Ultra-low emissions from steel companies are an important measure to promote industrial upgrading, improve atmospheric environmental quality, and resolve excess in the steel industry. At present, most of the newly built enterprises use dry purification methods for converter gas. This article discusses several new ultra-low emission technologies based on the analysis of existing ultra-low emission technologies.
Keywords: ultra-low emissions; variable resistance full DC; high temperature composite fiber filter tube; heat recovery gas cooler; sonic agglomeration
1 Problems existing in the existing converter gas dry purification system
1.1 Conventional converter gas dry purification and recovery process
The main process of conventional dry purification and recovery of converter gas is as follows: converter waste gas enters the evaporative cooler through the vaporization cooling flue, and the evaporative cooler sprays water. First, the gas at 900-1000℃ is cooled to 250-300℃, and the gas is coarsely dusted at the same time. Then the flue gas enters the cylindrical electrostatic precipitator for fine dust removal. The dust concentration at the outlet of the electrostatic precipitator is ≤15mg/Nm3. A gas fan is installed at the outlet of the electrostatic precipitator to send the gas to the switching station after passing through the muffler. When the O2 and CO gas content in the flue gas reaches the recoverable condition, the gas is introduced into the gas cooler through the switching station, and after water spray washing, the gas is cooled to below 70°C. The dust concentration at the outlet of the gas cooler is below 10mg/ Nm3, and finally enters the gas cabinet for storage. When the O2 or CO gas content in the gas does not meet the recovery conditions, the gas is introduced into the dispersion chimney through the switching station for dispersion mode. The gas is ignited and burned by the ignition device and then discharged to the atmosphere. The coarse ash separated by the evaporative cooler is transported out through the coarse ash conveying system, and the dust collected by the electrostatic precipitator is transported out through the fine ash conveying system. The flow chart is shown in Figure 1.
Figure 1 Flow chart of converter gas dry purification and recovery system
The dust concentration at the outlet of the electrostatic precipitator is ≤15mg/Nm3. When the gas is recovered, the dust concentration at the outlet of the gas cooler is lower than 10mg/Nm3. However, when the system is in gas dissipation, there is no gas cooler washing and dust reduction effect. As the equipment operation time becomes longer, the dust removal efficiency of the electrostatic precipitator will decrease, and the dust concentration at the outlet will further increase. In order to solve this problem, the system needs to be modified to achieve ultra-low emission requirements of ≤10 mg/Nm3 when the chimney is released.
2 Existing ultra-low emission technologies
2.1 Wet electrostatic precipitator technology
Wet electrostatic precipitators and dry electrostatic precipitators are electrostatic precipitators classified according to electrode cleaning methods. Both are dust removal equipment that use electrostatic force to separate dust in gas. The dust collector consists of two parts: the main body and the high-voltage power supply. Dry electrostatic precipitators use mechanical hammering and brushing methods to remove dust on the electrodes. The advantage is that dust post-processing is simple and convenient for comprehensive utilization. Mechanical and electromagnetic vibration are commonly used cleaning methods. However, dust will rise and accumulate during dust cleaning, or backflow will occur in a short period of time, which will affect the dust removal efficiency. Wet electrostatic precipitators use methods such as leaching, spraying, and overflow to clean the dust on the electrode surface. Dust is not raised during dust cleaning, but a certain amount of mud is produced, and the mud needs post-processing.
In the dry dust removal system, the dust concentration at the outlet of the dry electrostatic precipitator is ≤15mg/Nm3. Therefore, the wet electrostatic precipitator is only an extension of the electrostatic precipitator. It is installed in the dust removal system as a deep purification dust collector to ensure that the chimney emission concentration is ≤10mg/Nm3, see Figure 2.
Figure 2 Wet electrostatic precipitator ultra-low emission flow chart
In the current transformation project, the wet electrostatic precipitator has been successfully used in the wet dust removal of converter gas, but the application in the dry dust removal system requires further practice. Currently, the main shortcomings are as follows: ① Wet electrostatic precipitators require circulating water spraying, which increases energy consumption. ② After the flue gas is cooled by spraying water, “white smoke” will emit from the chimney outlet.
2.2 Gas cooler pre-installation plan
The gas cooler front-end solution is currently the most commonly used in converter gas dry dust removal and ultra-low emissions. Its process is shown in Figure 3. The converter gas from the electrostatic precipitator is washed by water spray from the gas cooler, and the dust concentration can be lower than 10 mg/Nm3. However, this solution has the following shortcomings:
① After water spray washing, the flue gas is close to saturation, and white mist will appear at the outlet of the chimney. Dewhitening treatment must be carried out, which increases the initial investment.
② Saturated flue gas will produce condensation water during the transportation process, which will affect the service life of the switching station valve.
③If the gas cooler fails, production must be stopped for maintenance, which will affect converter production.
④ When dissipating, the flue gas passes through the gas cooler, which increases the system resistance and increases the operating cost.
In existing projects, in order to solve the problem of high water content in flue gas after passing through the gas cooler, a high-efficiency dehydration device is usually installed at the outlet of the gas cooler. However, due to the limitation of the dehydration efficiency of the dehydration device, the phenomenon of white mist from the chimney cannot be fundamentally solved.
Figure 3 Gas cooler front flow chart
3 New ultra-low emission technologies
3.1 Variable impedance full DC power supply technology
The dust removal efficiency calculation formula of the electrostatic precipitator is:
In the formula, A—the total area of the dust collecting plate, m2; Q—the air volume of the dust collector, m3/s; ω—the effective driving speed of the electrostatic precipitator, m/s.
In the formula, β—constant, m2; Vp—voltage peak value, V; Vai—voltage average value, V.
It can be seen from equations (1) and (2) that when the total area of the dust collecting plate and the air volume are constant, the dust removal efficiency of the electrostatic precipitator is directly proportional to the effective driving speed. The effective driving speed is proportional to the product of the peak value and the average value of the applied voltage of the electric field. When the average voltage is equal to the peak voltage, the product of the peak voltage and the average voltage reaches the maximum value. Only with a nearly pure DC power supply can the average voltage be equal to the peak voltage. There are two ways to achieve a power supply that is close to pure DC: ① The higher the frequency, the closer the output of the power supply is to DC. ②The output adopts non-simple rectification method and uses variable impedance technology to achieve pure DC.
At present, the industry uses variable reactance full DC power supplies to achieve the above two approaches, and the output voltage is close to pure DC. The comprehensive comparison between variable reactance full DC power supplies and other power supplies is shown in Table 1.
Table 1 Comprehensive comparison between variable reactance full DC power supply and other power supplies
|Variable impedance full DC
|Comprehensive power conversion efficiency
|Get Vai and Vp
|Dust removal efficiency
|spark discharge shock
The variable reactance full DC power supply breaks through the capacity bottleneck of the current high-frequency power supply (the current maximum stable capacity of the high-frequency power supply is 2.2A) and achieves a large capacity of 3.5A. The use of variable impedance full DC technology, combined with the improvement of the dust collector itself, can improve the dust removal efficiency of the electrostatic precipitator and achieve ultra-low emissions from dry dust removal in converters of different tonnages.
The variable-reactance full DC power supply was upgraded and replaced in a steel plant for a 300t converter power supply. The comparison of operating data before and after the replacement is shown in Table 2.
Table 2 Comparison of operating data before and after power upgrade and replacement of a 300t converter in a steel plant
|Power supply type
|Maximum secondary voltage (kV)
|Maximum secondary current (mA)
|Maximum output power (kW)
|Industrial frequency power supply
|an electric field
|Second electric field
|Three electric fields
|Four electric fields
|Variable impedance full DC power supply
|an electric field
|Second electric field
|Three electric fields
|Four electric fields
In this project case, only the original industrial frequency power supply was upgraded and replaced. The actual operating voltage of the replaced variable-reactance full DC power supply is significantly higher than that of the original power supply. The driving speed is greatly increased, and through exit emission testing, the exit emission value is reduced by 33%. In addition, through the variable impedance full DC power supply technology, the rated capacity, maximum operating current and output power of the power supply have also been greatly improved, stimulating the operating potential of the dust collector and helping to achieve ultra-low emissions.
3.2 High temperature composite fiber filter tube dust removal technology
In the converter gas purification and recovery system, the fine dust collector behind the evaporative cooler is the key equipment for purification. Therefore, choosing an efficient fine dust collector is the key to achieving ultra-low emissions.
High-temperature composite fiber filter tube can meet the requirements of high temperature resistance and water repellency. Its main material is composite aluminum silicate, which has high strength, high void ratio, low density, and good thermal shock resistance (not affected by thermal expansion and contraction and fracture ), can operate stably at 750℃. Compared with traditional bag dust collectors, the structure is similar and the process structure is relatively mature. At the same time, the fiber filter tube has the following advantages: pulse cleaning does not deform, the dust cake layer control is stable, and the filtration accuracy always maintains a high level. There is no keel support, no mechanical friction is generated during the injection process, and the service life is long. The fiber filter tube has strong water-repellent properties. As long as there is no obvious condensation water, there will be no bagging phenomenon.
After using fiber filter tubes, the outlet concentration of the dust collector can be lower than 5 mg/Nm3. The specific process is shown in Figure 4. However, the resistance of the fiber filter tube dust collector is higher than that of the electrostatic precipitator. Therefore, when selecting a solution, it is necessary to comprehensively compare the initial investment and operating energy consumption to select a suitable purification solution.
Figure 4 Process flow chart of high temperature composite fiber filter tube dust collector
3.3 Heat recovery gas cooler
In order to solve the problem of white mist from the chimney in front of the gas cooler, a heat recovery gas cooler can be used. The essence of the heat recovery gas cooler is an indirect heat exchanger. It does not use spray water to directly cool the converter gas, but indirectly cools the gas through the circulating water in the heat exchanger. The circulating water is heated and gasified through the heat exchanger and then sent to the converter steam system for utilization.
The advantages of heat recovery gas coolers are:
①Solved the problem of white smoke coming out of the chimney.
② The moisture content of converter gas is low, and the gas holder can recover more gas, improve the quality of converter gas, and reduce its drainage load.
③The waste heat of the converter gas is further recycled.
However, the prerequisite for using the heat recovery type is that the fine dust removal device of the entire system can ensure that the emission concentration is ≤ 10mg/Nm3, because the heat recovery type gas cooler does not have the function of deep purification.
3.4 Sonic agglomeration dust removal device
Composite acoustic wave agglomeration technology uses high-intensity sound fields to cause relative motion of micron and sub-micron fine particles in aerosols in the sound field to increase their collision and agglomeration rate. Due to the strong van der Waals attraction on the particle surface, once the particles collide, they are likely to adhere and form larger agglomerates. This causes the particle size distribution of fine particles to rapidly migrate from small to large sizes within a short time range, and the number and concentration of particles is reduced. In order to enhance the removal efficiency of subsequent deep purification and dust removal facilities, deep purification and dust removal facilities can use high-efficiency tubular dust collectors, bag dust collectors, etc. The outlet concentration of dust collectors can be lower than 10mg/Nm3. The process flow is shown in Figure 5.
Figure 5 Process flow chart of sonic agglomeration dust removal device
The advantages of using the sonic wave agglomeration dust removal device in the converter gas deep purification system: ① The sonic wave agglomeration technology has strong adaptability and high reliability. ②The space occupied is small and the installation location is flexible.
Disadvantages of the sonic agglomeration dust removal device used in converter gas deep purification systems: ① Sonic agglomeration dust removal produces high sound pressure and high-frequency sound waves, which consumes a lot of energy. ②High-frequency sound waves may cause harm to people.
Therefore, it is a trend to develop low-frequency acoustic wave agglomeration dust removal devices.
With the advancement of ultra-low emission work, some targeted new ultra-low emission technologies for converter gas dry purification have emerged, such as ① variable resistance full DC power supply technology; ② high-temperature composite fiber filter tube dust removal technology; ③ heat recovery gas Cooler; ④ Sonic agglomeration dust removal device. The emergence of these technologies has provided some new ideas for converter gas dry purification with ultra-low emissions. However, when selecting a solution, a comprehensive comparison should be made based on the advantages and disadvantages of various technologies.