[Stoves] briquette air feed

Crispin Pemberton-Pigott crispinpigott at gmail.com
Thu Jan 6 11:06:22 CST 2011


Dear Kobus

It is pretty clear we need a suite of tests for different applications and
fuels. The burn-out test which is far more accurate than a WBT (according to
Penn Taylor) is an example of an off-the-wall, not-out-of-the-box test
method which is really easy to perform.

The SEET Lab in Ulaanbaatar (Stove emissions and efficiency testing lab) is
using a protocol that gives real time thermal efficiency during the burn. It
is really interesting to see the efficiency not only drop to zero, but to go
negative (sometimes 250% negative) late in the burn as the heat from the
stove and fuel remaining drives warm air up the chimney and pulls in cold
outdoor air to replace it.

As yet we do not have what I consider to be a good solution for the changing
composition of the fuel during the burn. If you are burning charcoal late in
the cooking session, are you using a different heat value per g burned? If
not, how large is the error involved?

With coal, the loaded fuel is about 12 MJ/kg at the start and 29.5 MJ/kg at
the end.  That is a very large change. Something I have noticed is that if
you correct the heat content per g burned continuously, the power level does
not actually drop off as one is given to think by the fuel burn rate.
Normally a plot of mass burned show that the rate drops off as the fuel gets
consumed. If the fuel mass burned is compensated for changing properties,
the power level is nearly flat for some stoves. This is much against most
expectations.

Here is something to consider:

There is a clear distinction between the way data is captured and processed,
and the conducting of an experiment to see which lighting technique or
refuelling sequence produces less PM.  The stove testing 'protocol' we have
been discussing for so long (WBT3 and its antecedents) is an experiment that
includes a protocol. Fixing the operation method for a stove is NOT part of
the recording and analysis.

The EPA has long favoured incorporating a fixed operation method in its
'test protocols' for stoves (and vehicles etc). Many sates or countries copy
EPA regulations because they don't know what else to do. The problem is that
fixing the operating method stifles innovation. If you have only one way to
operate a wood stove and only certain sizes of wood as specified in the
test, new fuels, fuel preparations and methods of use cannot be homologated
(made legal for general use). South Africa and New Zealand have been rather
better at avoiding this trap. Europe, not so successful.

In North America it has become difficult to think of a stove test without
including a strict specification of how the stove should be operated.
Surely it is obvious to all by now that how a stove is operated contributes
enormously to how it works. We have so many new stoves and fuels. The test
protocol must take cognisance of this reality and not stand in the way of
innovation.

Regards
Crispin

++++++++

Crispin,

It's taken 6 years for people to crit on it, well better late than never.
Just being cynical here:-) .  I appreciate your comments and will follow
your lead on WBT.  I used to assume the charcoal burning phase was the low
power phase, i.e. when the flame goes out you enter the glowing ember stage,
especially in the core of the briquette.  No visible flame results in poor
heat transfer hence the low power output and efficiency.  Looking forward to
proper, universally accepted WBT procedures or should that be looking
forward to applying a new universally accepted WBT in the correct manner.

--
Kobus Venter





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