CONTROLLED COMBUSTION OF
LOW-QUALITY GAS MIXTURES
Malinen Kaisua; Järvinen Mikab, Saari Karib, Lampinen Markkub, Fogelholm Carl-Johanb, Riikonen Artoc
a. Corresponding author. Helsinki University of Technology, Department of Energy Technology,
P.O. Box 4400, Sähkömiehentie 4, 02015 HUT, Finland. Currently at ÅF-Consult Ltd. email:
kaisu.malinen@afconsult.com, tel: +358 40 348 5372
b. Helsinki University of Technology, Department of Energy Technology, P.O. Box 4400,
Sähkömiehentie 4, 02015 HUT, Finland.
c. Gasum Oy, Miestentie 1, PL 21, 02151 ESPOO, Finland
ABSTRACT
The objective of this work was to experimentally determine the lowest levels of
methane content in gas mixtures that would still enable controlled combustion using a
commercial standard gas burner.
A gas mixture is of low quality when it is uncertain whether the mixture will ignite or
maintain stable combustion. Such low-quality gas mixtures can typically include
refinery gas, coke oven gas, blast furnace gas and biogas. This work focused on
studying and determining the properties of biogases. Biogas is a gas mixture comprised
of 35-70 % methane. The rest of the biogas consists of various inert gases, including
carbon dioxide, nitrogen, water vapour and sulphur compounds.
The testing equipment included a water-cooled boiler and a gas burner. A well insulated
cylinder with a glass window was built between the burner and the boiler for making
possible to videotape the form and the behaviour of the flame. The composition of the
flue gases were measured online. The oxygen level of the flue gas was kept on a range
of 3,5-4,0 %. The tests were performed with eight different nozzle pressures between
530 Pa and 3000 Pa.
Gas mixtures were composed by mixing natural gas with an inert gas, carbon dioxide or
nitrogen prior to combustion. Combustion was considered to be controlled when the
flame was stable, and the flue gases contained only small amounts of carbon monoxide
and/or hydrocarbons.
Tests of the carbon dioxide-methane gas mixtures revealed that combustion could still
be controlled at methane concentrations of 52-56 %. When nitrogen was used as the
inert gas, combustion was controlled until the methane content fell below 39-44 %.
Keywords: Biogas, low-quality gas, controlled combustion, experimental study.
INTRODUCTION
A gas mixture is of low quality when it is uncertain whether the mixture will ignite or
combustion will continue. Such low-quality gas mixtures can typically include refinery
gas, coke oven gas, blast furnace gas and biogas (Raiko et al. 1995). This research
focused on studying and determining the properties of biogases.
Biogas is a gas mixture comprised of 35-70 % methane. The rest of the biogas consists
of various inert gases, including carbon dioxide, nitrogen, water vapour and sulphur
compounds. (Hagen et al. 2001, Kuittinen et al. 2006, ETSU 1996, Clementson 2007).
Zabetakis presented the results of various flammability tests with combustible gas-inert
gas-air mixtures. The tests revealed that the carbon dioxide-methane mixture was still
flammable, when its methane concentration was 23,5% (7 % of CH4, 23 % of CO2, 70
% of air). When the inert gas was nitrogen, the mixture was flammable until the
methane concentration went below 15 % (6,5 % CH4, 37 % N2, 56,5 % air). Tia et al.
studied biogas combustion in a crater bed. Their research revealed that biogas
containing methane down to 7 % can be steadily burned with stabilized flame in the
crater bed due to its good heat circulation characteristics. According to Energy
Technology Support Unit, gas turbines can operate with 28 % of methane and possibly
less. Dual fuel engines are generally limited to about 35 % methane minimum. Spark
ignition engines have been limited to about 30 % methane minimum but current
developments may reduce this to 28 % with lean burn engines at λ = 1,75.
In this study, the main goal was to determine the lowest levels of methane content in gas
mixtures that would still enable controlled combustion without auxiliary fuel. Also, test
results of controlled combustion of biogas with standard gas burner could not be found.
Gas mixtures were composed by mixing natural gas with an inert gas, carbon dioxide or
nitrogen prior to combustion. Combustion was considered to be controlled when the
flame was stable (it did not flashback or blowoff), and the flue gases contained only
small amounts of carbon monoxide and/or hydrocarbons.
The tests were performed with different nozzle pressures when the oxygen level in the
flue gases was kept on the range of 3,5-4,0 %. The inert gas level was increased in the
gas mixture until the flame quenched. The flame was videotaped in order to observe the
changes in the combustion through the form and the behaviour of the flame. The
compositions of the flue gases were measured online with two analyzers.
All the gas amounts presented are volume-% if not stated otherwise.
METHODS
The testing equipment, shown on Figure 1, included a water-cooled, 85 kW Jäspi T-85
tube boiler manufactured by Kaukora Oy and a 16–34 kW Junior 2 G35 gas burner
manufactured by Oilon Oy. A well insulated cylinder with a glass window was built
between the burner and the boiler for making possible to videotape the form and the
behaviour of the flame. The composition of the flue gas was measured online with a
Gasmet portable sampling unit, FTIR-analyzer by Temet Instruments Oy. The oxygen
level of the flue gas was kept on a range of 3,5–4,0 % and it was monitored with PPM
IPA-Pro analyzer manufactured by PPM Systems. The tests were performed with eight
different nozzle pressures between 530 Pa and 3000 Pa.
Gas mixtures were composed by mixing natural gas with an inert gas, carbon dioxide or
nitrogen prior to combustion. Natural gas was provided to the system from the natural
gas network. The pressure of the natural gas was reduced from 4 bar to 175 mbar. The
flow rate of the gas was monitored mainly with two rotameters manufactured by Kytölä
Oy, but there was also a separate gas flowmeter (0,06-10 m3/h) attached to the pipeline.
Depending on the flow rate, the bigger, LA-NJ27-D (max. 180 l/min) rotameter or the
smaller, NP-G26 (max. 40 l/min) was used. The type of the gas flowmeter was BK-G6
and it was manufactured by Elster Instroment.
The inert gases, nitrogen and carbon dioxide, were in gas cylinders. The pipeline of the
inert gas was connected to the natural gas pipeline before the gas burner in order to mix
the two gases before the burner. The flow rate of the inert gas was monitored with two
rotameters manufactured by Kytölä Oy, A-5AR (max. 150 l/min) and NP-G26 (max. 40
l/min).
Figure 1. Testing equipment.
The bigger rotameter used for natural gas was already calibrated to methane but the
smaller rotameter and the rotameters used to measure the inert gas flow were calibrated
to air in STP, 20 °C and 1,013 bar.
Inlet and outlet temperature of the cooling water as well as the temperature of inert gas
and the temperature of the flue gases were measured with K-type thermocouples and
lead to datalogger. The signals from the flame electrode, cooling water flow and the
oxygen level in the flue gases were delivered also to the datalogger. The values were
measured in every 10 seconds by the datalogger and recorded to the computer.
The composition of the flue gases was measured with FTIR-analyzer in every 30
seconds and recorded to computer.
When measurements were executed with high rates of gas flow, the flow of the inert gas
cooled the control valve of the gas cylinder so extensively that the operation of the
control valve was impaired. Therefore the pipeline after the inert gas cylinder was
heated occasionally with a warm air heating device.
Figure 2. The window built between the burner and the boiler.
The form and the behaviour of the flame was videotaped with a standard video camera
through the window presented on Figure 2. Low flow rate of air was directed from two
opposite points to the window glass for cooling the window and keeping it clear. The
clock of the camera and the clocks of the two computers were calibrated for making
possible to retrieve exactly the correct time period from the videotape afterwards. The
pictures were generated from the videotape with an image processing program Pinnacle
Studio™ that makes able to divide the film into multiple pictures in a second.
For a safety procedure, there is a flame indicator electrode in the standard commercial
burners. If the electrode did not receive a current between 3-100 µA DC at any time of
the combustion it stopped the gas input. The flame indicator was bypassed temporary in
order to ensure that the electrode did not influence the combustion tests. The bypass
tests were executed several times and the results revealed that the indicator had no
influence in flame quenching.
RESULTS & DISCUSSION
The tests were performed with eight different nozzle pressures between 530 Pa and
3000 Pa with gas mixtures of methane-nitrogen and methane-carbon dioxide. On the
following chapters are presented the results of the five of the measurements performed
with nozzle pressures of 530/600 Pa, 1000 Pa, 1500 Pa, 2000 Pa and 2500 Pa.
Concentrations of various gases were measured from flue gases but the ones with most
significant changes, carbon monoxide and methane, are presented below.
Combustion of CH4-N2 mixtures
Due to the combustion air control system of the burner, it was not possible to start the
tests from the level of 0 % inert gas in the gas mixture when the nozzle pressure was
1500 Pa or higher. In the higher nozzle pressures the flow of the natural gas increased
higher than the burner was designed and it was not able to deliver enough combustion
air required. Due to the low air ratio in combustion, the flame quenched.
The combustion air control system affected also the combustion tests performed with
smaller nozzle pressures. When the nozzle pressure was 530-1000 Pa and the N2 content
in the gas mixture increased approximately to 50 %, the oxygen amount in the flue
gases started to increase substantially as the combustion air control system could not be
adjusted low enough. In these tests the carbon monoxide and methane content in the
flue gases increased significantly, together with the oxygen level.
Figure 3 presents the pictures of the flame in different situations when the nozzle
pressure is constant and the N2 concentration in the gas mixture increases. Figure 4 and
Figure 5 are presenting the carbon monoxide and methane contents in the flue gases in
corresponding situations.
As shown on the flame pictures, we were able to start the tests with 100 % of CH4 / 0 %
of N2 in the gas mixture with two of the lower nozzle pressures, 530 Pa and 1000 Pa. In
these tests the shape of flame was about the same. It is clearly shown on the pictures,
that when the level of nitrogen increased in the gas mixture, the flame shortened until it
divided into two parts and finally quenched flashing back to the burner.
When the nozzle pressure was 1500-2500 Pa, the combustion was possible to start when
the nitrogen content in the gas mixture was 10-35 %. The flames were almost identical
during these tests. The shape of the flame started to narrow in the middle when the
nitrogen level reached 60 % and the flame blowoff happened when there was 64-65 %
of nitrogen in the gas mixture.
N2
Nozzle pressure
Nozzle pressure
Nozzle pressure
Nozzle pressure
Nozzle pressure
530 Pa
1000 Pa
1500 Pa
2000 Pa
2500 Pa
*)
*)
*)
0%
20 %
40 %
60 %
64 65 %
*) Could not be measured due to the combustion air control system of the burner
Figure 3. The form of the flame when the nozzle pressure is constant and the N2 content in the gas
mixture increases.
1500
1000
500
CO content in
the flue gases, ppm
2000
3500
530 Pa
1000 Pa
1500 Pa
2000 Pa
2500 Pa
3000
2500
2000
1500
1000
500
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
N2 content in the gas mixture, %
Figure 4. The content of the carbon monoxide in
the flue gases with different nozzle pressures
when the nitrogen content in the gas mixture
increases.
CH4 content in
the flue gases, ppm
2500
530 Pa
1000 Pa
1500 Pa
2000 Pa
2500 Pa
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
N2 content in the gas mixture, %
Figure 5. The content of the methane in the flue
gases with different nozzle pressures when the
nitrogen content in the gas mixture increases.
The CO level in the flue gases started to increase at the nozzle pressure of 530 Pa, when
there was about 25 % of nitrogen in the gas mixture. The content of the carbon
monoxide started to increase exponentially when the content of the nitrogen in the gas
mixture exceeded 45 %. At the nozzle pressures of 1000-2000 Pa the carbon monoxide
level started to increase when there was about 35 % of nitrogen in the gas mixture. The
CO content started to increase exponentially when there was 55-57 % of nitrogen in the
gas mixture. On the highest nozzle pressure (2500 Pa) test the carbon monoxide content
in the flue gases stayed under 500 ppm. In the tests executed with nozzle pressures of
1000-2500 Pa, the CO content in the flue gases stayed under 100 ppm until there was 45
% of nitrogen in the gas mixture.
The figures of the methane content are accordant with the figures of CO content in the
flue gases. When there was 46 % of nitrogen in the gas mixture, started the methane
content to increase rapidly in the test performed with 530 Pa nozzle pressure. When
there was 55 % of nitrogen in the gas mixture at the nozzle pressure of 1000 Pa, started
the methane content increasing exponentially in the flue gases. In the measurements
performed with higher nozzle pressures the methane content in the flue gases did not
start to increase until the nitrogen content in the gas mixture exceeded 56-61 %.
Combustion of CH4-CO2 mixtures
At the end of the combustion tests the adiabatic combustion temperature of gas mixture
of methane and carbon dioxide was 1500-1600 °C, almost 300 °C higher than in the
combustion tests made with methane-nitrogen gas mixtures. Although the combustion
temperature was higher, the flame quenched much earlier, on average of 15 % of
smaller content of inert gas in the gas mixture than in the combustion tests performed
with nitrogen.
Figure 6 presents the pictures of the flame in different situations when the nozzle
pressure is constant and the CO2 concentration in the gas mixture increases. Figures 7
and 8 present the carbon monoxide and methane compositions in the flue gases in
corresponding situations.
The combustion tests were possible to start with 0 % CO2 / 100 % of CH4 when the
nozzle pressure was 600-1000 Pa. The form of the flame stayed at the same until there
was 50 % of carbon dioxide in the gas mixture. When the amount of carbon dioxide
reached 50 % in the gas mixture, started the flame get shorter in the test performed with
600 Pa nozzle pressure as the in the test performed with 1000 Pa nozzle pressure the
diameter of the flame started to narrow. It is clearly seen that at the nozzle pressure of
600 Pa, the flame flashed back to the burner, in the 1000 Pa measurements the flame
blew off.
In the nozzle pressures of 1500-2500 Pa, it was possible to start the tests at the CO2
levels of 13-30 % in the gas mixture. There are no differences in the forms of the flames
when the carbon dioxide content in the gas mixture stayed below 50 %. When the
carbon dioxide level reached 50 %, the flame had already quenched at the nozzle
pressure of 2500 Pa. At the same time the flame blew off at the nozzle pressure of 2000
Pa. When there was 51 % of carbon dioxide in the gas mixture, blew the flame off in the
combustion performed at 1500 Pa nozzle pressure.
CO2
Nozzle pressure
Nozzle pressure
Nozzle pressure
Nozzle pressure
Nozzle pressure
600 Pa
1000 Pa
1500 Pa
2000 Pa
2500 Pa
*)
*)
0%
*)
20 %
40 %
50 %
flame has quenched
51-
flame has quenched
55 %
*) Could not be measured due to the combustion air control system of the burner
Figure 6. The form of the flame when the nozzle pressure is constant and the CO2 content in the gas
mixture increases.
4000
1500 Pa
3500
2000 Pa
3000
2500 Pa
2500
2000
1500
1000
6000
600 Pa
1000 Pa
1500 Pa
2000 Pa
2500 Pa
CO content in
the flue gases, ppm
1000 Pa
5000
4000
3000
2000
1000
CH4 content in
the flue gases, ppm
4500
600 Pa
500
0
0
0
5
10
15
20
25
30
35
40
45
50
55
60
CO2 content in the gas mixture, %
Figure 7. The content of the carbon monoxide in
the flue gases with different nozzle pressures
when the carbon dioxide content in the gas
mixture increases.
0
5
10
15
20
25
30
35
40
45
50
55
60
CO2 content in the gas mixture, %
Figure 8. The content of the methane in the flue
gases with different nozzle pressures when the
carbon dioxide content in the gas mixture increases.
Contrary to the methane-nitrogen combustion tests, the oxygen level in the flue gases
did not increase as extremely. At the test performed with nozzle pressure of 530 Pa the
oxygen content in the flue gases rose above the limit of 3,5-4,0 %, but only at the end of
the tests, most of the measurement points are within the desired limits of O2.
In the lowest nozzle pressure test (530 Pa) the carbon monoxide level started to increase
already when there was only 20 % of CO2 in the gas mixture. The level of carbon
monoxide started to grow exponentially, when carbon dioxide content was 36-40 % in
the gas mixture. On the other tests performed with higher nozzle pressures, the CO
content in the flue gases started to increase coherently, when there was 30 % of CO2 in
the gas mixture. The CO level started to increase exponentially at the nozzle pressures
of 1000-2000 Pa when the CO2 level was 42-47 %. In all of the tests the CO level rose
at least to 500 ppm.
At the nozzle pressure of 530 Pa, the methane content started to grow exponentially,
much earlier than in the other measurements, when there was 36-40 % of carbon
dioxide in the gas mixture. In the other tests the methane concentrations started to
increase only after there was 43-48 % of carbon dioxide in the gas mixture.
Because of the mixing could have been interferenced by the excess amount of the
combustion air in the lower nozzle pressure levels, only the tests from 1000 Pa (CO2)
and 1500 Pa (N2) can be truly considered as the results of controlled combustion due to
the gas properties. When the amount of combustion air increased, the air-gas mixture
diluted which meant that the combustion velocity became slower, the temperature of the
flame reduced and the combustion became uncontrolled (as can be seen in the
concentrations of the CO and CH4 in the flue gases). If the air amount could have been
controlled, it is possible that controlled combustion with somewhat lower quality gas
mixtures could have been achieved.
SUMMARY AND CONCLUSIONS
Tests of the carbon dioxide-methane gas mixtures revealed that combustion could still
be controlled at methane concentrations of 52-56 %. When nitrogen was used as the
inert gas, combustion was controlled until the methane content fell below 39-44 %.
The flame quenched when there was less than 35-39 % of methane in the nitrogenmethane gas mixture. When the inert gas was carbon dioxide, the flame quenched at the
methane concentrations of less than 45-54 %. Comparing to the flammability tests made
by Zabetakis, the results seem to have excellent correspondence although the test
equipment and method of measurement were different. In our tests the methane content
in the methane-inert gas-air mixture was 6,6-6,7 % (6,5 % by Zabetakis) when the inert
gas was nitrogen and 6,9-7,1 % (7,0 % by Zabetakis) when the inert gas was carbon
dioxide. On the contrary, methane-inert gas compositions differ as the air ratio seems to
have been λ = 0,9-1,05 at the measurements performed by Zabetakis. The air ratio in
this study was fixed to λ = 1,28-1,29. With lower air ratio we could have achieved the
exactly same results but methane and carbon dioxide emissions in the flue gases would
have been even higher.
The most important thing in defining controlled combustion is to monitor the content of
carbon monoxide and hydrocarbons, such as methane, ethane and ethylene, in the flue
gases. When the combustion is controlled, the gas will mix and combust normally and
there should not be any above-mentioned gases found in the flue gases. In the test
performed, it was clearly seen from the contents of the CO and CxHy in the flue gases
when the combustion was not controlled anymore.
Landfill gas has the lowest methane content amongst the biogases. For controlled
combustion of landfill gas without auxiliary fuels, it is important that the methane
content of a landfill gas does not fall below current levels. However, in the future, the
methane content of landfill gases will certainly drop when the landfills are closed. One
solution to keep these low-quality gas mixtures combustible in standard gas burners
would be to upgrade or enrich the gas mixture with natural gas.
ACKNOWLEDGEMENTS
Gasum Oy is gratefully acknowledged for funding this work. We would also like to
thank Pasi Miikkulainen and Ari Kankkunen for the advises on videotaping the flame
and image processing, Loay Saeed for the guidance of using the FTIR flue gas analyzer
and most of all Mika Ahlgren for building the testing equipment.
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