[Stoves] Risk of CO poisoning with TLUDs

[ajheggie at gmail.com]

Hi Julien, Richard and Crispin

I am a bit late to the fray as I can only respond after reading the
archive as posts no longer arrive at my gmail inbox. I do get a few
spam posts daily that arrive via the moderation account..

My only hazard concern with TLUD char making is the stage at which the
descending combustion front has reached the bottom, most of the
volatiles have been burned off in the secondary flame but now the only
combustion is from primary air entering the bottom of the stove and
reacting with the recently formed char. At this stage the secondary
flame has died out and the only flue gases are from the char burning.
There are not necessarily any acrid fumes that would alert anyone to
the ongoing combustion.

Normally one would have dumped the char out and extinguished it to
prevent loss of weight of char but if left to burn it becomes an
updraught char burner.

As Crispin says when char burns in a traditional stove  most of the
combustion is from oxygen dissociating on the surface of the hot char,
to directly produce carbon dioxide but some of the oxygen will combine
with a carbon atom to form CO.

As the TLUD stove open top is now well above the char it becomes an
effective chimney and the buoyancy of the hotter flue gases causes an
increased depression at the point primary air enters, increasing the
primary air velocity. This has the effect of increasing the  height of
the combustion zone. The increased layer of hot char the gases pass
through coupled with the high temperature of char combustion (far
hotter than the initial TLUD burn) means some of the CO2 will become
reduced to CO, in the absence of a secondary flame the column of char
becomes a CO generator. Tom Reed gave a rule of thumb that to gasify
char to CO completely a depth of 20 particle diameters maintained
above 800C was necessary but an equilibrium exists  where the ratio of
CO to CO2 will be less favoured at lower amounts.

It is also the reason the bottom of a TLUD stove gets hot enough to
burn steel components if this is allowed to happen through inattention
at the end of a TLUD burn.

Andrew