On the Effect of Air Temperature on Mild Flameless Combustion Regime of High Temperature Furnace C. Rottier1*, C. Lacour1, G. Godard1, B. Taupin1, L. Porcheron2, R. Hauguel2, S.Carpentier2, A.M. Boukhalfa1, D. Honoré1 1
CORIA – CNRS, Université et INSA de Rouen, Saint Etienne du Rouvray, France 2 GDF SUEZ, Research and Innovation Division, Saint Denis La Plaine, France
Abstract This paper presents an experimental study of the effect of air preheating temperature on the main features of mild flameless combustion obtained in a laboratory-scale furnace. Mild flameless combustion regime is conserved whatever the air temperature varying from 838 K to ambient value. No visible flame is observed. Main aerodynamic features of the flow at the exit of the burner are similar with and without air preheating. Large momentum of methane and air turbulent jets favours the entrainment of recirculating flue gas and then the progressive dilution of reactant by flue gas. Main difference comes from the structures of the reaction zone which are more and more lifted and NOx emissions which decrease progressively with air temperature, down to a very low value (4 ppm @3%O2). Introduction Mild flameless combustion is an advanced combustion technique designed to reduce NOx emissions and increase energy efficiency of high temperature furnaces. The use of a recuperative or regenerative system in a burner allows the optimisation of the energy efficiency thanks to the heat transfer from hot exhaust gases to inlet combusting air [1, 2]. The main drawback of regenerative burner is the very high temperature that could be reached in the flame because of air preheating. This favours the NO formation from thermal route and thus very large NOx emissions, that may be over the regulation limits. Air and fuel staging is one of the most common primary strategy for reducing NOx emissions. It is used in several combustion applications notably in high temperature furnace . Mild flameless combustion has been developed when applying fuel or air staging to regenerative burners to the maximum level of a total staging, i.e. to a configuration where air and fuel injections are distant. Because of the large jet velocities of fuel and air associated to this staging, strong recirculation of flue gas occurs in the combustion chamber. Mixing of recirculating combustion products with air and fuel induces this specific diluted combustion regime named HiTAC combustion, flameless combustion or mild combustion [2, 4, 5]. Its main characteristics are the global homogeneity of heat release, the non visibility of the reaction zones, and the very low NOx emissions which can be divided by ten compared to a conventional regenerative burner . Although this technique has been applied successfully on industrial units, the impact of several key parameters on combustion characteristics are not fully understood yet. This has motivated several experimental studies at semi-industrial scale [7 – 12], as well as laboratory scale [2, 13 – 19].
Corresponding author: [email protected]
Proceedings of the European Combustion Meeting
Specific objectives Our approach is to reproduce mild flameless combustion regime with a burner configuration close to the real commercial industrial ones in a laboratory-scale facility enabling applications of several measurements techniques. This study follows a previous study performed at semi-industrial scale at the GDF SUEZ R&D center . Some results of the study of mild flameless combustion obtained in the laboratory-scale facility have been already presented when preheating combustion air . The present communication is focused on the experimental investigation of the effect of air temperature on the main features of this original combustion regime. The experimental setup The laboratory-scale mild flameless combustion facility and some measurement techniques have been already presented previously . Only main characteristics of the furnace are reminded here. Some specificities of the application of measurements techniques in such high temperature facility are presented. The mild flameless combustion facility The pilot facility, named “FOUR” for Furnace with Optical accesses and Upstream Recirculation, has a vertical plane-parallel combustion chamber (0.5 x 0.5 squared section and 1 m height), made with refractory materials for a maximum wall temperature of 1400 K (Figure 1). Several openings are set along in staggered rows on the four sides of the combustion chamber. These openings can receive an optical window or a probe stand for detailed measurements in the furnace, or a refractory block to preserve thermal confinement and measure wall temperature. The burner (Figure 2.) consists of three coplanar jets: two off-axis methane injectors (3mm dia.) spaced 101.4 mm apart and set on
either sides of a central air jet (25mm dia.) ending with a divergent shape (R = 11 mm).
Reactive zone visualisation by OH* chemiluminescence imaging OH* chemiluminescence imaging has been shown to be a very convenient technique for the visualisation of reaction zone structures in high temperature furnace compared to other radicals. Indeed in the visible range, CH* and C2* chemiluminescence signals are hardly discernable from strong global emissions of high temperature walls. As OH* chemiluminescence occurs in the ultraviolet spectral range, emissions from high temperature walls are several orders of magnitude less than in the visible range. Images of reaction zones can be then obtained . In the present experiment, OH* images are collected on an ICCD camera (Roper Princeton IMAX - 512 x 512 pixel - 16 bits), through a Goyo UV 25 mm f/2.8 lens and an interferential filter (310 ± 10 nm) through a 100 x 100 mm² UV silica window, set on one of the furnace opening. Because of the 100 mm width of the refractory wall, the collection system has to be placed as close as possible to the window to optimise the field of view obtained in the combustion chamber. However, the strong radiation heat from the furnace induces some damages on the optical filter when set directly in front of the windows. To avoid this problem, a dichroïc beam-splitter is used which reflects perpendicularly a part of the ultraviolet spectral range corresponding to OH* emissions bands and transmitting the visible and infrared ranges. Then the collection system (camera, lens and filter) are set perpendicularly to the window apart from the direct heat flux. This configuration ensures the safety for the material, a good spectral selection and a convenient field of view of 200 x 200 mm².
Figure 1 The mild flameless combustion furnace at laboratory scale. The centre of the air jet at the exit of the burner is defined as the origin. The vertical axis along the air jet corresponds to the y-axis, and the x-axis is the horizontal axis crossing the centres of the exits of air and methane injectors. side view
Figure 2 Geometry of the burner.
Velocity measurements by Particle Image Velocimetry The characterization of the aerodynamic mixing of high velocity jets and recirculating flue gas is performed by the application of Particle Image Velocimetry (PIV) on the FOUR facility. The setup consists of a double pulsed Nd:Yag laser Quantel Big Sky (120 mJ/pulse). The laser sheet formed by means of a periscope system and a set of lenses crosses the facility in the vertical plane of the three turbulent jets. Particle Mie scattering is collected on a CCD camera LaVision FlowMaster (1280 x 1024 pixel - 12bits) equipped with a ferro-electric liquid crystal shutter and a Nikkor 50 mm f/1.2 lens. Despite the use of a shutter, the wall radiation is not totally eliminated on the particle images and induces spurious velocities from the cross-correlation calculation. In order to avoid this correlation error, a background image is acquired without particles and is then subtracted to each particle image before the correlation processing . Another difficulty is to be able to combine sufficient dynamic range of the velocity measurement and suitable spatial resolution especially for the methane jets, because of their small width and high velocity. For this purpose, a direct crosscorrelation algorithm is used instead of a conventional FFT one , allowing the use of rectangular
Combustion air can be preheated up to 850 K thanks to an electric heater, in order to mimic the presence of a regenerative system in the burner and to allow continuous operating conditions. The reference operating conditions are a thermal input of 18.5 kW, an equivalence ratio of 0.85 and an air preheating of 838 K. In this case, wall temperature is homogeneous and equal to ~ 1320 K. No visible flame can be observed in this specific "colorless" combustion mode (Figure 3).
Figure 3 Photograph of the facility in flameless combustion regime.
interrogations windows (8 x 40 pixel²). For each case, mean and rms velocity fields are calculated over 300 instantaneous fields after global and local filtering of spurious vectors .
Results and discussion The evolution of NOx emissions with air preheating temperature is presented on Figure 4. Mild flameless regime is conserved whatever the air preheating temperature. For the maximum air temperature (Ta = 838 K), the value is already low: [NOx] = 25 ppm @ 3% O2. A gradual exponential decrease of NOx concentrations is observed when decreasing air temperature down to a very low value of 4 ppm @ 3% O2 for ambient air temperature. This can be attributed to the decrease of NO formation from thermal route, as the decrease of air preheated temperature induces a decrease of adiabatic flame temperature for a constant thermal power.
Temperature measurements with fine wire thermocouple Local temperature is measured in the combustion chamber thanks to 50 µm bare B-type (Pt – 6% Rh / Pt – 30% Rh) thermocouple. The signal is amplified (x100 – x1000) with a low noise preamplifier Stanford Research SR 560 and digitised by a 16 bit resolution ADC board National Instruments, AT-MIO-16-X), with a sampling frequency of 8 kHz during 20 sec per location. Difference between the measured bead temperature and the local gas temperature comes mainly from convection and radiation heat transfers between the hot junction and its environment. Indeed, the conduction can be neglected as the length of the wires is large enough compared to their diameters . Catalytic effect can be also neglected as no difference on temperature measurements are observed between two successive acquisitions on radial profiles nor between new and used thermocouple on the same locations. In this case, the resolution of the thermal balance between the thermocouple and its surroundings need the precise knowledge of wire diameter and emissivity, wall emissivity and convective heat transfer coefficient. A theoretical calculation of heat exchange in the ranges of expected velocity and temperature has been performed to estimate the difference between the measured bead temperature and the real gas temperature. The temperature is underestimated below 1700 K, and overestimated above. In our configuration, thanks to the small thermocouple diameter, the hot wall temperature and the global homogeneity of the mild flameless combustion, the temperature error is estimated to be no more than than ± 5 %, that is consistent with other experiments . Considering this uncertainty, temperature measurements are presented without correction in the following. Ta =838 K
Ta = 773 K
Ta = 673 K
[NOx] (ppm @3%O2)
Air tem perature T(K)
Figure 4 NOx emissions vs air preheating temperature. Figure 5 presents mean OH* chemiluminescence images in function of air preheating temperature for a constant thermal power of 18.5 kW and an equivalence ratio of 0.85. For the largest air preheating (Ta = 838 K), one can observed several reaction zones from the first optical access centred at y = 90 mm from the hearth of the furnace.
Ta = 573 K
Ta = 473 K
Ta = 373 K
Ta = 293 K
Figure 5 Mean OH* chemiluminescence images for different air preheating temperature Ta.
b. Figure 7 Mean velocity field obtained by PIV for the non air preheating conditions (Ta = 293 K). a: axial velocity (colors in m/s) and streamlines (whites lines). b: radial velocity (colors in m/s) and velocity vectors.
Figure 6 Mean velocity field obtained by PIV for the air preheating conditions (Ta = 838 K). a: axial velocity (colors in m/s) and streamlines (whites lines). b: radial velocity (colors in m/s) and velocity vectors. From the burner exit, an attached reaction zone appears on the internal side of the two methane jets. Similar reaction zone is also present on the other side of the methane jets but detached from the burner. These weak reaction zones occur in the shear layer between the methane jets and the hot recirculating flue gas as diffusion flames with excess oxygen present in the combustion products. The main reaction zone is lifted-off the burner. It starts from y* = 80 mm where the methane and air jets begin to merge. This reaction zone is developed along the stoechiometric line in the mixing layer of the reactant jets and ensures the most important heat release. It ends downstream before half-width of the furnace, as it is observed from the second optical access centred at y = 500 mm. When decreasing air preheating temperature, one can observed a progressive extinction of the weak reaction zone around the methane jets which is no more observable for Ta < 573 K. From Ta = 673 K, the liftoff position of the main reaction zone moves progressively upstream in the furnace. This shift is due to a change of local conditions in the mixing layer as an increase of the auto-ignition delay can be associated to the decrease of the inlet air temperature.
When air preheating temperature is decreasing, the intensity of OH* chemiluminescence signal is also decreasing, which indicates a decrease of local heat release density. The reaction zone is moving very downstream in the furnace. The core of the main reaction zone is above y = 400 mm for Ta < 473 K and reaches the half-width of the furnace for the non preheated air case. Figure 6 and Figure 7 present mean streamlines superimposed on the axial velocity field and the mean velocity vectors with radial velocity field obtained by averaging 300 instantaneous velocity field measured by PIV for respectively maximum air temperature (Ta = 838 K) and for ambient air temperature. The dimensions of the velocity field are limited by the field of view (120 x 100 mm²) achievable in the present configuration through the first opening. No velocity measurement can be done below 10 mm because of the laser reflection on the burner exit. Despite the size of the interrogation windows (0.8 x 4 mm²) which does not allow to fully resolve the methane jets, the maximum velocity is higher than the bulk flow velocity for the preheating case.
This can be attributed to the acceleration of the methane jet because of the presence of the attached reaction zone. One can see that there is no direct interaction between methane and air jets before y* = 80 mm as the axial velocity is close to zero beside them. Because of the large momentum of each jet, the entrainment process of recirculating flue gas by each reactant is important as can be seen from streamlines around the jets. Moreover, large values of radial velocity component around each methane jet show that the “aspiration” of flue gas by the jets occurs quickly from the burner exit. This is also true for the central air jet even if this region corresponds to its potential core as the divergent shape of the injection favours the turbulent mixing. One can also see that each methane jet is not symmetric. The internal side close to the air jet is more developed because of the momentum of the latter, which is typical of such parallel turbulent jets configuration . The interaction between each methane jet and the central air jet starts from y > 80 mm, where the main reaction zone is observed on OH* chemiluminescence imaging. Without air preheating, one observes the same aerodynamic features for the jets. However, the entrainment is less for the air jet as the momentum is lowered. The beginning of the merging of methane and air starts at the same position y* = 80 mm, which confirms that it is mainly controlled by the geometry of the burner. So, with or without air preheating, the entrainment process is already active at the exit of the burner for both jets thanks to their large momentum. This induces strong mixing of each reactant with recirculating combustion products inducing the diluted conditions where the main reaction begins [25, 26]. As the entrainment continues in the jets, the dilution is more and more enhanced, especially for the non preheating case as the reaction zone occurs only from 500mm.
(a) Ta =838 K
(b) Ta = 293 K
Figure 8 presents mean temperature maps in the furnace operating with air preheating (a) and without air preheating (b). The topology of the reaction zones obtained by OH* imaging (Figure 5) is found. For the preheated air case, one can observe a weak increase of temperature around methane jets, due to the attached reaction zone. In the present geometry of parallel turbulent jets, the entrainment of recirculating flue gas is a way to “condition” the fuel before it reacts with the air jet . Such weak reaction zone also participates to such preconditioning of the fuel jet by the formation of heat and radicals, without inducing large NOx emissions. The main increase of temperature is observed in the lifted reaction zone from the beginning of the mixing layer between the reactant jets as a typical jet flame.
Figure 8 Maps of mean temperature (a) with air preheating , (b) without air preheating. (each dot represents a measurement point)
Main difference is, even if it is underestimated by the thermocouple, the maximum of temperature reached in the core of the reaction zone, which stays relatively low (Tmax = 1717 K) compared to "standard" combustion, ensuring low NO production from thermal route. Without air preheating, no more reaction zone is observed around the methane jets nor at the beginning of the mixing layers between methane and air jets. In the first part of the combustion chamber, one observes a progressive smooth increase of temperature for the three jets surrounded by homogenous hot atmosphere. The temperature gradients remain low even in the reaction zone. The maximum temperature is only Tmax = 1556 K. These operating conditions correspond to a kind of ultimate case of mild combustion with a very large mixing of the reactants with recirculating flue gas ensuring that combustion occurs in highly diluted conditions and with small heat release density.
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Conclusions The effect of air temperature on mild flameless combustion regime is studied in a laboratory-scale facility. Velocity measurements obtained by PIV show the role of the entrainment process of air and methane turbulent jets to induce their dilution with a large amount of recirculating flue gas. Main heat release occurs then in hot diluted conditions in the mixing layers of air and methane jets downstream the burner exit. This induces very low temperature gradient as well as maximum value and thus explains the very low NOx emissions even with air preheating. When decreasing the inlet air temperature, the lifted reaction zone moves gradually downstream in the furnace and NOx emissions decrease. Without air preheating, the reaction zone occurs in the middle of the combustion chamber in large diluted conditions. In this case temperature peaks over the flue gas temperature are weak. Thermal NO formation is minimized, NOx concentrations are very low (4 ppm @ 3% O2). These results give new insights for the understanding of the mild flameless combustion regime and put forward some guidelines for the design of mild flameless combustion burners. Acknowledgements This work is done with the financial support of ANR (National Research Agency) – PAN-H Program and ADEME (French Agency for Environment and Energy Management). References  M. Flamme, M. Boss, M. Brune, A. Lynen, J. Heun, J.A. Wünning, J.G. Wünning, H.J. Dittman. Proc. International Gas Research Conference, San Diego, USA, 1998.  M. Katsuki, T. Hasegawa, Proc. Comb. Inst. 27:3135 (1998).  C.E. Baukal (Ed) Industrial Burners Handbook, CRC Press, Boca Raton, Florida, USA, 2004.