[Stoves] Two ways of testing combustion/emissions

[Crispin Pemberton-Pigott]

Dear Paul

The major difference between the methods is that you can't easily design and
improve a stove using a hood only. It is OK for analysing certain
performance criteria, but unless you know how much of each gas is present
inside the fire (meaning, passing through the stove) you can't really tell
what happened when you changed something. There is a big difference between
a 3 stone fire and a JIKO because you can easily get a gas sample from a
JIKO.

By far the most important thing to control is excess air, and where that air
is put. If you don't sample the gases as they emerge (as I did for example
with the Clean Cook stove at ETHOS 2007) I can't tell if the high CO was
caused by the flame touching the pot, or because of insufficient air.  If
you collect everything in a hood, you can't tell _why_ the CO was high, only
that it is. If you wander down an erroneous stove theory path, you can get
lost for years before finding out it is a niche or a dead end.

Elevating the pot 40mm cut the CO emissions by 75% because the meter showed
that the EA was fine, it was the flames touching the pot and chilling that
was the cause of the problem.  I understand that modification was
implemented immediately. Under a testing hood, different speculations would
have been equally believable.

In this simple example, the different between the methods is shown. Usually
things are a lot more complicated than that.  If you are making a space
heating no-chimney cooking fire (as is the case in many places) the 'stack
losses' can't be determined unless you have a gas sample from the point
where no more heat is being absorbed (usually the stove-pot gas exit point).
When comparing two stoves with different heat losses through the body,
different EA values, different gas velocities...it is difficult to know
_why_ they have different performances using a hood.

What I have seen around the world is that there is disagreement between
combustion device people and air quality people on how to test a stove and
what to look for because they have different goals.

Regards
Crispin