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Experience of High Hot Blast Temperature and Longevity of Yuxing Catenary Top Fired Hot Blast Stove for 1860m3 BF at Shanxi Tongcai Steel

Experience of High Hot Blast Temperature and Longevity of Yuxing Catenary Top Fired Hot Blast Stove for 1860m3 BF at Shanxi Tongcai Steel

(Summary description)Wu Huiqing Wei Hongyu Fu Zhengxue Liu Liming Liu Liyuan Shanxi Jianbang Group Henan Yuxing Engineering & Technology of Hot Blast Stove Co., Ltd

Experience of High Hot Blast Temperature and Longevity of Yuxing Catenary Top Fired Hot Blast Stove for 1860m3 BF at Shanxi Tongcai Steel

(Summary description)Wu Huiqing Wei Hongyu Fu Zhengxue Liu Liming Liu Liyuan

Shanxi Jianbang Group Henan Yuxing Engineering & Technology of Hot Blast Stove Co., Ltd

Information

Experience of High Hot Blast Temperature and Longevity of Yuxing Catenary Top Fired Hot Blast Stove for 1860m3 BF at Shanxi Tongcai Steel

Wu Huiqing Wei Hongyu Fu Zhengxue Liu Liming Liu Liyuan

Shanxi Jianbang Group Henan Yuxing Engineering & Technology of Hot Blast Stove Co., Ltd

Abstract: Since four Yuxing catenary top fired hot stoves for 1860m3 BF at Shanxi Tongcai have commissioned in July 2012, the average monthly hot blast temperature of 1300 oC was realized. After 6 years and 10 months’ high hot blast temperature operation, overhaul of blast furnace was performed in April 2019. So far, these four Yuxing catenary top fired hot stoves have been operating for 8 years. After hot blast stoves shut down, technical experts from Shanxi Tongcai Steel and Henan Yuxing Engineering and Technology of Hot Blast Stove Co carried out a systematic inspection on the condition of stove propers. Combined with the inspection results, the experience of long life of hot blast stoves was elaborated from two aspects: structure design and selection of refractory materials.

Key words: Catenary, top fired, hot blast stove, high hot blast temperature and longevity

1860m3 BF at Shanxi Tongcai Steel was equipped with 4 Yuxing catenary top fired stoves with design hot blast temperature of 1320 oC, blast time of 60 minutes, design lifetime of 25 years and operation mode 2 on gas and 2 on blast. Since BF went into production, stoves have delivered the highest monthly average hot blast temperature of 1314.7 oC and the highest hourly hot blast temperature of 1346 oC. Stoves have been operating for 6 years and 10 months with delivery of daily hot blast temperature over 1250 oC. Till April 2019, BF shut down. An expert team from Shanxi Tongcai Steel and Yuxing Limited conducted a systematic investigation on the internal structure of stove propers. It was found that hot blast outlet, burner, gas and air nozzles, dome and checker bricks of 4 stoves are in good conditions except for an area of 1.5 m2 silica bricks falling slightly in stove no1. Combined with inspection results, the experience of longevity of hot blast stoves was elaborated from two aspects: structure design and selection of refractory materials.

1 Basis design parameters and hot blast temperature record

1860m3 BF at Shanxi Tongcai Steel is equipped with 4 Yuxing catenary top fired stoves and 2 preheating stoves with usage of BFG only and preheating gas and air and air subject to the secondary preheating. Design hot blast temperature is 1320 oC and dome temperature is 1420 oC. High temperature zone of stoves is built from silica bricks and design lifetimes of stoves are 25 years. Five Yuxing proprietary technologies, blast-gas-changeover of stoves is realized via automatic and manual operation. Operation mode of stoves is 2 on 2 and 2 on blast. The main technical performance of catenary top fired stove is shown in Table 1. Hot blast temperature in October 2012 is shown in Table 2.

Table 1 Main design parameters of Yuxing catenary stove

Numbers of stoves 4
Stove height/m 35.88
Dia. of checker chamber/m 6. 8
Heating area per stove/㎡ 38426.6
Heating area of checker/(㎡·m-3 ) 48.56
Flue dia of checker/mm 30
Checkerwork height/m 21.8
Cross section of checker chamber/㎡ 36.298
Hot blast temperature/℃ 1350
Dome temperature/℃ 1420
Dome refractory design critical temperature/℃ 1550
Waste gas temperature/℃ Maximum 400, normal average 350
Air preheating /℃ 200
Gas preheating /℃ 200
Cold blast temperature/℃ 201
Cold blast flow rate/(N m3 • min-1 ) 3000
Cold blast pressure/KPa 355
Blast time/ min 60
Gas time/min 48
Changeover time/min 10
Unit blast heating area/(㎡·m-3 • m-1) 43.29
Checker mass per stove/t 1060
Unit blast checker mass/( 1•m3• min-1 ) 1.2

 Table 2: Hot blast temperature record in Oct 2012

Date 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Blast temp 1307 1294 1312 1306 1307 1313 1315 1308 1315 1309 1310 1304 1308 1308
Date 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Blast temp 1316 1305 1315 1310 1312 1314 1310 1292 1374 1301 1311 1314 1306 1313

2 Inspection during shutdown

On April 10, 2019, 1860m3 blast furnace was on medium repair during capacity expansion (BF hearth not repaired), and Yuxing catenary top fired stoves stop running successively and were cooled as planned; On April 19, experts from Shanxi Tongcai Steel and Yuxing Limited as well as maintenance specialists entered stoves for inspection. After careful examinations, it was found that hot blast outlet, burner, gas and air nozzles are in good conditions. Slight dome plug spalling in stove no1 happened with an area of 0.5 m2 and spalling depth of 10cm approximately. Top of checkerwork is flat without convex and concave phenomenon but is accompanied by slight slagging phenomenon. Checker holes have a slight dislocation phenomenon.

On April 20, stove no 2 and no 3 stopped running; On April 30, specialists entered no2 and no3 stove for inspection. It was seen that hot blast outlet, burner, gas and air nozzles, plug position of catenary dome are in good conditions. However, there was an area of 1.5m2 spalling of silica bricks with spalling depth of 100mm. Top of checkerwork is smooth without convex and concave phenomenon but is accompanied by slight slagging phenomenon. Checker holes have a slight dislocation phenomenon.

Inspection for no 4 stove was not carried out during shutdown. However, combined with the inspection of stove no1, 2 and 3, we were able to foresee that structure of no 4 was not seriously damaged. At the same time, hot blast duct inspection was also conducted. We found that hot blast duct bricks were in good conditions and casting area at T joint was intact. But the connection part between the casting area and hot blast duct was rugged.

Concerning the problem of slight spalling of dome plug brick for stove no1, experts believe that it is caused by the inadequateness of stress relief design. For silica bricks spalling for no2 stove dome, experts agree that the partial spalling is directly related to the improper operation of stove heating-up and cooling. In the daily use of hot stoves, attention should be paid to the flow rate and pressure of cold gas and the direct injection of flame with a large temperature difference shall be avoided to eliminate the occurrence of such phenomenon.

Although a few issues were found during shutdown inspection, it also shows that stove proper design of Yuxing catenary top fired stoves can meet the requirement of high hot blast temperature of 1300 oC. Besides the service life of the burner is even greater than that of stove proper. All these have shown the excellent performance of high hot blast temperature and longevity of Yuxing catenary top fired stove. This provides a good practical basis for further optimizing the design of hot blast stove and accumulating the application experience of 1300 oC hot blast temperature hot stove in China.

3 Reasons analysis for Yuxing catenary stoves featuring high hot blast and longevity

Yuxing catenary top fired stoves for 1860m3 blast furnace at Shanxi Tongcai Steel have been running safely and effectively for 8 years with the highest average monthly hot blast temperature of1314.7 oC, hourly hot blast temperature of 1346 oC and daily hot blast temperature above 1250 oC, and main stove body and components have no significant problems. This is directly related to the advanced stove proper design and the choice of high-quality refractories.

3.1 Structure design and material selection of important components of hot blast stove

3.1.1 Dome design and material selection

The space formed by dome of Yuxing catenary top fired stove is combustion chamber. The upper part is a bowl-shaped shell, and the lower part of dome is a cylinder shell with a certain height. Main design concept of catenary is as follows: dome is independent from ringwall subject to the thermal stress, and dome lining, dome insulation brick and dome shell employs a comprehensive stress relieving design concept. This renders non-existence of the thermal stress upon refractories after being heated so that a good load bearing capacity and stability of structure can be ensured.

The combustion chamber is made of layers of refractories of different properties in a steel shell. Stove ringwall structure from inside to outside is: high temperature anti-adhesion brick or andalusite brick as load-bearing brick with thickness of 450mm, insualtion silica bricks GGR-1.0 with thickness of 230mm, insulation fireclay bricks LG-0.8 with thickness of 230mm, fiber felt with thickness 50mm (or 80mm), gunnite with thickness of 50mm.

Although catenary dome of hot blast stove is a self-stabilizing structure in mechanics, it is necessary to select appropriate refractories to maintain stability in high temperature environment. Changes and unevenness of waste gas temperature in the combustion chamber will lead to local changes in stove ringwall or producing greater internal stress, which is often the root cause of damages of combustion chamber, so there is a higer requirement for thermal stability of refractories under high temeprature (RUL over 1450 oC). Refractories shall feature not only thermal shock resistance (over 20 cycles) but also have good cold crushing strength (45Mpa). So, the dome is normally made of andalusite high alumina bricks. Under the condition of high dome temperature and uniform stability, the anti-adhesion brick with high temperature and good thermal stability is also a choice to make the structure stable. The selection of high aluminum poly light ball brick is able to play the role of bearing and thermal insulation. Thermal insulation layer failures can be avoided when hot blast leak into insulating layer resulting in shrinkage of insulation bricks and softening of stove shell. Damages of stove shell, hot blast lekage or the dome failues can be avoided as well.

In order to ensure the accuracy of dome, stove dome adopts a variety of brick shapes. For the stability of dome, bricks with tongue and groove are used. Since combustion chamber is finally supported on burner wall, the special skew block is specially designed for the stability of dome base. Dome sits on stove shell and is connected with checker chamber wall by sliding labyrinth to ensure the stability of the whole combustion chamber in operation. This is the basis for the long-term safe and stable operation of stove.

3.1.2 Burner design and material selection

Burner of Yuxing catenary top fired stove adopts an annular structure with staggered arrangemnt of gas and air nozzles, which is located in the low temperature area between checker chamber wall and stove shell (temperature ranging from 500-900 oC), and will not be directly affected by high temperature of combustion chamber. The characteristics of temperature change in the burner is generally lower at gas stage but higher at blast period. Only gas and air nozzles are exposed to combustion chamber area with higher temperature. It should be pointed out that burner is a porous and multi-channel structure with uneven distribution of structural stress, which will cause structural damage under the action of thermal deformation caused by temperature change. In addition, the stability of burner is more important as it is the support foundation for combustion chamber. Therefore, considering the stability of the overall structure of burner, it is necessary to use refractory products with high thermal stability. The high thermal shock resistant brick (HRK) developed by Yuxing Limited can keep burner stable without local or overall deformation and damage under long-term temperature difference change. Interior parts of burner (ie. checker chamber inner wall) is built from andalusite high alumina bricks (upper area) and silica bricks (lower part). Due to its characteristics of high temperature resistance, thermal shock resistance, high pressure bearing capacity of HRK bricks, the stability of burner is ensured under high temperature.

Burner ringwall is also made of bricks with tongue and groove which makes the local contraction and expansion become overall contraction and expansion under the action of tongues and grooves, the occurrence of large local stress in burner can be avoided effectively, particular in gas and air ring and nozzles. In order to prevent the possibility of blast leakage between gas ring and air ring, stainless steel partitions are laid in related brick layers to ensure the safety of gas passage to the greatest extent.

3.1.3 Checker chamber wall design and material selection

Based on the working environment and structure of checker chamber wall, the temperature difference range of wall is relatively large from top to bottom, so the requirements for refractory materials are also different. At the same time, weight of combustion chamber and burner is supported on stove shell, and load on checker chamber wall is mainly from its own mass and related thermal stress. Therefore, on the premise of meeting the load bearing requirements and thermal stress bearing capacity of checker chamber, the inner layer of checker chamber from top to bottom are made of: high temperature anti-adhesion silica brick (GZF-95), andalusite brick (HRL-65), andalusite fireclay brick (HRN-45) with thickness of 230mm respectively; other layers from top to bottom are conducted from: insulation silica brick GGR-1.0 with thickness of 230mm, insulation fireclay brick (NG-0.8) with thickness of 230mm, fiber felt with thickness of 50mm and gunite with thickness of 50mm. By calculation, the maximum temperature of stove body and the supporting ring is about 80oC. The maximum temperature of the lower shell of stove body is below 50 oC. Compared with the traditional design, this arrangement has superior advantages in terms of load bearing capacity and thermal insulation performance.

3.1.4 Checker design and material selection

In order to ensure sufficient heat transmission and storage capacity of stove, checkerwork is made of checkers with passages mutually connected and flue size of 25mm. Ratio of space between flues S/d is 1.62. An individual checker brick height is 100-120mm, and there are 19 complete holes. The material of checker bricks is divided into three sections according to the temperature from top to bottom. The materials are: silica checker GZF-95, andalusite checker HRL-65, and fireclay checker HRN-45.

3.1.5 Stove shell design, material selection and thermal stress relief

Stove shell stability is one of the important factors affecting the stability and longevity of hot stove. The cracking of stove shell is a common factor restricting the service life of large hot stove in China. In order to ensure the stability and longevity of stove shell, shell dimension must be accurately calculated, and appropriate materials are selected. Blast pressure of stove is generally 4-5 kg. According to the diameter of stove shell, when it runs at high hot blast temperature of 1200-1300 oC, diameter of combustion chamber shell is the largest and the pressure it needs to bear is up to 3000-6000 tons per square meter. Therefore, stove shell is not only a structure bearing gravity, but also a large pressure vessel. The geometric size and thickness of stove shell must be calculated according to the standard of pressure vessel. According to the general empirical formula, the design value of the cone part of stoves applied at Shanxi Tongcai Steel is 20%-40% less than the design value calculated by Yuxing Limited in accordance with pressure vessel. As a result, stove shell is too thin to reach the expected service life. It is necessary to set the calculation software correctly and calculate the stove shell size accurately. In addition, steel quality of stove shell is one of the key factors affecting the quality of welding joints of stove shell. Compared with Q345C, Yuxing Limited propose to use Q345R with lower S and P content which effectively enhances weld strength of stove shell, eliminates the occurrence of hot brittleness phenomenon of weld seams, and improves the overall stability and safety of stove shell. When welding upper high temperature section stove shell, welding seam adopts special stress elimination vibration equipment. So far, there have been no cracks of weld joints.

3.1.6 Brickwork of hot blast duct and the connection with combustion chamber

Linings of hot blast duct adopts andalusite corundum brick (HRG-75) with stable structure and high temperature resistance. In order to maintain stability of hot blast duct under the condition of large pipe stress and deformation, the special “Z” shaped brick with tongue and groove is usually adopted. In this way, whether it is deformed or not under longitudinal or radial stress, brickwork of hot blast duct can eliminate the internal stress acting on it through sliding movement, and its sealing and the integrity of brickwork is not destroyed.

The stability of hot blast duct is more important in the process of delivering high hot blast temperature. Issues of T joint and hot blast outlet bricks falling, collapse and gettign red hot have happened in stoves for blast furnaces with different volume. As a result, optimization of T joint and hot blast outlet is extremely urgent. Yuxing Limited has performed T joint optimization of hot blast duct which is made of Z-shaped bricks. This allows hot blast duct to expand and contract free and blast leakage and duct getting red are avoided. T joint is able to sustain hot blast of 1250-1300 oC.

The design of the connection part between hot blast duct and combustion chamber joint design is very important for the safe and stable operation of combustion chamber, hot blast duct and hot bblast valve. Unreasonable design will lead to damages to brickwork of hot blast duct and combustion chamber wall as well as hot blast valve installed on hot blast duct under the interaction of load stress in stove shell and thermal stress in refractories. Consenquently, this will affect the overall service life of hot stove. Therefore, combined bricks are installed outside of two ring or one and a half ring bricks so that the inner ring bricks are connected with hot blast duct after retaining annular joints. Outer ring bricks become a part of ringwall of combustion chamber, and the outside of outer ring bricks are built from combined bricks. At the same time, a certain area of hot blast duct is made of heavyweight bricks. Therefore, various stresses at connection areas can be effectively absorbed or resisted during the upward movement of stove shell and downward movement of refractory linings of combustion chamber. Severe damages can be avoided and the stability of dome can be ensured. Hot blast valve cannot be installed at the area which is very close to hot blast outlet as it is always subject to high temperature state resulting from radiation. In addition, hot blast may leak due to unbalanced stress. The normal operation of hot stove will be ultimately affected. For the inlet pipe of gas and combustion air, there is also the problem of structural stress, and corresponding technical measures should be taken for the connection between gas/air inlet pipe and burner.

3.1.7 Connection between combustion chamber and chekcer chamber

Because combustion chamber is subject to high temperature for a long time and the temperature change is relatively small, so the periodic volume change is relatively small as well. But checker chamber differs. Although the periodic temperature change is not significant, the long cylindrical structure (ratio of height to diameter is bigger) results in temperature difference change so that the variation amount of bricks in the height direction is larger due to expansion by heat and contraction by cold. In order to ensure stability and safety of the overall stove structure, combustion chamber and checker chamber must be supported separately in order that there is no large force between each other. Combustion chamber is supported on the outer steel shell of stove, and checker chamber is seated on the stove foundation. Both are connected by sliding joints of labyrinth. As a result, expansion and contraction of combustion chamber and checker chamber are free, and there is no obvious force between them. Since combustion chamber is not connected with checker chamber, the change in height direction is relatively small and its relative stability does not impose additional stresses on the connected pipes.

3.2 Refractory selection for stove proper

As noted above, burner and combustion chamber are constructed from andaluse-65 bricks, thermal shock resistant (HRK) bricks, andalusite fireclay (HRN-45) bricks, and corresponding insulation bricks. In order to ensure the structural stability and quality of stove body, dome and annular burner, refractories for these areas are mainly produced and provided by Yuxing Limited. Main properties of refractories are shown in Table 3 and Table 4.

Table 3 Properties of refractories

Item Siica brick Insulation High alumina Insulation fireclay Burner brick Andalusite brick Andalusite fireclay
Mark GZF-95 LG-1.0 NG-0.8 HRK HRL-65 HRN -45
Application zone Wall, checker Wall Wall Burner Wall, checker Wall, checker
Refractoriness ≥1700     ≥1790 ≥1780 ≥1750
BDg/cm3 ≤2.3(true density ≤1.0 ≤0.8 ≥2.30 ≥2.40 ≥2.2
AP ≤24     ≤23 24 24
RUL 0.2Mpa ≥1650     ≥1400 ≥1450 ≥1410
CCS Mpa ≥45 ≥3.5 ≥3 ≥45 > 50 ≥40
A12O3 %   ≥48   ≥55 ≥65 ≥45
SiO2 % ≥95          
PLC         1450x2h±0.2 1400x2h ±0.2
Thermal conductivity (350 ±25)/ W(m.k)   ≤0.60 ≤0.45      
Shock resistance, 1100℃, cycles       ≥30 10 10

 

Table 4 Refractory list for one stove

Sr. No Zone Quality Weight, t  
1 Stove bottom HRN-45    
2 Stove body HRN-45    
LG-1.0    
NG-0.8    
HRL-65    
GZF-95    
GGR-1.0    
3 Burner HRK    
HRN-45    
4 Upper straight section GZF-95    
HRL-65    
GGR-1.0    
NG-0.8    
5 Dome GZF-95    
GGR-1.0    
NG-0.8    
6 19-flue checker HRN-45    
HRL-65    
GZF-95    
Subtotal
7 Heat resistance concrete
8 Steel fiber castable
9 High alumina cement castable
10 Mortar      
     
     
     
     
11 Fiber felt LYGX-212   70m³
12 Gunite FN-130    
13 Gunite CN-130    

 

4 Summary

Yuxing catenary top fired stoves for 1860m3 BF at Shanxi Tongcai Steel adopt catenary dome, ring burner located at dome base and checker bricks with passages mutually-connected and other connection bricklaying technology. This reasonable stove structure effectively disperses the thermal stress within internals of all components so that blast stove has a longevity and stability basis. Reasonable selection and collocation of refractory materials effectively ensures the safety and stability of stoves during its service life. These stoves have been running safely and stably for 6 years and 10 months under the average daily hot blast temperature of 1300 oC. No serious damage of stove proper and important structural elements has been found after inspection. High temperature and longevity of Yuxing catenary top-fired stove have been proven. It provides a good practical basis for further optimizing the design of hot blast stove and accumulating the application experience of 1300 oC high temperature hot blast stove in China.

Henan Yuxing Engineering & Technology of Hot Blast Stove Co.,Ltd.

Add: 23rd Floor, Kailin International, No. 226 Jinshui Road, Zhengzhou City, Henan Province, China

Tel:+86-371-86122361  

E-mail:yuxing@myyuxing.com

Henan Yuxing Technology of Hot Blast Stove Co.,Ltd.

Add:Yuecun Industrial Zone, Xinmi City, Henan Province, China

Tel:+86-371-63145281

E-mail:yuxing@myyuxing.com

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