High Hot Blast Temperature and Low NOx Combustion Technology of Catenary Top Fired Stove

Wei Qianlong¹ Ge Lijun1 Fu Zhengxue² Cheng Sushen³ Wang Longfei² Liu Liming³

1 Shanxi Jianlong Steel 2 Henan Yuxing Engineering and Technology of Hot Blast Stove Co 3 University of Science and technology Beijing

Abstract: How to improve hot blast temperature used to be the main direction of hot stove technology research. However, with the tightening of national environmental protection policies, how to both ensure high hot blast temperature delivery and low pollutant emission has become a new direction of stove technology research. Through the monitoring and data analysis of catenary top fired stoves for BF1 at Shanxi Jianlong Steel, the dual purposes of high hot blast temperature delivery and low NOx emission can be achieved through a rational arrangement of burner brick and checker bricks, optimization of waste gas flow distribution, improving thermal efficiency of checkerwork, decreasing the temperature difference between dome and hot blast.

Key words: Catenary top fired stove, high hot blast temperature, low NOx combustion

The increase of hot blast temperature is of great significance for iron and steel enterprises to reduce production cost, increase output and obtain better economic benefits. The practice of blast furnace ironmaking has shown that the coke ratio can be reduced by 15-20kg/t when hot blast temperature of stove is increased by 100 oC, and the output can be increased by 3% approximately [1]. Therefore, how to improve hot blast temperature used to be the main direction or even the sole direction of domestic stove technology research. But along with the environmental policy tightening gradually, especially with the introduction of an action plan for comprehensive control of air pollution policy in Beijing, Tianjin and Hebei in 2017, the atmospheric pollutant emissions targets for hot stove is receiving more and more public attention. Under the premise of delivering high hot blast temperature, how to reduce pollutant emissions of stove becomes the new direction of stove technology research.

1 High temperature Hot Stove Technology Development Status in China

As an important auxiliary facility for iron and steel enterprises, high temperature hot blast stove has been introduced into China since 1970s. After decades of development, great progress has been made in technology. As per the stove profile, stoves can be divided into ICC, ECC, ball-type and top fired stove. Hot blast temperature keeps increasing and fuel efficiency constantly keeps improving. Hot blast temperature of top fired stoves can stabilize at 1200 oC minimum at mega steel plants, and CO emissions can be neglected. From the perspective of auxiliary technology means, preheating of gas and air, accurate operation of hot stove through automatic control, reasonable use of refractories have been widely used to achieve high hot blast temperature.

In spite of this, China's current high temperature hot stove technology still met a number of challenges. On the one hand, 3-section top fired stoves with a conical dome installed in steel plants earlier in the century have been operating for nearly 10 years, and have revealed major defects of instable structure caused by thermal stress resulting in serious impact on high hot temperature performance of stove. Similar issues of these stoves have occurred in Zhongtie Steel, HBIS Laoting Steel, Tangshan Jinxi Steel and Hebei Zongheng Fengnan Steel. On the other hand, with China's increasingly strict restrictions on the emission of air pollutants, the pollutant emission of hot stove has become a rigid index that has to be considered. Although CO emission of current top fired stoves has been extremely low, the problem of high NOx emission has still not been solved. Therefore, it is necessary to develop a hot blast stove with stable structure, which can meet the requirements of high hot blast temperature and low NOx emission, to cater to the ironmaking field.

2 Optimizing Distribution of Waste Gas Flows - the Key to Achieve High Hot Blast Temperature and Low NOx Emission

Nitrogen oxides produced during combustion of blast furnace gas used in stoves are mainly temperature-thermal NO[2], while combustion temperature is the key to the production of temperature-thermal NO. When the combustion temperature (T) is less than 1500 oC, there is almost no temperature-thermal NO generation, and NO begins to be produced at temperature (T) over 1500 oC [3]. Since dome temperature of stove is closely related to the theoretical combustion temperature of blast furnace gas (according to production experience, when the temperature coefficient “ζ” of stove ranges from 0.90 to 0.98, dome temperature of stove is 0.90-0.98 times of the theoretical combustion temperature of blast furnace gas). Therefore, the relationship between NO production and combustion temperature can be further expressed as the relationship between NO production and dome temperature of stove, which is roughly shown in Table 2-1.

Table 2-1 Relationship between Dome Temperature and NO Emission

　As shown in Table 2-1, in order to obtain a better NOx emission index, dome temperature of stove should be kept below 1350 oC as far as possible. At present, temperature difference between hot blast and dome of 3-section top fired stoves widely applied in steel plant is between 80-100 oC. If a good nitrogen oxide emission index is to be obtained, the highest theoretical hot blast temperature of stove can only reach 1280 oC. If we want to obtain the theoretical hot blast temperature above 1300 oC, NOx emission will be as high as 240mg/m3 or more. Therefore, in order to obtain both higher theoretical hot blast temperature and lower NOx emission index, it is necessary to reduce temperature difference between hot blast and dome, and the main technical approach to reduce this difference is to optimize waste gas flow field distribution inside stove so that heat transfer efficiency of checkerwork can be improved.

The significance of reducing the difference between dome temperature and hot blast temperature by optimizing waste gas field distribution inside stove does not stop here. Because calorific value of blast furnace gas used for stove is below 3300kJ/m3 and the theoretical combustion temperature is below 1320 oC under normal circumstances, the theoretical hot blast temperature of stove using blast furnace gas only can reach about 1100 oC if no other measures are adopted. Therefore, in production practice preheating of blast furnace gas and combustion air is needed to increase the theoretical combustion temperature of blast furnace gas [4]. Assuming that hot blast temperature set by stove is 1200 oC and stove temperature coefficient is 0.9. Provided the difference between dome temperature and hot blast temperature is 100 oC, the theoretical combustion temperature of blast furnace gas is required to reach 1444 oC. If the difference between dome temperature and hot blast temperature is 50 oC, the theoretical combustion temperature of blast furnace gas only needs to reach 1388 oC, which can reduce preheating temperature of blast furnace gas and combustible air, thus reducing the expenditure of equipment and energy cost. In other words, under same conditions, hot stove with a small difference between dome temperature and hot blast temperature can obtain a higher hot blast temperature.

Therefore, it is of great significance to optimize the distribution of waste gas flow field in stove to achieve high hot blast and low NOx emission. As the catenary dome has a better waste gas flow distribution [5] and a better structural stability than other shapes of dome under the same blast flow and has re-entered the public's view. The effective combination of catenary dome and top fired structure has become a new breakthrough point of hot stove technology research.

3 Application of Catenary Top Fired Stove

Shanxi Jianlong No. 1 Blast Furnace (1860m3) is equipped with three top fired catenary dome stoves, which are contracted by Henan Yuxing Engineering and Technology of Hot Blast Stove Co., Ltd., and were put into operation on December 18, 2019. Operation mode is 2 on gas and 2 on blast. The basic design parameters are shown in Table 3-1. The operation data are shown in Table 3-2.

Table 3-1 Basic Design Parameters of Stoves

Table 3-2 Operating Monitoring Data of Stoves

Table 3-2 shows the operating parameters of top fired catenary dome stove for No.1 BF at Shanxi Jianlong Steel from May 6 to 18 on even days. The blast mixed valve is opened 31.28% on average, the stable hot blast temperature is achieved at 1250 oC, the initial average hot blast temperature is 1322 oC (blast start) and the final average hot blast temperature is 1263.5 oC (blast end). The mean temperature difference is 58.48 oC, mean hot blast temperature 1250 oC, mean dome temperature 1302.9 oC, and the average difference between dome and hot blast temperature 52.9 oC. The minimum NOx emission value is 18.04mg/m3, and the average emission value is 23.77 mg/m3, objectively demonstrating high hot blast temperature, low NOx emission and other characteristics of top fired catenary dome stove.

4 Technical Analysis

4.1 Catenary dome adopted. Although catenary is a self-stabilizing structure in mechanics, the change of waste gas temperature in the combustion chamber will lead to the local change of stove wall or produce a large internal stress, which will damage the structural stability of combustion chamber. Therefore, in order to disperse the thermal stress of combustion chamber and ensure the stability of catenary dome, top fired catenary dome stoves for BF1 at Shanxi Jianlong Steel adopt interlocking refractories, and sliding labyrinth is applied in checker chamber wall. Top fired catenary dome stoves have been running safely for 6 years and 10 months at a high hot temperature of more than 1300 oC on daily basis. However, dome plug has a slight spalling issue with an area of 0.5m2 approximately. These technical measures ensure the stability of catenary dome to the maximum extent so that stove can achieve stable performance of high hot blast temperature. Besides, the ratio between the height and diameter of catenary dome is over 1.15 [6]. Waste gas flow distribution is further improved by the annular burner installed at dome base (as shown in Fig. 4-1). It can be seen from the figure that high-temperature waste gas flows form vortexes in the center of combustion chamber, while local vortexes will form local high-temperature areas. Therefore, such distribution state of waste gas flows makes the center of combustion chamber become a real high-temperature area (as shown in Figure 4-2, section 3 of the center of combustion chamber has the most gorgeous color and the highest temperature). Thus, the structure of high temperature area of combustion chamber is symmetrical, the temperature range is distinct, the thermal efficiency is high, and the temperature difference between catenary dome and combustion chamber dome (i.e. the upper part of checker chamber) is small. It can also be seen from the figure that high temperature flue gas velocity at combustion chamber dome (that is, the upper part of checker chamber) is evenly distributed, which improves the heat transfer efficiency of stove and further reduces the difference between dome temperature and hot blast temperature, thus effectively increasing hot blast temperature of stove. Secondly due to the addition of a large amount of flows moving downwards into flows moving upwards, the concentration of fuel gas and air in the high temperature area is decreased (as shown in figure 4-3, combustion chamber center section 3, ie. figure 4-2 light color of high temperature area, gas concentration is lower), rising air temperature is reduced, flame length is long, making heating more uniform in height direction. Its role is similar to that of exhaust gas circulation technology of low NOx burner, which plays a role in reducing nitrogen oxide emissions.

4.2 Checker bricks with flues mutually connected adopted. Checker bricks (shown in Fig. 4-4) have optimized the traditional design and solved the disadvantages of dislocations of the conventional checkers. By taking 19-flue checker as an example, the checker bottom is provided with groove and tongue with depth of 15mm connecting 18 flues. When flows pass through 19 flues, connected 18 grooves and tongues are able to adjust flows leading to uniform distribution due to the action of resistance and pressure of fluid. For the checker chamber, it is subject to interconnection state, and the distribution state of waste gas flows and cold blast flows in checker chamber is optimized, heat exchange efficiency of stove is improved [7], and the difference between dome temperature and hot blast temperature is reduced. This plays a key role in increasing hot blast temperature and reducing nitrogen oxide emission. Professor Mr. Cheng Shusen from University of Science and Technology Beijing made a numerical model comparison of the distribution state of cold blast flow between checkerwork using pressure and flow equalizing checkers and that is made of the traditional checkers. The results are shown in Fig. 4-5, 4-6 and 4-7.

4.3 Annular ceramic burner with swapped arrangement of gas and air nozzles employed. This burner design is able to fully mix gas and air in a unique vortex disturbance mode so as to ensure complete combustion of blast furnace gas under the condition of excess air α=1.02-1.05. According to the relevant data, the smaller the value α is, the higher the theoretical combustion temperature of blast furnace gas is [4]. At the same time, according to the relevant knowledge of combustion chemical reaction kinetics, we can know that the smaller the value α, the smaller the concentration of oxygen. For the reversible chemical reaction of nitrogen oxide generation, the equilibrium of left shift can effectively reduce the amount of nitrogen oxide generation. In the meantime, because the smaller α creates short flame length at burner nozzles. By matching with the unique catenary dome, the mixed combustion flame has a long running distance. As a result, the annular burner features short flame mixing combustion and low NOx emission, further reducing the burner emission of nitrogen oxides (its analog combustion characteristics as shown in Figure 4 to 8).

Fig 4-8 Model for annular ceramic burner

5 Conclusion

In addition to stability and good waste gas flow field distribution, waste gas flow and cold blast flow distribution can be further optimised by improved catenary dome, ceramic burner and checker bricks so that the difference between dome temperature and hot blast temperature can be reduced, hot blast temperature be increased and NOx emission be decreased. This lays a good equipment foundation for clients to achieve the dual goals of economic benefits and social benefits.