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Lifted flame behavior of a non-premixed oxy-methane jet in a lab-scale slot burner
Fuel 103 (2013) 862–868
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Lifted flame behavior of a non-premixed oxy-methane jet in a lab-scale slot burner Jeongseog Oh ⇑, Dongsoon Noh Energy Efficiency Department, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
h i g h l i g h t s " The characteristics of oxy-methane flames were studied in a lab-scale slot burner. " The flame stability was enhanced when the slot width became narrower. " The empirical formula of the flame length was suggested for a combustor design.
a r t i c l e
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Article history: Received 27 April 2012 Received in revised form 18 September 2012 Accepted 19 September 2012 Available online 4 October 2012 Keywords: Oxy-fuel combustion Slot burner Non-premixed flame Flame stabilization Industrial furnace design
a b s t r a c t The characteristics of oxy-methane flames in atmospheric conditions (298 K and 1 bar) were studied in a lab-scale slot burner. As a diagnostic tool, OH⁄, CH⁄, and C2 measurements were used to analyze the flame structure. To estimate the flame stabilization, the flow velocity was varied from uF = 7–50 m/s for methane gas and uOx = 10–170 m/s for oxygen gas. From the experimental results, the flame stability was enhanced when the slot width became narrower. The flame length increased with increasing fuel quantity, while the lift-off height increased with increase in the fuel jet and oxygen velocity. Using the concept of effective diameter, an empirical formula for flame length was suggested for use in a combustor design. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the usage of fossil fuels has caused problems due to climate change and global warming [1]. Because the level of the usage of fossil fuels is related to the production of greenhouse gases (CO2, CH4, etc.), new technology in the field of combustion is needed in order to handle the crisis of climate change and the global warming. As one of the efforts to reduce the emission of greenhouse gases, the concept of carbon capture and storage (CCS) was suggested [2,3]. CCS technology consists of three components: pre-combustion, during-combustion, and post-combustion. The processes of precombustion and post-combustion are related to the usage of filters or absorbents. In the field of combustion, the method of duringcombustion is highly regarded. With regard to the techniques of during-combustion, one of the core technologies is oxy-fuel combustion. With oxy-fuel combustion, CO2 emissions can be captured directly because the products at stoichiometric conditions are composed of CO2 and H2O after the chemical reaction in the combustors. In particular, in the fields of the manufacturing of iron, ⇑ Corresponding author. Tel.: +82 42 860 3479; fax: +82 42 860 3133. E-mail address: [email protected] (J. Oh). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.055
cement, and ceramics as well as power generation, which is one of the major industries that emit CO2 gases, oxy-fuel combustion has a great ripple effect because it can be substituted into existing facilities that are currently based on combustion using air. In a similar way, the technology of oxygen-enriched combustion was investigated for and applied to coal-fired power generation [4–7]. In coal-fired power stations, the usage of oxygen-enriched combustion has the merit of flame stabilization, CO2 capture, and a more compact burner design than that of air combustion. However, due to the high flame temperature, additional equipment is needed, such as flue gas recirculation equipment. To improve the efficiency of coal-fired power plants, a combined cycle with supercritical and ultra-supercritical boilers is used. Recently, in order to reduce the recirculation of flue gases, the technique of fuel staging combustion was introduced [8]. With fuel staging techniques, the flame temperature can be controlled by combustion at non-stoichiometric conditions. Oxy-fuel combustion with natural gas has been studied by previous researchers [9–13]. Sautet et al. studied the normalized flame length of non-premixed oxy-methane jets in order to apply real-time flame control and derived an empirical formula that is expressed as a function of the fuel jet and coaxial air diameter [9]. Boushaki et al. investigated the effect of separation distance
J. Oh, D. Noh / Fuel 103 (2013) 862–868
863
Nomenclature a.u. CCS CH⁄ C2 DF dF dF,eff dOx f# FrF FWHM g H ICCD L l lpm MFC MFM OH⁄
auxiliary units carbon capture and storage chemiluminescence from methylidyne radicals (CH) chemiluminescence from diatomic carbon radicals (C2) mass diffusivity of fuel jet (m2/s). width of a fuel jet nozzle exit (mm) effective fuel jet nozzle width (mm) width of an oxygen nozzle exit (mm) F-number of a lens aperture Froude number of fuel jet ð¼ u2F =ðdF gÞÞ full width half maximum acceleration due to gravity (=9.8 m/s2) liftoff height (mm) intensified charge-coupled device flame length (mm) slot depth (mm) liters per minute mass flow controller mass flow meter chemilumenescence from hydroxide radicals (OH)
between the fuel jet and the oxygen nozzles on the flame length and liftoff height [11]. They reported that the mixedness of fuel and oxidant is an important parameter in determining flame behaviors. To control oxy-fuel flames, Boushaki et al. used tangential actuators in the fuel jet and oxygen nozzles [10]. By using swirl intensity in each nozzle, they could control the mixedness of reactants and the flame behaviors. In the current study, the characteristics of non-premixed oxymethane flames were investigated in a lab-scale slot burner: the flame stabilization and structure. The objectives are to study the effect of the slot width, temperature, and dilution gas on flame stabilization, to analyze the structure of non-premixed oxy-methane flames from OH⁄, CH⁄, and C2 images, and to derive the experimental approximation of the flame length for a combustor design.
2. Experimental methods 2.1. Experimental setup To study the behaviors of non-premixed oxy-methane flames, the flame stabilization and the flame structure were experimentally investigated by using OH⁄, CH⁄, and C2 measurements at an ambient temperature and atmospheric pressure (298 K and 1 bar). As shown in Fig. 1, the experimental setup consists of a lab-scale slot burner, an intensified charged couple device (ICCD) camera, and a data acquisition system. In the center of the facility setup, a lab-scale slot burner was employed with a delivery line and an exhaust duct. In the current study, a lab-scale slot burner was used. Compared with an axisymmetric Bunsen burner, a lab-scale slot burner has the benefit of allowing the observation of a 2-dimensional flame profile. According to Pun et al. [14], a 3-dimensional line-of-sight image cannot capture the fine flame structure, because the light emission of chemiluminescence is integrated through the depth of the flame. There are several methods by which to observe the cross section of the flame structure, such as planar laser induced fluorescence in terms of hardware or tomography in terms of software. However, a slot burner has another advantage: heat transfer in the form of radiation energy.
ReF Sc Se SL TAd tex uF uOx w /G
qb qeff qOx qF qu h
mF
Reynolds number of fuel jet (=uF dF/mF) Schmidt number (=mF/D) edge flame propagation velocity (m/s) laminar burning velocity (m/s) adiabatic flame temperature (K) exposure time of an ICCD camera (ls) fuel jet velocity at a nozzle exit (m/s) oxygen velocity at a nozzle exit (m/s) separation distance between two nozzles of fuel jet and oxygen (mm) global equivalence ratio density of burned gas (kg/m3) effective gas density (kg/m3) oxygen gas density (kg/m3) fuel gas density (kg/m3) density of unburned gas (kg/m3) angle of flame slope (°) kinematic viscosity of fuel jet (m2/s)
Fig. 2 shows the burner geometry and an interesting area (100 200 mm), used in the current study, which has two slot nozzles for oxygen (dOx = 0.52 mm) and methane (dF = 0.24 mm). The shape of a slot burner was cubic type (50 50 50 mm) with a slot width of 20 mm. Two stainless tubes were used for oxygen and methane supplying with an external diameter of 6.35 mm. The edge of each tube end was trimmed to prevent flow chocking. The delivery lines of oxygen and methane were connected with both side of the slot burner. Here, the flame length (L) was defined as the distance from flame base to flame tip. At that time, the threshold value was 25% of the maximum intensity in each image. With the same threshold value as that of the flame length, the liftoff height (H) was defined as the distance from the slot plate to the flame base and the flame slope (h) was defined as the flame turning angle from the axial direction normal to the slot plate to the parallel line connecting the flame base and the flame tip. The combustion section was partially shielded with four quartz plates (400 400 5 mm) to prevent external disturbances, allowing for optical assessments. Each plate was distanced from nozzle exits to prevent flame–wall interaction. Gas-phase oxygen and methane were used as reactants. The purity of reactants was 99.99% for methane and 99.95% for oxygen. Reactants were supplied into fuel and oxygen feeding lines, respectively, after being regulated by two mass flow meters (100 lpm; 5851E, Brooks Instrument Co., Hatfield, PA, USA), which had a linearity of over 97% in the range from 10 to 100 lpm. To ensure the reproducibility of the flow rate, the mass-flow-meter (MFM) was calibrated with a dry gas meter (100 lpm; DA-16A-T, Sinagawa Co., Tokyo, Japan), which has an accuracy of 95%. During calibration, the flow rate signal from the dry gas meter was taken for 100 s. At that time, the uncertainty of the time-averaged data was less than 0.3%. With the calibrated data of flow rate, the MFM was controlled by using a Lab-VIEW utility (NI cDAQ 9172 and 9205, National Instruments Co., Austin, TX, USA). For the chemiluminescence measurement of the OH, CH, and C2 radicals, an ICCD camera (PI-MAX 512; Princeton Instruments Inc., Trenton, NJ, USA) was used with an F-mount macro UV lens (105 mm; Nikor f/4.5 UV, Nikon Co., Tokyo, Japan). To capture the light emissions of the OH, CH, and C2 radicals, the macro lens was separately equipped with narrow band pass filters
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Fig. 1. Schematic diagram of the experimental setup: OH⁄, CH⁄, and C2 measurements.
the fuel jet (FrF) was defined as FrF = uF2/(dF g), where g is the acceleration due to gravity (=9.8 m/s2). The test conditions are summarized in Table 1. During chemiluminescence measurements with an ICCD camera, the exposure time (tex) was fixed at tex = 100 ls and F-number (f#) was f# = 8. In each case, 10 images were accumulated to take a highly-defined and time-averaged image. The gain value of an ICCD camera was 75 for OH⁄, 10 for CH⁄, and 20 for C2 to get high-quality images and to avoid local saturation of light intensity, which was dependent on radical selection. In case of CH⁄ and C2 measurements, even though the minimum transmittance of a C2 filter (50%) is higher than CH⁄ (45%), the gain value of an ICCD camera for C2 images (20) was higher than that of CH⁄ (10), because the absolute radiation intensity of C2 is lower than CH⁄. 3. Results and discussion 3.1. Flame stabilization
Fig. 2. Definition of the flame length (L) and the liftoff height (H).
(307FS10-50, 430FS10-50, and 515FS03-50; Andover Co., Salem, NH, USA). The center wave lengths of the filter were 307.1 ± 3 nm for OH⁄, 430.0 ± 3 nm for CH⁄, and 514.5 ± 0.5 nm for C2 . The full-width-half-maximums (FWHM) were 10 ± 2 nm for OH⁄, 10 ± 2 nm for CH⁄, and 3 ± 0.5 for C2 . The minimum transmittance was 15% for OH⁄, 45% for CH⁄, and 50% for C2 . 2.2. Experimental condition To create the flame stabilization curve, the velocity at the oxygen and fuel jet nozzle exits was varied from uF = 7–50 m/s for methane and from uOx = 10–100 m/s for oxygen. The Reynolds number of the fuel jet (ReF) was defined as ReF = uF dF/mF, where mF is the kinematic viscosity of the fuel jet. The Froude number of
Oxy-fuel combustion has more flame stability than combustion in air, due to its fast reaction rate. These characteristics of the fast reaction rate lead to intensive reactions and a small flame size, which have the advantages of lowering convection time and resisting external disturbances. Fig. 3 shows the effect of the separation distance between the fuel jet and the oxygen nozzles on the flame stabilization. The separation distance of slot width (w) was varied from w = 1–7 mm in Fig. 3a and from w = 8–20 mm in Fig. 3b. The flame stability curve was plotted with increasing fuel jet and oxygen velocity. The flame stabilization map of the non-premixed oxy-methane flames consists of the attached flame region, the liftoff flame region, and the blow-off and blowout region. However Fig. 3a was composed of the attached flame region and the blow-off region. In Fig. 3b, the attached flame region and lifted flame region were increased with decreasing slot width separation. Physically, the separation of slot width is thought to act as a dump plane which affects the mixedness of the fuel jet and oxygen gases. The usage of a slot burner has the benefit of allowing the observation of the 2-dimensional profile of non-premixed oxy-methane flames. Even though helical instability could occur at both slot edges of the nozzles, this effect was negligible compared to the whole flame’s behaviors. However, during experiments, the
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J. Oh, D. Noh / Fuel 103 (2013) 862–868 Table 1 Experimental conditions.
uF (m/s) uOx (m/s) ReF FrF (106) /G
ReOx FrOx (106)
Case 1
Case 2
Case 3
Case 4
Case 5
25 50 349 0.26 0.462
35 50 498 0.52 0.646
45 50 628 0.86 0.831
35 20 498 0.52 1.613
35 80 498 0.52 0.404
1731 0.49
1731 0.49
1731 0.49
692 0.07
2769 1.25
Fig. 4. Flame stabilization map when w = 8 mm: the attached flame region, the liftoff region, and the blow out region.
Fig. 3. Flame stability map as varying the separation distance of slot width (w): (a) w = 1–7 mm and (b) w = 8–20 mm.
fluctuation of the flame base was observed. This is thought to be due to the Carman vortex, which is generated on a slot plate between the fuel jet and oxygen nozzles. According to Chung [15], the stabilization point of the edge flames in the free jets moves along a stoichiometric line. The stability of the edge flames is dependent on the Schmidt number of the reactants (Sc). In cases in which Sc < 1, non-premixed lifted methane–air flames have an unstable nature and may be directly blown off when the flame base (edge) is perturbed. However, he mentioned that the lifted methane flame can be stabilized through the effects of buoyancy, near-nozzle flow, and the dilution of the fuel stream. 3.2. Flame structure On the basis of the flame stability curve, as shown in Fig. 3, a slot width (w) of w = 8 mm was selected because the flame stabilization at that width was better than it was at others and the liftoff flame region was the most broad. This means that a combustor with a slot width of w = 8 mm has a wide range of operations and has good performance in terms of flame stabilization. With a fixed slot width of w = 8 mm, the fuel jet velocity (uF) to uF = 25–45 m/s and oxygen velocity (uOx) to uOx = 20–80 m/s were
changed in order to observe the general trend of flame behaviors. The detailed experimental conditions were summarized in Table 1. Fig. 4 shows the flame stabilization map of w = 8 mm, which is divided into three regions: the attached flame region, the lifted flame region, and the blowout region. The experimental conditions, listed in Table 1, were marked in Fig. 4. A dotted line indicates the stoichiometric conditions and that the global equivalence ratio of injecting oxygen and methane gases is unity. In real industrial furnaces in work sites, these conditions are used for complete combustion and economical operation. In the current study, lifted flames were focused on. Due to the high flame temperature of oxy-fuel flames, the continual usage of the attached flame can damage burner nozzles and increase the maintenance expenditure in real industrial furnaces. Fig. 5 shows photos of non-premixed oxy-methane flames in a slot burner while varying the velocity of the fuel jet and oxygen in Cases 1–5, which were taken with the chemiluminescence measurements of OH, CH, and C2 radicals. In Fig. 5, flames in the OH⁄ images were longer and broader than those of CH⁄ and C2 . In general, flames became longer when fuel jet velocity increased at a fixed oxygen velocity. Meanwhile, flames became shorter when oxygen velocity increased at a fixed fuel jet velocity. In addition, flames were tilted slightly to the right (to the methane side). For a more detailed observation, the flame appearance of OH⁄ and CH⁄ images were compared. Fig. 6 shows the flame structure of a non-premixed oxy-methane flame in Case 2 (uF = 35 m/s and uOx = 50 m/s). The left-hand side is the OH⁄ image, while the right-hand side is the CH⁄ image. The flame appearance is generally feather-shaped. The flame base is narrow and sharp and it looks like the base of an edge flame. The chemiluminescence of OH and CH radicals is known to act as an indicator of heat release rate and reaction zone [16]. In general, the zone of OH radical indicates the presence of combustion products, while the zone of CH radical indicates the presence of fuel decomposition [17]. In other words, CH radical has a two-body reaction, while OH radical has a three-body reaction. As shown in Fig. 6, this is the reason why the flame in the OH⁄ photo is larger than that of the CH⁄ photo.
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Case 1
Case 2
Case 3
Case 4
Case 5
OH* (307.1±5 mm)
(a) CH* (430±5 nm)
(b) C 2* (514.1±2 nm)
(c) Fig. 5. Flame appearance of non-premixed oxy-methane jet in Cases 1–5, as a result of (a) OH⁄, (b) CH⁄, and (c) C2 measurements (photo size: 238 by 238 mm).
Fig. 6. Comparison of the flame structure in non-premixed oxy-methane jets in Case 2 (uF = 35 m/s and uOx = 50 m/s): chemiluminescence photos taken via OH radicals (left) and CH radicals (right).
3.3. Flame behavior To observe the behavior of oxy-methane flames, the flame base and the liftoff height were estimated from the measurement of OH chemiluminescence (OH⁄). Fig. 7 shows the propensity of the flame length and liftoff height behavior as varying fuel jet velocity and oxygen velocity. The flame length increased with the increase in fuel jet velocity, because the increase of fuel jet velocity led to the increase of fuel quantity and residence time for complete combustion. In Fig. 7a, the liftoff height increased with the increase in fuel jet velocity. The flame base is known to be stabilized at the point at which the flame propagation velocity is balanced to the local flow velocity [18]. With the similar flame propagation, the flame base is thought to move downstream, when the fuel jet nozzle velocity is increased. However, contrary to Fig. 7a, the flame length and the liftoff height decreased with increases in oxygen velocity, as shown in Fig. 7b. At a fixed fuel jet velocity, an increase in oxygen velocity led to the decrease of the global equivalence ratio and the increase of local mixing due to turbulence, which mean that the additional oxygen helps to enhance the chemical reaction with the injected fuel gas. As a result of the increase in oxygen quantity, it is thought
Fig. 7. Trend lines of the flame length (L) and the liftoff height (H), extracted from CH⁄ images: the effect of (a) fuel jet velocity and (b) oxygen velocity.
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that the mixedness of oxygen and methane became better and that the local reaction rate increased. If the local reaction rate increased due to better mixing, the flame propagation velocity at the flame base increased, the flame base moved upstream, and the liftoff height decreased. According to Chung [15], the propagation velocity at the edge flame base (Se) is affected by the density of unburned gas (qu) and burned gas (qb) at stoichiometric conditions. The equation for edge flame propagation is as follows:
Se ðqu =qb Þ1=2 SL
ð1Þ
where SL is the laminar burning velocity of oxy-methane flames. If the lifted oxy-methane flames have an edge flame structure, the liftoff height decreases with an increase in oxygen velocity, because the density of oxygen gas is larger than that of methane and the addition of oxygen gas increases the density of the unburned gas at a fixed fuel jet velocity. In general, the image of OH⁄ shows more distributed and broaden reaction zone than that of CH⁄. This is because the zone of OH radical indicates the presence of combustion products, while the zone of CH radical indicates the presence of fuel decomposition, as mentioned above. In particular, the distribution of OH radicals was effected by gravity at a flame tip, so that flame shapes in OH⁄ images were curved as going downstream. Fig. 8 shows the tendency of the flame slope from the results of the CH chemiluminescence (CH⁄) measurements. As shown in
Fig. 5, the flame is generally inclined to the fuel jet side. In Case 2, the degree of flame slope (h) was estimated as being about h = 9.8°. The flame slope decreased with the increase in fuel jet velocity at a fixed oxygen velocity in Fig. 8a. This is thought to be due to the movement of stoichiometric lines and the gradient of the mixture fraction [19]. Fig. 8b shows the effect of the variation of oxygen velocity on the flame slope. When oxygen velocity is increased at a fixed fuel jet velocity, the flame slope became more inclined toward the fuel jet side. Because the density of oxygen gas was larger than that of methane gas, the non-premixed oxy-methane flame was thought to lean toward the methane side, which had a lower momentum than that of oxygen. For a simple understanding of the behaviors of non-premixed oxy-methane flames, the concept of effective diameter was introduced. The idea of effective diameter was to treat the two separate nozzles for fuel and oxygen like a single diameter conceptually. The effective diameter is expressed as the ratio of momentum to mass flux of fuel and air [20]. The effective diameter was used to normalize the flame length, which was helpful in designing combustors and furnaces. In physical meaning, the normalized flame length using the effective diameter is the characteristic length of the conceptual equivalent source that would flow in the same quantity as the actual source, such as mass flux and momentum flux [21]. The scaling of the flame length can be derived from the scaling for the mixture fraction (c(x)) as c(x) (x/deff)1, where x is the axial distance from a nozzle exit. If the flame length is defined as the axial distance from a nozzle exit to the point where the mixture fraction reaches stoichiometric conditions, the equation for self similarity in the far field can be expressed as cðLÞ 1= 1 þ fs0 , 0 where fs is the stoichiometric ambient-to-source fluid mass ratio for a homogeneous source fluid mixture. From the combination of both equations, the equation of flame length scaling is derived as L 1 þ fs0 deff [21]. In a far field, the density difference between the fuel jet and the ambient air can be assumed to have same value as that of a premixed mixture. Meanwhile, in a near field, the density difference between the fuel jet and the ambient air is significant as that near the nozzles [20]. The equations for the effective diameter in the far-field concept and the near-field concept are as follows:
dF;eff ¼ dF
dF;eff ¼ dF
1=2
1=2
qeff qOx qF qeff
in the far-field concept
in the near-field concept
ð2Þ
where
qeff ¼
Fig. 8. Trend lines of the flame slope (h), extracted from CH⁄ images: the effect of (a) fuel jet velocity and (b) oxygen velocity.
qF u2F ðdF lÞ þ qOx u2Ox ðdOx lÞ u2F ðdF lÞ þ u2Ox ðdOx lÞ
where dF,eff is the effective fuel jet nozzle width, dF is the fuel jet nozzle width, dOx is the oxygen nozzle width, qeff is the effective gas density, qF is the fuel gas density, qOx is the oxygen gas density, uF is the velocity at a fuel jet nozzle exit, uOx is the velocity at an oxygen nozzle exit, and l is the slot depth. Fig. 9 shows the tendency of the normalized flame length, applying the effective diameter in the far-field concept and nearfield concept. Data for the flame length was sampled in Fig. 7. As a result of curve fitting on plotted data, the normalized flame length (L/dF,eff) decreased exponentially with the increase in the velocity ratio of oxygen and fuel (uOx/uF). Applying the far-field concept and the near-field concept to normalize the flame length, the experimental approximation is as follows:
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(4) By introducing the concept of the effective diameter (or momentum diameter), the experimental approximation of the flame length was suggested for a combustor design.
References
Fig. 9. Experimental approximation with the concept of the effective diameter.
L dF;eff
1=2 uOx uF
ð3Þ
In the current study of non-premixed oxy-methane flames, the normalized flame length in the far-field concept and the near-field concept showed similar trends, as plotted in Fig. 9. 4. Conclusion In the current study, the behaviors of non-premixed oxymethane flames in a lab-scale slot burner were investigated: the flame stabilization and structure. To observe the flame structure, photos from OH⁄, CH⁄, and C2 signals were taken. The trend of the flame length and the lift-off height were measured from CH chemiluminescence (CH⁄) images. From the above measurements, it was concluded as follows: (1) The flame stabilization became enhanced, when the separation distance between the fuel jet and the oxygen nozzles was narrower. (2) The flame length increased with an increase of fuel quantity, while the lift-off height increased with an increase of fuel jet and oxygen velocity. (3) The flame slope decreased with the increase in fuel jet velocity at a fixed oxygen velocity. And the flame slope became more inclined toward the fuel jet side when oxygen velocity is increased at a fixed fuel jet velocity.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Lifted flame behavior of a non-premixed oxy-methane jet in a lab-scale slot burner Jeongseog Oh ⇑, Dongsoon Noh Energy Efficiency Department, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
h i g h l i g h t s " The characteristics of oxy-methane flames were studied in a lab-scale slot burner. " The flame stability was enhanced when the slot width became narrower. " The empirical formula of the flame length was suggested for a combustor design.
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 18 September 2012 Accepted 19 September 2012 Available online 4 October 2012 Keywords: Oxy-fuel combustion Slot burner Non-premixed flame Flame stabilization Industrial furnace design
a b s t r a c t The characteristics of oxy-methane flames in atmospheric conditions (298 K and 1 bar) were studied in a lab-scale slot burner. As a diagnostic tool, OH⁄, CH⁄, and C2 measurements were used to analyze the flame structure. To estimate the flame stabilization, the flow velocity was varied from uF = 7–50 m/s for methane gas and uOx = 10–170 m/s for oxygen gas. From the experimental results, the flame stability was enhanced when the slot width became narrower. The flame length increased with increasing fuel quantity, while the lift-off height increased with increase in the fuel jet and oxygen velocity. Using the concept of effective diameter, an empirical formula for flame length was suggested for use in a combustor design. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the usage of fossil fuels has caused problems due to climate change and global warming [1]. Because the level of the usage of fossil fuels is related to the production of greenhouse gases (CO2, CH4, etc.), new technology in the field of combustion is needed in order to handle the crisis of climate change and the global warming. As one of the efforts to reduce the emission of greenhouse gases, the concept of carbon capture and storage (CCS) was suggested [2,3]. CCS technology consists of three components: pre-combustion, during-combustion, and post-combustion. The processes of precombustion and post-combustion are related to the usage of filters or absorbents. In the field of combustion, the method of duringcombustion is highly regarded. With regard to the techniques of during-combustion, one of the core technologies is oxy-fuel combustion. With oxy-fuel combustion, CO2 emissions can be captured directly because the products at stoichiometric conditions are composed of CO2 and H2O after the chemical reaction in the combustors. In particular, in the fields of the manufacturing of iron, ⇑ Corresponding author. Tel.: +82 42 860 3479; fax: +82 42 860 3133. E-mail address: [email protected] (J. Oh). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.055
cement, and ceramics as well as power generation, which is one of the major industries that emit CO2 gases, oxy-fuel combustion has a great ripple effect because it can be substituted into existing facilities that are currently based on combustion using air. In a similar way, the technology of oxygen-enriched combustion was investigated for and applied to coal-fired power generation [4–7]. In coal-fired power stations, the usage of oxygen-enriched combustion has the merit of flame stabilization, CO2 capture, and a more compact burner design than that of air combustion. However, due to the high flame temperature, additional equipment is needed, such as flue gas recirculation equipment. To improve the efficiency of coal-fired power plants, a combined cycle with supercritical and ultra-supercritical boilers is used. Recently, in order to reduce the recirculation of flue gases, the technique of fuel staging combustion was introduced [8]. With fuel staging techniques, the flame temperature can be controlled by combustion at non-stoichiometric conditions. Oxy-fuel combustion with natural gas has been studied by previous researchers [9–13]. Sautet et al. studied the normalized flame length of non-premixed oxy-methane jets in order to apply real-time flame control and derived an empirical formula that is expressed as a function of the fuel jet and coaxial air diameter [9]. Boushaki et al. investigated the effect of separation distance
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863
Nomenclature a.u. CCS CH⁄ C2 DF dF dF,eff dOx f# FrF FWHM g H ICCD L l lpm MFC MFM OH⁄
auxiliary units carbon capture and storage chemiluminescence from methylidyne radicals (CH) chemiluminescence from diatomic carbon radicals (C2) mass diffusivity of fuel jet (m2/s). width of a fuel jet nozzle exit (mm) effective fuel jet nozzle width (mm) width of an oxygen nozzle exit (mm) F-number of a lens aperture Froude number of fuel jet ð¼ u2F =ðdF gÞÞ full width half maximum acceleration due to gravity (=9.8 m/s2) liftoff height (mm) intensified charge-coupled device flame length (mm) slot depth (mm) liters per minute mass flow controller mass flow meter chemilumenescence from hydroxide radicals (OH)
between the fuel jet and the oxygen nozzles on the flame length and liftoff height [11]. They reported that the mixedness of fuel and oxidant is an important parameter in determining flame behaviors. To control oxy-fuel flames, Boushaki et al. used tangential actuators in the fuel jet and oxygen nozzles [10]. By using swirl intensity in each nozzle, they could control the mixedness of reactants and the flame behaviors. In the current study, the characteristics of non-premixed oxymethane flames were investigated in a lab-scale slot burner: the flame stabilization and structure. The objectives are to study the effect of the slot width, temperature, and dilution gas on flame stabilization, to analyze the structure of non-premixed oxy-methane flames from OH⁄, CH⁄, and C2 images, and to derive the experimental approximation of the flame length for a combustor design.
2. Experimental methods 2.1. Experimental setup To study the behaviors of non-premixed oxy-methane flames, the flame stabilization and the flame structure were experimentally investigated by using OH⁄, CH⁄, and C2 measurements at an ambient temperature and atmospheric pressure (298 K and 1 bar). As shown in Fig. 1, the experimental setup consists of a lab-scale slot burner, an intensified charged couple device (ICCD) camera, and a data acquisition system. In the center of the facility setup, a lab-scale slot burner was employed with a delivery line and an exhaust duct. In the current study, a lab-scale slot burner was used. Compared with an axisymmetric Bunsen burner, a lab-scale slot burner has the benefit of allowing the observation of a 2-dimensional flame profile. According to Pun et al. [14], a 3-dimensional line-of-sight image cannot capture the fine flame structure, because the light emission of chemiluminescence is integrated through the depth of the flame. There are several methods by which to observe the cross section of the flame structure, such as planar laser induced fluorescence in terms of hardware or tomography in terms of software. However, a slot burner has another advantage: heat transfer in the form of radiation energy.
ReF Sc Se SL TAd tex uF uOx w /G
qb qeff qOx qF qu h
mF
Reynolds number of fuel jet (=uF dF/mF) Schmidt number (=mF/D) edge flame propagation velocity (m/s) laminar burning velocity (m/s) adiabatic flame temperature (K) exposure time of an ICCD camera (ls) fuel jet velocity at a nozzle exit (m/s) oxygen velocity at a nozzle exit (m/s) separation distance between two nozzles of fuel jet and oxygen (mm) global equivalence ratio density of burned gas (kg/m3) effective gas density (kg/m3) oxygen gas density (kg/m3) fuel gas density (kg/m3) density of unburned gas (kg/m3) angle of flame slope (°) kinematic viscosity of fuel jet (m2/s)
Fig. 2 shows the burner geometry and an interesting area (100 200 mm), used in the current study, which has two slot nozzles for oxygen (dOx = 0.52 mm) and methane (dF = 0.24 mm). The shape of a slot burner was cubic type (50 50 50 mm) with a slot width of 20 mm. Two stainless tubes were used for oxygen and methane supplying with an external diameter of 6.35 mm. The edge of each tube end was trimmed to prevent flow chocking. The delivery lines of oxygen and methane were connected with both side of the slot burner. Here, the flame length (L) was defined as the distance from flame base to flame tip. At that time, the threshold value was 25% of the maximum intensity in each image. With the same threshold value as that of the flame length, the liftoff height (H) was defined as the distance from the slot plate to the flame base and the flame slope (h) was defined as the flame turning angle from the axial direction normal to the slot plate to the parallel line connecting the flame base and the flame tip. The combustion section was partially shielded with four quartz plates (400 400 5 mm) to prevent external disturbances, allowing for optical assessments. Each plate was distanced from nozzle exits to prevent flame–wall interaction. Gas-phase oxygen and methane were used as reactants. The purity of reactants was 99.99% for methane and 99.95% for oxygen. Reactants were supplied into fuel and oxygen feeding lines, respectively, after being regulated by two mass flow meters (100 lpm; 5851E, Brooks Instrument Co., Hatfield, PA, USA), which had a linearity of over 97% in the range from 10 to 100 lpm. To ensure the reproducibility of the flow rate, the mass-flow-meter (MFM) was calibrated with a dry gas meter (100 lpm; DA-16A-T, Sinagawa Co., Tokyo, Japan), which has an accuracy of 95%. During calibration, the flow rate signal from the dry gas meter was taken for 100 s. At that time, the uncertainty of the time-averaged data was less than 0.3%. With the calibrated data of flow rate, the MFM was controlled by using a Lab-VIEW utility (NI cDAQ 9172 and 9205, National Instruments Co., Austin, TX, USA). For the chemiluminescence measurement of the OH, CH, and C2 radicals, an ICCD camera (PI-MAX 512; Princeton Instruments Inc., Trenton, NJ, USA) was used with an F-mount macro UV lens (105 mm; Nikor f/4.5 UV, Nikon Co., Tokyo, Japan). To capture the light emissions of the OH, CH, and C2 radicals, the macro lens was separately equipped with narrow band pass filters
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Fig. 1. Schematic diagram of the experimental setup: OH⁄, CH⁄, and C2 measurements.
the fuel jet (FrF) was defined as FrF = uF2/(dF g), where g is the acceleration due to gravity (=9.8 m/s2). The test conditions are summarized in Table 1. During chemiluminescence measurements with an ICCD camera, the exposure time (tex) was fixed at tex = 100 ls and F-number (f#) was f# = 8. In each case, 10 images were accumulated to take a highly-defined and time-averaged image. The gain value of an ICCD camera was 75 for OH⁄, 10 for CH⁄, and 20 for C2 to get high-quality images and to avoid local saturation of light intensity, which was dependent on radical selection. In case of CH⁄ and C2 measurements, even though the minimum transmittance of a C2 filter (50%) is higher than CH⁄ (45%), the gain value of an ICCD camera for C2 images (20) was higher than that of CH⁄ (10), because the absolute radiation intensity of C2 is lower than CH⁄. 3. Results and discussion 3.1. Flame stabilization
Fig. 2. Definition of the flame length (L) and the liftoff height (H).
(307FS10-50, 430FS10-50, and 515FS03-50; Andover Co., Salem, NH, USA). The center wave lengths of the filter were 307.1 ± 3 nm for OH⁄, 430.0 ± 3 nm for CH⁄, and 514.5 ± 0.5 nm for C2 . The full-width-half-maximums (FWHM) were 10 ± 2 nm for OH⁄, 10 ± 2 nm for CH⁄, and 3 ± 0.5 for C2 . The minimum transmittance was 15% for OH⁄, 45% for CH⁄, and 50% for C2 . 2.2. Experimental condition To create the flame stabilization curve, the velocity at the oxygen and fuel jet nozzle exits was varied from uF = 7–50 m/s for methane and from uOx = 10–100 m/s for oxygen. The Reynolds number of the fuel jet (ReF) was defined as ReF = uF dF/mF, where mF is the kinematic viscosity of the fuel jet. The Froude number of
Oxy-fuel combustion has more flame stability than combustion in air, due to its fast reaction rate. These characteristics of the fast reaction rate lead to intensive reactions and a small flame size, which have the advantages of lowering convection time and resisting external disturbances. Fig. 3 shows the effect of the separation distance between the fuel jet and the oxygen nozzles on the flame stabilization. The separation distance of slot width (w) was varied from w = 1–7 mm in Fig. 3a and from w = 8–20 mm in Fig. 3b. The flame stability curve was plotted with increasing fuel jet and oxygen velocity. The flame stabilization map of the non-premixed oxy-methane flames consists of the attached flame region, the liftoff flame region, and the blow-off and blowout region. However Fig. 3a was composed of the attached flame region and the blow-off region. In Fig. 3b, the attached flame region and lifted flame region were increased with decreasing slot width separation. Physically, the separation of slot width is thought to act as a dump plane which affects the mixedness of the fuel jet and oxygen gases. The usage of a slot burner has the benefit of allowing the observation of the 2-dimensional profile of non-premixed oxy-methane flames. Even though helical instability could occur at both slot edges of the nozzles, this effect was negligible compared to the whole flame’s behaviors. However, during experiments, the
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J. Oh, D. Noh / Fuel 103 (2013) 862–868 Table 1 Experimental conditions.
uF (m/s) uOx (m/s) ReF FrF (106) /G
ReOx FrOx (106)
Case 1
Case 2
Case 3
Case 4
Case 5
25 50 349 0.26 0.462
35 50 498 0.52 0.646
45 50 628 0.86 0.831
35 20 498 0.52 1.613
35 80 498 0.52 0.404
1731 0.49
1731 0.49
1731 0.49
692 0.07
2769 1.25
Fig. 4. Flame stabilization map when w = 8 mm: the attached flame region, the liftoff region, and the blow out region.
Fig. 3. Flame stability map as varying the separation distance of slot width (w): (a) w = 1–7 mm and (b) w = 8–20 mm.
fluctuation of the flame base was observed. This is thought to be due to the Carman vortex, which is generated on a slot plate between the fuel jet and oxygen nozzles. According to Chung [15], the stabilization point of the edge flames in the free jets moves along a stoichiometric line. The stability of the edge flames is dependent on the Schmidt number of the reactants (Sc). In cases in which Sc < 1, non-premixed lifted methane–air flames have an unstable nature and may be directly blown off when the flame base (edge) is perturbed. However, he mentioned that the lifted methane flame can be stabilized through the effects of buoyancy, near-nozzle flow, and the dilution of the fuel stream. 3.2. Flame structure On the basis of the flame stability curve, as shown in Fig. 3, a slot width (w) of w = 8 mm was selected because the flame stabilization at that width was better than it was at others and the liftoff flame region was the most broad. This means that a combustor with a slot width of w = 8 mm has a wide range of operations and has good performance in terms of flame stabilization. With a fixed slot width of w = 8 mm, the fuel jet velocity (uF) to uF = 25–45 m/s and oxygen velocity (uOx) to uOx = 20–80 m/s were
changed in order to observe the general trend of flame behaviors. The detailed experimental conditions were summarized in Table 1. Fig. 4 shows the flame stabilization map of w = 8 mm, which is divided into three regions: the attached flame region, the lifted flame region, and the blowout region. The experimental conditions, listed in Table 1, were marked in Fig. 4. A dotted line indicates the stoichiometric conditions and that the global equivalence ratio of injecting oxygen and methane gases is unity. In real industrial furnaces in work sites, these conditions are used for complete combustion and economical operation. In the current study, lifted flames were focused on. Due to the high flame temperature of oxy-fuel flames, the continual usage of the attached flame can damage burner nozzles and increase the maintenance expenditure in real industrial furnaces. Fig. 5 shows photos of non-premixed oxy-methane flames in a slot burner while varying the velocity of the fuel jet and oxygen in Cases 1–5, which were taken with the chemiluminescence measurements of OH, CH, and C2 radicals. In Fig. 5, flames in the OH⁄ images were longer and broader than those of CH⁄ and C2 . In general, flames became longer when fuel jet velocity increased at a fixed oxygen velocity. Meanwhile, flames became shorter when oxygen velocity increased at a fixed fuel jet velocity. In addition, flames were tilted slightly to the right (to the methane side). For a more detailed observation, the flame appearance of OH⁄ and CH⁄ images were compared. Fig. 6 shows the flame structure of a non-premixed oxy-methane flame in Case 2 (uF = 35 m/s and uOx = 50 m/s). The left-hand side is the OH⁄ image, while the right-hand side is the CH⁄ image. The flame appearance is generally feather-shaped. The flame base is narrow and sharp and it looks like the base of an edge flame. The chemiluminescence of OH and CH radicals is known to act as an indicator of heat release rate and reaction zone [16]. In general, the zone of OH radical indicates the presence of combustion products, while the zone of CH radical indicates the presence of fuel decomposition [17]. In other words, CH radical has a two-body reaction, while OH radical has a three-body reaction. As shown in Fig. 6, this is the reason why the flame in the OH⁄ photo is larger than that of the CH⁄ photo.
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Case 1
Case 2
Case 3
Case 4
Case 5
OH* (307.1±5 mm)
(a) CH* (430±5 nm)
(b) C 2* (514.1±2 nm)
(c) Fig. 5. Flame appearance of non-premixed oxy-methane jet in Cases 1–5, as a result of (a) OH⁄, (b) CH⁄, and (c) C2 measurements (photo size: 238 by 238 mm).
Fig. 6. Comparison of the flame structure in non-premixed oxy-methane jets in Case 2 (uF = 35 m/s and uOx = 50 m/s): chemiluminescence photos taken via OH radicals (left) and CH radicals (right).
3.3. Flame behavior To observe the behavior of oxy-methane flames, the flame base and the liftoff height were estimated from the measurement of OH chemiluminescence (OH⁄). Fig. 7 shows the propensity of the flame length and liftoff height behavior as varying fuel jet velocity and oxygen velocity. The flame length increased with the increase in fuel jet velocity, because the increase of fuel jet velocity led to the increase of fuel quantity and residence time for complete combustion. In Fig. 7a, the liftoff height increased with the increase in fuel jet velocity. The flame base is known to be stabilized at the point at which the flame propagation velocity is balanced to the local flow velocity [18]. With the similar flame propagation, the flame base is thought to move downstream, when the fuel jet nozzle velocity is increased. However, contrary to Fig. 7a, the flame length and the liftoff height decreased with increases in oxygen velocity, as shown in Fig. 7b. At a fixed fuel jet velocity, an increase in oxygen velocity led to the decrease of the global equivalence ratio and the increase of local mixing due to turbulence, which mean that the additional oxygen helps to enhance the chemical reaction with the injected fuel gas. As a result of the increase in oxygen quantity, it is thought
Fig. 7. Trend lines of the flame length (L) and the liftoff height (H), extracted from CH⁄ images: the effect of (a) fuel jet velocity and (b) oxygen velocity.
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that the mixedness of oxygen and methane became better and that the local reaction rate increased. If the local reaction rate increased due to better mixing, the flame propagation velocity at the flame base increased, the flame base moved upstream, and the liftoff height decreased. According to Chung [15], the propagation velocity at the edge flame base (Se) is affected by the density of unburned gas (qu) and burned gas (qb) at stoichiometric conditions. The equation for edge flame propagation is as follows:
Se ðqu =qb Þ1=2 SL
ð1Þ
where SL is the laminar burning velocity of oxy-methane flames. If the lifted oxy-methane flames have an edge flame structure, the liftoff height decreases with an increase in oxygen velocity, because the density of oxygen gas is larger than that of methane and the addition of oxygen gas increases the density of the unburned gas at a fixed fuel jet velocity. In general, the image of OH⁄ shows more distributed and broaden reaction zone than that of CH⁄. This is because the zone of OH radical indicates the presence of combustion products, while the zone of CH radical indicates the presence of fuel decomposition, as mentioned above. In particular, the distribution of OH radicals was effected by gravity at a flame tip, so that flame shapes in OH⁄ images were curved as going downstream. Fig. 8 shows the tendency of the flame slope from the results of the CH chemiluminescence (CH⁄) measurements. As shown in
Fig. 5, the flame is generally inclined to the fuel jet side. In Case 2, the degree of flame slope (h) was estimated as being about h = 9.8°. The flame slope decreased with the increase in fuel jet velocity at a fixed oxygen velocity in Fig. 8a. This is thought to be due to the movement of stoichiometric lines and the gradient of the mixture fraction [19]. Fig. 8b shows the effect of the variation of oxygen velocity on the flame slope. When oxygen velocity is increased at a fixed fuel jet velocity, the flame slope became more inclined toward the fuel jet side. Because the density of oxygen gas was larger than that of methane gas, the non-premixed oxy-methane flame was thought to lean toward the methane side, which had a lower momentum than that of oxygen. For a simple understanding of the behaviors of non-premixed oxy-methane flames, the concept of effective diameter was introduced. The idea of effective diameter was to treat the two separate nozzles for fuel and oxygen like a single diameter conceptually. The effective diameter is expressed as the ratio of momentum to mass flux of fuel and air [20]. The effective diameter was used to normalize the flame length, which was helpful in designing combustors and furnaces. In physical meaning, the normalized flame length using the effective diameter is the characteristic length of the conceptual equivalent source that would flow in the same quantity as the actual source, such as mass flux and momentum flux [21]. The scaling of the flame length can be derived from the scaling for the mixture fraction (c(x)) as c(x) (x/deff)1, where x is the axial distance from a nozzle exit. If the flame length is defined as the axial distance from a nozzle exit to the point where the mixture fraction reaches stoichiometric conditions, the equation for self similarity in the far field can be expressed as cðLÞ 1= 1 þ fs0 , 0 where fs is the stoichiometric ambient-to-source fluid mass ratio for a homogeneous source fluid mixture. From the combination of both equations, the equation of flame length scaling is derived as L 1 þ fs0 deff [21]. In a far field, the density difference between the fuel jet and the ambient air can be assumed to have same value as that of a premixed mixture. Meanwhile, in a near field, the density difference between the fuel jet and the ambient air is significant as that near the nozzles [20]. The equations for the effective diameter in the far-field concept and the near-field concept are as follows:
dF;eff ¼ dF
dF;eff ¼ dF
1=2
1=2
qeff qOx qF qeff
in the far-field concept
in the near-field concept
ð2Þ
where
qeff ¼
Fig. 8. Trend lines of the flame slope (h), extracted from CH⁄ images: the effect of (a) fuel jet velocity and (b) oxygen velocity.
qF u2F ðdF lÞ þ qOx u2Ox ðdOx lÞ u2F ðdF lÞ þ u2Ox ðdOx lÞ
where dF,eff is the effective fuel jet nozzle width, dF is the fuel jet nozzle width, dOx is the oxygen nozzle width, qeff is the effective gas density, qF is the fuel gas density, qOx is the oxygen gas density, uF is the velocity at a fuel jet nozzle exit, uOx is the velocity at an oxygen nozzle exit, and l is the slot depth. Fig. 9 shows the tendency of the normalized flame length, applying the effective diameter in the far-field concept and nearfield concept. Data for the flame length was sampled in Fig. 7. As a result of curve fitting on plotted data, the normalized flame length (L/dF,eff) decreased exponentially with the increase in the velocity ratio of oxygen and fuel (uOx/uF). Applying the far-field concept and the near-field concept to normalize the flame length, the experimental approximation is as follows:
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(4) By introducing the concept of the effective diameter (or momentum diameter), the experimental approximation of the flame length was suggested for a combustor design.
References
Fig. 9. Experimental approximation with the concept of the effective diameter.
L dF;eff
1=2 uOx uF
ð3Þ
In the current study of non-premixed oxy-methane flames, the normalized flame length in the far-field concept and the near-field concept showed similar trends, as plotted in Fig. 9. 4. Conclusion In the current study, the behaviors of non-premixed oxymethane flames in a lab-scale slot burner were investigated: the flame stabilization and structure. To observe the flame structure, photos from OH⁄, CH⁄, and C2 signals were taken. The trend of the flame length and the lift-off height were measured from CH chemiluminescence (CH⁄) images. From the above measurements, it was concluded as follows: (1) The flame stabilization became enhanced, when the separation distance between the fuel jet and the oxygen nozzles was narrower. (2) The flame length increased with an increase of fuel quantity, while the lift-off height increased with an increase of fuel jet and oxygen velocity. (3) The flame slope decreased with the increase in fuel jet velocity at a fixed oxygen velocity. And the flame slope became more inclined toward the fuel jet side when oxygen velocity is increased at a fixed fuel jet velocity.
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