Combustion and Flame 157 (2010) 1132–1139

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Characterisation of an oxy-coal flame through digital imaging

John Smart a, Gang Lu b,*, Yong Yan b, Gerry Riley a

a RWE npower plc, Windmill Hill Business Park, Whitehill Way, Swindon SN5 6PB, UKb Instrumentation, Control and Embedded Systems Research Group, School of Engineering and Digital Arts, University of Kent, Canterbury, Kent CT2 7NT, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 August 2009Received in revised form 21 October 2009Accepted 28 October 2009Available online 1 December 2009

Keywords:Pulverised coalOxy-fuelFlame monitoringCCD cameraImage processing

0010-2180/$ - see front matter � 2009 The Combustdoi:10.1016/j.combustflame.2009.10.017

* Corresponding author. Fax: +44(0)1227456084.E-mail addresses: John.Smart@RWEnPower.com (J

Lu), y.yan@kent.ac.uk (Y. Yan), Gerry.Riley@RWEnPow

This paper presents investigations into the impact of oxy-fuel combustion on flame characteristicsthrough the application of digital imaging and image processing techniques. The characteristic parame-ters of the flame are derived from flame images that are captured using a vision-based flame monitoringsystem. Experiments were carried out on a 0.5 MWth coal combustion test facility. Different flue gasrecycle ratios and furnace oxygen levels were created for two different coals. The characteristics of theflame and the correlation between the measured flame parameters and corresponding combustion con-ditions are described and discussed. The results show that the flame temperature decreases with therecycle ratio for both test coals, suggesting that the flame temperature is effectively controlled by the fluegas recycle ratio. The presence of high levels of CO2 at high flue gas recycle ratios may result in delayedcombustion and thus has a detrimental effect on the flame stability.

� 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Oxy-fuel combustion with CO2 capture from flue gas is regardedas a near-zero emission technology that can be adapted to bothnew and existing coal-fired power plant. In an oxy-fuel firing sys-tem, pulverised fuel is fired in re-circulated flue gas mixed withpure oxygen to produce a high concentration of CO2 in the fluegas stream which can be sequestrated without costly flue gas sep-aration [1–4]. Oxy-fuel combustion can also reduce significantlythe NOx emission as the recycled NO is reduced or reburnt whenit is re-circulated through the system. SOx emissions per energyof fuel combusted will also be reduced by sulphur retention inash and deposits [1,5]. However, switching from conventionalair–fuel to oxy-fuel brings a number of technical challenges tocombustion engineers and plant operators. The increased concen-trations of CO2 and water vapour in the flue gas would substan-tially increase the emissivity of the furnace gas and thus increasethe radiative heat transfer in the furnace [2,3,6]. In addition, thedifferent heat capacity and densities of the main gases, i.e., N2

and CO2, will change the mass flows and velocities of the primaryand secondary flows (to attain a similar adiabatic flame tempera-ture to the air-firing situation), and thus burner aerodynamics,resulting in changes in fuel ignition properties, flame propagation,shape, and residence time [2,3,6]. Other problems with oxy-fuelcombustion include high concentrations of sulphur and mercury

ion Institute. Published by Elsevier

. Smart), g.lu@kent.ac.uk (G.er.com (G. Riley).

and changes in deposition and corrosion in the combustion cham-ber and flue pass [3]. Therefore, oxy-fuel combustion presents avery different picture from the conventional coal–air combustion.

Oxy-fuel combustion technology is still in its laboratory anddemonstration stages. Studies have been undertaken on the under-standing of the oxy-fuel combustion process in terms of thechanges in flame characteristics (in particular fuel ignition, flametemperature and stability), furnace radiative and convective heattransfer, and flue gas compositions in comparison with the air firedprocess [7–13]. Bejarano and Levendis [7] conducted a fundamen-tal investigation into the combustion of single coal particlesburned in a vertical drop-tube furnace under both O2/N2 and O2/CO2 environments. Andersson et al. [8] investigated the differencein radiative heat transfers between oxy- and air–coal flames on apilot-scale oxy-fuel test facility through studies on flame gas tem-perature (measured by a suction pyrometer) and line-of-sight totalradiation intensity (obtained using a narrow-angled radiometer).Khare et al. [9] studied the ignition characteristics of an oxy-coalflame on a 1.2 MWth test furnace, where the measured gas temper-ature and CFD (computational fluid dynamics) modelling wereused to infer the mechanisms of flame ignition changes. Toporovet al. [10] also conducted numerical and experimental investiga-tions into the underlying mechanisms of the aerodynamics of a pi-lot-scale oxy-coal swirl flame by measuring gas velocity, gas andparticle temperatures and gas compositions. Nozaki et al. [11] per-formed numerical simulations and experiments to investigate theignition stability of an O2/CO2 coal flame on a bench-scale burner,where the gas temperature and emissions along the furnace axiswere measured and compared with data for air combustion. Re-search work related to oxy gaseous fuel flames includes measure-

Inc. All rights reserved.

J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139 1133

ments of temperature and species concentrations of O2/H2 flamesusing Raman/Rayleigh spectroscopic techniques [12] and radiationintensity investigations on propane-fired O2/CO2 flames under dif-ferent flue gas recycle rates and oxygen concentrations [13]. Otheractivities related to oxy-fuels on large-scale furnaces were also re-ported by McCauley et al. [14], Zanganeh and Shafeen [15].

Although various advances have been made in oxy-fuel com-bustion, there is still a gap in knowledge between the currentstate and the requirement for future application, due to thecomplicated underlying mechanisms. Current flame visualisationand characterisation techniques have formed an essential basefor an in-depth understanding of the oxy-fuel combustion pro-cess. The visualisation techniques have been deployed to studycoal-fired flames in both laboratory and industrial environmentsfor a variety of applications [16,17,19–23]. Such techniques areconjoined with advanced optical sensing, digital image process-ing and soft computing algorithms, offering a two-dimensionaland non-intrusive means for on-line continuous monitoringand characterisation of flames. This paper is focused on theuse of flame visualisation techniques to investigate the charac-teristics of an oxy-fuel flame. The characteristic parameters ofthe flame including particle temperature and oscillation fre-quency were determined from flame images that were capturedusing a vision-based flame monitoring system. Experimentswere carried out on a 0.5 MWth coal combustion test facility.Two types of pulverised coal were tested under different fluegas recycle ratios and furnace oxygen levels. The characteristicsof the flames and the correlation between the measured flameparameters and the corresponding combustion conditions areinvestigated.

2. Methodology

2.1. Flue gas recycle

Flue gas recycle is the process where the flue gas is recycledback into the furnace to establish similar heat flux profiles in thefurnace as in conventional air-firing. Fig. 1 is the schematic of anoxy-fuel combustion system with flue gas recycle. In oxy-fuel com-bustion, the fuel is burnt in a mixture consisting of oxygen pro-duced in a cryogenic air separation unit (ASU) and recycled fluegas, giving a flue gas consisting mainly of CO2 and H2O. The recy-cled flue gas can be either wet or dry, depending on where therecycled flue gas is taken from in the system.

One of the most important parameters in a practical oxy-fuelsystem is the recycle ratio which is defined as,

Recycle ratio ¼ Recycled gas mass flowrateRecycled gas mass flow rateþ Product gas mass flow rate

� 100%:

ð1Þ

N2

Recycled flue gas

Air

Recycle

O2

O2

ASU

Primary flow

Secondary flow

Pulverized fuel

Fig. 1. Schematic of a typ

In the present study, recycled flue gas was simulated by usingCO2. The thermal input to the furnace (i.e., coal feed rate) andthe primary gas flow (CO2 + O2) were fixed. The secondary flow(CO2 + O2) was used as a rig variable in order to establish differentrecycle ratios, secondary flow rates and percentages of O2.

2.2. Combustion test facility

The experimental furnace used for the work was the RWEnpower’s 0.5 MWth combustion test facility (CTF). As shown inFig. 2, the CTF has a refractory lined combustion chamber withan inner cross-section of 0.8 m � 0.8 m and approximately 4 mlong. A water jacket layer is fitted to the outside of the chamberto remove the input energy. A 0.5 MWth International Flame Re-search Foundation (IFRF) aerodynamically air staged burner wasfitted and operated under the baseline operating condition, i.e.,without using the burner’s low NOx capability. The furnace alsohas a number of viewing ports on the centre line of the sidewall,allowing various measurements/sampling to be taken. The CTFwas originally designed for air-firing. It had been retrofitted tothe once-through oxy-fuel system so as to make it easier to useand more flexible to simulate different recycle configurations,e.g., both wet and dry flue gas, changing oxygen levels in variousgas streams. In the present study, however, only the dry casewas evaluated.

2.3. Flame parameters and measurement methods

A flame can be characterised by its physical parameters such assize, shape, brightness, temperature and oscillation frequency,depending upon the type of furnace and the target of interest. Inthis study, two parameters which were measured and used forflame characterisation are flame temperature and oscillation fre-quency. A vision-based multi-functional flame monitoring system,which was previously described [16], was used to measure the twoflame parameters. The system uses CCD (charge-coupled device)cameras to visualise the flame. The flame temperature is computedfrom flame images based on the two-colour pyrometry [17]. Astwo-colour pyrometry is derived from the Planck’s radiation law,the temperature computed is considered to be the weighted tem-perature of solid particles in the flame such as soot and char parti-cles. It has been recognised that, due to the physical and chemicalvariability of the particles involved in the combustion, solid parti-cles in a flame may have different temperatures. This difference de-pends upon many factors such as the physical and chemicalproperties of the particles, the rates of heat transfer between theparticles, and between particles and surrounding gas. In-depthstudies on the radiative properties of soot and coal particles andtheir effects on the flame temperature can be found elsewhere[18].

The oscillation frequency of the flame is the weighted averagefrequency of the reconstructed flame signal obtained from a high

Boiler

Wet recycle

d flue gas

Dry recycle

CO2 rich flue gas

H2O

ical oxy-fuel system.

Cooling loop

Flame imaging system and its location

PF & primary flow (CO2+O2)

Secondary flow (CO2+O2)

Gas gun

Flame

Water jacket

Fig. 2. Schematic of 0.5 MWth combustion test facility and the location of the flame imaging system.

1134 J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139

speed camera (up to 360 frames per second) [19]. The frequenciesin the root and middle regions of the flame are computed (shownas ‘Root’ and ‘Mid’ in results, respectively). The flame data, togetherwith furnace input data such as fuel type, recycle ratio and furnaceoxygen level are then used to assess the quality of the flame andhence the oxy-firing process.

In the tests the optical probe and CCD camera were installed atan existing viewing port close to the front wall of the furnace, asshown in Fig. 2. The resulting field of view of the imaging systemwas approximately 0.8 m in diameter along the burner axis. Undersuch an installation the root region of the flame, which was re-garded as the primary reaction zone of the combustion in termsof energy conversion and emission formation, was fully observed.The imaging system was calibrated prior to the tests using a tung-sten lamp as a standard temperature source. The relative error ofthe temperature measurement is no greater than 1% over the rangebetween 1280 and 1690 �C for a standard temperature source [16].However, due to the fact that there is no standard flame tempera-ture available, the actual accuracy of the flame temperature mea-surement is expected to be greater than this but it is verydifficult to quantify [20]. The reproducibility of the flame imagingsystem and the test furnace was evaluated by carrying out dupli-cate tests using a test coal (Russian coal, Table 2) at two differenttest times. The test results are shown in Table 1. The difference be-tween the measured flame temperatures for the same test condi-tions is no greater than 2.3% for the maximum temperature and1.8% for mean temperature of the flame measured. It is, however,noted that the flame temperature measured by the imaging systemis slightly affected by the furnace temperature, i.e., the tempera-ture measured later on the test day appears to be higher than thatrecorded earlier the same day [20]. Nevertheless, the results showthat the flame imaging system generally has a good reproducibil-

Table 1Reproducibility test (*: standard deviation).

Test Test condition

Recycle ratio (%) Furnace O2 (% mass) Total O2 (kg/h)

A 65 34.8 153.3B 65 34.8 153.3

ity, and therefore the variations in the flame characteristics ob-tained from the measurements in this study reflect the impact ofthe oxy-fuel operation on the flame.

3. Experimental programme

3.1. Properties of coals

Two supplies of coal, Russian and South African coals, weretested. The proximate and ultimate analyses of the coals are givenin Table 2. The coal particle size distribution was nominally 75%through 75 lm.

3.2. Test conditions

The test conditions for the two coals, together with the mea-sured furnace exit oxygen levels and temperature are summarisedin Tables 3 and 4. For all test conditions, the stoichiometry ratio(i.e., oxygen (kg) to coal (kg) ratio) remained effectively constantfor the low and high furnace exit O2 conditions respectively. Withvarying the recycle ratio for each of the corresponding furnace exitO2 conditions, the mass flow through the furnace changed but theO2 flow did not. As the recycle ratio was reduced the mass flowthrough the furnace reduced so the oxygen enrichment level in-creased. The thermal input to the furnace, i.e. the coal feeding rate,was fixed and the primary CO2 + O2 flow was set at 155 kg/h withan O2 content of 16.2% by mass. The secondary flow varied depend-ing upon the flow rate, recycle ratio and percentage of O2 (bymass). The total CO2 and O2 shown in the tables are the combina-tions of CO2 and O2 in the primary and secondary flows, respec-tively. The temperatures of the primary and secondary flowswere maintained at 70 �C and 270 �C respectively for all test condi-

Flame temperature (�C)

Total CO2 (kg/h) Max. r* Mean r*

369.7 1599 4.3 1502 7.9370.9 1571 15.7 1468 16.6

Table 2Proximate and ultimate analyses of coals (as received and averaged).

Russian coal South African coal

Moisture (%) 4.75 4.50Volatile matter (%) 29.08 25.71Ash (%) 14.12 12.62Fixed carbon (%) 52.05 57.17Carbon (%) 66.72 69.99Hydrogen (%) 4.52 3.90Nitrogen (%) 2.00 1.73Sulphur (%) 0.36 0.57Oxygen (%) 12.28 11.19Gross calorific value (kJ/kg) 26772.90 27288.30

J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139 1135

tions. During the tests, different simulated flue gas recycle ratios,i.e., 75%, 72%, 70%, 68%, 65% and 62%, were created for the two testcoals. For each recycle ratio, two oxygen enrichment levels (shownas high and low O2 settings in results) were generated. The second-ary flow rate, CO2 and O2 levels for the simulated flue gas recycleratio and exit O2 level were also calculated for each test condition.In addition to a wide range of oxy-fuel input conditions, the burnerwas also operated under air-firing conditions for the Russian coalto generate baseline comparative data.

4. Results and discussion

4.1. Temperature

The flame temperature profiles of the two test coals under oxy-firing conditions are shown in Figs. 3 and 4 whilst the temperature

Table 3Test conditions for the Russian coal.

Recycle ratio (%) Furnace O2 setting Total flow Total O2

kg/h % by mass

62 Low 477.9 31.8High 487.5 33.4

65 Low 523.0 29.3High 533.0 30.9

68 Low 577.7 26.6High 590.5 28.5

72 Low 669.6 23.4High 681.9 25.4

75 Low 757.1 21.0High 772.1 22.8

Air only 640.4 23.3Air only 667.9 23.3Air only (baseline) 698.2 23.3Air only 745.7 23.3

Table 4Test conditions for the South African coal.

Recycle ratio (%) Furnace O2 setting Total flow Total O2

kg/h % by mass

65 Low 554.8 30.3High 567.7 32.0

68 Low 615.7 28.2High 624.4 29.5

70 Low 656.9 25.9High 670.9 27.8

72 Low 709.1 24.3High 722.6 26.2

profiles of the Russian coal flame under air-firing are illustrated inFig. 5. Note that the temperature profile shown is the average of 10instantaneous results over a period of 120 s so as to represent asteady test condition. As can be seen, for both test coals, the flametemperature and its profile vary with the recycle ratio with clearpatterns, i.e., the temperature decreases with the flue gas recycleratio, indicating that the flame temperature is effectively con-trolled by the recycle ratio. Comparing oxy- and air-firing flamesunder similar oxygen inputs, for instance, the oxy-firing with a72% recycle ratio (low furnace O2 setting) and the air-firing base-line (i.e., 2.9% exit O2), indicates that the oxy-flame is darker thanthe air-flame due to CO2’s capability to absorb radiation. The oxy-flame is also more compact, i.e., smaller in size. As a result, if thecombustion feeding gas is composed of 23% of O2 in mass (similarto air) with the rest being flue gas, the flame temperature is foundto be significantly lower than that in the air-firing and, in certaincases, it is not even possible to maintain a stable flame.

Figs. 6 and 7 illustrate the variations of the maximum and meanflame temperatures with the recycle ratio, total flow and O2 for thetwo test coals. Each data point is an average of 10 instantaneousvalues and the error bar indicates the standard variation of thedata. The total flow (in mass) and total O2 (in mass) for the Russiancoal test (Fig. 6) are normalised to their respective values of the air-firing baseline (Table 2), i.e.,

Normalised total flow ¼ Total flow of oxy-firingTotal flow of air-firing baseline

: ð2Þ

Normalised total O2 ¼Total O2 of oxy-firing

Total O2 of air-firing baseline: ð3Þ

Total CO2 Exit condition

kg/h % by mass kg/h O2 (% by volume) Temp. (�C)

152.1 68.2 325.8 5.9 1157.9163.0 66.6 324.5 8.1 1194.2

153.3 70.7 369.7 5.8 1250.5164.4 69.2 368.6 7.9 1111.0

153.8 73.4 423.9 6.0 1235.0168.2 71.5 422.3 9.1 1220.9

156.7 76.6 512.9 5.2 1196.5172.9 74.6 509.0 7.6 1209.4

159.0 79.0 598.1 5.2 1154.2176.1 77.2 596.0 8.1 1178.1

1.0 1240.82.0 1227.22.9 1203.23.8 1253.7

Total CO2 Exit condition

kg/h % by mass kg/h O2 (% by volume) Temp. (�C)

168.0 69.7 386.8 7.9 1215.7181.9 68.0 385.8 11.0 1227.3

173.8 71.8 441.9 8.5 1193.7183.9 70.5 440.5 10.0 1208.6

170.2 74.1 486.7 7.0 1152.8186.2 72.2 484.7 9.5 1169.0

172.2 75.7 536.9 6.9 1126.5189.1 73.8 533.5 9.9 1140.1

62% 65% 68% 72% 75%

62% 65% 68% 72% 75%

(a)

(b)

Fig. 3. Temperature profiles for different recycle ratios for the Russian coal: (a) under low furnace O2 settings and (b) under high furnace O2 settings.

(a)

(b)65% 68% 70% 72%

65% 68% 70% 72%

Fig. 4. Temperature profiles for different recycle ratios for the South African coal: (a) under low furnace O2 settings and (b) under high furnace O2 settings.

3.8% 2.9% 2.0% 1.0%

Fig. 5. Temperature profiles for air-firing (volumes of exit O2 in mass) for the Russian coal.

(a)

(b)

100011001200130014001500160017001800

60 65 70 75 80Recycle ratio (%)

Max Mean Exit temp

Tem

pera

ture

(°°C

)

Tem

pera

ture

(°C)

100011001200130014001500160017001800

0.6 0.7 0.8 0.9 1.0 1.1 1.2Normalised total flow

Max MeanAir-firing: Mean Exit temp

100011001200130014001500160017001800

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6Normalised O2

Max MeanAir-firing: Mean Exit temp

Tem

pera

ture

(°C)

100011001200130014001500160017001800

60 65 70 75 80Recycle ratio (%)

Max Mean Exit temp

Tem

pera

ture

(°C

)

100011001200130014001500160017001800

0.6 0.7 0.8 0.9 1.0 1.1 1.2Normalised total flow

Max MeanAir-firing: Mean Exit temp

Tem

pera

ture

(°C

)

100011001200130014001500160017001800

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6Normalised O2

Max MeanAir-firing: Mean Exit temp

Tem

pera

ture

(°C

)

Fig. 6. Variation of the flame temperature with the recycle ratio, normalised total flow and normalised total O2 for the Russian coal: (a) low furnace O2 setting and (b) highfurnace O2 setting.

1136 J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139

(a)

(b)

100011001200130014001500160017001800

60 65 70 75 80Recycle ratio (%)

Max Mean Exit temp

100011001200130014001500160017001800

500 550 600 650 700 750 800 850Total flow (kg/h)

Max Mean Exit temp

1100

1200

1300

1400

1500

1600

1700

166 168 170 172 174 176

Total O2 (kg/h)

Max Mean Exit temp

100011001200130014001500160017001800

60 65 70 75 80Recycle ratio (%)

Max Mean Exit temp

100011001200130014001500160017001800

500 550 600 650 700 750 800 850Total flow (kg/h)

Max Mean Exit temp

1100

1200

1300

1400

1500

1600

1700

180 182 184 186 188 190 192Total O2 (kg/h)

Max Mean Exit temp

Tem

pera

ture

(°C)

Tem

pera

ture

(°C)

Tem

pera

ture

(°C)

Tem

pera

ture

(°C)

Tem

pera

ture

(°C)

Tem

pera

ture

(°C)

Fig. 7. Variation of the flame temperature with the recycle ratio, total flow and total O2 for South African coal: (a) low furnace O2 setting and (b) high furnace O2 setting.

J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139 1137

For a direct comparison, the mean temperature of the flame un-der the air-firing and the exit gas temperature (shown as ‘ExitTemp’) are also plotted in Fig. 6. It is clear that the decrease inflame temperature with the flue gas recycle ratio results in a stee-per gradient for the high O2 setting for both coals. For the Russiancoal flame under the low O2 setting, the mean temperature for alow recycle ratio (62%) can be up to 110 �C higher than that undera high recycle ratio (75%). In contrast, the difference can be up to160 �C under the high O2 input. A similar trend is also evident forthe South African coal flame. These differences are likely to origi-nate from the differences in CO2 and O2 inputs for these tests. Inaddition, the decrease in flame temperature with the total flowand O2 levels is due to the high mass ratio of gas to coal at the highrecycle ratios. The difference between the maximum and meantemperatures is also seen to decrease with the recycle ratio dueto the moderating effect of the increased total flow rate.

A comparison between Figs. 6 and 7 indicates that differences incoal properties may also have an impact on the flame temperature.

(a)

(b)

02468

101214161820

60 65 70 75 80Recycle ratio (%)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

101214161820

0.6 0.7 0.8Normal

RootAir-firing: Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

101214161820

60 65 70 75 80Recycle ratio (%)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

101214161820

0.6 0.7 0.8Normal

Osc

illat

ion

freq

uenc

y (H

z) RootAir-firing: Mid

Fig. 8. Variation of the flame oscillation frequency with the recycle ratio, normalised totafurnace O2 setting.

Higher flame temperatures are observed at high recycle ratios forthe Russian coal which has a high volatile content (Table 2) com-pared to those for the South African coal.

Comparisons between the oxy-firing and air-firing results alsosuggest that, to maintain equivalent flame temperatures in oxyand air combustion, the feeding gas needs to be around 30–35%volume oxygen and 65–68% by volume dry recycled CO2 (for thelow O2 setting). It is thus necessary to increase the concentrationof O2 in the feeding gas to raise the flame temperature and main-tain stable flames. The increased O2 concentration may, dependingupon the way fuel and O2 interact, also improve the flame stabilityand decrease the unburnt carbon content of the ash [3].

4.2. Flame stability

CO2 has distinctive thermodynamic and optical properties com-pared with those of air, and it is therefore important to know theimpact of CO2 on the flame stability. In general, flame stability

0.9 1.0 1.1 1.2ised total flow

MidAir-firing: Root

02468

101214161820

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Osc

illat

ion

freq

uenc

y (H

z) Root MidAir-firing: Mid Air-firing: Root

0.9 1.0 1.1 1.2ised total flow

MidAir-firing: Root

02468

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0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Osc

illat

ion

freq

uenc

y (H

z) Root MidAir-firing: Mid Air-firing: Root

Normalised O2

Normalised O2

l flow and normalised O2 for the Russian coal: (a) low furnace O2 setting and (b) high

(a)

(b)

02468

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60 65 70 75 80Recycle ratio (%)

Root MidO

scill

atio

n fr

eque

ncy

(Hz)

02468

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500 550 600 650 700 750 800 850Total flow (kg/h)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

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166 168 170 172 174 176Total O2 (kg/h)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

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60 65 70 75 80Recycle ratio (%)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

02468

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500 550 600 650 700 750 800 850Total flow (kg/h)

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

Total O2 (kg/h)

02468

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180 182 184 186 188 190 192

Root Mid

Osc

illat

ion

freq

uenc

y (H

z)

Fig. 9. Variation of the flame oscillation frequency with the recycle ratio, the total flow and O2 for the South African coal: (a) low furnace O2 setting and (b) high furnace O2

setting.

1138 J. Smart et al. / Combustion and Flame 157 (2010) 1132–1139

can be characterised by the stability of ignition and propagation ofthe flame. The oscillation frequency measured using the imagingsystem contains information about such characteristics and is thusused as an indicator of the flame stability [19]. Figs. 8 and 9 illus-trate the variation of flame oscillation frequency with the recycleratio, total flow and total O2 for the two test coals. The results fromthe air-firing baseline are also plotted against the normalised totalflow for the Russian coal. In general, the flame oscillation fre-quency decreases with the recycle ratio, indicating a high recycleratio has an adverse effect on the flame stability. The fact thatthe oscillation frequency is lower in the flame root region than inthe middle region in most cases also indicates that the oxy-firingresults in unstable ignition of the fuel. It is also noted that the oscil-lation frequency of the oxy-flame in both root and middle regionsis much lower than that under the air-firing. This would result inlower flame propagation speed in the oxy environment than inair-firing. The delayed flame ignition in the oxy-fuel combustionwould be attributed to the high heat capacity of CO2. The ignitiondelay in the oxy-firing can also be explained by the combined ef-fect of the properties of combustion gases [8].

5. Conclusions

Investigations into the impact of oxy-fuel combustion on flamecharacteristics in a coal combustion test facility have been pre-sented. The characteristic parameters of the flame, including tem-perature and oscillation frequency, are derived from flame imagesthat are captured using a vision-based monitoring system. Two dif-ferent coals were tested under a variety of recycle ratios and fur-nace oxygen volumes. The results have revealed that thetemperature of the flame is effectively controlled by the flue gas re-cycle ratio. The flame temperature decreases with the recycle ratiofor the two test coals. The decrease in flame temperature with theflue gas recycle ratio shows a steeper gradient for the high O2 levelsetting and is likely to be attributable to the differences in CO2 andO2 inputs. It is also found that the differences in coal propertieshave an impact on the flame temperature. High flame tempera-tures were observed for high volatile content coal (Russian coal)at high recycle ratios compared to those for the low volatile con-tent coal (South African coal). In addition, the flame oscillation fre-quency decreases with the recycle ratio, indicating that a high

recycle ratio has an adverse effect on the flame stability. In mostcases, the flame oscillation frequency in the root region is lowerthan that in the middle region, indicating ignition problems underoxy-fuel firing conditions. Comparisons between oxy-fuel and air-firing conditions have also suggested that, to maintain equivalentflame temperatures in oxy combustion and air combustion, thefeeding gas needs to be around 32–35% volume oxygen and 65–68% by volume dry recycled CO2. These results are useful additionsto the current body of knowledge on oxy-fuel combustion.Moreover, the flame imaging technique has been proven to be animportant diagnostic tool for the assessment of the impact ofswitching to oxy-fuel firing on existing large utility coal-firedplants.

Acknowledgements

The authors would like to acknowledge the funding of the UKTechnology Strategy Board. The work reported in this paper wascarried out as part of the OxyCoal-UK Phase 1 Project (TPC/00/00404/00/00, TP/5/LOW/6/I/H0205C).

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