[Stoves] Risk of CO poisoning with TLUDs

[ajheggie at gmail.com]

On Wed, 20 Sept 2023 at 21:06, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:
>
> Dear Andrew
>
> That was a good thought process.  Doi you remember John Davies from Secunda in South Africa?  He was making coal stoves that were TLUD coke-making and then coke burning. That is, TLUD  followed by BLUD. The former took about 2 hours and the latter 4 hours for his load of ~6 kg of coal.

Yes rather vaguely though.
>
> The difference between what you describe below and the Davies TLUD is that he did not lose the flame when the charring ended. If the flame goes out as the charring stops to be followed by char burning, it makes sense that there is more heat generated at the bottom, but there is also a loss of heat in the vertical volume above the char.

I note that even bituminous coal has rather less volatiles to coke
than woodgas has to char.

Yes the flame is lost because in order for it to be a complete
updraught gasifier the amount (and hence velocity) of primary air has
to increase . When the stove is first lit the chimney effect of the
stove body which contains the fuel is low. As the pyrolysis front
reaches the bottom the  char only occupies a small part of the space,
the volume of warm gases from the combustion of char occupies the rest
and it is this chimney effect that I am talking about. A TLUD flame
front is only in the order of 600C but as I said the combustion of the
char has a higher temperature (adiabatic temperature of carbon burning
is over 2000C which is why it could smelt iron whereas wood could
not).
>
> Given that the air allowed into the bottom of the column is normally highly restricted, are you sure the burn rate increaser the vertical flow (much)?

This is the crux, normal TLUD primary air is restricted as you say,
because the primary air is only necessary to burn enough fuel to
pyrolyse the wood at or just below the "flame" front. This is much
less than the air necessary to gasify the char, hence why up to 25% of
the initial dry matter remains as char at the end of burn. I never had
a means of detecting CO but I was always wary that there was some
emitted at the end of a burn once the secondary flame went out. I
experimented with tall steel tubes and fans so I also saw how
increasing primary air at end of burn meant I could maintain a
blue-purple flame but also the bottom of the tube would glow white
hot.


>
> If you are getting a flame-out, I suspect there is too much secondary air.
> If you add a bluff body (which I don't like in that position) will it retain the flame?
> If have "adequate" constriction of the gas path where the flame subsists, will it prevent the flame-out?

I tried many arrangements to maintain the secondary flame but once
most of the pyrolysis offgas was gone for the various reasons above it
was not possible simply, or desirable really as the main attribute of
TLUD for me was that the flame could be clean and low in particulates.
>
> I seem to recall John advising that the secondary air flow was to be adjusted once pyrolysation was complete.  Yes?

I'm not sure about this with a diffuse flame, generally with an
updraught stove you aim for a turndown such that the primary air
controls the evolution of the offgas which then entrains the correct
amount of secondary air for clean combustion, reduce primary air means
less off gas evolved and hence less secondary air entrained. The TLUD
is a special case with a poor turndown ratio in my experiments but
during the descending pyrolysis front the power was fairly constant.
>
> Your experience is mystifying.  Char burning requires substantially more air than burning of the volatiles to it should start running rich, not lean at the transition.

Yes we agree and I try to explain above. I was pointing out that
generally people using TLUD stoves do not want the char to burn. I was
conjectoring a situation where an unattended TLUD burn could result in
 higher levels of CO being produced. I certainly would not like to
remain in a room with one and there are many instances of barbecues
being used in or near a tent and killing sleeping occupants.
>
> I think it was Dr Riaz Achmad (CAU) who sampled gases from within the char column in various places to see what happened to CO produced at the bottom. It is quite possible (I am not sure) that a lot of it becomes CO2 and that extinguishes the flame.  Or some combination of deleterious conditions: high excess air because of too much secondary air;  low or no excess air snuffing the flame; a temporary carbon-to-CO2 burst just as the pyrolysation ends.  It needs only one slug of no-CO air in the column to put out the flame.

Yes I think this is why it is so difficult to re light a flame from a
TLUD where the flame front has become unstable because burning
particles have fallen below the pyrolysis front,  the water vapour and
carbon monoxide dilute the offgas too much for it to support a flame.
>
> We should look at this closely

It would be interesting to try in a lab.

Good to have a discussion again.

Andrew