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Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air
Energy 30 (2005) 385–398 www.elsevier.com/locate/energy
Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air Weihong Yang , Wlodzimierz Blasiak Royal Institute of Technology (KTH), Division of Energy and Furnace Technology, Brinellva¨gen 23, 100 44 Stockholm, Sweden
Abstract Combustion of a single jet of propane in a cross-flowing stream of preheated and oxygen deficient air is numerically analyzed with emphasis on influences of fuel temperature. Both Eddy-Break-Up and PDF/mixture fraction combustion models coupled with RNG k–e turbulent model were applied and the predicted results were compared. Thermal and prompt NO models were employed to calculate NO emissions. Results show that the Eddy-Break-Up model is more suitable for predicting temperature field and flame shape. It was showed that flame during high temperature air combustion condition is spread over a much larger volume. Flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. Additionally, it is almost constant at fixed oxygen concentration and fuel inlet temperature for the temperature of the preheated and oxygen deficient air equal to 1041–1273 K. Increase of the fuel inlet temperature results in smaller flame, shorter mean residence time, smaller temperature peaks, and lower emission of NO. # 2004 Elsevier Ltd. All rights reserved.
1. Introduction High temperature air combustion (HiTAC) applied to industrial furnaces offers energy savings, reduction of pollutant emissions, and high quality of the product at increased production rate [1,2]. Although this technology had been developed more than 10 years back and has been commercially applied in different types of furnaces as reported by Yasuda and Ueno [2], the basic chemical–physical phenomenon still needs to be better explained. Apart from semi-industrial tests, both experimental and numerical methods were employed to study the properties of single fuel jet behaviour during the HiTAC condition. When analyzing the literature of the subject, it is well seen that the study of a single fuel jet allowed explaining
Corresponding author. Tel.: +46-8-790-8402; fax: +46-8-20768. E-mail address: [email protected] (W. Yang).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.05.011
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Nomenclature A B k m M _ m n PDF RNG RSM RF Rf Ro Rtu T T Vf VF [O2] q e sR /
empirical coefficient empirical coefficient turbulence kinetic energy, m2 s2 mass fraction molecular weight, kg mol1 total mass rate of flue gas, kg s1 stoichiometric coefficient (number of moles) probability density function renormalization group Reynolds stress model fuel consumption rate, kg m3 s1 flame volume ratio oxidation mixture ratio temperature variance temperature, K average temperature in the furnace, K flame volume, m3 s1 volumetric flow rate of the fuel, m3 s1 oxygen concentration, % density, kg m3 turbulence kinetic energy dissipation rate, m2 s3 mean residence time, s fuel flammability limit
Subscripts a air ad adiabatic cr criteria value F fuel f flame g gas i calculation cell number j species O oxygen P combustion products Pre preheated
and measuring many unique features of the HiTAC. For example, experimental studies of single fuel jet were performed by Hasegawa et al. [1], Gupta et al. [3,4], Kitagawa et al. [5], Lille et al. [6] and Blasiak et al. [7].
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Numerical studies focused on studies of the structure of flame and different mathematical models using well-known general-purpose codes as FLUENT [8–10] and CFX [11]. In all the cases, a flame jet of propane or methane in a cross-flow of preheated and diluted combustion air was a subject of numerical modelling. Several modelling approaches were employed, for example, (a) Eddy-Break-Up model with a two (or three)-step chemical equilibrium and (b) PDF/mixture fraction model with equilibrium. The turbulent models employed included k–e model, RNG k–e, RSM and like large eddy simulation (LES). These studies were performed for different oxygen contents in the combustion air from 2% (vol.%) up to 21% as well as for various preheat air temperatures. It was concluded that the latter combustion model is suitable for predicting temperature and NO emission. Advanced turbulent models, like the LES and the RSM, give a small difference in the near field prediction of the flow, contrary to the empirical constants as, for example Cs, in the LES model which has a significant influence on the predictions [8]. The main features of HiTAC flame (uniform temperature distribution, chemical species profiles, flow pattern and NO emissions) were qualitatively obtained. Moreover, it was found that the flame temperature, the temperature uniformity, and the NO emissions are different when different air diluents were adopted [10]. The authors argued that the heat capacity of the diluted air plays a main role in the effect of air dilution on the combustion process, and the turbulent intensity also plays a role but less significantly compared with the heat capacity. In all works, the fuel gas inlet temperature was assumed to be equal to room temperature. However, in a furnace, the forepart of the fuel injection nozzle is always surrounded by hot flue gas. Since the nozzle is not cooled, the fuel temperature is elevated. Temperature of the fuel jet, as high as 500 K, was measured in the experimental single flame furnace, which is described, for example, by Lille et al. [6] and Blasiak et al. [7]. Therefore, influence of the fuel elevated temperature on results of mathematical modelling should be verified. On the other hand, preheating of the fuel gas can be of interest in new applications of HiTAC or when burning low calorific gas fuels. For these reasons, numerical study of influence of the fuel gas temperature on combustion performance was undertaken. In this work, HiTAC of a single propane jet at elevated temperatures was investigated. Two different combustion models were used. Influences of the empirical coefficients in the combustion model are discussed. Concepts of flame volume ratio Rf and temperature uniformity ratio Rtu are proposed to describe the physical changes of the HiTAC flame. A general-purpose code, STAR CD, was used in this work. 2. Numerical modelling To solve the time-averaged Navier–Stokes equations, a turbulence model is used to provide closure for the Reynolds stresses in the momentum equations, and a combustion model is used to obtain the time-averaged reaction rates in the species continuity and energy equations. In this work, the turbulence behaviour was modelled by the RNG k–e model [12] with a wall function. Two combustion models were applied to simulate the propane combustion with high temperature and oxygen deficient air. That is, the Eddy-Break-Up model with a three-step chemical equilibrium, and the PDF/mixture fraction model with equilibrium. Additionally, the radiation heat transfer was implemented by the discrete-beam method for inter-surface radiation transport. NO emissions were calculated by the use of thermal and prompt models. Computations of NO formation rates and NO concentrations were carried out using a post-processor based on
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previously calculated velocities, turbulence, temperature, and chemistry fields. In order to take into account turbulence fluctuations, a joint two-variable PDF was employed in the calculation of the mean NO formation rate. The reaction rates of EBU combustion model were calculated from the Arrhenius rate expression or by using eddy-dissipation concept of Magnussen and Hjertager [13]. According to the model, the volumetric ‘fuel’ consumption rate is given by qg e mO mP (1) ;B Amin mF ; RF ¼ sO sP k where sO ¼ nO MO =nF MF , sP ¼ nP MP =nF MF , A and B are empirical coefficients with nominal 4 and 0.5, respectively. In this regard, these were studied. The first two arguments in the square brackets determine the local rate-controlling concentration, while the third is intended to inhibit the reaction where the temperature is low. The micro mixing time scale is taken to be k=e, the dissipation time scale. The combustion is modelled with a three-step reaction scheme because HiTAC combustion is considered as a sort of staged combustion. In the first stage, fuel conversion to CO and H2 occurs in a diluted form, a further mixing in the reaction zone leads to complete conversion of CO and H2 to CO2 and H2O. The three-step reaction is as follows: C3 H8 þ 1:5O2 ! 3CO þ 4H2 CO þ 0:5O2 ! CO2 H2 þ 0:5O2 ! H2 O
ðR1Þ ðR2Þ ðR3Þ
The presume-PDF model employs the ‘fast-kinetics’ assumption, which results in having turbulent mixing between the streams as the rate-controlling process. In the present work, 11 species, such as C3H8, O2, CO, H2, CO2, H2O, N2, C2, OH, CH and O, have been considered. Furthermore to analyse the results of numerical experiments, the following parameters were used. 2.1. Flame volume ratio Flame volume ratio Rf is adopted to describe flame volume. Flame length is traditionally used to describe the turbulent jet flame size. This can often be determined visually. However, HiTAC combustion is more like a ‘volume combustion’, and the flame length is not so clearly defined or possible to measure. HiTAC flame is also less luminous or even flameless, thus, it can be agreed that the flame length is not enough to characterise the flame size. Therefore, to describe the flame under HiTAC, the flame volume ratio (reaction zone) is used in this work. Furthermore, determination of the HiTAC flame volume is useful to optimise the size of the combustion chamber or to determine optimal number of flames per combustion chamber. Flame volume is defined by means of the oxidation mixture ratio. It is calculated as mass fraction of oxygen to mass fraction of oxygen and the sum of oxygen needed to complete combustion at any point in combustion chamber, as follows: mO P (2) Ro ¼ mO þ sj mF;j j
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This ratio will have the value Ro ¼ 1 when it is at air inlet or combustion is completed. For fuel inlet, Ro ¼ 0. To determine flame border through this parameter, the critical Rcr must be given. Eq. (2) can be transferred as following: Ro ¼
1 P mF;j 1 þ sj mO j
(3)
For HiTAC technology, the preheated air temperature is above the fuel’s autoignition temperature, therefore, the fuel flammability limit plays a main role for combustion stability. Thus, the criterion Rcr can be determined through the flue flammability limit / in Eq. (4). Rcr ¼
1 P 1 þ sj /j
(4)
j
The lean flammability limits for different fuel species have been used to indicate the outside border of the flame, and their rich flammability limits are used to give the inside border of the flame. According to Glassman [14], the flammability lean limit of propane in air and oxygen is 2% (fuel volume percentage), and this value of CO in air is 12%, then Ro ¼ 0:99 is assumed to indicate a flame border. Thus, the flame volume can be approximately defined when 0 Ro 0:99. Dimensionless flame volume ratio, Rf, is defined as follows: Rf ¼
Vf VF
(5)
2.2. Mean residence time Mean residence time sR is defined as the time that fuel gas parcels need to pass through the flame (reaction) volume: _ sR ¼ qf Vf =m
(6)
Mean residence time is more suitable to describe the physical phenomena of combustion reaction and pollution emissions as the chemical reactions primarily occur inside the flame volume. 2.3. Gas temperature uniformity ratio Gas temperature uniformity ratio Rtu is used to describe the gas temperature field uniformity inside the furnace. Many published works in the literature stated that HiTAC technology gives much more uniform temperature field than traditional combustion. Furnace gas temperature uniformity ratio as defined below was used to describe the quality of temperature field in the furnace: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X ðTi T Þ 2 (7) Rtu ¼ T When Rtu ¼ 0, there is no gas temperature gradient inside the furnace.
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3. Results and discussion Combustion of a single fuel jet of propane in a laboratory furnace was studied numerically. The furnace combustion chamber has dimensions of 0:16 0:2 0:28 m. Computational domain and meshes of HiTAC test furnace can be seen in Fig. 1. Fuel nozzle is placed on the wall in a cross-flow to the main flow of oxygen deficient and preheated air. In a way similar to that during the experiments presented by Lille et al. [6], the preheated air was mixed with nitrogen in order to decrease oxygen concentration. Variables chosen for numerical studies are fuel injection temperature, oxygen concentration, and temperature of the preheated air. Flow rates of preheated air and fuel were kept constant and equal to 3:333 103 and 5:0 106 m3 s1 , respectively. Fuel preheat temperature was in the range from 288 to 873 K, and air preheat temperature from 1041 to 1273 K. Oxygen concentration in the preheated air varied from 2% (mass) to 18% (mass). Reynolds numbers were 1162 and 3800 for the preheated air and fuel gas inlet, respectively. The blowing ratio as defined in the paper by Hasselbrink and Mungal [15] is 160. At this stage of the study, the thermal decomposition of propane at elevated temperatures before the outlet from the gas nozzle was not considered. Adiabatic wall boundary conditions are assumed in the heat transfer model (i.e. zero heat flux). The computational domain, representing laboratory test furnace, was divided into 23 148 computational cells using unstructured meshes with embedded refinement to refine the fuel nozzle. Fig. 1 displays the spatial discretization of the computation domain. 3.1. Discussion of mathematical modelling Gas temperature profiles and flame zones are shown in Figs. 2 and 3. From Fig. 2 is seen that the maximum gas temperature predicted with the use of PDF combustion model is equal to 1952 K, but it is equal to 1846 K if the EBU combustion model is employed. Also, areas of maximum gas temperature within the flame are predicted to be larger when the PDF model was used. The lower peak temperature seems more realistic when oxygen concentration is only 10%.
Fig. 1. 3D-computational domain and meshes of HiTAC test furnace.
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Fig. 2. Predicted temperature profiles for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 473 K. (a) PDF model; (b) EBU model.
Moreover, one can see from Fig. 3 that the flame volume as defined by the means of the oxidation mixture ratio was predicted to be smaller when the PDF model is used. Also, it can be noticed that the flame size and shape predicted by the EBU model is visually similar to the flame presented in Fig. 3(c) [6]. Based on these facts, it was assumed that the HiTAC flame predictions with the EBU model are more realistic. Thus, the EBU model was further studied to be more applicable to the HiTAC flame modelling. Two empirical coefficients, A and B, are involved in the rate of reaction equation (1) in the EBU model. It has often proved to be necessary to adjust A and B to obtain good performance for a particular application. B is taken into account to inhibit reaction when the temperature is low. However, in HiTAC combustion, the air temperature is higher than fuel self-ignition temperature, the influence of empirical B is insignificant, so B is set as normal constant 0.5. Concerning A, it is known that the fuel consumption decreases when A decreases according to
Fig. 3. Predicted distributions of oxidation mixture ratio for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 473 K (the same scale as Fig. 5). (a) PDF model; (b)EBU model; (c) flame photograph.
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the reaction equation as the reaction rate will also decrease. Many experimental works (as for example, Refs. [2,3,6,16]) proved that the HiTAC combustion rate is slower than traditional combustion. This indicates that A should be less than nominal value 4. Thus, simulation for the following three different values of A, that is 4.0, 2.0 and 1.0, was performed. Fig. 4 shows that the predicted temperature field is influenced by a value of the A coefficient. Lower value of the A coefficient gives smaller temperature gradients in the flame. It is clearly seen that the area of flame with the highest temperature is reduced when a lower value of the A coefficient is applied. Influence of the A value on the oxidation mixture ratio is shown in Fig. 5. Numerical predictions demonstrate that the flame volume increases with the reduction in A value. It also shows more uniform distribution of the fuel inside the flame when smaller values of the A are used. It also confirms that the combustion rate is slow according to Eq. (1).
Fig. 4. Predicted gas temperature profiles for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 288 K. (a) A ¼ 4:0; (b) A ¼ 2:0; (c) A ¼ 1:0.
Fig. 5. Predicted distributions of the oxidation mixture ratio for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 288 K. (a) A ¼ 4:0; (b) A ¼ 2:0; (c) A ¼ 1:0.
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3.2. Flame volume ratio, Rf Consequently, the EBU model was used to study the influence of the fuel preheat temperature on the combustion. Fig. 6 shows that the flame volume ratio Rf depends a lot on oxygen concentration in the preheated combustion air, the temperature of the combustion air, and the fuel inlet temperature. Flame volume ratio always increases when the oxygen concentration in the preheated air is reduced. This is clearer if oxygen concentration in the preheated air is below 10%. Elevating fuel temperature leads to reduction of the flame volume ratio Rf at constant oxygen concentration. For example, when oxygen concentration is equal to 5% and combustion air temperature is equal to 1273 K, the Rf in the case of the fuel inlet temperature equal to 288 K is six times larger than in the case of the fuel inlet temperature equal to 873 K. At 10% of oxygen concentration, this relation is 5.3, and at 18% of oxygen concentration, it is equal to 8.7. The reason for this is the decrease of the fuel density with increasing temperature, thus leading to increase of fuel inlet velocity at constant fuel fluxes. Changes of density and fuel inlet velocity are proportional to the fuel temperature. Increase of initial velocity of the fuel jet improves the mixing between fuel and preheated air which results in flame volume decrease. Improvement of mixing is approximately proportional to the fuel inlet velocity. Therefore, reduction of Rf is also proportional to the fuel inlet temperature. Fig. 7 also shows that combustion air temperature has a much less significant influence on the flame volume at constant oxygen concentration and fuel temperature. For the investigated temperature range (1041–1273 K) of the preheated air, flame volume was found to be almost constant at fixed oxygen concentration and fuel inlet temperature. Generally, it can be concluded that the largest flames are obtained at the lowest oxygen concentration in the combustion air and at lowest fuel and preheated air temperatures. Assuming
Fig. 6. Rf versus oxygen concentration for various air and fuel temperatures (K).
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Fig. 7. Rf versus preheated air temperature for oxygen concentration and fuel temperatures (K).
the combustion air temperature equal to 1273 K and the fuel inlet temperature to be equal to 473 K, the HiTAC flame at oxygen concentration equal to 5i% is more than eight times larger than that at oxygen concentration equal to 18%. Flame volume in the case of 2% oxygen concentration and 288 K fuel temperature is 223 times larger than that in case of 18% oxygen concentration and 873 K fuel temperature. The performed numerical experiments show that the HiTAC process is spread over a much larger volume than conventional turbulent diffusion flame. It confirms that the HiTAC is a large volume combustion with reduced combustion rate. From the above, it can also be stated that three primary factors determine flame volume. These are as follows: (1)—initial momentum of the fuel jet; (2)—oxygen concentration in combustion air; (3)—ratio of fuel inlet density to ambient gas density, qF =q0 . 3.3. Mean residence time and flame peak temperature Fig. 8 shows that the mean residence time, sR, increases with decreasing oxygen concentration. This dependence is almost consistent with change of flame volume ratio, Rf, as showed in Fig. 7. It confirms the primary importance of oxygen concentration and fuel temperature on mean residence time. Fuel temperature influences the value of sR, but not as much as the molar fraction of oxygen. For example, for air temperature equal to 1273 K and fuel temperature equal to 473 K, the mean residence time in the case of 5% oxygen concentration is 6.7 times larger than that in the case of 18%. When oxygen concentration is 5% and combustion air temperature is 1273 K, the mean residence time in the case of fuel temperature equal to 288 K is 2.9 times larger than that in case of fuel temperature equal to 873 K. These values are somewhat less than changes of Rf at the same conditions as the flame density compensates. It can be concluded that the mean residence time increases with the reduction of oxygen concentration as well as with the decrease of fuel inlet temperature. The mean residence time also slightly increases with a decrease of the preheated air temperature. For example, assuming oxygen content to be equal to 5% and fuel temperature equal to 473 K, the changes of local residence time is 1.12 times for combustion air temperature from
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Fig. 8. Mean residence time (s) versus oxygen concentration and combustion air and fuel temperature (K).
1273 K down to 1041 K. This is because flame density increases with a reduction of combustion air temperature. It is obvious as shown in Fig. 9 that for any preheated air and fuel temperature, the peak temperature linearly decreases with the reduction of oxygen concentration. At any preheated air temperature, the peak temperatures fall slightly for constant oxygen concentration and at reduced fuel inlet temperature. The maximum temperature of flame, Tf,max, of the common fuel during preheated and diluted air combustion can be generalized using the following relationship: Tf;max ¼ Tpre þ
Tad 21=½O2 þ 1
(8)
Fig. 9. Peak flame temperature (Tmax) versus oxygen concentration for various combustion air and fuel temperature (K).
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The adiabatic flame temperature of propane combustion is 2267 K for stoichiometric combustion with air. 3.4. Gas temperature uniformity ratio, Rtu Gas temperature uniformity ratio, Rtu, versus oxygen concentration for various air and fuel temperatures is shown in Fig. 10. Rtu decreases with reducing oxygen concentration, which indicates the more uniform temperature field at lower oxygen content. Rtu, in the case of oxygen concentration, is equal to 5%, and at fuel inlet temperature equal to 288 K is two times lower than in the case of oxygen concentration equal to 18%. Increase of temperature field uniformity results from the reduction of flame peak temperature, and from increase of the flame volume Rf at lower oxygen concentrations. Influence of the fuel inlet temperature on the temperature uniformity ratio Rtu is difficult to detect. In general, it was noticed that at lower oxygen concentration, the influence of fuel inlet temperature is more distinct. At high oxygen levels, for example, at 18% oxygen concentration, the magnitudes of Rtu at different fuel and air temperatures are very close to each other. It is and the temperature of ith computational cell, Ti, in apparent that the average temperature T test furnace increase when the fuel temperature increases. 3.5. NO formation It can be clearly seen from Fig. 11 that NO emission decreases as oxygen concentration decreases besides there being a reduction of preheated air temperature. These changes have the same tendency as at the peak temperature since the NO emission mainly depends on the flame temperature. NO emission decreases with the increase of fuel inlet temperature (Fig. 11). It is the effect of fuel inlet temperature influence on the mean residence time, flame’s volume and flame’s uniformity. Increase of fuel inlet temperature increases its velocity, thus promoting better fuel jet
Fig. 10. Temperature uniformity ratio (Rtu) versus oxygen concentration for various air and fuel temperatures (K).
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Fig. 11. NO emission versus fuel temperature for various combustion air temperature and oxygen concentration and fuel temperature (K).
mixing with flue gases. It creates more uniform distribution of reactants inside the flame. As stated above, the HiTAC flame has a larger volume. Larger flame volume lowers the reactant concentration and temperatures. Since also the mean residence time is lowered with an increase of the fuel inlet temperature, the thermal NO formation is suppressed.
4. Conclusions 1. Concept of the flame volume ratio, Rf, introduced in this work was used to describe the flame volume changes during HiTAC. 2. It was showed that HiTAC is spread over much larger volume. Flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. Additionally, flame volume was found to be almost constant for the investigated temperature range (1041–1273 K) of the preheated air at fixed oxygen concentration and fuel inlet temperature. 3. Concept of gas temperature uniformity ratio Rtu introduced in this work was used to characterise the temperature uniformity changes inside the flame. The temperature profile becomes more uniform when oxygen concentration in preheated air decreases. 4. Mean residence time of fuel gas parcels inside the flame volume increases with a reduction of oxygen concentration as well as with a decrease of fuel inlet temperature and slightly increases with a decrease of the preheated air temperature. 5. Increase of the fuel inlet temperature results in a smaller flame, shorter mean residence time, smaller temperature peaks, and lower formation of NO. References [1] Hasegawa T, Tanaka R, Niioka T. Combustion with high temperature low oxygen air in regenerative burners. Proceedings of the First Asia–Pacific Conference on Combustion, Osaka, Japan May. 1997, p. 290–3.
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[2] Yasuda T, Ueno C. Dissemination project of industrial furnace revamped with HTAC. Proceedings of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [3] Gupta AK. Flame characteristics and challenges with high temperature air combustion. Proceeding of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [4] Gupta AK, Bolz S, Hasegawa T. Effect of air preheated temperature and oxygen concentration on flame structure and emission. Journal of Energy Resources Technology 1999;121:209–16. [5] Kitagawa K, Konishi N, Arai N, Gupta AK. Proceeding of International Joint Power Generation Conference. Two-dimensional distribution of flame fluctuation during highly preheated air combustion, Vol. 1. ASME; 1998, p. 239–42. [6] Lille S, Dobski T, Blasiak W. Visualisation of fuel jet in conditions of highly preheated air combustion. AIAA Journal of Propulsion and Power 2000;16(4):595–600. [7] Blasiak W, Szewczyk D, Dobski T. Influence of N2 addition on combustion of single jet of methane in highly preheated air. Proceedings of the International Joint Power Generation Conference, New Orleans (LA), USA, June. 2001. [8] Dong W, Blasiak W. Large eddy simulation of a single jet flow in highly preheated and diluted air combustion. Archivum Combustionis 2000;20:3–4. [9] Dong W. Design of advanced industrial furnaces using numerical modelling method. Doctoral Thesis. Royal Institute of Technology, Stockholm, Sweden, 1999. [10] Yuan J, Nause I. Effects of air dilution on highly preheated air combustion in a regenerative furnace. Energy and Fuel 1999;13:99–104. [11] Hsiao T, Yang W, Jiang SJ, Zhou JM. Experimental investigation and numerical simulation of HiTAC flames and flow phenomenon at its switching moment. Proceedings of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [12] Star CD manual, methodology version 3.05. Computation Dynamics Limited; 1998. [13] Magnussen BF, Hjertager BJ. On mathematical models of turbulent combustion with special emphasis on the soot formation and combustion. Proceedings of the 16th Symposium (International) on Combustion. The Combustion Institute; 1977, p. 719. [14] Glassman I. Combustion. Academic Press Inc; 1996. [15] Hasselbrink EF, Mungal MG. Observations on the stabilisation region of lifted non-premixed methane transverse jet flames. Proceedings of the 27th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1998, p. 1167–73. [16] Verlaan AL, Orsino S, Lallemant N, Weber R. Fluid flow and mixing in a furnace equipped with the low-NOx regenerative burner of Nippon Furnace Kogyo, IFRF Doc. NO. F46/y/1, Ijmuiden, 1998.
Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air Weihong Yang , Wlodzimierz Blasiak Royal Institute of Technology (KTH), Division of Energy and Furnace Technology, Brinellva¨gen 23, 100 44 Stockholm, Sweden
Abstract Combustion of a single jet of propane in a cross-flowing stream of preheated and oxygen deficient air is numerically analyzed with emphasis on influences of fuel temperature. Both Eddy-Break-Up and PDF/mixture fraction combustion models coupled with RNG k–e turbulent model were applied and the predicted results were compared. Thermal and prompt NO models were employed to calculate NO emissions. Results show that the Eddy-Break-Up model is more suitable for predicting temperature field and flame shape. It was showed that flame during high temperature air combustion condition is spread over a much larger volume. Flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. Additionally, it is almost constant at fixed oxygen concentration and fuel inlet temperature for the temperature of the preheated and oxygen deficient air equal to 1041–1273 K. Increase of the fuel inlet temperature results in smaller flame, shorter mean residence time, smaller temperature peaks, and lower emission of NO. # 2004 Elsevier Ltd. All rights reserved.
1. Introduction High temperature air combustion (HiTAC) applied to industrial furnaces offers energy savings, reduction of pollutant emissions, and high quality of the product at increased production rate [1,2]. Although this technology had been developed more than 10 years back and has been commercially applied in different types of furnaces as reported by Yasuda and Ueno [2], the basic chemical–physical phenomenon still needs to be better explained. Apart from semi-industrial tests, both experimental and numerical methods were employed to study the properties of single fuel jet behaviour during the HiTAC condition. When analyzing the literature of the subject, it is well seen that the study of a single fuel jet allowed explaining
Corresponding author. Tel.: +46-8-790-8402; fax: +46-8-20768. E-mail address: [email protected] (W. Yang).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.05.011
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Nomenclature A B k m M _ m n PDF RNG RSM RF Rf Ro Rtu T T Vf VF [O2] q e sR /
empirical coefficient empirical coefficient turbulence kinetic energy, m2 s2 mass fraction molecular weight, kg mol1 total mass rate of flue gas, kg s1 stoichiometric coefficient (number of moles) probability density function renormalization group Reynolds stress model fuel consumption rate, kg m3 s1 flame volume ratio oxidation mixture ratio temperature variance temperature, K average temperature in the furnace, K flame volume, m3 s1 volumetric flow rate of the fuel, m3 s1 oxygen concentration, % density, kg m3 turbulence kinetic energy dissipation rate, m2 s3 mean residence time, s fuel flammability limit
Subscripts a air ad adiabatic cr criteria value F fuel f flame g gas i calculation cell number j species O oxygen P combustion products Pre preheated
and measuring many unique features of the HiTAC. For example, experimental studies of single fuel jet were performed by Hasegawa et al. [1], Gupta et al. [3,4], Kitagawa et al. [5], Lille et al. [6] and Blasiak et al. [7].
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Numerical studies focused on studies of the structure of flame and different mathematical models using well-known general-purpose codes as FLUENT [8–10] and CFX [11]. In all the cases, a flame jet of propane or methane in a cross-flow of preheated and diluted combustion air was a subject of numerical modelling. Several modelling approaches were employed, for example, (a) Eddy-Break-Up model with a two (or three)-step chemical equilibrium and (b) PDF/mixture fraction model with equilibrium. The turbulent models employed included k–e model, RNG k–e, RSM and like large eddy simulation (LES). These studies were performed for different oxygen contents in the combustion air from 2% (vol.%) up to 21% as well as for various preheat air temperatures. It was concluded that the latter combustion model is suitable for predicting temperature and NO emission. Advanced turbulent models, like the LES and the RSM, give a small difference in the near field prediction of the flow, contrary to the empirical constants as, for example Cs, in the LES model which has a significant influence on the predictions [8]. The main features of HiTAC flame (uniform temperature distribution, chemical species profiles, flow pattern and NO emissions) were qualitatively obtained. Moreover, it was found that the flame temperature, the temperature uniformity, and the NO emissions are different when different air diluents were adopted [10]. The authors argued that the heat capacity of the diluted air plays a main role in the effect of air dilution on the combustion process, and the turbulent intensity also plays a role but less significantly compared with the heat capacity. In all works, the fuel gas inlet temperature was assumed to be equal to room temperature. However, in a furnace, the forepart of the fuel injection nozzle is always surrounded by hot flue gas. Since the nozzle is not cooled, the fuel temperature is elevated. Temperature of the fuel jet, as high as 500 K, was measured in the experimental single flame furnace, which is described, for example, by Lille et al. [6] and Blasiak et al. [7]. Therefore, influence of the fuel elevated temperature on results of mathematical modelling should be verified. On the other hand, preheating of the fuel gas can be of interest in new applications of HiTAC or when burning low calorific gas fuels. For these reasons, numerical study of influence of the fuel gas temperature on combustion performance was undertaken. In this work, HiTAC of a single propane jet at elevated temperatures was investigated. Two different combustion models were used. Influences of the empirical coefficients in the combustion model are discussed. Concepts of flame volume ratio Rf and temperature uniformity ratio Rtu are proposed to describe the physical changes of the HiTAC flame. A general-purpose code, STAR CD, was used in this work. 2. Numerical modelling To solve the time-averaged Navier–Stokes equations, a turbulence model is used to provide closure for the Reynolds stresses in the momentum equations, and a combustion model is used to obtain the time-averaged reaction rates in the species continuity and energy equations. In this work, the turbulence behaviour was modelled by the RNG k–e model [12] with a wall function. Two combustion models were applied to simulate the propane combustion with high temperature and oxygen deficient air. That is, the Eddy-Break-Up model with a three-step chemical equilibrium, and the PDF/mixture fraction model with equilibrium. Additionally, the radiation heat transfer was implemented by the discrete-beam method for inter-surface radiation transport. NO emissions were calculated by the use of thermal and prompt models. Computations of NO formation rates and NO concentrations were carried out using a post-processor based on
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previously calculated velocities, turbulence, temperature, and chemistry fields. In order to take into account turbulence fluctuations, a joint two-variable PDF was employed in the calculation of the mean NO formation rate. The reaction rates of EBU combustion model were calculated from the Arrhenius rate expression or by using eddy-dissipation concept of Magnussen and Hjertager [13]. According to the model, the volumetric ‘fuel’ consumption rate is given by qg e mO mP (1) ;B Amin mF ; RF ¼ sO sP k where sO ¼ nO MO =nF MF , sP ¼ nP MP =nF MF , A and B are empirical coefficients with nominal 4 and 0.5, respectively. In this regard, these were studied. The first two arguments in the square brackets determine the local rate-controlling concentration, while the third is intended to inhibit the reaction where the temperature is low. The micro mixing time scale is taken to be k=e, the dissipation time scale. The combustion is modelled with a three-step reaction scheme because HiTAC combustion is considered as a sort of staged combustion. In the first stage, fuel conversion to CO and H2 occurs in a diluted form, a further mixing in the reaction zone leads to complete conversion of CO and H2 to CO2 and H2O. The three-step reaction is as follows: C3 H8 þ 1:5O2 ! 3CO þ 4H2 CO þ 0:5O2 ! CO2 H2 þ 0:5O2 ! H2 O
ðR1Þ ðR2Þ ðR3Þ
The presume-PDF model employs the ‘fast-kinetics’ assumption, which results in having turbulent mixing between the streams as the rate-controlling process. In the present work, 11 species, such as C3H8, O2, CO, H2, CO2, H2O, N2, C2, OH, CH and O, have been considered. Furthermore to analyse the results of numerical experiments, the following parameters were used. 2.1. Flame volume ratio Flame volume ratio Rf is adopted to describe flame volume. Flame length is traditionally used to describe the turbulent jet flame size. This can often be determined visually. However, HiTAC combustion is more like a ‘volume combustion’, and the flame length is not so clearly defined or possible to measure. HiTAC flame is also less luminous or even flameless, thus, it can be agreed that the flame length is not enough to characterise the flame size. Therefore, to describe the flame under HiTAC, the flame volume ratio (reaction zone) is used in this work. Furthermore, determination of the HiTAC flame volume is useful to optimise the size of the combustion chamber or to determine optimal number of flames per combustion chamber. Flame volume is defined by means of the oxidation mixture ratio. It is calculated as mass fraction of oxygen to mass fraction of oxygen and the sum of oxygen needed to complete combustion at any point in combustion chamber, as follows: mO P (2) Ro ¼ mO þ sj mF;j j
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This ratio will have the value Ro ¼ 1 when it is at air inlet or combustion is completed. For fuel inlet, Ro ¼ 0. To determine flame border through this parameter, the critical Rcr must be given. Eq. (2) can be transferred as following: Ro ¼
1 P mF;j 1 þ sj mO j
(3)
For HiTAC technology, the preheated air temperature is above the fuel’s autoignition temperature, therefore, the fuel flammability limit plays a main role for combustion stability. Thus, the criterion Rcr can be determined through the flue flammability limit / in Eq. (4). Rcr ¼
1 P 1 þ sj /j
(4)
j
The lean flammability limits for different fuel species have been used to indicate the outside border of the flame, and their rich flammability limits are used to give the inside border of the flame. According to Glassman [14], the flammability lean limit of propane in air and oxygen is 2% (fuel volume percentage), and this value of CO in air is 12%, then Ro ¼ 0:99 is assumed to indicate a flame border. Thus, the flame volume can be approximately defined when 0 Ro 0:99. Dimensionless flame volume ratio, Rf, is defined as follows: Rf ¼
Vf VF
(5)
2.2. Mean residence time Mean residence time sR is defined as the time that fuel gas parcels need to pass through the flame (reaction) volume: _ sR ¼ qf Vf =m
(6)
Mean residence time is more suitable to describe the physical phenomena of combustion reaction and pollution emissions as the chemical reactions primarily occur inside the flame volume. 2.3. Gas temperature uniformity ratio Gas temperature uniformity ratio Rtu is used to describe the gas temperature field uniformity inside the furnace. Many published works in the literature stated that HiTAC technology gives much more uniform temperature field than traditional combustion. Furnace gas temperature uniformity ratio as defined below was used to describe the quality of temperature field in the furnace: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X ðTi T Þ 2 (7) Rtu ¼ T When Rtu ¼ 0, there is no gas temperature gradient inside the furnace.
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3. Results and discussion Combustion of a single fuel jet of propane in a laboratory furnace was studied numerically. The furnace combustion chamber has dimensions of 0:16 0:2 0:28 m. Computational domain and meshes of HiTAC test furnace can be seen in Fig. 1. Fuel nozzle is placed on the wall in a cross-flow to the main flow of oxygen deficient and preheated air. In a way similar to that during the experiments presented by Lille et al. [6], the preheated air was mixed with nitrogen in order to decrease oxygen concentration. Variables chosen for numerical studies are fuel injection temperature, oxygen concentration, and temperature of the preheated air. Flow rates of preheated air and fuel were kept constant and equal to 3:333 103 and 5:0 106 m3 s1 , respectively. Fuel preheat temperature was in the range from 288 to 873 K, and air preheat temperature from 1041 to 1273 K. Oxygen concentration in the preheated air varied from 2% (mass) to 18% (mass). Reynolds numbers were 1162 and 3800 for the preheated air and fuel gas inlet, respectively. The blowing ratio as defined in the paper by Hasselbrink and Mungal [15] is 160. At this stage of the study, the thermal decomposition of propane at elevated temperatures before the outlet from the gas nozzle was not considered. Adiabatic wall boundary conditions are assumed in the heat transfer model (i.e. zero heat flux). The computational domain, representing laboratory test furnace, was divided into 23 148 computational cells using unstructured meshes with embedded refinement to refine the fuel nozzle. Fig. 1 displays the spatial discretization of the computation domain. 3.1. Discussion of mathematical modelling Gas temperature profiles and flame zones are shown in Figs. 2 and 3. From Fig. 2 is seen that the maximum gas temperature predicted with the use of PDF combustion model is equal to 1952 K, but it is equal to 1846 K if the EBU combustion model is employed. Also, areas of maximum gas temperature within the flame are predicted to be larger when the PDF model was used. The lower peak temperature seems more realistic when oxygen concentration is only 10%.
Fig. 1. 3D-computational domain and meshes of HiTAC test furnace.
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Fig. 2. Predicted temperature profiles for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 473 K. (a) PDF model; (b) EBU model.
Moreover, one can see from Fig. 3 that the flame volume as defined by the means of the oxidation mixture ratio was predicted to be smaller when the PDF model is used. Also, it can be noticed that the flame size and shape predicted by the EBU model is visually similar to the flame presented in Fig. 3(c) [6]. Based on these facts, it was assumed that the HiTAC flame predictions with the EBU model are more realistic. Thus, the EBU model was further studied to be more applicable to the HiTAC flame modelling. Two empirical coefficients, A and B, are involved in the rate of reaction equation (1) in the EBU model. It has often proved to be necessary to adjust A and B to obtain good performance for a particular application. B is taken into account to inhibit reaction when the temperature is low. However, in HiTAC combustion, the air temperature is higher than fuel self-ignition temperature, the influence of empirical B is insignificant, so B is set as normal constant 0.5. Concerning A, it is known that the fuel consumption decreases when A decreases according to
Fig. 3. Predicted distributions of oxidation mixture ratio for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 473 K (the same scale as Fig. 5). (a) PDF model; (b)EBU model; (c) flame photograph.
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the reaction equation as the reaction rate will also decrease. Many experimental works (as for example, Refs. [2,3,6,16]) proved that the HiTAC combustion rate is slower than traditional combustion. This indicates that A should be less than nominal value 4. Thus, simulation for the following three different values of A, that is 4.0, 2.0 and 1.0, was performed. Fig. 4 shows that the predicted temperature field is influenced by a value of the A coefficient. Lower value of the A coefficient gives smaller temperature gradients in the flame. It is clearly seen that the area of flame with the highest temperature is reduced when a lower value of the A coefficient is applied. Influence of the A value on the oxidation mixture ratio is shown in Fig. 5. Numerical predictions demonstrate that the flame volume increases with the reduction in A value. It also shows more uniform distribution of the fuel inside the flame when smaller values of the A are used. It also confirms that the combustion rate is slow according to Eq. (1).
Fig. 4. Predicted gas temperature profiles for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 288 K. (a) A ¼ 4:0; (b) A ¼ 2:0; (c) A ¼ 1:0.
Fig. 5. Predicted distributions of the oxidation mixture ratio for 10% oxygen in the air preheated up to 1041 K and at fuel inlet temperature equal to 288 K. (a) A ¼ 4:0; (b) A ¼ 2:0; (c) A ¼ 1:0.
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3.2. Flame volume ratio, Rf Consequently, the EBU model was used to study the influence of the fuel preheat temperature on the combustion. Fig. 6 shows that the flame volume ratio Rf depends a lot on oxygen concentration in the preheated combustion air, the temperature of the combustion air, and the fuel inlet temperature. Flame volume ratio always increases when the oxygen concentration in the preheated air is reduced. This is clearer if oxygen concentration in the preheated air is below 10%. Elevating fuel temperature leads to reduction of the flame volume ratio Rf at constant oxygen concentration. For example, when oxygen concentration is equal to 5% and combustion air temperature is equal to 1273 K, the Rf in the case of the fuel inlet temperature equal to 288 K is six times larger than in the case of the fuel inlet temperature equal to 873 K. At 10% of oxygen concentration, this relation is 5.3, and at 18% of oxygen concentration, it is equal to 8.7. The reason for this is the decrease of the fuel density with increasing temperature, thus leading to increase of fuel inlet velocity at constant fuel fluxes. Changes of density and fuel inlet velocity are proportional to the fuel temperature. Increase of initial velocity of the fuel jet improves the mixing between fuel and preheated air which results in flame volume decrease. Improvement of mixing is approximately proportional to the fuel inlet velocity. Therefore, reduction of Rf is also proportional to the fuel inlet temperature. Fig. 7 also shows that combustion air temperature has a much less significant influence on the flame volume at constant oxygen concentration and fuel temperature. For the investigated temperature range (1041–1273 K) of the preheated air, flame volume was found to be almost constant at fixed oxygen concentration and fuel inlet temperature. Generally, it can be concluded that the largest flames are obtained at the lowest oxygen concentration in the combustion air and at lowest fuel and preheated air temperatures. Assuming
Fig. 6. Rf versus oxygen concentration for various air and fuel temperatures (K).
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Fig. 7. Rf versus preheated air temperature for oxygen concentration and fuel temperatures (K).
the combustion air temperature equal to 1273 K and the fuel inlet temperature to be equal to 473 K, the HiTAC flame at oxygen concentration equal to 5i% is more than eight times larger than that at oxygen concentration equal to 18%. Flame volume in the case of 2% oxygen concentration and 288 K fuel temperature is 223 times larger than that in case of 18% oxygen concentration and 873 K fuel temperature. The performed numerical experiments show that the HiTAC process is spread over a much larger volume than conventional turbulent diffusion flame. It confirms that the HiTAC is a large volume combustion with reduced combustion rate. From the above, it can also be stated that three primary factors determine flame volume. These are as follows: (1)—initial momentum of the fuel jet; (2)—oxygen concentration in combustion air; (3)—ratio of fuel inlet density to ambient gas density, qF =q0 . 3.3. Mean residence time and flame peak temperature Fig. 8 shows that the mean residence time, sR, increases with decreasing oxygen concentration. This dependence is almost consistent with change of flame volume ratio, Rf, as showed in Fig. 7. It confirms the primary importance of oxygen concentration and fuel temperature on mean residence time. Fuel temperature influences the value of sR, but not as much as the molar fraction of oxygen. For example, for air temperature equal to 1273 K and fuel temperature equal to 473 K, the mean residence time in the case of 5% oxygen concentration is 6.7 times larger than that in the case of 18%. When oxygen concentration is 5% and combustion air temperature is 1273 K, the mean residence time in the case of fuel temperature equal to 288 K is 2.9 times larger than that in case of fuel temperature equal to 873 K. These values are somewhat less than changes of Rf at the same conditions as the flame density compensates. It can be concluded that the mean residence time increases with the reduction of oxygen concentration as well as with the decrease of fuel inlet temperature. The mean residence time also slightly increases with a decrease of the preheated air temperature. For example, assuming oxygen content to be equal to 5% and fuel temperature equal to 473 K, the changes of local residence time is 1.12 times for combustion air temperature from
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Fig. 8. Mean residence time (s) versus oxygen concentration and combustion air and fuel temperature (K).
1273 K down to 1041 K. This is because flame density increases with a reduction of combustion air temperature. It is obvious as shown in Fig. 9 that for any preheated air and fuel temperature, the peak temperature linearly decreases with the reduction of oxygen concentration. At any preheated air temperature, the peak temperatures fall slightly for constant oxygen concentration and at reduced fuel inlet temperature. The maximum temperature of flame, Tf,max, of the common fuel during preheated and diluted air combustion can be generalized using the following relationship: Tf;max ¼ Tpre þ
Tad 21=½O2 þ 1
(8)
Fig. 9. Peak flame temperature (Tmax) versus oxygen concentration for various combustion air and fuel temperature (K).
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The adiabatic flame temperature of propane combustion is 2267 K for stoichiometric combustion with air. 3.4. Gas temperature uniformity ratio, Rtu Gas temperature uniformity ratio, Rtu, versus oxygen concentration for various air and fuel temperatures is shown in Fig. 10. Rtu decreases with reducing oxygen concentration, which indicates the more uniform temperature field at lower oxygen content. Rtu, in the case of oxygen concentration, is equal to 5%, and at fuel inlet temperature equal to 288 K is two times lower than in the case of oxygen concentration equal to 18%. Increase of temperature field uniformity results from the reduction of flame peak temperature, and from increase of the flame volume Rf at lower oxygen concentrations. Influence of the fuel inlet temperature on the temperature uniformity ratio Rtu is difficult to detect. In general, it was noticed that at lower oxygen concentration, the influence of fuel inlet temperature is more distinct. At high oxygen levels, for example, at 18% oxygen concentration, the magnitudes of Rtu at different fuel and air temperatures are very close to each other. It is and the temperature of ith computational cell, Ti, in apparent that the average temperature T test furnace increase when the fuel temperature increases. 3.5. NO formation It can be clearly seen from Fig. 11 that NO emission decreases as oxygen concentration decreases besides there being a reduction of preheated air temperature. These changes have the same tendency as at the peak temperature since the NO emission mainly depends on the flame temperature. NO emission decreases with the increase of fuel inlet temperature (Fig. 11). It is the effect of fuel inlet temperature influence on the mean residence time, flame’s volume and flame’s uniformity. Increase of fuel inlet temperature increases its velocity, thus promoting better fuel jet
Fig. 10. Temperature uniformity ratio (Rtu) versus oxygen concentration for various air and fuel temperatures (K).
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Fig. 11. NO emission versus fuel temperature for various combustion air temperature and oxygen concentration and fuel temperature (K).
mixing with flue gases. It creates more uniform distribution of reactants inside the flame. As stated above, the HiTAC flame has a larger volume. Larger flame volume lowers the reactant concentration and temperatures. Since also the mean residence time is lowered with an increase of the fuel inlet temperature, the thermal NO formation is suppressed.
4. Conclusions 1. Concept of the flame volume ratio, Rf, introduced in this work was used to describe the flame volume changes during HiTAC. 2. It was showed that HiTAC is spread over much larger volume. Flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. Additionally, flame volume was found to be almost constant for the investigated temperature range (1041–1273 K) of the preheated air at fixed oxygen concentration and fuel inlet temperature. 3. Concept of gas temperature uniformity ratio Rtu introduced in this work was used to characterise the temperature uniformity changes inside the flame. The temperature profile becomes more uniform when oxygen concentration in preheated air decreases. 4. Mean residence time of fuel gas parcels inside the flame volume increases with a reduction of oxygen concentration as well as with a decrease of fuel inlet temperature and slightly increases with a decrease of the preheated air temperature. 5. Increase of the fuel inlet temperature results in a smaller flame, shorter mean residence time, smaller temperature peaks, and lower formation of NO. References [1] Hasegawa T, Tanaka R, Niioka T. Combustion with high temperature low oxygen air in regenerative burners. Proceedings of the First Asia–Pacific Conference on Combustion, Osaka, Japan May. 1997, p. 290–3.
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[2] Yasuda T, Ueno C. Dissemination project of industrial furnace revamped with HTAC. Proceedings of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [3] Gupta AK. Flame characteristics and challenges with high temperature air combustion. Proceeding of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [4] Gupta AK, Bolz S, Hasegawa T. Effect of air preheated temperature and oxygen concentration on flame structure and emission. Journal of Energy Resources Technology 1999;121:209–16. [5] Kitagawa K, Konishi N, Arai N, Gupta AK. Proceeding of International Joint Power Generation Conference. Two-dimensional distribution of flame fluctuation during highly preheated air combustion, Vol. 1. ASME; 1998, p. 239–42. [6] Lille S, Dobski T, Blasiak W. Visualisation of fuel jet in conditions of highly preheated air combustion. AIAA Journal of Propulsion and Power 2000;16(4):595–600. [7] Blasiak W, Szewczyk D, Dobski T. Influence of N2 addition on combustion of single jet of methane in highly preheated air. Proceedings of the International Joint Power Generation Conference, New Orleans (LA), USA, June. 2001. [8] Dong W, Blasiak W. Large eddy simulation of a single jet flow in highly preheated and diluted air combustion. Archivum Combustionis 2000;20:3–4. [9] Dong W. Design of advanced industrial furnaces using numerical modelling method. Doctoral Thesis. Royal Institute of Technology, Stockholm, Sweden, 1999. [10] Yuan J, Nause I. Effects of air dilution on highly preheated air combustion in a regenerative furnace. Energy and Fuel 1999;13:99–104. [11] Hsiao T, Yang W, Jiang SJ, Zhou JM. Experimental investigation and numerical simulation of HiTAC flames and flow phenomenon at its switching moment. Proceedings of the Second International Seminar on High Temperature Combustion in Industrial Furnace, Stockholm, Sweden, January. 2000 [ISBN 83-912395-4-3]. [12] Star CD manual, methodology version 3.05. Computation Dynamics Limited; 1998. [13] Magnussen BF, Hjertager BJ. On mathematical models of turbulent combustion with special emphasis on the soot formation and combustion. Proceedings of the 16th Symposium (International) on Combustion. The Combustion Institute; 1977, p. 719. [14] Glassman I. Combustion. Academic Press Inc; 1996. [15] Hasselbrink EF, Mungal MG. Observations on the stabilisation region of lifted non-premixed methane transverse jet flames. Proceedings of the 27th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1998, p. 1167–73. [16] Verlaan AL, Orsino S, Lallemant N, Weber R. Fluid flow and mixing in a furnace equipped with the low-NOx regenerative burner of Nippon Furnace Kogyo, IFRF Doc. NO. F46/y/1, Ijmuiden, 1998.