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Experimental Study of the Impinging Flame Height in an Opposed Multi-burner Gasifier Puxing Fan, Yan Gong, Qing Zhang, Qinghua Guo, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: In an opposed multi-burner (OMB) gasifier, fuel is injected through four burners located approximately one-third down the length of the reactor and the flames impinge on each other in the center of the reactor. Understanding the impingement and flame interaction is important for system design, operation, and troubleshooting. On the basis of the benchscale opposed multi-burner gasifier, industrial cameras combined with high-temperature endoscopes and image processing technology are applied to study the characteristics of the impinging flame height of diesel and coal−water slurry (CWS) gasification. A new approach for flame image segmentation based on the pseudo-color enhancement is proposed. The results show that both the impinging flame heights of diesel and CWS increase with the increase of mole ratio of oxygen/carbon (O/C), while a turning point appears at O/C of 1.8 when it is diesel gasification. In comparison to diesel, the amplitude of the increase of CWS flame heights is smaller. The increase of gasification capacity results in the decrease of the fluctuation range of impinging flame heights. The results of fast Fourier transform (FFT) of instantaneous impinging flame heights show that the major frequencies of the impinging flame height are below 10 Hz for both diesel and CWS and the amplitudes of characteristic peaks decrease with the increase of O/C. The impinging flame heights of CWS fluctuate in a larger range compared to diesel. and Heskestad9 derived formulas for the dimensionless flame height based on their experiments. Mikofski et al.10 examined the relationship between inverse diffusion flame (IDF) heights obtained from images of flame luminosity and planar laserinduced fluorescence (PLIF) of hydroxyl radicals (OH) and compared these to flame heights calculated using Roper’s analysis.11 Consalvi et al.12 explored the viability of defining the flame height for upward flame spread from a threshold wall heat flux. Zhang et al.13 discussed the relationships between the oxygen/carbon (O/C) equivalence ratio and impinging flame heights in the experimental study of opposed impinging diffusion flames. As for the impinging flame height in the OMB gasifier, few studies exist because of the difficulty in detecting the flame. On the basis of the bench-scale OMB gasifier, Gong et al.14 studied the impinging flame height of the coal−water slurry (CWS) using the industrial light field camera and high-speed cameras combined with high-temperature endoscopes. However, the flame height images captured in the experiment are just the upper part of the entire flames, which cannot reflect the characteristics of the flame height near the burner plane. Otherwise, only the impinging flame height and pulsation frequency under a fixed CWS flow rate were investigated. To have a complete characterization of the flame height in the bench-scale OMB gasifier and explain its transformation feature, a combination of an industrial high-speed camera and a lateral 45° high-temperature endoscope is applied to capture the entire impinging flame height images along the radial direction of the gasifier and another combination of an

1. INTRODUCTION As a major component of energy, coal is playing and will continue to play an important role in meeting the growth of the world’s energy demand.1 However, the comprehensive utilization efficiency of coal is low, and the problem of environmental pollution is serious at present. Therefore, it is urgent to develop high-efficiency, clean coal utilization technologies. Since 1995, the Institute of Clean Coal Technology (ICCT) at the East China University of Science and Technology (ECUST) has worked on developing the opposed multi-burner (OMB) gasification technology.2 In an OMB gasifier, fuel is injected through four burners located approximately one-third down the length of the reactor and the flames impinge on each other in the center of the reactor. In 2005, ECUST’s OMB gasification technology was first demonstrated with the startup of two OMB units handling 750 and 1150 tons/day of coal.3 The flow field in the OMB gasifier can be divided into six regions: jet-flow region, impinging region, impinging-flow region, recirculation-flow region, re-entry-flow region, and plug-flow region.4 In the recirculation-flow region and reentry-flow region, the temperature can reach about 1800−2500 K.5 At such a high temperature, the upward impinging stream accompanied by a high-temperature flame and a large number of particles would cause erosion on the dome refractory bricks.6 This will lead to dome overtemperature and shorten the lifetime of dome refractory bricks, which is one of the key influencing factors of long-period operation for the gasifier.7 To provide a theoretical basis and guidance for the operation and design of the industrial OMB gasifier, the impinging flame behaviors including impinging flame height and pulsation frequency are studied. As an important characteristic of the combustion flame, the flame height has been studied by many researchers. Zukoski8 © 2014 American Chemical Society

Received: April 2, 2014 Revised: August 2, 2014 Published: August 4, 2014 4895

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Figure 1. Schematic diagram of the bench-scale OMB gasifier.

Figure 2. Combinations of industrial cameras and high-temperature endoscopes. (CCD) camera (JAI BB-500CL, hereafter referred to as JAI camera), which captures flame images along the axial direction of the gasifier. The other one laterally installed is composed of a CESYCO Φ19 mm lateral high-temperature endoscope of 45° optical axis−target angle with 55° view field and an industrial high-speed camera (Mikrotron MC1363, hereafter referred to as MC camera), which captures impinging flame height images along the radial direction of the gasifier. Each imaging system runs with water cooling and inert gas (argon) purging to avoid overheating and keep lenses clean. Diesel and CWS with a solid content of 61% were gasified under atmospheric pressure. The specific operation conditions of diesel and CWS are shown in Tables 1 and 2, respectively. Diesel, CWS, and oxygen were all fed under room temperature. Because CWS has been stirred before feeding, its temperature may rise to a certain extent. The

industrial high-resolution camera and an axial 0° hightemperature endoscope is applied to take the impinging flame images along the axial direction of the gasifier in this work. In combination with digital image processing technology, this paper makes both quantitative and qualitative analyses of the flames of diesel gasification and CWS gasification. Through summarizing and analyzing the relationship between operation conditions and flame characteristics, including impinging flame height and pulsation frequency in the bench-scale OMB gasifier, a theoretical basis and guidance for the operation and design of an industrial OMB gasifier are provided.

2. EXPERIMENTAL SECTION The schematic diagram of the bench-scale OMB gasifier is shown in Figure 1. The four non-premixed burners are side-mounted oppositely in a horizontal plane with the angle of 90° between each other. The inner diameter of the refractory bricks in the gasifier is 300 mm. Because the distance between the dome refractory brick and burner plane is 600 mm, the ratio of height/diameter of the gasifier (gasifier H/D) is 2.0. Supplied by the Dewar vessels, oxygen and argon are transported into the annulus of the burners and endoscopes, respectively, and their flow rates are controlled by the mass flow controllers. Thermocouples are installed at different positions of the gasifier to monitor the wall temperature of the gasifier. The impinging flame in the gasifier was visualized and captured by two sets of imaging systems, as shown in Figure 2. The one installed on the top of the gasifier is composed of a CESYCO Φ38 mm axial high-temperature endoscope of 0° optical axis−target angle with 60° view field and an industrial high-resolution charge-coupled device

Table 1. Operation Conditions of Diesel Gasification (Single Burner) flow rate of diesel (kg/h) 3

4

4896

O/C

flow rate of O2 (m3/h)

velocity of O2 (m/s)

1.6 1.7 1.8 1.9 1.6 1.7 1.8 1.9

3.84 4.08 4.32 4.56 5.16 5.46 5.76 6.12

111.33 118.29 125.25 132.21 148.44 157.72 167.00 176.27

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Table 2. Operation Conditions of CWS Gasification (Single Burner) flow rate of CWS (kg/h) 10

12

O/C

flow rate of O2 (m3/h)

velocity of O2 (m/s)

0.9 1.0 1.1 1.2 0.9 1.0 1.1 1.2

3.72 4.14 4.56 4.98 4.44 4.98 5.46 5.94

107.60 119.55 131.51 143.46 129.12 143.46 157.81 172.16

Figure 3. Impinging flame height image of diesel and CWS.

values in Tables 1 and 2 were chosen on the basis of the combination of scaling factors and stoichiometric ratios. The design flow rate of CWS is 10 kg/h for a single burner, which guarantees that the oxygen outlet velocity is about 120 m/s at O/C = 1.0 (typical gasification condition), similar to the industrial scale. On this basis, the other conditions were determined. The operation conditions of diesel gasification were chosen to keep the oxygen outlet velocity of diesel gasification similar to that of CWS gasification. Fluctuations in flow rates of slurry and oxygen can affect the performance of industrial gasifiers and downstream systems. Hence, we want to carry out research on this question through the bench-scale OMB gasifier to provide a theoretical basis and guidance for the industrial-scale OMB gasifier. In the preheating period, diesel was fed through the center of the two-channel burners by gear pumps. When the temperature in the gasifier reached about 1200 K, the operation condition was adjusted to the flow rate of diesel of 3 kg/h and O/C of 1.6. When the temperature was about 1250 K, the lateral imaging system was inserted into the gasifier and adjusted well to meet the demand of flame image acquisition. When the temperature was about 1300 K, the two imaging systems were controlled to record the flame images at the same time. The change of O/C was adjusted by the flow rate of oxygen, which was controlled by the mass flow controllers. After all of the flame images under the flow rate of diesel of 3 kg/h were collected, the flow rate of diesel was increased to 4 kg/h by adjusting the gear pump. The acquisition procedure was the same as above. When the temperature in the gasifier reached about 1500 K, the CWS pumped by a progressive cavity pump was switched to replace diesel through adjusting the valves under the condition of the flow rate of CWS of 10 kg/h and O/ C of 1.2. To ensure the success rate of CWS feeding and keep the lens of endoscopes clean, the amount of oxygen was large during the feeding period of CWS. After the feeding of CWS was successful, the flow rate of oxygen was reduced to the condition of O/C of 0.9. After 30 min of stable time, the flame images were collected. The acquisition procedures of the other conditions were the same.

impinging flame is fuzzy and difficult to be separated from the image. For this purpose, a new approach for flame image segmentation based on the pseudo-color enhancement is proposed. Pseudo-color16 is a color mapping of a monochrome image array, which is intended to enhance the detectability of details within the image. Pseudo-color enhancement can be achieved in many ways. In this paper, intensity segmentation, the most simple and intuitive method, was selected. The gray image f(x,y) was divided into N intervals Ii (i = 1, 2, ..., N) according to the gray level from black to white. All of the brightness values of the interval Ii were given the corresponding color Ci, so that we could convert a gray image into a color image. Then, the color level, which was most similar to the flame shape, would be used to identify the flame edge. Before performing the pseudo-color enhancement, histogram equalization was used to enhance the contrast of the flame image. The concrete realizing process is shown in Figure 4, in which the segmentation result is good. The flame can be extracted from the image accurately.

3. MEASUREMENT PRINCIPLES 3.1. Flame Image Processing. To obtain the whole impinging flame height image, the lateral endoscope was adjusted along the axial direction of the gasifier to make the lowest point of the view field of the endoscope located at the center of the burner plane. Typical impinging flame height images of diesel and CWS are shown in Figure 3. The upward stream formed by the impinging of four burner flames is mixed with a large number of high-temperature particles. Because the outlet velocity of the burner is high, the upward stream has a relatively high velocity and will entrain the surrounding fluids.15 At the same time, the closed furnace and dome refractory brick will also constrain the movement of the upward stream. Moreover, for the temperature of brick wall to be about 1400 K,5 the strong visible radiation generated causes great background disturbance to the illumination of impinging flames. Under multiple influencing factors, the edge of the

Figure 4. Sample of the flame image segmentation process.

Constrained by the jets from four burners, the hightemperature zone of the impinging flame cannot reach the refractory wall.5 Hence, the impinging flame concentrates in the middle of the field of view. Because the temperature of the upward flame formed by impinging decays in its rising process17 and the temperature has an approximate linear relationship with the gray value of the flame image,18 the intensity segmentation method is suitable to distinguish the impinging flame from the background of the refractory bricks. As a result, the pseudo-color enhancement method is valid in the segmentation of the impinging flame height image. The impinging flame height images of diesel gasification and CWS gasification were processed with the pseudo-color enhancement method. The segmentation process of four 4897

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Figure 5. Segmentation results of four consecutive frames.

consecutive frames is shown in Figure 5. In comparison to the diesel flame, the CWS gasification flame is more obscure and its difference with the background of refractory bricks is smaller; however, the pseudo-color enhancement method is still applicable. As seen from the segmentation results, the growth in the flame height of diesel gasification is relatively obvious. The contour of the CWS gasification flame is more obscure and irregular, and the rising trend of the CWS gasification flame height is not quite obvious. 3.2. Calibration of Image Pixel Scale. Because the lateral 45° endoscope has a 45° optical axis−target angle, the photographed object in the picture has the characteristic of being small in the distance but big in the vicinity. Therefore, a method of polynomial fitting was applied to calibrate the image pixel scale. To keep consistent with the experiment in the gasifier, the distance between the endoscope and the ruler was kept at 150 mm. The ruler image taken in the experiment is shown in Figure 6. The distance between two adjacent numbers is 10 mm. We can see that the resolution of the image in the far field of view decreases greatly, so that the graduated scale in the

lower area is difficult to distinguish. Moreover, the intrinsic characteristics of the endoscope also make the ruler image indistinct. Because it is designed to work under high temperatures, its aperture is small. Without enough light, the picture taken under a normal temperature is not clear. The image pixel number x and corresponding scale value y in the distinct area are obtained through the image processing software. A fitting curve shown in Figure 7 is obtained by

Figure 7. Polynomial fitting curve of the image scale.

means of polynomial fitting. The polynomial equation is shown as eq 1, of which the adjusted R2 value is 0.99949. y = −2.19154 + 0.31523x − 5.30558 × 10−4x 2 + 1.37040 × 10−6x 3

(1)

The measurable range calculated through eq 1 is 406 mm. Because the value calculated through the view field of the endoscope is 403 mm, the calibration method is feasible. Because the light path of the endoscope is constant, the impinging flame height can be calculated by eq 2.

H = 403 − y

(2)

The flame pulsates along both the axial and radial directions of the gasifier. As a result, it may deviate from the central axis of the gasifier occasionally, which can affect the accuracy of the calculation results.

Figure 6. Image of a ruler captured by the lateral 45° imaging system. 4898

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flame torches are not strictly impinged, the center of impact concentrates in the center of the gasifier, which shows that the upward impinging flames are constrained near the central axis of the gasifier. Therefore, in most cases, the upward impinging flame is on the calibration plane. The calibration method above is suitable for the calculation of the impinging flame height. 3.3. Definition of Flame Height. Various definitions of flame height exist.19,20 In this paper, the instantaneous impinging flame height (H) is defined as the distance between the luminous flame front point and the burner plane. Corresponding with gasifier H/D, flame H/D is defined as the ratio of the impinging flame height to inner diameter of the gasifier. In addition, the intermittency21 is introduced to define the average flame height. The intermittency, I, is defined as the fraction of time during which at least part of the flame lies above the burner plane located at elevation, H. The flame height when I equals 0.5 is defined as the average flame height, Havg. 3.4. Flame Pulsation Frequency. The impinging flame pulsates periodically, which could greatly affect the flame height, gasification efficiency, and flame structure. Many researchers22−24 have studied the flame pulse behavior of various flames and developed multiple empirical formulas for estimating flame pulsation frequency. Considering the complexity of the flame pulsation in the gasifier, it is difficult to use the existing formulas and methods. In this paper, fast Fourier transform25 (FFT) of the instantaneous impinging flame height is applied to study the flame pulsation frequency.

Figures 8 and 9 show the average flame images of diesel gasification and CWS gasification taken by the JAI camera,

Figure 8. Average images of the impinging flame of diesel gasification in different operation conditions.

4. RESULTS AND DISCUSSION 4.1. Impinging Flame Height. 4.1.1. Impinging Flame Height of Diesel Gasification. To satisfy the sampling demand, the frame rate and shutter speed of the high-speed camera were set as 400 fps and 1/2494 s, respectively. A total of 3000 frames of the flame image were taken in each operation condition. The impinging flame height of diesel gasification in different operation conditions is shown in Figure 10. The corresponding intermittency is shown in Figure 11. Figure 12 shows the average impinging flame height of diesel gasification in different operation conditions calculated by the intermittency. Under the experimental operation conditions, with the diesel flow rate unchanged, the impinging flame height rises with the increase of O/C, in general, until a turning point appears near O/C = 1.8. The fluctuation range of the impinging flame height

Figure 9. Average images of the impinging flame of CWS gasification in different operation conditions.

respectively. In diesel gasification, the frame rate of the JAI camera was 15 frames per second (fps) and its shutter time was 1 /10000 s. Because the temperature in the gasification of CWS was higher than that of diesel, the aperture of the Φ38 endoscope was adjusted to a smaller value. The frame rate of the JAI camera was still 15 fps, while its shutter time was set as 1 /2000 s. A total of 105 frames of images were captured in each operation condition. As seen from Figures 8 and 9, the impinging flame presents a cross structure in the burner plane, which is formed by the four flame torches from the four opposed burners. Although the four

Figure 10. Impinging flame height of diesel gasification in different operation conditions. 4899

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Figure 11. Intermittency of the impinging flame height of diesel gasification in different operation conditions.

that of diesel. To avoid image saturation, the frame rate and shutter speed of the high-speed camera were set as 600 fps and 1 /1661 s, respectively. The impinging flame height of CWS gasification in different operation conditions is shown in Figure 13. The corresponding intermittency is shown in Figure 14. Figure 15 shows the average impinging flame height of CWS gasification in different operation conditions calculated by the intermittency. In comparison to the gasification of diesel, a large number of high-temperature fly ashes and unburned carbon particles exist in the gasification process of CWS, which makes the flame contour of CWS more indistinct. In the gasification process of CWS, the impinging flame height varies in a larger range. The impinging flame can be detected from 50 to 380 mm above the burner plane, while this range in diesel gasification is 150−390 mm. Under the same CWS flow rate, as O/C increases, the impinging flame height increases, in general. However, the increase is less than that of diesel. Increasing the flow rate of CWS, the change of the impinging flame height is not quite obvious. However, the difference of the impinging flame height between different O/C decreases. As for the average impinging flame height, its variation is similar to that above. In addition, the variation range of the average impinging flame height decreases with the increase of the flow rate of CWS.

Figure 12. Average impinging flame height of diesel gasification in different operation conditions.

diminishes with the increase of O/C. Increasing the flow rate of diesel, the impinging flame height rises to a certain extent and the fluctuation range of the impinging flame height diminishes. As seen from Figure 12, the average impinging flame height follows the above change law. Both the increase of O/C and diesel flow rate diminish the fluctuation range of the impinging flame height. 4.1.2. Impinging Flame Height of CWS Gasification. The temperature of CWS gasification in the gasifier is higher than

Figure 13. Impinging flame height of CWS gasification in different operation conditions. 4900

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Figure 14. Intermittency of the impinging flame height of CWS gasification in different operation conditions.

operation conditions need to be determined according to their gasifier H/D. 4.2. Flame Pulsation Frequency. Figures 16 and 17 show the results of FFT of the instantaneous impinging flame height of diesel gasification in Figure 10 and CWS gasification in Figure 13, respectively.

Figure 15. Average impinging flame height of CWS gasification in different operation conditions.

Figure 17. Frequency spectra of the impinging flame height of CWS gasification in different operation conditions.

In conclusion, in the experimental operation conditions, the impinging flame cannot scour the dome refractory bricks directly when gasifier H/D is above 1.30. Because the impinging flame pulsates in the axial direction of the gasifier, it is likely to exceed the view field of the endoscope. In addition, the purge gas of the endoscope affects the flame to some extent. The possible two reasons make the measured values of the impinging flame height smaller than the real values. As a result, for the industrial gasifiers, the gasifier H/D is usually above 1.50. With regard to the running gasifiers with fixed gasifier H/ D, to reduce the possibility of overtemperature of the dome and prolong the lifetime of dome refractory bricks, suitable

As shown in Figures 16 and 17, the major frequencies of the impinging flame height locate in the low-frequency region and the characteristic frequencies are below 10 Hz for both diesel and CWS. Under the same flow rate of fuels, the amplitude of the characteristic peak is reduced and its reduction range decreases with the increase of O/C. Increasing the flow rate of fuels, the amplitude of the characteristic peak reduces greatly. Consequently, under the experimental operation conditions, both the increase of O/C and flow rate of fuels reduce the pulsation range of the impinging flame height and enhance the stability of the impinging flame.

Figure 16. Frequency spectra of the impinging flame height of diesel gasification in different operation conditions. 4901

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which shows that the average flame height begins to reduce. Figure 19 shows the gray value distributions in the central axis of average impinging flame height images of diesel gasification in different operation conditions. It is the quantitative expression of brightness in the central axis of average images of the flame height, and what it shows, supports the above analysis. 5.2. Average Impinging Flame Height Images of CWS Gasification. In the experiment of CWS gasification, the brightness of average flame images has no significant changes with the increase of the CWS flow rate. Under the same flow rate of CWS, the brightness of flame images increases with the increase of O/C, especially the brightness of the upper half region. The results in Figure 21 support the above analysis. In addition, Figures 18 and 20 show that the brightest region of the average impinging flame height images concentrates near the central axis of the average images, which is essentially coincident with the central axis of the gasifier. It can also prove that the flame is constrained on the calibration plane in most cases. 5.3. Result Analysis. Above all, in the gasification of diesel, with the increase of the diesel flow rate, with a constant O/C ratio, the fuel participating in the reaction increases; thus, the heat release increases and the temperature rises, leading to the increase of the brightness of the flame image. Under the same flow rate of diesel, with the increase of O/C, the heat release and the temperature increase; hence, the brightness of the flame image increases. However, when the amount of oxygen increases to a certain degree, part of diesel reacts with oxygen completely. The amount of diesel arriving in the impinging zone and rising up after impinging reduces. As a result, a turning point appears at O/C = 1.8 for the impinging flame height. In comparison to diesel, as a solid−liquid mixed fuel, the density, ignition point, and viscosity of CWS are higher. Thus, under similar conditions, its residence time and reaction time are longer than those of diesel. Hence, the fluctuation range of the impinging flame height of CWS gasification is larger than that of diesel gasification. With the increase of the flow rate of CWS, the amount of particles entering the gasifier increases and the particle concentration in the impinging region increases, which leads to the increase of the collision frequency of particles and, moreover, promotes the cohesion and agglomeration of particles.26 In addition, the reducing of burner

The difference of the impinging flame height between diesel gasification and CWS gasification is that the pulsation amplitude of the CWS impinging flame height is greater than that of diesel. That is to say, the process of CWS gasification is more complex and unstable.

5. RESULT VERIFICATION AND ANALYSIS To verify the results above, 3000 frames of impinging flame height images taken by the MC camera were averaged for each operation condition. Figures 18 and 20 show the average

Figure 18. Average impinging flame height images of diesel gasification in different operation conditions.

impinging flame height images of diesel gasification and CWS gasification, respectively. The gray value distributions in the central axis of the average impinging flame height images in different operation conditions are shown in Figures 19 and 21, respectively. 5.1. Average Impinging Flame Height Images of Diesel Gasification. As seen from Figure 18, the edge of the average impinging flame is more fuzzy and cannot be extracted from the flame image. However, a qualitative estimation on the flame height from the brightness of images is feasible. With the increase of the diesel flow rate, the brightness of average images increases. Under the same flow rate of diesel, the brightness of average images increases with the increase of O/C, especially the brightness in the upper half region, which shows that the occurrence probability of the flame at a higher position in the image increases and the average flame height grows. When O/ C is above 1.8, the brightness of images begins to decrease,

Figure 19. Gray value axial distributions of average impinging flame height images of diesel gasification in different operation conditions. 4902

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Figure 20. Average impinging flame height images of CWS gasification in different operation conditions.

Figure 21. Gray value axial distributions of average impinging flame height images of CWS gasification in different operation conditions.

atomization performance also results in the increase of the particle size. Because the following behaviors of large particles are worse than those of small particles,6 the rising height of particles entrained by the upward impinging stream has no significant change after capacity increase. Thus, the impinging flame height has no obvious change. The refractory bricks used in the laboratory are made of corundum. Figure 22 shows the refractory bricks after use for 2 years in the bench-scale OMB gasifier. The impinging flame

height image and the marks on the picture are used to indicate the relative position of refractory bricks and burner plane. As seen from Figure 22, the bricks are eroded most seriously between 0 and 200 mm above the burner plane, in which region the occurrence probability of the impinging flame height is maximum. Thus, for the bench-scale gasifier with a gasifier H/D of 2.0, the impinging flame will not affect the dome refractory bricks seriously under the experimental operation conditions. As for the industrial gasifier, it will be better to make the gasifier with a decided gasifier H/D work under design capacity, which can guarantee the carbon conversion efficiency and gasification efficiency. In addition, capacity fluctuation within a certain degree will have no severe effect on the dome refractory bricks. When a new gasifier is designed, an appropriate gasifier H/D should be confirmed through the required capacity and capacity range.

6. CONCLUSION On the basis of the bench-scale OMB gasifier, the impinging flame height and pulsation frequency of diesel gasification and CWS gasification are studied using the digital image processing technology, respectively. The main conclusions can be summarized as follows: (1) The impinging flame can be

Figure 22. Image of refractory bricks after use for 2 years in a benchscale OMB gasifier. 4903

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(14) Gong, Y.; Guo, Q. H.; Zhang, J.; Fan, P. X.; Yu, G. S. Ind. Eng. Chem. Res. 2013, 52, 3007−3018. (15) Turner, J. S. J. Fluid Mech. 1986, 373, 431−471. (16) William, K. P. Digital Image Processing: PIKS Inside, 3rd ed.; Pixel Soft, Inc.: Los Altos, CA, 2001; pp 285−286. (17) Sun, Z. H.; Dai, Z. H.; Zhou, Z. J.; Guo, Q. H.; Yu, G. S. Ind. Eng. Chem. Res. 2012, 51, 2560−2569. (18) Li, Z. H. Visual Method of Flame’s Development and Application; Institute of Engineering Thermophysics, Chinese Academy of Sciences: Beijing, China, 2006; pp 40−50 (in Chinese). (19) Du, J.; Axelbaum, R. L. Combust. Flame 1995, 100 (3), 367− 375. (20) Roper, F. G.; Smith, C.; Cunningham, A. C. Combust. Flame 1977, 29, 227−234. (21) Zukoski, E. E.; Cetegen, B. M.; Kubota, T. Symp. (Int.) Combust., [Proc.] 1984, 361−366. (22) Bejan, A. J. Heat Transfer 1991, 113 (1), 261−263. (23) Malalasekera, W. M. G.; Versteeg, H. K.; Gilchrist, K. Fire Mater. 1996, 20, 261−271. (24) Pagni, P. J. Appl. Mech. Rev. 1990, 43 (1), 153−170. (25) Wu, Y. C.; Wu, X. C.; Cen, K. F. Fire Technol. 2012, 48, 389− 403. (26) Li, C.; Dai, Z. H.; Li, W. F.; Xu, J. L.; Wang, F. C. Powder Technol. 2012, 225, 118−123.

extracted from the high-temperature background of the refractory wall accurately with the flame image segmentation method based on the pseudo-color enhancement. (2) Under the experimental operation conditions, both the impinging flame heights of diesel gasification and CWS gasification increase with the increase of O/C, while a turning point appears at O/C = 1.8 when it is diesel gasification. Increasing the flow rate of fuel, the increase of impinging flame heights of diesel is great, while the increase for CWS is not big. Both the increase of O/C and flow rate of fuels result in the decrease of the fluctuation range of the impinging flame height. In the experimental operation conditions, the impinging flame cannot scour the dome refractory bricks directly when gasifier H/D is above 1.30. (3) Under the experimental operation conditions, both of the major frequencies of the impinging flame height of diesel gasification and CWS gasification are located in the lowfrequency region and the characteristic frequencies are below 10 Hz. Both the increase of O/C and flow rate of fuels cause the decrease of the amplitude of characteristic peaks. In comparison to the diesel gasification flames, the amplitude of characteristic peaks is bigger for CWS gasification flames, which fluctuate in a larger range.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-64252974. Fax: +86-21-64251312. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Nature Science Foundation of China (21176078), the National Key State Basic Research Development Program of China (973 Program, 2010CB227004), the National High Technology Research and Development of China (863 Program, 2012AA053101), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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dx.doi.org/10.1021/ef5007287 | Energy Fuels 2014, 28, 4895−4904