Pure Oxygen Combustion

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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 TechnologyQueen’s University

Kingston, ON, K7L 3N6Canada

1IFM – KemiteknikLinkopings 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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IFRF Combustion Journal - 2 - Poirier, Grandmaison, Lawrence et. al.

Article No 200404 October 2004

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 asingle 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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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.,

Burner

3000

0

1000

500

1362

0

3362

-362 750

-362

0

Water-cooled floor panels

4500

Refractory

SideView

54625100

FurnaceExhaust

Plenum Wall

TopView

Figure 1: CAGCT research furnace shown with the single sidewall mountedburner used in the present study. All dimensions in mm.


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45000 750-362 54625100

3496

3000

T26

T25

T24

500

1000

0127

873

3362

1362

T31

T30

T29

1004496 1496

T27

T28

T36

2004 2750

T37

T32 T34

T33T35

T38

17501047453 1995 2996

890

2000

2854

2110

1000

1797

1500

1203

-362

0146

797

T2 T9 T14 T17

T13

T3 T10

T4 T11

T6T7

T15 T18

T19

T12T1

T5

T8

Burner

T16

4254

T40

T39

T41

3996

T21

T23

T22

T20

Figure 2: Location of refractory-wall thermocouples in the CAGCT Research Furnace – topfigure shows refractory roof, bottom figure shows blind-sidewall opposite the burner.

Pilot burner portUV scanner port

Fuel nozzle

Air/oxidant nozzle

Figure 3: Schematic diagram of theCGRI showing the locations for theair/oxidant and fuel nozzles.


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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 NO X levels.

Oxygen enrichment level,2O

ψ , is defined as

2

2

2 2

O

O

O O A

m 100

m + mψ

= ×

&

& &

where2O

m& and2O A

m& are the mass flow rates for the pure oxygen and oxygen associated with

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 O2-

enriched 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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rate) decreases linearly with increasing O2 enrichment level. At2O

ψ = 0% the average firing

rate required to maintain the furnace set point temperature was 353 kW and at2O

ψ = 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 2Oψ = 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,2O

ψ < ~30%. At higher

oxygen enrichment levels this trend was not evident within the experimental error associated

Figure 4: Furnace firing rate as a function of oxygen enrichment for various

stack oxygen levels. Furnace temperature between 1080 and 1120 C.

Table 1: Summary of the furnace firing rate conditions

(furnace target temperature of 1100 C) and potential fuel savings as a function oxygenenrichment level and stack oxygen concentration.

2Oψ = 0%

2Oψ = 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)

Potentialfuel 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 additionalobservations at stack oxygen concentrations exceeding 4.0% (w.b.). Furnace conditions were

near steady state with an average refractory temperature in the range = 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 O2-

enriched burners. The relatively low NOX emission levels observed for the CGRI O2-


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IFRF Combustion Journal - 9 -

Article No 200404

Figure 5: NOX emission as a function of stack oxygen level with

O2= 0 % and the furnace temperature between 1080 and 1120 C.

Figure 6: NOX production as a function

stack oxygen levels. Furnace tempera

Figure 7: NOX production as a function of stack oxygen concentration forvarious levels of oxygen enrichment. Furnace temperature between 1080

and 1120 C.

Figure 8: NOX production as a funconcentration for various levels of stac

temperature between 1


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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 isavailable 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 rate10% excess oxidant and 100% oxygen enrichment.

Furnace pressure, mmH2O

Firing rate, kW2Oψ , % Stack O

2, %w.b.

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

( )∫= S r arithr dST1/ST

and the radiative area-mean temperature

( )( ) 4/1S

4

r radr dST1/ST ∫=

at different stack oxygen levels are shown in Table 3. The standard deviation of the local

refractory temperatures from the arithmetic mean,σ r

T , are also shown in this table. At

2Oψ = 0%, estimates of the standard deviation ranged from 19 – 27 °C and slightly higher

values were observed at2O

ψ = 90% withσ r

T in the range 31 – 34 °C. The stack oxygen

concentration did not appear to have a significant effect onσ r

T .


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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

Figure 9: Difference in local temperatures from the arithmetic area-mean for the interior surface of thefurnace (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 O2= 0, 24.4 %, 49.8 % and 90.0 % respectively.


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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 at2O

ψ = 90 %.

Table 3: Arithmetic area-mean temperature, radiative area-mean temperature and standard deviationof 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.

2Oψ , % arith., °C rad., °C , °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

2Oψ , % arith., °C rad., °C , °C0.0 1106 1107 27.2

24.6 1108 1109 29.8

51.6 1110 1111 33.9

91.1 1123 1124 33.0



0.0 1100 1100 22.1

24.4 1103 1104 28.1

49.8 1108 1110 30.5

90.0 1113 1115 32.0

, % arith., °C rad., °C , °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



0.0 1083 1084 19.4

24.7 1101 1101 23.1

49.8 1108 1109 27.3

90.1 1103 1104 31.1


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.

REFERENCES

• Baukal, C.E., Oxygen-Enhanced Combustion, CRC Press, New York, 1998.

• Besik, F.K., Rahbar, S., Becker, H.A. and Sobiesiak, A., U.S. Patent Application No.08/562,999, Nov. 1995 and International Patent Application No. PCT/CA96/00334, May

24, 1996.

• Delabroy, O., Louédin, O., Tsiava, R., Le Gouefflec, G. and Bruchet, P., “Oxycombustionfor Reheat Furnaces” Major Benefits Based On ALROLLTM, A Mature Technology”,


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AFRC/JFRC/IEA 2001 Joint International Combustion Conference, Kauai, Hawaii, Sept.

9-12 (2001).

• De Lucia, M., Journal of Energy Resources Technology, 113: 122 (1991).

• Grandmaison, E.W., Yimer, I., Becker, H.A. and Sobiesiak, A., Combustion and Flame,114: 381 (1998).

• Marin, O., Bugeat, B., Macadam, S. and Charon, O., “Oxygen Enrichment in Boilers”,AFRC/JFRC/IEA 2001 Joint International Combustion, Kauai, Hawaii, Sept. 9-12 (2001).

• Milani, A. and Saponaro, A., Diluted Combustion Technologies, IFRF CombustionJournal, Article Number 200101, February 2001.

• Poirier, D., Grandmaison, Matovic, M.D., Barnes, K.R. and Nelson, B.D., “High

Temperature Oxidation of Steel in an Oxygen-enriched Low NOX Furnace Environment”,AFRC/JFRC 2004 Joint International Combustion Conference, Maui, Hawaii, Oct. 10-13

(2004).

• Riley, M.F., Ryan, H.M. and Kobayashi, H., “Application of dilute oxygen combustion(DOC) technology for steel reheating furnaces”, American Flame Research Committee

(AFRC) International Symposium, Newport Beach, CA, USA (2000).

• Sobiesiak, A., Rahbar, S. and Becker, H.A., Combustion and Flame, 115: 93 (1998).

• Wünning, J.A. and Wünning, J.G., Prog. Energy Combust. Sci., 23: 81 (1997).

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