Three-Dimensional Temperature Distribution of ... - ACS Publications

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Three-Dimensional Temperature Distribution of Impinging Flames in an Opposed Multiburner Gasifier Yan Gong, Qinghua Guo, Qinfeng Liang, Zhijie Zhou, and Guangsuo Yu* Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Based on the bench-scale opposed multiburner (OMB) gasifier, a new combination of the optical sectioning tomography (OST) and two-color method has been proposed and applied to reconstruct the three-dimensional (3-D) temperature distribution of the gasifier by employing a single charge-coupled device (CCD) camera installed on the top of the gasifier. The reconstruction method is first applied in diesel gasification and the reconstructed results are validated by calibrated thermocouples and a side imaging system; thus the 3-D temperature distribution of coal−water slurry (CWS) gasification is reconstructed. The results show that the temperature distribution is more spatially homogeneous and the average temperature is higher with blurry contours of flame than that of diesel. The overall temperature of the gasifier rises with the increase of oxygen to carbon ratio, and the high temperature region which ranges from 1700 to 2200 K remains in the axis of the gasifier steadily and remains a safe distance from the refractory wall, which makes the temperature of the refractory wall below 1550 K and makes the participating medium maintain a stable condition for gasification.

1. INTRODUCTION Clean coal technologies are now becoming popular because of their high efficiencies and minimal environmental impacts.1 As one of the key technologies, coal gasification offers one of the most versatile and cleanest ways to convert coal into electricity, hydrogen, and other valuable energy products.2 Developed during the 1990s, the opposed multiburner (OMB) gasification technology of the East China University of Science and Technology (ECUST) was first demonstrated in 2005 and has been selected for 20 projects that are either operating, under construction, or in the development phase.3 As a typical entrained-flow gasifier, the OMB gasifier operates at high temperatures (above ash slagging temperatures, generally above 1570 K) to ensure high carbon conversion and a syngas free of tars and phenols. However, the existence of abnormal operating conditions caused by high temperature could lead to unreasonable temperature distributions and axis aberrance of the flame in the gasifier, which would result in overprescriptive temperature in certain regions and additionally impact the refractory life along with the working duration of the burners. Therefore, the investigations of flame characteristics and temperature distributions in the gasifier are essential to the development and optimization of gasification technologies. Great progress has been made in diagnostic techniques for combustion furnaces.4−7 Since there are great differences between gasification and combustion, the high temperature and high pressure in industrial gasifiers make it extremely difficult to measure the actual status in gasification furnaces. In this case, investigations of the flame characteristics and temperature distributions which are based on bench-scale gasifiers become necessary. ECUST has set up a bench-scale OMB gasifier and Yan et al.8 reconstructed temperature distributions of flame sections in that gasifier by the method of filtered back projection. © 2012 American Chemical Society

Intrusive and nonintrusive temperature measurement techniques are the two major diagnostic techniques for monitoring and detecting the temperature in furnaces.9 The main problem with the traditional method using thermocouples focuses on its limit of special resolution and the difficulty in applying it to an industrial gasifier along with an unavoidable time lag. Therefore, nonintrusive temperature measurement techniques are widely used in furnaces. In order to avoid the difficulty of calibration, the two-color method is used by many scholars in the combustion research field and it has become a mature method for studying the projection temperature distribution of combustion flames. The method was originally used in the diagnostics for temperature measurement in diesel engines,10 and with years of developments it could be applied to online temperature measurements in furnaces along with several improvements.11−13 Studies on the reconstruction of two-dimensional (2-D) or three-dimensional (3-D) temperature distributions in furnaces are mainly based on the combustion process. Zhou et al.14 reconstructed the 3-D temperature distribution from the 2-D flame temperature images transformed from color flame images by using a modified Tikhonov regularization method. Wang et al.15 established a model for simultaneously measuring the twodimensional temperature and particle concentration distribution of a cross section from the images of the flame. Yuan et al.16 calculated the 3-D temperature field distribution by using the method of reconstruction images based on projection, the simultaneous algebraic reconstruction technique (SART), and the least squares estimation (LSE) algorithm. Received: Revised: Accepted: Published: 7828

December 7, 2011 May 7, 2012 May 17, 2012 May 17, 2012 dx.doi.org/10.1021/ie2028602 | Ind. Eng. Chem. Res. 2012, 51, 7828−7837

Industrial & Engineering Chemistry Research

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Such 3-D temperature reconstruction processes are mainly based on flame images which are simultaneously captured from several angles of view. In order to obtain the 3-D luminosity information of a semitransparent object by the limit of a single angle of view, optical sectioning tomography (OST) was first proposed by Agard and Sedat in 1983.17 A position fixed charge coupled device (CCD) camera was used to capture images from a fluorescent chromosome which was considered as a combination of N layers of 2-D sections. A series of projection images in a sequence of focus planes was gained and transformed into an original luminosity distribution of each section, by which the 3-D luminosity distribution of the chromosome was reconstructed. The OST method is combined with the relationship between the gray level of the flame image and the temperature; Zhou et al.18 reconstructed the 3-D flame temperature distribution of candlelight. Based on the bench-scale OMB gasifier, this study proposes a new combination of OST and the two-color method in order to reconstruct the 3-D temperature distribution in the gasifier.

It could be discretized to N

g (x , y , z′) =

∑ f (x , y , iΔz)∗h(x , y , z′ − iΔz)Δz (3)

i=1

where N = D/Δz refers to the number of sections of the object, each with the thickness of Δz. Since a 3-D object can be considered as a combination of parallel N-layer two-dimensional sections, the two-dimensional images captured by a camera contain 3-D information of the given luminous object. Keeping the object and the imaging system on the same direction of the optical axis, a series of projection images can be captured on different cross sections of focus in sequence: N

g (x , y , jΔz) =

∑ f (x , y , iΔz)∗h(x , y , jΔz − iΔz)Δz , i=1

j = 1, 2, ..., N

(4)

which can be simplified to N

2. MEASUREMENT PRINCIPLES 2.1. Optical Sectioning Tomography (OST). The principle of OST is shown in Figure 1.19 A 3-D luminous

gj =

∑ fi ∗hj − i ,

j = 1, 2, ..., N (5)

i=1

If the PSFs of the system at different defocusing distances are known, eq 5 becomes closed and solvable, which means the original luminosity distribution of each section f i (i = 1, 2, ..., N) can be obtained. 2.2. Two-Color Method. The two-color method detects the radiation emitted by the soot particles at two different wavelengths and calculates the flame temperature by detecting the unknown quantity of flame emissivity.20 According to Wien’s law of radiation, the spectral intensity emitted by a blackbody is defined by eq 6: Iλ =

1 C1 −C2 / λT ελ e π λ5

where Iλ is the monochromatic radiation intensity at a given wavelength λ and a given temperature T. ε λ is the monochromatic emissivity for a given wavelength λ, which approximately equals 1 for an artificial blackbody. C1 and C2 are known as Planck’s radiation constants, having values of 3.741 832 × 108 W·μm4/m2 and 1.438 769 × 104 μm·K, respectively. Calibration coefficients kr, kg, and kb are introduced in order to modify the red (R), green (G), and blue (B) values of a CCD camera, which form monochromatic radiation intensities at wavelengths of λr = 700 nm, λg = 546.1 nm, and λb = 435.8 nm, respectively:

Figure 1. Optical system schematic of OST.

object of thickness D emits monochromatic noncoherent light f(x, y, z), which forms images through a set of lens with object distance df and image distance dI. Assume that the coordinates of the object space originate from the left boundary of the object and teh z-axis coincides with the optical axis. The coordinates of the image plane are denoted as (x′, y′). When the lens focuses on the object plane z′, the projection with a luminosity distribution of g′(x′, y′, z′) on the image plane can be regarded as the superposition of the image of the focused plane z′ and other defocused planes. For a linear shiftinvariant optical imaging system, the luminosity projection on the imaging plane g1(x, y, z′) is the convolution of the luminosity function of the object f(x, y, z) and the point spread function (PSF) h(x, y, z1 − z′) of the optical system according to Fourier’s optics theory:

I λr = k rR ,

I λg = k gG ,

C 1 ε(λr) 15 e−C2 / λrT πR λr

(1)

kg =

Since the major concern is the object but not the image which zoomed in or out, the projection image is back-projected to the focused plane in object space; thus the total luminosity distribution g(x, y, z′) is formed when the plane z′ is focused on.

C 1 ε(λg ) 15 e−C2 / λgT πR λg

kb =

C 1 ε(λb) 15 e−C2 / λbT πR λb

g (x , y , z′) =

∫0

(7)

(8)

Based on the graybody assumption, the ratios of emissivities at different wavelengths could be simplified as 1. Keeping one of the three primary colors unchanged (take R, for example),

T

f (x , y , z) ∗h(x , y , z′ − z) dz

I λb = k bB

Combine eqs 6 and 7: kr =

g1(x , y , z′) = f (x , y , z) ∗h(x , y , z1 − z′)

(6)

(2) 7829

dx.doi.org/10.1021/ie2028602 | Ind. Eng. Chem. Res. 2012, 51, 7828−7837


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modify the other two primary colors which could reflect the relative spectral distribution of flame correctly: R′ = R ,

G′ = cgG ,

B′ = cbB

(9)

where the coefficients cg = kg/kr and cb = kb/kr. Thus, the temperature of any pixel in a flame image could be achieved: −1 ⎞⎡ ⎛ I λ λ r 5 ⎞⎤ ⎛1 1 r ⎢ ⎥ ⎟ T = −C2⎜⎜ − ⎟⎟ ln⎜⎜ λg ⎠⎢⎣ ⎝ I λgλg 5 ⎟⎠⎥⎦ ⎝ λr −1 ⎛1 ⎞⎡ ⎛ cgRλr 5 ⎞⎤ 1 ⎢ ⎥ ⎜ ⎟ = C2⎜⎜ − ⎟⎟ ln⎜ λr ⎠⎢⎣ ⎝ Gλg 5 ⎟⎠⎥⎦ ⎝ λg

(10)

2.3. Reconstruction of 3-D Temperature Distribution. In the process of achieving the cross-section images from projection images of different focuses in sequence, the superposed energy in projection images is reverted to every cross-section plane, which reflects on the luminosity distribution f i (i = 1, 2, ..., N) of each plane. Equation 11 is used to calculate the luminosity distribution in each cross section of flame: N−j

−1

f j = gj −

∑ i=1−j

gi + j ∗k 0∗hi −

∑ gi+ j ∗k 0∗hi i=1

(11)

where gj refers to the projection image captured by the CCD camera. k0 is the specially designed high-pass filter. The function is to block the low-frequency information of defocus and preserve the high-frequency information. hi is the PSF on different defocusing distances. The luminosity distribution of each cross-section image is split into a red component and a green component in order to gain the R and G values of every pixel in the flame image. The temperature of each pixel can be solved by eq 10, and the temperature distribution of each cross section in 3-D combustion space is achieved.

Figure 2. Schematic of the bench-scale OMB gasifier.

Before an experiment, CWS that is made of Shenfu coal is prepared with a concentration of 61%. During the gasification, oxygen is injected into the gasifier as oxidant, and diesel alternates with CWS as fuel. The diesel is gasified at a flow rate of 2.00 kg/h, along with the oxygen flow rate of 3.00 N m3/h for each burner, which are continuously measured by calibrated mass flow meters. CWS is gasified at atmospheric pressure, its average flow rate remains at 11.50 kg/h for each burner, and the oxygen flow rate varies from 5.70 to 6.00 and 6.30 N m3/h, which is controlled and measured by gas mass flow meters in order to change the mole ratio of oxygen and carbon (O/C) at 0.90, 0.95, and 1.00, respectively. The focus plane of the optical system changes in sequence in order to capture the origin images of the impinging flame under different focus sections in an axial direction which is for reconstructing the temperature distribution of each section. Meanwhile, the side imaging system captures impinging flame images in a radial direction with different heights by moving the endoscope up or down, which cooperates with thermocouples in order to test and verify the reconstructed 3-D temperature distribution. 3.2. Determination of the PSFs of Imaging System. For the purpose of reconstructing each cross-section image of flame by using the OST method, the exact PSFs at different defocusing distances must be acquired. However, the PSF of a typical imaging system is combined with the PSFs of the lens, CCD camera, image grabbing system, and displaying circuit, which is difficult to measure. For convenience, the whole

3. EXPERIMENTAL SETUP AND DETERMINATION OF PARAMETERS 3.1. Experimental Setup. The diagram of the bench-scale OMB gasifier is shown in Figure 2. The four nonpremixed burners are side mounted oppositely in a horizontal plane with the same angle of 90° between each other. Fuels, diesel and coal−water slurry (CWS), are injected into the gasifier by gear pumps and monopumps, respectively. Both are pumped through the center of the two-channel burner and atomized by a high velocity of oxygen flow which goes through the annulus. In the gasification chamber, four mixed streams of fuel and oxygen react under reducing conditions to produce raw syngas. The impinging flame in the gasifier is visualized and captured by two sets of imaging systems, including two specifically designed high temperature endoscopes (CESYCO) with optical assemblies which cooperate with two high resolution industrial CCD cameras (JAI BB-500CL, Camera Link standard), frame grabbers, associated image processing software, and a graphics workstation. Each imaging system runs with water cooling and inert gas purging to avoid excessive heat and keep the lens clean. The top imaging system is installed on the top refractory wall of the gasifier. 7830



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Figure 3. Actual captured images and calculated images.

Figure 4. Defocused PSFs under different distances from the lens.

Each calculated defocused image, which is generated by the convolution of a focused image and a specific PSF, is compared to the actual defocused image, which is captured by the imaging system in a specific defocusing distance in order to get the exact defocused PSF. The mean dual scale edge structure similarity (MDESSIM) method of image quality evaluation is used in the process of judging the similarity of two images,23 by which the similarity of a calculated defocused image and an actual defocused image could be judged quantitatively and objectively. The luminosity comparison function, the contrast comparison function, and the structural character similarity of macroedges combined with microedges comparison function are integrated in the MDESSIM evaluation method. The range of quantitatively evaluating similarity is from 0 to 1. The evaluative range of similarity in quantity is from 0 to 1 when two images are compared; the higher the similarity, the higher the value. As shown in Figure 3, Figure 3a is the captured focused image, Figure 3b is the captured defocused image, and Figure 3c is the calculated defocused image. The similarity of Figure 3b to Figure 3c is 0.951 when they are compared by the MDESSIM method, which means a high similarity of the two images. The assumed PSF involved in the calculation under this specific condition could be regarded as the actual defocused PSF. A focused image is calculated from a captured defocused image, shown in Figure 3d. The similarity of Figure 3a to Figure 3d is 0.937. In the same way, a set of totally 144 defocused PSF matrixes including nine object plane positions of red wavelength (700 nm) and green wavelength (546.1 nm) are obtained. A series of defocused PSF matrixes of different focuses (300, 600, and 900 mm between the focus plane and lens, the respectively) under the condition of the red wavelength (700 nm) and a distance of

imaging system could be considered as a black box with only an input image and an output image, and the exact PSF of the whole system could be obtained by experimental measurement. Since the PSF of an imaging system is the inverse Fourier transformation of the optical transfer function (OTF), the PSF could be obtained by calculating the OTF of the system. The defocused OTF can be expressed by the Stokseth approximation formula:21 Hs(ω) = 2(1 − 0.69ω + 0.0076ω 2 + 0.043ω3) J1(a − 0.5aω) a − 0.5aω

(12)

where a = (4π/λ)wω; w denotes the optical path difference, λ is the wavelength, the frequency ω = λf sin α, f is the spatial frequency, and α refers to the aperture angle. The ISO 12233 Test Chart22 is photographed by the imaging system which is mounted on the top of the gasifier under different conditions. Keeping the camera position unchanged and the distance between the object plane and the lens at 200 mm, the clear image C2−2 of the object plane is captured when the camera is focused. Changing the focus of lens, which makes the focus plane at different distances from the lens, eight defocused images (B2−3, B2−4, ..., B2−10) are captured in sequence where the focus plane is 300−1000 mm (at 100 mm intervals) from the lens. Changing the distance between the object plane and the lens from 300 to 1000 mm (at 100 mm intervals), another eight focused images concomitant with 64 defocused images under different distances are obtained. On the basis of the assumption of a series of optical path differences, the OTFs are calculated by the Stokseth approximation formula, by which a series of PSFs could be obtained with the use of the inverse Fourier transformation. 7831



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reconstructed and information from other layers is eliminated, which leaves only focused luminosity information. Because the depth of field increases with the increase of focus distance, more noise from other layers is introduced. The intensity of the drawing contours in sections also decreases with the increase of the focus distance. However, temperature is basically related to the ratio of the red and green matrixes of the flame image during the process of temperature calculation using the two-color method; thus the reconstruction results show that the OST method is effective and valid. 3.3. Calibration of the CCD Imaging System. Based on the two-color method, the CCD imaging system should be calibrated through a blackbody furnace (with temperature errors within ±5 K). The calibration results under typical conditions are shown in Table 1. The color radiant images of the blackbody furnace are captured under different temperatures and divided into red, green, and blue monochromatic images. Subsequently, all the pixel values in each monochromatic image are averaged to an R, G, or B value. kr and kg could be calculated by eq 8 and the coefficient cg = kg/kr. The relationship between cg and temperature data is fitted to a simple equation:

200 mm from the object plane to the lens are shown in Figure 4. In order to demonstrate the validity of the OST, the luminosity of a 3-D object with several 2-D sections is reconstructed. The schematic of the verification test is shown in Figure 5.

Figure 5. Schematic of the verification test for OST.

cg = f (T ) = 2.11343 × 10−4T − 0.21657

(13)

The calibration curve is linear and its adjusted R2 value is 0.999 81. Combined with eq 10, the radiant temperature of a blackbody could be calculated. Set a series of operating conditions, capture the radiant image, and calculate the corresponding temperature, which is compared with the set temperature. The relative error is less than 0.5%.

Five different drawings are hand painted on five glass layers which are separated 100 mm from each other in parallel. The imaging system focuses on each layer and captures images. Since the defocused PSFs of different focus distances are already obtained, the 2-D luminosity distributions of each section are reconstructed by the OST method. The reconstructed results are shown in Figure 6. The original captured images are divided into red and green monochromatic images, which are reconstructed separately. As shown in Figure 6, the drawings in each section are clearly

4. RESULTS AND ANALYSIS 4.1. Reconstruction of 3-D Temperature Distribution for Diesel Gasification and Validation of the OST Method. Based on the bench-scale OMB gasifier, diesel is gasified under different operating conditions mentioned in section 2.1. Adjust the focus plane of the imaging system at a series of specific positions, which are at 200−1000 mm (at 100 mm intervals) distances from the top of the gasifier, and a set of original data of nine sections is obtained by capturing flame images. For each section, 300 consecutive flame images are recorded (at a speed of 15 frames/s) and averaged. Figure 7 shows the average data from focusing the imaging system on planes at 300, 600, and 900 mm distance from the top. For the purpose of reducing unavoidable fluctuations in the process, measurements are taken at least stabilizing for 1 h. Images at different focusing distances are taken sequentially controlled by the stepping motor, and each set of data costs at most 3.5 min. Therefore, fluctuations appear much less in sequential averaged images. Each original flame image is composed of red, green, and blue monochromatic images, which are regarded as the original component images of each section. Equation 11 is applied in calculating the red or green monochromatic image ( f j) of each cross section, including the pixel values of R or G in each f j. Thus, the temperature distribution of each section in the gasifier could be calculated by eq 10. The results are shown in Figure 8. The impinging flame presents as a cross structure in the burner plane (600 mm from the top) which is formed by four flame torches extending from the burners to the axis with a maximum temperature of 2150 K, which appears at the outlet of the burners due to the high

Figure 6. Captured images and reconstructed images of each section. 7832



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Table 1. Calibration Results measd temp/K

shutter speed/s

R

G

B

1273 1373 1473 1573 1673 1773

1/30 1/100 1/500 1/1000 1/2000 1/4000

118.11 132.85 86.67 113.30 130.19 135.15

82.12 92.18 62.18 84.50 102.69 113.50

88.56 76.61 47.72 54.32 59.98 63.58

kr 5.832 1.681 7.118 1.322 2.513 4.840

× × × × × ×

kg 105 106 106 107 107 107

3.066 1.233 6.727 1.544 3.456 7.602

× × × × × ×

104 105 105 106 106 106

cg

calcd temp/K

0.0526 0.0734 0.0945 0.1167 0.1375 0.1571

1269.2 1375.4 1477.1 1570.4 1664.6 1780.7

Figure 7. Average images of diesel flame taken from different focal planes.

oxygen and diesel ratio. The maximum temperature reaches 1950 K at the center of the impinging area, and its spatial density appears to be sparse with some uncontinuous areas of 1500 K mingling in the center of this section. The refractory wall could keep from corroding by the high temperature flame and maintain a temperature around 1450 K together with the participating medium. Above the burner plane (200−500 mm from the top), the impinging height of gasifying flame remains at 200 mm, and the temperature of the participating medium declines with the distance getting close to the top. Due to the dimension limit in burners and the furnace diameter of the bench-scale gasifier, a small degree of jet flow deviation could result in straying from the axis during the rising of the impinging flame. Therefore, the high temperature area of 2100 K appears in the region between the angle of burner (2) and burner (3) (see Figure 8) as the four flame torches are not strictly impinged. However, the high temperature area could keep a certain distance from the refractory wall which remains around 1350 K. Below the burner plane (700−1000 mm from the top; see Figure 8), the impinging flame extends down. By the effect of unstrictly symmetrical impinging, a high temperature area of 1850 K appears in the region between the angle of burner (1) and burner (4) corresponding to the temperature distribution above the burner plane. The analysis is aimed at the region in and above the burner plane in order to maintain the validity of temperature reconstruction. Therefore, the validation processes are focused on the area in and above the burner plane. The side imaging system captures radial impinging flame images under different heights. Meanwhile, the calibrated thermocouples mounted at

Figure 8. Three-dimensional temperature distribution of diesel gasification.

the inner refractory wall indicate the actual temperature of each section. The impinging flame images captured by the side imaging system are processed by the two-color method in order to get the instantaneous projection temperature distribution of the axial flame which is shown in Figure 9. There are clear boundaries of the flame with the temperature range 1900−2200 K and the participating medium maintains at around 1300 K, which corresponds with the reconstructed temperature distribution. The average flame height is simultaneously measured by this system and in accordance with the reconstructed result. B-type thermocouples with corundum shells are used to obtain the independent local temperatures at different locations as shown in Figure 10. The measured errors are mostly caused by the conductive heat loss of thermocouple wire and the radiative heat loss between a thermocouple and the refractory wall. The length-to7833



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Figure 11. Measured temperature of thermocouples.

8, the results are in good agreement with the measured temperature data. In the burner plane (600 mm distance from the top), the center temperature (150 mm from the refractory wall) is 100 K lower than that at 100 mm from the wall in the reconstructed temperature field, but the measured results at the same locations achieve unity, which meet at 1600 K and 100 mm from the wall. The temperature at 50 mm from the wall and in the refractory shows little difference between the reconstructed and measured results, declining to 1550 and 1480 K, respectively. In the plane of the 500 mm distance from the top, the center temperature is 19 K lower than that of the 100 mm distance from the wall, which reflects the departing of the impinged flame from the center in the axis, and the phenomenon also presents in the reconstructed temperature field in Figure 8. In the upper space of the gasifier, the measured and reconstructed results are consistent with each other, and the temperature meets around 1380 K at the plane of 200 mm from the top. In the planes below the burner plane, the measured temperature distribution is more even, and so are the reconstructed results. The relative error between the measured and reconstructed temperatures is less than 6.25%, and the results show that the reconstruction method is feasible to be applied in reconstructing the 3-D temperature distribution in a gasifier. 4.2. Three-Dimensional Temperature Distribution of CWS Gasification. Figure 12 shows the average flame images of CWS gasification under different oxygen to carbon ratios (O/C = 0.90, 0.95, and 1.00, respectively) which are taken from a fixed focus plane at the burner plane. The dark pixels in the images are caused by particles adhering to the lens. The validated OST method is applied in reconstructing the 3-D temperature distribution of CWS gasification under a series of oxygen to carbon ratios of 0.90, 0.95, and 1.00, respectively. The results are shown in Figure 13. The flame structure is greatly determined by the characteristics of fuels as well as the oxygen to carbon ratio. Compared with diesel, the CWS contains a mass of solid particles with greater inertia and larger size, and the calorific capacity is lower due to the water in the CWS. As a result, the reaction time of the CWS is much longer than that of diesel during the gasification. The severe gasification reaction causes many high temperature particles within a range of 1500−2300 K which make the cross structure of the impinging flame in the burner plane become much thicker. Also, the temperature distribution

Figure 9. Projection temperature distribution of axial flame.

Figure 10. Locations of thermocouples.

diameter ratio of the thermocouple wire is kept at more than 200, at which the conductive heat loss is negligible. When the gasifier temperature arrives at heat equilibrium, the surface temperature of the refractory wall is adjacent to the surrounding gas; thus the radiative heat loss between the thermocouple and the refractory wall is negligible.8 The measured temperatures by seven thermocouples mounted at different locations are shown in Figure 11. Compared with the reconstructed temperature field in Figure 7834



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Figure 12. Average images of CWS flame captured under different oxygen to carbon ratios.

Figure 13. Three-dimensional temperature distribution of CWS gasification under different oxygen to carbon ratios.

the oxygen to carbon ratio, which rises from 1880 (O/C = 0.90) to 1920 K (O/C = 0.95) and then to the maximum of 2060 K (O/C = 1.00). The proportion taken by the high temperature area reduces from about 1/3 of the cross section (O/C = 0.90) to about 1/4 (O/C = 0.95) and then to a minimum proportion of about 1/6 (O/C = 1.00), which reflects the restriction on flame increases when oxygen speed rises in this cross section. For both CWS and diesel gasification, the temperature of the impinged flame declines when it moves upward. The gasifying flame of CWS travels much higher than that of diesel; the high temperature area of 1600−1800 K could reach above 400 mm from the burner plane (200 mm from the top) and the height of the impinged flame rises continuously with the increase of the oxygen to carbon ratio. The proportion taken by the high temperature area which ranges from 1700 to 2200 K increases, and the maximum of the flame height rises with the increase of oxygen speed in the space between 200 and 400 mm from the burner plane (400−200 mm from the top), which differs from the phenomenon of the cross section which is 100 mm above the burner plane. With the area influenced by high temperature increasing, the refractory wall could be corroded occasionally by

gets more spatially homogeneous in every cross section, and the overall temperature is higher with blurry contours of the flame which compares to diesel gasification. The high temperature area extends through the axis and occupies a large area of the section plane, but the maximum temperature is lower than that of diesel with a relatively small temperature gradient. As the operating condition changes, it is obvious that the average temperature in each cross section rises with the increase of the oxygen to carbon ratio. In the burner plane, higher oxygen speed means the enhancement of restriction on the flame structure and increase in reaction intensity. When the oxygen to carbon ratio increases from 0.90 to 1.00, the high temperature area becomes more centralized and the highest temperature rises from 2210 (O/C = 0.90) to 2250 K (O/C = 0.95) and then to the maximum of 2290 K (O/C = 1.00). The temperature of the refractory wall remains below 1550 K and is seldom influenced by the high temperature flame even under the condition of asymmetrical oxygen speed in each burner or unstrictly symmetrical impinging. When the flame rises axially to the cross section which is 100 mm above the burner plane (500 mm from the top) after impingement, the average temperature rises with the increase of 7835


Industrial & Engineering Chemistry Research the high temperature flame. This phenomenon appears in some instantaneous images of recorded data, but the time-averaged temperature distribution shows that the high temperature flame maintains around the central axis with a stable gasifying condition.

ACKNOWLEDGMENTS

■

REFERENCES

This work is financially supported by the National Nature Science Foundation of China (20876048) and the National Key State Basic Research Development Program of China (973 Program, 2010CB227004).

5. CONCLUSIONS Based on the bench-scale OMB gasifier, the OST method and the two-color method have been combined and applied to reconstruct the 3-D temperature distribution of the gasifier by employing a single CCD camera. During the reconstruction process, the precise series of PSFs of the imaging system that calibrated through the blackbody furnace are first obtained from the inverse Fourier transformation of a series of OTFs calculated by the Stokseth approximation formula, and accurately determined by the MDESSIM method. These PSFs and the original image data of each focal section participate in the calculation of the OST method; thus the 3D temperature distribution is reconstructed by the two-color method. The 3-D temperature distribution of diesel gasification is validated before the reconstruction method is applied to CWS gasification. The results obtained by the side imaging system and calibrated thermocouples have proved the reliability of the combined OST and two-color method, and the 3-D temperature distribution of CWS gasification is reconstructed. The results show that the average temperature in each cross section of the gasifier rises while the highest temperature in the burner plane rises from 2210 to 2290 K and its area becomes more centralized as the oxygen to carbon ratio increases from 0.90 to 1.00. When the flame rises axially after impingement, the proportion taken by the high temperature area reduces in the cross section which is 100 mm above the burner plane as the oxygen to carbon ratio increases. This restriction from high speed oxygen becomes relaxed when the flame moves upward between the cross sections 200 and 400 mm from the burner plane. The flame height in the gasifier rises with the increase of the oxygen to carbon ratio, and the refractory wall could be corroded occasionally by the high temperature flame, but the time-averaged temperature distribution shows that the flame maintains around the central axis of the gasifier and keeps the wall temperature below 1550 K. The reconstructed results are greatly helpful for the research of general temperature distributions in gasifiers, and for the impact of uncertainties in CWS gasification it could not strictly reflect every detail of the temperature information. For the industrial OMB gasifier, the peculiar flow field is very similar to that of the bench-scale gasifier and could maintain a more favorable circumstance for gasification. The high temperature areas in the gasifier always keep a safe distance from the refractory wall even in the dome and severely reacting section of the burner plane; in this way the durability of both burners and refractory wall would be extended, which could maintain a stable condition for gasification.

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