Energy & Fuels 2004, 18, 1329-1335
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Chemical Flame Length and Volume in Liquified Propane Gas Combustion Using High-Temperature and Low-Oxygen-Concentration Oxidizer W. Yang* and W. Blasiak Royal Institute of Technology, Division of Energy and Furnace Technology, S-100 44 Stockholm, Sweden Received April 6, 2004. Revised Manuscript Received June 11, 2004
In this paper, the effect of a high-temperature oxidizer at different oxygen concentrations on chemical flame length and volume has been numerically studied. Liquified propane gas (LPG) was used as the fuel. Hot exhaust flue gas was used as the dilution gas. The studied parameters include oxygen concentration and temperature of the oxidizer, fuel temperature, fuel firing rate, and diameter of the fuel nozzle. The following results were obtained: (1) Chemical flame length increased as either the oxygen content decreased, the oxidizer temperature increased, or the fuel temperature decreased; furthermore, the chemical flame length was independent of the fuel flow rate and the diameter of the fuel nozzle for the studied cases. (2) Chemical flame volume increased either as the oxygen content decreased and the oxidizer temperature increased, the fuel temperature was reduced, or the fuel firing rate was increased; chemical flame volume was dependent very much on the oxygen concentration in the oxidizer. (3) Influences of high temperature and low oxygen concentration in the oxidizer on the flame Froude number (Frf) were examined; regimes of momentum control or buoyancy control were determined on the assumption that the oxidizer temperature and oxygen concentration are changeable. (4) A simple correlation of the chemical flame length and volume, relative to the flow parameters, has been derived in terms of a Frf number for momentum-buoyancy transition jet flame under the hightemperature air combustion (HiTAC) condition. The criteria constants of the dimensionless chemical flame volume (V*) and the dimensionless chemical flame length (L*), to assess the momentum- or buoyancy-controlled flame, are given.
1. Introduction Preheating combustion air above 1000 °C, using exhaust flue gas, can provide a significant reduction of fuel consumption. However, one serious problem associated with high-temperature combustion air is high nitrogen oxide (NOx) emissions. An effective means of controlling NOx emissions is to control the oxygen concentration in the high-temperature oxidizer.1-7 This procedure has been termed high-temperature air combustion, or HiTAC.2 * Author to whom correspondence should be addressed. Telephone: 0046-8-790 8402. Fax: 0046-8-207681. E-mail:
[email protected]. (1) Tanaka, R. New Progress of Energy Saving Technology toward the 21st Century; Frontier of Combustion and Heat Transfer Technology. In Proceedings of the 11th IFRF Members Conference, May 1012, 1995. (2) Hasegawa, T.; Mochida, S.; Gupta, A. K. Development of Advanced Industrial Furnace Using Highly Preheated Air Combustion. J. Propul. Power 2002, 18 (2), 233-239. (3) Gupta, A. K. Thermal Characteristics of Gaseous Fuel Flames Using High-Temperature Air. J. Eng. Gas Turbines Power 2004, 126, 9-19. (4) Gupta, A. K.; Bolz, S.; Hasegawa, T. Effect of Air Preheat Temperature and Oxygen Concentration on Flame Structure and Emission. J. Energy Resour. Technol. 1999, 121, 209-216. (5) Tsuji, H.; Gupta, A. K.; Hasegawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M. High-Temperature Air Combustion: From Energy Conservation to Pollution Reduction; CRC Press: Boca Raton, FL, 2003. (6) Lille, S.; Dobski, T.; Blasiak, W. Visualisation of Fuel Jet in Conditions of Highly Preheated Air Combustion. J. Propul. Power 2000, 16 (4), 595-600.
The thermal characteristics of gaseous fuel flames using high-temperature and low-oxygen-concentration air were studies in the recent decade.1-15 Other than direct flame photography,2-7 laser-induced fluorescence (7) Lille, S. Experimental Study of Single Fuel Jet Combustion and High-Cycle Regenerative System; Royal Institute of Technology: Stockholm, Sweden, May 2002. (ISRN KTH/MSE-02/11-SE+METU/AVH, ISBN 91-7283-300-9.) (8) Kitagawa, K.; Konishi, N.; Arai, A.; Gupta, A. K. Temporally Resolved Two-Dimensional Spectroscopic Study on the Effect of Highly Preheated and Low Oxygen Concentration Air on Combustion. J. Eng. Gas Turbines Power 2003, 125, 326-331. (9) Hino, Y.; Sugiyama, S.; Suzukawa, Y.; Mori, I.; Konishi, N.; Ishiguro, T.; Kitawawa, K.; Gupta, A. K. Two-Dimensional Spectroscopic Observation of Nonluminous Flames in a Regenerative Industrial Furnace Using Coal Gas. J. Eng. Gas Turbines Power 2004, 126, 20-27. (10) Amagai, K.; Arai, M. Propane Flame Appeared in a Hot Air Heated over Self-Ignition Temperature. In International Symposium on Advanced Energy Conversion Systems and Related Technologies (RAN95), Nagoya, Japan, December 1995; pp 703-710. (11) Katsuki, M.; Hasegawa, T. In Proceedings of the 27th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; Vol. 27, pp 3135-3146. (12) Ishiguro, T.; Tsuge, S.; Furuhata, T.; Kitagawa, K.; Arai, N.; Hasegawa, T.; Tanaka, R.; Gupta, A. K. In Proceedings of the 27th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; Vol. 27, pp 3205-3213. (13) Yuan, J.; Naruse, I. Effects of Air Dilution on Highly Preheated Air Combustion in a Regenerative Furnace. Energy Fuels 1999, 13, 99-104. (14) Fujimori, T.; Riechelmann, D.; Sato, J. Experimental Study of NOx Reduction by Lifted Turbulent Jet Flame in Highly Preheated Flows. In Proceedings of the 1st Asia-Pacific Conference on Combustion, May 12-15, 1997, Osaka, Japan, pp 298-301.
10.1021/ef0499168 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/21/2004
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(LIF) spectroscopy2-5,8,9 has been used to describe the flame reaction zone by mean of detecting the spatial distribution of OH, CH, and C2 species during the HiTAC condition. Results of experimental studies of gas jet combustion with high-temperature and oxygendeficient air clearly showed an increase in the flame size, relative to a decrease of oxygen content.1-7,11,12,14,15 The influences of hot air temperature (over the selfignition temperature of the fuel) on the length of the propane flame were also studied.10 This work showed that the length of the laminar diffusion and premixed flames decreases as the air temperature increases. However, the length of the turbulent diffusion flame increases as the flame laminarization increases, because of high temperature. The studies of global flame features1-7 showed the flame color to change from yellow to blue to bluish-green to green, and, in some cases, hybrid and purple-colored flames were also observed. Under certain conditions, flameless or colorless oxidation of the fuel has also been demonstrated.2-7 It is generally accepted that the HiTAC flame length is not sufficient information to characterize the flame properties of high-temperature air combustion, because of its poor visibility and big chemical reaction volume. Quantitative numerical studies were performed in work,15 where the HiTAC flame volume was defined to demonstrate the HiTAC flame properties and its changes, depending on the process parameters that were studied. The concept was also used to obtain the multiflame volume numerically16 and experimentally17 in a semiindustrial HiTAC test furnace. It is known that the diffusion flame length can be generalized as the function of fuel jet momentum and furnace temperature by means of the Froude number (Fr).18-20 However, little knowledge is available about the influence of the preheat temperature and the oxygen concentration on the flame length. Studies1-7 have proved that the temperature profile in the combustion chamber is more uniform under the HiTAC condition, although the combustion air is preheated to a very high temperature. This implies that the buoyancy force in the furnace is smaller. Consequently, the Fr number is larger than that in the case of conventional combustion with the same initial jet momentum. Therefore, the parameters used to assess momentum control or buoyancy control must be optimized on the assumption that air temperature and oxygen concentration are changeable. The present study intends to fill this gap. The “flame” is determined in this paper according to the flammability limits of the combustible gases in the (15) Yang, W.; Blasiak, W. Numerical Study of Fuel Temperature Influence on Single Gas Jet Combustion in Highly Preheated and Oxygen Deficient Air. Energy 2004, article in press. (16) Yang, W.; Blasiak, W. Combustion Performance and Numerical Calculating of High-Cycle Regenerative System. Scand. J. Metall. 2004, 33, 113-120. (17) Blasiak, W.; Yang, W.; Rafidi, N. Physical Properties of a LPG Flame with High-Temperature Air on on a Regenerative Burner. Combust. Flame 2004, 136, 567-569. (18) Delichatsios, M. A. Transition from Momentum to BuoyancyControlled Turbulent Jet Diffusion Flames and Flame Height Relationships. Combust. Flame 1993, 92, 349-364. (19) Blake, T. R.; McDonald, M. An Examination of Flame Length Data from Vertical Turbulent Diffusion Flames. Combust. Flame 1993, 94, 426-432. (20) Becker, H. A.; Liang, P. Visible Length of Vertical Free Turbulent Diffusion Flame. Combust. Flame 1978, 32, 115-137.
Yang and Blasiak
combustion chamber during the HiTAC condition. The “flame” length and volume describe the physical properties of the chemical reaction zone in the combustion chamber. Consequently, they are referred to as a “chemical” flame length or height21 and volume, to distinguish them from luminous flame lengths and volumes determined based on visual observations. The flame length and volume in the latter part of this paper mean chemical flame length and volume. The HiTAC “flame” length and volume in coaxial flow with a hightemperature and low-oxygen-concentration oxidizer were numerically studied. The studied parameters included oxygen concentration in the oxidizer, fuel and oxidizer temperatures, firing rate, and diameter of the fuel nozzle. The HiTAC flame volume was also expected to be synthesized as a function of these various parameters. 2. Experimental Method and Numerical Modeling The subject of the experimental and numerical modeling was a single jet flame of LPG in a co-flow of high-temperature flue gas, as shown in Figure 1. In this work, real flue gases were used as the oxidizer, instead of using a mixture of air and inert gas, as studied in other works.1-6 A flue gas generator fired with LPG based on a conventional burner (normal firing rate, 25 kW) was used to create an oxidizer with controlled temperature and oxygen concentration. The fuel nozzle was positioned coaxially to the main flow of the hot flue gas and also was fed with LPG. In this work, only numerical studies were presented. Other details of the experimental setup and results of experimental studies can be found in the literature.7 The studies were conducted under steady-state conditions. Variables chosen for numerical studies were as follows: flow rate and temperature of fuel, oxygen concentration and temperature of the oxidizer, and diameter of the fuel nozzle (Table 1). The composition of the oxidizer (O2, CO2, H2O, and N2) was obtained according to the level of oxygen concentration, based on the chemical balance of the conventional burner that was used to produce the oxidizer. The Reynolds (Re) number of the oxidizer and fuel jet was in the range of Re ) 1273-2432 and Re ) 3316-13 263, respectively. Previous studies5,15,16 have already shown that the finiterate chemistry that combines the eddy-dissipation concept is suitable to predict the HiTAC performance. Therefore, this modeling concept was used in this work as well. The turbulence was modeled using the standard k- turbulence model, and radiation was handled using the discrete transfer method. Radiation properties of flue gas were assumed to be of the “gray body” type and were dependent on temperature, concentration, or both. A variable absorption coefficient modelscalled the weighted-sum-of-gray-gases (or WSGG) modelswas used to determine the absorption coefficient. Moreover, the heat flux on the combustion chamber wall was assumed to be zero. The oxidation mixture ratio15 was used to describe the borders of the chemical reaction zone. It is calculated as the mass fraction of oxygen relative to the mass fraction of oxygen added to the sum of oxygen needed to complete combustion at (21) Hawthorne, W. R.; Weddell, D. S.; Hottel, H. C. Mixing and Combustion in Turbulent Gas Jets. In Third Symposium on Combustion and Flame, and Explosion Phenomena; Williams and Wilkins: Baltimore, MD, 1949; p 266.
Length and Volume of an LPG Flame
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Figure 1. Schematic of combustion chamber design and its computational domain. Table 1. Values of Variables of Numerical Studies variable
value
QF TF To [O2] dF vo
0.01-0.04 g/s 299-1073 K 1073-1573 K 5-21 vol % 5 × 10-4-9.5 × 10-4 m 0.98 m/s
any point in the combustion chamber, according to the following expression:
mO
RO ) mO +
∑
(1)
scmF,c
c
where s ) nOMO/(nFMF). A ratio of RO ) 0.99 has been assumed to indicate an outside border of flame, and the rich fuel zone was assumed to be included in the flame volume. Thus, the flame volume can be approximately defined within the borders 0 e RO e 0.99; this approximation has been validated experimentally in our previous works.15-17 Flame length was calculated as the distance between the end of the fuel nozzle and the axial location of the oxidation mixture ratio equal to 0.99. The liftoff distance was negligible. There will be no any error for the calculation of flame length, and it is also apparent that the error for the prediction of flame volume from this assumption is acceptable.
3. Results and Discussion 3.1. Flame Appearance and Effect of Oxygen Concentration. The gas jet flame aligned coaxial with the flow of flue gases, at various oxygen molar fractions in the oxidizer, is shown in Figure 2. In this figure, the flame pictures were taken by a 60 mm × 60 mm Hasselblad camera through the window that was opened in the front wall of the combustion chamber. This experiment also showed that a reduced oxygen concentration ([O2]) increases the flame size and lift-off distance and decreases luminosity and visibility. The flame
Figure 2. Flame appearence versus oxygen concentration. Parameters were as follows: To ) 1173 K, dF ) 5 × 10-4 m, QF ) 0.01 g/s, and TF ) 299 K.
initially becomes bluish and then nonvisible. Experimental flame lengths were obtained by visual determination, as shown in Figure 2. For the cases of [O2] ) 16.8%, 12.8%, and 11.0%, the flame lengths were 0.25, 0.30, and 0.33 m, respectively. Figure 3 shows the flame shape and size as defined according to eq 1. The consequently calculated flame length and volume of LPG flames are shown in Figure 4. In comparison with the experimental results, the predicted values are larger. This is because of the difference of definitions for both predicted and measured values. However, the changing trend was similar and the agreement is acceptable. Figure 3 shows that one can further see that a decrease in oxygen concentration in the oxidizer increases the flame length as well as the flame volume. The influence of oxygen concentration on flame volume is more visible. When the oxygen concentration varies from 21% down to 5%, the flame volume increases 13.2 times, but the flame is barely 2.4 times longer. It is obvious because the flame volume is proportional to the cube of the flame length if the shape is similar. The
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Figure 3. Predicted flame shape and size for different oxygen concentrations. Parameters were as follows: To ) 1173 K, dF ) 5 × 10-4 m, QF ) 0.01 g/s, and TF ) 299 K.
Figure 4. Length and volume of the high-temperature air combustion (HiTAC) flame versus oxygen concentration. Parameters were as follows: To ) 1173 K, dF ) 5 × 10-4 m, QF ) 0.01 g/s, and TF ) 299 K.
rising trend in flame volume is clear if the oxygen concentration is
