- Email: [email protected]
Temperature Distribution of Iron Ore Pellet Bed in Grate
Available online at www.sciencedirect.com
.-" 'fIo.;_ ScienceDirect
JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2012, 19(2): 07-11
Temperature Distribution of Iron Ore Pellet Bed in Grate FENG j un-xiao ,
XIE Zhi-yin ,
CHEN Yan-mei
(School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China) Abstract: The temperature distribution of iron ore pellet bed in grate has a significant effect on pellet production and quality control, but the related work is scarce. A well-designed test was successfully carried out by means of tracking measurement and the temperature distribution and variation in pellet layers were obtained. The effects of blast temperature, blast velocity and oxidation reaction on the pellet layer temperature were studied. According to the analysis, the inlet air temperature in the up-draught drying zone (UDD) and blast temperature in the Preheating I (PH 1) zone should be raised, and the length of the down-draught drying zone (DDD) should be properly increased. Key words: grate; iron ore pellet bed; temperature distribution; industrial test
In recent years, the grate-kiln-cooler (GKC) used for pellet production has been developed rapidly at home and abroad. Compared with sinter ore, unit energy cost of pellet production could save 1/2-1/3 with less dust emission-". The main processes in the grate are drying and preheating of pellets by updraught and down-draught hot air from annular cooler and flue from rotary kiln. This process could remove the absorbed water or crystal water from green pellets and get a sufficient compressive strength <300-500 N), and the outlet temperature of pellets should reach 900 -1 000 'C, and then pellets are fed into rotary kiln[2]. The process parameters, such as the length of grate, the length ratio of four zones, the time of drying and preheating, the air quantity, the blast temperature and the blast velocity, have affected the pellet temperature and strength. On one hand, non-uniform temperature of pellet layers could lead to insufficient pellet strength which could not ensure the requirement to be fed into the rotary kiln. On the other hand, it could also cause the burnout of grate plate, shaft bending, chain breaking, and the deviation of grate motion. Therefore, it is necessary to research the temperature distribution in grate. In the latest literatures, industrial test is rarely reported due to high experiment cost, difficult operation, and low accuracy. In this paper, an industrial test is made by adopting the
method of tracking measurement and the temperature distributions of pellet layers are obtained.
1
Experimental
1. 1
Grate and process flow The main function of grate IS for drying and preheating of the green pellets. The grate is divided into four zones: up-draught drying (UDD) zone, down-draught drying (DDD) zone, preheating I (PH D zone and preheating II (PH II) zone. The process flows are shown in Fig. 1.
Gas from section m of annular cooler Fig. 1
To DDD section
Chart of grate process flows
The basic geometry size and operation parameters are shown in Table 1. The components of green pellet are iron ore powder 89. 8 %, bentonite 1. 1 % and water 9 %; and the basic properties and composition of magnetic iron ore are given in Table 2.
Foundation Item: Item Sponsored by National High Technology Research and Development Program of China (2007 AA05Z215) Biography:FENG Jun-xiao0960-), Male, Doctor, Professor; E-mail: ixfeng@ustb. edu, en; Received Date: December 30, 2010
Table 1
Basic data and operating conditions of grate
Parameter
Unit
Value 56
Length
m
Width
m
4.5
Bed height
222.1
Output
mm t > h- 1
272.3
Grate speed
m
2.68
Length of UDD zone
m
Length of DDD zone
m
15
Length of PH I
m
12
Length of PH II
m
21
Table 2
>
min " !
8
Composition of iron ore powder (mass percent, %)
TFe
FeO
SiO,
CaO
MgO
S
66.79
27. 1
4.65
0.32
0.31
0.04
The processes of each zone in the grate are as follows: 1) UDD. The hot air with temperature of 150200 'C from cooling zone III of annular cooler passes through the pellet layers so that the pellet is heated and attached water can be removed. 2) DDD. The hot air with temperature of 400 'C from PH II passes through the pellet layers, and then the pellet is dehydrated and preheated to ensure that the pellets could bear 650 'C in PH I. 3) PH I. The pellets are further preheated by hot air from cooling zone II of annular cooler, and oxidation reaction is also accelerated at 650 'C, so that the pellets could bear the temperature above 1000 'C in PH II. The aided burners are set in this zone. 4) PH II. The pellets are continuously preheated and oxidized, and the solidification and induration process are partially finished which could significantly increase the pellet strength for following process. The flue gas comes from kiln with temperature of 1050 'C, and the temperature of bottom layer must be controlled around 900 'C, otherwise, the grate plate could be easily destroyed-'". 1. 2
Vol. 19
Journal of Iron and Steel Research, International
• 8 •
Experimental design
Drying and preheating are very important processes for pellet production. Considering the continuity of production, large size of the equipment and high temperature operation condition, the industrial test research in this field still remains blank. In this paper. the influence of operation parameters on temperature distribution in pellet layers will be investigated. Furthermore. the production output can be increased
and pellet quality can be improved. Finally. the energy consumption could be lowered. Considering the continuous motion of pellet layers, the internal temperature can not be easily measured with traditional way of side-wall thermocouples. Therefore. a tracking measurement method is adopted; in this method, thermocouples will be arranged inside the pellet layers, meanwhile they are linked to a high temperature data recorder (HTDR) which is immerged inside layers. The data will be recorded instantly while the experimental apparatus moves with the pellet layers and successively passes through the UDD zone, DDD zone, PH I zone and PH II zone. The data will be output when the test is finished. The test system is shown in Fig. 2. Unit:mm High temperature Support frame data recorder
Thermocouples
No.3 No.4
2000
Fig. 2
Schematic diagram of thermocouple arrangement
The primary mission of this industrial test is to measure the temperatures at different heights of pellet layers. Four thermocouples are installed at different heights in the centre line of grate cross section. The distances from grate plate are 180. 120, 60. and 0 mm which are corresponding to No.1, No.2. No.3 and No.4, respectively. The support frame is made by high-temperature resistance angle iron with 10 mm in thickness and 300 mm apart from the measuring points. Therefore, the impact on internal flow in pellet layers can be ignored. The HTDR is 2000 mm from the measuring points for the sake of ignoring the impact of HTDR on the flow and heat transfer. The advantages of this method are as follows: 1) The temperature of four points at different heights of pellet layers can be measured successively. 2) The measurement error will be relatively small because of little impact on internal flow field due to the arrangement of thermocouples and HTDR. However. some loose contacts possibly exist between thermocouple probes and pellets that may cause some measurement errors. Nevertheless. it will not affect the integrated analysis of temperature
Issue 2
• 9 •
Temperature Distribution of Iron Ore Pellet Bed in Grate
distribution-f .
2
Experimental Results and Analysis
The HTDR is dragged out before entering the rotary kiln. Then, the data are obtained and analyzed. Overall temperature distribution It is shown in Fig. 3 that the temperature rise is rather small in UDD zone. It indicates that this zone has not been able to reflect its role due to the low mean temperature of inlet air with 154 ·C compared with the design requirement of 200 ·C. The blast temperature reaches 305 ·C when entering DDD zone, and the hot air firstly contacts with the pellets of upper layers which makes a rapid temperature rise, while temperature in lower layers has a slow rise, which results in energy degradation with bed height and impact of grate plate. The temperature in bottom layer sharply rises due to high temperature hot air of 729 ·C when pellets enter PH I zone. Meanwhile, FeO is gradually to be oxidized with large amount of heat released. The heat accumulated in the second layer leads to a higher temperature than upper layer in PH II zone. The temperatures of pellet layers at outlet of PH II zone finally reach around 1 000 ·C, whereas the bottom layer remains lower temperature that could protect grate plate. All analysis above show that the temperature distribution can meet the design requirement.
3
2. 1
Fig. 4
5
Distance/m
7
9
Temperature distribution of pellets layers in UDD zone
of the upper layers. Fig. 5 shows that the hot air blasted from top to bottom contacts with the first layer and makes a sharp curve of temperature rise at first half part of DDD zone, and then the temperature tends to be steady due to the surface evaporation mechanism of water within pellets-'". Thus, the heat is carried away by the surface water evaporation. Meanwhile, the second layer has a relatively stable temperature rise when blast temperature reaches 305 ·C. The overall temperature rise is not obvious because of heat lost caused by water evaporation. Moreover, lower layers remain in slow temperature rise due to small air quantity and relatively greater flow resistance. The suggestion for improvement is to increase the temperature of inlet air over 200 'C, which is beneficial for water evaporation. 300 r - - - - - - - - - - - - - - - - - - , _No.1 -No.2 .I. NO.3
1 200 ,...--,...------,-----,-----,---, _No.1
"No.4
-No.2 "No.3
P
" No.4
800
1!
o L...J~____'__--'-_ 1 2
400
2. 2
Fig. 5
o
10
Fig. 3
Overall temperature distribution
20
30
Distance/m
40
50
60
Temperature distribution in each zone Fig. 4 shows that the average temperature of pellet layers in ODD zone is below 40 ·C while the blast temperature is 154 ·C. The reason lies in the large initial moisture content of green pellets, and most of heat moves away by water evaporation-'". As a result of bottom-up direction of hot air, the temperature of the lower layer is apparently higher than that
4
6
__'__.L.___J~____'__
8
10
Distance/m
12
14
_!
16
Temperature distribution of pellets layers in DDD zone
In Fig. 6, a significant temperature increase arises in PH I zone. Water in pellets will be further decreased under the constant rate stage of evaporation when blast temperature reaches 729 'C. The oxidation rate of FeO within the pellets increases evidentlyC6] and the heat is obviously released to make a great temperature increase in the second layer. And there is a large temperature difference between upper layers and lower layers caused by the large flow resistance due to the heat degradation and lower oxidation rate in lower layers[7]. Furthermore, Fig. 3 shows that the temperature of the upper bed is holding at the beginning of PH I zone with the same trend of DDD zone
700. - - - - - - - - - - - - - - - - -No.1 eNo.2 P 500 "'No.3 "'No.4
l
!300
4
2
Fig. 6
6 8 Distance/m
10
12
Temperature distribution of pellets layers in PH I zone
which leads to a temperature-rise lag. To obtain further heating effect, blast temperature in PH I zone should be properly raised and the length of DDD zone should also be increased. Fig. 7 shows that the temperature of the upper layers increases faster under the blast temperature of 1103 'C in PH II zone, and then it tends to be steady and finally reaches 1050 'C, which can meet the requirement of feeding into rotary kiln. Meanwhile, oxidation rate of FeO reaches the maximum level in lower layers under high temperature condition, combined with the effect of heat storage of upper layers, the temperature of the third layer rises so rapidly even higher than that of upper layers. Furthermore, the temperature of grate plate is still in the safe level under large amount of air quantity at 328. 4 t/h. Above all, the average temperature of overall layers meets the requirement and the preheating process obtains a good performance.
p
11001---------:=::;;::;;;=;, 1000
1 4
2. 3
8
12 16 Distance/m
20
each zone can be obtained as aT = !::J. T / S, !::J. T is the temperature difference of each zone and S is the length of each zone. It is shown in Fig. 8 that aT of the lower layers is larger than that of the upper layers because hot air contacts with the lower layers" firstly at UDD zone while it is smaller than that of the upper layers at DDD zone and PH I zone due to the energy degradation of hot air. And aT of upper layers increases in PH II zone, which attributes to large amount of heat release caused by oxidation. Alternative arrangement of up-draught and "down-draught method decreases the temperature difference in bed height direction in the first two zones. Meanwhile, downdraught arrangement in PH I and PH II zone takes full advantage of oxidation to ensure the temperature rise of lower pellet layers. And finally, it makes a uniform temperature distribution in the overall pellet layers and thus enhances the energy utilization efficiency of the grate[8-10]. 35 r - - - - - - - - - . , , - - - - - - - - ,
25
e;
15 -No.1 eNo.2 "'No.3 "'No.4
5 0
-5 2
Fig. 8
3
Section
3
4
Thmperature variation rate of pellet layers in four zones
Conclusions 1) The testing results show a good agreement
-No.1 eNo.2 "'No.3 "'No.4
!
Fig. 7
Vol. 19
Journal of Iron and Steel Research, International
• 10 •
24
Thmperature distribution of pellets layers in PH II zone
Temperature variation in four zones
The pellet bed moves at a constant velocity and passes through four zones under various geometric and thermal conditions. The temperatures at four points have been recorded with passing time, because of the uniform motion of the grate, an average temperature variation rate aT is defined to demonstrate the temperature variation per unit distance. From the test data, the temperature variation of
with the expected requirement of drying and preheating process in the grate. The outlet temperature of pellets in the grate is relatively uniform and has reached 1050 'C, the requirement of feeding into the rotary kiln. 2) Based on the pellet characteristics, the design of four zones in the grate shows its rationality, that is, higher drying temperature and longer drying time contribute to high drying efficiency. 3) Rational utilization of high temperature waste gas and oxidation heat released from pellet layers have greatly decreased the energy consumption of pellet production, which leads to a high energy efficiency of the GKC system. 4) To reduce the temperature difference among
Issue 2
Temperature Distribution of Iron Ore Pellet Bed in Grate
the zones, the temperature of inlet air 10 UDD zone and the blast temperature in PH I zone should be raised, and the length of DDD zone could be properly increased.
[5J
[6J
References: [7J [IJ
[2J [3J
[4J
XU Iing-hai , FENG Iun-xiao , ZHANG Yong-rning , et al. Thermotechnical Measurement and Analysis for Grate-Kiln in Shougang Mining Company [JJ. Iron and Steel, 2009, 44(3): 90 (in Chinese). ZHANG Yi-rning, Theory and Technology of Pellet [MJ. Beijing: Metallurgical Industry Press, 2002 (in Chinese). ZHENG Hai-wei, The Research of Drying and Preheating Process in Grates [D]. Beijing: University of Science and Technology Beijing, 2007 (in Chinese). FENG Jun-xiao , XIE Xiao-yan , SUN Li-jia , et al. Experiment Research on Hot Air Cross-Flow Drying Through Thick-Layer Carbon-Containing Pellets [JJ. Industrial Heating, 2008(37):
[8J [9J
[10J
·11·
10 (in Chinese). Tsukerman T, Duchesne C, Hodouin D. On the Drying Rates of Individual Iron Oxide Pellets [JJ. Int J Miner Process, 2007,83(3/4): 99. Forsmo S P E, Forsmo S E, Samskog P 0, et al. Mechanisms in Oxidation and Sintering of Magnetite Iron Ore Green Pellets [JJ. Powder Technology, 2008, 183(2): 247. Libsch K D, Tarby S K. Magnetite Oxidation in a Traveling Grate Pellet Plant: a Computer Model [J]. Metallurgical Transactions, 1973, 4(5): 1347. Thurlby J A. Energy Cost Minimization in Grate/Kiln Induration [JJ. Metall Trans, 1988, 199B(l): 123. ZHANG Yu, FENG Jun-xiao , ZHANG Cai, et al. Energy and Exergy Analysis of Iron Ore Pellets Induration in Grate-KilnCooler [JJ. Sintering and Pelletizing, 2008, 33(5): 5 (in Chinese) . FENG Iun-xiao , SUN Zhi-bin, ZHANG Yu, et al. Mass and Thermal Balance and Energy-Saving Analysis of the GrateKiln System [JJ. Sintering and Pelletizing, 2007, 32(6): 29 (in Chinese).
.-" 'fIo.;_ ScienceDirect
JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2012, 19(2): 07-11
Temperature Distribution of Iron Ore Pellet Bed in Grate FENG j un-xiao ,
XIE Zhi-yin ,
CHEN Yan-mei
(School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China) Abstract: The temperature distribution of iron ore pellet bed in grate has a significant effect on pellet production and quality control, but the related work is scarce. A well-designed test was successfully carried out by means of tracking measurement and the temperature distribution and variation in pellet layers were obtained. The effects of blast temperature, blast velocity and oxidation reaction on the pellet layer temperature were studied. According to the analysis, the inlet air temperature in the up-draught drying zone (UDD) and blast temperature in the Preheating I (PH 1) zone should be raised, and the length of the down-draught drying zone (DDD) should be properly increased. Key words: grate; iron ore pellet bed; temperature distribution; industrial test
In recent years, the grate-kiln-cooler (GKC) used for pellet production has been developed rapidly at home and abroad. Compared with sinter ore, unit energy cost of pellet production could save 1/2-1/3 with less dust emission-". The main processes in the grate are drying and preheating of pellets by updraught and down-draught hot air from annular cooler and flue from rotary kiln. This process could remove the absorbed water or crystal water from green pellets and get a sufficient compressive strength <300-500 N), and the outlet temperature of pellets should reach 900 -1 000 'C, and then pellets are fed into rotary kiln[2]. The process parameters, such as the length of grate, the length ratio of four zones, the time of drying and preheating, the air quantity, the blast temperature and the blast velocity, have affected the pellet temperature and strength. On one hand, non-uniform temperature of pellet layers could lead to insufficient pellet strength which could not ensure the requirement to be fed into the rotary kiln. On the other hand, it could also cause the burnout of grate plate, shaft bending, chain breaking, and the deviation of grate motion. Therefore, it is necessary to research the temperature distribution in grate. In the latest literatures, industrial test is rarely reported due to high experiment cost, difficult operation, and low accuracy. In this paper, an industrial test is made by adopting the
method of tracking measurement and the temperature distributions of pellet layers are obtained.
1
Experimental
1. 1
Grate and process flow The main function of grate IS for drying and preheating of the green pellets. The grate is divided into four zones: up-draught drying (UDD) zone, down-draught drying (DDD) zone, preheating I (PH D zone and preheating II (PH II) zone. The process flows are shown in Fig. 1.
Gas from section m of annular cooler Fig. 1
To DDD section
Chart of grate process flows
The basic geometry size and operation parameters are shown in Table 1. The components of green pellet are iron ore powder 89. 8 %, bentonite 1. 1 % and water 9 %; and the basic properties and composition of magnetic iron ore are given in Table 2.
Foundation Item: Item Sponsored by National High Technology Research and Development Program of China (2007 AA05Z215) Biography:FENG Jun-xiao0960-), Male, Doctor, Professor; E-mail: ixfeng@ustb. edu, en; Received Date: December 30, 2010
Table 1
Basic data and operating conditions of grate
Parameter
Unit
Value 56
Length
m
Width
m
4.5
Bed height
222.1
Output
mm t > h- 1
272.3
Grate speed
m
2.68
Length of UDD zone
m
Length of DDD zone
m
15
Length of PH I
m
12
Length of PH II
m
21
Table 2
>
min " !
8
Composition of iron ore powder (mass percent, %)
TFe
FeO
SiO,
CaO
MgO
S
66.79
27. 1
4.65
0.32
0.31
0.04
The processes of each zone in the grate are as follows: 1) UDD. The hot air with temperature of 150200 'C from cooling zone III of annular cooler passes through the pellet layers so that the pellet is heated and attached water can be removed. 2) DDD. The hot air with temperature of 400 'C from PH II passes through the pellet layers, and then the pellet is dehydrated and preheated to ensure that the pellets could bear 650 'C in PH I. 3) PH I. The pellets are further preheated by hot air from cooling zone II of annular cooler, and oxidation reaction is also accelerated at 650 'C, so that the pellets could bear the temperature above 1000 'C in PH II. The aided burners are set in this zone. 4) PH II. The pellets are continuously preheated and oxidized, and the solidification and induration process are partially finished which could significantly increase the pellet strength for following process. The flue gas comes from kiln with temperature of 1050 'C, and the temperature of bottom layer must be controlled around 900 'C, otherwise, the grate plate could be easily destroyed-'". 1. 2
Vol. 19
Journal of Iron and Steel Research, International
• 8 •
Experimental design
Drying and preheating are very important processes for pellet production. Considering the continuity of production, large size of the equipment and high temperature operation condition, the industrial test research in this field still remains blank. In this paper. the influence of operation parameters on temperature distribution in pellet layers will be investigated. Furthermore. the production output can be increased
and pellet quality can be improved. Finally. the energy consumption could be lowered. Considering the continuous motion of pellet layers, the internal temperature can not be easily measured with traditional way of side-wall thermocouples. Therefore. a tracking measurement method is adopted; in this method, thermocouples will be arranged inside the pellet layers, meanwhile they are linked to a high temperature data recorder (HTDR) which is immerged inside layers. The data will be recorded instantly while the experimental apparatus moves with the pellet layers and successively passes through the UDD zone, DDD zone, PH I zone and PH II zone. The data will be output when the test is finished. The test system is shown in Fig. 2. Unit:mm High temperature Support frame data recorder
Thermocouples
No.3 No.4
2000
Fig. 2
Schematic diagram of thermocouple arrangement
The primary mission of this industrial test is to measure the temperatures at different heights of pellet layers. Four thermocouples are installed at different heights in the centre line of grate cross section. The distances from grate plate are 180. 120, 60. and 0 mm which are corresponding to No.1, No.2. No.3 and No.4, respectively. The support frame is made by high-temperature resistance angle iron with 10 mm in thickness and 300 mm apart from the measuring points. Therefore, the impact on internal flow in pellet layers can be ignored. The HTDR is 2000 mm from the measuring points for the sake of ignoring the impact of HTDR on the flow and heat transfer. The advantages of this method are as follows: 1) The temperature of four points at different heights of pellet layers can be measured successively. 2) The measurement error will be relatively small because of little impact on internal flow field due to the arrangement of thermocouples and HTDR. However. some loose contacts possibly exist between thermocouple probes and pellets that may cause some measurement errors. Nevertheless. it will not affect the integrated analysis of temperature
Issue 2
• 9 •
Temperature Distribution of Iron Ore Pellet Bed in Grate
distribution-f .
2
Experimental Results and Analysis
The HTDR is dragged out before entering the rotary kiln. Then, the data are obtained and analyzed. Overall temperature distribution It is shown in Fig. 3 that the temperature rise is rather small in UDD zone. It indicates that this zone has not been able to reflect its role due to the low mean temperature of inlet air with 154 ·C compared with the design requirement of 200 ·C. The blast temperature reaches 305 ·C when entering DDD zone, and the hot air firstly contacts with the pellets of upper layers which makes a rapid temperature rise, while temperature in lower layers has a slow rise, which results in energy degradation with bed height and impact of grate plate. The temperature in bottom layer sharply rises due to high temperature hot air of 729 ·C when pellets enter PH I zone. Meanwhile, FeO is gradually to be oxidized with large amount of heat released. The heat accumulated in the second layer leads to a higher temperature than upper layer in PH II zone. The temperatures of pellet layers at outlet of PH II zone finally reach around 1 000 ·C, whereas the bottom layer remains lower temperature that could protect grate plate. All analysis above show that the temperature distribution can meet the design requirement.
3
2. 1
Fig. 4
5
Distance/m
7
9
Temperature distribution of pellets layers in UDD zone
of the upper layers. Fig. 5 shows that the hot air blasted from top to bottom contacts with the first layer and makes a sharp curve of temperature rise at first half part of DDD zone, and then the temperature tends to be steady due to the surface evaporation mechanism of water within pellets-'". Thus, the heat is carried away by the surface water evaporation. Meanwhile, the second layer has a relatively stable temperature rise when blast temperature reaches 305 ·C. The overall temperature rise is not obvious because of heat lost caused by water evaporation. Moreover, lower layers remain in slow temperature rise due to small air quantity and relatively greater flow resistance. The suggestion for improvement is to increase the temperature of inlet air over 200 'C, which is beneficial for water evaporation. 300 r - - - - - - - - - - - - - - - - - - , _No.1 -No.2 .I. NO.3
1 200 ,...--,...------,-----,-----,---, _No.1
"No.4
-No.2 "No.3
P
" No.4
800
1!
o L...J~____'__--'-_ 1 2
400
2. 2
Fig. 5
o
10
Fig. 3
Overall temperature distribution
20
30
Distance/m
40
50
60
Temperature distribution in each zone Fig. 4 shows that the average temperature of pellet layers in ODD zone is below 40 ·C while the blast temperature is 154 ·C. The reason lies in the large initial moisture content of green pellets, and most of heat moves away by water evaporation-'". As a result of bottom-up direction of hot air, the temperature of the lower layer is apparently higher than that
4
6
__'__.L.___J~____'__
8
10
Distance/m
12
14
_!
16
Temperature distribution of pellets layers in DDD zone
In Fig. 6, a significant temperature increase arises in PH I zone. Water in pellets will be further decreased under the constant rate stage of evaporation when blast temperature reaches 729 'C. The oxidation rate of FeO within the pellets increases evidentlyC6] and the heat is obviously released to make a great temperature increase in the second layer. And there is a large temperature difference between upper layers and lower layers caused by the large flow resistance due to the heat degradation and lower oxidation rate in lower layers[7]. Furthermore, Fig. 3 shows that the temperature of the upper bed is holding at the beginning of PH I zone with the same trend of DDD zone
700. - - - - - - - - - - - - - - - - -No.1 eNo.2 P 500 "'No.3 "'No.4
l
!300
4
2
Fig. 6
6 8 Distance/m
10
12
Temperature distribution of pellets layers in PH I zone
which leads to a temperature-rise lag. To obtain further heating effect, blast temperature in PH I zone should be properly raised and the length of DDD zone should also be increased. Fig. 7 shows that the temperature of the upper layers increases faster under the blast temperature of 1103 'C in PH II zone, and then it tends to be steady and finally reaches 1050 'C, which can meet the requirement of feeding into rotary kiln. Meanwhile, oxidation rate of FeO reaches the maximum level in lower layers under high temperature condition, combined with the effect of heat storage of upper layers, the temperature of the third layer rises so rapidly even higher than that of upper layers. Furthermore, the temperature of grate plate is still in the safe level under large amount of air quantity at 328. 4 t/h. Above all, the average temperature of overall layers meets the requirement and the preheating process obtains a good performance.
p
11001---------:=::;;::;;;=;, 1000
1 4
2. 3
8
12 16 Distance/m
20
each zone can be obtained as aT = !::J. T / S, !::J. T is the temperature difference of each zone and S is the length of each zone. It is shown in Fig. 8 that aT of the lower layers is larger than that of the upper layers because hot air contacts with the lower layers" firstly at UDD zone while it is smaller than that of the upper layers at DDD zone and PH I zone due to the energy degradation of hot air. And aT of upper layers increases in PH II zone, which attributes to large amount of heat release caused by oxidation. Alternative arrangement of up-draught and "down-draught method decreases the temperature difference in bed height direction in the first two zones. Meanwhile, downdraught arrangement in PH I and PH II zone takes full advantage of oxidation to ensure the temperature rise of lower pellet layers. And finally, it makes a uniform temperature distribution in the overall pellet layers and thus enhances the energy utilization efficiency of the grate[8-10]. 35 r - - - - - - - - - . , , - - - - - - - - ,
25
e;
15 -No.1 eNo.2 "'No.3 "'No.4
5 0
-5 2
Fig. 8
3
Section
3
4
Thmperature variation rate of pellet layers in four zones
Conclusions 1) The testing results show a good agreement
-No.1 eNo.2 "'No.3 "'No.4
!
Fig. 7
Vol. 19
Journal of Iron and Steel Research, International
• 10 •
24
Thmperature distribution of pellets layers in PH II zone
Temperature variation in four zones
The pellet bed moves at a constant velocity and passes through four zones under various geometric and thermal conditions. The temperatures at four points have been recorded with passing time, because of the uniform motion of the grate, an average temperature variation rate aT is defined to demonstrate the temperature variation per unit distance. From the test data, the temperature variation of
with the expected requirement of drying and preheating process in the grate. The outlet temperature of pellets in the grate is relatively uniform and has reached 1050 'C, the requirement of feeding into the rotary kiln. 2) Based on the pellet characteristics, the design of four zones in the grate shows its rationality, that is, higher drying temperature and longer drying time contribute to high drying efficiency. 3) Rational utilization of high temperature waste gas and oxidation heat released from pellet layers have greatly decreased the energy consumption of pellet production, which leads to a high energy efficiency of the GKC system. 4) To reduce the temperature difference among
Issue 2
Temperature Distribution of Iron Ore Pellet Bed in Grate
the zones, the temperature of inlet air 10 UDD zone and the blast temperature in PH I zone should be raised, and the length of DDD zone could be properly increased.
[5J
[6J
References: [7J [IJ
[2J [3J
[4J
XU Iing-hai , FENG Iun-xiao , ZHANG Yong-rning , et al. Thermotechnical Measurement and Analysis for Grate-Kiln in Shougang Mining Company [JJ. Iron and Steel, 2009, 44(3): 90 (in Chinese). ZHANG Yi-rning, Theory and Technology of Pellet [MJ. Beijing: Metallurgical Industry Press, 2002 (in Chinese). ZHENG Hai-wei, The Research of Drying and Preheating Process in Grates [D]. Beijing: University of Science and Technology Beijing, 2007 (in Chinese). FENG Jun-xiao , XIE Xiao-yan , SUN Li-jia , et al. Experiment Research on Hot Air Cross-Flow Drying Through Thick-Layer Carbon-Containing Pellets [JJ. Industrial Heating, 2008(37):
[8J [9J
[10J
·11·
10 (in Chinese). Tsukerman T, Duchesne C, Hodouin D. On the Drying Rates of Individual Iron Oxide Pellets [JJ. Int J Miner Process, 2007,83(3/4): 99. Forsmo S P E, Forsmo S E, Samskog P 0, et al. Mechanisms in Oxidation and Sintering of Magnetite Iron Ore Green Pellets [JJ. Powder Technology, 2008, 183(2): 247. Libsch K D, Tarby S K. Magnetite Oxidation in a Traveling Grate Pellet Plant: a Computer Model [J]. Metallurgical Transactions, 1973, 4(5): 1347. Thurlby J A. Energy Cost Minimization in Grate/Kiln Induration [JJ. Metall Trans, 1988, 199B(l): 123. ZHANG Yu, FENG Jun-xiao , ZHANG Cai, et al. Energy and Exergy Analysis of Iron Ore Pellets Induration in Grate-KilnCooler [JJ. Sintering and Pelletizing, 2008, 33(5): 5 (in Chinese) . FENG Iun-xiao , SUN Zhi-bin, ZHANG Yu, et al. Mass and Thermal Balance and Energy-Saving Analysis of the GrateKiln System [JJ. Sintering and Pelletizing, 2007, 32(6): 29 (in Chinese).