[Stoves] Risk of CO poisoning with TLUDs

[Crispin Pemberton-Pigott]

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.

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.

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)?

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 seem to recall John advising that the secondary air flow was to be adjusted once pyrolysation was complete.  Yes?

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.

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.

We should look at this closely

Regards
Crispin





-----Original Message-----
From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of ajheggie at gmail.com
Sent: Wednesday, September 20, 2023 1:33 PM
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: [Stoves] Risk of CO poisoning with TLUDs

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

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