[Stoves] TLUD-ND transition from hydrocarbon flame tocharcoalflame / Temperature gap between self incending charcoal and woodgas

[Kirk H.]

Dr. Boll,

I hope I am understanding what you have said.  You have two concerns as I understand it, one is that to keep the coals hot enough to ignite the wood gas you would have to blow on it so strong that it would blow the wood gas flame out, and second that the wood gas flame must be held at or above its ignition temperature in order to keep it burning.  The burning temperatures are different and so the coals cannot ignite the wood gas.  This is good information.  

The TLUD-ND does not use the coals to ignite the wood gas.  The wood gas burns in the secondary flame.  The idea behind the perforated plate is to bring waste heat back in to the stove.  The coals need only be hot enough to pyrolyze the last of the wood.  That would be between 250C to 300C, not a problem.  Possibly the salvaged heat keeps the gasses hotter as they rise to the secondary burner, making them burn easier and keeping the secondary flame hotter.  

Crispin suggested that the plate spreads the flash of wood gas from the last of the wood over a longer period of time.  Rather than a quick flash it is more of a longer and lower bump.  This is like what I have seen in my tests, on my own and under the hood at Aprovecho.  Bringing the waste heat back into the stove must get the flash going sooner and lower, and also help keep the temperature of the secondary flame above the ignition temperature of the wood gas.  The two ideas you have presented help explain what is going on.  These two principles must be included in what is happening, since the transition is so smooth and clean.  

Thank You,

Kirk H.

Sent from Mail for Windows 10

From: Boll, Martin Dr.
Sent: Wednesday, August 15, 2018 3:56 PM
To: stoves at lists.bioenergylists.org
Cc: Crispin Pemberton-Pigott
Subject: Re: [Stoves] TLUD-ND transition from hydrocarbon flame tocharcoalflame / Temperature gap between self incending charcoal and woodgas

Kirk, Crispin and all,

by time I got aware that the different self-incending temperature of charcoal and woodgas makes a lot of problems in burning wood, especially in avoiding smoke.

All burning of any stuff takes only place above the special self-incending temperature. It does not work under that tmperature. - So simple, but was not aware of.

Guess charcoal about 600°C, guess woodgas about 1200°C. It does not matter if not exactely. There is enough temperature-difference to make trouble; - we all know  :-)

Charcoal burns ( and continues burning) still by small draft with dark-red colour.
To incend woodgas, charcoal has to have yellow- or white-glow. What draft-speed is necessary??
(key-word: Superficial speed ??)
In contrary to that pretty quick draft-speed, wood-gas has a very slow flame-speed. - Crispin I remind, that we talked about the tremendous low flame-speeds of different air-woodgas mixtures off stoves-list a lot of time ago.

The draft, making a bright charcoal-glow is fast enough to blow off a woodgas-flame!! I am sure you know that, as I saw long before understanding why.
I remind the ancient method of fire-starting: Glowing charcoal within a light "soft-ball" of dry grass or staw.
Blown by mouth, to make smoke and then fire. There is quick and slow moving air close together, so that the yellow-blown charcoal-glow incends the gas-flame, which gets not so fast blow to be blown off.
- My greatest respect to our ancestors for their knowledge, art and practise! 

Another important fact, as well respected by that old method of fire-staring,  is radiation and reflection of heat.
A small flame from a twig or leaf needs a same flame from another  twig to be heated by its radiation,  because the small flames loose so much heat by radiation. The small flame does not produce enough heat to maintain outgassing _and_ holding the self-ignition temperature of that woodgas, which is necessary to continue the burning.
- My conclusion since I understood this: I incend with a match always two or three small flames, burning close together. - A candle is another system than wood-burning.

I made some simplest experiments with just matches. After incening a match held it in different inclination-angles. (The match contains some wax and is not a mini-log!) 
A small wood-spoon, from leaf-wood, shows the differences by different angles clearer.


Kind Regards
Martin






_______________________________________________
Stoves mailing list

to Send a Message to the list, use the email address
stoves at lists.bioenergylists.org

to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org

for more Biomass Cooking Stoves,  News and Information see our web site:
http://stoves.bioenergylists.org/


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20180815/2c52bda4/attachment.html>