[Stoves] big TLUD

[Paul Anderson]

Rolf,

Fuel size is related to the dwell-time (time the fuel is exposed to the 
heat).   Tom Reed says it takes about one hour of exposure to pyrolyze 
through about one inch (2.5 cm) of wood FROM ONE SIDE.   So think of 
radius of the fuel or its smallest dimension.

And the TLUD height (which relates to the duration of the operation) 
will give some indication of the available time.

1.  So, you could have a 2 inch smallest-dimension piece of fuel near 
the top of the fuel pile in a one hour of operation TLUD.   But the same 
piece of wood if inserted vertically would have an hour of heat at the 
top but less than 30 minutes or even only 15 minutes for the end that is 
near the bottom.   The bottom part will be off-gasing (giving smoke) if 
it is removed when the majority of the pyrolysis has completed.   
Waiting for that piece to pyrolyze in a functioning unit will result in 
the loss of char that is burning to give the heat for pyrolysis.

Vertical pieces of wood work very well, but it is good to have a bottom 
layer of smaller pieces.

2.  The other big variable is the control of the two air supplies. The 
ability to SHUT DOWN the primary air is extremely important, and widely 
overlooked.   Ideally, the MPF (Migratory Pyrolytic Front) will descend 
rather uniformly.   But if it does not (and this problem increases in 
likelihood in larger TLUDs), pyrolysis and char making can be kept 
somewhat under control if the primary air is severely restricted.  Keep 
the HEAT (not the fire itself) inside the fuel chamber and the off-gases 
will be created, the fire at the top (burning the gases) can be 
sustained and also controlled for minimal smoke even though the fire 
inside the TLUD has dropped to the bottom of the fuel chamber.   Not a 
perfect run cycle, but probably some reasonable char production 
(compared with letting the fire race away inside the fuel chamber).

3.  The ability to supplement (increase) the air flows (both of them, 
but separately) is a major factor for control and for reducing the 
dependence on uniformity of fuel sizes.   Yesterday, in a TLUD of two 
small barrels, the final stages of a 45 minute operation had too much 
pyrolysis occurring, giving lots of flames (shooting 4 inches above the 
3 foot chimney) and some visible black smoke. Instead of cutting back 
the primary air, I used a portable blower (with a 12 V DC motorcycle 
battery) to increase only the secondary air, and the smokiness 
disappeared and the flames were only half way up the chimney.

NOTE:  This gave great heat supply, but for a shorter time period than 
if I had cut back on the primary air (giving more time for pyrolysis).

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 5/20/2015 2:30 PM, Crispin Pemberton-Pigott wrote:
> Dear Rolf
>
> I have a general rule (which results from empirical testing) which is that
> the fuel particle has to be smaller than 1/6th of the diameter of the
> chamber. At 6 it is iffy - problems abound with the fire going out and
> difficulty igniting, high excess air, poor potential for secondary air
> management etc.
>
> There is an upper limit too but I am not sure where it is. It is less than
> 25 and I suspect above 20 is a cause for concern.
>
> Packing density is an issue but it is an indicator, but 'the issue'.  "The
> issue" is the superficial and actual velocity of air moving through the
> system.
>
> The numbers are influenced by the temperature of the surrounds so it is not
> as simple as saying 'here are the hard numbers'.  When you get to mixing
> different sizes together you will have to work with the actual air flow
> rate.
>
> Regards
> Crispin
> .
>
> Hallo "big TLUDers",
>
> as I see from the various experiences and comments, the cross section of a
> big TLUD is to some extent limited.
> That means that in order to build a bigger unit, it has to grow by lenght,
> which in turn must enhance the resistance to the primary air flow. From
> Imberts we know that the relation between fuel size and hearth or throat is
> crucial.
> There must be enough space left between the particles to allow for a
> adequate air/gas flow.
>
> My question: Does anyone have a clue on the matter of fuel size in TLUDs ?
>
> Is it possible that larger diameters ask for larger chunks which in turn
> provide more space between them and ideally spread the upflowing air more
> uniformly?
>
> Rolf
>
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