Energy & Fuels 2006, 20, 2125-2130
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Emission Characteristics of the 0.03 MW Oxy-Fuel Combustor Ho Keun Kim and Yongmo Kim* Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-792, Korea
Sang Min Lee and Kook Young Ahn Eco-Machinery Engineering Department, Korea Institute of Machinery & Materials, Jang-Dong 171, Yusung-Ku Taejon 305-660, Korea ReceiVed July 26, 2005. ReVised Manuscript ReceiVed April 28, 2006
Emission characteristics of the 0.03 MW oxy-fuel combustor have been experimentally investigated under a wide operating range of velocity and quarl angle. To simulate the NO emission characteristics of industrial oxy-fuel furnaces, we mixed 3% nitrogen by supplied oxygen with pure oxygen. When a quarl is not fitted, the flame length decreased with increasing inlet fuel or oxidizer velocity, mainly because of the increase in turbulent intensity. In the case of the high-speed injection of fuel or oxidizer without a quarl, NO emission levels decrease because high-speed injection enhances the entrainment of product gas to the flame zone as well as decreases the overall combustion residence time. When a quarl is fitted, the flame length increases with expanding quarl angle because of the decrease in turbulent intensity. The NO emission level increases with expanding quarl angle because both the entrainment of product gas is prevented and the overall combustion residence time is increased with the installation of a quarl.
1. Introduction Oxy-fuel combustion technology is gradually gaining popularity in industrial combustion systems, such as glass, aluminum, iron, and steel production processes. Oxy-fuel combustion has the basic benefits, including high combustion efficiency, low exhaust-gas volumes, low fuel consumption, high melting capacity, and low NOx emission. Energy efficiency can be greatly enhanced by utilizing oxy-fuel combustion because the unnecessary heating of nitrogen in air is eliminated. In pure oxygen combustion that totally excludes nitrogen in the oxidizer, the volume of exhaust gas significantly decreases and zero NOx emission can theoretically be achieved. By varying the percentage of nitrogen in the oxidizer, we can calculate the flame propagation velocity and adiabatic temperature using PREMIX code,1 as illustrated in Figure 1. As shown in Figure 1, further advantages of oxy-fuel combustion are clearly identified in terms of higher temperature, faster flame-propagation speed and better flame stability. Moreover, it has been widely recognized that oxy-fuel combustion technology is one of the most effective ways to capture CO2 from exhaust gas as well as to cope with the future international CO2 regulation agreed upon in the 1997 Kyoto Protocol.2-5 * Corresponding author. Tel: 82-2-2220-0428. Fax: 82-2-2297-0339. E-mail:
[email protected]. (1) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A. A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames; Sandia National Laboratories Report SAND85-8240; Sandia National Laboratories: Albuquerque, NM, 1985. (2) Brown, J. T. Ceram. Eng. Sci. Proc. 1991, 12, 594-609. (3) Baukal, C. E. Industrial Burners Handbook; CRC Press: Boca Raton, FL, 2004. (4) Baukal, C. E. Oxygen-Enhanced Combustion; CRC Press: Boca Raton, FL, 1998. (5) Dankert, T. K.; Tuson, G. B. Ceram. Eng. Sci. Proc. 1996, 17-2, 47-54.
Figure 1. Effect of nitrogen-in-oxidizer percentage on laminar flame speed and adiabatic temperature.
It is expected that the progress in oxygen separation technologies will lead to ongoing decreases in the cost of oxygen. As such, the oxy-fuel combustors could be widely applied for hightemperature industrial combustion systems. Nevertheless, because it is impossible to completely eliminate nitrogen in oxyfuel combustion, the issue of NOx emission arises. Inherent nitrogen in oxygen or fuel and air infiltration through furnace gaps are mainly responsible for NOx formation in oxy-fuel furnaces. Therefore, in the design and application of the oxyfuel combustor, the reduction of NOx emission is the most important requirement. In air-fuel combustion, various methods such as flue gas recirculation (FGR) and staged combustion have been developed and applied to reduce NOx formation. Plenty of useful experimental data for various air-fuel combustion conditions are available. On the other hand, experimental data for oxy-fuel combustion are quite limited. Thus, the design of
10.1021/ef050232p CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
2126 Energy & Fuels, Vol. 20, No. 5, 2006
oxy-fuel combustors requires more comprehensive research efforts, including a sequence of trial and error testing, combustion measurements, and analysis. Ditaranto et al.6-8 showed that oxy-fuel combustion increases thermal efficiency and has potential for NOx emission reduction. They also showed that flame length and NOx emission are sensitive to air leaks into the combustion chamber. Tan et al.9 indicated that oxy-fuel combustion techniques based on O2/CO2 combustion with flue gas recycling offer excellent potential in terms of CO2 emission abatement, reduction of NOx emissions, and improved plant efficiency in conventional boilers. Baukal and Gebhart10 reported that thermal radiation increases dramatically with the enrichment of O2 in the oxidizer. They also found that oxygen enhancement causes the flame temperature to approach 3000 K so that NOx is produced mainly by the thermal mechanism. In view of soot formation in oxy-fuel combustion, Beltrame et al.11-14 reported that soot forms more easily upon an increase in the oxygen content of the oxidizer and soot particles agglomerate more readily with a rise in oxygen content. They provided an experimentally validated mathematical model of soot formation in oxygen-enhanced flames. In a semi-industrial scale test furnace, Hedley et al.15 showed temperature, species concentration, including NOx, and results corroborated with numerical prediction. Brink et al.16 reported that the prediction obtained with the EDC model was in better agreement with measurements than with the PDF model in oxyfuel combustion. The OXYFLAME project17-19 in IFRF provided much information on oxy-fuel combustion. The project gave us comprehensive in-flame data, including axial velocity and turbulence, temperature, various species, total radiance, and total radiative fluxes at the furnace wall, collected in flames of thermal input of 0.8 to 1 MW. This project showed that NOx emission is increased by increasing stoichiometry and by enriching N2 in the fuel and in the oxidizer (0-19%) but is decreased by further N2 enrichment (19- 47%) in the oxidizer. They also found that NOx emission is decreased by high(6) Ditaranto, M.; Sautet, J. C.; Samaniego, J. M. Exp. Fluids 2001, 30, 253-261. (7) Sautet, J. C.; Ditaranto, M.; Samaniego, J. M.; Charon, O. Int. Commun. Heat Mass Transfer 1999, 26, 647-656. (8) Sautet, J. C.; Salentey, L.; Ditaranto, M. Int. Commun. Heat Mass Transfer 2001, 28, 277-287. (9) Tan, Y.; Douglas, M. A.; Thambimuthu, K. V. Fuel 2002, 81 (8), 979-1091. (10) Baukal, C. E.; Gebhart, B. Int. J. Heat Mass Transfer 1997, 40, 2539-2547. (11) Beltrame, A.; Porshnev, P.; Merchan, W. M.; Saveliev, A.; Fridman, A.; Kennedy, L. A.; Petrova, O.; Zhdanok, S.; Amouri, F.; Charon, O. Combust. Flame 2001, 124, 295-310. (12) Lee, K. O.; Megaridis, C. M.; Zelepouga, S.; Saveliev, A. V.; Kennedy, L. A.; Charon, O.; Ammouri, F. Combust. Flame 2000, 121, 323333. (13) Naik, S. V.; Laurendeau, N. M.; Cooke, J. A.; Smooke, M. D. Combust. Flame 2003, 134, 425-431. (14) Naik, S. V.; Laurendeau, N. M. Combust. Flame 2002, 129, 112119. (15) Hedley, J. T.; Pourkashanian, M.; Williams, A. Combust. Sci. Technol. 1995, 108, 311-322. (16) Brink, A.; Hupa, M.; Breussin, F.; Lallemant, N.; Weber, R. J. Propul. Power 2000, 16, 609-614. (17) Lallemant, N.; Breussin, F.; Weber, R.; Ekman, T.; Dugue, J.; Samaniego, J. M.; Charon, O.; Van Den, A. J.; Van Der, J.; Fujisaki, W.; Imanari, T.; Nakamura, T.; Iino, K. J. Inst. Energy 2000, 73, 169-182. (18) Lallemant, N.; Dugue, J.; Weber, R. Analysis of the Experimental Data Collected during the Oxyflame-1 and Oxyflame-2 Experiments; IFRF Document F85/y/4 Part 1; International Flame Research Foundation: Velsen Noord, The Netherlands, 1997. (19) Bollettini, U.; Breussin, F.; Lallemant, N.; Weber, R. Mathematical Modeling of Oxy-Natural Gas Flames; IFRF Document F85/y/6; International Flame Research Foundation: Velsen Noord, The Netherlands, 1997.
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Figure 2. Schematic configuration of the oxy-fuel combustor.
momentum injection. However, these studies were conducted under limited experimental conditions. From a design perspective, these results do not clearly demonstrate whether NOx emission increases with varying fuel or oxidizer velocity. The main motivation of the present study has been to experimentally investigate the flame structure and emission characteristics of a 0.03 MW oxy-fuel combustor. The purpose of this study is to develop an improved understanding of the influences of fuel and oxidizer velocity with or without a quarl in NO emissions in oxy-fuel combustion. Last, we show the turndown capacity of the 0.03 MW oxy-fuel combustor designed by our research group. 2. Experimental Setup The 0.03 MW oxy-fuel combustor used in this paper supplied fuel through a central nozzle and oxidizer through an annular nozzle (Figure 2). Adjusting the diameters of the central and annular nozzles varies the inlet flow velocities of the fuel and oxidizer. Measurements were made on NO emission and flame length for various velocities of fuel and oxidizer without a quarl. The 0.03 MW oxy-fuel combustor is designed to easily equip a quarl. The four quarls (0, 3, 7, and 15° angles) were made of stainless steel. The quarl length was 2.5 times the outer diameter of the oxidizer and the inlet diameter of the quarl was equal to the outer diameter of the annular oxidizer nozzle. Measurements were made on NO emission and flame length for the four quarl angles. Detailed information about the combustor geometry and inlet conditions is listed in Table 1. Table 1. Dimension and Inlet Conditions of the 0.03 MW Oxy-Fuel Combustor diameter (mm) Df (in)
velocity (m/s)
Df (out)
Do (out)
Vo
Vf
2.7
4.7
9.52 10.8 12.98 17.89
34.2 24.8 16.0 7.9
146.2
3.65
5.65
9.52 10.8 12.98 17.89
39.9 27.7 17.2 8.1
79.9
4.21
6.21
9.52 10.8 12.98 17.89
45.0 30.0 18.0 8.3
60.0
5.16
7.16
9.52 10.47 10.8 12.98 17.89
59.6 40.2 35.9 20.0 8.7
40.0
7.29
9.29
10.8 12.98 17.89
77.3 28.5 10.0
20.0
Emission Characteristics of the 0.03 MW Oxy-Fuel Combustor
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Figure 4. Effect of nitrogen-in-oxidizer percentage on NO emission.
Figure 3. Experimental apparatus of the 0.03 MW oxy-fuel combustor. Table 2. Specifications of the Gas Analyzer species
accuracy (%)
principle operating range
CO (ppm) CO (%) CO2 (%) O2 (%) NO (ppm)
∼0.5 ∼0.5 ∼0.5 ∼0.5 ∼0.5
NDIR 0 to 100/1000 ppm NDIR 0 to 5/50% NDIR 0 to 10/100% Paramagnetic 0 to 25/100% NDIR 0 to 200/2000 ppm
As shown in Figure 3, the experimental apparatus consisted of a burner, combustion chamber, flow controlling system and gas analyzer. In this oxy-fuel combustor, there exists a sudden expansion step that leads to the enhancement of turbulent mixing and the entrainment of recirculated product gases. The fuel used was a chemically pure grade (>99.9%) methane. To simulate the NO emission characteristics of industrial oxy-fuel furnaces, we mixed 3% nitrogen by supplied oxygen with the pure oxygen for all combustor conditions. The flow rates of methane, oxygen, and nitrogen were fixed at 50 L/min (0.03 MW), 101.6 L/min (O2), and 3 L/min (N2), respectively. The overall equivalence ratio is 0.98 for all conditions shown in Table 1. A cylindrical quartz tube, 300 mm in diameter and 1200 mm in length, was used for visualizing the oxy-fuel flames and combustion processes. The diameter of the exhaust duct was 100 mm. Measurements are performed for various experimental conditions on flame length using video camera. A gas analyzer using a water-cooled sampling probe was installed to measure the emission levels for CO, CO2, O2, and NO. The gas analyzer specifications (HORIBA, VA-3000) are listed in Table 2.
3. Results and Discussion To examine NO emission characteristics for the nitrogen percentage in the oxidizer, at the inlet flow conditions specified with Vf ) 60 m/s and Vo ) 18 m/s, we measured NO emission levels for a variety of nitrogen conditions in the oxidizer. These experimental data are obtained by increasing the nitrogen flow rate in the oxidizer, whereas the fuel and oxygen flow rates are fixed. As shown in Figure 4, the NO emission level substantially increases with an elevation in the nitrogen-in-oxidizer percentage up to nearly 20%. Especially at percentages less than 10%, the
NO emission level drastically increases by enriching the nitrogen in the oxidizer. At nitrogen-in-oxidizer percentages between 10 and 20%, the NO emission level gradually increased. The NO emission level reached a peak value at around 20% nitrogenin-oxidizer. According to Wang et al.,21 in measurements of the turbulent jet flames without recirculation zone, NO emission was maximized at about 25%. This difference likely results from the different inflow conditions and combustor geometry, which might influence the residence time, turbulent mixing processes, and the entrainment of recirculated product gases. Experimental results also indicate that further increases in the nitrogen-inoxidizer percentage above 20% yield a decrease in NO emission. At a nitrogen-in-oxidizer percentage up to 27%, the NO emission level gradually decreases by enriching the nitrogen concentration. At percentages between 27 and 55%, the NO emission level substantially decreased by increasing the nitrogenin-oxidizer percentage. This considerable decrease of the NO emission level is mainly caused by the flame temperature decrease, and the nitrogen dilution effects in the flame zone and post-flame zone.17 Above 55% nitrogen-in-oxidizer, measurements were not performed because of the increased CO emission. These results clearly indicate that NO emission is quite sensitive with small nitrogen flow rates. Figure 5 shows the oxy-fuel flame images for four oxidizer velocities at the fixed fuel velocity, Vf ) 60 m/s without a quarl, whereas 3% nitrogen by supplied oxygen is mixed with the pure oxygen. Because the oxidizer flow rate is fixed for different oxidizer inlet velocity conditions, the smaller size of annular nozzle corresponds to a higher oxidizer inlet velocity. It is necessary to note that, in the case of the air-fuel combustion, flame stabilization is hardly achieved for fuel or air velocities higher than 10 m/s. Unlike air-fuel combustion, even at very high oxidizer velocity (45.0 m/s), the oxy-fuel flames are attached to the fuel nozzle without a swirler or quarl. This implies that oxy-fuel combustion significantly enhances flame stabilization because of a much higher burning velocity compared to air-fuel flames, as illustrated in Figure 1. The increase in oxidizer velocity decreases the yellowish luminosity from soot particles, flame length, and volume, because of the increase in turbulent intensity.8 These results are agree well with those of Wang et al.21 Figure 6 shows the variation of flame length for a wide range of fuel and oxidizer velocities without a quarl. Flame length (20) Weber, R. Scaling Characteristics of Aerodynamics, Heat Transfer, and Pollutant Emissions in Industrial Flames. Proceedings of the 26th Symposium on Combustion, Naples, Italy, 1996; The Combustion Institute: Pittsburgh, PA, 1996; pp 3343-3354. (21) Wang, L.; Endrud, N. E.; Turns, S. R.; D’agostini, M. D.; Slabejkov, A. G. Combust. Sci. Technol. 2002, 174 (8), 45-72.
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Figure 7. Dimensionless flame length of jet flames correlated with flame Froude number.
Figure 5. Flame images for four oxidizer injections at velocity Vf ) 60 m/s; (a) Vo ) 8.3, (b) Vo ) 18.0, (c) Vo ) 30.0, and (d) Vo ) 45.0 m/s.
jet diffusion flames. Thus, a correlation between characteristics of dimensionless flame lengths, including a fire Froude number and a burning fuel parameter for buoyancy and momentumdominant jets, is expressed in the following equations:
Frf )
Vefs3/2 (Fe/F∞)1/4(∆Tf/T∞gdj)1/2 L* )
L f fs df(Fe/F∞)1/2
(1)
(2)
The fire Froude number physically represents the ratio of initial jet momentum-induced force to buoyancy-induced force, as shown eq 1. The dimensionless flame length in eq 2 is expressed in terms of the stoichiometric mixture fraction, the ratio of the nozzle fuel density to ambient gas density, and the initial jet diameter. On the basis of these dimensionless parameters, one can define two regimes. In a buoyancy-dominated regime, the dimensionless flame length is expressed by the following correlation. Figure 6. Flame length versus oxidizer velocity for five fuel-injection velocities.
decreases as either fuel or oxidizer velocity increases, because of the increased turbulent intensity. In the case of relatively low fuel velocity (Vf < 40 m/s), the flame length substantially decreases with increasing oxidizer velocity. However, in case of very high fuel velocity (Vf > 80 m/s), the flame length slightly decreases with increasing oxidizer velocity up to 40 m/s. In the turbulent air-fuel non-premixed jet flames without recirculation, Turns et al.22 and Newbold et al.23 undertook extensive studies in order to find the relationships among flame radiation, NOx emissions, and residence time. For better understanding of the turbulent oxy-fuel flame characteristics, the present study has attempted to find a relationship between the length and velocity of the oxy-fuel flame, similar to the relationship of the airfuel flame. On the basis of dimensional arguments of physics for turbulent diffusion flames, Delichatsios24 analyzed the transition characteristics from momentum to buoyancy-controlled turbulent (22) Turns, S. R.; Myhr, F. H. Combust. Flame 1991, 87, 319-335. (23) Newbold, G. J. R.; Nathan, G. J.; Nobes, D. S.; Turns, S. R. Proc. Combust. Inst. 2000, 28, 481-487. (24) Delichatsios, M. A. Combust. Flame 1993, 92, 349-364. (25) Flamme, M. Energy ConVers. Manage. 2001, 42, 1919-1935. (26) Hu, Y.; Naito, S.; Kobayashi, N.; Hasatani, M. Fuel 2000, 79, 19251932.
L* ) 13.5Frf2/5 for Frf < 5
(3)
On the other hand, in a momentum-dominated regime, the dimensionless flame length L* is constant.
L* ) 23 for Frf g5
(4)
Therefore, the relationship of flame length to the oxy-fuel flame could be expressed by using eqs 1 and 2. Figure 7 shows the dimensionless flame length correlated with flame Froude numbers. The solid line and symbol show correlation (eqs 3 and 4) of Delichatsios24 and the present experimental data, respectively. As shown in Figure 7, the measured dimensionless oxy-fuel flame lengths are considerably longer than those estimated by Delichatsios’s correlation. Moreover, experimental data did not cluster on one line. This discrepancy is attributed to the shortcomings of Delichatsios’s correlation, on the basis of a mathematical similarity solution and experimental results of air-fuel flames. It also indicates that the improved correlation for the flame length of oxy-fuel flame should account for the effects of oxidizer coflow. Therefore, to realistically estimate oxy-fuel flame lengths, it is necessary to modify Delichatsios’s correlation. Although we cannot suggest any relationship because of limited experimental data, Figure 7 clearly shows that oxy-fuel flames exist within the momentum-dominated
Emission Characteristics of the 0.03 MW Oxy-Fuel Combustor
Figure 8. NO emission versus oxidizer velocity for five fuel-injection velocities.
regime. This implies that momentum-controlled oxygenmethane flames can easily be formed, whereas it is almost impossible to produce momentum-controlled air-methane flames because of flame stability limits. Thus, in the turbulent oxy-fuel flames with oxidizer coflow and recirculation, it is still a difficult task to find relationships among flame radiation, NOx emissions, and residence time. To resolve this issue, we need further extensive research. Figure 8 shows the measured NO emission level for a wide range of fuel and oxidizer velocity without a quarl. For all fuel velocities (20, 40, 60, 80, and 146 m/s), the NO emission level rapidly decreases with increasing oxidizer velocity up to 20 m/s. At oxidizer velocities higher than 20 m/s, the NO emission level gradually decreases with increasing oxidizer velocity. At a relatively low oxidizer velocity (8 m/s), the highest fuel velocity (146 m/s) yields the maximum NO emission level (670 ppm) because intense oxy-fuel burning occurs within the small flame volume. However, at the oxidizer velocity higher than 34 m/s, the highest fuel velocity produces a lower NO level (100 ppm) because of reduced flame temperature resulting from entrained product gas with increasing oxygen velocity.20 In this oxyfuel combustion process, the increase in oxidizer velocity especially at high-velocity fuel injection results in significant entrainment of the recirculated product gas to flame zone. These entrained products play a role in decreasing the temperature at the flame zone and the post-flame zone, where thermal NO is mainly formed. Consequently, the NO emission level decreases with increasing oxidizer velocities. Wang et al.21 stated that highmomentum flames produce more NO than low-momentum flames. Those trends are also reproduced in the present measurements if only fuel momentum is considered, but our results show that increased oxidizer momentum results in reduced NO emission. This different trend is mainly caused by the entrainment of the recirculated product gas encountered in the present oxy-fuel combustor. In air-fuel combustors, a quarl is often installed to stabilize the flame as well as protect the combustor inlet wall from the hot combustion zone.3 In the present study, measurements are made for a wide range of quarl angles and fuel velocities with medium oxidizer velocities (16-20 m/s). Figure 9 shows the effect of quarl angle on the flame length in the oxy-fuel combustor. By increasing the quarl angle up to 3°, the flame length is noticeably increased up to the threshold value. This trend is progressively apparent for the lower-velocity fuel injection. The increased flame length is directly tied to the decrease in turbulent intensity in a large-angle quarl. With a further increase in quarl angle, the flame length is almost
Energy & Fuels, Vol. 20, No. 5, 2006 2129
Figure 9. Effect of quarl angle on flame length at four fuel velocities.
Figure 10. Effect of quarl angle on NO emission at four fuel velocities.
invariable. This result indicates that the flame length is not affected by increased quarl angle over a threshold value. Figure 10 presents the effect of the quarl angle on NO emission in the oxy-fuel combustor. This result indicates that the NO emission level is increased with expanding quarl angle and increasing fuel velocity. In the case of the large quarl angle and high fuel velocity, a substantial increase in NO emission is mainly caused by the interception of entrained product gas and the increase residence time in the quarl. The minimum NO emission level corresponding to the zero angle quarl is higher than the minimum NO emission level for the oxy-fuel combustor without a quarl, as shown in Figure 8. These experimental results suggest that the installation of a quarl in the oxy-fuel combustor is not recommended for reducing NO emission. Optimal furnace operation requires a high capacity of the combustor as well as the product being heated, a certain type of furnace, temperature uniformity, the desired atmosphere, and favorable prevailing economic conditions.3 In this aspect, we need to know the turndown capacity of oxy-fuel combustors. Measurements estimating the turndown capacity of a designed 0.03 MW oxy-fuel combustor have been performed for the various turndown ratios at a fixed fuel/oxygen ratio. At this time, the 100% turndown ratio is 0.03 MW. At 100% turndown ratio, the corresponding oxy-fuel combustor flow conditions are Vf ) 60 m/s and Vo ) 45 m/s, among various combustor conditions of Table 1. Figure 11 shows the measured NO emission level versus the turndown ratio. The maximum turndown ratio is around 115% and the minimum is nearly 60%. Above the maximum turndown ratio, the CO emission increased
2130 Energy & Fuels, Vol. 20, No. 5, 2006
Figure 11. NO emission level versus turndown ratio for the 0.03 MW oxy-fuel combustor.
drastically, because the increased fuel and oxidizer velocity considerably reduced residence time and increased incomplete combustion. Under the minimum turndown ratio, the CO emission also increased drastically because of the poor turbulent intensity. By decreasing the turndown ratio, the NO emission level gradually increased, because the residence time at the hot combustion zone is increased by decreasing the fuel and oxidizer velocity.6-8 At the minimum and maximum turndown ratios, the measured NO emission levels are 106 and 88 ppm, respectively. 4. Conclusion Flame structure and NO emission characteristics of the 0.03 MW oxy-fuel combustor have been experimentally investigated under a wide operating range of velocities and quarl angles. On the basis of the experimental results obtained in this study, we can draw the following conclusions: 1. NO emission level substantially increases with elevating the nitrogen-in-oxidizer percentage up to nearly 20%. The NO emission level reaches a peak value at around 20%. Further increases in the nitrogen-in-oxidizer percentage higher than 20% yield a decrease in NO emission. This considerable decrease in NO emission level at the high nitrogen-in-oxidizer percentage is mainly caused by the decrease in flame temperature and the nitrogen dilution effects in the flame zone and post-flame zone. 2. Unlike air-fuel combustion, even at very high oxidizer velocity (45.0 m/s), without a swirler or quarl, the oxy-fuel flames attach to the fuel nozzle. This implies that oxy-fuel
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combustion significantly enhances flame stabilization because of a much higher burning velocity compared to that of airfuel combustion. 3. Flame length decreases as either fuel or oxidizer velocity increases because of the increased turbulent intensity. In the case of a relatively low fuel velocity (Vf < 40m/s), the flame length substantially decreases with increasing oxidizer velocity. However, in the case of very high fuel velocity (Vf > 80 m/s), the flame length slightly decreases with increasing oxidizer velocity up to 40 m/s. 4. The measured dimensionless oxy-fuel flame lengths are considerably longer than those estimated by Delichatsios’s correlation, which does not take into account the effect of the oxidizer coflow. Experimental results also indicate that momentum-controlled oxygen-methane flames are easily formed, whereas it is almost impossible to produce momentumcontrolled air-methane flames because of flame stability limits. 5. The increase in oxidizer velocity, especially at high-velocity fuel injection, results in significant entrainment of the recirculated product gas to flame zone. These entrained products play a crucial role in decreasing the temperature at the flame zone and the post-flame zone, where thermal NO is mainly formed. 6. With a quarl, the NO emission level increases with expanding quarl angle and increasing fuel velocity. In the case of a large quarl angle and high fuel velocity, the substantial increase in NO emission is mainly caused by the interception of entrained product gas and the increased residence time in the quarl. 7. The minimum NO emission level corresponding to the zero angle quarl is higher than the minimum NO emission level for the oxy-fuel combustor without a quarl. These experimental results suggest that the installation of a quarl in the oxy-fuel combustor is not recommended for reducing NO formation. 8. The maximum turndown ratio of the designed 0.03 MW oxy-fuel combustor is around 115% and the minimum is nearly 60%. At the minimum and maximum turndown ratios, the measured NO emission levels are 106 and 88 ppm, respectively. Acknowledgment. This research was supported by Grant AE2101-1-0-1 from the Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean Government. EF050232P
