[Stoves] Correcting a misconception that approaches myth status

[Crispin Pemberton-Pigott]

Dear Paul

 

You can't just jump in with such classifications without asking first. 

 

>.In 2004 - 05 I re-named it Top Lit UpDraft (TLUD) which is also not
totally accurate.   None of the names acknowledge the Migratory Pyrolytic
Front (MPF) that is the dominant feature of what is commonly called TLUD
micro-gasification.



I think you should have a look at a BLDD working and watch the migratory
pyrolytic front rising through the fuel instead of descending into it. You
might change your opinion.


Note:
A DD gasifier is ignited at the BOTTOM, and the fire (hot zone of
gasification) REMAINS at the bottom, and the unit can be operated with
continuous fuel entry into the top.



No that is not necessarily true at all!  One could make the same claim for
all top lit stoves with a high superficial air velocity.  In a BLDD there is
a gas production process started by a fire at the bottom. Same as TLUD.
There is a migratory pyrolytic front that works its way upwards. Same as a
TLUD. When it reaches the top, there is a fuel chamber full of char, same as
a TLUD. The gas produced can be burned immediately under the grate or taken
elsewhere to be burned in a gas burner. Same as a TLUD.


A IDD or TLUD or MPF gasifier is ignited at the TOP, and the fire (hot zone
of gasification) slowly MOVES to the bottom, after which time the fire zone
stays at the bottom and the unit operates as if it were a regular UpDraft
(UD) gasifier if anyone puts more fuel into the top.  

Fundamentally different.

 

Again, no. It is the same!  When the migratory pyrolytic front reaches the
top of a BLDD fuel chamber the combustion continues at the top drafting air
in (if allowed) and does exactly what you describe but upside down. The big
difference is that the BLDD can be refuelled by placing more fuel on top and
the MPF continues to work its way into the new fuel, upwards, until it again
reaches the fully charred condition.


Because of Tom Reed and also Paal Wendelbo who worked totally independently,
we have something new for cookstoves.   They did not invent pyrolysis.
They were not the first people to have a fire in a barrel making gases and
have the flames only up at the top where secondary air was available.   (I
saw that in the 1950s in a burn barrel at my home.) But Reed and Wendelbo
were the ones who CONTROLLED it and understood it and made it all small
enough to be useful in a cookstove.   And the rest is history.   And TLUD
history is still being made.

That may all be true, but the BLDD stoves with MPF have been in use
literally for centuries. BLDD combustors with large fuel chambers were used
for making coal gas and the only way a useful gas can be created is by
having a MPF in the fuel bed. Obviously it was controlled or they would not
have been able to get a reliable product.

 

A TLUD is a bottom-lit downdraft stove turned upside down so that can't be
refuelled. Deal with it.

 

The only practical difference is that the heat from pyrolysis in a DD stove
rises within the fuel supply, heating it and driving out moisture sooner
than it would in a TLUD where the heat all rises, save by radiation. There
is an advantage to this with coal or wet charcoal or wet wood. Otherwise,
not really any practical difference. I find that with a BLDD and wood
pellets, for example, the airflow velocity through the interstitial spaces
becomes more important in a DD stove than an UD one in order that the rising
heat does not create run-away gas generation inside the fuel bed. If too
much gas is produced it is hard to burn it all. Apart from this small
consideration, and having built lots of each, I observe no difference at all
between these architectures in terms of how the fuel is pyrolysed and the
options for burning the gas.

 

Regards

Crispin

 

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