[Stoves] RE:Low Cost Woodgas Stove

[Crispin Pemberton-Pigott]

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Dear Pottery Friends

>I first started working with Ron Larson on the natural draft WoodGas
>stoves about 1996 and he focused on wanting to build the stove from
>clay, Gretchen, his wife being a potter, and clay being cheap.

I am reminding people of the project under way in Maputo to make a ceramic
stove that can compete in the market against a metal stove.  This is a
formidable problem I admit because a) the metal stoves already dominate the
market and b) clay stoves are heavy, breakable and unknown.  The selling
point has to be fuel economy and perhaps appearance (pride of ownership).

>The "tincanium" stoves are easy to construct.  They have a
>very low heat capacity compared to clay.  They are lightweight
>and not fragile. They can be mass produced.  I have yet to see a
>clay stove that duplicates these virtues.

Exactly.

However high thermal mass offers other benefits, in particular the provision
of retained heat to the primary air late in the fire's burn increasing the
usually low combustion efficiency.  The reduction in CO later on is very
valuable both to reduce emissions and to increase the heat output basically
for free, though some of the heat might have been available earlier on which
could affect performance.

Overall clay stoves have long term advantages that may see them gain wide
acceptance.  First, they can be made cheap and in large quantities in
modestly sized businesses so that a standard product can reach the market
with a predictable performance.  This is valuable when scaling up production
in a city and when there is local or regional advertising touting them.
Artisanal stoves suffer from variability / inconsistency.

Clay stoves can have both a fairly high insulative value, giving early flame
temperature rise, as well as the ability to store some heat for later when
the amount of fuel is greatly diminished.  The effect can best be seen when
looking at a limited time 'cook' instead of  pretending that the stove works
in a continuum.  For example saying (as a criticism) that clay absorbs heat
from the fire in the early stages does not mean that it continues to do so
after a time.  It is a temporary thing and the heat can be returned later,
not to cook without a fire, but to heat the incoming air and assist the burn
over a 10 or 20 minute period as the stove body cools.  The effect on
emissions is pronounced.  Late in the burn when a charcoal stove is in the
dying-fire stage, heat transfer efficiencies frequently exceed 100% as the
heat is recovered from the stove body while simmering.  This effect is only
minimal with light metal bodies.  Thus clay stoves have some inherent
advantages over light metal ones.

As for the possibility of a hybrid clay+metal stove, the most efficient wood
or charcoal stove I have tested to date was a clay bodied stove with a Vesto
fire grate in it.  Its overall efficiency (PHU) was 52% and I can say it was
not optimised - just the parts we had available tossed together.  Joäo Da
Conceiçäo made the clay parts and the Vesto parts were standard.  I did the
testing with charcoal.

The reason for the efficiency is that the metal parts that do the air
preheating and fire manipulation are placed in the middle where these
attributes are most required.  The clay on the other hand is used to provide
stability and huge thermal insulation.  The stove did not get noticeably
warm on the outside after more than an hour.  The resulting stove looked a
great deal like a heavy clay Vesto.  It was stable and cheaper that an
all-metal Vesto.  A variation on this has been on sale by ICEMA in Maputo
for almost a year using locally made non-stainless steel inner parts.

Regards to all
Crispin