- Email: [email protected]
Research on the evolvement of morphology of coking coal during the coking process
Available online at www.sciencedirect.com
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(Suppl.) S186–S189
www.jesc.ac.cn
Research on the evolvement of morphology of coking coal during the coking process Xiangyun Zhong1 , Shiyong Wu2 , Yang Liu1 , Zhenning Zhao1 , Yaru Zhang1 , Jinfeng Bai1, ∗, Jun Xu1 , Bai Xi1 1. Key Lab of Advanced Coal and Coking Technology Liaoning, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114053, China. E-mail: [email protected] 2. School of Resource and Environment, East China University of Science and Technology, Shanghai 200237, China
Abstract The evolvement of morphology and structure of the coal with different metamorphic degrees during coking process in the vertical furnace was investigated by infrared Image detector. Moreover, the temperature distribution in the radial direction and the crack formation were also studied in heating process. The results show that the amount of crack and the shrinkage level of char decrease with the coal rank rising. In addition, the initial temperature of crack formation for char increases with the coal rank rising. Key words: coking coal; coking process; surface morphology; infrared imaging
Introduction
1 Experimental
The property and shrinkage behavior of coking coal in the coking process has an important influence on the properties of coke. In addition, the properties of coke depend on coal rank, petrological composition, mass fraction of mineral and coking technology. Researchers have studied the effects of the conventional coal quality index and coal petrologic parameters on the mechanical strength of coke, and the change of micropore structure of coke during the coking (Zhang et al., 2011; Shen et al., 2003; Shen and Wang, 2007). Optical microscope has been used to characterize coking coal macerals and microstructure of metallurgical coke (Enkins et al., 2010; Nandita et al., 2008). Fu et al. (2005) uses 4 g coal sample to analyze the morphological changes of the coal during the pyrolysis process by the digital video camera, and obtains the expansion and shrinkage performance of coal. However, the study on coking process and change of shrinkage behavior of coking coal using infrared radiation source has not been reported. In this work, the surface morphology and shrinkage mechanism of gas coal, 1/3 coking coal, fat coal, coking coal and lean coal in pyrolysis were studied by infrared imaging technology to guide the coking coal blending.
1.1 Raw coals
Corresponding author. E-mail: [email protected]
The properties of raw coals are shown in Table 1. In the experiment, the raw coals are pulverized, in which more than 85% amount of particles is less than 3 mm in size. 1.2 Experimental process A 20-g dry coal in a circular metal mould with a diameter 90 mm, whose stack density was 0.7 g/cm3 , was put into the vertical tube-type resistance furnace. The furnace was heated to 800°C at a ratio of 3°C/min. N2 of 700 mL/sec was inputted from the bottom of the furnace during heating process. An infrared thermal imager (mode AMTMP20) was used to characterize the surface morphology of coking coal since the furnace temperature was heated to 200°C.
2 Results and discussion 2.1 Analysis of the infrared images The thermal image characteristic of gas coal, 1/3 coking coal, fat coal, coking coal and lean coal at different temperature of coking process, and the quantitative analysis of crack caused in coking process were studied. The results are shown in Fig. 1. As shown in the figures, gas coal began to crack when the furnace temperature reached 563°C, and the highest and the lowest temperature of the coal surface in radial direction were 557°C and 481°C, respectively. 1/3
Suppl.
Research on the evolvement of morphology of coking coal during the coking process
S187
a1
a2
b1
b2
c1
c2
a3
a4
b3
b4
c3
c4
d1
d2
e1
e2
d3
d4
e3
e4
Fig. 1 Infrared images of gas coal (a), 1/3 coking coal (b), fat coal (c), coking coal(d), lean coal (d) in coking process. (1) 221°C; (2) 563°C; (3) 635°C; (4) 701°C. Table 1
Quality index of coal samples
Coal type (Coal mine)
Ad (%)
Vdaf (%)
G
Y (mm)
X (mm)
R¯ max (%)
Gas coal (Lao Wan) 1/3 Coking coal (Xin Jian) Fat coal (Long Hu) Coking coal (Di Dao) Lean coal (Zhang Taizi)
3.83 9.35 9.06 10.39 11.33
40.42 30.87 28.18 25.68 14.34
75 85 97 82 22
7.1 15.1 20.6 16.5 0
41.0 23.8 4.3 12.3 12.5
0.754 0.963 1.165 1.327 1.575
Coking coal began to crack when the furnace temperature reached 572°C, and the highest and the lowest temperature of the coal surface in radial direction were 566°C and 484°C, respectively. Fat coal began to crack when the furnace temperature reached 590°C, and the highest and the lowest temperature of the coal surface in radial direction were 584°C and 500°C, respectively. Coking coal began to crack when the furnace temperature reached 593°C, and the highest and the lowest temperature of the coal surface in radial direction were 587°C and 508°C, respectively. Lean coal has deep metamorphic grade and poor caking property, and no crack was observed in the whole process. Obviously, the crack-appearing temperature increased with metamorphic grade of coking coal deepening. Besides, gas coal had the most and the widest crack, because the thermal decomposition, hot polycondensation and char contraction of gas coal surface were intense during the coking process. When the temperature reached 701°C, the width of the
cracks was 1.50 mm, and the total length of the cracks was 275.6 mm. Both the amount and width of cracks of 1/3 coking coal were less than that of gas coal. At 704°C, the length of the cracks was 269.2 mm, and the shrinkage speed was 1.0 mm/min which was also fast, and the width of cracks was 1.45 mm with lots of tiny cracks in coking. The amount of cracks of fat coal was less than that of gas coal and 1/3 coking coal, for the fat coal have more plastic mass in coking. The length and the width of cracks were 265.0 mm and 1.30 mm, respectively. The volatility of coking coal is 25.68% which is medium in five coals, and the caking property is second to fat coal, and the metamorphic grade is higher than that of fat coal, and the quantity of macromolecules aromatic is more than that of others except fat coal, thus both the quantity and the speed of char shrinkage were small, and the thermal stability was high, and the the amount of cracks was less. For the metamorphic grade is the highest in the five kinds of coals,
Journal of Environmental Sciences 2013, 25(Suppl.) S186–S189 / Xiangyun Zhong et al.
S188
Fig. 2 The scheme of mesh model on coal briquette.
it is difficult for lean coal to represent characteristic of coking process. 2.2 Temperature distribution in radial direction The infrared image of coal briquette with the diameter of 80 mm was divided into sixty panes as shown in Fig. 2.
The temperatures of the segments of coal briquette were calculated by software (ThermaCAM QuickView), and the obtained temperature distributions in radial direction are shown in Fig. 3. For the coal briquette was heated from the surrounding to the interior, the temperature of coal briquette was increasing from the center to the edge by which the radial position of 40 mm to the center was considered. The temperature difference from the center to the edge of the coal briquette decreased with the heating temperature rising. The heat-conducting property of the coal at different coking time plays an important role on the temperature difference from the center to the edge of the coal briquette. Gas coal had a large temperature difference in radial direction at 7 min heating time. The cracks appeared at 519°C at which the radial temperature difference was 76°C. Most of cracks appeared when the average surface temperature of coal briquette reached 678°C at which the radial temperature difference was 34°C. The crack-appearing temperature of 1/3 coking coal of was 525°C at which the radial temperature difference of 800
800
600 500 t = 6 min t = 121 min t = 167 min
400 300
Temperapure (ºC)
Temperapure (ºC)
700 b
a
700
0
10
20 Distance (mm)
30
500 400
t = 0 min t = 124 min t = 156 min
300
100 0
40
800
800
700 c
700 d
Temperapure (ºC)
Temperapure (ºC)
600
200
200 100
600 500 t = 0 min t = 130 min t = 164 min
400 300
10
20 30 Distance (mm)
40
600 500 400
t = 7 min t = 131 min t = 163 min
300 200
200 100
Vol. 25
0
10
20 Distance (mm)
30
40
100
0
10
20 30 Distance (mm)
40
800 e
Temperapure (ºC)
700 600 500 400
t = 1 min t = 144 min t = 177 min
300 200 100
0
10
20 30 40 Distance (mm) Fig. 3 The temperature distribution in radial direction of different rank coals. (a) gas coal; (b) 1/3 coking coal; (c) fat coal; (d) coking coal; (e) lean coal.
Suppl.
Research on the evolvement of morphology of coking coal during the coking process
coal briquette was 82°C. Most of cracks appeared when the average surface temperature reached 642°C at which the radial temperature difference was 41°C. Fat coal began to appear the cracks when the temperature was 542°C at which the radial temperature difference of coal briquette was 84°C. Most of cracks appeared when the average surface temperature reached 661°C at which the radial temperature difference was 56°C. Coking coal began to appear the cracks at 548°C at which the radial temperature difference of coal briquette was 79°C. Most of cracks appeared when the average surface temperature reached 664°C at which the radial temperature difference was 38°C. For lean coal had not caking property, no crack was observed in the infrared image.
3 Conclusions It was intuitionistic and feasible to use a thermal imager to observe and quantitatively analysis the crack formation in the coking process. Imaging process could provide scientific data to represent the length and width of the cracks appearing on different kinds of coking coal. The appearing temperature of the cracks increases in the coking process with the metamorphic degree of coking coal increasing. The crack-appearing temperatures of gas coal, 1/3 coking coal, fat coal and coking coal were 519°C, 525°C, 542°C and 548°C, respectively. During the coking
S189
process, gas coal had the longest crack, and the following was 1/3 coking coal and fat coal. Acknowledgments This work was supported by the Hi-Tech Research and Development Program (863) of China (No. 2009AA063302).
References Enkins D R, Mahoney M R, Keating J C, 2010. Fissure formation in coke. 1: The mechanism of fissuring. Fuel, 89(7): 1654– 1662. Fu Z X, Guo Z C, Yuan Z F, Wang Z, 2005. Shrinkage characteristics of briquette during pyrolysis using online images collection. Journal of Fuel Chemistry and Technology, 33 (5): 525–529. Nandita C, Debadutta M, Prabal B, Saroj K, Sushanta K H, 2008. Microscopic evaluation of coal and coke for metallurgical usage. Current Science, 94(1): 74–81. Shen J, Li X Y, Zou G M, Wang Z Z, 2003. Measurement of coke micro-strength and its relation with property. Journal of China Coal Society, 28(3): 303–306. Zhang D L, Zeng T, Li W F, Wang P Z, Zheng M L, 2011. Relation between microstructure characteristics of coal and quality of coke. Iron and Steel, 46(1): 14–18. Shen J, Wang Z T, 2007. Study on variation of micro-pores (< 100 nm) and volatile of different rank coals during carbonization. Journal of China Coal Society, 32(6): 626– 629.
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(Suppl.) S186–S189
www.jesc.ac.cn
Research on the evolvement of morphology of coking coal during the coking process Xiangyun Zhong1 , Shiyong Wu2 , Yang Liu1 , Zhenning Zhao1 , Yaru Zhang1 , Jinfeng Bai1, ∗, Jun Xu1 , Bai Xi1 1. Key Lab of Advanced Coal and Coking Technology Liaoning, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114053, China. E-mail: [email protected] 2. School of Resource and Environment, East China University of Science and Technology, Shanghai 200237, China
Abstract The evolvement of morphology and structure of the coal with different metamorphic degrees during coking process in the vertical furnace was investigated by infrared Image detector. Moreover, the temperature distribution in the radial direction and the crack formation were also studied in heating process. The results show that the amount of crack and the shrinkage level of char decrease with the coal rank rising. In addition, the initial temperature of crack formation for char increases with the coal rank rising. Key words: coking coal; coking process; surface morphology; infrared imaging
Introduction
1 Experimental
The property and shrinkage behavior of coking coal in the coking process has an important influence on the properties of coke. In addition, the properties of coke depend on coal rank, petrological composition, mass fraction of mineral and coking technology. Researchers have studied the effects of the conventional coal quality index and coal petrologic parameters on the mechanical strength of coke, and the change of micropore structure of coke during the coking (Zhang et al., 2011; Shen et al., 2003; Shen and Wang, 2007). Optical microscope has been used to characterize coking coal macerals and microstructure of metallurgical coke (Enkins et al., 2010; Nandita et al., 2008). Fu et al. (2005) uses 4 g coal sample to analyze the morphological changes of the coal during the pyrolysis process by the digital video camera, and obtains the expansion and shrinkage performance of coal. However, the study on coking process and change of shrinkage behavior of coking coal using infrared radiation source has not been reported. In this work, the surface morphology and shrinkage mechanism of gas coal, 1/3 coking coal, fat coal, coking coal and lean coal in pyrolysis were studied by infrared imaging technology to guide the coking coal blending.
1.1 Raw coals
Corresponding author. E-mail: [email protected]
The properties of raw coals are shown in Table 1. In the experiment, the raw coals are pulverized, in which more than 85% amount of particles is less than 3 mm in size. 1.2 Experimental process A 20-g dry coal in a circular metal mould with a diameter 90 mm, whose stack density was 0.7 g/cm3 , was put into the vertical tube-type resistance furnace. The furnace was heated to 800°C at a ratio of 3°C/min. N2 of 700 mL/sec was inputted from the bottom of the furnace during heating process. An infrared thermal imager (mode AMTMP20) was used to characterize the surface morphology of coking coal since the furnace temperature was heated to 200°C.
2 Results and discussion 2.1 Analysis of the infrared images The thermal image characteristic of gas coal, 1/3 coking coal, fat coal, coking coal and lean coal at different temperature of coking process, and the quantitative analysis of crack caused in coking process were studied. The results are shown in Fig. 1. As shown in the figures, gas coal began to crack when the furnace temperature reached 563°C, and the highest and the lowest temperature of the coal surface in radial direction were 557°C and 481°C, respectively. 1/3
Suppl.
Research on the evolvement of morphology of coking coal during the coking process
S187
a1
a2
b1
b2
c1
c2
a3
a4
b3
b4
c3
c4
d1
d2
e1
e2
d3
d4
e3
e4
Fig. 1 Infrared images of gas coal (a), 1/3 coking coal (b), fat coal (c), coking coal(d), lean coal (d) in coking process. (1) 221°C; (2) 563°C; (3) 635°C; (4) 701°C. Table 1
Quality index of coal samples
Coal type (Coal mine)
Ad (%)
Vdaf (%)
G
Y (mm)
X (mm)
R¯ max (%)
Gas coal (Lao Wan) 1/3 Coking coal (Xin Jian) Fat coal (Long Hu) Coking coal (Di Dao) Lean coal (Zhang Taizi)
3.83 9.35 9.06 10.39 11.33
40.42 30.87 28.18 25.68 14.34
75 85 97 82 22
7.1 15.1 20.6 16.5 0
41.0 23.8 4.3 12.3 12.5
0.754 0.963 1.165 1.327 1.575
Coking coal began to crack when the furnace temperature reached 572°C, and the highest and the lowest temperature of the coal surface in radial direction were 566°C and 484°C, respectively. Fat coal began to crack when the furnace temperature reached 590°C, and the highest and the lowest temperature of the coal surface in radial direction were 584°C and 500°C, respectively. Coking coal began to crack when the furnace temperature reached 593°C, and the highest and the lowest temperature of the coal surface in radial direction were 587°C and 508°C, respectively. Lean coal has deep metamorphic grade and poor caking property, and no crack was observed in the whole process. Obviously, the crack-appearing temperature increased with metamorphic grade of coking coal deepening. Besides, gas coal had the most and the widest crack, because the thermal decomposition, hot polycondensation and char contraction of gas coal surface were intense during the coking process. When the temperature reached 701°C, the width of the
cracks was 1.50 mm, and the total length of the cracks was 275.6 mm. Both the amount and width of cracks of 1/3 coking coal were less than that of gas coal. At 704°C, the length of the cracks was 269.2 mm, and the shrinkage speed was 1.0 mm/min which was also fast, and the width of cracks was 1.45 mm with lots of tiny cracks in coking. The amount of cracks of fat coal was less than that of gas coal and 1/3 coking coal, for the fat coal have more plastic mass in coking. The length and the width of cracks were 265.0 mm and 1.30 mm, respectively. The volatility of coking coal is 25.68% which is medium in five coals, and the caking property is second to fat coal, and the metamorphic grade is higher than that of fat coal, and the quantity of macromolecules aromatic is more than that of others except fat coal, thus both the quantity and the speed of char shrinkage were small, and the thermal stability was high, and the the amount of cracks was less. For the metamorphic grade is the highest in the five kinds of coals,
Journal of Environmental Sciences 2013, 25(Suppl.) S186–S189 / Xiangyun Zhong et al.
S188
Fig. 2 The scheme of mesh model on coal briquette.
it is difficult for lean coal to represent characteristic of coking process. 2.2 Temperature distribution in radial direction The infrared image of coal briquette with the diameter of 80 mm was divided into sixty panes as shown in Fig. 2.
The temperatures of the segments of coal briquette were calculated by software (ThermaCAM QuickView), and the obtained temperature distributions in radial direction are shown in Fig. 3. For the coal briquette was heated from the surrounding to the interior, the temperature of coal briquette was increasing from the center to the edge by which the radial position of 40 mm to the center was considered. The temperature difference from the center to the edge of the coal briquette decreased with the heating temperature rising. The heat-conducting property of the coal at different coking time plays an important role on the temperature difference from the center to the edge of the coal briquette. Gas coal had a large temperature difference in radial direction at 7 min heating time. The cracks appeared at 519°C at which the radial temperature difference was 76°C. Most of cracks appeared when the average surface temperature of coal briquette reached 678°C at which the radial temperature difference was 34°C. The crack-appearing temperature of 1/3 coking coal of was 525°C at which the radial temperature difference of 800
800
600 500 t = 6 min t = 121 min t = 167 min
400 300
Temperapure (ºC)
Temperapure (ºC)
700 b
a
700
0
10
20 Distance (mm)
30
500 400
t = 0 min t = 124 min t = 156 min
300
100 0
40
800
800
700 c
700 d
Temperapure (ºC)
Temperapure (ºC)
600
200
200 100
600 500 t = 0 min t = 130 min t = 164 min
400 300
10
20 30 Distance (mm)
40
600 500 400
t = 7 min t = 131 min t = 163 min
300 200
200 100
Vol. 25
0
10
20 Distance (mm)
30
40
100
0
10
20 30 Distance (mm)
40
800 e
Temperapure (ºC)
700 600 500 400
t = 1 min t = 144 min t = 177 min
300 200 100
0
10
20 30 40 Distance (mm) Fig. 3 The temperature distribution in radial direction of different rank coals. (a) gas coal; (b) 1/3 coking coal; (c) fat coal; (d) coking coal; (e) lean coal.
Suppl.
Research on the evolvement of morphology of coking coal during the coking process
coal briquette was 82°C. Most of cracks appeared when the average surface temperature reached 642°C at which the radial temperature difference was 41°C. Fat coal began to appear the cracks when the temperature was 542°C at which the radial temperature difference of coal briquette was 84°C. Most of cracks appeared when the average surface temperature reached 661°C at which the radial temperature difference was 56°C. Coking coal began to appear the cracks at 548°C at which the radial temperature difference of coal briquette was 79°C. Most of cracks appeared when the average surface temperature reached 664°C at which the radial temperature difference was 38°C. For lean coal had not caking property, no crack was observed in the infrared image.
3 Conclusions It was intuitionistic and feasible to use a thermal imager to observe and quantitatively analysis the crack formation in the coking process. Imaging process could provide scientific data to represent the length and width of the cracks appearing on different kinds of coking coal. The appearing temperature of the cracks increases in the coking process with the metamorphic degree of coking coal increasing. The crack-appearing temperatures of gas coal, 1/3 coking coal, fat coal and coking coal were 519°C, 525°C, 542°C and 548°C, respectively. During the coking
S189
process, gas coal had the longest crack, and the following was 1/3 coking coal and fat coal. Acknowledgments This work was supported by the Hi-Tech Research and Development Program (863) of China (No. 2009AA063302).
References Enkins D R, Mahoney M R, Keating J C, 2010. Fissure formation in coke. 1: The mechanism of fissuring. Fuel, 89(7): 1654– 1662. Fu Z X, Guo Z C, Yuan Z F, Wang Z, 2005. Shrinkage characteristics of briquette during pyrolysis using online images collection. Journal of Fuel Chemistry and Technology, 33 (5): 525–529. Nandita C, Debadutta M, Prabal B, Saroj K, Sushanta K H, 2008. Microscopic evaluation of coal and coke for metallurgical usage. Current Science, 94(1): 74–81. Shen J, Li X Y, Zou G M, Wang Z Z, 2003. Measurement of coke micro-strength and its relation with property. Journal of China Coal Society, 28(3): 303–306. Zhang D L, Zeng T, Li W F, Wang P Z, Zheng M L, 2011. Relation between microstructure characteristics of coal and quality of coke. Iron and Steel, 46(1): 14–18. Shen J, Wang Z T, 2007. Study on variation of micro-pores (< 100 nm) and volatile of different rank coals during carbonization. Journal of China Coal Society, 32(6): 626– 629.