Radiation Intensity of Propane-Fired Oxy-Fuel Flames: Implications for

Energy & Fuels 2008, 22, 1535–1541

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Radiation Intensity of Propane-Fired Oxy-Fuel Flames: Implications for Soot Formation Klas Andersson,* Robert Johansson, Filip Johnsson, and Bo Leckner Department of Energy and EnVironment, DiVision of Energy Technology, Chalmers UniVersity of Technology, SE-412 96, Göteborg, Sweden ReceiVed August 13, 2007. ReVised Manuscript ReceiVed February 14, 2008

The changes in the soot-related radiation intensity between two different oxy-fuel flames and an air-fired flame were investigated in a 100 kW oxy-fuel test unit firing propane. The oxy-fuel test cases with 21 and 27 vol % O2 in the recycled flue gas (RFG) were run with different amounts of dry RFG, which in principle only consisted of CO2 from combustion and some excess O2. The stoichiometric oxygen-to-fuel ratio was kept at 1.15 in all cases. Total radiation intensity was measured with a narrow angle radiometer. Temperature and gas composition measurements served as inputs to computations of gas radiation. A comparison of the computed gas radiation with the measured total radiation intensity enabled estimation of the radiation related to soot. Clear differences were observed in the amount of soot formed in the two oxy-fuel flames (also compared to the air flame). In the oxy-fuel flame with 21 vol % O2 in the RFG, soot formation is almost completely suppressed, but when the total flow through the burner is reduced by about 20% (by volume) (i.e., from 21 to 27 vol % O2 in the RFG), the amount of soot present in the flame becomes significant. This change in soot volume fraction affects the radiation emitted from the flames; images of the flames qualitatively confirm these differences in the flame luminosity. Thus, carbon dioxide not only increases the gas radiation, but it can also drastically influence soot formation and the radiation originating from soot during oxy-fuel combustion.

Introduction Oxy-fuel combustion is gaining increasing interest due to its promising capability to provide a comparatively cost-effective CO2 capture from the flue gases of boilers.1–3 Instead of using air, highly concentrated oxygen is added to the process, which is mixed with externally recycled flue gas (RFG), to oxidize the fuel, and as a consequence, new options to optimize the combustion process arise. It is of interest to explore these options in order to discover cost reduction measures for the process. The radiative heat transfer is one of the important areas that needs to be addressed to obtain fundamental data for the development of predictive modeling tools for fuel conversion in O2/CO2 mixtures during recycling of flue gases. In an early review,4 the influence of gaseous additives on soot formation in diffusion flames is discussed. On the basis of the studies cited, the authors conclude that the addition of CO2 (and also other gases, such as H2O) to the oxidizer, as well as replacement of N2 by CO2 in the oxidizer, will reduce soot formation in the flame. Furthermore, based on the analysis of the effect on soot by a number of additives,5 it is discussed that the reduction in the soot formation due to additive gases can mainly be attributed to the reduced combustion temperature. * To whom correspondence should be addressed. Telephone: +46 31 772 5242. Fax: +46 31 772 3592. E-mail: [email protected]. (1) Singh, D.; Croiset, E.; Douglas, P. L.; Douglas, M. A. Energy ConVers. Manage. 2003, 44, 3073–3091. (2) Andersson, K.; Johnsson, F. Energy ConVers. Manage. 2006, 47, 3487–3498. (3) Kakaras, E.; Doukelis, A.; Giannakopoulos, D.; Koumanakos, A. Fuel 2007, 86, 2151–2158. (4) Haynes, B. S.; Wagner, H. G. G. Prog. Energy Combust. Sci. 1981, 7, 229–273. (5) Schug, K. P.; Mannheimer-Timnat, Y.; Yaccarino, P.; Glassman, I. Combust. Sci. Technol. 1980, 22, 235–250.

Experimental work on the influence on soot formation by CO2 and O2 addition in ethylene and propane counterflow diffusion flames6 separated three paths through which the soot formation could be affected by additives: thermal effects caused by the change in the flame temperature, dilution effects due to reduction of the concentration of the reactive species, and chemical effects from additives to the chemical reactions that interfere with the formation of soot. Du et al.6 conclude that CO2 addition to either the fuel or oxidizer suppresses soot inception via direct chemical effects in addition to the effects of dilution or change in temperature but that the exact reaction mechanism still had to be established. In a more recent study, the chemical effects of CO2 added to ethylene diffusion flames were investigated numerically.7 The authors concluded that CO2 reduces soot formation through chemical reactions (R1 and R2) that enhance the oxidative attack of soot precursors: CO2 + H f CO + OH

(R1)

CO2 + CH f CO + HCO

(R2)

al.8

discuss the implications of coal-derived soot Fletcher et in coal combustion systems in terms of its influence on the radiative heat transfer and the related effects on peak temperature and NOx formation. A modeling study of a 915 MW coal-fired furnace9 shows that, if soot is included in the radiative heat transfer calculations and if 10% of the volatile matter is (6) Du, D. X.; Axelbaum, R. L.; Law, C. K. Twenty-third Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1989; pp 1501–1507. (7) Liu, F.; Guo, H.; Smallwood, G. J.; Gülder, Ö. L. Combust. Flame 2001, 125, 778–787. (8) Fletcher, T. H.; Ma, J.; Rigby, J. R.; Brown, A. L.; Webb, B. W. Prog. Energy Combust. Sci. 1997, 23, 283–301. (9) Ahluwalia, R. K.; Im, K. H. J. Inst. Energy 1994, 67, 23–29.

10.1021/ef7004942 CCC: $40.75  2008 American Chemical Society Published on Web 05/01/2008

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Andersson et al.

transformed into soot, then the radiative heat transfer is increased by about 15% compared to a case when soot is not included in the calculations. Thus, also for industrial scale applications, the role of soot in the radiative heat transfer may be substantial. It is important to note that the overall influence of soot in coal combustion is also related to the presence and radiative properties of ash and char particles. Previous narrow angle radiometer measurements, performed in the test unit of the present work, showed that soot formation during the combustion of propane may change from air to oxyfuel combustion modes.10 However, the total radiation intensity data presented did not permit a comparison of all cases tested. Furthermore, the spatial resolution of the temperature and the total radiation intensity data has been improved in the present work. This enables a direct comparison of the modeled gas radiation with the measured total radiation intensity. As a consequence, an estimate is obtained of the contribution of soot to the measured intensity. During the oxy-fuel tests, dry flue gas recycling can be applied. This means that the flue gas is cooled and dried in two steps and that the flue gas has a temperature of 20–30 °C when it enters the burner registers. Then, the moisture content is similar to that of the combustion with ambient air. In summary, the effect of the dry recycle rate (i.e., the amount of recycled CO2) on the radiation intensity of propane-fired oxy-fuel flames is studied, and the total radiation intensity measurements are related to gas radiation modeling. Theory The radiation intensity has been modeled with the Malkmus statistical narrow band model (SNBM).11 The transmissivity of each narrow band (index k) is given by (1)

where the product YPL is the pressure path length of the absorbing species. The narrow band parameters, kk and dk, for H2O and CO2 needed in the model were taken from the work of Soufiani and Taine.12 The parameters were derived from lineby-line calculations and were tabulated as a function of temperature in the range 300–2900 K. The spectral range considered is 150–9330 cm-1, and the width of each band, ∆ν, is 25 cm-1, which is narrow enough to assume a constant blackbody radiation for the temperatures of interest. The halfwidths were calculated from eq 2, with Ps ) 1 bar, and Ts ) 296 K:12

{

() ()

Ts T P 0.462YH2O + Ps T Ts

0.5

[0.079(1 - YCO

}

2

- YO2) +

0.106YCO2 + 0.036YO2] γH2O )

()

P Ts Ps T

0.7

{0.07YCO

(2)

+

2

0.058(1 - YCO2 - YH2O) + 0.1YH2O}

For nonuniform paths, the Curtis-Godson approximation13 is applied, and the narrow band transmissivity of the mixture is given by the product of the transmissivities of the different absorbing species. The radiative path is discretized into homo(10) (11) (12) 991. (13) 2003.

geneous cells, as outlined in Figure 1, and the intensity equation averaged for one band is n-1

Ik,n ) Ik,0τk,0fn +

∑ (τ

k,i+1fn - τk,ifn)Ib,k,i+

i)0

1 2

(3)

In this correlated formulation, the spectrally averaged intensity Ik,n depends on the emitted radiation of all upstream cells as well as on the intensity of the radiation leaving the wall, Ik,0. τifn is the transmissivity from cell i to cell n. The spectral blackbody intensity, Ib, assumed to be constant within a band, is calculated at the center of the band. Finally, the total intensity can be determined as the sum of all bands according to the following: Itot,n )

∑I

k,n∆ν

(4)

k

The modeled gas radiation intensity is to be compared against the measured total intensity that to a different degree includes radiation from soot in addition to radiation from the combustion gases. Experiments

τk ) exp[-2γ/dk(√1 + YPLkkdk/γ - 1)]

γCO2 )

Figure 1. Discretization of the radiative path.

Andersson, K.; Johnsson, F. Fuel 2007, 86, 656–668. Malkmus, W. J. Opt. Soc. Am. 1967, 57, 323–329. Soufiani, A.; Taine, J. Int. J. Heat Mass Transfer 1997, 40, 987– Modest, M. F. RadiatiVe Heat Transfer, 2nd ed.; Academic Press:

The measurements were performed in the Chalmers 100 kW test facility (Figure 2). This work presents propane-fired tests with fuel properties and test conditions presented in Tables 1 and 2. The unit is down-fired and has a cylindrical refractory-lined furnace with an inner height of 2.4 m and an inner diameter of 0.8 m. The burner (Figure 3) consists of a fuel lance (diameter ) 34 mm) surrounded by cylindrical primary and secondary feed-gas registers. Direct oxygen injection can be applied in a separate inlet in the fuel lance (see Figure 3), but this option was not used in the present tests. The primary register is swirled with a fin angle of 45° and an outer diameter of 52 mm, whereas the secondary register (outer diameter ) 92 mm) has a swirl with a fin angle of 15°. According to the burner manufacturer, the swirl numbers of the primary and the secondary registers are 0.79 and 0.21, respectively. The burner and the furnace ceiling are in line. Out of the total feed-gas flow, roughly 40% enters the primary stream, and the remaining part is fed to the secondary stream. All tests were run at a stoichiometric ratio (λ) of 1.15. In the oxy-fuel tests, the global oxygen contents of the recycled flue gas (RFG) were 21 and 27 vol % (OF 21 and OF 27). The oxygen fraction was achieved by varying the recycle rate. The mass recycle rates (the ratio of recycled to total flue gas mass flow, including moisture) were kept to 0.82 and 0.77 for the OF 21 and OF 27 conditions, respectively. The OF 21 condition was chosen to yield the same volumetric flow through the burner as in the air-firing (see Table 1), and the OF 27 condition was chosen to give a similar peak temperature to that of the air-firing (in the range 1500–1600 °C for the two flames). Dry flue gas recycling was applied, since the effect of carbon dioxide on the total radiation intensity is of interest. During dry recycle operation, the flue gas exits the combustion chamber and passes the flue-gas cooler, the fabric filter, and the flue-gas condenser, in which the flue gases are cooled to 20–30 °C. The dried flue gas is then mixed with fresh oxygen and forced into the combustion chamber through the primary and secondary registers of the burner. The fuel and oxygen flows are controlled by mass

Radiation Intensity of Oxy-Fuel Flames

Energy & Fuels, Vol. 22, No. 3, 2008 1537

Figure 2. The Chalmers 100 kW oxy-fuel combustion test unit. Table 1. Test Conditionsa

a The notations primary and secondary denote the respective burner registers given in Figure 3. The oxidant composition is the same in both registers; for example, in the OF 27 case, the oxidant consists of 27% O2, and the rest is recycled flue gas.

The temperature, the total radiation intensity, and the gas composition were measured in the furnace. The temperature was measured with a suction pyrometer equipped with a B-type thermocouple with a diameter of 0.5 mm. A suction velocity of about 150 m/s was applied during the pyrometer measurements in order to minimize radiation losses from the thermocouple junction. The gas samples were collected with a water-cooled suction probe. The gas was dried and filtered after collection with the probe and then transported to online gas analysis. The moisture content of the gas was not measured, but for modeling purposes the moisture was estimated from its formation during combustion in parallel to the CO2 formed (the inlet concentration of CO2 to the combustor was measured during oxy-fuel operation). The narrow angle radiometer (Figure 4) is of IFRF-type, and its principle design is described by Radoux et al.14 The probe measures the line-of-sight radiation intensity (sometimes referred to as radiance), which is given by a thermopile in the detector end of the probe. The probe is calibrated by a blackbody source of high

Table 2. Fuel and Oxygen Properties

flow meters of high precision. The flue gas recycle rate (and the pressure balance inside the loop) is fixed by two frequencycontrolled induced-draft fans.

Figure 3. Propane burner.


Andersson et al.

Figure 4. The narrow angle probe14 used for the radiation intensity measurements.

Figure 5. Direct photographs of (a) air flame, (b) OF 21 flame, and (c) OF 27 flame, 215 mm from the burner inlet.

precision. In the measurements presented here, the focus was on the net radiation intensity of the flame and the gas in the combustion chamber. In order to avoid background radiation from the furnace wall, the radiometer was aligned to a nonreflecting water-cooled body covered with black paint, resistant to high temperature. During the measurements, a purge gas flow of argon was applied through the collimating tube (see Figure 4) to prevent fouling of the tube and absorption of radiation from the combustion products. The flow of purge gas was kept at a minimum, and consequently, only about 1–2 vol % of CO2 in the recycled flue gas was replaced by argon, according to the gas composition measurements. This was taken into account in the gas radiation modeling, although it proved to have a negligible effect on the modeled intensity.

Results and Discussion Figure 5 shows photographs of the flames 215 mm from the burner inlet. Both the air-fired flame (a) and the OF 27 flame (c) display a luminous appearance due to the soot formed. On the other hand, the soot volume fraction is drastically reduced from the air and OF 27 flames to the OF 21 flame (Figure 5b), with the latter being dominated by its blue/violet emission rather than a yellow/orange emission, a characteristic of soot. In fact, the OF 21 flame is close to transparent, and it is possible to see the opposite probe port as a black ring in the background; the overall soot formation is significantly reduced. The radial profiles were measured at two distances in the combustor, in Port R3 (384 mm from the burner) and Port R7 (1400 mm from the burner). Port R3 includes a region with intense combustion and presence of soot, similar to the photographs shown in Figure 5, whereas in Port R7 the soot is not visible and has burnt out (as well as other combustibles), and the radiative heat exchange is only due to gas in this location. The temperatures are different in the three flames, but they are all high enough for an intense visible flame to be seen when soot is present. This is the case at 384 mm from the burner where the soot loading of the three flames is compared. The (14) Radoux, F.; Maalman, T.; Lallemant, N. Narrow Angle Radiometer Probe; IFRF Doco C 73/y/10; IFRF: Ijmuiden, The Netherlands, 1998.

profile data recorded 1400 mm from the burner, on the other hand, enable a comparison of gas radiation modeling with measurements of the total radiation intensity without the presence of soot. Figure 6 presents temperature distributions from the two measurement planes, 384 and 1400 mm from the burner, for the three flames. The highest peak temperature occurs in the air-fired flame, and the lowest occurs in the OF 21 flame. The temperatures increase from the OF 21 to the OF 27 flame, since the amount of recycled flue gas is reduced, as was mentioned previously. The highest recorded temperatures in Port R3 are approximately 1550, 1430, and 1250 °C for air, OF 27, and OF 21 conditions, respectively. The recorded peak temperature was measured 215 mm from the burner. It is in the same range in the air and OF 27 flames but is significantly lower in the OF 21 flame. As indicated in Figure 6a-c, the gas temperature is cooled effectively by the refractory-lined combustor walls and by four water-cooled tubes inserted into the furnace, providing additional cooling of the flames. Figure 6d-f shows how the gas temperatures decrease to a uniform level at 1400 mm away from the burner in all cases. Figure 7 shows the CO2 and H2O concentrations for all flames at 384 mm from the burner. These data are used for the computation of the gas radiation intensity at the corresponding distance. The CO concentration is up to about 5 vol % in the proximity of the center line of the flames in Port R3, but calculations of the gas radiation intensity show that CO has a negligible influence on the results. The flue gas profile 1400 mm from the burner is almost uniform; and therefore, the stoichiometric composition (from conditions according to Tables 1 and 2) was used for the radiation modeling at this probe port. Since both air-fired and oxy-fuel fired conditions are close to axi-symmetric in Figure 6, the gas composition on both sides of the center line (400 mm) is taken to be equal in Figure 7. As seen, the CO2 partial pressure increases by about 8 times during oxy-fuel operation as compared to that of air-firing. The water vapor concentration also increases slightly from air/OF 21 to



Figure 6. Radial temperature distributions 384 and 1400 mm from the burner inlet. The error bars show the maximum and minimum recorded temperatures during one sampling sequence. The furnace walls are located at 0 and 800 mm, and the center line is located at 400 mm (the same as in Figures 7 and 8).

Figure 7. CO2 and H2O profiles at 384 mm from the burner.

OF 27 due to the change in flow conditions (see Table 1), which then slightly increases the gas emissivity from OF 21 to OF 27 conditions. Figure 8 presents measurements of the total radiation and the modeling of gas radiation. The results from Port R7 (Figure 8d-f), where the influence of radiation from soot can be disregarded in all cases, show that the modeling overpredicts the intensity of the gas radiation incident on the combustor wall by about 20% when the measured average gas temperatures (Tavg) are used as input. The measurement error of the narrow angle probe, calibrated in a high precision blackbody radiation source, should not exceed 15% of the measured intensity. The computed gas radiation intensity in Figure 8d-f, which corresponds to the measured minimum and maximum temperatures (represented by error bars in Figure 6d-f), shows that the measured temperature variations in the flue gas could cause a deviation of the same order of magnitude as the discrepancy between the computed and modeled intensities, owing to the

low intensities recorded at this location. The disagreement between the modeling and the experimental results should, therefore, be considered acceptable in Port R7. The intensities shown in Figure 8a-c are much higher than those of Figure 8d-f because of higher temperatures and the presence of soot. There are also important differences between the three flames investigated. Both soot and gas contributes to the radiation emitted in the air-fired flame, but gas radiation dominates the total intensity signal (Figure 8a). In the OF 21 flame, the computed gas radiation is similar to the measured total radiation, and the results correspond well to the image of the OF 21 flame shown in Figure 5b, where the soot formation was almost completely suppressed. Furthermore, the total intensity of the OF 21 flame is close to that of the air flame; the OF 21 flame partially compensates for the decrease in temperature and the reduction of soot in the flame by enhanced radiation from CO2. Figure 8c shows a significant increase in radiation intensity from the OF 21 and the air case to the OF


Andersson et al.

Figure 8. Measured total radiation intensity and modeled gas radiation intensity. 0 mm corresponds to the position of the cold blackbody target, a replaced segment of the wall opposite to the probe opening. Measured average and minimum and maximum gas temperatures (see Figure 6) are used as input to the modeling.

27 case. In analogy, the photographs in Figure 5 show that the formation of soot is drastically enhanced when the recycle rate is reduced from OF 21 to OF 27 conditions. It was discussed previously that additives and replacement gases in the oxidizer (e.g., when N2 is replaced by CO2) can cause a reduction in the formation of soot and that the reduction can be brought about by thermal, dilution, and chemical effects. The obvious differences between the OF 21 and the OF 27 flame are the amount of recycled CO2 and its subsequent effect on the flame temperature. Yet, the effect of soot on the total radiation intensity is enhanced also for OF 27 as compared to airfired conditions, despite similar maximum flame temperatures. Table 1 shows that the inlet volumetric gas flow to the burner is reduced by about 20% in the OF 27 case as compared to the air and the OF 21 case, but the fuel input is kept the same in all cases. Thus, in the OF 27 case, the inlet fuel concentration is increased compared to the two other cases. During OF 27 conditions, the concentration and the residence time of soot precursors in the near-burner zone may be increased as compared to those of the air-firing for the same reasons. This would promote the formation of soot in the OF 27 case as compared to that of the OF 21 and air conditions. In addition, the change in flow conditions and the enhanced inlet oxygen concentration in the OF 27 case as compared to the air/OF 21 cases influence the mixing in the near-burner zone, which may contribute to the differences in soot formation. The evaluation of the radiative properties of the flames is based on average values. However, the nonlinearity of the temperature dependence of the radiative transfer may give rise to inaccuracies while averaging; εT 4 is not equal to the same quantity estimated from εT 4 . Instead, we have that

Itot,n ) εT 4 ) φεT 4

(5)

Coelho.15

The correction where φ is a correction factor, given by factor is greater than unity and depends on the magnitude of the fluctuations. According to Figure 6a-c, the temperature in Port R3 fluctuates in the range of a few percent up to 10% that results in φ e 1.06. Figure 8a-c shows that the deviation from the mean is in the same range for the maximum and minimum temperatures; the maximum and minimum intensities both deviate up to 10% from the mean. The temperatures are measured by suction pyrometer, employing a high suction velocity that reduces radiative losses, but it also deforms the local flame structure during sampling, and the measured temperature fluctuations are, therefore, just a coarse reflection of the actual flame structure. Also, the thermocouple has a response time, which dampens the amplitude of the actual fluctuations. There are, furthermore, fluctuations in concentration, that is, in emissivity, whose correlation with temperature was not recorded. Temperature fluctuations in the order of 20–30% are reported from laboratory scale diffusion flames.16,17 A temperature fluctuation of about 20% over the cross section, corresponding to φ ≈ 1.25, can therefore serve as an estimate of the largest error in the present flame. The effect of the temperature fluctuations may be seen from the flame with low soot content (Figure 8b); the calculated values are slightly lower than the measured ones, which could be caused by minor (15) Coelho, P. J. Prog. Energy Combust. Sci. 2007, 33, 311–383. (16) Krebs, W.; Koch, R.; Ganz, B.; Eigenmann, L.; Wittig, S. 26th Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2763–2770. (17) Masri, A. R.; Kalt, P. A. M.; Barlow, R. S. Combust. Flame 2004, 137, 1–37.


quantities of soot present but also from the nonlinearity. The calculated radiative intensities (Itot,calc) are compared with the narrow angle incident radiation (Iitot,meas) from a corresponding path across the flame to estimate the impact of soot; Iitot,meas Itot,calc ) the intensity of soot radiation, including a soot emissivity. Because φ > 1, this difference is actually smaller than what is represented by the mean values of Figure 8. However, even in the case where the temperature fluctuations would increase the gas emission by 25% (φ ) 1.25), as discussed above, the conclusion of the present work would remain unchanged; the soot formation changes drastically with CO2 recycle rate in oxy-fuel combustion. Conclusions CO2 recycling influences the intensity of flame radiation by changing the temperature and the emissivity in the oxy-fuel flames. For example, the radiation intensity emitted by the OF 21 flame is close to that of the air-fired flame, despite lower temperatures and less soot present in the flame; that is, the level of the radiation intensity is partially recovered by the high CO2 concentration in the OF 21 flame. Yet, the temperature gradients between the maximum temperature region of the flame and the combustor walls reduce the overall influence of the high CO2 partial pressure on the gas radiation incident on the combustor walls in the oxy-fuel flames tested. However, a more important observation from the measurement of the radiation intensity and the gas radiation modeling is that the soot formation enhances the flame radiation markedly in the OF 27 flame as compared to that of the OF 21 flame. Images of the oxy-fuel flames confirm significant differences in the amount of soot present in the flames; the OF 21 flame appears transparent in contrast to the OF 27 flame, in which a significant amount of soot is


produced. Thus, in addition to its effect on the flame temperature and gas emissivity, the recycled carbon dioxide can also cause drastic changes in the soot formation, thereby affecting the continuous radiation emitted by the flame. Acknowledgment. This work was cofinanced by the Swedish Energy Agency, the EU within the 6th framework program in the ENCAP project (Contract No. SES6-CT-2004-502666), and AGA gas AB.

Nomenclature d ) mean line spacing, cm-1 I ) spectral intensity, W m-2 sr-1 cm-1 Itot ) total intensity, W m-2 sr-1 Ib ) spectral blackbody intensity, W m-2 sr-1 k ) mean band absorption coefficient, cm-1 bar-1 L ) geometrical length, cm P ) pressure, bar T ) temperature, K Y ) mole fraction ε ) emissivity γ ) mean line half-width, cm-1 λ ) stoichiometric ratio τ ) transmissivity ν ) wave number, cm-1 Subscripts 0 ) wall, starting point of radiative pathway b ) blackbody value i ) cell number k ) band k n ) cell number nh ) nonhomogeneous ν ) spectral property EF7004942

Radiation Intensity of Propane-Fired Oxy-Fuel Flames: Implications for

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