Article pubs.acs.org/EF
Distribution Characteristics of OH*, CH*, and C2* Luminescence in CH4/O2 Co-flow Diffusion Flames Ting Zhang, Qinghua Guo, Qinfeng Liang, Zhenghua Dai, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: Information on luminescence of the excited-state radicals is of significance for spectrometer-based diagnostics of practical diffusion flames. In this paper, the emission spectrometry was applied to detect the distribution characteristics of three excited-state radicals (OH*, CH*, and C2*) in CH4/O2 co-flow jet diffusion flames with relatively high flow velocities (66 m/s CH4 and 18 m/s O2/N2), which is the basis to rationalize chemiluminescence flame diagnostics in gasification and combustion processes. This method allowed for the online measurement of luminescence intensity in flames. Results showed that there exited two peaks along the flame propagation direction for OH* when [O/C]e was greater than 0.90. The first peak formed near the nozzle exit because of the relatively high flow velocities pushing the interdiffusion of fuel and oxygen in the flame bottom, and the second peak was located near the boundary region of blue and yellow zones, indicating where the other reaction zone is. In contrast, CH* and C2* demonstrated only one chemiluminecent peak near the nozzle exit. OH*/CH* intensity ratio exhibited an exponential correlation with [O/C]e in the blue region, where the background spectra could be negligible, which can be regarded as the same as those found in premixed flames, and it is valuable to characterize the local [O/C]e quantitatively. entrained flow gasifiers, such as Texaco, Shell, and OMB gasifiers.11 The understanding of the diffusion flame characteristics and their correlations with operating parameters is important for optimizing the gasification process. In comparison to the studies on the premixed flame, relatively limited work has been performed with respect to this specific diffusion flame. Some researchers numerically and experimentally quantified OH* and CH* in laminar diffusion flames and the formation kinetics of these radicals.12−14 Panoutsos et al. studied the chemiluminescence of OH* and CH* in laminar premixed and non-premixed counterflow methane−air flames, and they found that the premixed and non-premixed reactions in turbulent flames could be distinguished between each other in terms of the OH*/CH* intensity ratio.15 Leo et al. performed an investigation into the excitation pathways of OH* and CH* in opposed flow methane oxy-flames, and they observed that the equivalence ratio was an important factor affecting the chemical ́ et al. and thermal excitation, especially for OH*.16 Verissimo examined the emission characteristics of a small-scale combustor, and the OH* images revealed that the structure of the main reaction zone changed with the excess air coefficient.17 Ikeda and Beduneau investigated the chemiluminescence along the centerline of the diffusion laminar flame using the cylindrical Bunsen burner, but only seven points were chosen.18 Therefore, no exhaustive observation has been spectrometrically made to reveal the distributions of excited radicals in the co-flow jet diffusion flames under high flow velocity conditions and their relationships with combustion conditions. The interest of this paper is mainly centered on the distribution characteristics along the diffusion flame axis with different
1. INTRODUCTION Spontaneous electromagnetic emission of flame has different forms:1 Solid body produces a continuous blackbody spectrum; gas molecules form rotation-emission bands at high temperatures; and excited radicals generate radiation because of spontaneous transition. Because of the feature of spontaneous emission, the distribution characteristics of excited radicals form the basis to rationalize chemiluminescence flame diagnostics in gasification and combustion processes as a useful tool, with the advantages of being non-intrusive and inexpensive relative to laser-based flame diagnostics. Spectrometry of OH*, CH*, and C2* has been used as a common approach for characterizing the flames by examining the relationships of luminescence intensity with varying process parameters, such as fuel type, equivalence ratio, strain rate, and flame structure.2−5 In premixed flames, in general, the OH*/ CH* intensity ratio displays a regular change with a growing equivalence ratio.6,7 Tinaut et al. studied the OH* and CH* chemiluminescence emitted during the combustion of different primary fuels at different initial conditions, and they found that the maximum of OH* chemiluminescence coincided with the highest rate of heat release inside the combustion bomb.8 Blevins et al. measured the radial temperature profiles and CH* radical locations in partially premixed co-flow methane/air flames and found that both the line-of-sight CH* chemiluminescent signal and the centerline point-by-point CH LIF signal exhibited maxima at locations coincident with the temperature of the rich premixed hydrocarbon/air component flame.9 Tripathi et al. proposed a multivariate sensing approach to measure fuel/air equivalence ratios in a premixed atmospheric methane−air flame.10 Diffusion flame is a prevalent flame form occurring in entrained flow gasifiers because the fuel and oxidizing agent are always fed to the gasifier through a coaxial burner in a nonpremixing way in high flow velocities, despite various types of © 2012 American Chemical Society
Received: June 7, 2012 Revised: July 27, 2012 Published: July 30, 2012 5503
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focal length symmetrical crossed Czerny−Turner type), and each device has a specific optical bench (Table 2), detecting a certain wavelength range synchronously to achieve a high spectral resolution. A fiberoptics probe with 600 μm core diameter and 25° field of view (FOV) is directed to the flame to collect luminescence signals. The distance from the probe to the flame is fixed in all measurements to ensure the detection of a constant projected flame area within the field of view of the fiber. Because of the flame radial dimension changing with the height above the nozzle, the detected volume is not an invariant. Figures 2 and 3 show the radial dimension and detected volume for each condition, and the final data are the average radiation intensity per unit volume. The spectrum is an average of 10 spectra obtained repeatedly under the same conditions. The dark background spectrum obtained before ignition or without flame luminescence emissions for each experiment is subtracted from the raw spectrum. The raw intensity data collected by the spectrometer are subject to some errors: (1) losses in the intensity when the light passes through the lens, (2) attenuation losses in the optical fiber, and (3) losses caused by the grating efficiency and CCD sensitivity of the spectrometer. Using a standard light source (deuterium−tungsten halogen lamp, Ocean Optics, Inc.), with certain spectral ranges of 210−400 nm (deuterium) and 360−1500 nm (tungsten halogen) and reference color temperatures of 5500 K (deuterium) and 3150 K (tungsten halogen), allows for the calibration of the optical system. To correct for the effects as mentioned above, a calibration factor Ci for each wavelength using actual measured intensities from the lamp and the intensity derived from Planck’s law with the reference temperature of the lamp is used, which is calculated by the following equation:
equivalence ratios, and some new observations on the distributions of excited radicals are made in this work.
2. EXPERIMENTAL SECTION 2.1. Combustion System. Figure 1 shows a schematic diagram of the experimental setup, consisting of two main parts: a stainless-steel
Figure 1. Schematic diagram of the experimental setup. combustion chamber and an optical measurement system. A jet burner is mounted inside the chamber at the bottom. The burner consistes of two coaxial tubular layers: the inside tube has an inner diameter of 0.8 mm, and the outer tube has an inner diameter of 3 mm. Fuel is transported through the inside tube, and the oxidizer is transported through the annular pathway. The flow rate is measured with a mass flowmeter (Sevenstar, Inc., D07-19B). The chamber is designed to have a side quartz window to permit the optical measurement of the entire flame by a spectrometer and to record the visible flame image by a high-resolution charge-coupled device (CCD) camera (JAI, Inc., BB-500CL). Combustion is carried out in the chamber at atmospheric pressure. Pure methane (>99.9%) is used as fuel, and the flow rate of methane is kept at 2 L/min. The mixture of O2 and N2 with varying ratios is used as an oxidizer, and the flow rate of the mixture is kept at 5 L/min. The equivalence ratio ([O/C]e) is changed in a range of 0.80−1.25, which is defined by the following equation:
[O/C]e = [O/C]a /[O/C]s
Ci = Imi /Ipi = Imi /[C1/(λi 5 exp(C2/λiT − 1))]
(2)
where Imi is the measured intensity at a given wavelength λi, Ipi is the intensity calculated by Planck’s law of radiation at λi, C1 = 2πhc2, C2 = hc/k, and the constants h, k, and c are Planck’s constant, Boltzmann’s constant, and the speed of light, respectively. The values are then normalized on the basis of the maximum line intensity value. Figure 4 is the calibration curve for the ultraviolet (UV) and visible (vis) ranges. The actual intensity of the object will then be calibrated by dividing the measured intensity at λi by Ci from Figure 4. The detailed procedure refers to the publication by Keyvana et al.19
3. RESULTS AND DISCUSSION 3.1. Typical Emissions of OH*, CH*, and C2*. Excited radicals are formed in the flame by two paths: thermal excitation and chemical excitation. Thermal excitation is related to the flame temperature and the number of ground-state radicals. The chemical excitation reactions are similar to other reactions that produce ground-state radicals, resulting in specified regions of the flame. In hydrocarbon flames, the major excited radical radiations come from OH*, CH*, and C2*. For OH*, the primary emission
(1)
where [O/C]a is the actual O/C molar ratio calculated from the amount of fuel and oxygen fed and [O/C]s is the stoichiometric O/C molar ratio. The equivalence ratio was adjusted by changing the O2 content in the O2/N2 mixture, with the total flow rate unchanging. Table 1 lists the experimental conditions. 2.2. Optical Measurement Method. The optical measurement is carried out using an Ocean Optics spectrometer, which is equivalent to four HR2000+ devices combined (the optical bench is a 101.6 mm
Table 1. Flame Conditionsa Used in Different Experiment Numbers O2 content (mol %) [O/C]e a
1
2
3
4
5
6
7
8
9
10
64 0.80
68 0.85
72 0.90
76 0.95
80 1.00
84 1.05
88 1.10
92 1.15
96 1.20
100 1.25
The flow velocities of CH4 and the mixture of O2 and N2 used in all experiments were 66 and 18 m/s, respectively.
Table 2. Optical Bench and Settings for Each HR2000+ grating device number
slit (μm)
groove density (lines/mm)
blaze wavelength (nm)
wavelength range (nm)
optical resolution (nm)
integration time (ms)
1 2 3 4
5 5 5 5
1200 1200 1200 1200
300 holographic UV holographic UV 750
200−420 400−605 590−780 760−950
0.16 0.15 0.14 0.14
500 500 500 500
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(R2)
OH + M → OH *+ M
(R3)
The reverse reactions of R1−R3 also act as dominant OH* consumption paths. Reaction R3 is the thermal excitation way of OH* formation, which becomes more dominant at temperatures above 2800 K.20 The luminous flame lengths are quantified by gray images with gray-threshold segmentation, and the OH* radiation can be seen until the end of flame luminescence, as shown in Figure 6. Because of the limited spatial resolution, the end positions of OH* radiation are located slightly higher than luminous flame lengths. The OH* distributions along the axis of the flames with different [O/C]e are shown in Figure 7. It is can be clearly seen that, for gasification conditions (rich-fuel case), there is only one peak near the nozzle exit, but when [O/C]e is up to 0.90, a second peak appears near the flame front with increased emission levels, and as [O/C]e increases, the peak is more obvious. Because chemiluminescence is produced directly by chemical reactions, it marks the location of the initial reactions during the combustion processes.8 When [O/C]e is low, the formation of OH* is concentrated near the nozzle exit, forming the first OH* peak. The first peak position is unchanged basically (Figure 8), indicating that there is always a reaction zone near the nozzle exit. Along with the oxygen content being increased to a fairly high level, under which the flame is near a stoichiometric condition, an extreme OH* peak is found at another position along the flame axis. Figure 8 also shows the boundary region (the position where flame luminous changes from blue
Figure 2. Radial dimension along the flame axis with different [O/C]e.
Figure 3. Detected volume along the flame axis with different [O/C]e.
occurs at approximately 283, 306, and 309 nm. CH* radiates at about 390 and 431 nm, and the C2* Swan Systerm has five emission bands, with the strongest band near 516 nm. Using the optical measurement system, the typical OH*, CH*, and C2* emissions in the UV and vis ranges (200− 600 nm) with different transitions are obtained, as shown in Figure 5. The precise radiation data are shown in Table 3. 3.2. Change of OH* Distributions with Increasing [O/C]e. The primary radical radiation extent will move toward the flame upstream along the flame propagation direction if the oxidizer velocity is increased. In the present experiment, the oxidizer velocity is constant, and then [O/C]e is the only factor affecting radiation. OH* generation can be modeled by a set of elementary reactions, given as follows: O + H + M → OH *+ M
CH + O2 → CO + OH*
Figure 5. Typical spectrum of CH4/O2 diffusion flame (condition 1, 3 mm above the nozzle exit).
(R1)
Figure 4. UV and vis calibration curves. 5505
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Table 3. Radiation Data of the Major Excited Radicals radical wavelength (nm) transition vibrational level comment
OH* 282.9 (0, 1) Q2-Linie
306.72 A2∑+ − X2∏ (0, 0) R2-Linie
C2*
CH* 308.9 (0, 0) Q2-Linie
388.9 B2∑g − X2∏ (0, 0) Q-Head
Figure 6. Comparison of flame lengths and the end positions of OH* radiation.
431.42 A2Δ − X2∏ (0, 0) Q-Head
471.52 (1, 2)
473.71
512.93 516.52 A3∏g − X3∏u (0, 1) (1, 1) (1, 2) Swan System
563.55 (1, 0)
Figure 9. Images with threshold (the boundary region of different flames).
OH* is located near the boundary region of blue and yellow zones. Because of the change of the O2 content, the flame-front position cannot maintain stability, so that it moves toward the flame downstream first and then moves toward upstream when the O2 content is excessive ([O/C]e > 0.95). The second OH* peak position is changed resulting from the movement of the flame front. There is a clear increasing tendency of OH* peak intensity when [O/C]e increases, which could be explained by the fact that the flame is hotter with oxygen enriched and the dissociation of molecular oxygen into atomic oxygen is greatly facilitated; thus, more oxygen atoms and molecules are involved in reactions R1 and R2. The probability of the thermal excitation way in reaction R3 effecting OH* formation may be small, which could be ignored. The double-peak distribution shows that there are two reaction zones for these specific diffusion flames. 3.3. Change of CH* and C2* Distributions with Increasing [O/C]e. The formation pathways of CH* and C2* are shown by the following reactions:
Figure 7. OH* distributions in flames with different [O/C]e.
C2 + OH → CO + CH*
(R4)
C2H + O → CO + CH*
(R5)
C + H + M → CH *+ M
(R6)
C2H + O2 → CO2 + CH*
(R7)
Figure 8. Positions of OH* peaks and the boundary region.
CH + M → CH *+ M
(R8)
to yellow) of different flames, which is the flame front. It is difficult to distinguish the boundary region with the naked eye; therefore, the region is abtained by threshold segmentation in this study: First take out the image of the flame blue component from the RGB image recorded by the high-resolution industrial CCD camera, then translate the blue component image into the gray image, finally segment the gray image with threshold, and obtain the boundary region. The images with threshold are shown in Figure 9, and Table 4 lists the thresholds for each images. It can be seen that the second peak of
CH 2 + C → C2 *+ H 2
(R9)
C2 + M → C2 *+ M
(R10)
The distributing areas of CH* and C2* along the axis show a clearly single peak in all flames (Figure 10). At the flame tip, OH* is measured but no remarkable CH* or C2* is measured. The CH* radiation position is near the nozzle exit, and the radiation is relatively weak compared to OH*. It is reported that thermal excited CH* never constitutes more than 30% of the total excited population.16 The emission of CH* is 5506
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Table 4. Thresholds Used To Segment Gray Images [O/C]e threshold
0.80 110
0.85 119
0.90 133
0.95 145
1.00 156
1.05 167
1.10 176
1.15 185
1.20 192
1.25 193
Figure 10. CH* and C2* distributions in flames with different [O/C]e.
extremely weak even in pure-oxygen flame (flame X), which suggests that the effect of thermal excitation on CH* generation is relatively weaker. According to these results, it can be speculated that CH* formation mainly results from the chemical process as shown by reactions R4−R7. Similar to CH*, the leading way of C2* formation is chemical excitation. Both CH* and C2* peaks are located near the nozzle exit, because of the interdiffusion of fuel and oxygen under high flow velocity conditions. OH* radiation is significantly higher than CH* and C2* for all flames. CH3 and C2H2 are the precursors for the CH radical involved in the formation of OH*, and C2H4 and C2H2 are the precursors for the CH2 and C2H radicals that affect the formation of CH* and C2* radicals, respectively.21 In the combustion of methane fuels, CH3 is produced in larger quantities, but the amount of C2H4 and C2H2 is very limited, which leads to a higher level of OH* radiation. 3.4. Luminescent Emissions for [O/C]e Measurements. The emissions from OH*, CH*, and C2* formed within the flame reaction zone depend upon [O/C]e, which could be defined when the luminescence of those three radicals is measured with an insufficient background spectrum (associated with CO2* emission at 391−543 nm). For premixed flames, the intensity ratio of OH*/CH* usually exhibits a monotonic relation with the equivalence ratio.6,7,9 In comparison to premixed flames, fuel and oxygen are not well-mixed for diffusion flames, especially near the nozzle exit; thus, the dependence of the intensity ratio upon [O/C]e may be different in these kinds of flames. No remarkable CO2* emission is measured in the obvious blue flame region (0.90), which shows that there are two reaction zones for these specific diffusion flames. The first peak of OH* forms near the nozzle exit because of the relatively high flow velocities pushing the interdiffusion of fuel and oxygen in the flame bottom, and the second peak is located near the boundary region of blue and yellow zones, indicating where the other reaction zone is. (2) CH* and C2* profiles along the flame axis show a clearly single peak located near the nozzle exit with relatively weaker radiation intensities compared to OH*, which have slight variations when [O/C]e increases. (3) The OH*/CH* intensity ratio exhibits an exponential increase with [O/C]e in the blue region, where the background spectra could be negligible, which is in agreement with the result observed in 5508
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