[Stoves] Use of furnace emissions testing equipment

Wed Feb 2 12:17:22 CST 2011

Dear Paul'n'All

Sorry not to be able to get to ETHOS - it was my intention to get here this
year but things in Ulaanbaatar are moving fast than expected and several
major advances in products, understanding and financing have popped. That's
life, at least some of the time. What this space because it is going to have
effects beyond UB.

>2.  To the best of my knowledge, neither Aprovecho nor any  
>installation of the PEMS equipment does not have TSI or Testo testing  
>equipment, so no correlations have been made between the results of  
>these different types of equipment.

There are two approaches used to defining combustion efficiency: the
CO/Total Carbon method and the CO/CO2 method. Both use only a portion of
actual combustion efficiency and this is done because they are easy to
measure and/or calculate.

CO/CO2 is a measure of combustion IN-efficiency.

Measures of combustion inefficiencies include:
CO/Carbon and CO/CO2
H2/H2O
H2S/S and H2S/SO2

I am hoping that Steve Garrett will come up with a way of measuring H2O at
high temperatures (700) using an inexpensive thermoacoustice device. He is
optimistic.

The direct measure of combustion efficiency is often written as:

CO2/(CO2+CO)

That is the ratio of CO2 (proper combustion) to the sum of CO and CO2 (total
carbon-containing gases measured). The higher the number, the more complete
the burn. 

Examples are: smouldering dung in an Indian open fire might be 85% (reported
by K Smith), and burning lignite in a Mongolian GTZ 7.5 stove averages
99.44% (reported by SEET Lab).

Legislation is rarely written this way. It is not written with a high
combustion efficiency in mind, it is written with a permitted 'inefficiency'
in mind, meaning emissions of something we don't want.

So the most commonly used measure of combustion is CO/CO2. It is also common
to have 2% as the target value for combustion because it has been shown that
in a normally ventilated house, the CO level does not build up to dangerous
levels if the burner is working well enough to keep the CO below that level.

98% combustion efficiency is NOT the same as CO/CO2=2%. The latter means 1
molecule of CO per 50 molecules of CO2. 98% means a ratio of 1/48.  Thus it
is technically to refer to the CO2/(CO2+CO) as an efficiency, and CO/CO2 as
a ratio. It is often referred to as COr (the CO ratio) because it is easier
to say. We ask, 'What is the combustion efficiency?' and get the answer,
'The COr is 3.51%.'

Combustion analysers often give the COr as a % or as a fraction. My TSI
reports the fraction in decimal form: 0.0200 is 2%. Testo reports 2% on the
screen.

The reason they do not report the combustion efficiency is the technician is
testing a burner like an apartment building furnace and has been told to
tune the air supply until the number is 'below xx%' so that it is operating
within the manufacturer's stated range.

Lab results sometimes report the CO2/Total Carbon = CO2/(CO2+CO) =
combustion efficiency.

Russian republics (incl former USSR) normally have different COr values
permitted for different fuels. Examples are:
Wood 4%
Coal 2%
Anthracite 0.5%

There is an article on the use of a combustion analyser to test and develop
stoves here:

http://www.hedon.info/docs/BP55-PembertonPigott.pdf

The upcoming Domestic Use of Energy (DUE) Conference in Cape Town in April
will have three papers on the test lab use of combustion analysis to develop
stoves in Ulaanbaatar. One is on the problem analysis (air quality and
particle analysis to attribute the problem to domestic coal combustion).
Another is on the test protocol used to make measurements and process them.
The third is on the use of the measurements to adapt the product to minimise
the PM produced. Over a period of 6 months, these methods were applied to
reduce emissions by a factor of 1000 (99.9%). The baseline and improved
stove PM measurements are attached. The black lines are real time emissions
of PM2.5.

Key to this result was the use of real time calculations to discover when
things were working well and expand that zone of perfection until it covered
nearly the entire burn period. 

The stove development was not actually guided by the PM numbers, but by the
CO/CO2 ratio. As the ratio goes very low when there are still volatiles
present in the fuel, the PM disappears as a matter of course. You don't have
a CO/CO2 ratio of 0.02% (1/100th of the permitted ratio) and still have lots
of PM.  The reverse is not true. If the PM is absent, it doesn't mean the CO
is gone. A coke fire can produce no PM and vast quantities of CO.

PM emissions are often expressed in g/kg of fuel burned which can be very
inconvenient when making comparisons between stoves and fuels.
Labs/engineering facilities usually express it in g or mg per MJ or per
useful MJ (MJ absorbed, work done, used etc).

Stove rating involves testing the whole system, as used, so it should be
mg/MJ absorbed (in the pot, if it is a cooking test). A stove with worse
combustion but better heat transfer will rate better in terms of emissions
per task accomplished. 

Regards
Crispin

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