[Stoves] Influence of fuel-bed temperatures on CO and condensed matter emissions from packed-bed residential coal combustion

Sun Sep 20 14:14:51 CDT 2015

Dear Paul

Well, you are going to have a classification problem with a TLUD (which is a
flow direction) if it doesn't have a migrating pyrolysis front. 

>So, please do not confuse the TLUD (which is MPF) with something that is
lit at the top maintains its burning at the top.   

Huh?  If the fire burns 'openly' meaning large particles and has air flowing
through and does not have a 'coking stage' following by a coke burning
stage, what is it? It is a normal fire that is top lit that slowly burns
down - but there are flames and smoke and CO and PM being produced
throughout the fuel bed. The fire works its way to the bottom. It is not
'pyrolysing' in any sense similar to a TLUD pyrolyser creating a combustible
gas that is burned above the fuel bed. 

This research paper is clearly NOT referring to what has become known as the
TLUD micro-gasifier.   

Well they are certainly TLUD's but they are definitely not micro-gasifiers.
The reason they burn about 80% cleaner (multiple tests) is because they have
a coke bed on top of the fuel pile. But here are flames throughout the fuel,
eventually. 

A fire of that type can be created by making the particles and air gaps
larger. You could use as a metric the combustion chamber diameter (or area)
and the fuel particle diameter (or area). If the ratio is 6:1, it can work
as a TLUD or BLDD and be pretty clean. If the ratio is smaller, it won't
work properly.  At 10-15 times it is going to be a pyrolyser if the air flow
is controlled.

>Yes, people can light fires at the top.   They can be just regular fires.

When I was a Boy Scout they taught us to light really big fires on top. That
was to reduce smoke. Is that a regular fire? A BLUD fire makes a heck of a
lot more smoke and we didn't do that.

>That is why I am emphasizing MPF as being the important characteristic of
TLUD stoves.   Others that are LIT on the top or bottom are not necessarily
gasifiers. 

Agreed, and I would not classify the mbaula as a gasifier in the sense you
use. You are drawing a thick line between types and there needs to be some
finer lines in there. A TLUD ordinary fire burns cleaner than a BLUD with
the same fuel.  A TLUD in a closed space with good primary air control can
be a gasifier or a pyrolyser, the latter having a narrower definition, I
think, about its hot zone. 

Because all coal and wood fires are 'gas fires' where do you place a fire
that is burning a particle of fuel for two hours? The fire works its way
down the pile, long before it finishes pyrolysing or burning the upper
portion to ash. 

>The designations of UP and DD for updraft and downdraft are term for
GASIFIERS (PHYSICAL DEVICES) WHERE THERE IS AN INTENTIONAL AND CLEAR
SEPARATION OF WHERE AND WHEN THE GASES ARE CREATED AND WHERE AND WHEN THE
GASES ARE BEING BURNED.   

I don't think so. Where is that from? Updraft is a direction of draft. Up,
cross, down. The Franklin stove is both down and cross drafting, i.e. on an
angle.

A TLUD is a gasifier.   Lighting coal or wood in a bucket is not a gasifier.

Well, not quite - a TLUD could be a gasifier or a fire. Some TLUD's are
gasifiers. Some are pyrolysers. Some are neither. The critical difference
for the last case is that the fuel particles have to have access to adequate
air to burn completely but do not do so in a short period of time.

>All fires involve gasification (a process), but that does not make the
device (bucket, open air, tube, whatever) automatically to be a gasifier
DEVICE.   

Agreed. If it is making combustible gases which are aflame from the fuel
particle, through the whole fuel bed and above it towards the pot, that is
not a gasifier. But it is still without question a TLUD device and we
encourage people to light it like that.

There is a down-side to this BMG mbaula: If it is lit on top, it gives of
significant PM throughout most of the burn - certainly far longer than if it
is bottom lit. The total PM emitted is much reduced, but it is not a clean
fire to cook on. That is mostly to do with the device, not the fuel. Fuel
size is a factor, but the cause is the device. For the cook, they may prefer
to have the coal coked in the beginning as rapidly as possible, then cook on
the coke, indoors on occasion. This is definitely the preference for space
heating using the same device.

I am pointing out that running the mbaula as a TLUD means having significant
PM emissions for a much longer time which may affect the cook and the taste
of food more than the BLUD method which gets rid of the volatiles in one
horrendous go.

In order to burn the coal 'cleanly' meaning without small or soot, one has
to use a proper stove tuned to that fuel.  I can't see that happening
without a chimney which is needed to get the proper velocities and
pressures.

Regards

Crispin

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