[Stoves] RE: improving charcoal stoves

[Crispin Pemberton-Pigott]

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?

Regards
Crispin