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Flame Blowout Limits of Landfill Gas Mixed Fuels in a Swirling Nonpremixed Combustor Cheol-Hong Hwang, Chang-Eon Lee,* and Jong-Hyun Kim School of Mechanical Engineering, Inha UniVersity, 253, Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea ReceiVed January 21, 2008. ReVised Manuscript ReceiVed April 28, 2008
Recently, landfill gas (LFG) has attracted considerable interest as a source of regenerative energy for the production of heat and power. In the present study, the characteristics of flame blowout limits for landfill gas-liquified propane gas (LFG-LPG) mixed fuels in a swirling nonpremixed combustor were investigated. Validations of existing blowout models were conducted, and global parameters were suggested to explain the characteristics of the blowout limit for LFG-LPG mixed fuels in swirling nonpremixed flames. It was found that the flame stability of LFG-LPG mixed fuels was lower than that of CH4 under weak swirl conditions. Under strong swirl conditions, however, LFG-LPG mixed fuels had a similar or larger stable flame region with an increase of the air flow rate compared to those of CH4. Blowout velocities estimated by the Kalghatgi model as well as the Dahm-Mayman model, in simple jet flames and nonswirling flames with a coaxial air flow, respectively, were in good agreement with measurements from CH4-CO2 mixed fuels. For LFG-LPG mixed fuels, however, these models presented highly inaccurate estimations. Finally, it was identified that the flame blowouts of LFG-LPG mixed fuels were affected mainly by the CO2 ratio and stoichiometric air-fuel ratio of the fuels under weak swirl conditions and by the higher heating value of the fuels under strong swirl conditions. From these results, new global parameters, CO2(%)/(A/F)st, under weak swirl condition and higherheating value under strong swirl condition, were suggested to characterize the flame blowout velocity of LFG-LPG mixed fuels.
1. Introduction Landfill gas (LFG) is generated through anaerobic decomposition of municipal solid waste (MSW) deposited in landfills and consists mainly of CH4 and CO2 together with smaller amounts of oxygen and nitrogen, as well as trace amounts of a large number of volatile organic compounds (VOCs).1,2 Recently, LFG has attracted considerable interest as a source of regenerative energy for the production of heat, power, and fuels.3 The quality of LFG as a fuel may vary significantly in accordance with the proportion of CH4 in LFG. Typically, LFG has a lower heating value (about 3000-6000 kcal/Nm3) than liquefied natural gas (LNG) and liquefied propane gas (LPG).4 Therefore, utilization methods and operating conditions may vary in accordance with LFG quality. There are two approaches to the utilization of LFG in various thermal systems. In one approach, high quality LFG may be used as a sole fuel. In this case, the fraction of CO2 is the most important factor with respect to determining the combustion characteristics of the LFG fuel. Fundamental studies have been carried out on the effects of CO2 on flame speed,5 extinction * Corresponding author. Tel: +82-32-860-7323. Fax: +82-32-868-1716. E-mail address:
[email protected]. (1) http://www.eia.doe.gov/, (accessed Jan 1, 2008). (2) Robinson, M. G. Landfill gas in use as a fuel for process firing and power generation. Proceedings of the D & E Conference on Landfill Gas: Energy & the EnVironment, Bournemouth, March 27, 1990; pp 551-572. (3) Porteous, A. IEE Proc. Part A 1993, 140, 86–93. (4) Zach, A.; Binner, E.; Muna, L. Waste Manage. Res. 2000, 18, 25– 32. (5) Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. A detailed study of the combustion characteristics of landfill gas. WSS/CI 1999 Fall Meeting, October 26, 1999; 99F-033.
and flammability limit,6 and pollutant emissions.7 Qin et al.8,9 also carried out comprehensive studies on the fundamental and environmental aspects of LFG gas utilization for power generation. For utilization of low quality LFG, the heating value is raised through the addition of a higher-grade fuel such as liquefied natural gas (LNG) and/or liquefied petroleum gas (LPG). If LFG is mixed with LPG, the LFG-LPG mixed fuel (simply called LFG-LPG fuel) can be used as an interchangeable gas with LNG. To use the LFG-LPG fuel in place of LNG, major factors, such as the Wobbe index (WI) or the heating value (HV), should be rendered equivalent to those of LNG. This approach is currently being investigated with the aim of using LFG generated from the Gimpo landfill in South Korea as an alternative or interchangeable gas with LNG. In previous research, Lee et al. proposed a method for determining the burning velocities of LFG and LFG-LPG fuels10 and studied the effect of CO2 on the flame structure and NOx formation in laminar nonpremixed flames.11 Lee and (6) Ju, Y.; Masuya, G.; Ronney, P. Proc. Combust, Inst 1998, 27, 2619– 2626. (7) Locke, T. W. Ultra-low emission enclosed landfill gas flare—a full scale factory test. 21st Annual Landfill Gas Symposium, Austin, TX, March 26, 1998. (8) Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. Chem. Eng. J. 2001, 82, 157–172. (9) Qin, W. Dissertation Thesis, Experimental and numerical study of combustion of landfill gas. Department of Chemical Engineering, University of Southern California, 2000. (10) Lee, C. E.; Oh, C. B.; Jung, I. S.; Park, J. Fuel 2002, 81, 1679– 1686. (11) Lee, C. E.; Lee, S. R.; Han, J. W.; Park, J. Int. J. Energy Res. 2001, 25, 343–354.
10.1021/ef8000407 CCC: $40.75 2008 American Chemical Society Published on Web 07/01/2008
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Figure 1. Schematic of the experimental apparatus and fuel blending system (unit: millimeters).
Hwang12 compared the flame stability of two kinds of LFG-LPG fuels that were made to have a Wobbe index or heating value nearly equivalent to that of LNG in South Korea. They confirmed the possibility that these fuels may be used interchangeably with LNG for domestic appliances using a premixed flame, and swirl combustors using a nonpremixed flame under strong swirl conditions. In addition, Lee et al.13 conducted a study on the interchangeability of LFG-LPG fuels in accordance with the quality of LFG used in producing mixed fuel for domestic combustion appliances. They provided detailed information on the variation of the stable flame region for various mixed fuels. To successfully utilize LFG or LFG-LPG fuels in existing industrial combustors using a nonpremixed flame, however, additional information on the differences in blowout stability limit for a swirl-stabilized flame between LFG-LPG fuels and existing fuels is required. Various models and underlying physical mechanisms have been proposed to delineate the blowout stability limit and to develop predictive techniques for this phenomenon, including the premixed model,14 the flamelet extinction model,15 the largescale mixing model,16,17 the combined premixed flame propagation and flamelet extinction model,18 the eddy dissipation model,19 and the recent triple-flame model.20 Verifications of associated theories have also been conducted, and most of the noted models and theories have been reviewed in detail by Pitts.21 Recently, Chao et al.22 experimentally validated the (12) Lee, C. E.; Hwang, C. H. Fuel 2007, 86, 649–655. (13) Lee, C. E.; Hwang, C. H.; Lee, H. Y. Fuel 2008, 87, 297–303. (14) Kalghatgi, G. T. Combust. Sci. Technol. 1981, 26, 233–239. (15) Peters, N.; Williams, F. A. AIAA J. 1983, 21, 423–429. (16) Broadwell, J. E.; Dahm, W. J. A.; Mungal, M. G. Proc. Combust. Inst. 1984, 20, 303–310. (17) Dahm, W. J. A.; Mayman, A. G. AIAA J. 1990, 28 (7), 1157– 1162. (18) Miake-Lye, R. C.; Hammer, J. A. Proc. Combust. Inst. 1988, 22, 817–824. (19) Byggstφyl, S.; Magnussen, B. F. Turbulent Shear Flow 4; Springer: Berlin, 1985, 381. (20) Mu¨ller, C. M.; Breitbach, H.; Peters, N. Proc. Combust. Inst. 1994, 25, 1099–1016. (21) Pitts, W. M. Proc. Combust. Inst. 1988, 22, 809–816. (22) Chao, Y. C.; Wu, C. Y.; Lee, K. Y.; Li, Y. H.; Chen, R. H.; Cheng, T. S. Combust. Sci. Technol. 2004, 176, 1735–1753.
blowout models presented by Kalghatgi et al.14 and Broadwell et al.16 as representative blowout limit models in inert-diluted methane, propane, and hydrogen jet turbulent flames. They found satisfactory estimations of the blowout velocity using these models with proper modifications. However, these models were not validated in relation to LFG-mixed fuels. Furthermore, considering that most studies have been conducted on simple configurations such as a jet flame without swirl flow, there is little detailed information regarding the blowout mechanism of LFG-LPG mixed fuels in swirling nonpremixed flames. In view of these considerations, the present study focused on the flame blowout limit of LFG-LPG fuels in a swirling nonpremixed combustor. In particular, the characteristics of the flame blowout limits under conditions of weak and strong swirl intensities were examined. To identify the CO2 effect in LFG-LPG fuels having complex composition, the blowout limits of CH4-CO2 mixed fuel (simply referred to as CH4-CO2 fuel) were also measured. Finally, an examination and verification of existing blowout models suggested by Kalghatgi14 and Dahm and Mayman17 were carefully conducted for LFG-LPG flames, and global parameters were suggested to explain the characteristics of the blowout limit in weak and strong swirl nonpremixed flames. 2. Description of Experiments and Blowout Theories 2.1. Experimental Apparatus. Although the combustors used for industrial boilers, furnaces, and dryers have a range of configurations and characteristics, they typically use nonpremixed flames, the general stability of which are well-represented using a swirl burner flame. Figure 1 shows a schematic of the blending system along with the experimental apparatus used for testing a swirling nonpremixed-type combustor. LFG was made by blending four pure gases: CH4, CO2, N2, and O2; LFG-LPG fuel was made by adding LPG to the LFG. The flow rates of these gases were controlled using a series of thermal sensor-style mass flow controllers, each connected to a gas cylinder. Air supplied from a compressor was introduced into a reservoir and dryer, where water vapor was extracted. In addition, the air was regulated using a mass flow controller. Fuels supplied from the blending system were injected into the combustion zone via a central fuel nozzle. The inner diameter of the fuel nozzle was 1.95 mm. The air supplied
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Figure 2. Schematic of the movable-block swirl generator (unit: millimeters).
Figure 3. Flame stability regions of CH4 versus the swirl number.
from the compressor was introduced into the swirl generator via eight vertical holes, and a swirling flow was then injected through a coaxial air nozzle. The inner diameter of the air nozzle was 36 mm. The burner quarl had an inclination of 25°. Figure 2 shows a schematic of a movable-block swirl generator.23 This generator consists of two annular plates with wedge-shaped blocks. Interlocked, the blocks form alternating radial and tangential flow channels, where the air flow splits into radial and tangential streams, which combine downstream into one swirling flow. The upper block of this device can be continuously rotated from 0° to 20°. The operator can change the swirl intensity by changing the rotating angle of the upper block. The degree of swirl intensity is typically characterized by the swirl number S, a nondimensional number representing the axial flux of angular momentum divided by the axial flux of axial momentum, multiplied by the exit radius of the burner nozzle.24 In this experiment, the swirl number (S) calculated from the geometry of the swirl generator23 was changed from 0 to 1.02, which corresponded to a 11° rotation angle of the movable plate. Detailed information regarding the burner and the definition of the swirl number can be found in the literature.25 Flame blowout limits were measured by observation and photography. To measure the limits of flame blowout accurately, the fuel velocity was increased gradually by 1.0 m/s and the air velocity was increased by 0.05, 0.25, 0.5, and 1.0 m/s, according (23) Fricker, N.; Leuckel, W. J. Inst. Fuel 1976, 49, 152–158. (24) Beer, J. M.; Chigier, N. A. Combustion aerodynamics; Applied Science Publishers: London, 1972. (25) Leuckel, W.; Fricker, N. J. Inst. Fuel 1976, 49, 103–144.
Figure 4. Flame images of CH4 under the conditions near the leanblowout limit (Uf ) 6.0 m/s, Ua) 8.0 m/s) and the rich-blowout limits (Uf ) 84.0 m/s, Ua ) 8.0 m/s) under a swirl number of 1.02.
to the range of the flame stability. The measurements were repeated three times at the same operating conditions. All the measured blowout limits had the maximum standard deviation of 7% and 10% for rich-blowout limits and lean-blowout limits, respectively. 2.2. LFG-LPG and CH4-CO2 Fuels Used. The compositions and properties of LFG, LFG-LPG fuel, CH4, CH4-CO2 fuel, and the LPG used in this experiment are shown in Table 1. The composition of standard LNG used in South Korea is also presented to estimate the interchangeability of LFG-LPG fuels. The values in parentheses for the mixed fuels, such as (70:30) for LFG-LPG fuels, represent the volumetric mixing ratio of LFG to LPG. Although the composition of LFG may vary significantly in accordance with the landfill site, lapse of filled time, or method of pretreatment,4 the average value of LFG generated at the Gimpo landfill26 was used in this study. The interchangeability of LFG-LPG fuels according to the variation of the LFG quality was investigated in a previous study.13 As representative LFG-LPG fuels, two compositions were considered: LFG-LPG (70:30) and (50:50) fuels were produced such that they had equivalent higher heating values (HHV) and Wobbe index (WI) values to those for standard LNG in South Korea. Pure C3H8 was used in place of commercial LPG in order to avoid changes in the composition during the experiment. To show the effects of CO2 and the heating value in LFG-LPG fuels on flame stability, the compositions and properties of CH4-CO2 fuels are presented in Table 1. 2.3. Review of Existing Flame Blowout Theories. The estimation of the blowout limit for LFG-LPG fuels is an important issue for the design of combustor and combustion system. As mentioned previously, although various models have been proposed to estimate the blowout limit, existing models have yet to be validated for LFG-LPG fuels in jet nonpremixed flames with or without the swirling condition. In this study, the estimations of the blowout limit based on the premixed flame model14 and the extended largescale vortex model17 were carried out and compared with experimental data. These models are convenient, because they require only the nozzle conditions to estimate the blowout velocity. The physical mechanisms and formulas used in this study are introduced briefly below.21,22 The concept of the premixed flame model was adopted by Kalghatgi14 for a simple jet flame without a coaxial air flow. He reported that the flame will blow out when the change in the local turbulent burning velocity cannot keep pace with the change in the local flow velocity anywhere in the jet as the flame moves downstream from the nozzle exit. On the basis of experimental data, Kalghatgi was able to empirically scale the nondimensionalized blowout velocity with a Reynolds number, based on the mean stoichiometric envelope, and a universal formula was proposed:
Uf ) 0.017RH(1 - 3.5 × 10-6RH)
(1)
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Table 1. Components and Properties of the LFG-LPG and CH4-CO2 Mixed Fuels CH4-CO2 mixed fuels (CH4%: CO2%)
LFG-LPG mixed fuels (LFG%: LPG%) component
LFG
CH4 (%) C2H6 (%) C3H8 (%) C4H10 (%) CO2 (%) N2 (%) O2 (%) HHV (kcal/Nm3) (A/F)st (m3/m3) γ WI (kcal/Nm3)
54.5 0.0 0.0 0.0 37.5 7.0 1.0 5202.01 5.14 0.9536 4684.93
(70:30)
(50:50)
38.15 0.00 30.00 0.00 26.25 4.90 0.70 10834.99 10.74 1.1317 8980.79
27.25 0.00 50.00 0.00 18.75 3.50 0.50 14623.08 14.47 1.2514 11546.24
CH4 100.0 0.0 0.0 0.0 0.0 0.0 0.0 9530.95 9.52 0.5549 11243.80
j f is the nondimensionalized blowout velocity with respect Here, U to the laminar burning velocity and RH is the Reynolds number based upon H, SL, νe, and (Fe/Fa). H denotes the distance from the nozzle exit to the stoichiometric envelope of the flame. Other parameters are given in the Nomenclature section. The extended large-scale vortex model was developed by Dahm and Mayman (hereafter, the D-M model)17 for a nonswirling jet flame with a coaxial air flow. In this model, the effect of a coaxial air flow on the blowout limit was incorporated in the original model suggested by Broadwell, Dahm, and Mungal (hereafter, the BDM model).16 In the BDM model, flame blowout is assumed to occur when the vortical mass transport rate exceeds the chemical reaction rate. The predicted shape of the flame blowout curve by the D-M model under a zero swirl condition is as follows:
Ua2 ) (ξβ4⁄3ηda2⁄3)Uf4⁄3 - (βη)Ua2
(2)
where the parameters ξ, β, η, and ε equate to [(SL2/Rst)(1 + AF)2/ ε]2/3, (Ff/Fa)(df/da)2, 1/[1 - (df,o/da)2], and dfSL2(1 + AF)2(Ff/Fa)1/2/ (UfRst), respectively. Other parameters appear in the nomenclature. Estimations of the blowout velocity in a swirling nonpremixed flame were also suggested by Feikema et al. (hereafter, the Feikema model).27,28 The Feikema model, extended from the BDM model, requires measurement of the critical angular velocity at the onset of recirculation. Therefore, the Feikema model was not validated directly due to absence of experimental data on the critical angular velocity in this study. However, considering that the Feikema model also uses a physical mechanism with the BDM model, it can be inferred that the validation of the Feikema model will be estimated by validation of the D-M model. Therefore, in what follows, we present only validation of the D-M model. In order to calculate the various fuel parameters required in the premixed flame model and the D-M model, the data for kinetic viscosity, thermal diffusivity, density, and air-fuel mass ratio were calculated using a program developed using the libraries of Chemkin29 and the Tranfit package.30 The maximum laminar flame speed was calculated using the premixed code.31
3. Results and Discussion 3.1. Flame Stability of Swirling Nonpremixed Flames. (26) Kim, D. Y.; Lee, C. E.; Park, J. W. Final Report 1998N-B102-P08, Ministry of Commerce, Industry and Energy: South Korea, Actual proof verification of landfill (Gimpo in Korea) gas for 10 000 m3/year: 2000. (27) Feikema, D.; Ghen, R.; Driscoll, J. F. Combust. Flame 1990, 80, 183–195. (28) Feikema, D.; Chen, R. H.; Driscoll, J. F. Combust. Flame 1991, 86, 347–358. (29) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A fortran chemical kinetics package for the analysis of gas phase chemical kinetics: Sandia Report, SAND89-8009B, Sandia National Laboratories: Livermore, CA, 1989. (30) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. A fortran computer code package for the evaluation of gas-phase multicomponent transport: Sandia Report, SAND86-S8246, Sandia National Laboratories: Livermore, CA, 1994. (31) Kee, R. J.; Garcar, J. F.; Smook, M. D.; Miller, J. A. Sandia Report, SAND85-8240, Sandia National Laboratories: Livermore, CA, 1994.
(95:05)
(85:15)
95.00 0.00 0.00 0.00 5.00 0.00 0.00 9056.36 9.04 0.6034 10246.68
85.00 0.00 0.00 0.00 15.00 0.00 0.00 8106.55 8.09 0.7005 8514.74
LPG
LNG
0.0 0.0 100.0 0.0 0.0 0.0 0.0 24210.01 23.80 1.5545 17227.02
89.78 7.48 2.02 0.70 0.00 0.02 0.00 10470.32 10.49 0.6219 11675.52
Although the combustors used for practical appliances have a range of configurations and characteristics, they usually adopt nonpremixed flames with swirl flow motion to enhance the flame stability. Therefore, changes in the flame stability and flame blowout limit of LFG-LPG fuels compared to typical gas fuel, such as CH4, were investigated using a nonpremixed combustor with a movable-block swirl generator under nonswirl and swirl conditions. To identify the general behaviors of the flame blowout limit with varying swirl numbers in a nonpremixed flame, the flame stability diagrams of CH4 (a representative fuel) were investigated, as shown in Figure 3. The x axis (Uf) and y axis (Ua) represent the air and fuel stream mean velocities calculated from the mass flow rate and nozzle area at the exit of each nozzle, respectively. The blowout velocity was defined as the mean axial exit velocity of fuel and air at which complete extinction occurs. The dashed-dotted line represents the stoichiometric equivalence ratio (Φ ) 1) of CH4; this classifies the fuel-air ratio supplied by each nozzle into rich or lean combustion conditions. In this experiment, the flame blowout was classified according to the rich-blowout limit and the lean-blowout limit. Stable flames exist between the lines of the rich-blowout limit and leanblowout limit at a given swirl intensity. To provide a clearer understanding of the flame conditions near the blowout limit, Figure 4 shows images of flames formed near the lean-blowout limit (a) and the rich-blowout limit (b) under a swirl number of 1.02. Figure 4a shows the flame shape that formed near the lean limit condition. The flame is very short and appears blue in color. The flame has a strong recirculation vortex in its central region and the fuel jet does not penetrate the recirculation vortex. Therefore, the flame blows out due to excessive stretching of the lean flame by the strong recirculation vortex, rather than by excessive fuel velocity.28 Figure 4b shows the flame shape that formed near rich-blowout limit condition. The flame is a long, attached, jet-like flame. A blue flame is observed upstream due to rapid fuel-air mixing and a red flame is generated downstream due to reaction of the remaining fuel and the surrounding air. As the fuel velocity continues to increase, the flame is lifted and then blows out when the fuel velocity exceeds the richblowout limit.27 From the viewpoint of practical operating conditions, the fuel can be supplied up to the rich-blowout limit. Therefore, information on the change in the rich-blowout limit of new fuels provides information on the change in operating conditions for a fuel supply system when one type of fuel is replaced with alternative fuels. From observation of the flames shown in Figures 3 and 4, the flames for swirl numbers S ) 0.68 and 1.02 appear to be stabilized with the recirculation vortex formed in the central upstream region. This acts as a heat source by forcing hot products to move upstream and mixing the partially premixed reactant. However, for the flames having a swirl number below
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Figure 5. Stable flame regions of LFG-LPG mixed fuels and CH4 under the weak swirl conditions (S ) 0-0.58).
0.58, flame stability with the recirculation vortex could not be definitively observed. Therefore, in this study, the shape and size of stable flame regions can be classified into two groups: within the present experimental conditions, one group has a closed region of small size when the swirl number is below 0.58 (so-called weak swirl conditions), and the other group has an open region of large size when the swirl number is above 0.68 (so-called strong swirl conditions). Under weak swirl conditions, the size of the stable flame region increases with the swirl number, but the ratios of the increase are small; that is, the swirl affects the flame stabilization but it does not significantly improve the flame stability. Under strong swirl conditions, the shapes of the blowout curve are very different from those under weak swirl conditions. That is, the richblowout limit increases linearly with an increase of air velocity while the lean-blowout limit decreases with an increase of air velocity. Thus, the size of the stable flame zone increases suddenly with an increase in the swirl number. On the basis of these results, the flame stability in swirling nonpremixed flames has different characteristics according to weak and strong swirl conditions. Thus, the following discussion on the flame stabilities, including flame blowout phenomena for LFG-LPG fuels, was classified under weak swirl conditions and strong swirl conditions. 3.2. Blowout Limits under Weak Swirl Conditions. Figure 5 represents the variation of stable flame regions of LFG-LPG fuels and CH4 with an increase in the swirl number under weak swirl conditions. On the basis of variation of the blowout curves of CH4, the rich-blowout limit and lean-blowout limit are maintained at higher air stream velocities as the swirl number increases, although the rich-blowout limit is slowly reduced as the air velocity increases. These characteristics of the blowout limit create a peninsula-shaped stable flame region and enlarge the stable region as the swirl number increases. In addition, coaxial air destabilizes the flame and causes the rich-blowout limit to be lower than that with no swirl flame, as the stabilizing effects of the swirl do not yet exceed the destabilizing effects of the coaxial air under weak swirl conditions. LFG-LPG fuels also show a similar variation of the stable flame region with the swirl number, but the stable flame regions differ greatly in size. CH4 has the largest stable flame region, and the stable regions of LFG-LPG fuels greatly decrease with increases in the LFG ratio, despite the fact that CH4 and LFG-LPG fuels have nearly equivalent burning velocity and flame temperature, as reported in previous studies.10–13 Although these results indicate that the use of LFG-LPG fuels as an
Figure 6. Stable flame regions of CH4-CO2 mixed fuels under weak swirl conditions.
alternative to LNG should be avoided for a combustor using no or weak swirl nonpremixed flames, the cause of the difference in the blowout limits between CH4 and LFG-LPG fuels has not yet been clarified. As presented in Table 1, LFG-LPG fuels mainly consist of CH4, C3H8, and CO2, and the fraction of CO2 increases with LFG mixing ratio. Therefore, the decrease in the stable flame region associated with the increase in the LFG ratio may be attributed to the fraction of CO2 in fuel. Additionally, the differences in the blowout limit among fuels may be explained by the fraction of CO2 in fuel. Thus, we carried out additional experiments to systematically examine the effect of CO2 included in the LFG-LPG fuels on the flame blowout limit. Figure 6 shows the variation of the stable flame regions of CH4 and CH4-CO2 fuels with an increase in the swirl number under weak swirl conditions. To obtain similar stable flame regions as with those of LFG-LPG fuels, amounts of CO2 corresponding to 5% and 15% by volume were blended with pure CH4. The characteristics of flame stability with swirl intensity in CH4-CO2 fuels are similar to those of the LFG-LPG fuels, as shown qualitatively in Figure 5. In particular, the stable flame region of CH4-CO2 (85:15) fuel is nearly equivalent to that of LFG-LPG (70:30) fuel, including CO2 of 26%, although the amount of CO2 in the fuel differs by more than 10%. From this figure, it can be observed that CO2 in fuel is an important factor that can affect the blowout limit. The information on the difference in flame blowout velocity between the usual fuels and LFG-LPG fuels is very important if we want to use LFG-LPG fuels in existing combustors in place of usual fuels, because this information can set the supply conditions of fuel and air flows. The results presented in Figures 7 and 8 show the validation results for whether existing blowout models can be also applied to LFG-LPG fuels. In these figures, the blowout limits of CH4-CO2 fuels were compared to verify the existing models and identify the CO2 effects. Figure 7 shows the normalized blowout velocities and universal formula proposed by Kalghatgi versus the Reynolds number for simple jet flames without a coaxial flow. Symbols denote the experimental data and the line indicates the universal formula, as given in eq 1. The normalized blowout velocities of the pure CH4 and CH4-CO2 fuels calculated by Kalghatgi’s model are in good agreement with the measurements. In the case of pure LFG (+ symbol), the universal formula for normalized blowout velocity is underestimated significantly and, in the case of the LFG-LPG fuel (O and b), the universal
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Figure 7. Blowout velocities versus the Reynolds number in simple jet flames.
Figure 8. Blowout velocities versus air velocity under nonswirl conditions with a coaxial air flow.
formula overestimates the measured blowout velocity. In particular, as the fraction of LPG in LFG-LPG fuels increases, the discrepancy also increases considerably. These reasons may be inferred from the background of Kalghatgi’s model. The Kalghatgi’s model was suggested based on the experimental results of fuels including CH4, C2H4, C3H8, C4H10 and H2 with a narrower range of inert dilution. Thus, some of predictions were found to deviate from the experiment data, especially for diluted turbulent flame, such as CO2/CH4, CO2/C3H8.16,22 These discrepancies may be attributed to inaccurate prediction of the axial distance corresponding to the stoichiometric level (H) including entrainment, molecular mixing and kinetic effects due
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to change in fuel composition. In result, the adoption of Kalghatgi’s model to estimate the blowout velocities of LFG-LPG fuels having very complex composition requires a considerable caution. Figure 8 shows the normalized blowout velocities under nonswirling conditions with a coaxial air flow for CH4-CO2 mixed fuels and LFG-LPG mixed fuels. The symbols indicate the measured data and the lines denote the predicted data by the Dahm and Mayman (D-M) model. In the case of the CH4-CO2 fuels, the estimations of the blowout limits by the D-M model are in reasonably good agreement with the present measurements. In the case of the LFG-LPG fuels, however, the discrepancies between the measured and predicted data increase significantly with an increase in the amount of LPG included in the LFG-LPG fuels. In particular, the case of LFG-LPG (50:50) fuel shows a significant difference from that of the D-M model. It can be inferred that the D-M model cannot reasonably reflect the effects of LPG addition, such as the change of flame location, mixing rate of fuel/air, turbulence level, etc. Thus, the D-M model extended from the original BDM model shows limitations regarding estimation of blowout limits for LFG-LPG fuels under nonswirling conditions with a coaxial air flow. Actually, as mentioned before, Feikema et al.27,28 proposed a model to estimate the blowout velocity in a swirling nonpremixed flame, but this model is also based on the same physical mechanism used in the D-M model. Therefore, it can be inferred from the validation of the D-M model that the Feikema model may provide an incorrect estimation in the case of LFG-LPG fuels in no swirl or swirl conditions. The above validation results indicate that existing models give no reasonable information on flame blowout velocity and, therefore, a new model applicable to LFG-LPG fuels is necessary. However, setting up a new formula requires a detailed understanding of the physical mechanisms underlying the flame stability of LFG-LPG fuels. In this study, we attempted to introduce new global parameters which can relate the difference of flame blowout velocity for LFG-LPG fuels. From Figures 7 and 8, it is evident that the discrepancies between existing models and experimental data can be attributed to the amount of CO2 and C3H8 included in LFG-LPG fuels. If we consider the change in the stoichiometric air-fuel ratio of LFG-LPG fuels, the high CO2 ratio in fuels causes the flame surface corresponding to a location of stoichiometric air-fuel ratio to move toward the center direction. On the other hand, the high C3H8 ratio in fuels causes the flame surface to move toward the outer direction, because the stoichiometric air-fuel ratio of C3H8 is much higher than that of CH4, as presented in Table 1. Considering that the fuel velocity near the rich-blowout limit is generally higher than the air velocity, if the flame surface moves toward the centerline of the fuel nozzle in the case of high CO2 ratio in fuels, the flame surface will be located at a region of relatively higher velocity and strain. Thus, the flame can blow out easily at a relatively lower fuel velocity. On the contrary, if the flame surface moves toward the outer direction in the case of high C3H8 ratio in fuels, the flame surface will be located at a region of relatively lower velocity and strain, and the flame will blowout at relatively higher fuel velocity. Thus, the flame blowout velocity decreases in proportion to the CO2 ratio in fuels and in reversely proportion to the stoichiometric air-fuel ratio. In this study, the CO2 volume fraction in fuel divided by the stoichiometric air-fuel ratio “(CO2(%)/(A/ F)st)” was adopted under weak conditions as a new global
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Figure 9. Blowout velocities versus CO2(%) in fuel divided by (A/F)st under weak swirl condition (S ) 0.58, Ua ) 0.6 m/s).
Figure 10. Stable flame regions of LFG-LPG mixed fuels and CH4 for the swirl number S ) 1.02.
parameter relating the difference of flame blowout velocity of LFG-LPG fuels. Figure 9 presents the fuel velocities at which flame blowouts of LFG-LPG and CH4-CO2fuels occur as a function of CO2(%)/(A/F)st. In this case, the mixing ratio of LFG and LPG in LFG-LPG fuels was changed, and experimental data obtained under the same conditions of swirl number 0.58 and air flow velocity 0.6 m/s were used. In this figure, all experimental data can be related linearly as a function of the new parameter. That is, we can identify a new global parameter, CO2(%)/(A/F)st, which can give information on the difference in blowout limits for LFG-LPG fuels under weak swirl conditions. 3.3. Blowout Limits under Strong Swirl Conditions. Figure 10 shows the stable flame regions of LFG, the LFG-LPG fuels, and CH4 under a strong swirl number of 1.02. The shapes of the blowout curves are very different from those under weak swirl conditions, as shown in Figure 5. In particular, under weak swirl conditions, the rich-blowout limits decreased slowly with an increase in the air velocity; however, under strong swirl conditions, they increase linearly with an increase in air velocity. In addition, the lean-blowout limits decrease abruptly under strong swirl conditions and remain at the low lean limit even when the air velocity is increased. Thus, the stable flame regions
Figure 11. Stable flame regions of CH4-CO2 mixed fuels for the swirl number S ) 1.02.
extend to a higher fuel velocity and a lower air velocity compared to those of the weak swirl conditions. Furthermore, if the air stream has sufficient velocity to generate a strong swirl flow, the stable flame region of the LFG-LPG (50:50) fuel will be more extended than that of CH4, and that of the LFG-LPG (70:30) fuel is similar to that of CH4. From these results, under strong swirl conditions, it is noted that the flame stability is proportional to the heating value of each fuel. The ratio of the two blowout limits represents the turndown ratio, which should be maximized in practical combustors. The turndown ratios of LFG-LPG fuels and CH4 can reach 10, and that of LFG can reach 5 under the present experimental conditions. From these results, it is noted that the LFG-LPG (50:50) and (70:30) fuels can be used interchangeably with LNG with respect to flame stability; in addition, LFG itself can also be used as an alternative to LNG, except that the turndown ratio will be reduced by half. To examine the effect of CO2 included in the LFG-LPG fuels on the flame blowout limit under strong swirl conditions, Figure 11 shows the stable flame regions of pure CH4 and CH4-CO2 fuels for a strong swirl number of 1.02. The richblowout limit under no air streamflow condition, corresponding to the y-axis intercept, significantly decreased with an increase of CO2 ratio, and the difference is more clearly identified in Figure 6. However, the difference in the rich-blowout limit decreases gradually with the increase of air velocity regardless of CO2 ratio in fuel. That is, the CO2 included in the fuel does not have a significant effect on the flame blowout under the sufficient air streamflow in strong swirl conditions; on the contrary, CO2 significantly influences the blowout limits under weak swirl conditions. To find a global parameter that can explain the difference in rich-blowout limit for LFG-LPG fuels under strong swirl conditions, Figure 12 presents the fuel velocities corresponding to the rich-blowout limit as a function of a higher heating value (HHV). In this case, the mixing range of LFG-LPG was changed largely and experimental data obtained under the same condition of swirl number 1.02 and air flow velocity 8.0 m/s were used for all types of mixed fuels. As can be seen, the variation in the rich-blowout limits can be related linearly as a function of the HHV of fuels regardless of CO2 concentration and stoichiometric air/fuel ratio. Therefore, under strong swirl conditions, it can be said that the flame stability of LFG-LPG fuels is dominated by the HHV. Furthermore, the change of flame blowout velocity can be inferred with the simple information on the HHV of the mixed fuels.
2940 Energy & Fuels, Vol. 22, No. 5, 2008
Figure 12. Fuel blowout velocities versus higher heating values under strong swirl condition (S ) 1.02, Ua ) 8.0 m/s).
From the comparisons of stable flame region and blowout limit of LFG-LPF fuels with additional information on pure CH4 and CH4-CO2 fuels, we confirmed that the flame stability in swirling nonpremixed flames has different characteristics, and the dominating parameters on the flame blowout velocity also differ according to swirl conditions. That is, in weak swirl conditions, the change in flame blowout velocity is related to the CO2(%)/(A/F)st of fuels, and in strong swirl conditions, the change is related linearly as a function of the HHV of fuels. To attain a more detailed understanding of the physical mechanism underlying the flame stability of LFG-LPG fuels, additional information regarding the combustion field, including the flame behaviors and recirculation phenomena by the swirl motion, is required. These systematic analyses, however, are left for further studies. 4. Conclusions The flame stability of LFG-LPG fuels in a swirling nonpremixed combustor was investigated. In particular, the characteristics of the flame blowout limits under the conditions of weak and strong swirl intensities were examined. Examination and validation of existing blowout models were conducted for LFG-LPG fuels, and new global parameters were proposed to explain the difference in the blowout limit in swirling nonpremixed flames. The conclusions are as follows: (1) Under weak swirl conditions, the stabilizing effects of the swirl do not exceed the destabilizing effects of the coaxial air. Moreover, the flame stabilities of the LFG-LPG fuels are
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lower than that of CH4, although LFG-LPG fuels and CH4 have nearly equivalent burning velocities and flame temperatures. (2) Under strong swirl conditions, LFG-LPG fuels have similar or larger stable flame regions with an increase of the air flow rate compared to those of CH4. Furthermore, LFG itself can be used as an alternative to LNG, except that the turndown ratio is reduced by half in this case. (3) Blowout velocities estimated by the Kalghatgi model for a simple jet flame and the Dahm and Mayman (D-M) model for a nonswirl flame with a coaxial air flow are in good agreement with the measurements for the CH4-CO2 fuel assessed here. In LFG-LPG fuels, however, these models provide highly inaccurate estimations of the blowout velocities. (4) Under weak swirl conditions, the blowout velocity decreases in proportion to the CO2 ratio and in reversely proportion to the stoichiometric air-fuel ratio of LFG-LPG fuels. A new global parameter, CO2(%)/(A/F)st, affecting the above characteristics, can give information on the difference in blowout limits of LFG-LPG fuels. (5) Under strong swirl conditions, the flame blowout of LFG-LPG fuels is governed mainly by the higher heating value of the fuel. The change of blowout velocity can be related linearly as a function of the higher heating value of the fuel. Acknowledgment. This work was supported by an Inha University Research Grant.
Nomenclature AF ) air-fuel ratio based on mass A/F ) air-fuel ratio based on volume da ) air nozzle inner diameter df, dfo ) fuel nozzle inner, outer diameter HHV ) higher heating value (kcal/Nm3) S ) swirl number SL ) burning velocity U ) initial axial velocity WI ) Wobbe index Greek Symbols R ) thermal diffusivity β,η,ε,ξ ) parameters defined in eq 2 F ) density θ ) fuel mass fraction νe ) kinematic viscosity γ ) relative density based on air Subscripts a ) air e ) at burner exit f ) fuel st ) stoichiometric condition EF8000407
