[Stoves] Changing rate of pyrolysis front in TLUDs

[Crispin Pemberton-Pigott]

Dear Paul

You cover quite a lot of ground so I can't explain everything in one go.

First, principles:

The hot portion of the gases in the internal chumney do not interest me 
because that remains constant.  The only change is the height of the portion 
between the top of the fuel (initial level) and the bottom (which is not the 
top of the char but the bottom of the char because it is hot above the 
charring level.

>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.

I don't think thsi is important.  The gas velocity is not what I can 
calculate, only the draft.  The gas velocity depends on the combusted volume 
and that is not known (yet) because it is smoke.

>...does the draft calculator maintain its application
>for the low-temperture
>conditions in the TLUD gasifiers?

For the draft value, given the correct temperature for the zone and its 
height, yes I think it does because it is a simple calculation.

>You estimate 300 g of wood for an hour of TLUD gasification

That is not an important value because I am not really dealing with gas 
volume / resulting velocity.

>> 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.

The average is not important.  The temperature of the volume of fuel/air 
below the pyrolysis front, and the temperature of the hot smoke space above 
it.  The hotter zone above the fire is not important because the it remains 
the same height and temperture (pretty much) throughout.

>> 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?

Very little.  And I thought of a new issue.  As there is some packing of the 
charcoal as the pile reduces and as teh draft increases slightly, the air 
flow is restricted more by ash/ breakdown pieces.

>> 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.

OK.  There is some radiant heating of the fuel below what is burning (or it 
wouldn't light).  This affects the total draft.  I guess it isn't much.  The 
choking by collapse may be more more significant.

The total draft of the hot internal chimney (at perhaps 700) is quite a bit 
mroe than the shorter cooler smoke zone.

In terms of % the variation from full to burned out is not much, AND it is a 
low number in absolute terms.

A Hot fire zone 700 degrees 400mm high 3.746 pascals
B Smoke zone at 400 C 40mm high 0.312 Pa
C Smoke zone at 400 C 100mm high 0.780 Pa
D Smoke zone at 400 C 200mm high 1.560 Pa
E Primary air zone ambient temp 200mm high 0.000 Pa
F Primary air zone 50 C, 100mm high 0.206 Pa
G Primary air zone 100 C 20mm high 0.072 Pa

Starting the fire it is A+B+E = 4.058
After 1/2 is burned it is A+C+F = 4.732
Nearly burned out it is A+D+G = 5.378

The increase is 33% over the run of the fire, and if the collapsing char 
blocks the airflow much at all, it will negate the gain. That is my main 
point.

Conclusion:  It is not unreasonable to expect that the burn will remain 
nearly constant throughout the cooking. Also, variations in fuel moisture 
will have far more effect that any change in draft.

Regards
Crispin