[Stoves] Changing rate of pyrolysis front in TLUDs

[Paul S. Anderson]

Crispin,  (and asking Tom Reed and Dale Andreatta to comment on the numbers,
etc.)

Thanks for doing that calculations (and please thank your son Nigel again on
behalf of all of the Stovers for making the "draft calculator"!!!)

My comments are placed below:

Quoting Crispin Pemberton-Pigott <crispinpigott at gmail.com>:

> Dear Friends
>
> The draft change is not very significant between the top and bottom of the
> fuel (refering to an increase in draft when the fuel burns down).

You start with a statement that, in fact, is the question to be 
answered. Mainly I agree with that statement, but the question is still 
open.
>
> Remembering that this is calculated for burning all the gases, not creating
> them, Nigel's Chimney draft calculator gives (for these numbers):

I suspect that the above true comment is more of the answer than we 
recognize. Just in terms of air, the gasifier uses 1 unit of primary 
air for about 5 units
of secondary air for the combustion that occurs later.

Therefore, a QUESTION:  Does that 1 to 5 ratio also apply to the "heat" 
released
by what the primary air accomplishes versus heat from the secondary 
combustion? If so, does the draft calculator maintain its application 
for the low-temperture
conditions in the TLUD gasifiers?

> Type of Fuel:  Wood
> AIR FACTOR, actual air flow / ideal air flow
> 110%
> Diameter of chimney
> 100.00mm
> Time to burn fuel
> 3600seconds
> Mass of wood used in time above
> 300g

You estimate 300 g of wood for an hour of TLUD gasification that proceeds
downward for some number of millimeters (see my comment on the TLUD height
further down).  Dale and Tom, what is the weight and height and burn time and
burn rate (in mm or cm) for woodchips (or specify pellets) in a 10 cm (4 inch)
diameter TLUD?  Are these numbers about right or realistic?  I will need to do
some checking on this.

> Temp inside chimney in this case a gasification chamber (average of entry
> and exit temperatures)
> 400C

Here I can disagree.  If the ambient temp is 20 C for entry of air, a 400 C
average would mean 760 deg C exit temperature.  We are speaking of the inside
of the TLUD where pyrolysis is occuring between 250 and 400 C, and the smokey
gases would cool a little before reaching the top.  We should not be using the
temperature of the combustion area, should we?  The average of 20 C and 360 C
would be 200 C.

But do we need to allow for some drafting even if, for example, the secondary
combustion was extinguished and simply a large plume of smoke was rising from
the pyrolytic gasifier?  See below:

> % carbon in the wood
> 51%
> % water contained in wood
> 12%
> Height of chimney
> initially 40mm and later 200mm as the fuel burns down

Big issue here:  It is fine to allow 40 mm from the top of the initial 
fuel pile
to the top of the canister.  But maybe here we should enter in some additional
millimeters to simulate the necessary chinmey to sustain the flow of the
incoming primary air.  IF you add on even 400 mm MORE (to match my recommended
height of the internal chimney in the Champion stove that is natural draft),
the calculations might reveal something interesting.  And then try only 200 mm
to see what would happen to the numbers.

Okay, Crispin's calculation allows for 160 mm drop in the level of the top of
the char (or are you referring to the drop of the level of the pyrolysis
front?).  My "eyeball" experience is that the top of the char will drop
approximately 50 to 70 percent of the total height of the fuel pile, assuming
that the batch is terminated when the pyrolysis is completed rather than
continuing on to burn the char.  (Tom and Dale, would you agree or not?)

Assuming that the 160 mm drop represents 60% of the total initial fuel pile,
then the gasifier chamber is equal to 260 mm + the 40 mm above the 
initial fuel
pile, equaling 300 mm (12 inches).

> Heat content of wood/biomass
> 19.300MJ/Kg
>
> The draft changes from 0.312 to 1.56 Pascals.  OK, so it appears to go up
> by a factor of 4,

With any of the new numbers, what is the impact?  Is the factor getting larger
or smaller, and by how much?  I will let Crispin crank through the numbers and
report back, please.  Personally, I would consider a factor of 4 to be very
high, so I hope the new calculations can bring the factor down to maybe 1.5  .

> but as you can see in the field, the difference in so
> little

Is the difference really quite small?  I never paid much attention to 
any change
in the rate.  Dale's experiments with metered primary air cannot help us here,
but his observations from when the air in his TLUD was free-flowing might
suggest something.

> that taking into account the heating of the air under the fuel in the
> column when it is full of fuel and the loss of that pre-heating when cold
> air is entering next to the last portion of the fuel, it seems the change
> will not be as big as first appears to be.
>
> The radiant heat from the pyrolosis zone surely heats the fuel chunks
> underneath and the temperature of the inside air is not 'ambient'.

I will question that statement.  I can place my hand on the cool side of the
lower part of an operating single-walled TLUD, and slowly move upward and find
that the wall gets very hot in a very short distance.  Until the 
pyrolysis zone
reaches a level, that level seems to be cool (ambient).

> Heat
> conducts down the metal skin on the inside assisting this further.

I do not detect much of this conduction in the thin-walled sheetmetal ducting
tubes that I use.

> It
> should be pretty easy to get some air temperatures from inside the
> combustion chamber.  Paul can you do this?  I can re-do the calc.

Do not wait for me to give inside temperatures before you do some
recalculations.  I think that Dale's experiments show that the inside air
temperatures are very stable until precisely when the pyrolysis zone reaches
the level in question.

Crispin, with your next round of calculations and with some comments from Tom
and Dale and any others, we should have some light shed on this topic within a
few more exchanges of messages.

THANKFULLY, at least for this topic we have a "Subject:" title that 
says what we
are discussing!!!!

Paul
>
> Regards
> Crispin


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