[Stoves] RE: improving charcoal stoves

[Peter Verhaart]

Crispin Pemberton-Pigott wrote:
> Dear Charcoal burners
>
> I am interested in assisting this consultation about burning charcoal better 
> but a bite at a time, not the whole meal because you will choke on the 
> length of the replies.
>
> Peter Verhaart asked:
>   
>> Do we know how much CO escapes from a charcoal stove?
>>     
>
> Having been testing charcoal curners for a couple of years now I can at 
> least try to give an answer.  The typical CO production of a small JIKO-like 
> stove is something like this:
>
> 15 to 95% of the carbon emerges as CO, when dividing the CO by the CO2. That 
> is, the CO/CO2 ratio (COr) is 15 to 95%.
>
> As you can imagine that is pretty horrific and presents a clear danger to 
> the users, millions of them, because people take their stoves indoors when 
> it rains.  They think it is a 'clean burning stove' because the emissions 
> are invisible.
>
> The reason people like JIKOs and other similar stoves is not that they are 
> perfect, but that they are more efficient overall than some methods of 
> burning the same fuel.  Even though the JIKO leaves much of the fuel 
> unburned (in the form of CO) it is better at transferring the heat and thus 
> it saves some fuel.
>
> The problem with the JIKO is the little door which is closed to 'turn the 
> stove down'.  It lowers the power output by choking the fire of oxygen which 
> if course generates massive amounts of CO.
>
> If you want low CO in a turned down condition, you have to maintain a small 
> fire generating CO and add enough hot secondary air to the output of that 
> fire to burn the CO to CO2.  This is possible to achieve, but only until the 
> fire, in its latest stages of burnout, fails to generate enough concentrated 
> heat to ignite the CO. Some heat can be stored in the grate to extend the CO 
> burn.
>
> Flames were mentioned by Dean and yes, flames assist the ignition of CO, but 
> heating the CO hot enough results in flames from the CO alone, without the 
> need for additional flames from volatiles or H2.  In other words, it is is 
> not necessarily flames we need to burn CO, but heat that ignites CO to 
> produce flames (blue ones).  The temperature of a CO flame is quite high. 
> The low temperature Dean mentioned is the result of the CO not burning 
> properly (i.e. a cold fire).  The Cobb Cooker claims a temperature of only 
> 400 and this is in a very low air flow with high CO.
>
> A JIKO with a small hinged door loaded with 300 gm of charcoal at 4 or 5 
> gm/minute has a much lower CO level than a BBQ, however if the burn rate of 
> the charcoal is raised, it quickly runs out of air.
>
> The JIKO was designed by a potter, not a stove maker.  It is almost always 
> starved of air.  The consequences of this are two: it has high CO level and 
> it has a very low excess air ratio.  The low excess air ratio increases 
> flame temperature, at least for a while.  Increasing the burn rate without 
> increasing the air flow results in excess air levels that eventually reach 
> zero.  After that, the flames cannot be maintained and the CO stops burning 
> altogether and the fire power drops, even as the fuel burn rate increases! 
> Blowing on the fire may not help at all because it does not solve the 
> problem of burning the CO, if just burns more carbon at a low temperature.
>
> The approach taken with the Maputo Ceramic Stove (MCS) is to try to limit 
> the production of CO by having a 'lazy primary fire' and a more vigorous 
> secondary burn.  It is not perfect (without moving parts) but it seems to 
> work very well.  I have managed to get CO below 2% occasionally and below 6% 
> most of the time, provided it is on 'high'.  The basic approach has been, as 
> others have commented, to try to light the charcoal in a way that it does 
> not all burn at once, to starve the middle of air more than the ring around 
> it, and to shape the airflow to create a cone of flame with a point at the 
> top.  Drafting air over the top of a conical fire seems to give the best 
> chance for burning the CO emerging from the choked middle.
>
> A major contributor to the CO production is the very low 'headspace' of most 
> charcoal stoves.  The pot is FAR too close to the fuel and there is no space 
> for the CO flames to stay alight.  Often the pot sits directly on the 
> burning fuel killing flames completely.
>
> The JIKO's emission of CO can be reduced by making the little door about 
> twice its normal size.  This greatly increases the chance of the CO being 
> burned (as there is enough air) but as it is a flat plate with raised edges, 
> the whole charge of fuel tries to burn at once 'in parallel' rather than 'in 
> series' like Dr Tom's gasifier.  'Parallel' means burning it all at once so 
> it gives a lot of heat that might not be wanted, high exit temperatures and 
> at the fire's end, it results in high excess air, lower flame temperatures, 
> more CO extinguishing, higher CO levels sooner (as the fire dies it happens 
> anyway) and finally, a lower heat transfer efficiency due to a lower Delta 
> T.
>
> I found that there must be 50 to 60 mm of clear space between the top of the 
> charcoal and the bottom of the pot, preferably more.  As the fire burns 
> down, more space is created so that helps, as does a conical grate like a 
> Mali Stove or an MCS.
>
> Peter, if a charcoal stove is burning 90% of its fuel to CO2 I would say 
> that is 'improved' for stove rating purposes.  Would you agree?
>   
I certainly would, Crispin.
Our, or at least my thinking about charcoal fires has changed thanks to 
reports on the List.
Apparently charcoal stoves have more in common with wood stoves than we 
previously thought.
As in woodburning stoves, with charcoal we also have to try and prevent 
fuel not immediately involved in combustion to remain unaffected by the 
heat of the burning fuel. Interesting.
Why did it take so long for enlightened stovers to come up with the idea 
of a central cylindrical space where the fuel burns, not seriously 
affecting fuel in the vicinity. Now that it has been done I wonder why 
we never thought of it before, even though we had examples of sawdust 
burning stoves built on that principle.
Perhaps a short pipe like extension would help concentrate the flames 
and prevent them from affecting the top layer of fuel.
Just a thought.
Keep up the good work,

Peter Verhaart
> Regards
> Crispin 
>
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