[Stoves] Heat to sustain TLUD process

[AJH]

On Thu, 31 May 2007 12:34:12 -0500, Paul S. Anderson wrote:

>  I enlist the powers of Tom Reed
>and others to help solve our questions.

Fair enough, Tom and I have been quietly disagreeing about this for
about 10 years ;-).

>
>Andrew says the heat comes (primarily?) from char-gasification plus (??).

No! From low temperature char combustion to CO2

>>
>> It is the reason I moved away from TLUD (not Talmud, which could be 
>> Top-always-lit-multifuel-up-draft) 

 :-)

>If my explanation is correct, I win because Andrew might move back to
>considering TLUD for char making.

Probably not because it doesn't seem to scale up, again because of
this Biot number thing and loss of stratification, not that I'm not
familiar with losing :-), I've expounded on this before and no one has
chosen to follow up on the ideas

>I suspect that Andrew also thinks that he will win with either 
>explanation being
>correct, or even with some third explanation that is not yet evident.

Yes, a good explanation is enough reward.

>  But we must get past this
>ignition stage to see what sustains the TLUD operation later.

Yes, although the ignition stage is short, convection currents at the
surface probably mean it acts little different from a normal updraught
fire until the char layer forms.
>
>2.  A layer of char is created.  The char is hot when it is in the pyrolysis
>zone, but is relatively cool in the middle and top of the char layer that is
>thick and getting thicker.  Can we agree that the char that is more than one
>centimeter (maybe 2 or 3 cm??) above the pyrolysis front is too cool to be
>undergoing char-gasification, 

Definitely, from my observations the pyrolysis front never gets hot
enough for the equilibrium to favour gasification. I don't see why you
think the char immediately cools down, it has no reason to as it is
still bathed in pyrolysis offgas and the only heat losses are through
the sides of the containment.


>and that essentially no more pyrolysis gases can
>be liberated/released/created from that char?  Temperature inside that char
>layer can be measured.  I suspect it is only in the 200 to 300 deg C 
>range, too
>cool for pyrolysis or char-gasification.

Measured at the outside I see temperatures around 500C, so surmise
that temperatures in the middle rise above this, still too low to
favour much CO2 to turn back to CO. Mostly I don't think the reaction
zone (of hot char) is thick enough either, the CO2 simply hasn't the
opportunity to contact enough hot char and even if it did the
endothermy would drop the temperature too quickly for much CO to form.
>
>3.  At the bottom edge of the cool-char layer (and at the top edge of the raw
>fuel pile) is the pyrolysis front.  The thickness of that front is 
>perhaps 1 or
>2 cm unless much primary air is being forced or drafted into it from 
>below.

Yes, my assumption is that primary air is strictly controlled,
otherwise the whole device simply becomes an updraught gasifier
sitting on top of raw fuel and we would expect to see temperatures of
up to 2000C at this level and the top of the CO generator at 1100C
until the Char layer was too thin top act as a gasifier. Tincanium
would last a few minutes at most.

>  In
>and at the pyrolysis front can be found:
>a.  raw fuel below,
>b.  cool (300 degree C) char above

I don't think it cools that quick and some pyrolysis will still be
occurring in thicker pieces.

>c.  hot char, glowing red

I doubt it is glowing much, the surface of the metal certainly doesn't
glow in daylight (400C metal just about glows in the dark but 600C
before you'd see it in daylight IIRC.).

I've not experimented with a quartz or pyrex tube, least of all a gold
coated one which Tom Reed used for his experiments, perhaps it's still
available for tests?

>d.  hot fuel sending off combustible pyrolysis gases

Yes
>
>4.  The only entrance of oxygen into the pyrolysis front is the primary air
>coming from below.  When oxygen meets red hot char, the char-gasification
>reaction is:  2C + O2 => 2CO  (I believe this is endothermic, requiring heat,
>resulting in cooling of the hot char.) 

Yes but I don't think this happens, the reaction you have shown must
occur in stages, the first is the oxygen needs to meet an atom of
carbon, now oxygen comes in lumps of two atoms bonded together, the
hot carbon atom must first provide enough energy for these two atoms
to dissociate, then we have two oxygen atoms  available to oxidise
something and the only nearby thing offering itself is the 1 carbon
atom.

02 + C => CO2 plus lots of heat i.e. exothermic

So everything nearby gets hot.

For the next reaction to occur:

2CO2 + 2C => 2CO endothermic so it cools things down

You need to maintain a good bed of hot char and the reaction only tips
in favour of CO at above 800C

So I don't think this latter reaction is a big player here.


> But in the next instant and slightly
>higher up in the pyrolysis zone, there is another possible reaction:
>2CO + O2 => 2CO2  (which is very exothermic, giving heat to make things hot).
>There is no H2 present, so no H2O is created.)

As has been pointed out no oxygen  survives passing through the hot
char. 
<snip description of pyrolysis the splitting of chemicals by heat>

>long-chain hydrocarbons such as tars, plus some methane (?), and other "smokey
>stuff."  As these become hot gases or vapors or droplets, they escape from the
>fuel particles (leaving behind the C of char) and move upward.  They are
>combustible, but there is very little O2 available. 

Yes the salient point is they quickly leave upward where there is
little oxygen present.


> But a few molecules of O2
>are present, and at the molecular level a small amount of the combustible
>pyrolysis gases can be combusted (oxydized), releasing heat and the chemical
>by-products including H2O and shorter-chain hydrocarbons.  Everything 
>is moving
>upward in the TLUD. 

Which is the nub, do conditions exist at this level and temperature
for the true gases and vapours to oxidise. I think this because:

1) the char acts as an "oxygen mop"
2) The gases do not have enough energy to break oxygen bonds ( their
autoignition temperatures are all higher than that of char)
3) there is not a means to hold a flame for those gases that are above
their flash points (all of them probably)
4) this off gas has a limited range of flammability in air when
premixed and by its nature this is all very premixed if any oxygen did
survive into the offgas
5) for any given temperature, in the regions we are considering,
retort charcoal yields a higher mass than the tlud.
6) water present in a fuel costs heat to vapourise, the higher the
moisture content of a tlud fuel the lower the char yield.


> But radiant heat from this combustion of CO or pyrolysis
>gases does radiate downward and helps keep the reactions going.

It's probably both conduction and radiation, bearing in mind that once
a particle has been dried and reaches pyrolysis temperature it is
exothermic in the region 300-440C but I'd like to see further writings
on this, especially some where the constituent parts of wood can be
treated separately.
>
>6.  Is it the CO from the char or is it the combustible hydrocarbons that give
>the heat?  

No I believe it's chiefly the CO2 from the char.


>Two pieces of information:
>a.  CO has a notoriously slow flame speed; 

It's still too fast for this to be significant at these small scales.


> at least some of the hydrocarbons
>have faster flame speeds; (H2 has the fastest flame speed).  The O2 molecules
>are not fussy.  They go with the first-encountered and fastest-speed
>combustible molecules. 

I don't think speed comes into it, they go with the first combustible
molecule that has sufficient energy to dissociate them, for a many
gases at these temperatures this would need a flame or spark.


> I suspect that CO looses in most of these match-ups of
>molecules.

I agree as CO has a high autoignition temperature but I do not think
CO is a feature here.

>b.  The char-gasification reaction needs temperatures substantially 
>higher than
>the temperatures for pyrolysis. 

Yes but the surface dissociation of oxygen on char only requires
temperatures around 300C, check it you can ignite char with a hot air
gun, try the same with wood and first the offgases are driven off and
then the char ignites, it's only once the char is burning that any
offgas ignites and forms the first flame. look at any pellet stove
igniting!


> Those temperatures are not reached in the
>pyrolysis zone unless it is exactly at the edge of the char particle.  When a
>TLUD is finished with the pyrolysis (that is, when the pyrolysis front has
>reached the bottom of the fuel column), THEN the same amount of entering air
>(and O2) is all going to the char-gasification (plus some possible CO
>combustion).  And the base of the TLUD gets substantially hotter.  It can even
>make the metal glow red-hot (and hurt the TLUD fuel chamber).

Perzactly! QUED??
>
>Anyone, please correct my errors and challenge my assumptions.
>
>Andrew, it is your turn to explain the processes.

I hope I have but my reply has been interrupted by several long 'phone
calls so sorry for any delay.

AJH