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Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2957–2964

SCALING PROPERTIES OF SWIRLING PULVERIZED COAL FLAMES:FROM 180 kW to 50 MW THERMAL INPUT

ROMAN WEBER and FREDERIC BREUSSINInternational Flame Research Foundation

P.O. Box 10 000

1970 CA IJmuiden, The Netherlands

The objective of this work is to provide guidelines for scaling of swirl-stabilized burners fired withpulverized coal with emphasis on NOx emissions. The experimental data spanning the thermal input rangefrom 0.9 to 12 MW are used. At the high thermal input end, the scaling range has been extended to a 50-MW power representing a full industrial-size burner. At the low thermal input end, our considerationshave approached flames as small as 176 kW, representing a laboratory scale. For the extrapolation, amathematical model for predicting properties of pulverized coal flames is used. Constant velocity andconstant residence time-scaling criteria have been employed.

When the prototype experiments are carried out at thermal inputs larger than 4 MW, the NOx emissionsare representative of full industrial-scale applications. The scaling can be successfully carried out usingeither the constant velocity or the constant residence time principle. When the prototype experiments arecarried out at thermal inputs lower than 2–3 MW, the NO emissions decrease with thermal input. Thisdecrease has been attributed to the solid-phase aerodynamics (particle trajectories). To obtain identicalNO emissions in prototype experiments carried out at a thermal input lower than 1 MW, the constantresidence time scaling would be recommended, and the pulverized fuel would have to be milled finer.

Introduction

Most combustion equipment manufacturers carryout burner development work using semiindustrial-scale furnaces for testing prototype burners. Thethree basic questions that the manufacturers for-mulate are [1]:

1. What is the minimum thermal input for the pro-totype combustion tests?

2. What scaling laws should be used on the proto-type test results?

3. What would be the performance of both the pro-totype and the final design in furnaces/boilers ofdifferent shape and thermal characteristics?

The objectives of this paper are to attempt to pro-vide answers to the preceding questions, with em-phasis on the first two. We want to derive guidelinesfor scaling of swirl-stabilized burners fired with pul-verized coal with emphasis on NOx emissions. Tothis end, the International Flame Research Foun-dation (IFRF) experimental data spanning the ther-mal input range from 0.9 to 12 MW are used. Thedata set contains both unstaged (type-2, high-NOx

flames) as well as staged (type-1, low-NOx flames)flames. At the high thermal input end, the scalingrange has been extended to a 50-MW power, rep-resenting a full industrialsize burner. At the low ther-mal input end, our considerations have approachedflames as small as 176 kW, representing a laboratory

scale. For the extrapolation, a mathematical modelfor predicting properties of pulverized coal flames isused (see also Ref. [2]).

Experimental Data

The experimental data used in this paper weregenerated using the Aerodynamically Air StagedBurner [3]. The burner has been well documentedin the literature. It can function under high- and low-NOx operation, dependent on the coal particle tra-jectories in the near burner zone. The low-NOx

operation (type-1 flames) is achieved by inserting thecoal injector into the burner throat, whereas high-NOx (type-2) flames are generated when the coalinjector is positioned at the inlet to the burner quarl[3,4].

Constant Velocity Flames

The basic formula for the burner thermal input(Q) is as follows:

2Q 4 K q U D (1)0 0 0

where q0, U0, and D0 relate to the inlet fluid density(typically combustion air density), characteristicburner (combustion air) velocity, and burner char-acteristic diameter (burner throat diameter); Kstands for a proportionality constant. When a burner

2958 COAL AND CHAR COMBUSTION

TABLE 1Summary of experimental conditions for constant velocity scaling

ThermalInput(MW)

Overallstoichiometry

Coal name, ASTMvolatiles (%),

Ult. Analysis (% dry),HT volatiles1(%)

Heart SinkCharacteristics References

12—unstaged 1.16 hvBb Gottelborn Coal;ASTM36; C83, H5, N1.8,S1.4, O8.8; HT54

First 7 mrefractory lining,otherwise water-cooled surface

[7,8]

12—staged as above as above as above as above3.4—unstaged 1.20 hvCb Coal Valley Coal;

ASTM36;C67, H4, N0.76, S0.3,O17; HT56

11 cooling loops;5 loops near the burner

[9]

3.4—staged as above as above as above as above2.5—unstaged 1.19 hvBb Gottelborn Coal;

ASTM33;C72, H5, N1.51, S0.9,O12, HT54

as above [10]

2.5—staged as above as above as above as above2.4—unstaged 1.22 hvBb Gottelborn Coal;

ASTM37;C74, H5, N1.33, S0.86,O11, HT55

7 cooling loopslocated 1.5–6 mdownstream

[11]

2.2—unstaged 1.20 hvCb Coal Valley Coal;ASTM36; C67, H4,N0.76, S0.3, O17; HT56

9 cooling loops;3 loops near theburner

[9]

2.2—staged as above as above as above as above0.9—unstaged 1.20 as above 8 cooling loops;

1 loop near theburner

[9]

0.9—staged as above as above as above as above

1High-temperature volatile as measured in coal characterization experiments.

is scaled to a different thermal input using the con-stant velocity criterion (U0 4 const), the character-istic burner diameter can be easily evaluated

0.5D } Q (2)0

and so, if the geometrical similarity of the burner ismaintained, the other burner dimensions can be cal-culated.

Table 1 lists both burner dimensions and experi-mental conditions for the constant velocity flames.The confinement, which can be defined as the fur-nace over burner outlet diameter (2D0) ratio, is al-most identical (4.67–4.83) for the 12-, 2.5-, 2.4-, and2.2-MW tests. It is 3.76 and 7.46 for the 3.4- and0.9-MW tests, respectively. On the importance ofmaintaining the confinement while scaling flamessee Refs. [1,5,6]

Constant Residence Time Flames

The principle of this scaling criteria is to maintainD0/U0 ratio constant while changing the burner ther-mal input. The ratio represents the inertial timescale

of the flow often called the convective timescale. Athigh enough Reynolds numbers, all time measuresin the flow, except these associated with the (final)molecular dissipation processes, are proportional toD0/U0. It has been demonstrated that for simpleburners, the D0/U0 ratio represents the flame resi-dence time. Recalling equation 1 and invoking therequirement of maintaining D0/U0 constant resultsin the relationship:

0.33D } Q (3)0

For this scaling principle, experiments at 12-MW[7,8] and 2.5-MW [12] thermal inputs are available.From the baseline 12-MW burner, the characteristicburner throat diameter for a 2.5-MW constant resi-dence time version of the burner is calculated to be0.3 m. This is substantially larger than the throatdiameter (0.234 m) of the constant velocity burnerof the same thermal input. Consequently, the com-bustion air velocity drops to 26 m/s, and the trans-port air velocity drops to 12 m/s. In both the 12-MW

SCALING OF PULVERIZED COAL FLAMES 2959

and 2.5-MW experiments, Gottelborn coal was com-busted. Its nitrogen content was 1.8% and 1.43%,respectively. For both flames, the flame residencetime is 110 ms.

For all the constant velocity flames (Table 1) andcorresponding constant residence time flames [12],detailed in-flame measurements as well as furnaceexit measurements are available. The furnace exitvalues contain temperature, O2, CO, CO2, NO, andchar burnout. In-flame measurements of tempera-ture and gas composition (O2, CO, CO2, NOx) weremade using IFRF probes. The most comprehensiveset of data exists for the 2.4-MW, unstaged flame(Table 1). In addition to the temperature and gasspecies mapping, the data include laser Doppler ve-locimetry (LDV), concentrations of nitrogen precur-sors (HCN, NH3), in-flame char burnout, and radi-ative fluxes. The 12-MW, unstaged flame datacontain also the LDV measured velocities.

Mathematical Model

The mathematical model used is basically identicalto the one described in our previous publications[13,14]. The gas-phase balance equations are solvedin an Eulerian frame of reference, whereas the solidphase is calculated using Lagrangian stochastic par-ticle tracking. In addition to the fluid flow equations(see later) the transport equations for enthalpy, massfraction of oxygen, volatiles, carbon monoxide, car-bon dioxide, and water are solved for. Radiation iscalculated using the discrete transfer method. Thedevolatilization and char combustion submodels arespecific for the coal combusted (see Ref. [14]). TheNOx postprocessor includes both the thermal andfuel routes. The prompt NO is omitted. The thermalNO is calculated using the extended Zeldovichmechanism and a presumed single-variable proba-bility density function (PDF) approach with the gastemperature being the only fluctuating variable [14].An additional transport equation is solved for thenitrogen oxide precursors. The coal nitrogen is givenoff into the gas phase with the rate identical to thedevolatilization rate. The high-temperature volatilesdetermine the fraction of the coal nitrogen releasedinto the gas phase. The overall fuel–NO formationand destruction mechanism is that of De Soete [15]and Pohl and Sarofim [16]. For a comparison withother models, see Refs. [17,18].

For predicting the flowfield of unstaged high-NOx

type-2 flames, the standard k–e turbulence model isused. In our experience, there is no compelling per-formance advantage to replacing k–e with second-order models when predicting type-2 flames [13].However, the story is different for type-1 flames[19], which can be characterized by a fuel jet pen-etrating through the swirl-induced recirculationzone. The physical size of the fuel-rich region and

subsequent NOx predictions are strongly dependenton the calculated jet penetration. The latter is defi-nitely better predicted when a Reynolds stressmodel is used, and all the stresses are calculatedfrom their transport equations. The corollary of thepreceding observations is that the unstaged flameshave been computed using the k–e turbulencemodel, whereas the staged flames have been com-puted with the Reynolds stress model.

Quality of the Model Predictions

Table 2 lists the measured and computed furnaceexit parameters for the constant velocity flames. Thepredictions for the unstaged flames are of very goodquality, showing that the furnace exit temperaturecan be predicted typically within a 50 8C accuracyand oxygen concentrations within 0.2% margin. TheNOx predictions do not differ in absolute values bymore than 51 ppm and 196 ppm, corresponding toa 5.14% and 22% accuracy for Coal Valley and Got-telborn coal, respectively. For the staged flames, themodel predictions are of less quality, in particularwith respect to NO emissions. The maximum differ-ences between the measured and predicted NOx

emissions are 147 ppm (36%) and 43 ppm (9.4%)for the Canadian and German coals, respectively.The lower prediction’s quality for the staged flamesis attributed to two factors. First, the near burnerzone properties, mainly fluid dynamics and globalcombustion chemistry (O2, CO, CO2, volatiles), arenot predicted as accurately as for the unstagedflames. Second, the NO postprocessor does not in-corporate the NOx reburning with CHi radicals,which is certainly more important in the stagedflames. We have thoroughly validated the predic-tions using measured in-flame data, and in our pre-vious publications [11,13,14,19] one may find the ap-propriate comparisons.

Scaling Considerations

Scaling considerations are extended, at the highthermal input end, to a 50-MW input and to a 176-kW input at the lower end. Thus, two extra flamesare computed for each burner scaling principle. The50-MW constant velocity burner has a 1.045-mthroat diameter (D0), and the combustion air veloc-ity is 40 m/s at 300 8C temperature. The furnacediameter is chosen to be 10.1 m to secure the burnerconfinement of 4.83. To provide a thermal similaritywith the other flames, 40% of the burner thermalinput is extracted by an appropriately sized and po-sitioned heat sink. The 176-kW burner has a 62-mmthroat and combustion air velocity of 40 m/s. Thefurnace diameter is 0.60 m, resulting in a confine-ment ratio of 4.83. The 50-MW constant residence

2960 COAL AND CHAR COMBUSTION

TABLE 2Measured and predicted furnace exit parameters for constant velocity flames (see Table 1)

Temperature (8C) NOx (ppm) dry

Thermal input (MW) Measured Predicted Measured Predicted

50—unstaged (CV) — 1358 — 105550—staged (CV) — 1120 — 52012—unstaged (G) 1188 1160 884 108012—staged (G) 1179 1120 557 580*)12—unstaged (CV) — — 99312—staged (CV) — — 9933.4—unstaged (CV) 1143 1321 900 8563.4—staged (CV) 1377 1345 403 5502.5—unstaged (G) 1293 1260 991 9402.5—staged (G) 1283 1260 458 501*)2.4—unstaged (G) 1354 1310 820 7352.2—unstaged (CV) 1323 1320 839 8082.2—staged (CV) 1353 1308 514 4830.9—unstaged (CV) 1153 1140 794 8100.9—staged (CV) 1227 1247 480 4070.176—unstaged (CV) — 1135 — 7870.176—staged (CV) — 1156 — 320

CV, Coal Valley coal; G, Gottelborn coal; *) with estimated thermal NO.

time burner has a 0.825-m throat and a 95 m/s com-bustion air velocity at 300 8C, whereas the coal in-jection velocity is 45 m/s at 70 8C. For the 176-kWconstant residence time burner, the following is ap-plicable: 0.125-m throat, a 17 m/s combustion airvelocity. The four extra flames have been computedusing hvCb Coal Valley coal as the fuel.

Two different coals were combusted to generatethe constant velocity flames listed in Table 1. At ther-mal inputs of 0.9, 2.2, and 3.4-MW, a hvCb CoalValley coal from Canada was burned, and a hvBbGottelborn coal from Germany was burned at 12MW. We expect that global flame characteristics likeflame length, shape, and volume as well as coal par-ticles’ trajectories and subsequent devolatilizationregions must be almost identical, regardless ofwhether the Canadian or the German coal is used.This is so, because both coals have almost identicalhigh-temperature volatile yields with only a slightalteration to the volatile matter composition. How-ever, their NO emissions may be different becausenitrogen contents are 1.8% and 0.76% for the Ger-man and Canadian coal, respectively. Therefore, anextra computational run has been made simulating12-MW baseline flame of Coal Valley coal at 20%excess air. The predicted NOx emissions were 993ppm, that is 87 ppm lower that those predicted forGottelborn coal (Table 2). These figures are to becompared with both the measured and computedNOx emissions listed in Table 2 under 12-MW ther-mal input. Forthcoming scaling considerations have

been carried out using the Canadian coal as a basis.The experimental data and predictions for Gottel-born coal have been added wherever available.

Flame Length and Volume

We arbitrarily define here a flame volume (Vf) asa region where the concentration of unburned hy-drocarbons (volatiles) is greater than or equal to0.5% (vol, dry), and the flame length (Lf) as a dis-tance where the 0.5% contour line crosses theburner centerline. Fig. 1 shows how the flame lengthand flame volume (both derived from the flame pre-dictions) vary with thermal input. For constant ve-locity flames, the flame length scales as Q0.58 andQ0.56, with thermal input (Q) for the staged and un-staged flames, respectively. The staged flames arearound 20% longer than the unstaged ones. For con-stant residence time scaling, Lf ; Q0.26 and Lf ;Q0.3 relationships are applicable for baseline andpenetration flames, respectively. Fig. 2 shows alsohow the flame volume scales with thermal input. Thederived laws agree very well with the correlationsexpected from the theory. Assuming that the flamelength is proportional to the burner diameter (D0),and using relationships (1) and (2), the following ap-plies: Lf ; Q0.5, Vf ; Q1.5 for constant velocityflames, and Lf ; Q1/3, Vf ; Q for constant residencetime flames.

SCALING OF PULVERIZED COAL FLAMES 2961

Fig. 1. Scaling of pulverized coal flames; left-flame length versus thermal input; right-flame volume versus thermalinput. l, constant velocity baseline flames; C, constant velocity penetration flames; ❖, constant residence time baselineflames; ▫, constant residence time penetration flames. Dotted regression lines show constant velocity scaling, and solidregression lines show constant residence time scaling.

Fig. 2. Constant velocity flames. NOx emissions above600 ppm correspond to baseline (high-NOx) flames; NOx

figures below 600 ppm correspond to penetration (low-NOx) flames. All flames at 20% excess air unless indicatedotherwise. The small numbers show flame residence time.C, , measurements—Gottelborn coal [7,8,10]; ● predic-tions—Gottelborn coal; ▫, n measurements—Coal Valleycoal [9]; n, m predictions—Coal Valley coal.

NOx Emissions

The NOx emission data for all the flames are plot-ted in Figs. 2 and 3 as a function of thermal input.The predicted NO emissions are accompanied byerror bars showing the estimated uncertainty. Fig. 2shows that for the constant velocity scaling of un-staged (high-NOx) flames, the NOx emissions in-crease with the thermal input. For thermal inputsgreater than 4–5 MW, the emissions become prac-tically independent of the thermal input. A similar

trend is observed for the staged flames. The NOx

emissions of baseline flames of Gottelborn coal arehigher, by around 100 ppm, than those of Coal Valleycoal. A dependence of the emissions on nitrogencontent of the coal fired is observed. On the otherhand, emissions of the staged flames seem to be in-dependent of coal nitrogen content.

It is interesting to observe that the NO emissionsof the baseline flames, either scaled using constantvelocity (Fig. 2) or the constant residence time prin-ciple (Fig. 3), are very similar indeed. This is cer-tainly true for thermal inputs larger than 4 MW.There are indications that when the constant resi-dence time scaling is applied, it should be possibleto carry out prototype experiments at thermal inputssmaller than 1–2 MW and obtain NO emissions only10% lower than those corresponding to a 50-MWinput. Manipulating further with the size of coal par-ticles (see the following) may result in identical NOemissions in the prototype and full-scale experi-ments. Regrettably, we do not posses any hard (ex-perimental) evidence to substantiate this observa-tion. For the penetration flames, we also observe arelatively little difference in the NO emissions ofconstant residence time and constant velocityflames.

Constant velocity scalingOne of the main scaling parameters for such

burner design is the penetration distance of coal par-ticles. The particle trajectories determine the loca-tion and size of the region where volatiles are givenoff, as illustrated in Fig. 4. In laboratory-scale(176 kW) experiments, the degree of penetration(penetration distance normalized by the length ofthe swirl-induced reverse flow region that is formedin the burner vicinity) is substantially longer than in

2962 COAL AND CHAR COMBUSTION

Fig. 5. Devolatilization region of constant residencetime flames; top—baseline flames; bottom—penetrationflames; Convective timescale 4 0.6D0/U0 4 7 ms; Flameresidence times are 110 ms and 315 ms for baseline andpenetration flames, respectively.

Fig. 4. Devolatilization region of constant velocityflames; top—baseline flames; bottom—penetration flames;U0 4 40 m/s; 50 MW D0 4 0.512 m; 2.5 MW D0 4 0.234m; 0.176 MW D0 4 62 mm.

Fig. 3. Constant residence time flames; NOx emissionsabove 700 ppm correspond to baseline (high-NOx) flames;NOx figures below 700 ppm correspond to penetration(low-NOx) flames. All flames at 20% excess air. ● predic-tions—Gottelborn coal; n predictions—Coal Valley coal; m

predictions—Coal Valley coal; . predictions—Gottelborncoal; , measurements—Gottelborn coal [12,9].

semiindustrial (2.5 MW) or full-industrial scale (50MW) burners. At large thermal inputs, the particlesin the baseline flames are directly entrained by thesecondary air stream. Fig. 4 shows the smaller thescale is, the larger the volume of the devolatilization

region, if compared with the quarl volume. The 176-kW flame is predicted lifted and the devolatilizationregion of the baseline flame (Fig. 4, top) is compa-rable to that of the penetration flame (Fig. 4, bot-tom).

For the baseline (high-NOx) flames, the emissionsincrease up to around 4-MW input to gradually leveloff at larger thermal inputs (Fig. 2). The NOx emis-sions of the penetration (low-NOx) flames also in-crease with the thermal input for inputs lower thanaround 2 MW. There is a direct relationship betweenthe penetration pattern and the NOx emissions forthe constant velocity flames. The shorter the pene-tration depth into the reverse flow region, the higherthe NO emissions. This feature is also visible whencomparing the 150-kW flames of Abbas et. al. [20]and Costa at al. [21] with the flames of Smart [9],although different burners have been used. This dis-parity can be overcome if the coal is pulverized fineror micronized for low thermal input experiments.

Constant residence time scalingFigure 5 shows how the penetration distance, and

the location of the region where volatiles are givenoff, scales with the thermal input. The convectivetime scale (0.6 2 D0/U0) for all the flames shown inFig. 5 is around 7 ms. The residence time in theflame is 110 ms and 315 ms for the baseline andpenetration flames, respectively. For both baseline

SCALING OF PULVERIZED COAL FLAMES 2963

and penetration flames, there are relatively few dif-ferences in the devolatilization pattern. Conse-quently, the NOx emissions do not depend stronglyon the thermal input as shown in Fig. 4. For thisburner scaling methodology, one can easily show thatthe momentum flux (mass flow rate 2 velocity/area)is a function of the thermal input to the power 2/3.This results in difficulties in stabilizing such flame atlow thermal inputs.

Conclusions

During the last decade, a number of IFRF trialshave been carried out to address the issue of flamesscaling. The experiments on swirl-stabilized flamesof pulverized coal have spanned a 0.9- to 12-MWthermal input range. Two scaling criteria, constantvelocity and constant residence time, have been em-ployed. Sets of detailed flame data, which were gen-erated during these experiments, have been used todevelop and validate a mathematical model for pre-dicting properties of the flames, including NO emis-sions. The mathematical model has been used to ex-tend the considerations on flame scaling to 50 MWand 176 kW at the high and low end of the thermalinput range, respectively. The analysis of the exper-imental data and the model predictions led to a num-ber of conditions required for achieving, in proto-type experiments, NOx emissions representative offull industrial-scale applications. These conditionsare as follows:

(a) When the prototype experiments are carried outat thermal inputs larger than 4 MW, the NOx

emissions are representative of full industrial-scale applications. The scaling can be success-fully carried out using either the constant veloc-ity or the constant residence time principle. Ithas been demonstrated that none of these scal-ing principles is superior.

(b) When the prototype experiments are carried outat thermal inputs lower than 2–3 MW, the NOemissions decrease with thermal input. This de-crease has been attributed to solid-phase aero-dynamics (particle trajectories). To obtain iden-tical NO emissions in prototype experimentscarried out at a thermal input lower than 1 MW,the constant residence time scaling would berecommended, and the pulverized fuel wouldhave to be milled finer.

(c) For low-NOx penetration flames, both scalingmethodologies were equally successful over thewhole thermal input range. In this case, the keyto NOx scaling is the location of the devolatili-zation zone inside the reverse flow region.

The preceding conclusions are valid for internallyair staged burners where NOx reduction is achievedby a proper interaction between the swirl-induced

recirculation zone and the coal particles. In the pro-totype experiments, both the burner confinement(furnace over burner diameter ratio) and the totalheat extraction (heat extracted to thermal input ra-tio) should be maintained constant. Small departuresfrom these requirements do not obscure the NOx

emission similarity; however, for large deviations, theprototype results may not resemble the full indus-trial-scale emissions.

REFERENCES

1. Weber, R., in Twenty-Sixth Symposium (International)

on Combustion, The Combustion Institute, Pittsburgh,1996, pp. 3343–3354.

2. Truelove, J. S. and Williams, R. G., in Twenty-Second

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62(453):237–245 (1989).4. Smart, J. P., Knill, K. J., Visser, B. M., and Weber, R.,

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bustion, The Combustion Institute, Pittsburgh, 1988,pp. 1117–1125.

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thews, K. J., Sarjeant, M., and Godridge, A. M., inPrinciples of Combustion Engineering for Boilers.

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Kamp, W. L., and Ereaut P. R., Evaluation of a 12 MW

Aerodynamically Air-Staged Burner. IFRF doc. no. F37/y/33, 1994.

8. Smart, J. P., Woycenko, D. M., Morgan, D. J., and Vande Kamp W. L., J. Inst. Energy 16 (480):131 (1996).

9. Smart, J. P., “On the Effect of Burner Scale and CoalQuality on Low NOx Burner Performance.” Ph.D. the-sis, University of London, 1992.

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100:331–342 (1994).11. Weber, R., Dugue, J., Sayre, A., and Visser, B. M., in

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bustion, The Combustion Institute, Pittsburgh, 1992,pp. 1373–1380.

12. Woycenko, D. M. and Smart, J. P., Evaluation of a 2.5

MW Constant Residence-Time Scaled Version of a 12

MW Aerodynamically Air Staged Burner, IFRF doc.no. F 37/y/31, 1993.

13. Weber, R., Peters, A. A. F., Breithaupt, P. P., and Vis-ser, B. M., ASME J. Fluids Eng. 117:289–297 (1995).

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(122):131–182 (1997).15. De Soete, G. G., “Fundamentals of NOx and N2O For-

mation and Destruction, Utilisation of Solid Fuels,”The IFRF Third Flame Research Course on Fuel Util-ization and Environment, Noordwijkerhout, TheNetherlands, 1990.

16. Pohl, J. H. and Sarofim, A. F., in Sixteenth Symposium

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(International) on Combustion, The Combustion In-stitute, Pittsburgh, 1976, pp. 491–501.

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Clean and Efficient Use, Elsevier Biomedical Amster-dam, 1994.

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COMMENTS

Philippe Ngendakumana, University of Liege, Belgium.

For NO emissions scaling, you recommend to keep theresidence time constant. Is that residence time based onthe total volume of the furnace? If yes, that means that youimplicitly assume that the NO formation occurs at the samerate in the whole furnace.

Author’s Reply. The principal of the constant residencetime scaling is to maintain D0/U0 ratio constant whilechanging the burner thermal input. The ratio representsthe inertial time scale of the flow, often called the convec-tive time scale. At high enough Reynolds numbers, all timemeasures in the flow, except those associated with the(final) molecular dissipation process, are proportional toD0/U0. It has been demonstrated that for simple burners,the D0/U0 ratio represents the flame residence time.

●

Lixing Zhou, Tsinghua University, China. I think, fromthe point of view of particle dynamics, you should have kepta constant Stoke’s number rather than a constant residencetime. Are you keeping the swirl number unchanged?

Author’s Reply.

1. Particle dynamics is indeed important, as it has beenhighlighted in the text and in our previous publications(Refs. [3,9] in the paper). When a coal particle is in-jected into a region of reverse flow, it will come to restat a distance governed by Stokes number. Scaling theburners using the constant velocity principle implicitlyinvokes constant Stokes number because, in our case,the inlet swirl and the size distribution of coal particlesare kept the same for all thermal inputs. For constantresidence time scaling, the Stokes number varies with

thermal input. From our understanding, the best scalingapproach would be to maintain the constant residencetime and mill the fuel finer (see text) for small thermalinput experiments. Thus, the residence time and Stokesnumber would be maintained constant.

2. Yes, the inlet swirl number and the type of the inletvortex (constant tangential velocity) are maintained un-changed.

●

Yann Rogaume, Laboratorie de Combustion et Deto-

nique, France. The problem of scaling is always difficult totreat, and this paper gives a lot of ideas and solutions—thatway, it is very interesting. When the thermal input varies,what about the temperatures in the flame? As a part ofNOx is in this case formed via the thermal NOx mechanism,do you not think different levels of temperature can changethe NOx emissions level?

Author’s Reply. The scaling experiments were designedto achieve a thermal similarity. For each thermal input, theamount of the energy extracted constituted the same frac-tion of the input. This resulted in similar values of the fur-nace exit temperature for all experiments. Furthermore, inthe flame zone, the heat sinks were manipulated to achievesimilar flame temperatures. Measurements have demon-strated that the peak values of the in-flame temperatureswere similar for each thermal input. For the unstaged (highNOx) flames, the figure of 1850 K is applicable while forthe staged (low NOx) flames, the peak temperatures werearound 1720 K. The mathematical model predictions in-dicated that around 30% of the NOx emissions were gen-erated via the thermal NO mechanism. For the stagedflames, a figure of 20% is applicable.