- Email: [email protected]
Investigations of CH∗ chemiluminescence and blackbody radiation in opposed impinging coal-water slurry flames based on an entrained-flow gasifier
Fuel 211 (2018) 688–696
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Investigations of CH∗ chemiluminescence and blackbody radiation in opposed impinging coal-water slurry flames based on an entrained-flow gasifier ⁎
Chonghe Hu, Yan Gong, Qinghua Guo , Lei He, Guangsuo Yu
MARK
⁎
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: CH∗ chemiluminescence Blackbody radiation Impinging flame Coal-water slurry Entrained-flow gasifier
The characteristics of excited CH radical (CH∗) chemiluminescence and blackbody radiation in opposed impinging coal-water slurry (CWS) flames were investigated in this paper based on a bench-scale opposed multiburner (OMB) gasifier. CH∗ chemiluminescence is a significant radical species in flame spectral diagnostics, and blackbody radiation can reflect the spatial distribution of solid particles produced in flame. A fiber-optic spectrometer and a CCD camera coupled with multiple bandpass filters are used to obtain the spectral emission lines and two-dimensional spectral distributions, respectively. The results show that CH∗ emission peak at 431 nm and continuous blackbody radiation can be detected in CWS flames. The differences of spectral distributions between diesel and CWS impinging flame are analyzed. Impinging zone is the core chemical reaction region, and solid particles are also concentrated in impinging zone, indicating that four-burner impinging can well restrict the reactants and flames in the impinging zone, thereby greatly reduce the damage to the refractory walls. Moreover, according to the time-averaged distributions, the intensity and area of CH∗ emission and blackbody radiation are enhanced with the increase of oxygen and carbon molar ratio (O/C), demonstrating that improving O2 velocity promotes the chemical reactions, and flame temperature plays more dominant role than solid particle quantity in blackbody radiation. The time-dependent blackbody radiation evolution presents periodical change. Besides, it is found that O/C or syngas concentration can be feasibly estimated using CH∗ chemiluminescence in OMB gasifier.
1. Introduction The efficient and clean utilization of coal resource is of significant interest in recent years. Coal gasification technologies, especially the opposed multi-burner (OMB) gasification technology using coal-water slurry (CWS) as feedstock, can achieve the clean production of syngas from coal. OMB gasification technology has been widely used by over 50 companies all over the world [1], due to the high efficiency, large scale and good economic benefit. Moreover, impinging stream can effectively strengthen heat and mass transfer, improve particle residence time and reaction efficiency [2]. Hence it has been utilized in many kinds of industrial reactors, also including the OMB gasifier. This paper was to explore the CWS impinging flame characteristics through flame spectral diagnostics, based on a bench-scale OMB gasifier. Flame spectral diagnostics is an important and promising method to obtain the flame information and monitor the combustion or gasification systems [3], because the diagnostic method is non-intrusive and
⁎
the spectral signals are easily measured. Excited CH radical (CH∗) chemiluminescence is one of the most significant radical species, and much research has been reported on the capability of CH∗. It was found that CH∗ can be used to predict flame structure, chemical reaction region, equivalence ratio, etc. [4–6]. However, the ability of CH∗ to estimate total or local flame heat release rate is still uncertain through the work of Hossain and Nakamura [7]. Most of such cases were focused on gas or oil flames, less work has been done on coal flames, especially the impinging-stream coal flames. Because the experiment and measurement systems are more complex, and the interference of broadband blackbody radiation on chemiluminescence is relatively stronger. In the heterogeneous flames, high-temperature solid particles can emit strong and continuous blackbody radiation [8], and blackbody radiation is usually used to evaluate flame temperature on the basis of two-color, multi-color and other pyrometry methods [9–11]. Besides, some researchers have reported their study on blackbody measurements in flames using optical techniques. Pourhoseini and Moghiman
Corresponding authors. E-mail addresses: [email protected] (Q. Guo), [email protected] (G. Yu).
http://dx.doi.org/10.1016/j.fuel.2017.09.094 Received 21 June 2017; Received in revised form 16 August 2017; Accepted 25 September 2017 Available online 03 October 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
Fuel 211 (2018) 688–696
C. Hu et al.
and 700 nm are used in the current work, and all filters can completely cover the camera sensor. The full width and peak transmission are approximately 10 nm and 50%, respectively.
[12] used an IR filter coupled with a digital camera to photograph the qualitative distribution of soot particles in nature gas flame which was injected with pulverized coal. Liu et al. [13] provided a generalized method to estimate the temperature distribution and wavelength-dependent emissivity in sooty flames on the basis of spectroscopic radiation intensity. Liu et al. [14] also calculated the soot temperature, absorption coefficient and volume fraction in ethylene flames using hyperspectral imaging device and the iterative process. In this work, the two-dimensional blackbody radiation distributions of impinging flames were analyzed based on a bench-scale gasifier. For the CWS flame, the blackbody radiation is mainly derived from many kinds of solid particles such as soot particles, coal particles, coal char, ash and slag, etc. [15]. Should explain here the actual solid particle is not the idealized blackbody which can absorb all incident radiation. In the actual industry, the particle is generally regarded as the graybody which has lower emissivity independent of frequency [16]. Hence the statement “blackbody” in this paper was used to represent the solid particles produced in flame. For the OMB gasifier, some research on the spectroscopic characteristics has been conducted. Hu and Song et al. [17,18] investigated the spectral characteristics in diesel and CWS flames using a HR2500+ spectrometer. It is a point measurement in the impinging zone, and only one-dimensional emission information can be measured. In this article, two-dimensional distributions of CH∗ chemiluminescence and blackbody radiation in CWS impinging flames were firstly derived, through image processing method and a CCD camera coupled with multiple bandpass filters (420, 430, 440 and 700 nm). Using CWS as feedstock, the findings can be directly extended to the industrial OMB gasifier. Furthermore, the differences of spectral emissions between diesel and CWS impinging flame are analyzed. The effects of O/C on CWS impinging flames are discussed. And the feasibility of using CH∗ chemiluminescence to predict O/C or syngas concentration is evaluated.
2.2. Operating conditions Diesel and CWS are used as feedstock, and oxygen is the oxidizer in this article. The analysis of diesel and CWS are given in Table 1 and Table 2, respectively. The CWS is prepared using Shenhua bituminous coal, and the solid mass content is 61%. The coal analysis meets the National Standards of PRC (GB/T 212-2008 and GB/T 31391-2015). The operating conditions of each burner are listed in Table 3. All conditions are under gasification atmosphere. O/C is the calculated molar ratio of oxygen and carbon, and it is changed by varying the O2 flow rate. To analyze the differences between diesel and CWS flames under the same O/C, should note that condition 1 and 8 have the same O2 flow rate, and so on. 3. Results and discussion 3.1. Processing method of spectral image The spectral lines of CH∗ chemiluminescence and blackbody radiation exist in CWS flame, as shown in Fig. 2. Take condition 3 for instance, the emission lines around 367 nm arise from H∗, and the emission peak at 409 nm is from NO∗. CH∗ chemiluminescence is taken at 431 nm. Continuous background radiation also can be observed. In the gasifier, the background radiation mainly includes blackbody radiation and CO2∗ emissions [21]. As mentioned above, blackbody radiation was caused by soot particles in diesel flame and solid particles in CWS flame. The broadband range of CO2∗ emissions are from CO2 excitation mainly at 310–600 nm [22–23]. From Fig. 2, CH∗ emission intensity is significantly influenced by the background radiation. Hence, in order to obtain correct CH∗ chemiluminescence image, the blackbody radiation and CO2∗ emissions should be subtracted according to Eq. (3.1):
2. Experimental work 2.1. Experimental setup
∗
CO2 blackbody measurement ICH ∗ = I430 −(I430 + I430 nm nm nm )
As Fig. 1 shows, the experimental work was carried out in a benchscale entrained flow gasifier operated in the ambient atmosphere. Two pairs of burners were oppositely mounted with 90° in the same horizontal plane (impinging plane). The distances of two opposed burners and impinging plane to endoscope are 300 mm and 600 mm, respectively. The burner has two coaxial channels, CWS is transported into the central channel by helical rotor pump, and oxygen is supplied into annular channel from Dewar tank. Diesel is used to preheat the gasifier, then the CWS is injected into gasifier. The temperature is monitored by several B-type thermocouples. Moreover, raw syngas flows downward from gasification chamber to quenching chamber, and the syngas concentration was online measured by a mass spectrometer (Type thermostar GSD 320 T2). The sampling and pretreatment processes of raw syngas have been reported by Niu [19] in detail. Spectroscopic measurement system includes a fiber-optic spectrometer and a high resolution CCD camera coupled with multiple bandpass filters. The spectrometer (HR2500+, Ocean Optics Inc.) was used to detect the spectral signals of impinging zone. The fiber probe was stretched into the sampling port and protected by the cooling jacket. The detailed descriptions have been presented by Zhang and Song [18,20]. Besides, the flames in the gasifier can be visualized through a CCD camera (XCL-500, SONY) combined with a high temperature endoscope (CESYCO, Φ38 mm). The spatial resolution of CCD camera is 2448 × 2048 pixels. Cooling water and purging gas (Ar) can make the endoscope avoid high temperature and pollutants. The spectral images were obtained by setting the bandpass filters in the endoscope. As shown in Fig. 1, a filter slot is located in front of the CCD camera, which is convenient for changing different filters to satisfy experimental request. The filters at central wavelengths of 420 nm, 430 nm, 440 nm
(3.1)
where I means the emission intensity. On the basis of this principle, Karnani et al. [24] put forward the filtering and image subtraction method to produce the soot-free image of CH∗ chemiluminescence in ethylene flame. Zhang et al. [25] verified the processing method and obtain CH∗ image of diesel flame. In this work, the method was adopted to obtain CH∗ chemiluminescence in diesel and CWS flames. Taking condition 6 for example, the detailed subtraction procedure is illustrated in Fig. 3. The first step is to obtain the background radiation at 430 nm, and it can be calculated by the flame images through 420 nm and 440 nm filters according to Eq. (3.2). The specific derivation processes are also reported by Karnani and Zhang [24,25]. background background background I430 = 0.6361 × I420 + 0.3486 × I440 nm nm nm
(3.2)
The second step was to subtract the background radiation from the filtering image captured at 430 nm. Then the CH∗ chemiluminescence distribution in CWS flame was derived. Should mention that the timeaveraged filtering images are processed by averaging at least 50 timedependent images. Besides, as for blackbody radiation images, a 700 nm filter was used to photograph the flames in order to avoid the interference of CO2∗ emissions. 3.2. Spectral emissions in diesel and CWS flames In the experimental process, diesel was used to heat the furnace up. Usually, when the furnace temperature reached about 1400 °C, CWS was injected into a pair of opposed burners, then into the other two burners. Diesel flame is gas-liquid two phase, and CWS flame is gas689
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 1. Schematic sketch of the bench-scale gasifier.
Table 1 Analysis of the diesel. Molecular formula
Relative molecular mass
C (wt%)
H (wt%)
O (wt%)
CxHy
190–220
86.2%
13.7%
0.1%
Table 2 Analysis of the coal. Proximate analysis (wt%)
Ultimate analysis (wt%)
Mad
Vad
Aad
FCad
Cad
Had
Nad
Sad
Oad
2.40
33.20
9.01
55.39
73.49
4.81
1.13
0.61
10.95
ad: air dried base; M: moisture; V: volatile matter; A: ash; FC: fixed carbon. Table 3 Operating conditions of each burner. Condition Number
O/C
1 2 3 4 5 6 7
0.8 0.9 1.0 1.1 1.2 1.3 1.4
Fig. 2. Spectral emission lines in CWS flame under condition 3.
CWS Flow Rate (kg/h)
O2 Flow Rate (L/ min)
O2 Velocity (m/s)
Condition Number
Diesel Flow Rate (kg/h)
10.0
54.9 61.8 68 75 82 88 94
94.4 106.2 118.0 129.8 141.6 153.4 165.3
8 9 10 11 12 13 14
5.1
liquid-solid three phase. Hence, under the same O/C, the spectral emissions in diesel and CWS flames present significant differences. Taking O/C = 1.2 for example, the distinctions were particularly analyzed in this section. 3.2.1. CH∗ chemiluminescence in diesel and CWS flames Based on the processing method above, Fig. 4 shows the two-dimensional CH∗ distributions in diesel and CWS flames. Fig. 4(a) and (b) are under condition 12, Fig. 4(c) and (d) are under condition 5. CH∗ radicals are primarily excited by chemical excitation [26], thus CH∗ distributions can reflect the chemical reaction region in flames. From Fig. 4(a) and (b), the generation of CH∗ radicals was concentrated near 690
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 3. Subtraction procedure diagram of CWS impinging flame.
reflect the spatial distribution of solid particles produced in the flame [12,28,29]. In this section, the particle distributions in diesel and CWS flames were analyzed. From Fig. 5(a) and (b), the blackbody radiation intensity of diesel flame was much weaker than that of high-temperature refractory walls. It can be observed that very weak blackbody radiation in diesel flame existed only in the impinging zone, because it is basically covered by the radiation from the hot refractory walls. After CWS is injected into the gasifier, the blackbody radiation in CWS flame is much stronger than not only the hot walls, but also the diesel flame. Compared with CWS flame, diesel flame is relatively clean and produces less solid particles, indicating solid particle quantity has a great influence on blackbody radiation intensity captured by CCD camera. Besides, as Fig. 5(c) and (d) show, the particles intensely disperse in the entire furnace for two-burner CWS flame, whereas the particles mainly concentrated in the impinging zone for four-burner CWS flame, suggesting that the restriction effects of four-burner impinging on solid particles are stronger than two-burner impinging. It also can be
the burner exits, indicating the chemical reactions mainly occur near the burner exits. When changing the fuel of two burners from diesel to CWS, the generation of CH∗ radicals was mainly located in the impinging zone, rather than near the burner exits. After CWS was injected into the other two burners, the emission intensity and area of CH∗ increased obviously. Which indicated that the core reaction region transferred to the impinging zone for the CWS impinging flame. Diesel is a kind of liquid fuel, which is easier to react with oxygen than CWS, and CWS gasification processes are more complex than diesel. Hence, the diesel reacted with oxygen immediately after it entered into the gasifier, whereas the chemical reactions of CWS and oxygen focused in the impinging zone due to the promotion effects of impinging flames. 3.2.2. Blackbody radiation in diesel and CWS flames For diesel flame, blackbody radiation is mainly emitted by soot particles [27]. For CWS flame, blackbody radiation is derived from many kinds of solid particles mentioned above. According to the previous study, the photograph through infrared filter can qualitatively
Fig. 4. CH* distributions of diesel and CWS flames.
691
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 5. Blackbody distributions of diesel and CWS flames.
Fig. 6. Time-averaged CH* distributions under different conditions.
C2 H + O2 → CH ∗ + CO2
concluded that four-burner impinging can well restrict the reactants and flames in the impinging zone, then greatly reduce the damage to the refractory walls.
∗
Therefore, CH radical is a good marker of reaction region, and CH∗ chemiluminescence is greatly influenced by operating conditions. In this section, the effects of O/C on CH∗ emission distribution were investigated. As shown in Fig. 6, under different O/C, CH∗ radicals are mainly formed in four jet flow zones and impinging zone. The CH∗ emission intensity in impinging zone is much stronger than that in four jet flow zones, indicating that impinging zone is the core reaction region in OMB gasifier. Accordingly, the peak emission intensity and core reaction region area were also analyzed. In order to obtain the correct peak intensity, a circular region with a diameter of 50 mm (size factor is 0.158 mm/pixel) was chosen in the core reaction region. The peak intensity in Fig. 7 is the mean value calculated by computer software based on all the intensities in the circular region. Moreover, the core reaction region area was obtained with Image J software through
3.3. Spectral emissions under different O/C in four-burner CWS flames 3.3.1. Time-averaged CH∗ chemiluminescence under different O/C CH∗ radical is an important intermediate product in flames, hence much research has been reported on the reaction mechanisms of CH∗, and several detailed mechanisms have been proposed [30,31]. Generally, for hydrocarbon/oxygen flames, the following pathways are the major chemical reactions of CH∗ formation:
C2 + OH → CH ∗ + CO
(R1)
C2 H + O → CH ∗ + CO
(R2)
(R3)
692
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 7. CH* peak intensities and core reaction region areas under different O/C.
Fig. 8. Time-averaged blackbody radiation distributions under different conditions.
setting the scale and selecting the uniform threshold. From Fig. 7, CH∗ peak emission intensity is enhanced remarkably with the increase of O/ C. When O/C = 0.9, the peak intensity is only 325 mW Sr−1 m−2. Whereas, when O/C = 1.4, the peak intensity reaches 2261 mW Sr−1 m−2. According to (R1), (R2) and (R3), increasing oxygen amount promotes the chemical reactions and the formation of CH∗. Meanwhile, increasing oxygen velocity improves the atomization effect of burners, then the reactions are further facilitated. Similarly, the area of core reaction region also increases with the rise of O/C.
From Fig. 9, both peak radiation intensity and core radiation region area increase with increasing O/C. In CWS gasifier, the blackbody radiation captured by CCD camera is mostly influenced by flame temperature and solid particle quantity. When the flame temperature is higher, or the particle quantity is larger, the radiation intensity will get stronger. In this work, with the increase of O2 flow rate, more CWS reacts with oxygen and more heat is generated, then the flame temperature is higher [32]. More coal particles are consumed, and the formation of soot particles is weakened when the condition gets closer to complete combustion [33]. It can be inferred that the quantity of solid particles is reduced. Under the joint effects of two factors, the radiation intensity and area still increase, demonstrating that flame temperature plays more dominant role in the blackbody radiation.
3.3.2. Time-averaged blackbody radiation under different O/C The time-averaged blackbody radiation distributions were investigated in this section. As shown in Fig. 8, under different O/C, the core blackbody radiation region is always located in the impinging zone, indicating that the solid particles are restricted in the impinging zone by four-burner impinging flames. With the increase of O/C, the distributions are obviously different, and the peak radiation intensities and core radiation region areas were also calculated through the same ways described in Section 3.3.1.
3.3.3. Evolution of time-dependent blackbody radiation at O/C = 1.2 CH∗ emission distribution cannot be captured directly, hence only time-averaged CH∗ distribution can be derived through image processing method. However, the time-dependent blackbody radiation distribution can be directly captured by CCD camera of which the 693
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 9. Blackbody peak intensities and core radiation region areas under different O/C.
Fig. 10. Time-dependent blackbody radiation distributions under condition 4.
evolution process of blackbody radiation presents periodical change, and the evolution trend mainly includes two stages. One stage is from (c) to (a) and (a) to (b), the radiation intensity and area keep decreasing during approximately ten frames time, indicating the solid particles are gradually consumed by reacting with oxygen at that time. Hence we named this process gradual weakening stage. The other one was named sudden enhancement stage, as observed from (b) to (c). Because the radiation intensity and area dramatically increase in only one frame time due to the flame pulsation and pump fluctuation, indicating more
operating frame rate was 20 fps. Take condition 4 as an example, the evolution of blackbody radiation was illustrated in Fig. 10. The arrows describe the development with time of impinging flame. The marks (a), (b) and (c) correspond to the images in green, blue and red boxes, respectively. Should note that the time intervals of (a) to (b), (b) to (c) and (c) to (a) are about 250 ms, 50 ms and 250 ms, respectively. To more clearly observe and analyze the evolution trend, the timedependent core radiation areas were calculated also using Image J software, as shown in Fig. 11. From Figs. 10 and 11, it is found that the 694
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 11. Time-dependent radiation area evolution trend under condition 4.
inevitably lower than those in the industrial gasifier, whereas the evolution trend is consistent with the industrial data [34]. Through Peak /(CO + H ) ratio and O/C attempts and comparison, we found that ICH ∗ 2 Peak /(CO + H ) and O/C have the best correlation. As Fig. 13 shows, ICH ∗ 2 present a good linear relationship, and it can be fitted by the following line: Peak /(CO + H ) = 185.47 × O / C −168.05 ICH ∗ 2
(3.3)
The fitting degree of Eq. (3.3) is 0.983. Based on Eq. (3.3), for the given CH∗ peak intensity and O/C, the uncertainties on syngas concentration measurements are within 5%. Therefore, it is a feasible way to predict the syngas concentration according to CH∗ peak intensity and operating condition, or to estimate the operating condition if CH∗ peak intensity and syngas concentration are known. 4. Conclusions This experimental study focuses on the spectroscopic characteristics of CH∗ chemiluminescence and particle radiation in opposed impinging CWS flames, based on an entrained flow bench-scale gasifier. The results can be summarized into the following conclusions: The emission peak of CH∗ chemiluminescence at 431 nm and continuous blackbody radiation can be detected in CWS flames. On the basis of CH∗ distributions, for diesel flame, the chemical reactions mainly occur near the burner exits. For CWS flame, CH∗ radicals are formed in four jet flow zones and impinging zone, and impinging zone is the core reaction region. According to blackbody radiation, the radiation intensity of CWS flame is much stronger than that of diesel flame, because diesel flame is relatively clean and produces less particles. In the four-burner CWS flame, solid particles are mainly restricted in impinging zone. Four-burner impinging can well restrict the reactants and flames in the impinging zone, thereby greatly reduce the damage to the refractory walls. The time-averaged CH∗ distribution and blackbody radiation with O/C were analyzed. With the increase of O/C, CH∗ emission intensity and area are remarkably enhanced, because improving O2 velocity promotes the chemical reactions and atomization effect. Under the joint influence of flame temperature and solid particle quantity, the blackbody radiation intensity and area also increase with rising O/C, indicating that flame temperature plays more dominant role in blackbody radiation. Time-dependent blackbody radiation presents periodical change. The evolution trend includes two stages: a gradual weakening stage and a sudden enhancement stage. Moreover, CH∗ peak intensity/ (CO+H2) ratio and O/C present a good linear relationship, hence it is a feasible way to estimate O/C or syngas concentration using CH∗ chemiluminescence.
Fig. 12. Syngas concentrations under different O/C.
Peak /(CO + H ) ratio and O/C. Fig. 13. Relationship of ICH ∗ 2
coal particles have been supplied into the gasifier. Afterwards, the intensity and area of blackbody radiation are gradually weakened (i.e. the gradual weakening stage). 3.4. Relationship among CH∗ peak intensity, O/C and syngas concentration In our experimental system, changing O/C has a great influence on Peak ) and syngas concentration (CO+H ). Then we CH∗ peak intensity (ICH ∗ 2 attempted to explore the relationship among the three parameters. The ∗ CH peak intensities and syngas concentrations under different O/C are shown in Fig. 7 and Fig. 12, respectively. Both CO and H2 concentrations decrease with increasing O/C. The highest syngas concentration reaches 68.61% at O/C = 0.9, while the lowest value is only 23.55% at O/C = 1.4. The syngas concentrations in this bench-scale gasifier are
Acknowledgments The present work was supported by the National Natural Science Foundation of China (21676091), the Fundamental Research Funds for the Central Universities (222201514336) and the Shanghai Pujiang Program (15PJD011). 695
Fuel 211 (2018) 688–696
C. Hu et al.
[18] Song X, Guo Q, Hu C, Gong Y, Yu G. Optical experimental study on the characteristics of impinging coal-water slurry flame in an opposed multi-burner gasifier. Fuel 2017;188:132–9. [19] Niu M, Yan Z, Guo Q, Liang Q, Yu G, Wang F, et al. Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier. Fuel Process Technol 2008;89:1060–108. [20] Zhang T, Guo Q, Liang Q, Dai Z, Yu G. Distribution characteristics of OH∗, CH∗ and C2∗ luminescence in CH4/O2 Co-flow diffusion flames. Energy Fuels 2012;26:5503–8. [21] Lauer M, Sattelmayer T. On the adequacy of chemiluminescence as a measure for heat release in turbulent flames with mixture gradients. J Eng Gas Turb Power 2010;132:061502. [22] Ballester J, García-Armingol T. Diagnostic techniques for the monitoring and control of practical flames. Prog Energy Combust Sci 2010;36:375–411. [23] Docquier N, Lacas F, Candel S. Closed-loop equivalence ratio control of premixed combustors using spectrally resolved chemiluminescence measurements. Proc Combust Inst 2002;29:139–45. [24] Karnani S, Dunn-Rankin D. Visualizing CH∗ chemiluminescence in sooting flames. Combust Flame 2013;160:2275–8. [25] Zhang Q, Song X, Yu G. Experimental study on CH∗ chemiluminescence characteristics of impinging flames in an opposed multi-burner gasifier. AICHE 2016;7:405–10. [26] Kathrotia T, Fikri M, Bozkurt M, Hartmann M, Riedel U, Schulz C. Study of the H +O+M reaction forming OH∗: kinetics of OH∗ chemiluminescence in hydrogen combustion systems. Combust Flame 2010;157:1261–73. [27] Solomon PR, Best PE. Fourier transform infrared emission/transmission spectroscopy in flames. Combust Meas 1991;43:385–444. [28] Molina A, Shaddix CR. Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion. Proc Combust Inst 2007;31:1905–12. [29] Pastor JV, García-Oliver JM, García A, Micó C, Durrett R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl Energy 2013;104:568–75. [30] Nori VN, Seitzman JM. CH∗ chemiluminescence modeling for combustion diagnostics. Proc Combust Inst 2009;32:895–903. [31] Devriendt KE, Peeters J. Direct identification of the C2H(X2Σ+)+O(3P)→CH(A2△) +CO reaction as the source of the CH(A2△→X2Π) chemiluminescence in C2H2/O/ H atomic flames. J Phys Chem A 1997;101:2546–51. [32] Gong Y, Guo Q, Liang Q, Zhou Z, Yu G. Three-dimensional temperature distribution of impinging flames in an opposed multiburner gasifier. Ind Eng Chem Res 2012;51:7828–37. [33] Lee KO, Megaridis CM, Zelepouga S, Saveliev AV, Kennedy LA, Charon O, et al. Soot formation effects of oxygen concentration in the oxidizer stream of laminar coannular nonpremixed methane/air flames. Combust Flame 2000;121:323–33. [34] Yu Z, Wang F. Coal gasification technology. Beijing: Chemical Industry Press; 2010.
References [1] Guo Q. Proceedings of the 2016 Gasification and Syngas Technologies Conference. Vancouver, British Columbia, Canada, Oct 16–19; 2016. [2] Tamir A. Impinging-stream reactors: fundamentals and applications. Amsterdam: Elsevier; 1994. [3] Docquier N, Candel S. Combustion control and sensors: a review. Prog Energy Combust Sci 2002;28:107–50. [4] Kojima J, Ikeda Y, Nakajima T. Spatially resolved measurement of OH∗ CH∗ and C2∗ chemiluminescence in the reaction zone of laminar methane/air premixed flames. Proc Comb Inst 2000;28:1757–64. [5] Oh J. Spectral characteristics of a premixed oxy-methane flame in atmospheric conditions. Energy 2016;116:986–97. [6] Giassi D, Cao S, Bennett BAV, Stocker DP, Takahashi F, Smooke MD, et al. Analysis of CH∗ concentration and flame heat release rate in laminar coflow diffusion flames under microgravity and normal gravity. Combust Flame 2016;167:198–206. [7] Hossain A, Nakamura Y. A numerical study on the ability to predict the heat release rate using CH∗ chemiluminescence in non-sooting counter flow diffusion flames. Combust Flame 2014;161:162–72. [8] Romero C, Li X, Keyvan S, Rossow R. Spectrometer-based combustion monitoring for flame stoichiometry and temperature control. Appl Therm Eng 2005;25:659–76. [9] Fu T, Wang Z, Cheng X. Temperature measurements of diesel fuel combustion with multicolor pyrometry. J Heat Transfer 2010;132:51602. [10] Khatami R, Levendis YA. On the deduction of single coal particle combustion temperature from three-color optical pyrometry. Combust Flame 2011;158:1822–36. [11] Yuan Y, Li S, Zhao F, Yao Q, Long MB. Characterization on hetero-homogeneous ignition of pulverized coal particle streams using CH∗ chemiluminescence and 3 color pyrometry. Fuel 2016;184:1000–6. [12] Pourhoseini SH, Moghiman M. Effect of pulverized anthracite coal particles injection on thermal and radiative characteristics of natural gas flame: an experimental study. Fuel 2015;140:44–9. [13] Liu H, Zheng S, Zhou H, Qi C. Measurement of distributions of temperature and wavelength-dependent emissivity of a laminar diffusion flame using hyper-spectral imaging technique. Meas Sci Technol 2016;27:25201. [14] Liu H, Zheng S, Zhou H. Measurement of soot temperature and volume fraction of axisymmetric ethylene laminar flames using hyperspectral tomography. IEEE Trans Instrum Meas 2017;66:315–24. [15] Hu C, Gong Y, Guo Q, Song X, Yu G. An experimental study on the spectroscopic characteristics in coal-water slurry diffusion flames based on hot-oxygen burner technology. Fuel Process Technol 2016;154:168–77. [16] Yang S, Tao W. Heat transfer. Beijing: Higher Education Press; 2006. [17] Hu C, Gong Y, Guo Q, Wang Y, Yu G. Experimental study on the spectroscopy of opposed impinging diesel flames based on a bench-scale gasifier. Energy Fuels 2017;31:4469–78.
696
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Investigations of CH∗ chemiluminescence and blackbody radiation in opposed impinging coal-water slurry flames based on an entrained-flow gasifier ⁎
Chonghe Hu, Yan Gong, Qinghua Guo , Lei He, Guangsuo Yu
MARK
⁎
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: CH∗ chemiluminescence Blackbody radiation Impinging flame Coal-water slurry Entrained-flow gasifier
The characteristics of excited CH radical (CH∗) chemiluminescence and blackbody radiation in opposed impinging coal-water slurry (CWS) flames were investigated in this paper based on a bench-scale opposed multiburner (OMB) gasifier. CH∗ chemiluminescence is a significant radical species in flame spectral diagnostics, and blackbody radiation can reflect the spatial distribution of solid particles produced in flame. A fiber-optic spectrometer and a CCD camera coupled with multiple bandpass filters are used to obtain the spectral emission lines and two-dimensional spectral distributions, respectively. The results show that CH∗ emission peak at 431 nm and continuous blackbody radiation can be detected in CWS flames. The differences of spectral distributions between diesel and CWS impinging flame are analyzed. Impinging zone is the core chemical reaction region, and solid particles are also concentrated in impinging zone, indicating that four-burner impinging can well restrict the reactants and flames in the impinging zone, thereby greatly reduce the damage to the refractory walls. Moreover, according to the time-averaged distributions, the intensity and area of CH∗ emission and blackbody radiation are enhanced with the increase of oxygen and carbon molar ratio (O/C), demonstrating that improving O2 velocity promotes the chemical reactions, and flame temperature plays more dominant role than solid particle quantity in blackbody radiation. The time-dependent blackbody radiation evolution presents periodical change. Besides, it is found that O/C or syngas concentration can be feasibly estimated using CH∗ chemiluminescence in OMB gasifier.
1. Introduction The efficient and clean utilization of coal resource is of significant interest in recent years. Coal gasification technologies, especially the opposed multi-burner (OMB) gasification technology using coal-water slurry (CWS) as feedstock, can achieve the clean production of syngas from coal. OMB gasification technology has been widely used by over 50 companies all over the world [1], due to the high efficiency, large scale and good economic benefit. Moreover, impinging stream can effectively strengthen heat and mass transfer, improve particle residence time and reaction efficiency [2]. Hence it has been utilized in many kinds of industrial reactors, also including the OMB gasifier. This paper was to explore the CWS impinging flame characteristics through flame spectral diagnostics, based on a bench-scale OMB gasifier. Flame spectral diagnostics is an important and promising method to obtain the flame information and monitor the combustion or gasification systems [3], because the diagnostic method is non-intrusive and
⁎
the spectral signals are easily measured. Excited CH radical (CH∗) chemiluminescence is one of the most significant radical species, and much research has been reported on the capability of CH∗. It was found that CH∗ can be used to predict flame structure, chemical reaction region, equivalence ratio, etc. [4–6]. However, the ability of CH∗ to estimate total or local flame heat release rate is still uncertain through the work of Hossain and Nakamura [7]. Most of such cases were focused on gas or oil flames, less work has been done on coal flames, especially the impinging-stream coal flames. Because the experiment and measurement systems are more complex, and the interference of broadband blackbody radiation on chemiluminescence is relatively stronger. In the heterogeneous flames, high-temperature solid particles can emit strong and continuous blackbody radiation [8], and blackbody radiation is usually used to evaluate flame temperature on the basis of two-color, multi-color and other pyrometry methods [9–11]. Besides, some researchers have reported their study on blackbody measurements in flames using optical techniques. Pourhoseini and Moghiman
Corresponding authors. E-mail addresses: [email protected] (Q. Guo), [email protected] (G. Yu).
http://dx.doi.org/10.1016/j.fuel.2017.09.094 Received 21 June 2017; Received in revised form 16 August 2017; Accepted 25 September 2017 Available online 03 October 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
Fuel 211 (2018) 688–696
C. Hu et al.
and 700 nm are used in the current work, and all filters can completely cover the camera sensor. The full width and peak transmission are approximately 10 nm and 50%, respectively.
[12] used an IR filter coupled with a digital camera to photograph the qualitative distribution of soot particles in nature gas flame which was injected with pulverized coal. Liu et al. [13] provided a generalized method to estimate the temperature distribution and wavelength-dependent emissivity in sooty flames on the basis of spectroscopic radiation intensity. Liu et al. [14] also calculated the soot temperature, absorption coefficient and volume fraction in ethylene flames using hyperspectral imaging device and the iterative process. In this work, the two-dimensional blackbody radiation distributions of impinging flames were analyzed based on a bench-scale gasifier. For the CWS flame, the blackbody radiation is mainly derived from many kinds of solid particles such as soot particles, coal particles, coal char, ash and slag, etc. [15]. Should explain here the actual solid particle is not the idealized blackbody which can absorb all incident radiation. In the actual industry, the particle is generally regarded as the graybody which has lower emissivity independent of frequency [16]. Hence the statement “blackbody” in this paper was used to represent the solid particles produced in flame. For the OMB gasifier, some research on the spectroscopic characteristics has been conducted. Hu and Song et al. [17,18] investigated the spectral characteristics in diesel and CWS flames using a HR2500+ spectrometer. It is a point measurement in the impinging zone, and only one-dimensional emission information can be measured. In this article, two-dimensional distributions of CH∗ chemiluminescence and blackbody radiation in CWS impinging flames were firstly derived, through image processing method and a CCD camera coupled with multiple bandpass filters (420, 430, 440 and 700 nm). Using CWS as feedstock, the findings can be directly extended to the industrial OMB gasifier. Furthermore, the differences of spectral emissions between diesel and CWS impinging flame are analyzed. The effects of O/C on CWS impinging flames are discussed. And the feasibility of using CH∗ chemiluminescence to predict O/C or syngas concentration is evaluated.
2.2. Operating conditions Diesel and CWS are used as feedstock, and oxygen is the oxidizer in this article. The analysis of diesel and CWS are given in Table 1 and Table 2, respectively. The CWS is prepared using Shenhua bituminous coal, and the solid mass content is 61%. The coal analysis meets the National Standards of PRC (GB/T 212-2008 and GB/T 31391-2015). The operating conditions of each burner are listed in Table 3. All conditions are under gasification atmosphere. O/C is the calculated molar ratio of oxygen and carbon, and it is changed by varying the O2 flow rate. To analyze the differences between diesel and CWS flames under the same O/C, should note that condition 1 and 8 have the same O2 flow rate, and so on. 3. Results and discussion 3.1. Processing method of spectral image The spectral lines of CH∗ chemiluminescence and blackbody radiation exist in CWS flame, as shown in Fig. 2. Take condition 3 for instance, the emission lines around 367 nm arise from H∗, and the emission peak at 409 nm is from NO∗. CH∗ chemiluminescence is taken at 431 nm. Continuous background radiation also can be observed. In the gasifier, the background radiation mainly includes blackbody radiation and CO2∗ emissions [21]. As mentioned above, blackbody radiation was caused by soot particles in diesel flame and solid particles in CWS flame. The broadband range of CO2∗ emissions are from CO2 excitation mainly at 310–600 nm [22–23]. From Fig. 2, CH∗ emission intensity is significantly influenced by the background radiation. Hence, in order to obtain correct CH∗ chemiluminescence image, the blackbody radiation and CO2∗ emissions should be subtracted according to Eq. (3.1):
2. Experimental work 2.1. Experimental setup
∗
CO2 blackbody measurement ICH ∗ = I430 −(I430 + I430 nm nm nm )
As Fig. 1 shows, the experimental work was carried out in a benchscale entrained flow gasifier operated in the ambient atmosphere. Two pairs of burners were oppositely mounted with 90° in the same horizontal plane (impinging plane). The distances of two opposed burners and impinging plane to endoscope are 300 mm and 600 mm, respectively. The burner has two coaxial channels, CWS is transported into the central channel by helical rotor pump, and oxygen is supplied into annular channel from Dewar tank. Diesel is used to preheat the gasifier, then the CWS is injected into gasifier. The temperature is monitored by several B-type thermocouples. Moreover, raw syngas flows downward from gasification chamber to quenching chamber, and the syngas concentration was online measured by a mass spectrometer (Type thermostar GSD 320 T2). The sampling and pretreatment processes of raw syngas have been reported by Niu [19] in detail. Spectroscopic measurement system includes a fiber-optic spectrometer and a high resolution CCD camera coupled with multiple bandpass filters. The spectrometer (HR2500+, Ocean Optics Inc.) was used to detect the spectral signals of impinging zone. The fiber probe was stretched into the sampling port and protected by the cooling jacket. The detailed descriptions have been presented by Zhang and Song [18,20]. Besides, the flames in the gasifier can be visualized through a CCD camera (XCL-500, SONY) combined with a high temperature endoscope (CESYCO, Φ38 mm). The spatial resolution of CCD camera is 2448 × 2048 pixels. Cooling water and purging gas (Ar) can make the endoscope avoid high temperature and pollutants. The spectral images were obtained by setting the bandpass filters in the endoscope. As shown in Fig. 1, a filter slot is located in front of the CCD camera, which is convenient for changing different filters to satisfy experimental request. The filters at central wavelengths of 420 nm, 430 nm, 440 nm
(3.1)
where I means the emission intensity. On the basis of this principle, Karnani et al. [24] put forward the filtering and image subtraction method to produce the soot-free image of CH∗ chemiluminescence in ethylene flame. Zhang et al. [25] verified the processing method and obtain CH∗ image of diesel flame. In this work, the method was adopted to obtain CH∗ chemiluminescence in diesel and CWS flames. Taking condition 6 for example, the detailed subtraction procedure is illustrated in Fig. 3. The first step is to obtain the background radiation at 430 nm, and it can be calculated by the flame images through 420 nm and 440 nm filters according to Eq. (3.2). The specific derivation processes are also reported by Karnani and Zhang [24,25]. background background background I430 = 0.6361 × I420 + 0.3486 × I440 nm nm nm
(3.2)
The second step was to subtract the background radiation from the filtering image captured at 430 nm. Then the CH∗ chemiluminescence distribution in CWS flame was derived. Should mention that the timeaveraged filtering images are processed by averaging at least 50 timedependent images. Besides, as for blackbody radiation images, a 700 nm filter was used to photograph the flames in order to avoid the interference of CO2∗ emissions. 3.2. Spectral emissions in diesel and CWS flames In the experimental process, diesel was used to heat the furnace up. Usually, when the furnace temperature reached about 1400 °C, CWS was injected into a pair of opposed burners, then into the other two burners. Diesel flame is gas-liquid two phase, and CWS flame is gas689
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 1. Schematic sketch of the bench-scale gasifier.
Table 1 Analysis of the diesel. Molecular formula
Relative molecular mass
C (wt%)
H (wt%)
O (wt%)
CxHy
190–220
86.2%
13.7%
0.1%
Table 2 Analysis of the coal. Proximate analysis (wt%)
Ultimate analysis (wt%)
Mad
Vad
Aad
FCad
Cad
Had
Nad
Sad
Oad
2.40
33.20
9.01
55.39
73.49
4.81
1.13
0.61
10.95
ad: air dried base; M: moisture; V: volatile matter; A: ash; FC: fixed carbon. Table 3 Operating conditions of each burner. Condition Number
O/C
1 2 3 4 5 6 7
0.8 0.9 1.0 1.1 1.2 1.3 1.4
Fig. 2. Spectral emission lines in CWS flame under condition 3.
CWS Flow Rate (kg/h)
O2 Flow Rate (L/ min)
O2 Velocity (m/s)
Condition Number
Diesel Flow Rate (kg/h)
10.0
54.9 61.8 68 75 82 88 94
94.4 106.2 118.0 129.8 141.6 153.4 165.3
8 9 10 11 12 13 14
5.1
liquid-solid three phase. Hence, under the same O/C, the spectral emissions in diesel and CWS flames present significant differences. Taking O/C = 1.2 for example, the distinctions were particularly analyzed in this section. 3.2.1. CH∗ chemiluminescence in diesel and CWS flames Based on the processing method above, Fig. 4 shows the two-dimensional CH∗ distributions in diesel and CWS flames. Fig. 4(a) and (b) are under condition 12, Fig. 4(c) and (d) are under condition 5. CH∗ radicals are primarily excited by chemical excitation [26], thus CH∗ distributions can reflect the chemical reaction region in flames. From Fig. 4(a) and (b), the generation of CH∗ radicals was concentrated near 690
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 3. Subtraction procedure diagram of CWS impinging flame.
reflect the spatial distribution of solid particles produced in the flame [12,28,29]. In this section, the particle distributions in diesel and CWS flames were analyzed. From Fig. 5(a) and (b), the blackbody radiation intensity of diesel flame was much weaker than that of high-temperature refractory walls. It can be observed that very weak blackbody radiation in diesel flame existed only in the impinging zone, because it is basically covered by the radiation from the hot refractory walls. After CWS is injected into the gasifier, the blackbody radiation in CWS flame is much stronger than not only the hot walls, but also the diesel flame. Compared with CWS flame, diesel flame is relatively clean and produces less solid particles, indicating solid particle quantity has a great influence on blackbody radiation intensity captured by CCD camera. Besides, as Fig. 5(c) and (d) show, the particles intensely disperse in the entire furnace for two-burner CWS flame, whereas the particles mainly concentrated in the impinging zone for four-burner CWS flame, suggesting that the restriction effects of four-burner impinging on solid particles are stronger than two-burner impinging. It also can be
the burner exits, indicating the chemical reactions mainly occur near the burner exits. When changing the fuel of two burners from diesel to CWS, the generation of CH∗ radicals was mainly located in the impinging zone, rather than near the burner exits. After CWS was injected into the other two burners, the emission intensity and area of CH∗ increased obviously. Which indicated that the core reaction region transferred to the impinging zone for the CWS impinging flame. Diesel is a kind of liquid fuel, which is easier to react with oxygen than CWS, and CWS gasification processes are more complex than diesel. Hence, the diesel reacted with oxygen immediately after it entered into the gasifier, whereas the chemical reactions of CWS and oxygen focused in the impinging zone due to the promotion effects of impinging flames. 3.2.2. Blackbody radiation in diesel and CWS flames For diesel flame, blackbody radiation is mainly emitted by soot particles [27]. For CWS flame, blackbody radiation is derived from many kinds of solid particles mentioned above. According to the previous study, the photograph through infrared filter can qualitatively
Fig. 4. CH* distributions of diesel and CWS flames.
691
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 5. Blackbody distributions of diesel and CWS flames.
Fig. 6. Time-averaged CH* distributions under different conditions.
C2 H + O2 → CH ∗ + CO2
concluded that four-burner impinging can well restrict the reactants and flames in the impinging zone, then greatly reduce the damage to the refractory walls.
∗
Therefore, CH radical is a good marker of reaction region, and CH∗ chemiluminescence is greatly influenced by operating conditions. In this section, the effects of O/C on CH∗ emission distribution were investigated. As shown in Fig. 6, under different O/C, CH∗ radicals are mainly formed in four jet flow zones and impinging zone. The CH∗ emission intensity in impinging zone is much stronger than that in four jet flow zones, indicating that impinging zone is the core reaction region in OMB gasifier. Accordingly, the peak emission intensity and core reaction region area were also analyzed. In order to obtain the correct peak intensity, a circular region with a diameter of 50 mm (size factor is 0.158 mm/pixel) was chosen in the core reaction region. The peak intensity in Fig. 7 is the mean value calculated by computer software based on all the intensities in the circular region. Moreover, the core reaction region area was obtained with Image J software through
3.3. Spectral emissions under different O/C in four-burner CWS flames 3.3.1. Time-averaged CH∗ chemiluminescence under different O/C CH∗ radical is an important intermediate product in flames, hence much research has been reported on the reaction mechanisms of CH∗, and several detailed mechanisms have been proposed [30,31]. Generally, for hydrocarbon/oxygen flames, the following pathways are the major chemical reactions of CH∗ formation:
C2 + OH → CH ∗ + CO
(R1)
C2 H + O → CH ∗ + CO
(R2)
(R3)
692
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 7. CH* peak intensities and core reaction region areas under different O/C.
Fig. 8. Time-averaged blackbody radiation distributions under different conditions.
setting the scale and selecting the uniform threshold. From Fig. 7, CH∗ peak emission intensity is enhanced remarkably with the increase of O/ C. When O/C = 0.9, the peak intensity is only 325 mW Sr−1 m−2. Whereas, when O/C = 1.4, the peak intensity reaches 2261 mW Sr−1 m−2. According to (R1), (R2) and (R3), increasing oxygen amount promotes the chemical reactions and the formation of CH∗. Meanwhile, increasing oxygen velocity improves the atomization effect of burners, then the reactions are further facilitated. Similarly, the area of core reaction region also increases with the rise of O/C.
From Fig. 9, both peak radiation intensity and core radiation region area increase with increasing O/C. In CWS gasifier, the blackbody radiation captured by CCD camera is mostly influenced by flame temperature and solid particle quantity. When the flame temperature is higher, or the particle quantity is larger, the radiation intensity will get stronger. In this work, with the increase of O2 flow rate, more CWS reacts with oxygen and more heat is generated, then the flame temperature is higher [32]. More coal particles are consumed, and the formation of soot particles is weakened when the condition gets closer to complete combustion [33]. It can be inferred that the quantity of solid particles is reduced. Under the joint effects of two factors, the radiation intensity and area still increase, demonstrating that flame temperature plays more dominant role in the blackbody radiation.
3.3.2. Time-averaged blackbody radiation under different O/C The time-averaged blackbody radiation distributions were investigated in this section. As shown in Fig. 8, under different O/C, the core blackbody radiation region is always located in the impinging zone, indicating that the solid particles are restricted in the impinging zone by four-burner impinging flames. With the increase of O/C, the distributions are obviously different, and the peak radiation intensities and core radiation region areas were also calculated through the same ways described in Section 3.3.1.
3.3.3. Evolution of time-dependent blackbody radiation at O/C = 1.2 CH∗ emission distribution cannot be captured directly, hence only time-averaged CH∗ distribution can be derived through image processing method. However, the time-dependent blackbody radiation distribution can be directly captured by CCD camera of which the 693
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 9. Blackbody peak intensities and core radiation region areas under different O/C.
Fig. 10. Time-dependent blackbody radiation distributions under condition 4.
evolution process of blackbody radiation presents periodical change, and the evolution trend mainly includes two stages. One stage is from (c) to (a) and (a) to (b), the radiation intensity and area keep decreasing during approximately ten frames time, indicating the solid particles are gradually consumed by reacting with oxygen at that time. Hence we named this process gradual weakening stage. The other one was named sudden enhancement stage, as observed from (b) to (c). Because the radiation intensity and area dramatically increase in only one frame time due to the flame pulsation and pump fluctuation, indicating more
operating frame rate was 20 fps. Take condition 4 as an example, the evolution of blackbody radiation was illustrated in Fig. 10. The arrows describe the development with time of impinging flame. The marks (a), (b) and (c) correspond to the images in green, blue and red boxes, respectively. Should note that the time intervals of (a) to (b), (b) to (c) and (c) to (a) are about 250 ms, 50 ms and 250 ms, respectively. To more clearly observe and analyze the evolution trend, the timedependent core radiation areas were calculated also using Image J software, as shown in Fig. 11. From Figs. 10 and 11, it is found that the 694
Fuel 211 (2018) 688–696
C. Hu et al.
Fig. 11. Time-dependent radiation area evolution trend under condition 4.
inevitably lower than those in the industrial gasifier, whereas the evolution trend is consistent with the industrial data [34]. Through Peak /(CO + H ) ratio and O/C attempts and comparison, we found that ICH ∗ 2 Peak /(CO + H ) and O/C have the best correlation. As Fig. 13 shows, ICH ∗ 2 present a good linear relationship, and it can be fitted by the following line: Peak /(CO + H ) = 185.47 × O / C −168.05 ICH ∗ 2
(3.3)
The fitting degree of Eq. (3.3) is 0.983. Based on Eq. (3.3), for the given CH∗ peak intensity and O/C, the uncertainties on syngas concentration measurements are within 5%. Therefore, it is a feasible way to predict the syngas concentration according to CH∗ peak intensity and operating condition, or to estimate the operating condition if CH∗ peak intensity and syngas concentration are known. 4. Conclusions This experimental study focuses on the spectroscopic characteristics of CH∗ chemiluminescence and particle radiation in opposed impinging CWS flames, based on an entrained flow bench-scale gasifier. The results can be summarized into the following conclusions: The emission peak of CH∗ chemiluminescence at 431 nm and continuous blackbody radiation can be detected in CWS flames. On the basis of CH∗ distributions, for diesel flame, the chemical reactions mainly occur near the burner exits. For CWS flame, CH∗ radicals are formed in four jet flow zones and impinging zone, and impinging zone is the core reaction region. According to blackbody radiation, the radiation intensity of CWS flame is much stronger than that of diesel flame, because diesel flame is relatively clean and produces less particles. In the four-burner CWS flame, solid particles are mainly restricted in impinging zone. Four-burner impinging can well restrict the reactants and flames in the impinging zone, thereby greatly reduce the damage to the refractory walls. The time-averaged CH∗ distribution and blackbody radiation with O/C were analyzed. With the increase of O/C, CH∗ emission intensity and area are remarkably enhanced, because improving O2 velocity promotes the chemical reactions and atomization effect. Under the joint influence of flame temperature and solid particle quantity, the blackbody radiation intensity and area also increase with rising O/C, indicating that flame temperature plays more dominant role in blackbody radiation. Time-dependent blackbody radiation presents periodical change. The evolution trend includes two stages: a gradual weakening stage and a sudden enhancement stage. Moreover, CH∗ peak intensity/ (CO+H2) ratio and O/C present a good linear relationship, hence it is a feasible way to estimate O/C or syngas concentration using CH∗ chemiluminescence.
Fig. 12. Syngas concentrations under different O/C.
Peak /(CO + H ) ratio and O/C. Fig. 13. Relationship of ICH ∗ 2
coal particles have been supplied into the gasifier. Afterwards, the intensity and area of blackbody radiation are gradually weakened (i.e. the gradual weakening stage). 3.4. Relationship among CH∗ peak intensity, O/C and syngas concentration In our experimental system, changing O/C has a great influence on Peak ) and syngas concentration (CO+H ). Then we CH∗ peak intensity (ICH ∗ 2 attempted to explore the relationship among the three parameters. The ∗ CH peak intensities and syngas concentrations under different O/C are shown in Fig. 7 and Fig. 12, respectively. Both CO and H2 concentrations decrease with increasing O/C. The highest syngas concentration reaches 68.61% at O/C = 0.9, while the lowest value is only 23.55% at O/C = 1.4. The syngas concentrations in this bench-scale gasifier are
Acknowledgments The present work was supported by the National Natural Science Foundation of China (21676091), the Fundamental Research Funds for the Central Universities (222201514336) and the Shanghai Pujiang Program (15PJD011). 695
Fuel 211 (2018) 688–696
C. Hu et al.
[18] Song X, Guo Q, Hu C, Gong Y, Yu G. Optical experimental study on the characteristics of impinging coal-water slurry flame in an opposed multi-burner gasifier. Fuel 2017;188:132–9. [19] Niu M, Yan Z, Guo Q, Liang Q, Yu G, Wang F, et al. Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier. Fuel Process Technol 2008;89:1060–108. [20] Zhang T, Guo Q, Liang Q, Dai Z, Yu G. Distribution characteristics of OH∗, CH∗ and C2∗ luminescence in CH4/O2 Co-flow diffusion flames. Energy Fuels 2012;26:5503–8. [21] Lauer M, Sattelmayer T. On the adequacy of chemiluminescence as a measure for heat release in turbulent flames with mixture gradients. J Eng Gas Turb Power 2010;132:061502. [22] Ballester J, García-Armingol T. Diagnostic techniques for the monitoring and control of practical flames. Prog Energy Combust Sci 2010;36:375–411. [23] Docquier N, Lacas F, Candel S. Closed-loop equivalence ratio control of premixed combustors using spectrally resolved chemiluminescence measurements. Proc Combust Inst 2002;29:139–45. [24] Karnani S, Dunn-Rankin D. Visualizing CH∗ chemiluminescence in sooting flames. Combust Flame 2013;160:2275–8. [25] Zhang Q, Song X, Yu G. Experimental study on CH∗ chemiluminescence characteristics of impinging flames in an opposed multi-burner gasifier. AICHE 2016;7:405–10. [26] Kathrotia T, Fikri M, Bozkurt M, Hartmann M, Riedel U, Schulz C. Study of the H +O+M reaction forming OH∗: kinetics of OH∗ chemiluminescence in hydrogen combustion systems. Combust Flame 2010;157:1261–73. [27] Solomon PR, Best PE. Fourier transform infrared emission/transmission spectroscopy in flames. Combust Meas 1991;43:385–444. [28] Molina A, Shaddix CR. Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion. Proc Combust Inst 2007;31:1905–12. [29] Pastor JV, García-Oliver JM, García A, Micó C, Durrett R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl Energy 2013;104:568–75. [30] Nori VN, Seitzman JM. CH∗ chemiluminescence modeling for combustion diagnostics. Proc Combust Inst 2009;32:895–903. [31] Devriendt KE, Peeters J. Direct identification of the C2H(X2Σ+)+O(3P)→CH(A2△) +CO reaction as the source of the CH(A2△→X2Π) chemiluminescence in C2H2/O/ H atomic flames. J Phys Chem A 1997;101:2546–51. [32] Gong Y, Guo Q, Liang Q, Zhou Z, Yu G. Three-dimensional temperature distribution of impinging flames in an opposed multiburner gasifier. Ind Eng Chem Res 2012;51:7828–37. [33] Lee KO, Megaridis CM, Zelepouga S, Saveliev AV, Kennedy LA, Charon O, et al. Soot formation effects of oxygen concentration in the oxidizer stream of laminar coannular nonpremixed methane/air flames. Combust Flame 2000;121:323–33. [34] Yu Z, Wang F. Coal gasification technology. Beijing: Chemical Industry Press; 2010.
References [1] Guo Q. Proceedings of the 2016 Gasification and Syngas Technologies Conference. Vancouver, British Columbia, Canada, Oct 16–19; 2016. [2] Tamir A. Impinging-stream reactors: fundamentals and applications. Amsterdam: Elsevier; 1994. [3] Docquier N, Candel S. Combustion control and sensors: a review. Prog Energy Combust Sci 2002;28:107–50. [4] Kojima J, Ikeda Y, Nakajima T. Spatially resolved measurement of OH∗ CH∗ and C2∗ chemiluminescence in the reaction zone of laminar methane/air premixed flames. Proc Comb Inst 2000;28:1757–64. [5] Oh J. Spectral characteristics of a premixed oxy-methane flame in atmospheric conditions. Energy 2016;116:986–97. [6] Giassi D, Cao S, Bennett BAV, Stocker DP, Takahashi F, Smooke MD, et al. Analysis of CH∗ concentration and flame heat release rate in laminar coflow diffusion flames under microgravity and normal gravity. Combust Flame 2016;167:198–206. [7] Hossain A, Nakamura Y. A numerical study on the ability to predict the heat release rate using CH∗ chemiluminescence in non-sooting counter flow diffusion flames. Combust Flame 2014;161:162–72. [8] Romero C, Li X, Keyvan S, Rossow R. Spectrometer-based combustion monitoring for flame stoichiometry and temperature control. Appl Therm Eng 2005;25:659–76. [9] Fu T, Wang Z, Cheng X. Temperature measurements of diesel fuel combustion with multicolor pyrometry. J Heat Transfer 2010;132:51602. [10] Khatami R, Levendis YA. On the deduction of single coal particle combustion temperature from three-color optical pyrometry. Combust Flame 2011;158:1822–36. [11] Yuan Y, Li S, Zhao F, Yao Q, Long MB. Characterization on hetero-homogeneous ignition of pulverized coal particle streams using CH∗ chemiluminescence and 3 color pyrometry. Fuel 2016;184:1000–6. [12] Pourhoseini SH, Moghiman M. Effect of pulverized anthracite coal particles injection on thermal and radiative characteristics of natural gas flame: an experimental study. Fuel 2015;140:44–9. [13] Liu H, Zheng S, Zhou H, Qi C. Measurement of distributions of temperature and wavelength-dependent emissivity of a laminar diffusion flame using hyper-spectral imaging technique. Meas Sci Technol 2016;27:25201. [14] Liu H, Zheng S, Zhou H. Measurement of soot temperature and volume fraction of axisymmetric ethylene laminar flames using hyperspectral tomography. IEEE Trans Instrum Meas 2017;66:315–24. [15] Hu C, Gong Y, Guo Q, Song X, Yu G. An experimental study on the spectroscopic characteristics in coal-water slurry diffusion flames based on hot-oxygen burner technology. Fuel Process Technol 2016;154:168–77. [16] Yang S, Tao W. Heat transfer. Beijing: Higher Education Press; 2006. [17] Hu C, Gong Y, Guo Q, Wang Y, Yu G. Experimental study on the spectroscopy of opposed impinging diesel flames based on a bench-scale gasifier. Energy Fuels 2017;31:4469–78.
696