Practice of increasing gas utilization rate of No.1 1080mm blast furnace in Laigang

　　Abstract: by optimizing the distribution system, reducing the amount of small-size ore into the furnace, increasing the iron tapping rate at the iron port, and reducing the slow wind down production operation caused by equipment reasons, the gas utilization rate of No. 1 1080m3 blast furnace of Laiwu Iron and Steel Co., Ltd. has been effectively increased to 45.96%, which provides a strong support for stable furnace conditions and production cost control.

　　Key words: blast furnace; Gas utilization rate; Furnace temperature; alkalinity

　　There is a direct corresponding relationship between the gas utilization rate and the fuel consumption of blast furnace. Improving the gas utilization rate of blast furnace can significantly reduce the fuel consumption of blast furnace, and then reduce the production cost of blast furnace. In 2017, the annual fuel ratio of No. 1 1080m3 blast furnace of Laigang exceeded the planned value by 9kg/t, and the annual average gas utilization rate was only 45%, and the monthly value fluctuated greatly. Increasing the utilization rate of blast furnace gas can effectively reduce the fuel consumption of blast furnace. According to the empirical data, every 1% increase in the utilization rate of blast furnace gas can reduce the fuel ratio of 8kg/t. Compared with the same type of blast furnace, combined with the actual situation of No. 1 blast furnace, a series of key problems were organized and solved, gradually increasing the utilization of gas to 45.96%, providing strong support for stable furnace conditions and production cost control [1].
 

　　1. Analysis of key influencing factors

　　1.1 current situation investigation

　　The monthly average gas utilization rate of No. 1 blast furnace in 2017 is calculated (see Table 1).

　　It can be seen that in 2017, the lowest gas utilization rate of No. 1 blast furnace was 44.3%, the highest was 46.2%, and the annual average was 45.1%. Not only is there a certain gap from the best level in the industry, but also the process control value fluctuates greatly.

　　1.2 cause analysis

　　Theoretically speaking, there are two macroscopic reasons affecting the utilization rate of gas: one is the contact time between the charge and the gas flow, and the other is the reaction time between the charge and the gas flow.

　　1.2.1 large fluctuation of furnace temperature alkalinity

　　The boiler temperature alkalinity and gas utilization rate in January 2018 were compared (see Figure 1 and Figure 2). Through data comparison, it was found that the gas utilization rate in areas with large fluctuation of boiler temperature alkalinity was relatively low, so it was determined as the main reason.

　　1.2.2 unreasonable distribution system

　　It is found through the observation of the charge surface on the top of the furnace after the blowing off that the flatness of the charge surface on the top of the furnace is not enough, and the edge air flow is too developed, which is not conducive to the full and reasonable contact between the charge and the gas flow, and this charge surface is not conducive to improving the utilization rate of gas.

　　1.2.3 poor control of raw material screening

　　The particle size analysis results of the 11 day cut-off charge are as follows (see Table 2):

　　It can be seen that the proportion of blast furnace charge greater than 5mm is high. The small particle size of the incoming charge will lead to poor permeability of the charge column, affect the reasonable and stable distribution of the air flow, and in serious cases, lead to hearth accumulation and other adverse consequences affecting the stable and smooth operation of the furnace. These results will affect the reasonable contact between burden and gas flow, thus affecting the utilization rate of blast furnace gas.

　　1.2.4 poor quality of blasting mud and low operation level in front of furnace

　　For the double port blast furnace, under the current smelting intensity, the iron tapping rate must reach more than 73% to meet the requirements of blast furnace slag and iron emission. But now the tapping rate of No. 1 BF obviously can not meet this requirement. The low tapping rate at the iron port will lead to poor discharge of blast furnace slag and iron, resulting in the operation of holding slag and iron in the blast furnace, resulting in disordered distribution of air flow, affecting the full and reasonable contact between the gas flow and raw materials, thus reducing the utilization rate of blast furnace gas.
 

　　2. Implement improvements

　　2.1 strengthen raw material supervision and timely adjust according to raw material composition

　　Strengthen supervision over raw materials and fuels. Strengthen the foreman's material viewing system, check the raw materials at the belt before entering the warehouse, especially the coke moisture, to reduce the abnormal fluctuation of furnace temperature caused by insufficient coke dry out rate; Timely check the raw ore composition of sinter and pellet, especially the sinter alkalinity, timely verify the fluctuation trend of sinter alkalinity, and timely contact the sintering side for correction in case of abnormal conditions, so as to make the sinter alkalinity return to the normal range. Predict and predict according to the basicity of sinter, sulfur in iron and the basicity of final slag. The fluctuation of furnace temperature alkalinity will cause abnormal fluctuation of blast furnace. Especially under the current raw fuel conditions, the content of aluminum oxide is high and the amount of slag is large. We should be more careful in operation. According to the actual situation, No. 1 blast furnace actively took Countermeasures in operation, frequently looked at the tuyere, frequently took the final slag, and adjusted the coal amount according to the feed rate and tuyere heat. Adjust the sintering ratio in time according to the alkalinity of final slag and the composition of incoming materials. In order to ensure sufficient hearth heat, we stipulate that the furnace temperature range is 0.3% ~ 0.5%, the theoretical combustion temperature is 2250 ~ 2350 ℃, and the physical heat of slag iron reaches 1480 ℃.

　　2.2 optimize the distribution system to ensure the reasonable distribution of gas flow

　　1) Gradually expand the ore angle difference, increase the distribution width, increase or decrease the ore belt width, and fully contact and react with the gas flow

　　The reasonable distribution of gas flow will directly affect the stable and smooth operation of blast furnace. In actual production, due to the large amount of slag, delayed slag discharge or short slag discharge time, the phenomenon of holding slag iron occurs from time to time, resulting in poor central air flow, enhanced edge gas flow, unstable air flow and abnormal increase in fuel ratio. The increase of slag under the tuyere blocks the tuyere, resulting in wind pressure rising and material suspension. Adjust the distribution matrix by observing the temperature difference of cooling stave water and cross temperature measurement on the furnace top, as shown in Table 3, 4 and 5:

　　Through the above adjustment, gradually expand the ore angle difference, increase the distribution width, increase or decrease the ore belt width, fully contact and react with the gas flow, and improve the utilization rate of blast furnace gas.

　　2) By optimizing the coke load at the center and edge, reasonably guide the gas flow at the center and edge to ensure the smooth operation of the furnace

　　According to the performance of furnace conditions, timely adjust and optimize the coke load at the center and the edge, reasonably guide the gas flow at the center and the edge, significantly enhance the anti fluctuation ability of the blast furnace, reduce the phenomenon of air hold in front of the iron and slag skin in front of the tuyere, stabilize the furnace temperature, reduce the rate of three categories of pig iron, and the obvious "V" shape of the charge surface at rest is conducive to the improvement of the gas utilization rate, which increases from about 45% to 46%.

　　2.3 reduce the feeding amount of small size ore

　　Under the condition of ensuring the feeding speed, the screening time shall be strictly determined to ensure that the sintering screening time is greater than 20s/t and the coke is greater than 1min/t. If the time is too short, it shall be handled by adjusting the amplitude of the vibrating screen motor or inserting a stop bar at the feeding port. In case of partial crushing of raw materials, the screening time can be extended or the screen with a larger screen hole can be temporarily replaced. Under the condition that the normal feeding is not affected, each vibrating screen shall be air vibrated regularly to reduce the powder entering the furnace, Improve column permeability [2].

　　Strictly control the raw fuel level management. If the normal level is greater than 5m, it is a low level. While reducing the secondary crushing, prevent the powder from being concentrated into the furnace due to the particle size segregation of raw materials. Through the improvement, the amount of powder into the furnace reaches the control target value (less than 6%), and the blast furnace can adapt to the phased excessive powder.

　　2.4 improve the iron tapping rate at the iron port and reduce the operation times of slag holding iron

　　The quality of the production organization in front of the furnace directly affects the stability and smooth operation of the furnace. The iron mouth in front of the furnace is broken and leaked before improvement. The stability of the top temperature of the gas flow is often affected by the poor slag iron. It is an effective method to improve the operation rate of the iron port in front of the furnace in order to avoid the influence of air hold in the furnace on the air flow. By strengthening the study and training of the four classes in front of the furnace, the theoretical knowledge and operation skills of the furnace workers are enhanced, and labor competition activities are carried out to mobilize the enthusiasm of the workers and improve the team cooperation ability.

　　Through the development of standardized system for furnace front operation, reduce the randomness of operation of furnace front employees, standardize the production organization in front of furnace, operate in three shifts in a unified manner, reduce the fluctuation of iron mouth work caused by human operation differences, improve the iron tapping rate at iron mouth, and reduce the occurrence of slag holding iron operation.

　　Through 12 months' practice, the operation rate of No. 1 furnace iron port has been increased from 70% to 72.5%, and the operation times of slag holding iron in blast furnace have been significantly reduced.

　　3. Improvement effect

　　In 2018, the average utilization rate of No. 1 blast furnace gas from January to December was 45.96%, basically achieving the expected goal.
 

　　4. Conclusion

　　The highlight measures to improve gas utilization rate of No. 1 1080m3 blast furnace of Laiwu Iron and Steel Co. are summarized as follows:

　　1) Optimize the charging system, improve the accuracy of furnace top distribution, eliminate human errors, and ensure the stable and normal air flow.

　　2) Strengthen the balance of furnace temperature and alkalinity, stabilize the heating system and slagging system, and maintain the smooth operation of furnace conditions and all air operation.

　　3) Pay close attention to the raw material warehouse and screening management, adhere to the principle of half warehouse charging, and reduce the incoming powder; Control the reasonable screening speed, strictly determine the screening time under the condition of ensuring the feeding speed, and improve the furnace stability.

　　4) The iron port shall be well maintained in front of the furnace, and the slag iron shall be stably and evenly discharged.