Measurement and modelling of coal flame stability in a pilot-scale combustor

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 963-971

M E A S U R E M E N T A N D M O D E L L I N G OF COAL FLAME STABILITY IN A PILOT-SCALE C O M B U S T O R J. S. TRUELOVE BHP Central Research Laboratories PO Box 188, Wallsend, N S W 2287 AUSTRALIA AND

D. HOLCOMBE Australian Coal Industry Research Laboratories North Ryde, N S W 2113 A US TRALIA

A mathematical model of a coal flame is applied to simulate and interpret pilot-scale test data on flame stability and combustion efficiency of three coals with volatile contents varying from 19-41% daf. The flame model is based on the equations governing conservation of mass, momentum, species and energy and includes physical models for turbulence, volatiles combust'ion, radiation heat transfer and particle dispersion together with phenomenological models to describe coal devolatilization and char combustion. Measurements and predictions suggest that the flame stability is primarily determined by the burner aerodynamics with little influence of coal volatile content, while the ignition behaviour is determined by both the coal volatile content and the specific energy of the volatiles. Combustion efficiency is strongly influenced by the coal volatile content. Detailed in-flame measurements of flow, temperature and gas composition are reported and used to evaluate the coal flame model. Discrepancies in the prediction of near-burner-zone properties appear to be attributable to defects in the modelling of turbulent transport and/or volatiles combustion. Modifications to either component of the model are shown to yield improved agreement with the measured data.

Introduction Pilot-scale test furnaces are now used routinely to rank the relative flame-stability behaviour of coals. The direct utilization of the data is, however, limited by scaling problems because the pilot-scale test does not exactly simulate the thermal and kinematic conditions in a full-scale boiler flame. In a previous investigation, 1 the potential of pulverizedcoal flame models for interpretation and scaling of pilot-scale data, and selection of operating conditions necessary to promote flame stability and acceptable combustion efficiency in boiler plant, was demonstrated, The present paper describes a further application of the coal-flame model, 1 to interpret pilot-scale data on flame stability and combustion efficiency of lowvolatile and conventional steaming coals. The objective of the investigation is to simulate the routine test data for flame stability and burnout in a pilot-scale combustor. For the purpose of model evaluation, detailed measurements are obtained for flow, temperature and species distributions in the 963

near-burner zone. Relevant coal-dependent properties, such as the high-temperature volatile yield and char reactivity are measured in a laboratoryscale reactor.

Pilot-Scale Coal Combustor Test Furnace:

The ACIRL pilot-scale test furnace is a refactorylined vertical cylinder 0.66 m in diameter and 2.5 m long, downfired by a double-concentric swirl burner at a thermal input up to about 150 kW. External cooling of the cylindrical walls and internal cooling panels control furnace heat absorption and extract about 40% of the thermal input. The facility is fully instrumented to monitor all fuel and air flows, and ports located in the cylindrical wall allow flame observation and access for in-flame measurements of temperature, species concentrations and flow direction, and-solids sampling. The gas temperature is measured using a water-

964

COAL BOILERS/FURNACES

cooled suction pyrometer with a single ceramic shield and a ceramic-sheathed thermocouple. Species concentrations and flow direction are measured using a combined solids/gas sampling and Hubbard probe designed by ACIRL. A schematic of the furnace, showing the location of the measuring ports, is given in Fig. la. The geometry and relevant dimensions of the variable swirl burner are given in Fig. lb.

Coals Fired: The characteristics of the coals fired are given in Table I. Coals J and F are low-volatile bituminous with proximate volatile contents less than 20% db; coal H is a high-volatile bituminous coal used for steaming purposes.

Measurements: The routine pilot-scale testing consists of four test procedures to determine the flame-stability and combustion-efficiency behaviour of the test coal. The test procedures are briefly outlined below:

Burner Measurement Pods ---- x / l ] : 2.2 -'35 --5-6 -'6"9

Refractory

"-

Water Jacket~

9"1

-'--

12"3

Exit

9 Flame stability measurements to determine the minimum, or critical, level of secondary swirl required to establish a stable flame with ignition in the burner quarl. Nominal load of 150 kW, varying stoichiometric ratios for the primary air stream (SR~) and fixed stoichiometric ratio for the secondary air stream (SRs) of 1.0. 9 Flame standoff measurements at zero swirl to assess the ignitability of the coal. Nominal load of 150 kW, varying SI~ and fixed SRs of 1.0. 9 Turndown measurements to determine the critical swirl for flame stability at low load. Load reduced to 60 kW while maintaining a fixed pri-

TABLE I Coal characteristics, volatile yields and char reactivities Coal

(a) ACIRL TEST

FURNACE

secondary air ----primary a i r + c o a l - - - .

D=%mm

.

.

.

.

.

.

.

.

.

-

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Dp/D=(>53 De/D=0-21 LID=0.85 13=22.5~

Property

J

F

H

Moisture, % as received Ash, % db Volatile matter VM, % daf Specific energy (net), MJ/kg daf Stoichiometric air, kg/kg daf Specific energy of volatiles (net), MJ/kg (a) Vtooo/VM (b) V~o/VM (b) V~oo/VM (b) A~, kg/m~.s.atm w~ (c) Ec, MJ/kmol (c)

1.1 12.2

0.9 19.2

3.2 9.6

18.8

23.5

40.9

35.2

35.5

32.6

11.8

12.0

11.0

44.2 1.27 0.95 1.03 40.5 82.2

40.7 1.19 1.32 1.44 13.9 66.6

32.4 1.45 1.31 1.40 162.0 93.4

[b) VARIABLE-SWIRL BURNER FIG. 1. Schematic of ACIRL pilot-scale test furnace and variable-swirl burner: x = axial distance from burner throat; D = burner throat diameter; Dp = primary nozzle diameter; Dc = core nozzle diameter; L = quarl length; 13 = quarl half angle.

a. Estimated from net specific energy of coal and measured high-temperature volatile yield. b. High-temperature volatile yields at 1000, 1300 and 1500~ C. c. /L and Ec are the Arrhenius parameters for the char reaction rate coefficient.

COAL FLAME STABILITY mary air flowrate corresponding to SI~ of 0.2 at 150 kW and a fixed overall stoichiometrie ratio of 1.2. 9 Burnout measurements to determine combustion efficiency. Nominal load of 150 kW, SRn of 0.2 and SR, of 1.0. Here, the stoichiometric ratio for a particular stream is defined as the air flowrate relative to the stoiehiometric air flowrate. The routine tests are carried out using a standard size distribution of pulverized coal, of 70% below 75 p,m. The burner operating conditions for the detailed flame probing are given in Table II. The burner is operated at a nominal load of 150 kW, SP~ and SRs of 0.2 and 1.0 respectively, and a swirl level just above the critical setting for a stable, attached flame.

The mathematical model used in the present work has been described previously. 1 Briefly, the model is based on the time-averaged equations for conservation of mass, momentum, species and energy for gas and particle phases. Physical models used include the standard k-e turbulence model, a mixing-limited volatiles combustion model, a discreteordinate radiation model and a stochastic simulation of particle dispersion. Phenomenological models are used to describe coal devolatilization and char combustion.

TABLE II Burner operating conditions Coal J

Coal Devolatilization: The high-temperature volatile yields for the three coals were measured in a laboratory-scale, laminarflow pyrolysis furnace employing techniques established in earlier studies.2 The data at temperatures of 1000, 1300 and 1500~ C are given in Table I. The data for coal F show a trend of increasing yield with increasing temperature and are represented by the two-competing-reactions model using rate constants for bituminous coalsz and stoichiometric factors adjusted to reproduce the observed temperature dependence of the volatile yield.2 The data for the other two coals do not show a uniform trend with temperature and are represented by the single-reaction model4 with an average value for the ultimate yield.

Char Combustion:

Mathematical Model

Property

965

F

H

Coal flow (dry), kg/h 16.9 18.3 17.6 Coal size, wt % < 75 I~m(a) 74 79 69 Primary air flow, kg/h 35.3 35.5 35.0 Primary air temperature, ~ 42 47 47 Primary air velocity, m/s 5.3 5.6 5.6 Secondary air flow, kg/h 177 177 175 Secondary air temperature, ~ 267 272 267 Secondary air velocity, m/s 14.7 15.0 14.7 Secondary swirl number 0.22 0.27 0.28 Furnace exit temperature, ~ 1230 1245 1265 a. Size distribution measured by Malvern particle size analyser.

The char reactivities for the three coals were determined from measurements in a laboratory-scale reactor~ for a range of temperatures 1000-1400~ C and oxygen concentrations. The kinetic parameters for the rate coefficient based on external surface area are given in Table I. At any given temperature within the measurement range, the reactivities for the three chars differ by less than a factor of two.

Gas-Phase Combustion: Gas-phase combustion is one of the least-well understood aspects of coal-flame modelling. Here, the volatiles are presumed to react by a single global reaction to form carbon monoxide and water vapour, the rate of reaction being physically controlled by turbulent mixing and quantified in terms of the turbulence decay rate. 6 The carbon monoxide from volatile and char combustion is converted to carbon dioxide at a rate equal to the smaller of the mixing-controlled and chemically-controlled rates. In practice, the chemical influence is small and the rate of carbon monoxide conversion is effectively mixing controlled.

Results

Flame Stability: The predicted and measured flame stability maps for the three coals are compared in Fig. 2. At swirl levels above the critical value the flame is stable and attached to the burner quarl; below the critical value the flame is 'unstable' and lifted. The maps exhibit the expected increase in critical swirl with increasing SRv (ie. primary velocity). At a typical SRn of 0.2, the critical swirl number is about 0.25, a relatively low value which is a direct consequence

966

COAL BOILERS/FURNACES

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Ss,c FIG. 2. Predicted and measured flame stability. Measurement uncertainty in critical swirl (S,.c) estimated as -+0.03. of the rather high secondary-to-primary velocity ratio (3:1) for the present burner geometry. Stable flames are predicted to ignite within the burner quarl and are stabilized by a swirl-induced internal-recirculation zone. The onset of flame instability is abrupt and is associated with the collapse of the internal-recirculation zone and flow separation off the burner quarl, leading to a shift in the ignition front well away from the burner. The flames for SRp less than about 0.25 are predicted to be short and narrow with intense combustion of volatiles in the region extending to about three burner diameters from the quarl-discharge plane. At SP~ greater than about 0.3, the flames are predicted to be very wide with a low-velocity internalrecirculation zone extending to about ten burner diameters from the quarl-discharge plane. In practice, the latter flame types are likely to be only marginally stable because the internal-recirculation zone is very weak and not strongly attached to the burner nozzle. The predictions suggest, and the measurements tend to confirm, that the flame stability is relatively insensitive to the coal volatile content: indeed, the critical swirl appears to be primarily a function of the burner aerodynamics, at least for the present burner/furnace configuration and range of operating conditions encompassed by the testing procedure.

Flame Standoff.. The prediction of the zero-swirl flame requires a non-equilibrium calculation wherein the refractorytemperature distribution is fixed at that established

by the 150 kW flame at SI~ of 0.2 and critical swirl. The non-swirling coal jet ignites at some distance (the standoff distance) downstream of the burner as a result of radiative heating and entrainment of hot externally-recirculated gas. For constant coal size and flowrate, the standoff distance is generally determined by the refractory temperature; the yield, specific energy and rate of release of volatile matter; and the mass of primary air to be heated. For prediction purposes, the flame standoff is taken as the distance at which three percent of the coal mass is devolatilized. The selection of three percent devolatilization as the criterion for predicting flame standoff is arbitrary; a criterion of one (five) percent devolatilization would decrease (increase) the standoff distance by about five percent. The refractory temperatures in the near-burner zone are predicted to be about 1130~ C, 1190~C and 1160~ C for coals J, F and H, respectively. This trend with coal type is supported by the gas temperature measurements adjacent to the refractory. The predictions and measurements of flame standoff are compared in Fig. 3. The flame standoff increases with increasing SR~ as expected. At low SP,~, the measurements show a decrease in standoff distance with increasing coal volatile matter. At high SRp, the trends in the measurements are masked by scatter in the data. The measurement uncertainty is illustrated by the two data sets for coal F. The trends predicted with coal type are at variance with those measured. The predictions show a

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967

COAL FLAME STABILITY variation in standoff distance which correlates with the refractory temperature rather than coal volatile matter. In fact, the predictions show no significant difference between the coals when the refractory temperatures are presumed to be the same. That the standoff predictions are obtained using measured volatile yields and refractory temperatures supported by experimental evidence suggests that the discrepancy may be due to the assumed devolatilization kinetics, based on measurements for high-volatile coals, 3'4 being inappropriate for lowvolatile coals. Tu rn~ot~

:

The predicted and measured turndown stability maps are compared in Fig. 4. The critical swirl increases with decreasing load, primarily as a result of the increase in the primary-to-secondary velocity ratio during turndown. The predicted and measured turndown curves are generally in good agreement, except for the lowest-volatile coal J at 90 kW, where the measurements indicate an unstah]e flame, even at the highest swir[ of 1.5, and the predictions show a very wide flame stabihzed by a large lowvelocity internal-recircu]ation zone. As noted above, such flames are likely to be only marginally stable in a practical situation. The predictions show no significant differences in turndown behaviour among the coals.

Burnout: The predicted and measured burnout curves are compared in Fig. 5. Both the predictions and measurements exhibit the trends expected on the basis of the coal volatile matter: in the near-burner zone, rapid burnout for the high-volatile coal H and relatively slow burnout for the low-volatile coal J. Beyond axial distance 300 mm, at which point the predictions and measurements both indicate the coals are essentially completely devolatilized, the rate of burnout depends on the char reactivity and available oxygen. At the furnace exit, the predictions tend to overestimate the burnout. Finally, it is interesting to note that in the near-burner region the burnout in the coal jet (located between the recirculation zone and the secondary-air jet) appears to be significantly higher than on the centreline of the flame. This is presumably due to the higher temperature and oxygen levels in the coal jet.

Detailed Flame erojrdes: The predictions and measurements for the detailed probing of coal flames J and H are presented in Figs. 6-8. The results, for axial velocity, temperature and oxygen concentration, are shown as profiles through the flame at each of the measurement ports. The data for the axial velocity are estimates based on the magnitude of the pressure differential indicated by the Hubbard probe. The velocity estimates are subject to considerable uncertainty due to probe interference effects, particularly in regions of low velocity.

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H I

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AXIAL DISTANCE (m) FIG. 5. Predicted and measured burnout: closed symbols in the coal jet; open symbols along the flame centreline; predictions integrated over furnace crosssection.

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FIC. 6. Predicted and measured axial velocity profiles for coal flames J and H. Flame symmetry is generally satisfactory. It should be noted that some asymmetry is unavoidable due to probe interference effects and slight misalignments in the burner nozzles. The axial velocity profiles (Fig. 6) clearly indicate the presence of the flame-stabilizing internal-reverse-flow zone at the measurement port nearest the burner and the strong forward flow associated with the swirling jet from the secondary annulus of the burner. In general, the predicted and measured flow patterns are in fair qualitative agreement. Discrepancies in detail, for example at measurement distance x/D = 3.5 where the observed trend with coal type is not predicted, may be attributable to measurement uncertainty and/or failure of the model to capture the turbulent flow and combustion interaction. It should also be noted that for flames near critical swirl, small changes in operating conditions can result in large changes in the flow and combustion patterns in the near-burner region. The temperature profiles (Fig. 7) clearly show the structure of the flames in the near-burner zone, The temperature maxima lie in the narrow region adjacent to the internal-reeireulation zone where coal volatiles are evolved and burn; the temperature minima correspond to the relatively cool secondaryair jet; and the regions of uniform temperature between the flame and the furnace wall eorrespond to the well-mixed external-reeireulation zone. By

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Y (m) FIG. 7. Predicted and measured gas temperature profiles for coal flames J and H. comparison with the measurements, the predictions show a more pronounced variation in temperature through the flame in the near-burner zone and a less rapid evolution to a uniform distribution with increasing distance from the burner. The measurements show relatively little difference between the temperatures for the two flames. The oxygen concentration profiles (Fig. 8) also

COAL FLAME STABILITY

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Sensitivity to Model Parameters: Possible defects in the modelling which may account for the discrepancies in temperature and oxygen concentration, particularly in the near-burner zone where the coal ignites, are investigated by means of a sensitivity study of the model predictions for coal flame F. The modelling of turbulent transport is an obvious area which impacts on mixing flames. Increasing the turbulence levels at the burner nozzle produced no significant i m p r o v e m e n t in the reeireulation-zone properties. Implementation of an algebraic stress model in place of the k-r model produced marginally worse predictions. The modelling of devolatilization kinetics is an area of considerable uncertainty. Plausible variations in rate parameters for the single-reaction model (2 • 105 - 2 • 104 s-l), implementation of a multiple-parallel-reactions model, and changes in volatile yield within the measured range yielded no improvement. Radiation from soot formed in the fuel-rich regions of a flame may influence the temperature field. The arbitrary assumption that the local soot mass fraction is equal to the mass fraction of unreacted volatiles resulted in a decrease in the temperature close to the flame axis but otherwise little change. With regard to radiation modelling, it should be noted that the standard treatment of radiation neglecting soot yields incident radiation fluxes at the furnace wall in good agreement with the data for coal flame F. It is well known that all current turbulence models are deficient in predicting swirling flows. Cold flow studies of swirling jets 7 have shown that the strength of the internal-recirculation zone may be underpredicted by a factor of about two. Furthermore, the mass fraction of secondary fluid within the internal-recirculation zone may be under-estimated by a similar factor. There is also evidence that turbulence levels are frequently under-predicted in swirling flows, s In reacting flows, the deficiencies in the turbulence model are compounded by uncertainties in flame-generated turbulence. In a coal flame, any one of these effects will result in oxygen levels being underpredicted and temperatures overpredicted in the internal-recirculation zone. As noted above, there is no simple method to rectify the deficiencies in the turbulence model. However, some of the change to he expected of an improved turbulence model may be very crudely simulated by increasing the swirl, which has the effect of increasing reeireulation and turbulence levels in the nearburner zone while leaving the far-field relatively unchanged. Figure 9 shows the effect on the oxygen profiles for coal flame F of an arbitrary increase in the swirl level from 0.27 to about 0.6. The oxygen concen-

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FIG. 8. Predicted and measured oxygen concentration profiles (% vol dry) for coal flames J and H. show the structure of the flame in the near-burner zone. Within the central-recirculation zone, the predictions show vanishingly small levels of oxygen while the data generally show levels of about 5%. The predictions also indicate significant levels of carbon monoxide and unreacted volatiles within and immediately downstream of the central-recirculation zone. The presence of unreacted fuel species within the central-recirculation zone is supported by the data for both flames. The predictions and measurements both show a more rapid drop in oxygen concentration with increasing distance from the burner in the case of the high-volatile flame H.

969

COAL BOILERS/FURNACES

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Y (rn) Ftc. 9. Predicted and measured oxygen concentration profiles for coal flame F: - standard model; - - - - - - modified swirl level; . . . . modified volatile reaction scheme. tration within the central-recirculation zone is increased from zero to about 4% and the profiles evolve more rapidly to a uniform distribution. The predictions are generally in better agreement with the data. The representation of volatiles combustion is an area where far more model evaluation and refinement is needed. In the present work, the rates of combustion of volatiles and carbon monoxide are related to the turbulence time scale through an

empirical constant, A, which is taken as 4.0. 6 Not surprisingly, the predictions of temperature and species concentration in the near-burner zone of volatiles combustion are rather sensitive to A. However, if A is simply reduced to obtain the observed oxygen levels in the central-recirculation zone at the first measurement port, then the levels of carbon monoxide in the far field are grossly overpredicted. A recent mathematical modelling study at the IFRF 9 also identified the problem of simultaneously predicting the observed levels of oxygen and carbon monoxide in the near-burner region of coal flames and suggests a refinement to the gas-phase combustion model whereby the constant A is reduced to 0.5, and the reaction scheme is modified so that carbon in any fuel species reacts to carbon monoxide when the local conditions are sub-stoichiometric (fuel rich) and otherwise to carbon dioxide. The modification is shown to yield improved predictions of species concentrations throughout the flame. A modified volatiles reaction scheme similar to that proposed by the IFRF is examined in the present work. The constant A in the mixing-controlled model is taken as 0.5 for the reaction of voiatiles to carbon monoxide and for the oxidation of carbon monoxide under locally fuel-rich conditions. Under fuel-lean conditions, carbon monoxide is assumed to react in a premixed state at the kinetically-controlled rate. The oxygen profiles for coal flame F predicted using the modified gas-phase reaction scheme are also shown in Fig. 9. As can be seen, the modification yields improved predictions in the near-burner zone. However, in the absence of data for the concentration of unreacted fuel species in the near-burner zone, it is not possible to evaluate fully the modified volatiles-reaction scheme. Finally, it should be noted that the prediction of mixing-controlled reaction rates from the turbulence decay rate may be compromised by deficiencies in the turbulence model.

Conclusion

The flame stability and combustion efficiency of three coals with volatile contents varying from 1941% daf have been measured in a pilot-scale test furnace and the data interpreted using a mathematical model of the coal flame. The measurements and predictions suggest that the flame stability is primarily determined by the burner aerodynamics with little influence of coal volatile content, while the ignition behaviour is determined by the yield, specific energy and rate of release of volatiles. Detailed probing of the flames has established an extensive data base for evaluation of the coal flame model. Discrepancies in the detailed predictions of

COAL FLAME STABILITY the near-burner zone appear to be attributable to defects in the modelling of turbulent transport and/ or volatiles combustion.

Acknowledgment The support of the Australian National Energy Research Development and Demonstration Programme is gratefully acknowledged.

REFERENCES 1. TRUELOVE,J. S.: Twenty-Second Symposium (International) on Combustion, p. 155, The Combustion Institute, 1989. 2. JAMALUODIN,A. S., TRUELOVE, J. S. AND WALL, T. F.: Comb. Flame 63, 329 (1986).

971

3. KIMBER, G. M. AND GRAY, M. D.: Comb. Flame 11, 360 (1967). 4. BADZIOCH, S. AND HAWKSLEY,P. G. W.: Ind. Eng. Chem. Process Des. Dev. 9, 521 (1970), 5. YOUNC, B. C.: The Chemistry of Low Rank Coals (H. H. Schubert, Ed.), Amer. Chem. Soc. Symp. Series 264, p. 243, 1984. 6. MAGNUSSEN, B. F. AND HJERTAGER, B. H.: Sixteenth Symposium (International) on Combustion, p. 719, The Combustion Institute, 1979. 7. MAHMUD,T., TRUELOVE,J. S. AND WALL, T. F.: J. Fluids Engineering 109, 275 (1987). 8. SLOAN, D. G., SMITH, P. J. AND SMOOT, L. D.: Prog. Energy Combust. Sci. 12, 163 (1986). 9. V1SSER, B. M. AND WEBER, R.: Computations of near-burner-zone properties of swirling pulverized-coal flames, I F R F Report F 3 3 6 / a / 1 3 , IJmuiden, 1989.