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Pollutant emissions reduction and performance optimization of an industrial radiant tube burner
Experimental Thermal and Fluid Science 30 (2006) 605–612 www.elsevier.com/locate/etfs
Pollutant emissions reduction and performance optimization of an industrial radiant tube burner Gianfranco Scribano, Giulio Solero *, Aldo Coghe Dipartimento di Energetica, Politecnico di Milano, via La Masa, 34, 20156 Milano, Italy Received 25 May 2005; received in revised form 15 November 2005; accepted 13 December 2005
Abstract This paper presents the results of an experimental investigation performed upon a single-ended self-recuperative radiant tube burner fuelled by natural gas in the non-premixed mode, which is used in the steel industry for surface treatment. The main goal of the research activity was a systematic investigation of the burner aimed to find the best operating conditions in terms of optimum equivalence ratio, thermal power and lower pollutant emissions. The analysis, which focused on the main parameters influencing the thermal efficiency and pollutant emissions at the exhaust (NOx and CO), has been carried out for different operating conditions of the burner: input thermal powers from 12.8 up to 18 kW and equivalence ratio from 0.5 (very lean flame) to 0.95 (quasi-stoichiometric condition). To significantly reduce pollutant emissions ensuring at the same time the thermal requirements of the heating process, it has been developed a new burner configuration, in which a fraction of the exhaust gases recirculates in the main combustion region through a variable gap between the burner efflux and the inner flame tube. This internal recirculation mechanism (exhaust gases recirculation, EGR) has been favoured through the addition of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gas entrainment into the flame tube. The most important result of this solution was a decrease of NOx emissions at the exhaust of the order of 50% with respect to the original burner geometry, for a wide range of thermal power and equivalence ratio. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Natural gas burners; Radiant tube burners; Pollutant emissions
1. Introduction Owing to recent strict legislation [1], environmental impact of combustion processes is nowadays one of the most important problems involving both scientific community and burner manufacturers. In the case of natural gas combustion, nitric oxides (NOx) reduction is the most critical aim to achieve, often requesting for industrial applications heavy and high-cost plant revamping (for instance, adoption of low-NOx burners or burned gases de-NOx treatment [2,3]). Moreover, although NOx formation mechanisms are today quite understood [4], many unsolved questions are connected to the interaction between fluid dynamic and chemistry inside the burner device [5] and this *
Corresponding author. Tel.: +39 2 23998562; fax: +39 2 23998566. E-mail address: [email protected] (G. Solero).
0894-1777/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2005.12.006
makes the CFD codes not yet fully predictive. Radiant tube burners are characterised by the presence of a flame confined inside a hot space; therefore NOx formation is dominated by the thermal mechanism and NOx reduction techniques are based on minimizing flame temperature, fulfilling at the same time the thermal requirements of the heating process. This requisite can be obtained by diffusion mixing of the fuel and air to slow heat release and providing combustion of a relatively low-intensity to distribute the heat along the length of the tube and reduce the formation of potential local hot spots. A careful design of this type of burners to meet emission regulations requires a deeper knowledge of the fluid-dynamic, combustion and heat transfer processes. Although the first single-ended radiant tube was developed in the 1940s, there is scarce scientific information on this peculiar type of burner and the combustion process involved [6]. Therefore, a detailed research
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activity (both experimental and numerical) is anyway necessary to deepen our knowledge in this field and to obtain a reasonable practical solution for pollutant emissions reduction. This work presents the results of an extensive investigation performed upon a radiant tube natural gas nonpremixed burner, used in the steel industry for surface treatment. The burner behaviour has been investigated in a configuration very similar to the real working one, analysing through different experimental techniques (flame imaging, laser Doppler velocimetry, temperature measurements by thermocouples, pollutant emissions analysis at the exhaust) the burner sensitivity to input thermal power and equivalence ratio inside a wide range of operative conditions. The motivation of this analysis was primarily to provide input data for CFD simulations and also to experiment new burner designs and combustion strategies aimed to reduce NOx emissions which are usually very high in this type of burners. Consequently, the original burner has been improved introducing a self-induced exhaust gases recirculation mechanism. Exhaust gases recirculation (EGR) is in fact a well known procedure [7] used mainly in automotive applications for NOx abatement (especially thermal NOx), lowering the peak temperature and oxygen concentration in the flame region. In the new burner configuration, a fraction of the exhaust gases can recirculate in the main combustion region; the recirculation mechanism developed in the present application is promoted by the momentum of the hot jet issuing from a converging nozzle located downstream of the burner efflux. This improvement is based on the addition of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gases entrainment into the flame tube. The most important result of this solution is a decrease of NOx emissions at the exhaust of the order of 50% with respect to the original burner, due to intense internal EGR, for a wide range of thermal power and equivalence ratio, without compromising the burner performances.
2. Experimental set-up Fig. 1 reports a schematic view of the investigated burner, a single-ended self-recuperative radiant tube. As it can be seen, the burner is operated in non-premixed mode, fuelled by natural gas and air in a coaxial configuration characterised by three gas outflows at 120° and the swirled air injected through an annular opening with six lobes. On the central hole a pilot premixed flame is used to ignite the mixture and to prevent burnout. The main features of this burner typology are the presence of a swirl component in the reacting air flow to improve mixing and flame stability and the confinement of the flame inside a tube. Swirl motion is imparted to the air stream by a screw type swirl generator. Combustion is confined inside a long inner tube (L/D 15, D = 60 mm); the hot combustion gases are transported to the closed end of the outer tube, where they are diverted back along the external annulus to the exit. Therefore, the annular enclosure between the inner and outer tubes is used to carry the combustion products out and to heat up the incoming air stream through an integrated heat exchanger. A natural draught hood provides the exhaust and sampling of the burned gases. A laboratory model burner was also made with an inner quartz tube (L 0.9 m) in order to have full optical access to the combustion process for flame visualization and velocity measurements by laser Doppler velocimetry (LDV). The nominal operating conditions of the burner, determined as described in Section 3.1 of the present paper, are listed in Table 1. The air stream has been supplied by the laboratory air compressed line; natural gas was provided by the network distributor in the laboratory area (90% methane, 4% ethane, 1.5% propane, 4% N2 and a small percentage of higher hydrocarbons). Both air and fuel flowrates were metered and stabilised by calibrated thermal mass flowmeters and controllers.
Fig. 1. The burner geometry and reactants burner efflux cross-section (empty = white).
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612 Table 1 Nominal operating conditions of the burner
Flow rate (N l/s) Reynolds number Swirl number S Pilot Burner flow rate (N l/s) Fuel–air momentum ratio MR Input thermal power (kW) Equivalence ratio of the main burner U
Air
Natural gas
4 ffi8000 0.85 0.4 0.005 12.8 0.85
0.36 ffi500 0 0.042
607
A parallel numerical study of the burner (not reported in this paper, see [10,11]) has been started using a commercial code, first validated by the experimental data (particularly, velocity measurements) under both the isothermal and reactive cases. Subsequently, the numerical activity has been used mainly for the fluid-dynamic analysis of the pre-combustion chamber in the improved burner configuration. 3. Experimental results
Swirl number of the air stream is defined as [8] S¼
G# ; G x Rb
where Gh = axial flux of angular momentum; Gx = axial flux of axial momentum; Rb = 30 mm = burner radius. The value 0.85 of the swirl number has been estimated using the above mentioned definition by direct measurement through LDV and subsequent integration of mean axial and tangential radial velocity profiles at the efflux, in isothermal conditions. The swirl number resulted an increasing function of Reynolds for Re < 4000 and then remained constant to the value fixed by the burner geometry. Under reacting conditions, the high gas temperature may reduce the swirl number due to viscosity and density changes which also affect the Re number and turbulence. As previously outlined, owing to the high complexity of the flow inside the combustion chamber (due to the high swirl intensity imparted to the air stream and flame confinement), the experimental characterisation has been carried out through different techniques. Measurement by LDV of the reactive flow field has been performed at increasing distances h from the efflux, in order to analyse the formation of the recirculating regions and their possible influence upon flame development. Velocity fields were measured using a two-component fiber optics laser Doppler velocimeter equipped with an Argon ion laser and a Bragg cell with 40 MHz frequency shift for directional ambiguity resolution. The optical system was operated in the backscatter mode and the signal processors were two Burst spectrum analysers (BSA—Dantec). Micrometric Al2O3 particles were used as scattering centres: at least 10,000 instantaneous velocity data were acquired for statistical analysis, with estimated statistical errors of less than 2% in the mean values and 5% in the rms fluctuations. Mean temperature was measured using a B-type thermocouple with 0.3 mm diameter bead. The amplified signals were sampled at a 500 Hz sampling frequency and the mean value was based on 5000 instantaneous data. A correction was made for the radiation error, following [9] and using the measured velocity values for the evaluation of the local convective heat transfer coefficient. Finally, burned gases have been sampled for the measurement of pollutant emissions (NOx and CO), through a system based, respectively, upon chemiluminescence and infrared analysis.
As previously outlined, the research activity can be divided in two steps: first, the analysis and tuning of the burner in its original configuration to identify the optimal operating conditions and then the improvement (especially under the point of view of pollutant emissions) through the EGR mechanism. 3.1. Analysis and tuning of the burner At first, qualitative flame imaging has been performed on the model burner equipped with the quartz tube to analyse the flame length as a function of the main operating parameters: equivalence ratio /, Reynolds number and momentum ratio of fuel to air, MR. A color digital photocamera and a digital image processing software were used to acquire and analyse the images of the flame in the different conditions. All the tests were performed at constant mass flow rate of natural gas (and, consequently, at constant input thermal power), varying the air mass flow rate. This operating procedure implies that the three parameters /, Re and MR are inter-connected. The results (see Fig. 2) indicated that the operating parameters /, Re and MR strictly influence flame length: by increasing the inlet air flow Reynolds number the flame length reduces, due to the increased shear stress between the reactants. At fixed Re, the flame length increases with the MR, in agreement with some semi-empirical correlation available for similar flames [12,13]. Near the burner exit, a more intense blue colour, typical of quasi-premixed flames, indicates the main mixing zone. Qualitatively, the obtained results identify the possible optimal operating conditions of the burner: under this point of view, the range 0.7 < / < 0.85 seems to represent the best compromise for flame length and, consequently, temperature distribution inside the flame tube. This has been quantitatively deepened through temperature measurements along the burner axis to verify the thermal distribution inside the flame tube, at different equivalence ratio. Temperature was measured in the inner tube of the real burner by a B-type thermocouple inserted from the closed end of the outer tube. The results (Fig. 3) generally revealed a rapid temperature increase within few mm from the burner head, followed by a second zone where the rate of increase was strongly affected by the equivalence ratio. A more uniform region follows the peak value and extends for the major part of the tube length, before a rapid
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Fig. 2. Flame length for different operating conditions, at fixed input thermal power (12.8 kW).
Φ = 0.80
1300
Φ = 0.85
1200
Φ = 0.90
1100
Φ = 0.95
1000
Φ = 1.00
]
T [°C]
Φ = 0.75
1400
900 800 700 600
0
0.2
0.4
0.6
0.8
1
h [m]
Fig. 3. Axial temperature profile inside the inner tube for different equivalence ratio (input thermal power = 12.8 kW).
decrease near the closed end of the outer tube. The temperature distribution was found as a good indicator of thermal efficiency and surface temperature of the radiant tube. Particularly, the condition / 0.85 shows the faster rate of temperature increase downstream the reactants efflux with the highest peak and average temperatures. A reduction of the equivalence ratio (/ = 0.7) gives rise to a slight temperature gradient and a subsequent uniform region (characterised by temperature values of about 1100 °C) followed by a rapid decrease close to the tube end plate, due to the shorter flame. In this case the mixture is quite lean, which slows heat release and provides combustion of relatively low-intensity within the inner tube. This behaviour, as reported in [13,14] for burners used in similar applications, optimizes the temperature uniformity of the burner outer surface, enhancing thermal efficiency of the radiant tube. On the contrary, approaching the stoichiometric condition (see also Fig. 2) induces a worsening of the mixing
efficiency with increase of flame length, reducing both the average temperature inside the flame tube and the thermal efficiency. Pollutant emissions at the exhaust have been measured to have a complete characterisation of the burner and to find a possible correlation with the temperature distribution. NOx and CO emissions were measured by continuous sampling of the exhaust gases, and studied as a function of equivalence ratio, input thermal power and Reynolds number. The results are shown in Figs. 4 and 5. Input thermal power presents only a slight influence upon pollutant emissions (more evident as for NOx emissions, which are strictly connected to local temperature in the combustion chamber and, consequently, to local heat release). As for the equivalence ratio, it can be observed a NOx decrease and a rapid increase of CO emissions close to the stoichiometric value (/ > 0.85). This can be partially explained by the previous results (increasing flame length and lower temperature distribution) and induces to suspect the generation of a fuel
10 kW 12.8 kW 16 kW 18 kW
350 NOx [mg/Nm 3% O2]
Φ = 0.70
Temperature - Φ
300 250 200 150 100 50 0 0.4
0.5
0.6
0.7 Φ
0.8
0.9
1
Fig. 4. Nitric oxides emissions as a function of equivalence ratio and input thermal power.
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612
the burner head. This influence disappears at progressive increasing distance h from the burner efflux (typically h > 6 mm). Fig. 6 reports the mean axial velocity and
10 kW 12.8 kW 16 kW
20000
18 kW
15000
h = 46 mm
10000
14
5000
10
0 0.4
0.5
0.6
0.7 Φ
0.8
0.9
1
Fig. 5. Carbon monoxide emissions as a function of equivalence ratio and input thermal power.
U, RMS [m/s]
CO [mg/Nm 3% O2]
30000 25000
609
6 2 -36
-24
-12
-2 0
12
24
36
12
24
36
12
24
36
-6 -10 -14
r [mm] h = 11.5 mm 14 10
U, RMS [m/s]
6 2 -36
-24
-12
-2 0 -6 -10 -14
r [mm] h = 5.75 mm 14 10
U, RMS [m/s]
6 2 -36
-24
-12
-2 0 -6 -10 -14
r [mm] h = 3.5 mm 14 10
U, RMS [m/s]
rich flame characterised by incomplete mixing and extinction at the closed end of the outer tube. A reduction of the equivalence ratio (/ < 0.85) gives rise to an initial increase in NOx emissions and a subsequent decrease towards / = 0.5, the minimum value assuring flame stability. The maximum was found around the value / = 0.7, while thermal efficiency and surface temperature uniformity resulted well optimized, as previously outlined. To understand these results it must be considered that the reduction of /, at constant input thermal power, was achieved by increasing the Re of the air flow. The consequence was a better mixing and a shorter flame with higher temperatures (see Fig. 3 for / = 0.8). Further dilution of the mixture significantly reduces the average flame temperature, but probably induces hot spots and larger disuniformities with local extinctions not revealed by the thermocouple measurements. The conclusion was that to reduce pollutant emissions while ensuring the thermal requirements of the heating process and surface temperature uniformity it would be necessary to modify the combustion process. The most effective solution to reduce peak temperatures and oxygen concentration in the reaction zone appeared to dilute the combustion process by the use of exhaust gases recirculation. An attempt was made by opening a gap between the burner head and the flame tube to help exhaust gases entrainment, but the results indicated a poor efficiency of this solution and suggested the need of a better understanding of the flow pattern in the near field of the burner exit. Therefore, the operating condition at / = 0.85 (reported in Table 1) has been further on analysed by flow field characterisation under reacting conditions in the region near the burner exit. Velocity measurements were performed to provide a more detailed information on the internal fluid dynamics in the main combustion zone and to verify the possibility of promoting exhaust gases recirculation. Application of the LDV technique inside the inner tube required the use of a quartz tube without the outer envelope, thus making impossible to simulate the entrainment process. Near the burner exit, the axial velocity profile depends on the angular position, due to the complicated geometry of
6 2 -36
-24
-12
-2 0 -6
12
24 U (m/s) RMS (m/s)
-10 -14
r [mm]
Fig. 6. Mean axial velocity and turbulence intensity radial profile.
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turbulence intensity radial profiles for different values of h. Fig. 7 refers to the tangential mean velocity component and turbulence intensity. It is clearly evidenced the presence of a
h = 46 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-2 0
-12
12
24
36
-6 -10 -14
r [mm] h = 11.5 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-12
-2 0
12
24
central toroidal recirculation zone (CRTZ) due to the air swirl, which develops around the inner fuel jet and extends up to about 30 mm. A smaller corner recirculation zone close to the wall is caused by the sudden expansion at the burner exit and the flame confinement which has a strong effect also on the inner flow structure. Tangential velocity profiles put into evidence the formation of a central rigid vortex core due to swirl motion and slightly affected by flame development. Reactions should be primarily confined to the inner shear layer between the air stream and the recirculation zone, where thermal expansion produces higher axial velocities compared to the non-reacting flow. This seems in agreement with the observed blue zone near the burner exit and the measured steep temperature gradient, favoured by the recirculation of hot gases in the central zone. The peak temperature observed in Fig. 3 should correspond to the end of the CRTZ. The rms velocity was found higher in the inner shear layer with a broad region of high axial and tangential rms velocities also in the inner vortex core, indicating the unsteady nature of the heat release and large flow fluctuations. The observed flow pattern explains the poor efficiency of the flue gas recirculation and suggested a new more efficient solution.
36
3.2. Improvement of the burner by EGR
-6 -10
The flow field structure generated in the original configuration just downstream the burner head is not favourable for burned gases entrainment by the outflowing reactant mixture inside the flame tube. Therefore, to improve the pollutant emission performance it was studied the possibility of enhancing the internal gas recirculation in order to lower peak flame temperature and reduce the oxygen content in the reaction zone. After many attempts (performed both experimentally and by CFD simulation), the best solution was found to be the insertion of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to improve exhaust gas entrainment into the flame tube (Fig. 8). The effect was to promote exhaust gases recirculation by the momentum of the hot jet issuing from the nozzle of the pre-combustion chamber where the combustion starts and pre-heat the gas. The exhaust flue gases flowing in the annulus were allowed to be entrained in the hot jet through a variable gap between the nozzle and the entrance of the inner tube. This solution proved to be the most effective
-14
r [mm]
h = 5.75 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-12
-2 0
12
24
36
-6 -10 -14 r [mm]
h = 3.5 mm 14
W, RMS [m/s]
10 6 2
Burner exit cross-section
-36
-24
-12
-2 0 -6
12
24
36
W (m/s)
Flame tube
Radiant tube
RMS (m/s)
-10 -14
r [mm]
Fig. 7. Mean tangential velocity and turbulence intensity radial profile.
Pre-combustion chamber
Fig. 8. Burner geometry with pre-combustion chamber terminated by a converging nozzle.
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612
4. Conclusions
300 NOx [mg/Nm3 3% O2]
611
250 200 150 100 50 0 0.4
0.6
0.8
1
E.R. with EGR
orig. Burner
Fig. 9. NOx emissions as a function of equivalence ratio for the two cases: with and without internal EGR (input thermal power 12.8 kW).
in reducing NOx emissions without penalizing the thermal efficiency and the surface temperature uniformity which is very important for this type of burner. Results obtained by CFD simulation (not reported here, see [11]) put into evidence that the amount of recirculating combustion products can be estimated around 30% of the air mass flow at the inlet and increases with the inlet Reynolds number. The NOx reduction with respect to the original burner configuration (see Fig. 9) is of the order of 50% for a wide range of thermal power (12–18 kW) and equivalence ratio (0.5–0.8). For 0.5 < / < 0.8 NOx emissions are almost insensitive to equivalence ratio, being the EGR process rather than the nominal feeding equivalence ratio the governing parameter upon local mixture fraction and temperature values and, consequently, nitric oxide formation. Moreover, CO emissions (not represented here) are very low (<50 mg/Nm3 3% O2) and almost identical to those measured in the original burner release. Approaching stoichiometric condition (/ > 0.8), similarly to the original burner, CO levels increase rapidly with a parallel NOx reduction, probably owing to a less effective mixing efficiency, not fairly opposed by the EGR mechanism. The insertion of the mixing chamber allows to break the combustion process up into multiple stages. In the initial portion of the combustion process the reactants are mixed and partially burned in the chamber and a fraction of the heat of reaction is converted into momentum of the hot jet issuing from the nozzle. The resulting combustion products are then delivered to a second combustion zone where cooled flue gases are entrained and the reactions are completed; however the flame temperature will be lower than that of a single combustion stage without flue gases recirculation. High recirculation ratios within the inner tube result in improvement of both gas and tube wall temperature uniformity, with a subsequent reduction of thermal NOx emissions. Nevertheless, as the primary combustion zone is located upstream of the point of EGR introduction, substantial thermal NOx may already have been generated, with the present burner design.
The research activity reported in this paper was mainly finalised to the analysis and optimization of a prototype radiant tube burner as a function of operating conditions and to the assessment of possible improvements in terms of pollutant emissions by means of an internal exhaust gases recirculation mechanism. The first part examined flame behaviour as a function of burner operating conditions, such as Reynolds number, input thermal power, equivalence ratio and fuel to air momentum ratio. All these parameters have a significant effect on flame length, gas temperature axial distribution, radiant tube temperature uniformity and pollutant emissions. A laboratory model with a quartz inner tube has been also utilised to investigate the gas velocity distribution in the primary combustion zone by laser Doppler velocimetry. An important outcome of the research activity was the assessment that substantial NOx reduction could be achieved by means of intense exhaust gases recirculation and a methodology was implemented to directly recirculate the flue gases internally into the combustion zone through a variable gap between the burner head and the flame tube. The recirculation mechanism developed for the present application relies on a mixing chamber inserted downstream of the burner efflux and terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gases entrainment into the flame tube by the momentum of the hot jet issuing from the nozzle of the pre-combustion chamber. The solution with internal EGR (exhaust gases recirculation) proved very effective in reducing NOx emissions (up to 50% with respect to the original burner design) without compromising thermal efficiency, CO emissions and surface temperature uniformity of the radiant tube, for a wide range of thermal power and equivalence ratio. Acknowledgements The authors would like to acknowledge the support of CITT Impianti Srl and are grateful for the technical assistance provided by Dr. Paolo Gennaro during the conduction of the whole activity. References [1] Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air, Official Journal L 163, 29/06/99, pp. 0041–0060. [2] T. Kolb, P. Jansohn, W. Leuckel, Reduction of NOx emission in turbulent combustion by fuel-staging: effects of mixing and stoichiometry in the reduction zone, 22nd Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, 1988, p. 1193. [3] H. Bosch, F. Janssen, Catalytic reduction of nitrogen oxides: a review on the fundamentals and technology, Catalysis Today 2 (4) (1987) 369–401. [4] J.A. Miller, C.T. Bowman, Mechanism and modeling of nitrogen chemistry in combustion, Prog. Energ. Combust. Sci. 15 (1989) 287– 338.
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[5] K.N.C. Bray, The interaction between turbulence and combustion, 17th Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, 1979, p. 223. [6] D.E. Quinn, J. Newby, Radiant Tube Burners, in: C.E. Baukal Jr. (Ed.), Industrial Burners Handbook, CRC Press, 2003 (Chapter 14). [7] G.H. Abd-Alla, Using exhaust gas recirculation in internal combustion engines: a review, Energ. Convers. Manage. 43 (8) (2002) 1027–1042. [8] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, Abacus Press, Tunbridge Wells, 1984. [9] T.V. Morgan, Thermal Behaviour of Electrical Conductors, John Wiley and Sons, 1991. [10] G. Scribano, G. Solero, A. Coghe, Numerical simulation of an industrial radiant burner, Joint Meeting of the Greek and Ital-
[11]
[12]
[13]
[14]
ian Sections of The Combustion Institute, Corfu` 17–19, June 2004. G. Scribano, Study of a radiant burner: performance optimization and reduction of pollutant emissions, Ph.D. Thesis, Energetics Dept. Politecnico di MIlano, 2004. D. Feikema, R.H. Chen, J.F. Driscoll, Blowout of non-premixed flames: maximum coaxial air velocities achievable, with and without swirl, Combust. Flame 86 (1991) 347–358. H. Ramamurthy, S. Ramadhyani, R. Viskanta, Development of fuel burn-up and wall heat transfer correlations for flows in radiant tubes, Numer. Heat Transfer 31 (1997) 563–584. R.F. Harder, R. Viskanta, S. Ramadhyani, Gas-fired radiant tubes: a review of literature, Topical Report GRI-87/0343, Gas Research Institute, Chicago Ill 1987.
Pollutant emissions reduction and performance optimization of an industrial radiant tube burner Gianfranco Scribano, Giulio Solero *, Aldo Coghe Dipartimento di Energetica, Politecnico di Milano, via La Masa, 34, 20156 Milano, Italy Received 25 May 2005; received in revised form 15 November 2005; accepted 13 December 2005
Abstract This paper presents the results of an experimental investigation performed upon a single-ended self-recuperative radiant tube burner fuelled by natural gas in the non-premixed mode, which is used in the steel industry for surface treatment. The main goal of the research activity was a systematic investigation of the burner aimed to find the best operating conditions in terms of optimum equivalence ratio, thermal power and lower pollutant emissions. The analysis, which focused on the main parameters influencing the thermal efficiency and pollutant emissions at the exhaust (NOx and CO), has been carried out for different operating conditions of the burner: input thermal powers from 12.8 up to 18 kW and equivalence ratio from 0.5 (very lean flame) to 0.95 (quasi-stoichiometric condition). To significantly reduce pollutant emissions ensuring at the same time the thermal requirements of the heating process, it has been developed a new burner configuration, in which a fraction of the exhaust gases recirculates in the main combustion region through a variable gap between the burner efflux and the inner flame tube. This internal recirculation mechanism (exhaust gases recirculation, EGR) has been favoured through the addition of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gas entrainment into the flame tube. The most important result of this solution was a decrease of NOx emissions at the exhaust of the order of 50% with respect to the original burner geometry, for a wide range of thermal power and equivalence ratio. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Natural gas burners; Radiant tube burners; Pollutant emissions
1. Introduction Owing to recent strict legislation [1], environmental impact of combustion processes is nowadays one of the most important problems involving both scientific community and burner manufacturers. In the case of natural gas combustion, nitric oxides (NOx) reduction is the most critical aim to achieve, often requesting for industrial applications heavy and high-cost plant revamping (for instance, adoption of low-NOx burners or burned gases de-NOx treatment [2,3]). Moreover, although NOx formation mechanisms are today quite understood [4], many unsolved questions are connected to the interaction between fluid dynamic and chemistry inside the burner device [5] and this *
Corresponding author. Tel.: +39 2 23998562; fax: +39 2 23998566. E-mail address: [email protected] (G. Solero).
0894-1777/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2005.12.006
makes the CFD codes not yet fully predictive. Radiant tube burners are characterised by the presence of a flame confined inside a hot space; therefore NOx formation is dominated by the thermal mechanism and NOx reduction techniques are based on minimizing flame temperature, fulfilling at the same time the thermal requirements of the heating process. This requisite can be obtained by diffusion mixing of the fuel and air to slow heat release and providing combustion of a relatively low-intensity to distribute the heat along the length of the tube and reduce the formation of potential local hot spots. A careful design of this type of burners to meet emission regulations requires a deeper knowledge of the fluid-dynamic, combustion and heat transfer processes. Although the first single-ended radiant tube was developed in the 1940s, there is scarce scientific information on this peculiar type of burner and the combustion process involved [6]. Therefore, a detailed research
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activity (both experimental and numerical) is anyway necessary to deepen our knowledge in this field and to obtain a reasonable practical solution for pollutant emissions reduction. This work presents the results of an extensive investigation performed upon a radiant tube natural gas nonpremixed burner, used in the steel industry for surface treatment. The burner behaviour has been investigated in a configuration very similar to the real working one, analysing through different experimental techniques (flame imaging, laser Doppler velocimetry, temperature measurements by thermocouples, pollutant emissions analysis at the exhaust) the burner sensitivity to input thermal power and equivalence ratio inside a wide range of operative conditions. The motivation of this analysis was primarily to provide input data for CFD simulations and also to experiment new burner designs and combustion strategies aimed to reduce NOx emissions which are usually very high in this type of burners. Consequently, the original burner has been improved introducing a self-induced exhaust gases recirculation mechanism. Exhaust gases recirculation (EGR) is in fact a well known procedure [7] used mainly in automotive applications for NOx abatement (especially thermal NOx), lowering the peak temperature and oxygen concentration in the flame region. In the new burner configuration, a fraction of the exhaust gases can recirculate in the main combustion region; the recirculation mechanism developed in the present application is promoted by the momentum of the hot jet issuing from a converging nozzle located downstream of the burner efflux. This improvement is based on the addition of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gases entrainment into the flame tube. The most important result of this solution is a decrease of NOx emissions at the exhaust of the order of 50% with respect to the original burner, due to intense internal EGR, for a wide range of thermal power and equivalence ratio, without compromising the burner performances.
2. Experimental set-up Fig. 1 reports a schematic view of the investigated burner, a single-ended self-recuperative radiant tube. As it can be seen, the burner is operated in non-premixed mode, fuelled by natural gas and air in a coaxial configuration characterised by three gas outflows at 120° and the swirled air injected through an annular opening with six lobes. On the central hole a pilot premixed flame is used to ignite the mixture and to prevent burnout. The main features of this burner typology are the presence of a swirl component in the reacting air flow to improve mixing and flame stability and the confinement of the flame inside a tube. Swirl motion is imparted to the air stream by a screw type swirl generator. Combustion is confined inside a long inner tube (L/D 15, D = 60 mm); the hot combustion gases are transported to the closed end of the outer tube, where they are diverted back along the external annulus to the exit. Therefore, the annular enclosure between the inner and outer tubes is used to carry the combustion products out and to heat up the incoming air stream through an integrated heat exchanger. A natural draught hood provides the exhaust and sampling of the burned gases. A laboratory model burner was also made with an inner quartz tube (L 0.9 m) in order to have full optical access to the combustion process for flame visualization and velocity measurements by laser Doppler velocimetry (LDV). The nominal operating conditions of the burner, determined as described in Section 3.1 of the present paper, are listed in Table 1. The air stream has been supplied by the laboratory air compressed line; natural gas was provided by the network distributor in the laboratory area (90% methane, 4% ethane, 1.5% propane, 4% N2 and a small percentage of higher hydrocarbons). Both air and fuel flowrates were metered and stabilised by calibrated thermal mass flowmeters and controllers.
Fig. 1. The burner geometry and reactants burner efflux cross-section (empty = white).
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612 Table 1 Nominal operating conditions of the burner
Flow rate (N l/s) Reynolds number Swirl number S Pilot Burner flow rate (N l/s) Fuel–air momentum ratio MR Input thermal power (kW) Equivalence ratio of the main burner U
Air
Natural gas
4 ffi8000 0.85 0.4 0.005 12.8 0.85
0.36 ffi500 0 0.042
607
A parallel numerical study of the burner (not reported in this paper, see [10,11]) has been started using a commercial code, first validated by the experimental data (particularly, velocity measurements) under both the isothermal and reactive cases. Subsequently, the numerical activity has been used mainly for the fluid-dynamic analysis of the pre-combustion chamber in the improved burner configuration. 3. Experimental results
Swirl number of the air stream is defined as [8] S¼
G# ; G x Rb
where Gh = axial flux of angular momentum; Gx = axial flux of axial momentum; Rb = 30 mm = burner radius. The value 0.85 of the swirl number has been estimated using the above mentioned definition by direct measurement through LDV and subsequent integration of mean axial and tangential radial velocity profiles at the efflux, in isothermal conditions. The swirl number resulted an increasing function of Reynolds for Re < 4000 and then remained constant to the value fixed by the burner geometry. Under reacting conditions, the high gas temperature may reduce the swirl number due to viscosity and density changes which also affect the Re number and turbulence. As previously outlined, owing to the high complexity of the flow inside the combustion chamber (due to the high swirl intensity imparted to the air stream and flame confinement), the experimental characterisation has been carried out through different techniques. Measurement by LDV of the reactive flow field has been performed at increasing distances h from the efflux, in order to analyse the formation of the recirculating regions and their possible influence upon flame development. Velocity fields were measured using a two-component fiber optics laser Doppler velocimeter equipped with an Argon ion laser and a Bragg cell with 40 MHz frequency shift for directional ambiguity resolution. The optical system was operated in the backscatter mode and the signal processors were two Burst spectrum analysers (BSA—Dantec). Micrometric Al2O3 particles were used as scattering centres: at least 10,000 instantaneous velocity data were acquired for statistical analysis, with estimated statistical errors of less than 2% in the mean values and 5% in the rms fluctuations. Mean temperature was measured using a B-type thermocouple with 0.3 mm diameter bead. The amplified signals were sampled at a 500 Hz sampling frequency and the mean value was based on 5000 instantaneous data. A correction was made for the radiation error, following [9] and using the measured velocity values for the evaluation of the local convective heat transfer coefficient. Finally, burned gases have been sampled for the measurement of pollutant emissions (NOx and CO), through a system based, respectively, upon chemiluminescence and infrared analysis.
As previously outlined, the research activity can be divided in two steps: first, the analysis and tuning of the burner in its original configuration to identify the optimal operating conditions and then the improvement (especially under the point of view of pollutant emissions) through the EGR mechanism. 3.1. Analysis and tuning of the burner At first, qualitative flame imaging has been performed on the model burner equipped with the quartz tube to analyse the flame length as a function of the main operating parameters: equivalence ratio /, Reynolds number and momentum ratio of fuel to air, MR. A color digital photocamera and a digital image processing software were used to acquire and analyse the images of the flame in the different conditions. All the tests were performed at constant mass flow rate of natural gas (and, consequently, at constant input thermal power), varying the air mass flow rate. This operating procedure implies that the three parameters /, Re and MR are inter-connected. The results (see Fig. 2) indicated that the operating parameters /, Re and MR strictly influence flame length: by increasing the inlet air flow Reynolds number the flame length reduces, due to the increased shear stress between the reactants. At fixed Re, the flame length increases with the MR, in agreement with some semi-empirical correlation available for similar flames [12,13]. Near the burner exit, a more intense blue colour, typical of quasi-premixed flames, indicates the main mixing zone. Qualitatively, the obtained results identify the possible optimal operating conditions of the burner: under this point of view, the range 0.7 < / < 0.85 seems to represent the best compromise for flame length and, consequently, temperature distribution inside the flame tube. This has been quantitatively deepened through temperature measurements along the burner axis to verify the thermal distribution inside the flame tube, at different equivalence ratio. Temperature was measured in the inner tube of the real burner by a B-type thermocouple inserted from the closed end of the outer tube. The results (Fig. 3) generally revealed a rapid temperature increase within few mm from the burner head, followed by a second zone where the rate of increase was strongly affected by the equivalence ratio. A more uniform region follows the peak value and extends for the major part of the tube length, before a rapid
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Fig. 2. Flame length for different operating conditions, at fixed input thermal power (12.8 kW).
Φ = 0.80
1300
Φ = 0.85
1200
Φ = 0.90
1100
Φ = 0.95
1000
Φ = 1.00
]
T [°C]
Φ = 0.75
1400
900 800 700 600
0
0.2
0.4
0.6
0.8
1
h [m]
Fig. 3. Axial temperature profile inside the inner tube for different equivalence ratio (input thermal power = 12.8 kW).
decrease near the closed end of the outer tube. The temperature distribution was found as a good indicator of thermal efficiency and surface temperature of the radiant tube. Particularly, the condition / 0.85 shows the faster rate of temperature increase downstream the reactants efflux with the highest peak and average temperatures. A reduction of the equivalence ratio (/ = 0.7) gives rise to a slight temperature gradient and a subsequent uniform region (characterised by temperature values of about 1100 °C) followed by a rapid decrease close to the tube end plate, due to the shorter flame. In this case the mixture is quite lean, which slows heat release and provides combustion of relatively low-intensity within the inner tube. This behaviour, as reported in [13,14] for burners used in similar applications, optimizes the temperature uniformity of the burner outer surface, enhancing thermal efficiency of the radiant tube. On the contrary, approaching the stoichiometric condition (see also Fig. 2) induces a worsening of the mixing
efficiency with increase of flame length, reducing both the average temperature inside the flame tube and the thermal efficiency. Pollutant emissions at the exhaust have been measured to have a complete characterisation of the burner and to find a possible correlation with the temperature distribution. NOx and CO emissions were measured by continuous sampling of the exhaust gases, and studied as a function of equivalence ratio, input thermal power and Reynolds number. The results are shown in Figs. 4 and 5. Input thermal power presents only a slight influence upon pollutant emissions (more evident as for NOx emissions, which are strictly connected to local temperature in the combustion chamber and, consequently, to local heat release). As for the equivalence ratio, it can be observed a NOx decrease and a rapid increase of CO emissions close to the stoichiometric value (/ > 0.85). This can be partially explained by the previous results (increasing flame length and lower temperature distribution) and induces to suspect the generation of a fuel
10 kW 12.8 kW 16 kW 18 kW
350 NOx [mg/Nm 3% O2]
Φ = 0.70
Temperature - Φ
300 250 200 150 100 50 0 0.4
0.5
0.6
0.7 Φ
0.8
0.9
1
Fig. 4. Nitric oxides emissions as a function of equivalence ratio and input thermal power.
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612
the burner head. This influence disappears at progressive increasing distance h from the burner efflux (typically h > 6 mm). Fig. 6 reports the mean axial velocity and
10 kW 12.8 kW 16 kW
20000
18 kW
15000
h = 46 mm
10000
14
5000
10
0 0.4
0.5
0.6
0.7 Φ
0.8
0.9
1
Fig. 5. Carbon monoxide emissions as a function of equivalence ratio and input thermal power.
U, RMS [m/s]
CO [mg/Nm 3% O2]
30000 25000
609
6 2 -36
-24
-12
-2 0
12
24
36
12
24
36
12
24
36
-6 -10 -14
r [mm] h = 11.5 mm 14 10
U, RMS [m/s]
6 2 -36
-24
-12
-2 0 -6 -10 -14
r [mm] h = 5.75 mm 14 10
U, RMS [m/s]
6 2 -36
-24
-12
-2 0 -6 -10 -14
r [mm] h = 3.5 mm 14 10
U, RMS [m/s]
rich flame characterised by incomplete mixing and extinction at the closed end of the outer tube. A reduction of the equivalence ratio (/ < 0.85) gives rise to an initial increase in NOx emissions and a subsequent decrease towards / = 0.5, the minimum value assuring flame stability. The maximum was found around the value / = 0.7, while thermal efficiency and surface temperature uniformity resulted well optimized, as previously outlined. To understand these results it must be considered that the reduction of /, at constant input thermal power, was achieved by increasing the Re of the air flow. The consequence was a better mixing and a shorter flame with higher temperatures (see Fig. 3 for / = 0.8). Further dilution of the mixture significantly reduces the average flame temperature, but probably induces hot spots and larger disuniformities with local extinctions not revealed by the thermocouple measurements. The conclusion was that to reduce pollutant emissions while ensuring the thermal requirements of the heating process and surface temperature uniformity it would be necessary to modify the combustion process. The most effective solution to reduce peak temperatures and oxygen concentration in the reaction zone appeared to dilute the combustion process by the use of exhaust gases recirculation. An attempt was made by opening a gap between the burner head and the flame tube to help exhaust gases entrainment, but the results indicated a poor efficiency of this solution and suggested the need of a better understanding of the flow pattern in the near field of the burner exit. Therefore, the operating condition at / = 0.85 (reported in Table 1) has been further on analysed by flow field characterisation under reacting conditions in the region near the burner exit. Velocity measurements were performed to provide a more detailed information on the internal fluid dynamics in the main combustion zone and to verify the possibility of promoting exhaust gases recirculation. Application of the LDV technique inside the inner tube required the use of a quartz tube without the outer envelope, thus making impossible to simulate the entrainment process. Near the burner exit, the axial velocity profile depends on the angular position, due to the complicated geometry of
6 2 -36
-24
-12
-2 0 -6
12
24 U (m/s) RMS (m/s)
-10 -14
r [mm]
Fig. 6. Mean axial velocity and turbulence intensity radial profile.
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turbulence intensity radial profiles for different values of h. Fig. 7 refers to the tangential mean velocity component and turbulence intensity. It is clearly evidenced the presence of a
h = 46 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-2 0
-12
12
24
36
-6 -10 -14
r [mm] h = 11.5 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-12
-2 0
12
24
central toroidal recirculation zone (CRTZ) due to the air swirl, which develops around the inner fuel jet and extends up to about 30 mm. A smaller corner recirculation zone close to the wall is caused by the sudden expansion at the burner exit and the flame confinement which has a strong effect also on the inner flow structure. Tangential velocity profiles put into evidence the formation of a central rigid vortex core due to swirl motion and slightly affected by flame development. Reactions should be primarily confined to the inner shear layer between the air stream and the recirculation zone, where thermal expansion produces higher axial velocities compared to the non-reacting flow. This seems in agreement with the observed blue zone near the burner exit and the measured steep temperature gradient, favoured by the recirculation of hot gases in the central zone. The peak temperature observed in Fig. 3 should correspond to the end of the CRTZ. The rms velocity was found higher in the inner shear layer with a broad region of high axial and tangential rms velocities also in the inner vortex core, indicating the unsteady nature of the heat release and large flow fluctuations. The observed flow pattern explains the poor efficiency of the flue gas recirculation and suggested a new more efficient solution.
36
3.2. Improvement of the burner by EGR
-6 -10
The flow field structure generated in the original configuration just downstream the burner head is not favourable for burned gases entrainment by the outflowing reactant mixture inside the flame tube. Therefore, to improve the pollutant emission performance it was studied the possibility of enhancing the internal gas recirculation in order to lower peak flame temperature and reduce the oxygen content in the reaction zone. After many attempts (performed both experimentally and by CFD simulation), the best solution was found to be the insertion of a pre-combustion chamber terminated by a converging nozzle acting as a mixing/ejector to improve exhaust gas entrainment into the flame tube (Fig. 8). The effect was to promote exhaust gases recirculation by the momentum of the hot jet issuing from the nozzle of the pre-combustion chamber where the combustion starts and pre-heat the gas. The exhaust flue gases flowing in the annulus were allowed to be entrained in the hot jet through a variable gap between the nozzle and the entrance of the inner tube. This solution proved to be the most effective
-14
r [mm]
h = 5.75 mm 14
W, RMS [m/s]
10 6 2 -36
-24
-12
-2 0
12
24
36
-6 -10 -14 r [mm]
h = 3.5 mm 14
W, RMS [m/s]
10 6 2
Burner exit cross-section
-36
-24
-12
-2 0 -6
12
24
36
W (m/s)
Flame tube
Radiant tube
RMS (m/s)
-10 -14
r [mm]
Fig. 7. Mean tangential velocity and turbulence intensity radial profile.
Pre-combustion chamber
Fig. 8. Burner geometry with pre-combustion chamber terminated by a converging nozzle.
G. Scribano et al. / Experimental Thermal and Fluid Science 30 (2006) 605–612
4. Conclusions
300 NOx [mg/Nm3 3% O2]
611
250 200 150 100 50 0 0.4
0.6
0.8
1
E.R. with EGR
orig. Burner
Fig. 9. NOx emissions as a function of equivalence ratio for the two cases: with and without internal EGR (input thermal power 12.8 kW).
in reducing NOx emissions without penalizing the thermal efficiency and the surface temperature uniformity which is very important for this type of burner. Results obtained by CFD simulation (not reported here, see [11]) put into evidence that the amount of recirculating combustion products can be estimated around 30% of the air mass flow at the inlet and increases with the inlet Reynolds number. The NOx reduction with respect to the original burner configuration (see Fig. 9) is of the order of 50% for a wide range of thermal power (12–18 kW) and equivalence ratio (0.5–0.8). For 0.5 < / < 0.8 NOx emissions are almost insensitive to equivalence ratio, being the EGR process rather than the nominal feeding equivalence ratio the governing parameter upon local mixture fraction and temperature values and, consequently, nitric oxide formation. Moreover, CO emissions (not represented here) are very low (<50 mg/Nm3 3% O2) and almost identical to those measured in the original burner release. Approaching stoichiometric condition (/ > 0.8), similarly to the original burner, CO levels increase rapidly with a parallel NOx reduction, probably owing to a less effective mixing efficiency, not fairly opposed by the EGR mechanism. The insertion of the mixing chamber allows to break the combustion process up into multiple stages. In the initial portion of the combustion process the reactants are mixed and partially burned in the chamber and a fraction of the heat of reaction is converted into momentum of the hot jet issuing from the nozzle. The resulting combustion products are then delivered to a second combustion zone where cooled flue gases are entrained and the reactions are completed; however the flame temperature will be lower than that of a single combustion stage without flue gases recirculation. High recirculation ratios within the inner tube result in improvement of both gas and tube wall temperature uniformity, with a subsequent reduction of thermal NOx emissions. Nevertheless, as the primary combustion zone is located upstream of the point of EGR introduction, substantial thermal NOx may already have been generated, with the present burner design.
The research activity reported in this paper was mainly finalised to the analysis and optimization of a prototype radiant tube burner as a function of operating conditions and to the assessment of possible improvements in terms of pollutant emissions by means of an internal exhaust gases recirculation mechanism. The first part examined flame behaviour as a function of burner operating conditions, such as Reynolds number, input thermal power, equivalence ratio and fuel to air momentum ratio. All these parameters have a significant effect on flame length, gas temperature axial distribution, radiant tube temperature uniformity and pollutant emissions. A laboratory model with a quartz inner tube has been also utilised to investigate the gas velocity distribution in the primary combustion zone by laser Doppler velocimetry. An important outcome of the research activity was the assessment that substantial NOx reduction could be achieved by means of intense exhaust gases recirculation and a methodology was implemented to directly recirculate the flue gases internally into the combustion zone through a variable gap between the burner head and the flame tube. The recirculation mechanism developed for the present application relies on a mixing chamber inserted downstream of the burner efflux and terminated by a converging nozzle acting as a mixing/ejector to promote exhaust gases entrainment into the flame tube by the momentum of the hot jet issuing from the nozzle of the pre-combustion chamber. The solution with internal EGR (exhaust gases recirculation) proved very effective in reducing NOx emissions (up to 50% with respect to the original burner design) without compromising thermal efficiency, CO emissions and surface temperature uniformity of the radiant tube, for a wide range of thermal power and equivalence ratio. Acknowledgements The authors would like to acknowledge the support of CITT Impianti Srl and are grateful for the technical assistance provided by Dr. Paolo Gennaro during the conduction of the whole activity. References [1] Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air, Official Journal L 163, 29/06/99, pp. 0041–0060. [2] T. Kolb, P. Jansohn, W. Leuckel, Reduction of NOx emission in turbulent combustion by fuel-staging: effects of mixing and stoichiometry in the reduction zone, 22nd Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, 1988, p. 1193. [3] H. Bosch, F. Janssen, Catalytic reduction of nitrogen oxides: a review on the fundamentals and technology, Catalysis Today 2 (4) (1987) 369–401. [4] J.A. Miller, C.T. Bowman, Mechanism and modeling of nitrogen chemistry in combustion, Prog. Energ. Combust. Sci. 15 (1989) 287– 338.
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[5] K.N.C. Bray, The interaction between turbulence and combustion, 17th Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, 1979, p. 223. [6] D.E. Quinn, J. Newby, Radiant Tube Burners, in: C.E. Baukal Jr. (Ed.), Industrial Burners Handbook, CRC Press, 2003 (Chapter 14). [7] G.H. Abd-Alla, Using exhaust gas recirculation in internal combustion engines: a review, Energ. Convers. Manage. 43 (8) (2002) 1027–1042. [8] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, Abacus Press, Tunbridge Wells, 1984. [9] T.V. Morgan, Thermal Behaviour of Electrical Conductors, John Wiley and Sons, 1991. [10] G. Scribano, G. Solero, A. Coghe, Numerical simulation of an industrial radiant burner, Joint Meeting of the Greek and Ital-
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