[Stoves] Misuderstanding of TLUD stoves Re: Refuelable TLUD Coal Stove developed in Mongolia that is not a batch process.

[Crispin Pemberton-Pigott]

Dear Paul

I am still not convinced you agree that it is possible to make an upside
down TLUD. The description you have provided (several times) can be
inverted. 

Let me see if I can follow this closely enough to satisfy you. I t is my
view that you are ascribing to all devices something that happens with some
fuels and not others. Your descriptions may apply to chopped biomass, or
some chopped biomass, but it does not apply to all fuels in all
configurations running at all air flow rates in all directions. I think you
are attempting to classify all stove or fire configurations and you reserve
a special case for TLUD's as being essentially different from an inverted
TLUD. While I do not understand why you want to do that, I will try to
assist because I find your conclusions mostly correct.

"In a true downdraft gasifier, there is a hot zone that stays at the bottom
all the time, and the fuel moves downward to it, becomes char and is later
subjected to char gasification.  The air flow is also downward."

This only true is some cases. A downdraft gasifier can gasify all the fuel.
I don't think we can say 'a true downdraft gasifier'. What's that? You
describe a situation in which the fuel moves down all the time. Well, some
gasifiers work like that but I would not call that a gasifier at all unless
there was a limited air supply that produced a combustible gas. Increasing
the air supply would make it a fire, not a gasifier. 

 Then there is the case I described (which I have built and run many times)
in which the fuel is NOT falling continuously into the hot zone. There is a
pyrolysis front that moves upwards through the fuel, just as you describe
one moving downwards through the fuel, reaching the 'end' of the furl
charge. That is on top in the downdraft and the bottom in the updraft.

They are literally identical in operation save for the 'down-ness' of the
construction. In the case of a downdraft, more fuel can be added on top of
the pyrolysed fuel and the reaction can continue. In the case of a TLUD fuel
can be pumped under the pile to achieve the same result. There are large
Russian coal burners that work this way. They can make gas or burn
everything completely, and/or make coke depending on what you want. I have
seen drawings for a 320 kW one here in Ulaanbaatar.

"In a TLUD, the hot zone starts at the top, but then the hot pyrolysis  
front migrates downward until reaching the bottom of the fuel pile.   
That is FUNDAMENTALLY DIFFERENT from the downdraft gasifiers."

Well, it is different from the one you described above, but it is identical
to the ones I have built. It is identical to a downdraft gasifier that
operates in a char producing mode. You described a downdraft fire that burns
the char and which has more fuel falling into the hot zone. I have built
those too. Depends on what you want! If you choke the air passing through
the fuel in a downdraft stove, just as you choke the under-air in a TLUD,
the result is the same: same fire, same gases, same pyrolysis front, same
char. The difference is that you can toss a little fuel on top of the hot
zone. I can toss fuel on top of the pyrolysis done. Both have the effect of
continuation of the fire.

This continuation is however slightly different in nature. Tossing small
amount of fuel on top of TLUD is not continuing a TLUD gasifier operation as
there is now no moving pyrolysis front. Opening the cover of a DD gasifier
and tossing on some fuel on top of the still reacting pyrolysis front lets
the 'true gasification' continue. You could in fact add a 1 foot deep pile
of fuel on top of the fuel and the pyrolysis zone would move into it as it
did before. If the air supply remained controlled (limited) it would
continue to produce gases below the grate at the bottom.

If you like I can demonstrate this to you.

>Note:  In the case of the Chip Energy Biomass Furnace (www.chipenergy.com)
which is a true updraft gasifier with continuous operation (and the hot zone
is at the bottom where the air is entering), the new fuel enters in small
amounts, avoiding the problem mentioned in the above paragraph about water
vapor."

This does not sound like a TLUD, it sounds like a gasifier that has fuel
dropped onto a burning layer in a restricted air environment (which
automatically produces a combustible gas). I agree that it is updraft. I
agree that it is a gasifier. It is of course no longer 'top-lit' as the hot
zone is under the new fuel, right?

Interesting.

Regards
Crispin