[Stoves] Particulate emissions

[IPC]

Dear Crispin

Yes, I think there should be some sort of link between CO generation and
fine particles.  

The "smoke" (fine particles of combustible material) takes a surprisingly
long time to burn.  You need excess air for starters, then you need
retention at temperature. If you don't have sufficient retention, then you
have incomplete combustion, and if you have incomplete combustion, then you
will also have CO - even with excess air. 

The smaller the scale, the more difficult it is to get enough retention at
temperature. That is in essence why stoves tend to smoke with solid fuels.
The point of a top-lit updraft stove is that the smoke generated from
heating of the fuel below the hot zone has to pass through the hot zone,
thus giving more retention than a bottom-lit updraft.  Thus less smoke.

The whole point of steam, about which we were so concerned a few days back,
is that the gasification reactions are much faster than the combustion
reactions and can carry on in the absence of air.  So by adding steam you
gasify the smoke rapidly, within the flame, and generate CO and hydrogen
that will burn once they get to adequate air.  

Re "Potassium in the fuel is the most important indicator- -" I did tell you
the alkalis were more fusible than the alkaline earths, so yes!

This could give you some guide to a testing procedure - but I am inclined to
think the best test is to get particulates down very low, period.  PM2.5 is
nasty stuff, because the breathing system does a real good job on filtering
out particles >2,5 micron in size (around 99% efficiency at 3 microns, we
measured at the Chamber). "Don't smoke!" is advice to give to stovers as
well as nicotine addicts.

Best regards
(Dr)Philip Lloyd
Energy Research Centre
University of Cape Town
Private Bag Rondebosch 7701
South Africa
Tel +27 (0)21 650 3896
Fax +27 (0)21 650 2830


-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: 04 January 2008 01:09
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Particulate emissions and testing - was Water in coal

Dear Philip Again

I had another think about this and had a good read of a paper presented at
the 15th European Biomass Conference & Exhibition a few months ago called,
"Fine particulate emissions from modern Austrian small-scale biomass
combustion plants" (Obernberger, Brunner and Barnthaler) May 2007.

In it there is a description of the process of formation and growth of fine
particulates.  It appears that CO presents a simple indicator for the
conditions leading to the formation of organic aerosols.  They show for a
number of different biomass fuels and furnaces a lock-step relationship
between CO, TOC and PM10 levels. (Proceedings p.1552)

They relate this to the combustion temperature, particularly the burnout
conditions near the end of the flame.

"Since organic aerosols are a consequence of an incomplete gas phase
burnout, elevated TOC emissions can be used as an indicator for the
increased formation of these particles." (p.1552)

I like that they report the particulates (PM10, PM2 and PM1) in terms of
mg/MJ instead of per task accomplished.  This allows us to examine heat
production separately from heat utilisation. At present the stove testing
protocols do not contain enough information to be able to distinguish
between the two so we are not sure if a particular stove is a good combustor
or a good structure.  The UCB-WBT tests the package.  Varying both parts of
the package may give a different performance but the reporting does not show
which part was responsible for the change.  We should re-visit this because
the WBT is not appropriate for space heating and cooking stoves - some also
have water boilers attached. Combined heating and cooking (CHC) stoves may
lead us to more meaningful stove tests.

Back to the burners: The authors show a stable relationship between the
three PM fractions and the total, and a stable relationship between the
total and the TOC, and again between the TOC and the CO level.

Decreasing furnace temperatures are directly linked to the TOC and CO
levels, and the presence of 'Lambda controls' (excess air monitors that
automatically adjust the airflow in reduced power conditions) were credited
with the improvement of furnace performance over the past 5 years.

In conclusion, CO is a pretty good indicator of PM levels in combustors
which are already pretty clean burning.  Potassium in the fuel is the most
important indicator predicting the formation of inorganic aerosols.

Huh, I said to myself and the giant pot plant beside me, there being no one
else in the room at the time.

I recall that Tami concluded that the CO and PM10's were not very related in
wood stoves, however the tests she did in the field were not exactly on
devices with 'already pretty clean burning' combustors. Can both
observations be true? Perhaps.

I am wondering then, can we conclude that where the combustion is lousy, CO
is not a predictor of PM10 (and therefore PM2.5 or PM anything else) but if
the TOC or CO level is very low, it is?

The formation of PM10's seems to be pretty well understood, according to
these guys.

What would be wonderful is to be able to say to a programme, for a very low
fee, that their improved stove shows a CO level (combustion efficiency) of
xxx and this is a big improvement on the previous technology, they should
continue to work on the device monitoring the CO.  The particulates would be
rising and falling with the CO level now that they have entered the realm of
'pretty good combustors' (PGC's).

A PGC could be defined as one in which the combustion so clean that the PM
total, PM10, PM2 and PM1 are related to the CO and TOC levels.

Thoughts?