IFRF Combustion Journal
Article Number 200404, October 2004
ISSN 1562-479X
Oxygen-Enriched Combustion Studies with the
Low NOx CGRI Burner
D. Poirier, E.W. Grandmaison*, A.D. Lawrence1, M.D. Matovic and E. Boyd
Centre for Advanced Gas Combustion Technology
Queen’s University
Kingston, ON, K7L 3N6
Canada
1
IFM – Kemiteknik
Linkopings Universitet,
581 83 Linkoping
Sweden
*Corresponding Author(s):
Ted Grandmaison,
Department of Chemical Engineering,
Queen's University,
Kingston, ON K7L 3N6,
Canada.
Tel.:
+1 613 533 2771
Fax : +1 613 533 6637
E-mail: [email protected]
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ABSTRACT
An oxygen-enriched/natural gas combustion study with a modified low NOx CGRI burner
has been completed. Effects of oxygen enrichment, at various stack oxygen levels and a
single furnace operating temperature, on NOx and CO2 emissions, fuel efficiency and furnace
temperature distribution, were determined. Combined effects of oxygen enrichment and air
infiltration were also studied. A single sidewall mounted burner was employed in the pilot
scale CAGCT research furnace. The firing rate required to maintain the furnace temperature
at 1100°C decreased linearly with increasing oxygen enrichment. At full oxygen enrichment,
a reduction of 40-45% in the firing rate was needed to maintain constant furnace temperature.
NOx emissions (< 12 mg/MJ) were relatively constant with changes in oxygen enrichment
levels below ~ 60% and decreased at higher oxygen enrichment. NOx emission increased
with increasing stack oxygen concentration at all oxygen enrichment levels. Air infiltration
resulted in NOx emissions similar to those observed with no air infiltration but with similar
stack oxygen concentrations. The standard deviation of the temperature distribution for the
furnace roof and blind sidewall was in the range, 19 – 27 °C with no oxygen enrichment and
31 – 34 °C with 90% oxygen enrichment.
Keywords:
oxygen-enriched combustion, low NOx combustion, energy efficiency
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INTRODUCTION
Improvements in energy efficiency coupled with reduced emissions are an ongoing objective
in many industrial sectors employing combustion technologies.
Dilute combustion
technology (Milani and Saponaro, 2001) has been found to reduce NOx emissions by mixing
the fuel and oxidant streams with inert combustion product gases. This technique leads to
lower oxygen and fuel concentrations along with lower temperatures in the combustion or
reaction zones of industrial furnaces. A burner conceived by the Canadian Gas Research
Institute (CGRI) and tested jointly with the Centre for Advanced Combustion Technology
(CAGCT) falls into this category of technology (Besik et al., 1996; Sobiesiak et al., 1998;
Grandmaison et al., 1998).
Oxygen-enhanced combustion is a relatively well developed technology (Baukal, 1998)
employed in the combustion industry (e.g. De Lucia, 1991; Delabroy et al., 2001; Marin et al.,
2001). This study combines dilute combustion and oxygen-enriched combustion, with the
goal of optimizing the beneficial characteristics of both technologies: energy efficiency (low
CO2), low NOx emissions and good heat transfer.
CAGCT FURNACE SYSTEM
Testing and development of the O2-enriched furnace system were conducted at the Centre for
Advanced Gas Combustion Technology (CAGCT), Research Furnace Laboratory, Queen’s
University. The interior of the furnace, Figure 1, is divided into two unequal size chambers
by a checker-work, brick end-wall. The first chamber is the main furnace cavity with internal
dimensions of 4.5 m long, 3 m wide and 1 m high (177 in. x 118 in. x 39 in.). The second
chamber serves as an exhaust plenum with interior dimensions of 0.6 m long, 3 m wide and 1
m high (24 in. x 118 in. x 39 in.). The checker wall, 215 mm thick (8.5 in.), with an 8 x 3
array of openings, 75 mm x 115 mm (3 in. x 4.5 in.), separates these two chambers. The
refractory lining for the furnace walls and roof are ceramic fibre blocks, 305 mm (12 in.)
thick. The furnace wall structure and refractory is a combined 362 mm (14 in.) thick, as
shown in Figure 1.
Instrumentation for the furnace includes fixed thermocouples, static pressure taps, orifice
meters for gas and air flow measurement and control. Refractory wall surface-thermocouples
are located at positions T1 – T41 as shown in Figure 2. These thermocouples, 0.254 mm
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Burner
Plenum
Wall
-362
0
Top
View
Furnace
Exhaust
3000
3362
-362 0
750
4500 5100 5462
Refractory
1362
1000
Side
View
500
0
Water-cooled floor panels
Figure 1: CAGCT research furnace shown with the single sidewall mounted
burner used in the present study. All dimensions in mm.
diameter Pt/Pt-10%/Rh, are embedded about 5 mm into the refractory walls. The size and
positioning of these thermocouples help minimize measurement error. The furnace is also
equipped with water-cooled floor panels for heat flux measurements, sampling ports for
internal furnace measurements and recuperators for air preheat.
In the present work, a single burner was fired from the furnace sidewall, Figure 1. The
furnace was operated at positive pressure for the primary set of tests with a selected set of
trials performed at negative pressure to study the effect of air infiltration. The burner design,
Figure 3, was a modified form of the ultra-low NOX burner initially developed at the
Canadian Gas Research Institute (CGRI) and CAGCT (Besik et al., 1996, Sobiesiak et al.,
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Poirier, Grandmaison, Lawrence et. al.
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Burner
-362
0
T20
T16
T12
T5
797
890
1000
1203
1500
1797
2000
2110
3000
T8
T1
146
T9
T2
T21
T14 T17
T6
T7
2854
T22
T15 T18
T3
T10
T4
T11 T13
T23
T19
3362
453
-362 0
1047 1750 1995
2996
3996
750
4500 5100 5462
1362
1000
873
500
0
127
T26
T28 T31 T33
T25
T30
T24
T27 T29 T32
496
1004 1496 2004
T35
T34
T38
T41
T37
T40
T36
T39
2750
3496
4254
Figure 2: Location of refractory-wall thermocouples in the CAGCT Research Furnace – top
figure shows refractory roof, bottom figure shows blind-sidewall opposite the burner.
UV scanner port
Pilot burner port
Air/oxidant nozzle
Fuel nozzle
Figure 3: Schematic diagram of the
CGRI showing the locations for the
air/oxidant and fuel nozzles.
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Poirier, Grandmaison, Lawrence et. al.
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1998, Grandmaison et al., 1998) was used in this work; hereafter this burner is referred to as
the CGRI burner. The burner consists of a ring-array of alternating fuel and oxidant nozzles
directed at different angles to the burner axis. The burner was modified to include oxygen
supply tubes and jets running coaxially to the air supply tube and jets. The oxygen nozzle
diameter and the air nozzle annulus were sized so that the momentum of the combined
oxidant stream would remain relatively constant with changing O2-enrichment level for a
constant firing rate. The air and O2 nozzle angle (10°), air-nozzle annulus size, fuel jet angle
(20°) and fuel nozzle diameter were maintained at constant values for the results reported in
this work
The firing rate was adjusted to maintain a constant furnace temperature of 1100 °C as O2
enrichment and excess oxidant was varied. This clearly demonstrated fuel savings gained by
O2 enrichment and provided a better basis for comparison of other data including NOX levels.
Oxygen enrichment level, ψ O 2 , is defined as

&O

m
ψO = 
 × 100
m
& O A 
 &O +m
2
2
2
2
& O2 and m
& O2 A are the mass flow rates for the pure oxygen and oxygen associated with
where m
the air feed streams, respectively. Concentrations of O2, CO2, CO, NOX and CH4 in the
exhaust gases were continuously measured. Refractory surface temperatures of the furnace
walls and ceiling and heat flux to water-cooled floor panels were also continuously monitored.
Quasi-steady-state furnace conditions for gas composition measurements were assumed once
the furnace control temperature reached the operator set point (1100 °C in these trials) and
gas analysis readings stabilized. A large number of the data reported in this work were
obtained during steel scaling tests reported by Poirier et al. (2004) in which the furnace was
operated at fixed conditions over 4 – 8 hour periods.
RESULTS AND DISCUSSION
To demonstrate the reduction in fuel usage and CO2 emissions that can be expected with O2enriched combustion, the burner firing rate was monitored at the furnace set point
temperature (1100 ± 20 °C) under constant furnace load and stack O2 level for various levels
of O2 enrichment. Results of these tests, shown in Figure 4, indicate that fuel usage (firing
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Figure 4: Furnace firing rate as a function of oxygen enrichment for various
stack oxygen levels. Furnace temperature between 1080 and 1120 °C.
rate) decreases linearly with increasing O2 enrichment level. At ψ O 2 = 0% the average firing
rate required to maintain the furnace set point temperature was 353 kW and at ψ O 2 = 100%
the firing rate decreased to an average value of 212 kW. This represents a potential fuel
savings of ~40% with full oxygen enrichment. A summary of the data at ψ O 2 = 0 and 100%
are also given in Table 1 showing the firing rate data as a function of excess oxidant levels
with stack oxygen concentrations in the range of 0% < O2 < 2.0% and 2% < O2 < 4%. These
results and the data in Figure 4 show a modest effect of excess oxidant level on the required
firing rate. As expected, the firing rate tends to increase with increased stack O2, but this
trend was only evident at lower values of oxygen enrichment, ψ O 2 < ~30%. At higher
oxygen enrichment levels this trend was not evident within the experimental error associated
Table 1: Summary of the furnace firing rate conditions
(furnace target temperature of 1100 °C) and potential fuel savings as a function oxygen
enrichment level and stack oxygen concentration.
ψ O 2 = 0%
ψ O 2 = 100%
Stack O2,
% w.b.
Firing rate range, kW,
Average firing rate, kW,
(number of tests)
Firing rate range, kW,
Average firing rate, kW,
(number of tests)
Potential
fuel savings
0 < O2 < 2.0%
331 – 358 kW,
344 kW,
(17)
214 – 224 kW,
219 kW,
(2)
36%
2.0 < O2 < 4.0%
365 – 372 kW,
366 kW,
(10)
202 – 224 kW,
211 kW,
(10)
42%
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with these measurements. The corresponding potential fuel savings was 36% at lower excess
oxidant levels (0.0% < stack O2 < 2.0%) and 42% at higher excess oxidant levels (2.0% <
stack O2 < 4.0%).
A large portion of the experimental work was dedicated to examining effects of excess
oxidant and O2 enrichment level, on NOX emissions. Results for the base case of NOX
emissions as a function of excess oxidant with no oxygen enrichment are shown in Figure 5.
These emission levels are consistent with the results reported by Sobiesiak et al. (1998) for
the CGRI burner with low air preheat (the air temperature in the present work was relatively
constant at ~ 50 °C). The results were typically 8 – 10 ppm (w.b.) at low stack oxygen
concentrations, increasing linearly up to about 14 ppm at 4 % stack oxygen concentration.
The firing rate was adjusted to maintain a constant furnace temperature as the O2 enrichment
and the excess oxidant was varied. This provided a good basis for comparison of NOX levels
across the data set. Figures 6 and 7 display the data in two slightly different, but revealing
ways. Figure 6 permits one to examine the effects of oxygen enrichment on NOX production,
while effects of excess oxidant level on NOX emissions can be more clearly seen in Figure 7.
The data in Figure 7 include all the data shown in Figures 4 and 6 as well as additional
observations at stack oxygen concentrations exceeding 4.0% (w.b.). Furnace conditions were
near steady state with an average refractory temperature in the range <Tr> = 1080 – 1120 °C,
no cooling panels were exposed and no air infiltration was permitted.
Figure 6 shows how NOX emissions varied with O2 enrichment levels for various ranges of
stack oxygen level. The graph demonstrates that there is no dramatic increase in NOX
emissions with increasing O2 enrichment. NOX emissions, in fact, appear to remain relatively
constant in the O2 enrichment range of 0 – 60%. This is somewhat different from the case
with conventional O2-enriched burners, where a sharp increase in NOX emissions is
encountered.
Conventional oxygen-enriched burners produce a much hotter flame than
conventional air-only burners. Emissions of NOX are sensitive to temperature and although
nitrogen available for conversion to NOX decreases with increased O2 enrichment, NOX
emissions rise due to the increased peak temperature. The CGRI O2-enriched burner is a
dilute combustion technology which exhibits much lower peak temperatures than typical O2enriched burners. The relatively low NOX emission levels observed for the CGRI O2-
IFRF Combustion Journal
Article No 200404
Figure 5: NOX emission as a function of stack oxygen level with
ψO2= 0 % and the furnace temperature between 1080 and 1120 °C.
Figure 7: NOX production as a function of stack oxygen concentration for
various levels of oxygen enrichment. Furnace temperature between 1080
and 1120 °C.
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Poirier, Grandmaison, Lawrence et. al.
October 2004
Figure 6: NOX production as a function of oxygen enrichment for various
stack oxygen levels. Furnace temperature between 1080 and 1120 °C.
Figure 8: NOX production as a function of the furnace nitrogen
concentration for various levels of stack oxygen concentration. Furnace
temperature between 1080 and 1120 °C.
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enriched burner with 0 – 60% O2 enrichment are due to these lower peak “flame”
temperatures.
While local gas temperatures were not measured in the furnace, the
temperature distribution of the refractory surfaces, described later in this section, and the
results of Wünning and Wünning (1997) support this observation.
As O2 enrichment levels increase beyond 60% enrichment, Figure 6 shows that NOX
emissions decrease for all levels of excess oxidant. This is expected since, as even with
conventional burners, when firing with nearly pure oxygen, nitrogen available for conversion
to NOX is significantly reduced, resulting in lower NOX production. One expects NOX
production to drop to zero when pure O2 (100% O2 enrichment) is the only oxidant used.
This is not the case for the results displayed in Figure 6. Although no nitrogen from air is
available for conversion to NOX, there is nitrogen entering the furnace from the fuel, natural
gas. In our case, approximately 1.6% of the fuel is nitrogen. This fuel-nitrogen is sufficient
for production of the NOX levels observed at 100% O2 enrichment.
Although there is no pronounced trend in NOX production with O2 enrichment level, the
difference in the NOX levels between different excess oxidant levels is obvious. Higher
levels of NOX emissions are observed as the stack oxygen level increases. This trend is
clearly demonstrated in Figure7 where the NOX emissions are presented as a function of the
stack oxygen level for various ranges of O2 enrichment. This figure clearly shows the
relative effects of excess oxidant and O2 enrichment on NOX emission levels. It is evident
that excess oxidant is influential for all levels of O2 enrichment, while O2 enrichment is only
influential at levels above 60% enrichment.
The NOX production rate as a function of the stack N2 level is shown in Figure8. Riley et al.
(2000) reported results of an oxygen enrichment study with dilute oxygen combustion. They
suggested that an increase of 10% nitrogen in the furnace gas leads to an increase of about
60% in NOX emissions. In the present work this trend appears to be valid up to nitrogen
concentrations of about 50% N2 (w.b.) after which the NOX levels remain relatively constant
or decrease slightly.
The results of tests to examine the effect of air infiltration on NOX emissions are shown in
Table 2. For these trials, the furnace was operated at a firing rate of 212 kW, 100% oxygen
enrichment and 10% excess oxidant.
The first row of data corresponds to the no-air-
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infiltration case with a furnace operating pressure of +1.27 mm H2O, the second row of data
corresponds to the same furnace settings with a negative pressure of –1.27 mm H2O. The
resulting air infiltration, quantified by the increase of stack O2 from 5.5% to 7.5% by volume
corresponds to an air infiltration rate equal to 20% of the volumetric burner feed. The third
row of data in Table 2 corresponds to an infiltration rate equal to 43% of the burner
volumetric feed. The increase in NOX with these levels of air infiltration corresponds well
with the results for similar stack O2 levels and effective O2 enrichment levels without air
infiltration shown in Figures 6 – 8.
Table 2. NOx emissions at three furnace operating pressures at constant firing rate
10% excess oxidant and 100% oxygen enrichment.
Furnace pressure, mm
H2O
Firing rate, kW
ψ O2 , %
Stack O2, %
w.b.
<Troof>arith, °C
NOX, mg/MJ
+1.27
212
100
5.5
1117
5.7
-1.27
211
100
7.5
1115
7.2
-2.54
211
100
9.0
1113
10.2
Temperature distribution is an important aspect of furnace performance and is of particular
interest here, since oxygen-enriched combustion typically results in intensified (hotter)
combustion zones. The best of the current data sets available for studying the effects of
oxygen enrichment on furnace temperature distribution were trials where the furnace
operating conditions where maintained at constant levels for extended periods of 4 – 8 hours.
Refractory surface-temperatures were continually logged throughout each trial for the furnace
roof and blind sidewall (opposite the burner sidewall), Figure 2. Data showing the values for
the roof and sidewall arithmetic area-mean temperature
Tr
arith
= (1/S)∫ Tr dS
S
and the radiative area-mean temperature
Tr
rad
(
= (1/S)∫ Tr4 dS
S
)
1/ 4
at different stack oxygen levels are shown in Table 3. The standard deviation of the local
refractory temperatures from the arithmetic mean, Tr
σ
, are also shown in this table. At
ψ O 2 = 0%, estimates of the standard deviation ranged from 19 – 27 °C and slightly higher
values were observed at ψ O 2 = 90% with Tr
σ
in the range 31 – 34 °C. The stack oxygen
concentration did not appear to have a significant effect on Tr
σ
.
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Figure 9: Difference in local temperatures from the arithmetic area-mean for the interior surface of the
furnace (furnace roof in top diagram; blind sidewall in bottom diagram) for various levels of oxygen
enrichment. Stack oxygen level of 2% w.b.; temperature units are °C. The data at each location are
ordered (top-to-bottom) with the results for ψO2= 0, 24.4 %, 49.8 % and 90.0 % respectively.
Temperature mapping for the furnace operating at 2 % stack oxygen (w.b.) expressed in
terms of the difference in local temperature from the arithmetic area-mean value is shown in
Figure 9.
With the single burner operation employed in this work, the refractory
temperatures exhibited positive deviations from the arithmetic area-mean value along furnace
roof downstream from the burner. A maximum positive deviation was observed in the corner
junction of the furnace roof and the blind sidewall directly opposite from the burner.
Minimum values were observed near the exhaust plenum and the lower parts of the blind
side-wall. This trend was consistent for all levels of oxygen enrichment and the deviations
from the arithmetic area-mean were larger at higher oxygen enrichment levels. This trend is
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indicated by the results shown in Figure 9 and the higher standard deviations from the mean
value noted in Table 3 at ψ O 2 = 90 %.
Table 3: Arithmetic area-mean temperature, radiative area-mean temperature and standard deviation
of the arithmetic area-mean temperature from the mean value for the roof and blind sidewall refractory
surfaces. Results are shown for different oxygen enrichment and stack oxygen levels.
Stack oxygen = 1 %, w.b.
ψ O2 , %
<Troof>arith., °C
<Troof>rad., °C
<Troof> , °C
0.0
1099
1099
23.0
24.6
1100
1101
28.2
51.6
1106
1107
33.0
91.1
1118
1119
31.9
ψ O2 , %
<Tsidewall>arith., °C
<Tsidewall>rad., °C
<Tsidewall> , °C
0.0
1106
1107
27.2
24.6
1108
1109
29.8
51.6
1110
1111
33.9
91.1
1123
1124
33.0
Stack oxygen = 2 %, w.b.
ψ O2 , %
<Troof>arith., °C
<Troof>rad., °C
<Troof> , °C
0.0
1100
1100
22.1
24.4
1103
1104
28.1
49.8
1108
1110
30.5
90.0
1113
1115
32.0
<Tsidewall>arith., °C
<Tsidewall>rad., °C
<Tsidewall> , °C
0.0
1107
1107
24.6
24.4
1108
1109
33.0
49.8
1116
1117
31.4
90.0
1117
1118
33.9
,%
Stack oxygen = 4 %, w.b.
ψ O2 , %
<Troof>arith., °C
<Troof>rad., °C
<Troof> , °C
0.0
1083
1084
19.4
24.7
1101
1101
23.1
49.8
1108
1109
27.3
90.1
1103
1104
31.1
ψ O2 , %
<Tsidewall>arith., °C
<Tsidewall>rad., °C
<Tsidewall> , °C
0.0
1091
1092
22.8
24.7
1108
1108
26.8
49.8
1114
1115
29.5
90.1
1081
1090
30.8
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CONCLUSIONS
The objective of this work was to test a modified version of the CGRI low NOX burner with
oxygen-enriched combustion in the CAGCT research furnace at 1100 °C. This technology
has potential to reduce energy costs and emissions of CO2 and NOX. The reduction in CO2
emissions arises directly from the expected savings in fuel with O2-enrichment and potential
savings of ~ 40% in fuel usage at 100% oxygen enrichment were observed. NOX emissions
up to ~12 mg NOx/MJ were observed with this modified version of the CGRI burner.
Oxygen-enrichment had little effect on NOX emission up to an enrichment level of about
~60%. At higher oxygen-enrichment, emission levels decreased but not to zero because of
fuel nitrogen present in the natural gas supply. NOX emission increased with increasing stack
oxygen concentration (up to ~ 6% O2 w.b. in the present work) at all oxygen levels. Air
infiltration also had an effect on NOX levels leading to emissions similar to those observed
with no air infiltration but with similar stack oxygen concentrations. Oxygen enrichment
level had the most significant effect on the temperature distribution of the roof and blind side
wall of the furnace. The standard deviation of the temperature variation was in the range, 19
– 27 °C with no oxygen enrichment and 31 – 34 °C with 90% oxygen enrichment.
ACKNOWLEDGEMENT
This work was performed under the U.S. Department of Energy (DOE) / American
Iron
and
Steel
Institute
(AISI)
Cooperative
Agreement
DE-FC07-97ID13554,
Technology Roadmap Research Program for the Steel Industry.
The support and
participation of Air Liquide Corporation, BOC Gases, Dofasco Inc., Fuchs Systems
and Stelco Inc. in this program is greatly appreciated.
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•
Baukal, C.E., Oxygen-Enhanced Combustion, CRC Press, New York, 1998.
•
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•
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