A simple measurement method of temperature and emissivity of coal ...

Fuel 88 (2009) 980–987

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A simple measurement method of temperature and emissivity of coal-fired flames from visible radiation image and its application in a CFB boiler furnace Zhi-Wei Jiang, Zi-Xue Luo, Huai-Chun Zhou * State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Hongshan District, Wuhan, Hubei 430074, PR China

a r t i c l e

i n f o

Article history: Received 12 October 2008 Received in revised form 27 November 2008 Accepted 15 December 2008 Available online 9 January 2009 Keywords: Temperature measurement Radiation property Image processing Coal combustion Circulating fluidized bed

a b s t r a c t The two-color method is widely used in non-contact temperature measurement of combustion flames from radiation images, traditionally based on the spectroscopic characteristics of the image sensor, and/or the representative wavelengths for the red, green and blue filters of the image-forming devices, for example, Charge Coupled Device (CCD) cameras. In this paper, a new method to derive the temperature and emissivity images from a color CCD image is presented for coal-fired combustion processes, which is independent of the spectroscopic characteristics and the representative wavelengths of the CCD and the image processing system. In this method, it is only necessary to capture image information by the image processing system from a blackbody furnace at different temperatures which cover the possible temperature range of the combustion processes to be measured. In a 480 ton/h coal-fired circulating fluidized bed (CFB) boiler experiments are conducted. The temperature measured by this method is validated by that obtained by thermocouple, and it varies obviously with the load the boiler. The flame emissivity provided by this system is worthy for further study. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction As an advanced coal combustion technology, CFB technology has been developed rapidly and globally due to its insensitivity to fuel quality and its low emission of SO2 and NOx. More and larger capacity CFB boilers are in commercial operation in China [1]. Thus, there is an increasing demand for developing advanced combustion monitoring, control, and optimization technologies applicable in CFB boilers. A number of methods for flame temperature measurements have been studied in the past. At present, the most widely applied methods use physical probes, such as thermocouples or gas-sampling probes, with obvious disadvantages including intrusive nature, degradation in harsh environments, and singlepoint measurement. As a widely applied non-intrusive measurement method, the two-color method has been used for the temperature measurement of particle-laden flames. There has been a continuing effort in applying this technique to various situations. Hottel and Broughton [2] pioneered the two-color technique and applied it to determine the flame temperature in utility furnaces. The most extensive application of the approach has been found in combustion engines where not only the flame temperature but also the soot concentration was measured [3,4]. The method has been expanded over recent years to various open flames, either premixed or diffusion flames [5,6]. Similar to the two-color method principle, the multi* Corresponding author. Tel.: +86 27 87557815; fax: +86 27 87540249. E-mail address: [email protected] (H.-C. Zhou). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.12.014

color method has also been developed [7–9]. All these applications detect only single-point temperature within a flame. Recently, Tago et al. [10] determined the 2-D temperature and emissivity distribution of high emissivity materials using two-color thermometry principle, and compared narrow-bandpass filters with wide-bandpass filters in thermometry. Brown et al. [11] reported a method to determine combustion flame temperature for premixed lean burn conditions using ultraviolet emission. Berry et al. [12] obtained the soot surface temperature in flames with ethylene fuels with a calibrated two-color soot pyrometer technique. CCD sensors can reveal spectroscopic radiation information of combustion flames and are suitable for the temperature distribution measurement. Shimoda et al. [13] reported a method of predicting unburnt carbon in a coal-fired utility boiler using an image processing unit and a furnace model unit. Zhou et al. [14] employed a CCD sensor to measure the temperature image from a laboratory-scale, coal-fired combustor in order to monitor the flame condition. Huang et al. [15] developed a novel optical instrumentation system for on-line continuous measurement of temperature distribution in a furnace. The system consisted of a single CCD camera and optical filters mounted on a rotatable holder to acquire alternatively the radiation images at two different wavelengths. All above methods of temperature measurements with CCD cameras have the shortcoming of complex system structures. Wang et al. [16] used a color CCD camera to measure temperature field of an experimental furnace. This method utilized the three primary colors of color images captured by a color CCD camera

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Z.-W. Jiang et al. / Fuel 88 (2009) 980–987

Nomenclature b b C1 C2 E g g k r r

blue component blue spectral tristimulus value the first Planck’s constant, 3.741832 108 W m4/m2 the second Planck’s constant, 1.4388 104 m K monochromatic radiation power, W m2 m1 green component green spectral tristimulus value proportional coefficient red component red spectral tristimulus value

to calculate the temperature field, which does not need extra devices. But this method needs to know the representative wavelengths of the three primary colors and ignore the influence of the bandwidth of color CCD cameras. Ten years ago, it was not possible to visualize the combustion condition in a utility boiler furnace due to its large size [17]. In recent years, the radiation image processing technique has been utilized to reconstruct 2-D/ 3-D temperature distributions inside large-scale, coal-fired boiler furnaces [18,19]. As a first step in this technique, it is necessary to get 2-D temperature images from the color flame images based on the two-color method. Most recently, vision-based measurement techniques for the on-line monitoring and characterization of coal-biomass co-firing flames on an industrial-scale combustion test facility was reported [20]. In this paper, a simple method is developed to obtain the temperature and emissivity images from a color flame image with the assumption of gray radiation for particle-laden combustion processes. In this method, it is not necessary to know the spectroscopic characteristics of the CCD camera, even independent of any wavelength information associated with the image processing system. Experiments were conducted on a 480 ton/h CFB boiler. The measurement principle and the experiments will be described in the following sections.

T e k

temperature, K emissivity wavelength, m

Subscripts b blue component b blackbody g green component r red component k wavelength

With the assumption of gray radiation for coal-fired flames [23], the emissivity of the flame is independent of wavelength, i.e., ek ðTÞ ¼ eðTÞ, and Eq. (2) becomes

8 rðTÞ ¼ eðTÞr b ðTÞ > < gðTÞ ¼ eðTÞg b ðTÞ > : bðTÞ ¼ eðTÞbb ðTÞ

where rb, gb and bb represent the color images captured by the image processing system from a blackbody with known temperatures

8 R 780 > r ðTÞ ¼ kr 380 rðkÞEb ðk; TÞ dk > < b R 780 g b ðTÞ ¼ kg 380 gðkÞEb ðk; TÞ dk > > R 780 : bb ðTÞ ¼ kb 380 bðkÞEb ðk; TÞ dk

C1 Eb ðk; TÞ ¼ 5 k exp CkT2 1

ð1Þ

where Eb ðk; TÞ is the monochromatic radiation power of a blackbody (W m2 m1), k is the wavelength of the radiation (m), T is absolute temperature (K), C1 and C2 are the first and second Planck’s constants. For a particle-laden flame, the red (r), green (g), and blue (b) primary color signals in a color radiation image are received by a CCD camera in an image processing system. These obey the Planck’s radiation law, and can be expressed as [22]

8 R 780 > rðTÞ ¼ kr 380 rðkÞek Eb ðk; TÞ dk > < R 780 gðTÞ ¼ kg 380 gðkÞek Eb ðk; TÞ dk > > R 780 : bðTÞ ¼ kb 380 bðkÞek Eb ðk; TÞ dk

ð2Þ

where kr, kg and kb are the instrument constants which are affected by many factors including the radiation attenuation due to the image processing system and atmosphere, the observation distance, the lens properties and the signal conversion, etc. ek is the mono chromatic emissivity of the flame, r ðkÞ; gðkÞ and bðkÞ represent the red, green, and blue spectroscopic characteristics of the CCD camera, which play a key role in the measurement of temperature.

ð4Þ

These can be calculated easily before the measurements in industrial furnaces as a calibration procedure. The relationship between rb and T can be fitted by a polynomial function as

rb ðTÞ ¼

M X

ai T i

ð5Þ

i¼0

which will serve as a basis to calculate the emissivity of a flame as its temperature is known. From Eq. (3) it is easily yielded that

2. Measurement principle The radiation of a blackbody is governed by Planck’s radiation law [21]:

ð3Þ

eðTÞ ¼

rðTÞ gðTÞ bðTÞ ¼ ¼ r b ðTÞ g b ðTÞ bb ðTÞ

ð6Þ

Selecting any two of the three primary colors (red and green colors are selected in this paper) and rearranging Eq. (6) yields

rðTÞ rb ðTÞ ¼ ¼ K rgðTÞ gðTÞ g b ðTÞ

ð7Þ

which means that the colorimetric parameter Krg(T), the ratios of the red and green image data for a gray flame and a blackbody, is consistent at the same temperature T. Then the relationship between T and Krg(T) can also be fitted by a polynomial function as

T¼

N X

bj K jrg

ð8Þ

j¼0

After the flame temperature T is calculated using Eq. (8), its emissivity can be obtained by

rðTÞ

eðTÞ ¼ PM

i¼0 ai T

i

ð9Þ

The procedure of the measurement is summarized below: (i) Capture blackbody images at different temperatures with the image processing system with different CCD shutter speeds, and determine polynomial functions of Eqs. (5) and (8).

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Stainless steel pipe

Video out connector

Image guide

Lens

CCD camera Power input connector Cooling air

Viewing angel field of 78 in the vertical direction (90 in the horizontal direction)

Fig. 1. Schematic structure of the flame image detector.

(ii) Capture flame images with the image processing system with a certain CCD shutter speed, and calculate Krg from r and g in the images by Eq. (7). (iii) Calculate temperature T from Krg obtained from the flame images by Eq. (8). (iv) Calculate emissivity e from r and T by Eq. (9). It is obvious that the measurement procedure is independent of rðkÞ; gðkÞ; and bðkÞ and the wavelength. Besides the temperature, the emissivity is determined by the present method, which is also excellent than the general two-color method.

age detectors. Because the temperature range of the combustion flame is very large, the differences between the monochromatic radiations in the R, G, and B images are very large, which cause either the R image to be saturated or the B image to have little sensitivity. The white balance control (WBC) function of the CCD camera was utilized to extend the measurable temperature range of the detectors by changing the gain of R and B, as done in [16]. According to the practical condition, the gain setting for R and G in this work is set to be 95 and 117, respectively, and the auto gain control of the CCD camera is disabled. Table 1 gives the image data of R, G, and B obtained from the images of the blackbody furnace with different temperatures by

3. Calibration of the flame image detectors 1000

Y =435.30015+153.3156 X+278.40623 X

2

950

Temperature/

The flame image detector consists of a lens, an image guide, and a Samsung SCC-B2303P color CCD camera, as shown in Fig. 1. The image guide is fixed in the center of a stainless steel pipe that is inserted into the water-wall of the furnace. The image guide is cooled by air to avoid overheating. The lens’ surface is also purged by the cooling airflow to keep it dust-free. The radiation image conveyed by the image guide enters the color CCD camera. The color flame image detectors are used to get the approximately monochromatic radiation images of red (R), green (G) and blue (B). A blackbody furnace with a temperature range from 973 to 3273 K (with errors within ±5 K) is used to calibrate the flame im-

900

850

800

750

Blackbody temperature (°C)

Shutter speed (s)

Red (r)

Green (g)

Blue (b)

Colorimetric data (r/g)

725.00 750.00 750.00 775.00 800.00 800.00 825.00 825.00 850.00 850.00 875.00 875.00 900.00 900.00 925.00 925.00 950.00 950.00 975.00 975.00 999.00 999.00

1/30 1/30 1/50 1/120 1/30 1/50 1/50 1/120 1/50 1/120 1/120 1/250 1/120 1/250 1/250 1/500 1/250 1/500 1/500 1/1000 1/500 1/1000

79.41 120.28 69.85 56.54 207.36 143.29 186.13 107.80 213.20 135.30 184.39 116.33 229.73 157.86 192.09 126.11 228.22 159.62 196.10 126.56 232.09 158.28

102.32 146.70 84.94 64.98 232.14 159.18 200.47 114.21 221.89 138.96 182.42 115.29 217.92 149.19 178.39 116.08 204.57 142.74 172.96 110.08 197.79 136.03

23.98 35.05 19.14 17.68 37.37 43.65 73.75 45.97 77.43 54.88 67.32 44.12 75.28 55.72 63.40 44.86 69.31 53.88 60.37 42.36 64.72 51.03

0.7761 0.8199 0.8223 0.8701 0.8933 0.9002 0.9285 0.9439 0.9608 0.9737 1.0108 1.0090 1.0542 1.0581 1.0768 1.0864 1.1156 1.1183 1.1338 1.1497 1.1734 1.1636

700 0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

r/g Fig. 2. Variation of the temperature with the colorimetric data (r/g), and the corresponding polynomial fitting function.

1600 2 Y =-2987.78765+18.40576 X-0.03433 X +2.0404E-5 X

3

1400 1200 1000

rb

Table 1 Red (r), green (g) and blue (b) data (0–255) obtained from the images of a blackbody furnace with different temperatures by the image processing system with different shutter speeds, and the colorimetric data r/g.

800 600 400 200 0 700

750

800

850

900

950

1000

Temperature/ Fig. 3. Variation of the red color (rb) of the blackbody with the temperature, and the corresponding polynomial fitting function.

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the image detector with different shutter speeds. The ratio of R and G is also given in Table 1. As known from Table 1, the ratios of R and G with different shutter speeds are close to each other at the same temperature, so the relationship of the temper-

ature and the ratio could be obtained. Fig. 2 shows the variations of T, the temperature, with Krg, the ratio of R and G, and its polynomial fitting. The relationship between T and Krg is fitted to be

Table 2 Comparison of the temperature and emissivity between the setting values and the calculated values, and the relative errors. Blackbody temperature (°C)

725.00 750.00 750.00 775.00 800.00 800.00 825.00 825.00 850.00 850.00 875.00 875.00 900.00 900.00 925.00 925.00 950.00 950.00 975.00 975.00 999.00 999.00

Blackbody emissivity

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

Shutter speed (s)

1/30 1/30 1/50 1/120 1/30 1/50 1/50 1/120 1/50 1/120 1/120 1/250 1/120 1/250 1/250 1/500 1/250 1/500 1/500 1/1000 1/500 1/1000

Temperature

Emissivity

Calculated values (°C)

Relative errors (%)

Calculated values

Relative errors (%)

721.98 748.16 749.62 779.47 794.42 798.92 817.67 828.06 839.61 848.54 874.72 873.44 906.33 909.22 923.20 930.46 952.83 954.93 967.02 979.57 998.53 990.65

0.4164 0.2456 0.0502 0.5773 0.6974 0.1345 0.8884 0.3709 1.2220 0.1720 0.0316 0.1788 0.7032 1.0245 0.1944 0.5898 0.2982 0.5186 0.8183 0.4684 0.0471 0.8358

0.94 1.08 0.97 0.92 1.05 1.06 1.10 0.97 0.98 0.97 1.01 1.01 0.93 0.97 1.04 0.91 0.96 0.94 1.05 0.94 0.98 1.08

6.4340 7.9601 2.7928 8.0092 4.6655 5.6120 9.6403 2.9041 2.1402 2.6827 1.3017 0.7168 6.5789 3.4238 3.5887 9.4854 4.3321 6.2654 4.7423 5.7625 1.9494 8.0083

Fig. 4. Schematic of the furnace and the image processing system.

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T ¼ 435:30015 þ 153:3156 K rg þ 278:40623 K 2rg

ð10Þ

Because of the limitation of the 8-bit resolution in the image framegrabber, the range of R, G and B data is limited to value within 0– 255, thus, the images of R, G, or B obtained with lower shutter speeds will easily be saturated at higher temperatures. However, it is easy to calculate the equivalent values of R, G, and B at a fixed, lower shutter speed from those got at higher temperatures by the detector with faster shutter speeds. For example, from Table 1 it can be seen that at 750 °C, the values for r are 120.28 and 69.85 at shutter speeds of 1/30 (s) and 1/50 (s), respectively. That means, the equivalent value for r at 1/30 (s) can be got from that at 1/50 (s) being multiplied by 120.28/69.85 = 1.722. By this procedure, the

equivalent values for r at 1/30 (s) can be got from those at all faster shutter speeds. Fig. 3 shows the variations of r of the blackbody with the temperature at the shutter speed of 1/30 s, and the fitted polynomial function is followed below:

rb ¼ 2978:78675 þ 18:40576 T 0:0343 T 2 þ 2:0404 105 T 3 ð11Þ The comparisons for the temperature and the emissivity between the calculated and set values are shown in Table 2. From Table 2, the errors of the temperature are all less than 1.3%, and the average relative error is only 0.4765%. Although the maximum error of the calculated emissivity is 9.49%, its average relative error is just 4.95%, showing acceptable for industrial applications.

Fig. 5. The locations of the 6 detectors mounted in the three layers on the wall of the boiler.

Fig. 6. Representative flame images captured by the detectors at two loads, (a) 120 MW and (b) 140 MW.

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4. Experimental measurements and results Experiments were conducted on a coal-fired CFB boiler with a steam capacity of 480 ton/h. The boiler furnace with horizontal– cross dimensions of 15,240 mm (width) 7492 mm (depth) is shown schematically in Fig. 4. The configuration of the measurement system is also shown in Fig. 4. The system mainly consists of flame image detectors, a frame-maker and an industrial computer with a frame-grabber. Six flame image detectors are mounted on the front wall at different heights. Fig. 5 gives the schematic of the locations of the six detectors. By using a frame-maker and a frame-grabber, the six analogue video signals are combined into one video signal that is converted into one digital color image in the computer. From the color images, the temperature and emissivity images of a flame are calculated by the system.

In experiments, many flame images were captured, and the shutter speeds were set as 1/50 s for all detectors. Fig. 6 shows a group of flame images. The corresponding flame temperature and emissivity images calculated from the six detectors were shown in Figs. 7 and 8, respectively, and the left diagram is for the unit load of 120 MW while the right one for 140 MW. Fig. 9 shows the history records of the unit load and average temperatures of each layer with time. It can be seen that the average temperatures in the lower layer was higher than those in the upper layer, and the trends of the average temperatures of each layer were consistent with the unit load. These were in accord with the actual operating condition of the CFB boiler. Comparison between the average temperature measured by the image processing system at the height of 13.5 m and the

a 0.10

a

0.20

700.0

20 0.30

725.0 20

0.40

750.0 40

0.50

775.0 40

0.60

800.0 825.0

60

0.70 60

850.0

0.80

875.0

0.90

900.0

80

1.0

80 920.0

10 10

20

30

40

50

20

30

40

50

60

60

b

b

0.10 700.0 0.20 725.0

20

20 0.30

750.0

0.40

775.0 40

800.0

40

0.50 0.60

825.0

0.70

850.0

60

60

80

875.0

0.80

900.0

0.90

925.0

10

20

30

40

50

80

1.0

60

10 Fig. 7. The flame temperature images (°C) at two loads, (a) 120 MW, and (b) 140 MW.

20

30

40

50

60

Fig. 8. The emissivity images at two loads, (a) 120 MW, and (b) 140 MW.

Z.-W. Jiang et al. / Fuel 88 (2009) 980–987 200

1000

180

975

160

950

140

925

120

900

at 10.5m

875

100

at 13.5m

80

850

reason maybe due to the lower temperatures close to the waterwall surface where intensive convective heat transfer occurs. This phenomenon is worthy of study further but is outside the scope of this work. Temperature /

Unit Load /MW

986

825

60

at 27.5m

40

800 775

20 0 00:00

02:00

04:00

06:00

08:00

10:00

12:00

14:00

750 16:00

Time(hh:mm) Fig. 9. The average temperatures at three layers and the unit load varied with time.

950

by present system by thermocouple

925

Temperature /

900 875 850 825

This work addressed the shortcomings of using traditional twocolor method to calculate flame temperature with CCD cameras. To acquire the images of a flame at two different wavelengths, either the structures of measurement systems were complex, or the central wavelengths of red (R), green (G) and blue (B) would be known. A new method for temperature and emissivity measurements was presented in this paper. In this method, only the colorimetric information obtained by the flame image detector with a CCD camera is needed for data processing. That information is independent of the spectroscopic characteristics and/or the representative wavelengths of the camera. Validation of the method has shown its accuracy and reliability. The experiments were conducted on the CFB boiler with a steam capacity of 480 ton/h to obtain temperature and emissivity images of the combustion flames using the simple method mentioned above. Temperature computed by the image processing system varies accordingly with the load of the boiler, and the largest difference compared with the temperature got by a thermocouple is less than 10%. The emissivity obtained by the present system shows valuable information for the combustion and heat transfer processes inside the furnace.

800 775 750 00:00

Acknowledgement 02:00

04:00

06:00

08:00

10:00

12:00

14:00

16:00

Time (hh:mm) Fig. 10. Comparison between the temperature measured by the present system and a thermocouple in experiments.

temperature measured by a thermocouple installed at the height of 15.0 m is shown in Fig. 10. It can be seen that the agreement was good with the largest difference less than 10%. Also as shown in Fig. 10, the trend of the temperature measured by the detector was consistent with the thermocouple. Fig. 11 shows the variations of the average emissivity of each layer with time. It was found that the emissivity in the upper layer is higher than that in the lower layers, even the particle concentration in the lower layers located in the dense-phase zone of the boiler is higher than that in the higher layer with lean-phase zone. The

1.0 0.9

at the height of 27.5m 0.8

Emissivity

5. Conclusions

0.7

at the height of 13.5m 0.6 0.5

at the height of 10.5m 0.4 0.3 00:00

02:00

04:00

06:00

08:00

10:00

12:00

14:00

Time(hh:mm) Fig. 11. The average emissivity of each layer varied with time.

16:00

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