pyrolysis gas-energy, causes polluting, and drives deforesting
to get large chunks of wood to make charcoal.  On the other hand, if a T-LUD
stove was used to make the charcoal, the picture of the negative impacts
changes dramatically.  But even then, why use charcoal stoves if a good T-LUD
could let urban dwellers use dry biomass without the energy wastes, the
polluting, and deforesting (because T-LUDs use small pieces of wood from
sustainable forest practices, or non-wood biomass fuels.)  In that "ideal
world" [  ;-)  ], the char produced by the T-LUD could be permanently
sequestered in the soil (soil building) and CDM carbon credits could be paid
for the Carbon not sent back to the atmosphere.     NOTE:  If you want 
to reply
to THIS paragraph, please change the Subject line of your message to "T-LUD
stove claims" or some other clear Subject line.

But I will help you make a better charcoal burning stove.

Paul

Quoting Dean Still <dstill at epud.net>:

> Dear Lanny and the List,
>
> We are trying to improve charcoal stoves but are not having much success.
>
> Here's what I think about charcoal but I'm sure that others will enlighten
> me a lot more since I'm just starting to play with it.
>
> 1.) Charcoal does not burn very hot. A thermometer an inch above the burning
> charcoal only reads 600C or so. Wood fires are a lot hotter more like 900C?
>
> 2.) Charcoal emits a lot of CO but not much PM. The CO escapes because there
> are few flames to burn it up. Flame makes PM so maybe for the same reason
> there's low PM?
>
> 3.) The pot needs to be almost touching the charcoal because radiation is
> dependent on distance. A steak at one inch burns but at 4 inches cooks
> really slowly not because the radiation sees a smaller target but that
> radiation diminishes with distance?
>
> 4.) Making flame above the charcoal should help cut CO but would increase
> PM?
>
> 5.) Increasing air flow doesn't clean up emissions only increases firepower?
>
> Does anyone know how to use least amount of charcoal to make most amounts of
> food/least emissions?
>
> HELP! I'M STUCK...
>
> Best,
>
> Dean
>
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>



-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to (below) and click on my name:
http://www.repp.org/discussiongroups/resources/stoves/contributions.html

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