general So you finally bought a Combustion Analyser!

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```					                                                                                          Boiling Point / 55 / 2008

general

So you finally bought a Combustion Analyser!
Author
Crispin Pemberton-Pigott
New Dawn Engineering, P.O. Box 3223 Manzini, MZ200, Swaziland, Southern Africa
Email: crispinpigott@gmail.com

The goal of many household energy projects is to develop an improved stove with
a good combustion and heat transfer performance. This article describes how with a
basic combustion analyser, along with a few mathematical tools, a stove developer
is well equipped to work wonders in improving a stove’s performance.

Developing an improved stove is the          oxygen (o2)

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primary goal of many domestic energy         Oxygen is also easy to detect and is
projects. Most developers know it is not     usually reported in percent (%). The air
easy to produce a clean burning stove        entering a stove can be thought of in
without using emissions measuring            two components, the amount required
equipment like a combustion analyser. But    for combustion (the air demand) and the
along with having the correct equipment      air not theoretically needed to burn the
you also need to know how to extract         fuel (excess air)
useful information from the raw numbers.
A stove developer is looking for better      Excess air (EA) is calculated as follows:
combustion and better heat transfer. A
basic combustion analyser along with                         [ O2% – (CO%/2)] x 100
EA (%) =                                  Figure1 A TSI CA-6203 Combustion Analyser
a few mathematical tools will produce                        20.95 – [O2% – (CO%/2)]

[                  ]
useful information from a surprisingly                                                                      20.95 – (O2%+ CO%)
small number of measurements.                Summing the combustion and excess air        CO2% = CO2 Max % x               2
gives the total air supplied, also called                              20.95
Analysing combustion                         the Air Factor, represented by the symbol    Note: CO2 Max for Wood is 19.4%
Lambda, λ. Lambda is excess air plus one.
Improving combustion has two aspects:                                                     the Co/Co2 Ratio (CoR)
burning the fuel completely and                 EA% + 1                                   A measure of how completely the fuel
λ=
minimizing harmful emissions. Similarly,        100                                       is being burned can be determined
a better heat transfer also has two main                                                  by dividing the CO by the CO2. Fully
factors: getting the heat into the pot or    Example                                      combusted carbon emerges as CO2,
the room, and limiting the amount that is                                                 partially burned carbon as CO. The
wasted either up the chimney or into the     If EA = 160%, λ = 160/100 + 1.00 = 2.60      better the combustion, the lower the
air. The combustion analyser will help                                                    proportion of CO. This calculation can
with all of these.                           i.e. the total air entering the stove is     be made with any level of dilution,
2.6 times greater than that required         provided both are determined from
First you need to find the level of carbon   for combustion.                              readings taken at the same time. As the
monoxide (CO) in the emissions, the                                                       CO is usually given in ppm and the CO2
oxygen (O2) level and the temperature.       Calculating Co2                              in %, a conversion factor is needed to
These three measurements are key. If you     Because the composition of fuels like coal   determine their relative abundance.
have a scale you can also determine the      or wood is usually known, the amount of
mass of fuel being burned at the time the    CO2 in the stack (chimney) sample can be     COR = CO
measurements were taken and from this        calculated from the O2 and CO. If there            CO2
calculate the quantity of CO produced        is 20.95% oxygen in the air going into
when burning a kilogram of fuel.             a stove, and 10% in the gases that come      Example
out, then approximately half of it has
Carbon monoxide (Co)                         been used during combustion. Some            Suppose the levels are 500 ppm CO,
If CO or CO2 is found in the gas flowing     of it will have reacted with hydrogen        and 10% CO2
from a stove, there is combustion taking     in the fuel to make H2O (water). This
place. Detecting CO2 is more difficult       happens easily so analysers usually          First convert the CO ppm to CO%
than CO, so simple gas analysers will        assume that all the hydrogen has been
only measure the latter. It is normally      burned. Another portion of the oxygen        500ppm CO
reported in parts per million (ppm) or       combines with carbon to make CO.               10,000  = 0.05% CO
milligrams per cubic metre of gases (mg/     So based on the fuel composition, the
m3). To convert mg/m3 to ppm, multiply       initial and post-combustion oxygen           Then divide the CO by the CO2
mg/m3 by 0.81075. To convert CO ppm          levels, and the CO level, the rest of
to CO%, divide by 10,000.                    the oxygen can be assumed to have            COR = 0.05% CO = 0.005 = 0.5%
been burned to CO2. Using this logic, a             10% CO2
Example                                      reasonable calculation of the CO2 level,
expressed in %, can be made without          The target of a stove developer is to
500 ppm CO = 0.05% CO                        measuring it directly, useful if you have    achieve a COR of 2% or less. Very low
10,000                                     that simple gas analyser.                    readings are possible in modern stoves.
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Correcting the Co reading                      Analysing heat
undiluted gas concentration                    transfer efficiency
The COR is calculated using the readings
taken directly from the analyser and can       A combustion analyser can measure
compare the combustion efficiency of           the chimney gas temperature and
different stoves. However it is not correct    calculate the amount of heat lost up the
to make comparisons between stoves             ‘chimney stack’.
The presence of excess air, as indicated       The air feeding a stove has to be drawn
by the oxygen level, means that the            from outdoors. The initial temperature
CO measurements will be incorrect,             (T1) is the outdoor temperature and the
with valid comparisons for individual          final temperature (T2) is the temperature
gases only being made using                    inside the chimney.                                Figure 2 A Testo 350 XL Combustion Analyser
EA-corrected figures.
T2-T1 = Delta T = ΔT
Example: Compare these measurements
from the stack and determine which             Stack losses are a combination of gas
theme

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version of the stove has lowest CO level:      volume and ΔT.

Test 1 CO = 2561 ppm, O2 = 8.00%               Recording the temperature in the chimney
Test 2 CO = 1981 ppm, O2 = 10.60%              will not, alone, tell you what the loss is.
Test 3 CO = 2144 ppm, O2 = 11.25%              You need to know, as before, the amount
of excess air that is diluting and expanding
Test 1 shows the EA is 60.19%, so λ is         the volume of emissions from the fire. The
1.6019. The undiluted CO level is 1.6019       combustion analyser will calculate the
x 2561 = 4301 ppm.                             amount of heat contained in the gases              Figure 3 A Lufft temperature logger
and combine this with the quantity of
Test 2 shows the EA is 100.50%, so λ is        excess air to produce a percentage heat
2.0050. The undiluted CO level is 2.0050       loss. If the exit temperature was the same
x 1981 = 3972 ppm.                             as the outdoor temperature, the loss               Using a combustion analyser to track the
would be 0%.                                       undiluted gas and particulate levels, the
Test 3 shows the EA is 113.62%, so λ is                                                           heat loss and the COR a stove developer
2.1362. The undiluted CO level is 2.1362       To determine the loss in Watts, you have           is well equipped to work wonders
x 2144 = 4580 ppm.                             to weigh the fuel being burned and                 improving a stove’s performance.
determine the heat generated, then
The stove in Test 2 is the cleanest burning,   multiply that times the percentage of              Profile of the Author
and Test 3 is the dirtiest, something not      heat being lost. This heat loss feature is         Crispin Pemberton-Pigott has worked with
obvious from the CO reading alone. It is       helpful even if you are working on stoves          Appropriate Technologies for 30 years,
very important to make this correction to      without a chimney. Take a sample of                largely in rural water and manual production
obtain the undiluted gas concentration. It     gases from the point at which they exit            equipment. A stove maker for 25 years, he
makes meaningful comparisons between           past the pot and you get the percentage
Chairman’s Award 2004 for the ‘Vesto’, a semi-
different stoves and fuels possible.           of heat being lost at that point. The              gasifing stove now manufactured in Swaziland
inputs used are the room temperature,              at New Dawn Engineering, a producer of
Particulates                                   the exit temperature and the Excess Air            labour-based manufacturing systems for
Suppose we want to know the PM 2.5             level. Care must be taken to ensure no air         rural employment. He is a co-founder of the
particulate emission level and how clean       from the room enters the sample being              Eastern Cape Appropriate Technology Unit
the burn is when a stove is used with two      drawn or you will get an inflated Excess           (RSA), the Renewable Energy Association
different fuels.                               Air figure.                                        of Swaziland and the Industrial Designers
Association of South Africa. Presently the
Example:                                       For small stoves with a short gas path, the exit   he is also on the Board of the Sustainable
Test 1 CO = 3566 ppm, O2 = 13.05%, PM          temperature will give a general indication         Energy Society of Southern Africa (SESSA)
2.5 = 135 µg/m3                                of losses: the higher the temperature, the         and chairs its daughter organisation, the
Test 2 CO = 2911 ppm, O2 = 11.40%, PM          greater the loss. Unfortunately, this is only      Association for Renewable Energy Cooking
2.5 = 161 µg/m3                                true in certain cases. For example, if you         Appliances (AFRECA). He is a member of the
increase the excess air supply significantly,      South African Bureau of Standards technical
The calculated EA, λ, CO2 and COR levels       you may see a drop in temperature but              committees writing national standards and test
protocols for coal, paraffin and gel fuel stoves.
for the tests are:                             a large increase in heat loss because the
extra air is cooling the fire and rushing
Test 1 EA = 159.34%, λ = 2.5934, CO2           the heat past the pot in a larger volume of
7.15%, COR = 4.99%                             cooler gas.
Test 2 EA = 116.08%, λ = 2.1608, CO2
www.hedon.info/KUJA
8.71%, COR = 3.34%                             The thermal efficiency of a small stove
The undiluted PM 2.5 concentrations are:       chimney stove. Exceptions to this are                • Author profile and latest
Test 1 135 x 2.5934 = 350 µg/m3                some institutional stoves with pots sunk
Test 2 161 x 2.1608 = 348 µg/m3                completely into an all-enclosing, insulated            contact details
body. In such a stove, decreasing the
The fuel in Test 2 has a better combustion     excess air can show a constant or even
efficiency indicated by a lower COR but they   a decreasing exit temperature and a
have the same level of PM 2.5 emissions.       substantial increase in efficiency.                Meet us @HEDON
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