[Stoves] RE: improving charcoal stoves

[jason marshall]

I have a general question on this topic:

All else being equal* how does the CO output of 'good' charcoal stoves
compare to the sorts of fuels typical of a rocket stove?  I was under
the impression that the rocket stove produced very low levels of CO,
if any.

Thanks,
Jason

* All else is never equal.



On 5/31/06, Peter Verhaart <pverhaart at iprimus.com.au> wrote:
> 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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>
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-- 
- Jason