Experimental Study on the Basic Phenomena of Flame Stabilization

Article pubs.acs.org/EF

Experimental Study on the Basic Phenomena of Flame Stabilization Mechanism in a Porous Burner for Premixed Combustion Application Neda Djordjevic,*,† Peter Habisreuther,‡ and Nikolaos Zarzalis‡ †

Institute for Technical Chemistry (ITC), Karlsruhe Institute of Technology (KIT), Germany Engler-Bunte-Institute Division of Combustion Technology (EBI-VBT), Karlsruhe Institute of Technology (KIT), Germany

‡

ABSTRACT: One solution to intensify heat recirculation in a premixed burner and, thus, to enhance the flame stability is to employ porous inert material. Through the highly stable premixed flames low temperature combustion can be achieved employing high excess air ratios which beneficially results in a reduction of the NOx formation by the thermal pathway. Additionally, minimization of temperature and flow inhomogeneities, insensitivity to the fuel supply fluctuation, and damping of flow pulsations are beneficial properties of a porous burner leading to high power dynamic range. Heat transport properties of the solid porous inert media and the strongly tortuous flow path through this structure play a key role for the internal heat recirculation and for flame stabilization as its macroscopic manifestation. These properties strongly depend on the structure geometry and/or physical properties of the material. With an aim to quantify the contribution of the basic physical processes in a porous burner and to optimize its performance the present work reveals a comprehensive experimental study on the flame stability and emissions of such a burner containing different reticulate ceramic sponge structures. It was shown that in order to quantify the contribution of each heat transport mechanism of the global heat recirculation phenomenon and to estimate its relative importance experiments along a three-dimensional matrix (geometry, material, thermodynamic conditions) are required. The quantification of each relevant heat transport mechanism contribution was achieved using one-dimensional volume averaged analysis and comparison with experiments. Furthermore, such comprehensive experimental data with defined boundary conditions provide a necessary prerequisite for numerical validation cases.

1. INTRODUCTION Solid sponges are highly porous, monolithic materials with an open-cell structure consisting of stiff, interconnected struts. Struts and voids represent continuous networks which penetrate each other. The manufacturing of such structures can be achieved using ceramic, metal, glass, or polymer. Due to their excellent properties (high porosity, high specific surface, low flow resistance, and high mechanical stability) solid sponges exhibit a great potential as internals in process engineering equipment e.g. as a catalyst carriers in exhaust and waste gas purification1 or in partial and preferential oxidation,2,3 as solar receivers,4 in photobioreactors,5 etc. In general, a potential deployment of the solid sponges (also referred to as open-cell foams) in process engineering is for the reactions where high fluid flows are employed, strong evolution of heat occurs, and/or mass transport influences the reaction kinetics.6 A particularly critical application of the spongelike structures is in high temperature process engineering e.g. in a porous burner. Employing the porous burner concept, combustion takes place within an inert solid matrix exhibiting significantly better heat transport properties in comparison to the combustible gas mixture alone. The ceramic porous body in the reaction zone provides highly efficient heat transport by means of solid radiation and conduction resulting in a preheating of the incoming gas mixture and, thus, increasing the burning velocity. Therefore, low temperature combustion can be achieved through high excess air ratios beneficially resulting in a reduction of the formation of NOx by the thermal pathway. Additionally, the overall permeable highly heat © 2012 American Chemical Society

conductive solid structure tends to homogenize the temperature profiles diminishing eventual hot spots that promote NOx formation and therefore enables an optimization of the operating characteristics.7 Another advantage of the porous burner is that it is relatively insensitive to the fluctuations in fuel supply and even to its short interruptions due to the high thermal inertia of the system resulting from the high heat capacity of the solid body.8 Furthermore, the solid matrix acts also as a flow pulsations damper. Besides the excellent emission and flame stability characteristics, the porous burner concept offers an advantage of the three-dimensional solid matrix located in the high temperature combustion zone. Thus, the solid matrix represents a volumetric radiation source that emits very high infrared radiation. This opens up a broad field of application of porous burner in industrial drying and heating. These beneficial properties of a porous burner motivated several application-orientated studies that showed a great potential of porous burner deployment in energy- and heating engineering,8,9 in a ZEE (zero emission engine),10 as a burner for boilers and furnaces,11 as a pilot burner for gas turbines,12 etc. Current changes in energy policy stimulated the ongoing developments targeting more efficient and low pollution energy conversion systems. This was the driving force for investigation of porous burner application in exothermic thermal partial oxidation (POX) for production of syngas feed for MCFCs Received: August 3, 2012 Revised: October 15, 2012 Published: October 18, 2012 6705

dx.doi.org/10.1021/ef3013008 | Energy Fuels 2012, 26, 6705−6719

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The SiSiC sponge demonstrated a better flame stabilization effect for both fuels. All these findings indicate that the porous burner performance is strongly influenced by the choice of the porous body. The numerous studies in the literature are either application oriented or they do not deal with the quantification of the burning velocity and of the effect of sponge properties on the flame stability. Nevertheless, there is a great need for extensive and systematic investigations of such kind as indicated in a review article by Wood and Harris.25 1.1. Combustion within a Ceramic Sponge Structure. In the focus of the present work is a porous burner containing spongelike structures (Figure 1). These structures are favored

(Molten Carbonate Fuel Cells) and SOFCs (Solid Oxide Fuel Cells)13 or for combustion of low calorific gases from landfills and waste pyrolysis.14 Combustion in porous media is an example of an excess enthalpy flame (superadiabatic flame) where enhanced heat transport from the postflame zone through the solid matrix leads to a preheating of the incoming reactants without dilution.15−17 High flame stability observed at such burner configurations motivated investigations on porous burners employing various porous structures. A cylindrical porous burner containing a spongelike structure made of partially stabilized zirconia (PSZ) was investigated in refs 18−20, and enhanced burning velocities relative to laminar flame were reported. Very limited access for both optical measurement techniques and extractive probe measurements due to the solid matrix in the reaction zone makes it difficult to observe the combustion process in a porous burner. Typically, the flame stability limits are detected by recording a temperature profile perpendicular to a flame front by means of thermocouples inserted only up to the outer edge of the solid porous body.20,21 The influence of the pore size on the flame stability in a PSZ porous burner was investigated in ref 20. It was reported that an increase in pore density (pores per inch − PPI) results in a decrease of the flame stability for the given PSZ burner. The study on the flame stability in two different porous burners containing open-foam structures made of yttria-stabilized zirconia/alumina composite (YZA) or zirconia toughened mullite (ZTM) was conducted in ref 21. It was shown that the material of the solid matrix has a strong influence on flame stability. However, no physical explanations were given due to the very limited amount of information about the material properties of YZA ceramics available in the literature. In ref 22 the YZA burner was fired with methane or propane showing that for the investigated range of equivalence ratios both lower and upper operating limit were lower in case of methane relative to propane. The authors reported very low NOx, CO, and UHC emissions sampled at the burner exit. Unlike the combustion without porous structure, in a porous burner it is very difficult to define a flame blow off limit, since with an increase in flow rate the flame does not extinguish abruptly but the blow off occurs rather as a gradual process.22 To overcome this difficulty Diezinger23 suggested a modified design of a porous burner containing conical ceramic foam made of SiSiC. With this approach the burning velocity could be directly determined from the position of the flame within the cone. Burning velocity in a porous burner is determined by the balance between heat release, heat recirculation, and heat loss. Under certain conditions an excess enthalpy flame (superadiabatic flame) can be achieved, and the resulting flame speed is greater than the laminar flame speed. A change in the heat balance resulting in higher heat losses and thus lower heat recirculation can lead to flame speeds lower than the corresponding laminar flame speed i.e. a subadiabatic flame is achieved. In this case the maximized heat losses from the system can be recovered as usable energy in the form of radiative heat or exhaust gas enthalpy. A study on a porous burner containing FeCrAlY metal foam showed that this burner can be operated in both superatiabatic and subadiabatic regimes.24 Al-Hamamre et al.14 applied two different porous burners: an alumina mixer structure and a SiSiC spongelike structure to study combustion of low calorific synthesis gas and landfill gas.

Figure 1. Ceramic sponge.

due to their high porosity i.e. low pressure drop, excellent heat transport properties, and high thermal resistance. The spongelike geometries are overall permeable for fluid flow, nevertheless the resulting flow field in such structures is very complex (Figure 2) influencing accordingly heat and mass transport processes.27 In order to demonstrate the basic principle of the flame stabilization in a porous burner, temperature profiles of the gas and the sponge as well as concentration profiles of some important species calculated using a 1-dimensional model of the combustion process in porous inert media (PIM) are

Figure 2. 3D numerical simulation: Flow field through the representation of a real sponge structure obtained by means of magnet resonance imaging.26 6706


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properties again depend on the structure geometry and/or thermophysical properties of the material. In order to optimize the porous burner performance there is a need for systematic investigation of the influence of the structural and material parameters on the flame stability. Nevertheless, due to the complex interaction between physical processes governing heat recirculation and the PIM, a response on change in one parameter of influence will depend on the other parameters. Hence, the macroscopic result (flame stability) of the variation of e.g. one structural parameter such as pore size (determined by the pore density PPI) will depend on the material and on the thermodynamic conditions chosen for the investigation. In this way observed influence of pore density is valid only for the investigated configuration and conditions and does not provide a global insight into the heat recirculation mechanism. In order to observe a global heat recirculation mechanism it is necessary to conduct systematic measurements along the three-dimensional matrix with following dimensions: structure geometry, material, and thermodynamic condition. While there are studies reporting on the influence of one parameter e.g. pore size20 or material,14,21 a systematic study with variation of all relevant parameters conducted on the same burner, that would enable a global insight into the stabilization mechanism, is still missing. With this aim the present work reveals an experimental study on the flame stability and emissions of a porous burner containing various reticulate ceramic sponge structures. In order to account for the influence of the material thermal properties sponges made of aluminum oxide (Al2O3) and silicon infiltrated silicon carbide (SiSiC) were investigated. Furthermore, the pore diameter as a parameter of influence for all heat transport processes was varied for both sponge materials. The influence of the macroscopic porosity of the sponge structures on the flame stability was only investigated for the Al2O3 sponges. Thus, five different burner configurations were investigated for different thermodynamic conditions achieved by variation of excess air ratio and air preheat temperature. Based on this study heat transport processes contributing to the simplified mechanism that can describe the global heat recirculation phenomenon are to be determined. One way to quantify each relevant heat transport mechanism contribution is by using one-dimensional volume averaged analysis and comparison with experiments. In order to enhance the reliability of the analysis the geometrical parameter used should be obtained by characterization of the investigated structures via magnet resonance imaging (MRI) or computer tomography (CT) instead of being calculated based on the PPI as given by the manufacturer. Furthermore, according to the literature there is a difference in opinions regarding the significance of the flow dispersion contribution to the flame stabilization e.g. refs 27−29 vs refs 30−32. If the contribution of the dispersion is considered in the literature it is typically described using the correlations derived for the packed beds.33 As the flow pattern in a packed bed significantly defers from that in a spongelike structure, it is important to quantitatively describe the hydrodynamic dispersion in the investigated structures in order to analytically observe its contribution to the global heat recirculation mechanism. Finally, an additional objective was to present the experimental data in such way that they could be easily used for validation cases of numerical simulations.

shown in Figure 3. In the reaction zone the gas temperature is higher than the sponge temperature and the heat released from

Figure 3. Temperature of gas (T) and solid (TS) and concentration profiles calculated using a 1D model of combustion in PIM.

the combustion reaction is transferred effectively from the gas to the solid due to the high specific surface of PIM. The temperature gradient in the solid phase created in this way leads to an intensive upstream heat flow by means of solid-tosolid radiation and conduction. As a result of the upstream heat flow, in the preheating zone the temperature of the sponge is higher than the gas temperature. Thus, the heat is transferred from the sponge to the gas i.e. a preheating of the combustible mixture occurs. This process results in an increase of the local combustion temperature above the equilibrium value which, in turn, leads to higher burning velocities and an extension of the stable operating range relative to laminar flame. Besides the high conductive and the radiative heat transport a very important contribution to the flame stabilization within the solid sponge is given by the hydrodynamic dispersion.27 The enhanced thermal and species diffusivity due to dispersion lead to a flattening of the temperature and concentration profiles relative to a laminar flame resulting in a thickening of the flame front analogous to turbulent flames. Given the known proportionalities a/δ ∝ Sl ∝ (a/τc)1/2 a variation of the thermal diffusivity a at approximately constant chemical time scale τc results in a variation of flame speed and thickness as follows: δ|τc = const. ∝ Sl|τc = const. ∝ √a. Thus, thickening of the flame front leads to higher conversion rate and higher burning velocity. 1.2. Mechanical Stability. Application of ceramic sponges in high temperature process engineering gives rise to certain problems untypical for other applications such as catalyst carrier, particles filter, etc. Their limited thermal shock resistance reduces significantly the durability of ceramic sponges employed in a porous burner, since they exhibit large temperature gradients across the bulk of material during the ignition and the termination of the combustion process. This imposes a limitation of ceramic sponge application on long-term stationary processes or further optimization of the transitional processes mentioned above in order to avoid high temperature gradients. 1.3. Problem Definition and Objectives of the Study. From the previous considerations it is obvious that the key factors for internal heat recirculation and for flame stabilization as its macroscopic manifestation are the heat transport properties of the PIM and the strongly tortuous flow path through this structure (hydrodynamic dispersion). These 6707


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2. EXPERIMENTAL SECTION 2.1. Experimental Setup. Figure 4 depicts a scheme of the experimental setup used in this work. Natural gas flowing

samples to be piled up in premix/preheating zone. These sponges are centered and sealed against bypass flow with a 3 mm thick fiber mat. The large pore sponge (D = 160 mm) is placed within the combustion zone. This zone is insulated using refractory bricks and fiber mat and is water cooled to prevent damaging of the steel containment through the high combustion temperatures. The large pore sponge in the combustion zone has a height of 200 mm requiring piling up of 4 or 8 sponge samples, depending on their height. In order to avoid a bypassing gas flow, the sponges in the combustion zone have been laterally coated with ceramic mortar and sintered before insertion into the combustion chamber. The different sponges used in the combustion zone for the present experiments are listed in Table 1. Furthermore photographs of the employed structures are given in Figure 4. A ring made of refractory brick provides a distance of 10 mm between the small pore and the large pore section, in order to avoid heat transfer from solid to solid by means of conduction and reduce the warming of the flame arrestor surface. In order to record the axial temperature profile a special effort has been done to measure the temperature within the solid sponge and not only at its outer edge. Sixteen thermocouples have been inserted, each 40 mm deep in both small and large pore sponge. K-type thermocouples are applied for the measurement within the premixing/preheating zone. High temperatures in the combustion zone are recorded using 12 S-type thermocouples. Due to manufacturing constraints for the employed thermocouple sealings, the S-type thermocouples are installed in 3 columns displaced by an angle of 15°. The output signals of all measuring devices (flow, pressure, temperature measurement) have been recorded and evaluated continuously with the time using a LabView based program. 2.2. Experimental Procedure. With respect to the experimental determination of the flame stability limits there are two possible approaches as depicted in Figure 5. Here, the experimentally determined flame stability limit of the alumina burner at air inlet temperature of 25 °C is shown, indicating that for the conditions above the line the flame is unstable. The operational points at the stability limit can be determined either by keeping the excess air ratio constant at a certain value and successively increasing the volumetric flow rate (and therewith firing rate) or by a stepwise increase of the excess air ratio at constant firing rate. Being less time-consuming, the latter approach has been used in the present work.

Figure 4. Experimental setup (left) and investigated large pore sponges (right).

through the fuel injection system enters the static mixer. The efficiency of the designed mixing unit was proved prior to the flame stability investigations in order to ensure that homogeneous gas mixture enters the combustion zone. Potential mixture inhomogeneities lead to local deviations of the excess air ratio from the adjusted one which, in turn, may lead to a local increase of the flame temperature above the operational one and, furthermore, to destruction of the sponges. After mixing with the combustion air the combustible gas mixture passes through the flow homogenizer (steel wool) and enters the small pore sponge in the premixing/preheating zone. The small pore sponge (zirconia partially stabilized with magnesia; 45 PPI and D = 160 mm) has a function of the flame arrestor, based on the principle of the quenching distance. Since the flame quenching distance is temperature dependent, this premixing/preheating zone is intentionally not insulated in order to ensure heat flow out of the small pore sponge. The small pore PIM samples have a height of 25 mm requiring two

Table 1. Investigated Burner Configurations and Operational Conditions combustion zone burner configuration

material

PPI

porosity

K01

99,5% Al2O3 (Vesuvius)

10

85%

K02


20

85%

K03


10

80%

K04

SiSiC (Erbicol)

10

87%

K05

SiSiC (Erbicol)

20

87%

6708

flame arrestor

Tair,in [°C]

MgO/ZrO2 45 PPI porosity 80%

25 350 25 350 25 350 25 85 350 25 85 350

Pth [kW] 30, 30, 30, 30, 30, 30, 20, 20, 30, 20, 20, 30,

40, 50, 50 50, 40, 50, 30, 30, 40, 30, 30, 40,

50, 60 70, 90, 110 70 50, 70, 40, 40, 50, 40, 40, 50,

60, 70 90, 110 50 50 60 50 50 60


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means of thermocouples is to observe the combustion process in order to determine flame stability limits and not to determine an exact flame temperature. The variation range of excess air ratio was determined by the flame stability limit (depending on the burner configuration) for λmax and by the maximal operational temperature of the PIM in the combustion zone (1400 °C for SiSiC and 1600 °C for Al2O3) for λmin. Simultaneously with the flame stability investigations the pollutant emissions were investigated sampling the exhaust gas above the sponge (Figure 4) by means of a water-cooled probe. The probe temperature was maintained at 160 °C in order to provide fast cooling of the exhaust gas and thus quench further chemical reactions while keeping the temperature above the dew-point of the exhaust. Concentrations of CO and CO2 were determined by commercial gas analyzers (Leybold-Heraeus Binos) working on the basis of infrared absorption with an accuracy of ±7 ppm and ±0.2%, respectively. O2 was measured with a Leybold-Heraeus Oxinos gas analyzer on the basis of paramagnetism (±0.25%) and nitrogen oxides with an Eco Physics CLD 700 EL h analyzer based on chemiluminiscence (±0.1 ppm).

Figure 5. Schematic representation of possible approaches for determination of flame stability limits.

Stability investigations including the determination of the burning velocity of natural gas/air flames in the PIM for particular conditions have been conducted using the procedure mentioned above. After ignition at predefined thermal load and certain excess air ratio the flame was allowed to stabilize for the given conditions. Keeping the firing rate constant, the excess air ratio was increased in steps of 0.05 allowing between two steps enough time for the flame to stabilize. An increase of excess air ratio (λ) results in a decrease in both laminar burning velocity and combustion temperature. At a certain λ, the velocity of unburned mixture became higher than the burning velocity in the sponge for the given conditions and the flame front migrated slowly to the burner exit indicating flame blow off. Figure 6 shows measured temperature profiles at different

3. RESULTS 3.1. Flame Stability Investigations. The flame stability investigations were conducted according to the measurement procedure described in Section 2 for all burner configurations listed in Table 1 and for a variation of the air inlet temperature and thermal load. The obtained lean stability limits for different burner configurations are displayed in stability diagrams (Figures 7 − 13) as a function of the maximal firing rate

Figure 7. Stability diagram of Al2O3 burner for air inlet temperature 25 °C: influence of pore size.

Figure 6. Measured temperature profiles in K04 at 995 kW/m2 for the variation of excess air ratio.

excess air ratios in the K04 burner at constant firing rate of 995 kW/m2. The dashed line (λ = 1.9) marks a momentary position of the unstable, slowly moving flame. For comparison, the gray line displays the value of adiabatic flame temperature at λ = 1.7 demonstrating that the combustion conditions were very close to adiabatic. Nevertheless, the measuring points of the thermocouples are located within the porous structure i.e. heat exchange between the thermocouple pearl and sponge surface occurs by means of radiation. Thus, the measured temperature probably represents a value between the gas and the sponge temperature i.e. it is possible that the gas temperature maximum is higher than the value given in Figure 6. However, the purpose of the temperature measurement by

(thermal load/unit area) for a stable operation at given excess air ratio and air inlet temperature. For comparison, the corresponding flame data calculated for laminar freely propagating flames are also included. The laminar burning velocities were obtained by CHEMKIN based numerical simulations using the PREMIX code.34 PREMIX solves the steady state transport equations for energy and mass fractions of species for the set of given thermodynamic inlet conditions. The reaction terms in the mass fraction balance equations were calculated on the basis of a detailed reaction mechanism according to Egolfopoulos.35 The stable operating range (gray area in stability diagram) of the porous burner was constrained for small excess air ratios (i.e., high burning velocities) by the 6709


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maximal operating temperature of the sponge (dash-dot line in stability diagram). All stability diagrams (Figures 7 − 13) demonstrate a substantial enhancement of the flame stability in the porous burner compared with laminar flames. 3.1.1. Al2O3 Burner. The influence of pore size of the Al2O3 sponges on the flame stability was investigated varying the pore density (PPI) while keeping the porosity constant (K01 and K02 configuration in Table 1). The obtained stability diagrams for two different air inlet temperatures of 25 and 350 °C are given in Figures 7 and 8. For all investigated thermodynamic conditions a decrease in pore size (increase in PPI) results in a reduction of flame stability in the Al2O3 sponge. Figure 10. Stability diagram of Al2O3 burner for air inlet temperature 350 °C: influence of porosity.

across the bulk of the material during the ignition and the termination of the combustion process. Every time the burner was heated up and cooled down the effect became more pronounced. However, no significant effect of the sponge degradation on the flame stability was noticed. To overcome these difficulties, there is a possibility to segment the used Al2O3 sponges having a relatively large cross section or to employ the sponges made of the materials exhibiting higher thermal shock resistance such as SiSiC for example. 3.1.2. SiSiC Burner. Sponges made of SiSiC were examined in order to investigate the influence of the material’s thermal properties on the flame stability. Again, the pore size was varied while keeping the porosity constant (K04 and K05 in Table 1). The results obtained from stability investigations for both SiSiC burners are summarized in stability diagrams (Figures 11

Figure 8. Stability diagram of Al2O3 burner for air inlet temperature 350 °C: influence of pore size.

Influence of the Sponge Macroporosity. In order to investigate the influence of the sponge macroporosity, two burners containing the sponges with the same PPI number but different porosity (K01 and K03 in Table 1) were investigated for different air preheating temperatures, and the obtained experimental data are shown in stability diagrams given in Figures 9 and 10. For the set of investigated thermodynamic conditions no difference in the flame stabilization effect of the burners K01 and K03 was detected. The reason is probably the relatively small difference between the two investigated porosities. Mechanical Stability. Degradation of the Al2O3 sponges due to cracking occurred as a result of large temperature gradients

Figure 11. Stability diagram of SiSiC burner for air inlet temperature 25 °C: influence of pore size.

− 13). It is obvious that the influence of the pore size on the stability behavior of the SiSiC burners differs significantly from that of the Al2O3 burners. For the lower air inlet temperatures (Figure 11 and Figure 12) no clear difference in the flame stabilizing effect of the SiSiC sponges with two different pore sizes can be noticed. For the high air preheat temperature of 350 °C (Figure 13) 20 PPI burner demonstrate better performance than the one with 10 PPI. Mechanical Stability. The investigated SiSiC sponges demonstrated very good mechanical behavior with respect to the cracking due to the high temperature gradients during some operational phases in the porous burner. The silicon infiltrated

Figure 9. Stability diagram of Al2O3 burner for air inlet temperature 25 °C: influence of porosity. 6710


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Figure 14. Measured NOx emission levels for various burner configurations and operating conditions.

nitric oxides determined in the set of experiments conducted with air inlet temperature of 350° do not differ from those measured low initial gas temperatures since the operating temperature window was nearly the same for the given sponge material due to the constraint of the maximal operating temperature. An intermediate species of the combustion process of hydrocarbons is carbon monoxide. Therefore, low concentration of oxygen, low temperature level due to e.g. quenching at cold wall, and/or low retention time may prohibit the further oxidation and represent the main reason for high CO concentrations in exhaust gas. In order to inspect whether the combustion reaction is completed within the porous burner, the sampled exhaust gas was also examined on carbon monoxide. The obtained experimental data are expressed based on 15% O2 analogous to NOx concentrations and are shown in Figure 15 for all


in the voids of the struts provides higher mechanical stability and higher thermal shock resistance of the sponges. However, it is the reason for the limited operational temperature of 1400 °C. A long-term operation requires temperatures lower than 1400 °C being dangerously close to the melting point of Si. Melting of Si would result in its leakage out of the struts. 3.2. Pollutant Emissions. Simultaneously with the flame stability investigations the pollutant emissions investigations were conducted for all investigated cases as listed in Table 1. The measured concentrations of nitric oxides (NOx) are expressed based on 15% O2 as recommended in EU-Directive 2001/80/EC and for all investigated burner configurations and operating conditions are ploted in Figure 14 vs adiabatic flame temperature. An approximation of the adiabatic flame temperatures were extracted from CHEMKIN based numerical simulations using the PREMIX code.34 The NOx concentration level is very low and similar for all investigated burners as being mainly a function of the process temperature. According to the thermal mechanism of NO formation that dominates in hightemperature combustion over a fairly wide range of excess air ratios postulated by Zeldovich, temperatures above 1700 K are required in order to significantly activate the NO formation.36 The extended flame stability limits (Figures 7-13) provide stable burning of the premixed flames at high excess air ratios ensuring the low temperature combustion followed by very low NOx emissions. Increasing of the excess air ratio is followed by diminishing NOx emission due to the decrease of the combustion temperature (Figure 14). The concentrations of

Figure 15. Measured CO emission levels for various burner configurations and operating conditions.

investigated burner configurations and operating conditions. In a porous burner, conversion of the fuel takes place in a relatively thin zone within the sponge, whereas the oxidation of CO into CO2 occurs in a broad burnout zone also located within the hot porous body. Thus the gas temperature remains high for a relatively long time prohibiting quenching of the reactions that lead to equilibrium. The residence time within the hot sponge varies depending on the experimental condition in the range 40−330 ms. The increase of the process 6711


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fluid and solid phase continuous (e.g., solid sponges) with the scheme presented in Figure 17.

temperature due to the decrease of excess air ratio leads to higher CO concentrations (Figure 15) due to the dissociation reactions and partial equilibrium of water-gas-shift reaction. From Figures 14 and 15 it can be observed that the porous burner can be operated under conditions, while both nitric oxides and carbon monoxide concentrations are very low.

4. DISCUSSION The measured flame stability diagrams for all investigated burner configurations demonstrate an excellent flame stabilization performance (Figures 7-13). The magnitude of burning velocity approaches even the order of aerodynamically stabilized diffusion flames37 but without the accompanying high emission levels of the latter. Figure 16 demonstrates that

Figure 17. The dominating heat transport mechanisms in porous burner employing spongelike structures.

In the scheme in Figure 17 the coupling of different heat transport processes in solid sponges is presented as a connection of resistances. In such a porous burner, besides the here not displayed downstream convective heat transport with the bulk flow, there is an upstream heat transport allowing for a flame to stabilize (see Section 1.1). Between the hot gas in the combustion zone and the cold gas in the preheating zone the heat can be transported along the two alternative pathways: −Path G: Directly through the gas phase by means of heat conduction and radiation − Path S: By heat transfer to the solid phase, then through the solid phase by means of solid conduction and radiation, and finally by heat transfer in the preheating zone from the solid to the gas. Heat conduction and radiation of a gas phase is negligible in comparison to that of a solid body and therefore typically not considered. However, the diffusive heat transport in gas is considered in the block diagram since it is being strongly enhanced due to the hydrodynamic dispersion in the sponge.27 The heat transport terms along two alternative pathways act in parallel. The necessary interaction between two phases is provided through the convective heat transfer, being serially coupled with the heat transport through the solid phase. All considered heat transport mechanisms are functions of the sponge geometry.40 Furthermore, for some of these mechanisms the heat transport properties of the material itself additionally play an important role. In order to estimate what is a total effect of a change in one sponge parameter on the heat recirculation and therewith on the flame stability, certain basic rules can be postulated according to the coupling scheme (Figure 17). The order of magnitude of the total heat recirculation is determined by − the highest of the parallel connected terms and − the lowest of the serially connected terms. This applies only when the considered heat transport mechanisms are of different orders of magnitude. In case that the heat transport terms are of the same order of magnitude this consideration becomes more complex since in that case all the heat transport mechanisms have the same relevance.

Figure 16. Flame stabilization effect: comparison between coflow swirl flame30 and K05 burner.

the achieved firing rate in the porous burner (K05 in Table 1) is of the same order of magnitude as the one achieved in a burner operating with a coflow swirl flame, especially at high excess air ratios. The detected increase of the burning velocity i.e. the enhancement of the flame stability can be explained based on the thermal theory of laminar flame propagation showing that the laminar burning velocity is proportional to thermal diffusivity38(see also Section 1.1). Varying the effective thermal diffusivity of the combustion system the burning velocity can be manipulated. Thus, it is possible to optimize a porous burner with respect to flame stability by influencing its effective thermal diffusivity. The effective thermal diffusivity in the porous burner depends on the heat transport properties of the sponge (heat conductivity and emissivity) and on the hydrodynamic dispersion of the gas phase rising from the complex flow pattern in such structures. Those parameters again exhibit complex correlations with the sponge geometry. The measured flame stability data along the three-dimensional matrix with the dimensions structure geometry, material, and thermodynamic condition were presented in Section 3 and can be used for further analysis (see Section 1.3). To obtain information about the relative contribution of various heat transport processes to heat recirculation i.e. flame stabilization from the experimental data it is necessary to use a scheme of the global heat recirculation mechanism. A complex scheme of heat transport in porous media derived for industrial refractory materials39 has been applied for porous burners in refs 23 and 30. This scheme can be simplified for structures that have both 6712


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Table 2. Parameter for the Estimation of the Order of Magnitude of Relevant Heat Transport Mechanisms in Al2O3 Burners PPI

u0[m s−1]

ε [%]

SV [m−1]

dPe [mm]

Pe [-]

dRe [mm]

ks [W/m2K]

β [m−1]

CNu [-]

mNu [-]

10 20

0.6 −7.5 0.6 −7.5

85 85

629 1109

2.0 1.8

54−677 49−609

1.95 1.14

5 6

210 300

0.146 0.139

0.96 0,92

In order to quantify an effect of a change in certain sponge parameter on the total internal heat recirculation it is necessary to estimate its effect on each of the considered heat transport mechanisms. Therefore, one-dimensional volume averaged analysis was conducted for each investigated large pore PIM (Table 1). Since the exact thermophysical parameters of the investigated structures are not known and the models used to describe different heat transport processes are only simplified expressions, this estimation was conducted with an aim to obtain orders of magnitudes only, and no attempt was made to compare the terms of the same order of magnitude. Nevertheless, for modeling of the property data the real geometrical parameters (mean pore diameter, specific surface) were used. The geometrical characterization of the investigated porous structures was done by means of magnet resonance imaging (MRI) for the sponge structures made of aluminum oxide or by means of microcomputer tomography (μCT) for the sponges made of silicon infiltrated silicon carbide (SiSiC). The details regarding the geometrical characterization can be found in ref 41. A significant reduction of the thermal conductivity of a sponge structure relative to that of the solid nonporous body made of the same material was determined according to the correlation from Boombsma and Poulikakos42 to account for the effect of the longer tortuous paths along which the heat is conducted and the smaller cross-sectional area. The thermal conductivity was calculated from eq 1 for each investigated sponge structure using a thermal conductivity of solid material ks.

The contribution of the solid-to-solid radiation to the total heat recirculation was determined using the Rosseland approximation for optically thick media. The radiation density at location x of a one-dimensional system can be expressed with following equation: qṙ =

f=

(

5

x = 0, T = 300 K x = 0.035 m, T = 1700 K

∫0

16σB dT 3β dx

∫0

δ

T 3(x)dx ,

W m

(6)

(7)

As recommended in ref 45 the real pore diameter obtained from the MRI and CT measurements41 was used as the characteristic length for the definition of the Re and Nu number. The convective heat exchange between the two phases over the sponge length was then calculated as follows:

)

qĠ − S = α(ΔT )G − S SV APIM ,

W m

(8)

According to eq 8 a large uncertainty in estimation of the heat transfer originates from determination of the local temperature difference between gas and solid (ΔT)G−S. Simultaneously conducted numerical investigations27,46 of the exactly same burner configurations and thermodynamic conditions as experimentally studied in the present paper showed that the spatial maximum of the temperature difference between two phases varies for different process conditions (λ, Tin) and for different sponge structures. Comparing the computed gas and solid temperature profiles for all the investigated configurations and conditions,27 the temperature differences between the two phases were chosen for the purpose of the present estimation as follows: (ΔT)G−S = 40 K for Al2O3 sponges and (ΔT)G−S = 10 K for SiSiC sponges. A complex and substantially three-dimensional flow pattern in spongelike structures leads to an enhanced mixing by means of the flow dispersion.26,47 This results in a strong enhancement

In order to account for the change in pore size according to ref 43 ks was slightly varied in eq 1 (see Tables 2 and 4). The heat conduction through the solid phase per unit of the length of the sponge structure can be then calculated with the following equation: (2)

ΔT denotes here the temperature difference between the reaction and the preheating zone (Figure 17). Analogous the heat conduction in fluid per unit of the length of the sponge can be estimated: W m

qṙ dx=

NuV = CRem

(1)

qġ , cond = kg ΔT ,

δ

The extinction coefficient β is inversely proportional to the pore diameter and was calculated based on sponge pore diameter and porosity as given in ref 44. The convective heat transfer between the two phases was modeled with the empirical Nu-Re correlation suggested by Younis and Viskanta.45

e = 0.339

W m

(5)

The total heat flow by means of radiation over the flame thickness is then

π (3 − 4e 2 − e)

qṡ , cond = kPIM ΔT ,

(4)

The temperature profile necessary for this rough estimation of the order of magnitude of solid-to-solid radiation in a sponge was described with the linear approximation over the flame thickness of 0.035 m (as estimated from the measured temperature profiles) with the following boundary conditions:

kPIM 1 = ks 2 −1 ⎧ 4f 3e − 2f ( 2 − 2e)2 ⎫ ⎨ 2 ⎬ + + 2πf 2 (1 − 2e 2 ) ⎭ e2 ⎩ 2e + πf (1 − e) 2 2 − 8 e 3 2 − 2ε

4σB ∂T 4 W , 3β ∂x m2

(3) 6713


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of the fluid thermal diffusivity. To account for this effect gas thermal diffusivity was calculated as follows:29,33

ag* ag

parameters used for the calculation (eqs 1−10) are given in Table 2. Since the certain heat transport mechanisms are dependent on the flow velocity (eqs 7 and 9) the investigated velocity range is also indicated in Table 2. In order to get an impression about the magnitude of the flow dispersion in various structures Pe numbers calculated for given dPe and investigated velocity range are also shown. The calculated orders of magnitude for each investigated transport process are given in Table 3. According to the obtained orders of magnitude and coupling scheme (Figure 17) the following conclusions regarding the heat recirculation in Al2O3 sponges can be drawn: • Considering the transport path S, the heat transport is governed by the transport through the solid phase since the gas−solid transfer is for at least 1 order of magnitude higher. • The heat transport through the solid is governed by the radiation as the solid conduction is for an order of magnitude lower. Thus, the dominant mechanism for the heat recirculation along the path S is solid-to-solid radiation. Accordingly, the higher extinction coefficient of the 20 PPI burner (K02) relative to the K01 burner leads to the lower stabilizing effect as detected in the stability investigations for both air inlet temperatures (Figures 7 and 8). This statement agrees well with the experimental results obtained in ref 20 for a PSZ burner featuring also low thermal conductivity and emissivity. Furthermore, in case of Al2O3 sponges dispersion and radiation are of the same order of magnitude and therefore both alternative pathways for the upstream heat transport are similarly important for the flame stabilization. The Pe number describing hydrodynamic dispersion is somewhat higher for 10 PPI sponge (Table 2) also leading to better flame stability. An increase of the air preheating temperature (Figure 8) results in a strong enhancement of the laminar burning velocity for a given excess air ratio (λ), thus moving the flame stability limit to higher λ at constant thermal load (i.e., firing rate). The operating temperature window was nearly the same in all investigations regardless of the air inlet temperature due to the constraint of maximal operational temperature of the sponge on one side and the lean stability limit of the flame on the other side. 4.2. SiSiC Burner. The order of magnitude estimation of the relevant heat transport mechanisms was conducted for the SiSiC sponges using the sponge parameters listed in Table 4 , and the obtained data are given in Table 5. For this material it is important to emphasize that sponges feature very high heat conductivity, unlike the previously investigated aluminum oxide structures. Furthermore as compared to alumina sponges the investigated SiSiC structures also have lower extinction coefficient (i.e., better transport by radiation), demonstrate higher flow dispersion (Pe number), and are characterized with lower specific surface (i.e., lower gas/solid heat transfer for a given velocity; eq 8). The results shown in Table 5 indicate that all the heat transport mechanisms are of the same order of magnitude and

= 0.5 Pe (9)

In order to apply this primarily for the packed beds derived correlation (eq 9) on the sponge structures it is necessary to determine the characteristic length defining the Pe number. For the present analysis the dispersion coefficients were obtained from the three-dimensional numerical simulations of the flow through the real sponge structures used in the porous burner. The structures for the simulation were generated from the three-dimensional MRI and CT data. The dispersion coefficients determined for all investigated structures were then used to determine the characteristic length dPe for the calculation of Pe number. More details regarding the threedimensional numerical simulations of the flow through the sponge can be found in ref 27. The heat conduction term resulting from the flow dispersion can be calculated as follows: W qdisp ̇ = ag*ρg cp , g ΔT , (10) m The coupling scheme of the dominant heat transport mechanisms (Figure 17) together with the one-dimensional analysis of the order of magnitude of each heat transport term represent the basis for the interpretation of the experimentally obtained flame stability data given in Section 3. At this point it is important to emphasize that although alumina and SiSiC sponges are characterized with the same PPI-number and approximately the same porosity (different manufacturer), the difference in their geometry is obvious already by optical inspection (Figure 18) and is caused by the differences in the

Figure 18. Photograph of Al2O3 and SiSiC sponges (10 PPI) illustrating obvious differences in structure topology.

manufacturing process. The different morphology of the sponges will have a strong influence on the sponge property data and also on the flow field inside the sponges and thus on the hydrodynamic dispersion. Therefore, it is very important for the analysis that the geometric parameters as well as the hydrodynamic dispersion were determined for the actual investigated structures. 4.1. Al2O3 Burner. The detected flame stability behavior of the porous burners containing the Al2O3 sponge (Figures 7 and 8) can be explained using the introduced analysis. The physical

Table 3. Estimated Orders of Magnitude of Relevant Heat Transport Mechanisms in Al2O3 Burners PPI

conduction solid [W m−1] (eq 2)

radiation solid [W m−1] (eq 6)

dispersion [W m−1] (eq 10)

gas−solid convection[W m−1] (eq 8)

10 20

102 102

103 103

102−103 102−103

104−105 104−105

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Table 4. Parameter for the Estimation of the Order of Magnitude of Relevant Heat Transport Mechanisms in SiSiC Burners PPI

u0[m s−1]

ε [%]

SV [m−1]

dPe [mm]

Pe [-]

dRe [mm]

ks [W/m2K]

β [m−1]

CNu [-]

mNu [-]

10 20

0.6 −7.5 0.6 −7.5

87 87

473 683

3.24 3.05

88−1100 83−1030

2.18 1.603

40 42.3

178 244

0.146 0.139

0.96 0.92

Table 5. Estimated Orders of Magnitude of Relevant Heat Transport Mechanisms in SiSiC Burners PPI

conduction solid [W m−1] (eq 2)

radiation solid [W m−1] (eq 6)

dispersion [W m−1] (eq 10)

gas−solid convection [W m−1] (eq 8)

10 20

103 103

103 103

102−103 102−103

103−104 103−104

A graphical comparison of the solid transport terms with the ones of gas/solid transfer is given in Figure 19 for various

therefore relevant for the internal heat recirculation in the sponge. The performed one-dimensional volume averaged analysis does not allow for comparison of the terms of the same order of magnitude. Nevertheless the following conclusions can be drawn: • The heat conduction term of SiSiC sponges is for an order of magnitude higher than that of alumina structures (compare Table 3 and 5) and of the same order of magnitude as the radiation term (Table 5). Therefore it is possible that their sum exceeds the magnitude of the convective heat transfer between the two phases. Thus, taking into account the rules introduced in section 4.1 a conclusion whether the gas−solid convection or the heat transport by means of solid conduction and radiation will limit the upstream heat transport along the path S depends on the flow velocity. • The change in the pore size corresponding to the variation of the PPI number from 10 to 20 results in an increase of the sponge thermal conductivity.43 This influences positively the flame stability but is followed by a simultaneous increase in the extinction coefficient,44 which has a negative effect on the flame stability. Since those parallel connected heat transport terms are of the same order of magnitude in case of SiSiC sponges, they are both equally relevant for the heat recirculation and thus for the flame stabilization. Furthermore, as being of the same order of magnitude it is not possible to compare the value of total heat transport through the solid phase for the two investigated SiSiC structures. Due to the excellent heat transport properties of SiSiC sponges the heat is effectively transported to the preheating zone in both burner configurations (K04 and K05). • Preheating the combustion air up to 350 °C (Figure 13) moves the stable operating range of the porous burner to higher excess air ratios, thus higher gas velocities are applied. Under these conditions the K05 burner exhibits a better stabilizing effect for the whole investigated range of thermal loads. The operating temperature window was nearly the same for the stability investigations in the SiSiC burners at different preheating temperatures due to the constraint of the maximal operating temperature of the sponge. Hence, the temperature dependence of the sponge thermal conductivity and emissivity may be neglected in case of the three different air inlet temperatures. Therefore, the different behavior of the two investigated burner configurations at different air preheating temperatures can only be attributed to the variation of the gas velocity and, thus, to the velocity dependent sponge parameters such as heat transfer coefficient α and dispersion effect ag*. The similar phenomenon has been observed in ref 48 where the increase in burning velocities was attributed to elevated influence of turbulence fluctuations on reaction zone. In the present paper the “turbulence in the porous media” is referred to as a flow driven enhanced mixing i.e. dispersion, but the phenomenology behind it is the same as in ref 48.

Figure 19. Qualitative representations of the estimated magnitudes of heat transport terms for SiSiC and Al2O3 sponges.

burner configurations and variation of gas velocity. Intentionally no values are included in the graphical representation as due to the uncertainty of the analytical estimation only orders of magnitude can be compared. The term solid transport corresponds to the sum of the heat transport flows by means of conduction and radiation (full symbols). Open symbols denote heat transfer between gas and solid phase. The gray lines mark Al2O3 burners, and the black lines stand for SiSiC burners. From the first sight it can be seen, that for the Al2O3 case the gas/solid transfer is always much bigger than the solid transport. In contrast, for small velocity values and for the SiSiC case solid transport can be as big as or even bigger than the gas/solid transfer. Furthermore, it can be seen that the gas/ solid transfer curve for 20 PPI SiSiC sponge (K05) is steeper than the one of the 10 PPI SiSiC configuration (K04) due to its higher specific surface (see eq 8). Thus, the intersection point between solid transport and gas/solid transfer curve for 20 PPI SiSiC sponge corresponds to somewhat lower values of the gas velocity than in the case of 10 PPI sponge. Therefore, the velocity range can be subdivided in three domains: I) solid transport is larger than gas/solid transfer for both SiSiC sponges; II) gas/solid transfer is equal or larger than solid transport for 20 PPI sponge but still lower than solid transport for 10 PPI sponge; III) gas/solid transfer is larger than solid transport for both SiSiC sponges. Here one should bear in mind that based on the conducted analysis it cannot be concluded if the solid transport is higher in 10 PPI or in 20 PPI as the changes of solid conduction and radiation with pore density are of the same order of magnitude. 6715


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same PPI (Figure 18) has a strong influence on the flow field in sponge and, thus, on the hydrodynamic dispersion. In order to quantify the above explained flame stability behavior in different sponges, flame velocity ratios FVR were determined as given by eq 11 for all investigated conditions, and the obtained values are listed in Table 6.

4.3. Porous Burner Optimization. For the purpose of comparison, the stability diagrams of all investigated burner configurations are summarized in Figure 20 and Figure 21.

FVR =

SPIM Sl

(11)

Table 6. FVR- and FVR′-Parameter for All Investigated Burner Configurations and Operation Conditions burner configuration

Tair,in [°C]

λ [-]

FVR [-]

FVR′ [-]

K01

25

1.6 1.55 1.5 1.45 2.05 2.0 1.9 1.85 1.8 1.55 1.45 1.95 1.9 1.85 1.6 1.55 1.5 1.45 2.05 1.95 1.9 1.85 1.8 1.85 1.75 1.65 1.55 1.9 1.8 1.7 1.6 2.15 2.1 2.0 1.95 1.9 1.75 1.65 1.55 2 1.8 1.7 1.6 2.2 2.15 2.05 2.0

5.79 6.72 7.32 7.67 4.4 6.73 7.71 9.05 10.13 4.7 4.76 3.64 5.57 7.02 5.81 6.71 7.34 7.74 4.4 6.13 7.71 9.05 10.13 7.69 8.41 8.31 7.86 6.73 7.50 7.60 7.34 5.32 6.45 6.65 7.26 9.06 8.41 8.31 7.86 9.12 7.50 7.60 7.34 5.84 7.08 7.31 7.98

6.82 7.91 8.61 9.02 5.18 7.92 9.08 10.65 11.92 5.56 7.02 4.29 6.56 8.26 7.27 8.38 9.17 9.67 5.51 7.66 9.64 11.32 12.66 8.83 9.66 9.55 9.04 7.73 8.63 8.74 8.44 6.12 7.41 7.65 8.35 10.42 9.66 9.55 9.04 10.49 8.63 8.74 8.44 6.72 8.14 8.41 9.17

Figure 20. Stability diagram for all investigated burner configurations and air inlet temperature 25 °C. 350

K02

25 350

K03

25

350

Figure 21. Stability diagram for all investigated burner configurations and air inlet temperature 350 °C.

Although, due to the different maximal operational temperatures of the sponges it was not possible to perform all the investigations for the same set of conditions, a comparison between different burners can be made for the overlapping areas of the stable burning regions. It can be observed that for all investigated conditions common for all the burner configurations sponges made of SiSiC demonstrate more effective flame stabilization than those made of Al2O3. The observed trend was expected due to the superior heat transport properties of SiSiC sponges to that of Al2O3 structures. However, the maximal operational temperature of Al2O3 sponges (1600 °C) is significantly higher than that of SiSiC sponges (1400 °C), and, thus, for certain applications alumina sponges can be favorable. Figure 20 and Figure 21 reveal that the slopes of the flame stability curves for the SiSiC sponges differ significantly from those of Al2O3 sponges. Besides the fact that the different ranges of excess air ratios were investigated for these two different sponge categories, the difference in the slope of the flame stability curves can be explained with different temperature dependencies of the heat transport coefficients for two different materials.23,49,50 Furthermore, the different morphology of the alumina and SiSiC sponges characterized with the

K04

25

85

350

K05

25

85

350

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reticulate ceramic sponge structure and to optimize its performance, an experimental study on the flame stability and emissions was conducted. The key factors for internal heat recirculation and for flame stabilization as its macroscopic manifestation are heat transport properties of the PIM and the hydrodynamic dispersion. These properties depend on the structure geometry and/or thermophysical properties of the material. The experimental methodology used is based on the variation of the all relevant parameters along the measurement matrix with the dimensions structure geometry, material, and thermodynamic condition. Altogether five different burner configurations were investigated for different thermodynamic conditions achieved by variation of excess air ratio and air preheat temperature. In order to obtain information about the relative contribution of various heat transport processes to heat recirculation and flame stabilization a simplified scheme of global heat recirculation mechanism is used together with the experimental data. The quantification of each relevant heat transport mechanism contribution was achieved using onedimensional volume averaged analysis and comparison with experiments. It was shown that the effect of the variation of pore density (PPI) on the flame stability differs significantly for the two materials investigated (Al2O3 and SiSiC). The conducted analysis showed that the global mechanism of the heat recirculation applies for both materials, but the different stabilizing behavior results from the different relative importance of various heat transport processes contributing to the global scheme. Furthermore, the variation of thermodynamic conditions through the variation of air inlet temperature allowed varying the fluid velocity without changing process temperature. This permitted to draw conclusions regarding the influence of the velocity dependent heat transport processes (gas/solid transfer and hydrodynamic dispersion) and their relative importance for the flame stabilization. Measured flame stability diagrams for all investigated burner configurations and conditions demonstrate an excellent flame stabilization performance. The magnitude of burning velocity approaches even the order of aerodynamically stabilized diffusion flames but without the accompanying high emission levels of the latter. Sponges made of SiSiC demonstrate more effective flame stabilization than those made of Al2O3 for the investigated range of excess air ratios. The observed trend was expected due to the superior heat transport properties of SiSiC sponges to that of Al2O3 sponges playing a key role for the flame stabilization. However, the maximal operational temperature of Al2O3 sponges (1600 °C) is significantly higher than that of SiSiC sponges (1400 °C), and, thus, for certain applications alumina sponges can be favorable. Furthermore, it was found that the temperature operational range achieved in the porous burner ensures that both nitric oxides and carbon monoxide concentrations are very low.

From the experimental data, considering that in case of a stable flame the incoming mixture velocity equals the burning velocity, it was possible to quantify the burning velocity of natural gas/air mixtures in the porous burner as follows

v0 = SPIM =

⎛ AFR stoic·λ Pth ⎜ LHVnat . gas ⎝ ρair (p , T )

+

2 π ·DPIM 4

⎞ 1 ⎟ ρnat . gas (p , T ) ⎠ (12)

where v0 is velocity of incoming gas mixture in the empty tube. SPIM denotes the burning velocity in the porous burner, and AFRstoic = 15.65(kg air/kgf uel) is the air-to-fuel ratio at stoichiometric condition with respect to the natural gas composition. It is common practice to define the burning velocity based on the parameter of the fresh gas mixture and as a volume flow rate per unit of burner cross section. This velocity is easily accessible and is technically relevant for the throughput of the burner. However, the burning velocity in a porous burner is often defined in the literature as the interstitial velocity of the unburned gas mixture.28 In order to enable a comparison between the literature and the present data an additional burning velocity S′PIM as an interstitial velocity of the gas at inlet conditions and the corresponding acceleration factor FVR′ are defined as follows: S′PIM =

SPIM ε

(13)

FVR′ =

S′PIM Sl

(14)

The obtained values for FVR′ are given in Table 6 together with the acceleration factor FVRwhich is relevant for the dimensioning and scaling of the porous burner. The data in Table 6 clearly demonstrate that porous burner technology can provide more than ten times higher burning velocities for the investigated range of excess air ratios and burner configurations in comparison to laminar flames permitting the design of small combustor units for commercial applications and achieving low temperature combustion followed by low NOx emissions. Furthermore, it can be seen that in case of Al2O3 sponges FVR increases sharply with decreasing excess air ratio. The decrease in λ corresponds to the increase in combustion temperature. This confirms the conclusion drawn in section 4.1 that the dominant mechanism for the flame stabilization is heat transfer by solid-to-solid radiation, as it exhibits a strong dependency on process temperature. Nevertheless, the variation of FVR with excess air ratio is less pronounced in the case of SiSiC sponges due to the complex interaction of all relevant heat transport mechanisms and their often opposed changes with λ. The relative importance of solid conduction compared to solid-to-solid radiation increases with increasing excess air ratio due to the corresponding decrease in process temperature. This was demonstrated in ref 28. SiSiC sponges feature very high heat conductivity providing an intensive heat recirculation through the solid phase also for the lean mixtures where the contribution of the solid-to-solid radiation sinks.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0) 721 - 60 82 − 4423. E-mail: neda.djordjevic@ kit.edu.

5. CONCLUSION In order to gain a basic knowledge about physical processes underlying combustion in porous burner that contain a

Notes

The authors declare no competing financial interest. 6717


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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the German Research Foundation (DFG) in the frame of the Research Group FOR 583: Solid Sponges-Application of Monolithic Network Structures in Process Engineering.

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NOMENCLATURE A = cross section [m2] a = thermal diffusivity [m2/s] AFR = air-fuel ratio [kgair/kgfuel] cp = specific heat capacity at constant pressure [J/(kg K)] D = diameter [m] d = characteristic length [m] FVR = flame velocity ratio [-] k = thermal conductivity [W/(m K)] LHV = lower heating value [J/m3] NOx = nitric oxides (NO+NO2) Nuv = volumetric Nusselt number, (α Sv d2)/kg Pe = Peclet number, (d v0)/a PPI = pores per inch Pth = thermal load [W] q̇ = heat flow [W/m] Re = Reynolds number, (ρ v0 d)/μ S = burning velocity [m/s] Sv = specific surface [1/m] T = temperature [K] v0 = gas mixture velocity at sponge inlet [m/s]

Greek symbols

α β ε λ μ ρ σ

heat transfer coefficient [W/(m2 K)] extinction coefficient [1/m] porosity [-] excess air ratio [-] dynamic viscosity [Pa s] density [kg/m3] Stefan−Boltzmann constant [W/(m2 K4)]

Subscripts

cond disp G-S g l nat.gas PIM r s stoic v

conduction dispersion gas−solid gas laminar natural gas porous inert media (sponge) radiation solid stoichiometric volumetric

Superscripts

* enhanced by dispersion ‘ interstitial

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REFERENCES

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Experimental Study on the Basic Phenomena of Flame Stabilization

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