[Stoves] Stove comparison/ PM

Tue Mar 22 14:22:12 CDT 2011

Dear Paul

Thanks for the contribution of ideas.

>I believe there are two (probably more?) sources of PM.  One is from the
NON-combustion of the volatiles, which is the case IF the fire goes out at
the upper combustion of the gases from the TLUD.  And Alex stated that also
below.

There is definitely some escaping, unburned volatiles that condense, and of
course there is water which does condense as well. The method used at
Aprovecho does not, as far as I know, limit the chance that the particle
measured is water, not something from the fuel. The method we are using goes
to some trouble to ensure that there are no water particles condensing in
the gas stream and being measured. We do this because there is a huge amount
of water vapour come from lignite stoves (620 g of water /kg fuel burned).

>The second source is the point of confusion with the comment from Crispin.
In the TLUD when char is created, the carbon even when very free from
volatiles is also holding much of the ASH that is in the raw fuel.  When the
char/carbon is burner (char gasification), that ash is being released and
some (the smallest particles?) is able to be carried upward by the air
currents, giving the PM reading.  The carbon does not produce the PM.  It
was just holding the matter that could escape as PM in some circumstances.

I will list what I think are the sources of PM in the measurements:

- Black carbon particles formed in the tips of flames from incomplete
combustion

- Organic carbon particles formed in the tips of flames

- BC and OC resulting from sneakage - they avoid the secondary flames after
being created by primary flames/combustion/pyrolysis

- condensed water vapour

- condensed vapours from flames, largely non-carbon (i.e. formaldehyde H2CO)

- ash blown up from the bed

- fuel particles blown up from the bed

- condensed raw vapours that were never in the flame at all

>Note that the PM from TLUDs is lowest when the char is note char-gasifier,
is higher when the TLUD combusts everything down to only ash remaining, 

That is a stove-related statement, not a fuel or combustion type determined
observation. The fact that the current crop of stoves behaves that was does
not determine what can be achieved by other combustors. Paul it is
specifically this misunderstanding that I am trying to expound on. Building
a stove for which the statement holds true, that PM is lower when the char
is not burned, does not mean someone else can't build a different TLUD which
makes no more PM when burning the char. Having just see several TLUD's which
make nearly no PM at all whether burning volatiles or char proves my point.
I will share the results which include real time PM emission charts.

>.and both of those amounts of PM are LOWER than what is the PM from a
charcoal burning stove (when compared by the WBT:  The data are in my graph
of CO and PM emissions that reports data from WB tests at Aprovecho.

That again is a stove-specific statement. The fact that they tested a
charcoal stove that produced more PM than a TLUD does not mean all charcoal
burned in any stove will do so. It depends on the stove. If you wanted to
make more PM from the ash of the carbon fuel in a charcoal stove, just
increase the vertical height, let the air run freely, have a high excess air
ration and presto - PM will appear that was not there before. 

>But the vast majority of potential for PM is from "smoke" that is not
combusted, and a proper burner (such as the concentrator lid) on a TLUD does
a very good job of getting that smoke combusted.

That is one approach. Limiting the excess air in a very much larger but
exceedingly hot environment with no concentrator disk/cone also works very
well. Time and temperature.

Personally I like the concentrator approach but I am seeing things which are
clean and do not have one.

It is my hope that the generality of stove developers will see the
distinctions clearly explained above. Just because a set of stoves have a
certain performance profile does not mean the fuel can be labelled with
certain inherent emissions. Emissions are produced by the stove, not the
fuel, unless it is something chemical in the fuel like sulphur which has to
emerge as some sort of sulphur compound like SO2.

Ask particles are of course a separate category of particle and have little
if anything to do with the combustion. It is my limited experience that
stoves with natural draft up to about 2 metres of chimney take very little
ash up the chimney unless they are allowed a great deal of excess air. Low
velocity gases do not pick up much ash. This applies to chimney stoves with
good excess air control (i.e. an improved stove). Open fire can pull up
large chunks of ash - even flaming embers! In terms of combustion though,
biomass and coal flames do not produce large particles near the fire. I hear
from Prof Annegarn that 1 km downwind from a bad coal fire large dendritic
particles (called giants) form by bumping into each other and agglomerating.
They are made up of a large number of really small original particles. 

Fan stoves probably send up a lot of ash particles, like open wood and
charcoal fires. 

Regards

Crispin reporting from the (flame) front

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