Dear Friends
From
http://www.mfe.govt.nz/withyou/funding/smf/results/2205_woodburner_report.pd
f page 37/73.
“These figures demonstrate that regardless of the units used to express
emissions, relative performance between the standard laboratory test and
home operation may be quite different.”
Where have we heard that before?
The document is a really interesting real-world read on the outputs of
different test protocols. It is interesting to me how little agreement there
is between different test outputs.
Table 4-1 has a chart of different emissions from wood burning appliances
and there are pithy notes below indicating that it is really hard to make
meaningful comparisons because of the number of variables that affect
performance (which varies widely).
The challenge is how to report the emissions from the stoves we are testing
in Ulaanbaatar and the lack of agreement on this (internationally, or even
within the US or Europe) is still a problem.
In order for test results to be meaningful, the best number for a space
heating stove seems to be mass of PM 2.5 emitted per MJ that is effectively
remaining in the room. That means factoring in the thermal efficiency of the
stove. If a home needs 80 MJ per sq metre per month to be warm, and the main
interest is reduction in outdoor air pollution (which is the case is many
well populated countries) then the emissions produced when delivering a
MegaJoule is the most interesting number.
In Mongolia we have many different fuels on common use so comparing them
with ‘PM emitter per kg burned’ means no direct comparison can be made
because the fuels have widely different heat values per kg. The immediate
calculation that has to be made is to divide the PM value (in milligrams,
usually) by the number of MJ in the fuel per dry kg. But if the thermal
efficiency of the stove is very different from another, one would have to
burn very different amounts of the fuel in order to be warm so the PM
emitted would be quite different.
The number ‘PM2.5 emitted per MJ retained’ (PM/MJret) incorporates the PM
emitted over a burn (still to be defined), the net heat content of the fuel
and the thermal efficiency of the stove.
If you know the climate profile (which is easy to obtain) and the heat
demand of the building (ditto) the PM emitted to heat it can be directly
calculated.
Much of the discussion in the US seems to be about what reporting method is
to be used. The coal people all prefer PM/kg burned on a dry, ash-free
basis. That is sort of useful in that it incorporates the heat per burnable
kg of coal (the rest being ash and moisture). However if the moisture is not
taken into consideration and the fuel is always wet (like lignite is) then
the PM actually emitted when heating a home is not really represented in the
reported number. In other words a stove emission rating on that basis cannot
be used to decide which is cleaner burning.
I hope all of this will soon be rendered moot by the development of stoves
that produce nearly now PM emissions at all! That would be nice.
Having got the systems working at the stove testing laboratory in
Ulaanbaatar we are receiving a stream of data with really interesting things
learned in the first week.
1. There are nearly no particles emitted after the volatiles are gone
no matter what the fuel or stove (with certain caveats).
2. In some cases, the combustion is so complete that the PM rating is
negative – for one stove in particular dubbed the GTZ-7, the stack is
cleaner than the ambient air a great deal of the time. As it is still summer
and the PM is only 20-30 micrograms per cubic meter, that is a good sign. In
early winter the level rises to as much as 4200. It seems entirely possible
the stove will be taking in more particles than it emits, averaged over the
whole burn. The implications are obvious.
3. So far we have detected few particles above PM2.5 using chimneys of
3 metres unless there is a considerable fire in the stove driving enough air
to loft ash particles.
4. Top lighting the wood during ignition of the coal (where the
architecture permits) reduces the smoke produced during the first 10 minutes
quite a bit.
5. The operation of the stove is a more important determinant of
emissions than minor changes to the architecture. Wholesale changes to the
architecture are needed to make really big savings.
6. It is possible to dramatically reduce the particles emitted after
ignition by changing the architecture alone, even if the operator does not
have much skill at lighting.
7. There is a general relationship between the PM and CO during the
early stages of the burn (ignition and getting to the flaming pyrolysis
stage).
8. There is a strong correlation between the CO and H2 emitted – both
being a measure of combustion efficiency. It seems that in general people
have not been looking for hydrogen in the stack. The combustion analysers
usually assume (mathematically) that it is 100% burned and it definitely is
not. For different fuels it is (for example) equal to, or ½ or 1/3 or ¼ of
the CO level and remains pretty much fixed as that level throughout the
test. As far as I observe there is no way to predict the CO2 level by
tracking them closely (they are both easy and cheap to measure so it might
have been the poor man’s CO2 meter).
When it comes to performance, we tested a few stoves and included two
lighting methods with traditional ones. Mostly it was about shaking out
problems in the data recording system and procedures. It seems to be working
consistently now. So we got interesting info along the way.
The GTZ-7, about which more is no doubt going to be heard (see attached)
showed an enormous drop in PM emitted compared with a well designed
traditional stove in good condition operated in a normal manner. We can
speculate about why the traditional method and layout is not very clean, but
the more interesting thing is why the GTZ-7 works so well. It has the
general layout of a European home heating furnace of about 50 years ago but
with a few tweaks and ideas. It is tiny and is I think the 4th version in a
series getting better all the time, particularly at lighting which is where
the emissions are concentrated. This is the stove I mentioned a couple of
months ago referring to the depth of the fuel bed being used to control
excess air. It is optimised now.
The indications are that it reduces PM 2.5 by about 98% when burning 5 kg of
lignite, measured from beginning to end, the end being when the gas analyser
can’t tell the difference between the stack and the ambient air (EA =
2000%). As mentioned above, the stack concentration is often less than the
(pretty clean) ambient air so there is a net reduction in particulate
matter. The CO/CO2 ratio was frequently unmeasureable, meaning that the CO
level in the stack was so low and the CO2 so high that it would not register
on the 4th decimal place which read COr = 0.0000 (that is in fractions not
%: 0.0200 = 2%).
If anyone wants to have a crack at analysing the raw data they are welcome
to have a copy. I am particularly interested in how people will treat the PM
measurements for reporting purposes. I have done the following:
PM measured in the diluter per standard cubic metre (we are getting dry and
condensed particles) multiplied by the level of dilution which varies
between 1100 and 3.7 depending on the PM concentration in the stack a the
time. (The variation in the PM mass/m^3 is about 20,000,000:1 during the
burn.) This gives a mass per cubic metre that is not yet corrected for
dilution by excess air. This calculation is done per 10 seconds.
The mass of fuel burned in that 10 seconds is noted and the stoichiometric
air demand added to the excess air present to give a volume of stack gas
including excess air. That volume of gas is multiplied by the undiluted mass
per cubic metre to get a total mass in the stack. In the very beginning of
the fire the volume of gas is small so a high concentration is compensated
for by the tiny volume to give what is often a low mass. Emitting a high
concentration later on with a big fire is a much larger total mass.
Over the whole test (burning 5 kg of raw fuel) the total PM emitted per 10
seconds is summed. This is then divided by the net number of MJ the fuel
burned contained, factored for the moisture content. At the moment the net
MJ value is not factored for chemical losses (CO emitted instead of CO2, H2
instead of H2O, etc). The final number is PM emitted per net MJ.
Multiplying that by the thermal efficiency of the stove gives the number
mentioned above: PM emitted during the burn per MJ delivered into the room.
Obviously we will report both figures.
The CO mass emitted can be calculated in the same manner. The early
indications are that the GTZ-7 reduces CO over 80% and PM 98%, and that the
lighting technique matters. Each burn includes one refuelling episode. The
timing of the refuelling makes a large difference to the CO but not much to
the PM. Hot stoves are pretty clean compared with cold ones.
What great fun!
Regards
Crispin kicking back his heels in Seoul
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