[Stoves] stoves 101

[Crispin Pembert-Pigott]

Dear Juanito

 

>Would you point me to introductory posts re:

 

>>"...to give the flames a change (sic) to burn out before getting to the
pot"

 

Thanks for the question. 

 

Carbon monoxide and particulate matter (partially burned fuel or carbon
particles that grow from free elemental carbon liberated during combustion
are both 'products of incomplete combustion'.  When the combustion
conditions are not perfect, there are inevitably components of the fuel
which do not burn to completion. This can be quantified as how inefficient
(2%) or now efficient (98%) the combustion is. Usually the number relates to
the heat lost that 'could have been' or it is a ratio of how well something
like Carbon or Sulphur was burned.

 

When a flame touches something cooler than itself, it fails to burn the fuel
completely. A pot is always cold because what is inside (usually) is liquid.
If no space is left between the fire and the pot, it is inevitable that the
pot interferes with the combustion. This can easily be shown in a couple of
ways. The first is that some stoves make a heck of a lot more smoke with a
pot on that off - that is the first sign. The second is that elevating the
pot can reduce substantially the PM produced (try it and see - don't work
about the heat transfer efficiency). This is most evident in ethanol stoves
where one can get a very clean fire quite easily, or a dirty one by
preventing proper combustion.

 

A third is to pass a saucer (without the tea cup) across the top of a candle
flame. Make several passes starting well above the flame tip and descending
slowly with each subsequent pass. Below a certain clearance, the saucer will
starting blackened as it crosses over the flame. That is the point at which
it the interference happens. It is not in a visible region of the
combustion, but carbon is still burning there. Below that point, the saucer
will get very dirty with a single quick pass because it massively interferes
with the combustion.

 

Creating a TLUD is quite easy. Creating one with no flame space means having
dirty pots and a lot of smoke. Nearly every charcoal stove in the world has
insufficient space to burn the charcoal properly. People even put the pot
directly on top of the burning charcoal thinking it 'helps heat transfer'.
It is in fact creating a cloud of CO from unburned carbon. The absolute
minimum space between the fuel and the pot (on a charcoal stove) is 50mm.
You will see in that space the blue flame of burning CO. Lower the pot and
it disappears. 

 

You will see from the video Jock provided, the flame burning above the
secondary air gap is completed well below the pot. The amount of air
entering is far more than is needed to burn the gases indicating that there
is a high excess air condition and this a cool flame (sorry, Jock). The best
heating efficiency will be achieved when the secondary air quantity is 'just
right' and the pot is immediately above the flame. The difference in fuel
consumption can be large, a facto of three, between an ideal system and one
with too much air and too close a pot.

 

When you see flames running along the bottom of a pot and up the sides, you
know that is an inefficient, smoky device.  To get a clean, short burn, the
secondary air should be introduced hot enough, low enough and fast enough to
get good mixing. That does not require a fan, just right-sizing.

 

Regards

Crispin

 

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