[Stoves] Why doesn't charcoal burn in the Champion TLUD?

[ajheggie at gmail.com]

[Default] On Wed, 15 Oct 2014 16:54:50 -0700,"kgharris"
<kgharris at sonic.net> wrote:

>Huck,
>
>I was not planning to venture an answer to you question since it was addressed to Paul Anderson, but he answered and opened it to the list so here goes.

Hi Kirk, good answer and I offer some of my thoughts:
>
>Your question asks why the heat generated to drive off the gasses from the fuel doesn't burn the charcoal in the Champion stove. The reason is that there is not enough oxygen. It also doesn't burn most of the wood gas. Only a small amount of primary air is allowed into the stove providing enough oxygen to support the pyrolysis process to heat the fuel and continue producing more wood gas, but not enough oxygen to support burning either the char or most of the gas.

This is dependant on the velocity of the primary air. In his early
experiments Tom Reed said he was able to see a flame at this primary
combustion region but he had a quartz cylinder coated with a
transparent layer of metal. A recent (2012) thesis by S Varunkumar
seems to confirm this at very low air velocities with dry wood.

This flame is important as a flame is an area of combining gases,
combustion in the absence of a flame is likely a dissociation and
combining directly with atoms on the solid fuel surface. For a flame
to occur the primary air has to be able to mix with offgas fuel
molecules and enough energy be available (from the flame above) to
dissociate the oxygen molecule. So the velocity of the offgas must be
lower than the rate oxygen can diffuse into it, as this velocity
increase so fuel molecules will be swept upward before they can react
and the oxygen will fid a char particle instead.

I have never had access to lab facilities which would enable me to
measure or see such phenomena but in the field with higher mc fuel
than bone dry I feel it is not possible to sustain a clean burn at
such low primary air velocities.

As primary air velocity increases there is a region where the heat for
the pyrolysis front is provided by oxygen dissociating on the hot
nascent char surface and producing heat and CO AND/OR CO2. This heat
powers the pyrolysis front by first raising the temperature of fuel
particles below  above 100C, boiling off any residual water and then
raising them above 330C to initiate autopyrolysis and evolve offgas
and produce a fresh layer of char. Perceived wisdom is that at this
stage there is a slight exothermy in the pyrolysis and temperatures of
the particle will rise to about 440 when an endothermic region
returns. In fact we see substantially higher temperatures than this in
TLUD which suggests more complete combustion takes place. All the
region above the pyrolysis front is depleted of oxygen and the
increased volume of combustion products rise away from the pyrolysis
faster than oxygen can diffuse into any gases produced so there is
little or no flame. It will be seen that if the fuel contains more
moisture the combustion at the pyrolysis front has to provide more
heat to vaporise the moisture and we tend to see this is the case as
there is less char residue with higher moisture content fuels, above
30%mc wwb it seems a TLUD burn cannot be sustained. I thing this last
region of primary air velocities is what most of us have when burning
TLUD.

The third stage is when the air velocity is increased further and
enough oxygen passes through the pyrolysis front to be able to combine
with all the nascent char and its offgas.


>Only a small amount of wood gas burns with the limited primary air to keep the process going. 

See above, I believe offgas only burns at the pyrolysis front at very
low primary air velocities.



>The three components needed for a fire are fuel, heat, and oxygen, and oxygen is limited here. Most of the gas, which contains flamable hydrogen and carbonmonoxide rises. It is hot but mostly unburned. Secondary air is injected into it above the char and that is where the gas begins burning. Once all the volitiles are driven out of the fuel, then the small amount of primary oxygen will start supporting the burning of the char,


Yes and if the primary air remains at the level TLUD pyrolysis took
place then the char burns out mostly to CO2 and any CO that is
produced probably escapes un burnt as it is too dilute in the mixture
of nitrogen and CO2 to find a free oxygen to react with.  At this
stage as air velocity is increased the fire morphs into an updraught
gasifier and the offgas becomes predominantly CO and nitrogen which
then burns with a blue secondary flame, however temperatures in the
firebed can rise to a damaging high level. Charcoal can burn up to
2000C, the endothermy of an updraught  char gasifier will still have
temperatures in the region 1100C-800C.

>producing both CO and CO2. The charcoal fire is adjustable with the primary adjustment, so increasing the primary air increases the charcoal fire. If the char is hot enough and there is adiquate oxygen from the secondary, there will be a CO flame. It is bluish and very dim in a lighted area, but quite visible in darkness. I do not know if this flame burns all of the CO forming CO2, or leaves a lot of CO unburned. The charcoal fire can provide good heat for a time after the hydrocarbon flame goes out. I have even brought a small amount of water to boil using the charcoal fire. As the others have stated, the red hot charcoal is very hot and damaging to the metal of the stove. 

Yes and char burning in restricted air supply was used in pocket hand
warmers, though I suspect some of the required oxygen came from
nitrates impregnated in the char.
>
>The pyrolysis process is interesting as well. The heated fuel produces wood gas. Most of this gas rises to the secondary burn area, but a small amount of it is burned at the pyrolysis front using the primary air. This small wood gas flame is what keeps heating the fuel and keeps the process going. The mechanism by which the pyrolysis front moves down into the fuel is mostly radiant heat. The small primary wood gas flame and hot char will radiate heat downward to a piece of fuel, heating it. 

I agree this has been seen at very low primary air velocities but
don't think most of us operate in that region, also what other
explanation is there for the loss of char when TLUD takes place in
higher mc fuels?


>At some point the temperature of this piece reaches the the flash point. At this point a very small flame moves around the surface of the piece and chars the surface. This tiny flame lasts maybe 2 seconds and then goes out and the piece is now producing wood gas, a small amount of which is burned using the primary air, thus continuing the process.

Interesting observation has anyone measured the flash point of
pyrolysis offgas? I had always assumed it was much lower than ambient
temperature and probably that of any free hydrogen or CO in the mix.
The flash point presupposes a flame, we know smouldering charring can
exist without a flame and what is needed to initiate the flame is for
enough oxygen to reach the nascent char to raise the temperature above
the autoignition point of the offgas. We see this when we blow on
smouldering char to initiate a flame.
>
>The making of charcoal from wood is a pyrolysis process. The wood is put into a container which keeps oxygen out. Then some heat source heats the container to the point where the volatiles are driven off. The wood gas must have some way of exiting the container to avoid explosion. Without oxygen, the charcoal does not burn. The gas is often vented to the atmosphere where it cools and becomes smoke, a very wasteful process. 

Yes but the making of char in a retort differs from that in a kiln, of
which a TLUD fire is a subset. The smoke you see being given off is a
sol of the tarry droplets and condensed vapours suspended in the true
gases of predominately H2, CO and methane, this is what you see rising
from the char bed and under the secondary flame in TLUD combustion
plus of course the combustion products and nitrogen from the initial
primary air (which is not present in the gas evolved from a retort).

All the oxygen that was previously in the wood is accounted for in the
offgas and any higher tars remaining in the char, I reject the idea
that this oxygen bonded in the wood is able to contribute any
meaningful energy to the process because a surrogate for the total
energy available in burning wood is the energy in burning the
equivalent of carbon in the original sample.

AJH