Energy Fuels 2009, 23, 5331–5337 Published on Web 10/08/2009
: DOI:10.1021/ef900383s
Emission Characteristics of the 0.2 MW Oxy-fuel Combustor Ho Keun Kim,† Yongmo Kim,*,† Sang Min Lee,‡ and Kook Young Ahn‡ †
Department of Mechanical Engineering, Hanyang University, Seoul 133-791, Korea, and ‡ Korea Institute of Machinery & Materials, 171 Jang-Dong, Daejeon 305-343, Korea Received April 30, 2009. Revised Manuscript Received August 14, 2009
The emission characteristics of the 0.2 MW LNG oxy-fuel combustor have been experimentally investigated for the wide operating range of oxidizer velocity. The detailed in-flame data of temperature and concentrations, such as O2, CO2, CO, and NO, are measured for the oxy-fuel combustor. Nitrogen (3%) is mixed with oxygen to experimentally simulate the NOx emission characteristics in the industrial oxy-fuel furnaces, where nitrogen comes from air infiltration or inherent nitrogen in fuel or oxygen as impurities. Special emphasis is given to the effects of the oxidizer velocity on entrainment of product gases, NO emission, and flame structure in the oxy-fuel combustors. The experimental results obtained in the present study suggest that oxidizer velocity at the oxy-fuel combustor could be one of the crucial design parameters to control the NO emission.
are mainly responsible for the NOx formation in the real oxyfuel combustion systems. Therefore, in the design and application aspects of oxy-fuel combustor, the NOx emission should be minimized. In the case of air-fuel combustion,5 various methods, such as flue gas recirculation (FGR) and staged combustion, have been developed and applied to reduce NOx emission. Plenty of useful experimental data for various air-fuel combustion conditions are also available. On the other hand, experimental data for oxy-fuel combustion are quite limited. Ditaranto et al.6-8 showed that the oxy-fuel combustion considerably increases thermal efficiency and has a great potential to reduce NOx emission. It was also observed that the NOx emission is sensitively influenced by air leaks into the combustion chamber. Experimental results obtained by Tan et al.9 indicated that the oxy-fuel combustion techniques with the flue gas recycle offer excellent potential for CO2 emission abatement, reduction on NOx emissions, and plant efficiency in conventional boiler. According to Baukal10 and Naik,11,12 thermal radiation increases dramatically by enriching oxygen in the oxidizer as
1. Introduction Oxy-fuel combustion technology has gradually gained popularity in industrial combustion systems, such as the glassmaking, aluminum, iron, and steel production processes.1-3 The oxy-fuel combustion has the basic advantages, including high flame temperature, enhanced stability, low exhaust gas volumes, low fuel consumption, high melting capacity, and low NOx emission.1 For heating and melting furnaces, productivity and energy efficiency can be greatly enhanced by using the oxy-fuel combustion because the unnecessary heating of nitrogen in air is eliminated.2 In the oxy-fuel combustion, the volume of exhaust gas significantly decreases and the zero NOx emission can be achieved theoretically if no nitrogen is injected through air infiltration or inherent nitrogen in fuel or oxygen. It has been widely recognized that the oxy-fuel combustion technology is one of the most effective ways to capture CO2 from exhaust gas as well as to comply with the future CO2 regulation internationally agreed upon by the 1997 Kyoto Protocol.4 Because the progress of the oxygen separation technologies leads to continuously cut down the cost of oxygen, the oxyfuel combustion might gradually gain popularity for the application of the high-temperature industrial combustion systems. Because it is difficult to perfectly eliminate nitrogen in the oxy-fuel combustion, the NOx emission control could be one of the crucial research issues.1 Both inherent nitrogen in oxygen or fuel and the air infiltration through gap of furnace
(5) Flamme, M. Low NOx combustion technologies for high temperature applications. Energy Convers. Manage. 2001, 42, 1919– 1935. (6) Ditaranto, M.; Sautet, J. C.; Samaniego, J. M. Structural aspects of coaxial oxy-fuel flames. Exp. Fluids 2001, 30, 253–261. (7) Sautet, J. C.; Ditaranto, M.; Samaniego, J. M.; Charon, O. Properties of turbulence in natural gas-oxygen diffusion flames. Int. Comm. Heat Mass Transfer 1999, 26, 647–656. (8) Sautet, J. C.; Salentey, L.; Ditaranto, M. Large-scale turbulent structures in non-premixed oxygen enriched flames. Int. Comm. Heat Mass Transfer 2001, 28, 277–287. (9) Tan, Y.; Douglas, M. A.; Thambimuthu, K. V. CO2 capture using oxygen enhanced combustion strategies for natural gas power plants. Fuel 2002, 18 (8), 979–1091. (10) Baukal, C. E.; Gebhart, B. Oxygen-enhance/natural gas flame radiation. Int. J. Heat Mass Transfer 1997, 40, 2539–2547. (11) Naik, S. V.; Laurendeau, N. M.; Cooke, J. A.; Smooke, M. D. Effect of radiation on nitric oxide concentration under sooting oxy-fuel conditions. Combust. Flame 2003, 134, 425–431. (12) Naik, S. V.; Laurendeau, N. M. Quantitiative laser-saturated fluorescence measurements of nitric oxide in counter-flow diffusion flames under sooting oxy-fuel conditions. Combust. Flame 2002, 129, 112–119.
*To whom correspondence should be addressed: Department of Mechanical Engineering Hanyang University, 17, Haengdang-Dong, Seongdong-Ku, Seoul 133-172, Korea. Telephone: þ82-2-2220-0428. Fax: þ82-2-2297-0339. E-mail:
[email protected]. (1) Baukal, C. E. Industrial Burners Handbook; CRC Press: Boca Raton, FL, 2004. (2) Brown, A.; Ekman, T.; Axelsson, C. L. The development and application of oxy-fuel technology for use in heating furnace applications. Proceedings of 2001 Joint International Combustion Symposium, Kauai, Hawaii, 2001 (3) Baukal, C. E. Oxygen-Enriched Combustion; CRC Press: Boca Raton, FL, 1998. (4) International Energy Agency (IEA) Greenhouse Gas R&D Programme. Greenhouse Issues, 2006; pp 11-12. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 5331–5337
: DOI:10.1021/ef900383s
Kim et al.
well as by increasing a firing rate. It was also found that the NOx formation for the oxygen-enhanced gaseous flames is mainly controlled by a thermal mechanism because of the substantially elevated flame temperature. The OXYFLAME project13-15 provided the useful inflame data, which were collected for the oxy-fuel flames with thermal input from 0.8 to 1.0 MW. Their measurements13-15 include the profiles of axial velocity, temperature, species concentrations, total radiance, and total radiative fluxes at the furnace wall. Moreover, the effects of nitrogen concentration and momentum injection strength on the NOx emission characteristics were also experimentally analyzed. Recently, to achieve the quite low emission of nitrogen oxides, Blasiak et al.16 suggested the flameless oxy-fuel combustion technology, which is characterized by a lower temperature, more uniform temperature distribution, lower oxygen concentration, a lower level of nitrogen inside the combustor. They presented the main features and NOx emission characteristics of the flameless oxy-fuel combustion and made the comparative study for the flameless oxy-fuel and air-fuel combustion processes. However, those previous studies for the oxy-fuel flames were conducted under the limited conditions. The design of the oxyfuel combustor requires the detailed information of the oxyflame structure as well as the more comprehensive research efforts including sequence of trial and error testing, combustion measurements, and analysis. In this aspect, the present study has been mainly motivated to investigate the flame structure and emission characteristics of the 0.2 MW oxy-fuel combustors for the wide range of operating conditions. In the case of the small-scale 0.03 MW oxy-fuel combustor,17 measurements were performed for the various conditions of the fuel and oxygen velocity. In the present study, additionally, the detailed in-flame data of the 0.2 MW oxy-fuel combustor are measured for the wide range of the oxidizer velocity.
Table 1. Dimension and Inlet Conditions of the 0.2 MW Oxy-fuel Combustor fuel case 1 2 3 4
oxidizer
diameter (mm)
velocity (m/s)
diameter, in (mm)
diameter, out (mm)
velocity (m/s)
11.5 11.5 11.5 11.5
60 60 60 60
17.0 17.0 17.0 17.0
25.5 29.0 34.6 47.5
45 30 18 8.3
Figure 1. Combustor configuration and schematic of the 0.2 MW oxy-fuel furnace.
rate is mixed with the pure oxygen. Detailed information about the geometry and inlet conditions is listed in Table 1. The experimental arrangement for the 0.2 MW oxy-fuel furnace is illustrated in Figure 1b. In this furnace, the six cooling air tubes are installed for steady in-furnace operation. The diameter and thickness of the cooling air tube are 77 and 3 mm, respectively. The cooling air is supplied using a blower, and the flow rate of air is 800 kg/h. This flow rate corresponds to the steady in-furnace temperature of 1020 °C. The diameter and length of the furnace is 700 and 3100 mm, respectively. The furnace wall is made of the insulator material, and the diameter of the exhaust duct is 300 mm. In the 0.2 MW oxy-fuel combustor, natural gas is used as fuel and pure oxygen is supplied by evaporating the liquid oxygen. The precise composition of natural gas includes 88.9% CH4, 8.9% C2H6, and 1.3% C3H8. The flow rates of natural gas, oxygen, and nitrogen are 22.0, 50.1, and 1.5 N m3/h, respectively. This inflow mixture condition corresponds to the overall equivalence ratio, 0.97. The NO emission level is measured at the exhaust duct, and the flame length is obtained by visual observation. In the case of the 0.2 MW oxyfuel combustor, the detailed in-flame data are measured at six radial locations (r = 0.0, 6.0, 12.0, 18.0, 24.0, and 30.0 cm) and five axial stations (x = 10, 50, 90, 150, and 230 cm). At each location, the gas analyzer is used to analyze the sampling gas extracted by inserted the water-cooled sampling probe. A gas analyzer (HORIBA, VA3000) using a water-cooled sampling probe is installed to measure the emission levels for CO, CO2, O2, and NO. The specifications of the present gas analyzer are listed in Table 2. The temperature was measured by the R-type thermocouple.
2. Experimental Section The schematic configuration of the 0.2 MW oxy-fuel combustor is shown in Figure 1a. The oxidizer velocity is varied by changing the oxidizer diameter. The 0.2 MW oxy-fuel combustor is designed to operate under the given oxidizer velocity conditions at fixed fuel velocity (Vf = 60 m/s).18,19 In the case of the 0.2 MW oxy-fuel combustor, NO emission characteristics, flame lengths, and detailed in-flame data are measured for four oxidizer velocity conditions at fixed fuel velocity. In terms of an oxidizer formulation, to simulate the NO emission characteristics of the industrial oxy-fuel furnaces, 3% nitrogen of the total oxygen flow (13) Lallemant, N.; Breussin, F.; Weber, R.; Ekman, T.; Dugue, T.; Samaniego, J. M.; Charon, O.; Van Den Hoogen, A. J.; Van Der Bemt, J.; Fufisaki, W.; Imanari, T.; Nakamura, T.; Iino, K. Heat transfer and pollutant emissions characteristics of oxy-natural gas flames in the 0.7-1 MW thermal input range. J. Inst. Energy 2000, 73, 169–182. (14) Lallemant, N.; Dugue, J.; Weber, R. Analysis of the experimental data collected during the oxyflame-1 and oxyflame-2 experiments. IFRF Doc 1997, F85/y/4 Part 1. (15) Bollettini, U.; Breussin, F.; Lallemant, N.; Weber, R. Mathematical modeling of oxy-natural gas flames. IFRF Doc 1997, F85/y/6. (16) Blasiak, W.; Yang, W. H.; Narayanan, K.; von Scheele, J. Flameless oxyfuel consumption and nitrogen oxides emissions reductions and productivity increase. J. Energy Inst. 2007, 1, 3–11. (17) Kim, H. K.; Kim, Y.; Lee, S. M.; Ahn, K. Y. Emission characteristics of the 0.03 MW oxy-fuel combustor. Energy Fuels 2006, 20, 2125–2130. (18) Weber, R. Scaling characteristics of aerodynamics, heat transfer, and pollutant emissions in industrial flames. Proc. Combust. Inst. 1996, 26, 3343–3354. (19) Hedley, J. T.; Pourkashanian, M.; Williams, A. NOx formation in large-scale oxy-fuel flames. Combust. Sci. Technol. 1995, 108, 311–322.
3. Result and Discussion According to our previous experimental study of the 0.03 MW oxy-fuel combustor,16 flame length decreases by 5332
Energy Fuels 2009, 23, 5331–5337
: DOI:10.1021/ef900383s
Kim et al.
Table 2. Specifications of the Gas Analyzer species
method
operating range
accuracy (%)
CO (ppm) CO (%) CO2 (%) O2 (%) NO (ppm)
NDIR NDIR NDIR magneto-pneumatic chemi-luminescence
0-100/1000 ppm 0-5/50% 0-10/100% 0-25/100% 0-200/2000 ppm
∼0.5 ∼0.5 ∼0.5 ∼0.5 ∼0.5
Figure 3. Centerline profiles of the CO mole fraction (%, dry) for various oxidizer velocities.
shows the NO emission characteristics for various oxidizer velocities in the 0.2 MW oxy-fuel combustor. The NO emission level is considerably reduced with increasing the oxidizer velocity. At the oxidizer velocity range from 18 to 45 m/s, the NO emission level is gradually and linearly decreased with the oxidizer velocity. However, especially at the oxidizer velocity range from 8.3 to 18 m/s, the NO emission level is drastically reduced. According to the previous studies,20-23 the NO emission level has a certain proportional relationship with the flame length. However, in the present oxy-fuel flames, when the oxidizer velocity is increased, the NO emission level is rapidly decreased, particularly at the oxidizer velocity, 18 m/s. This implies that there is another physical process that influences the NO emission. In this oxy-fuel flame, there exists the sudden expansion step, which leads to the enhancement of turbulent mixing and the entrainment of recirculated product gases, and the entrained product gases reduce the temperature at the flame zone and the post-flame zone.17 Consequently, the NO emission level is progressively reduced with increasing the oxidizer velocity because of entrainment of the product gas.24 Especially at the oxidizer velocity range from 8.3 to 18 m/s, which has the relatively long flame length and large flame size, the NO emission level is sensitively reduced by the entrained product gases. In comparison with the present 0.2 MW oxy-fuel combustor, the 0.03 MW oxy-fuel combustor17 yields similar NO emission characteristics. These results imply that the NO emission level and flame length for the scaled-up oxy-fuel combustor can also be reduced with increasing the oxidizer velocity. The detailed in-flame data have been measured for various oxidizer velocities in the 0.2 MW oxy-fuel combustor. Figure 3 shows the centerline profiles of the CO mole fraction for various oxidizer velocities. In the case of the relatively high
Figure 2. Flame lengths and NO emission versus oxidizer velocity in the 0.2 MW oxy-fuel combustor.
increasing either fuel or oxidizer velocity. Elevation in fuel or oxidizer velocity results in the increase of turbulent mixing and fluctuations of scalar variables, including temperature and concentration. Also, when the oxidizer velocity was varied at the fixed fuel velocity, the NO emission level was reduced with increasing the oxidizer momentum. Therefore, in the present study, the flame length and NO emission characteristics of the large-scale 0.2 MW oxy-fuel combustion have been experimentally investigated for the wide operating range of the oxidizer velocity. Moreover, in this study, the detailed in-flame data are measured for the 0.2 MW oxy-fuel combustor. Figure 2a shows the flame length for various oxidizer velocities in the 0.2 MW oxy-fuel combustor. The flame length is almost linearly reduced with increasing the oxidizer velocity because of the increase of turbulent mixing. The smallest oxidizer velocity yields the longest flame length (Vo =8.3 m/s), while the largest oxidizer velocity does the shortest flame length (Vo=45 m/s). This trend is quite similar to experimental results17 for the 0.03 MW oxy-fuel combustor. Figure 2b
(20) Kim, S. K.; Kang, S.; Kim, Y. Flamelet modeling for combustion processes and NOx formation in the turbulent nonpremixed CO/H2/N2 jet flames. Combust. Sci. Technol. 2001, 168, 47–83. (21) Turns, S. R.; Myhr, F. H. Oxides of nitrogen emissions from turbulent jet flames: Part I;Fuel effects and flame radiation. Combust. Flame 1991, 87, 319–335. (22) Newbold, G. J. R.; Nathan, G. J.; Nobes, D. S.; Turns, S. R. Measurement and prediction of NOx emissions from unconfined propane flames from turbulent-jet, bluff-body, swirl, and precessing jet burners. Proc. Combust. Inst. 2000, 28, 481–487. (23) Costa, M.; Parente, C.; Santos, D. A. Nitrogen oxides emissions from buoyancy and momentum controlled turbulent methane jet diffusion flames. Exp. Therm. Fluid Sci. 2004, 28, 729–734. (24) Turns, S. R. An Introduction to Combustion, 2nd ed.; McGraw-Hill Companies: New York, 1999.
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Figure 5. Radial profiles of the temperature for four oxidizer injection velocities at Z = 90 cm.
Figure 4. Centerline profiles of the NO mole fraction (ppm, dry) for various oxidizer velocities.
oxidizer velocities (30 and 45 m/s), the CO mole fraction has the peak level in the proximity of the slightly fuel-rich region (x = 50 cm) close to the fuel nozzle. On the other hand, at the relatively low oxidizer velocities (8.3 and 18 m/s), the peak CO level zone is created around the slightly fuel-rich region (x = 90 cm), far from the fuel nozzle. Except the near injector region, in comparison to the relatively high oxidizer velocity conditions (30 and 45 m/s), the relatively low oxidizer velocity conditions (8.3 and 18 m/s) yield the much higher CO level, owing to the much lower turbulent mixing and the much longer hot-reaction zone. The locations of the CO peak level are directly in line with the flame length, corresponding to the various oxidizer velocities. After reaching the peak CO level, the CO level for all oxidizer injection velocities decreases along the downstream region mainly because of the CO burnout. Figure 4 presents the centerline profiles of the NO mole fraction for various oxidizer velocities. For all oxidizer velocity conditions, the NO mole fraction increases along the downstream region. In comparison to three oxidizer velocity conditions (18, 30, and 45 m/s), the lowest oxidizer velocity case (8.3 m/s) yields the much faster increase of the NO level along the centerline of the furnace and the highest NO emission level. The experimental results clearly indicate that the NO level is substantially decreased with increasing the oxidizer velocity, because of the much shorter residence time in the hot temperature zone as well as the entrainment of the product gas to the flame zone. Especially, when the oxidizer velocity is increased from 8.3 to 18 m/s, the NO level drastically decreases. As mentioned above, it is speculated that the high sensitivity of the NO emission level to this oxidizer velocity range is mainly caused by the entrainment of the recirculated product gases. Figure 5 shows the radial profiles of the temperature for four oxidizer injection velocities at Z = 90 cm. As would be expected, the lowest oxidizer injection velocity causes the highest temperature distribution mainly because of the much weaker entrainment of the product gas as well as the smaller departure from the equilibrium condition. However, in the central high-temperature zone (R 90 cm), the higher oxidizer velocity conditions yield a much lower NO level at all radial locations because of the much shorter residence time in the hot temperature zone as well as the entrainment of the product gas to the flame zone. At the furnace exit, the lowest oxidizer velocity case yields 5334
Energy Fuels 2009, 23, 5331–5337
: DOI:10.1021/ef900383s
Kim et al.
Figure 7. Contours of the O2 mole fraction (%) in the 0.2 MW oxy-fuel combustor.
Figure 8. Contours of the CO2 mole fraction (%) in the 0.2 MW oxy-fuel combustor.
observed that the dilution effect of product gas is progressively apparent. As a result, the temperature in the flame field could be deceased by the increased dilution effect. Particularly at the upstream recirculation zone (about R = 20 cm and Z = 30 cm), the CO2 level and the size of the high CO2 concentration zone are increased for the higher oxidizer velocity. Figure 9 presents the contours of the CO mole fraction for the wide range of the oxidizer velocity in the 0.2 MW oxy-fuel combustor. The CO level and the high CO concentration zone are substantially reduced with increasing the oxidizer velocity. Because CO is highly distributed in the hot flame zone, where the chemical dissociation actively occurs, the size of the CO distribution zone roughly reflects that of the actual hot-flame zone. This result confirms that the flame zone and flame length are reduced with increasing the oxidizer velocity because of the enhanced turbulent mixing. It is also necessary to note that the CO level in the flame zone is considerably influenced by the turbulent mixing. Figure 10 displays the contours of the in-furnace NO concentration for four oxidizer
the highest NO emission level followed by the oxidizer velocity conditions (18 m/s). It can be clearly seen that the NO level drastically decreases by increasing the oxidizer velocity from 8.3 to 18 m/s. To clearly examine in-flame data, the experimental data are displayed by contour graphs. Figure 7 shows the contours of the O2 mole fraction for various oxidizer velocities. When the oxidizer velocity is increased, the O2 level is considerably reduced, especially at the upstream recirculation zone (about R = 20 cm and Z = 30 cm), because of the enhanced entrainment process of the product gas. These entrained product gases decrease the temperature at the flame zone and the post-flame zone, where the thermal NO is mainly formed. Consequently, the entrainment of the product gas can be partially contributed to reduce the NO emission. Figure 8 displays the contours of the CO2 mole fraction for various oxidizer velocities. These results indicate that the CO2 level at the flame field increases with increasing the oxidizer velocity. When the oxidizer velocity is elevated, it is clearly 5335
Energy Fuels 2009, 23, 5331–5337
: DOI:10.1021/ef900383s
Kim et al.
Figure 9. Contours of the CO mole fraction (%) in the 0.2 MW oxy-fuel combustor.
Figure 10. Contours of the NO mole fraction (ppm) in the 0.2 MW oxy-fuel combustor.
velocities of the 0.2 MW oxy-fuel combustor. As would be expected, the concentration of NO is substantially decreased with increasing the oxidizer velocity because of the decrease of the residence time in hot combustion zone as well as the significant entrainment of the product gas to the flame zone. These results suggest that the oxidizer velocity at the oxy-fuel combustor is the crucial design parameter to control the NO emission.
substantially decreased with increasing the oxidizer velocity, because of the much shorter residence time in the hot temperature zone as well as the entrainment of the product gas to the flame zone. These results suggest that the oxidizer velocity at the oxy-fuel combustor is the crucial design parameter to control the NO emission. (3) In the case of the relatively high oxidizer velocities (30 and 45 m/s), the CO mole fraction has the peak level in the proximity of the slightly fuel-rich region (x = 50 cm) close to the fuel nozzle. On the other hand, at the relatively low oxidizer velocities (8.3 and 18 m/s), the peak CO level zone is created around the slightly fuel-rich region (x = 90 cm) far from the fuel nozzle. Except this near injector region, the lower oxidizer velocity conditions generate the much higher CO level at all radial locations because of the much lower turbulent mixing and the much larger hot combustion zone. (4) When the oxidizer velocity is increased, the O2 level is considerably reduced, especially at the upstream recirculation zone (about R = 20 cm and Z = 30 cm), because of the enhanced entrainment process of the product gas. Experimental results also indicate that the CO2 level at the
4. Conclusion Emission characteristics of the 0.2 MW oxy-fuel combustors have been experimentally investigated for the wide operating velocity range. On the basis of the results obtained in this study, the following conclusions can be drawn: (1) When the oxidizer velocity is increased, the flame length is almost linearly reduced. This result implies that the flame zone and flame length are reduced with increasing the oxidizer velocity because of the enhanced turbulent mixing. (2) The experimental results clearly indicate that the NO level is 5336
Energy Fuels 2009, 23, 5331–5337
: DOI:10.1021/ef900383s
Kim et al.
flame field increases with increasing the oxidizer velocity. Particularly at the upstream recirculation zone (about R = 20 cm and Z = 30 cm), the CO2 level and the size of the high CO2 concentration zone are increased for the higher oxidizer velocity.
Acknowledgment. This work was supported by a Grant (CH2101-01) from the Carbon Dioxide Reduction and Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean Government.
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