[Stoves] improving charcoal stoves

Tue May 30 05:48:44 CDT 2006

*Dear Paul and All:

Charcoal (C plus small amounts of volatiles) is VERY different in its 
combustion characteristics from wood, CH1.4O0.6.  Complete 
(stoichiometric) combustion of charcoal requires an air/fuel ratio of 
~12, while wood has an air/fuel ratio of 6. 

**Charcoal:          C + (O2 + 3.76N2) ==> CO2 + 3.76 N2   A/F = 137/12= 
11.4 ~12*
*
CH1.4O0.6 + 1.05 (O2 + 3.76N2) ==> CO2 + 0.7 H2O + 4 N2    A/F = 
137.3/23 = 5.96 ~ 6
(To find A/F ratio, convert formulas to molecular wt, (C = 12, H=1, O = 
16, N=14, so wood = 23))
A/F = 6

Pyrolytic gasification of wood typically has an A/f ratio of <1.5, while 
gasification of charcoal to CO has an A/F ratio of ~6.  So after the 
WoodGas campstove finishes the pyrolytic gasification of the wood, it 
requires 4 times as much air to gasify the charcoal. 

*Paul S. Anderson wrote:
> 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.
>
*If we measured the CO emissions of charcoal barbecues we would NEVER 
barbecue indoors.  I would guess that at least half of the charcoal 
becomes unburnt CO.  Charcoal barbecues are certainly the least 
efficient of all cooking methods.  *
> 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.
>
*Sometimes I see dancing blue flames of CO above the remaining charcoal 
- sometimes I don't and know that CO is being generated.  (I have a CO 
meter in my hood over my test space and sometimes see > 100 ppm going 
outdoors.)  Fortunately, the stove is small enough that total CO 
emissions are small. I have NEVER had a CO headache, but learned long 
ago not to breathe much woodsmoke.  *
> 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.
>
*In the WoodGas Cookstove the fan is analogous to the throttle on an 
engine and generates more or less heat depending on fan speed.  In any 
more advanced WoodGas stove design I would recommend having a control on 
the air/fuel ratio "choke" as well to change the air/fuel ratio to 
optimize the flame for either biomass or charcoal gasification/combustion.*
> 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.)
>
*Part of this question depends on generating CO in a deep char bed, then 
burning it over the bed.  Combustion requires not only fuel, but a flame 
holder.  Sometimes I see the dancing CO flames over the bed - sometimes 
I don't. *
> 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.
>
*The WWII gasification era started with charcoal gasifiers in the 1930s 
and morphed to wood gasifiers as the war progressed and consumed more 
and more trees.  Air passed through a deep bed of characoal produces a 
combustible gas to run engines, but producing the charcoal wastes 2/3 of 
the wood energy.  If wood is added on top of the charcoal, the characoal 
pyrolyses the wood at very high temperature, replenishing the charcoal 
and destroying the volatile tars.*
> 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.
>
*Pure CO is VERY difficult to burn, but fortunately charcoal has enough 
volatiles to give some H2 in the mix which makes it much more combustible.*
> 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.
>
*Right on. *
> 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  *WHY?   *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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