Reduce pollution of stainless steel industry
Abstract: In recent years, the Party Central Committee and the State Council have attached great importance to the ultra-low emission transformation of iron and steel. In many important meetings and government work reports, it has been proposed to promote ultra-low emission transformation of the iron and steel industry. “Opinions on Promoting the Implementation of Ultra-Low Emissions in the Iron and Steel Industry” ( After the issuance of Huan Taiqi  No. 35), all regions are required to carry out ultra-low emission transformation of the steel industry in phases and regions. The emission limits in the “Opinions” are also called “the strictest standards in history” by industry experts. Under this general situation, by combing through the index requirements for particulate emissions in the document and the current status of my country’s dust removal technology, compare the advantages and disadvantages of the main dust removal technology with high recognition in the industry, and discuss the selection of dust removal technology routes under the new requirements. And upgrade ideas for reference by relevant steel companies and help win the battle against the blue sky.
In order to implement the decisions and deployments of the Party Central Committee and the State Council, in April 2019, the Ministry of Ecology and Environment, together with the Development and Reform Commission and the Ministry of Industry and Information Technology, jointly issued the “Opinions on Promoting the Implementation of Ultra-Low Emissions in the Steel Industry” (hereinafter referred to as the “Opinions”). The “Opinions” have once again tightened the original emission standards for particulate matter in various steel processes, and proposed that ultra-low emissions refer to ultra-low throughout the process. It also puts forward progress requirements for ultra-low transformations in various regions, which further promotes the dust removal and treatment technology of the steel industry. Change. However, at present, most domestic iron and steel enterprises have long blast furnace-converter process, with many and complicated processes. It is no easy task to meet the particulate matter emission standards of the entire production process. Moreover, the development of domestic iron and steel enterprises is uneven, and the production capacity that does not meet environmental protection standards is still rare. Therefore, upgrading and upgrading of dust removal facilities is imperative. Therefore, under the current environmental protection policy situation, seeking to achieve the ultra-low emission limit of dust particles in a short period of time is undoubtedly the most urgent problem faced by steel companies.
1. Particulate matter control requirements in ultra-low emission transformation
In April 2019, the “Opinions” was officially launched, setting off a storm of steel environmental protection, declaring that my country’s steel industry as a whole has entered the general situation of ultra-low emission transformation. Regarding particulate matter indicators, the “Opinions” require exhaust gas in the form of organized emissions, sintering machine head and pellet roasting flue gas (including shaft furnace, grate-rotary kiln, belt roaster), coking process coke oven chimney exhaust gas , Other major pollution sources (including the tail of the sintering machine, coal charging, coke dry quenching, hot blast stoves, blast furnace pits and tapping houses, hot metal pretreatment, converter secondary flue gas, etc.) The hourly average emission concentration of particulate matter is not high At 10 mg/m3, the hourly average emission concentration of at least 95% of the time per month meets the standard; the waste gas is in an unorganized form, the material conveying and blanking points, sintering, pelletizing, ironmaking, coking and other processes of material crushing, screening, Dust removal facilities should be provided for mixing equipment and scrap cutting. In addition, the “Opinions” also pointed out that enterprises should choose mature and applicable environmental protection transformation technologies in accordance with factory conditions, and encourage the use of advanced dust removal facilities such as film-coated filter bag dust collectors and filter cartridge dust collectors, which points out the direction for the choice of dust removal treatment technology. .
2. Current status of dust removal technology application
After investigating more than 20 iron and steel enterprises, it is found that almost all iron and steel enterprises use high-efficiency bag filter or cartridge filter to treat dust-containing exhaust gas, and some processes that produce wet exhaust gas use wet electrostatic precipitators. The company believes that these mature processes have the best dust and waste gas treatment effect, which is the same as the dust removal technology mentioned in the “Opinions”. In addition, in accordance with the feasible technologies for the treatment of particulate matter in exhaust gas specified in the “Technical Specifications for Pollution Permit Application and Issuance”, except for the exhaust gas produced by the hot rolling mill finish rolling, the other exhaust gas pollution generation nodes can be treated with bag dust (covering). Membrane filter material) and filter cartridge dust removal process. Therefore, this article mainly analyzes the advantages and disadvantages and application of the bag and filter cartridge dust removal technologies.
The bag filter appeared earlier and was used as early as the end of the Westernization Movement. It was mainly used to filter dry, dusty gas with small particle size. The filter bag is made of various filter fibers (chemical fiber or glass fiber) through weaving or needle punching, and uses the filtering function of the fiber fabric to filter dust-containing gas. The cartridge type dust collector appeared relatively late. In the 1970s, some users appeared in Western countries. They believed that this type of dust collector was relatively small in size, significantly improved in processing efficiency, and easy to maintain. However, if it is necessary to treat dusty gas with a larger air volume, the treatment effect will be poor due to the small capacity of the precipitator, which is difficult to apply in large industrial enterprises, so it has not been widely promoted for many years. Since the 21st century, the world’s material technology has developed rapidly. Some foreign companies have taken the lead in improving the structure and filter material of the dust collector, increasing the overall capacity by several times and becoming a large dust collector with a filter area of more than 2,000 m2.
3. Comparative analysis of dust removal technology
1. Bag dust collector
(1) Working principle of bag filter
The dust-containing gas enters the ventilation duct from the dust removal hood, and when it reaches the outlet, it is induced by the induced draft fan, and then the fibrous dust removal filter bag is used to capture the smoke and dust with the help of gravity and inertia.
(2) The main factors affecting the performance of the bag filter
The performance of bag filter mainly includes dust removal efficiency, pressure loss and service life. The main factors that determine the dust removal efficiency and service life of the bag filter include the air-to-cloth ratio, the type of filter material, and the choice of dust removal methods.
The filter material of the bag filter has evolved from conventional fibers to superfine fibers, then to special-shaped cross-section fibers, and then to ePTFE membrane structure. Conventional fibers cannot control fine dust particles, so it is necessary to change the fiber structure or use external force to achieve ultra-low dust emission control; ultra-fine fiber shaped cross-section fibers have a larger specific surface area, resulting in a larger filtration area, thereby reducing Air-to-cloth ratio; ePTFE membrane can intercept dust particles on the membrane surface. At present, the choice of membrane filter material for filter bag material is the choice with higher dust removal efficiency.
2. Cartridge dust collector
The working principle of the filter cartridge dust collector: The dust-containing gas enters the ventilation duct through the dust collector, and is introduced into the box by the external induced draft fan. Because the box has a much larger radius than the pipe, the airflow expands, and the heavier large particles of dust settle by gravity , The lighter small particles of dust enter the filter cartridge with the airflow, and are blocked by the filter element through a series of comprehensive effects and then separated from the air.
3. Comparison of advantages and disadvantages of bag filter and cartridge filter
Bag type dust collector and filter cartridge dust collector have their own advantages and disadvantages in the use process. When selecting the dust removal process, comprehensive consideration should be given to the company’s own situation. The advantages and disadvantages are shown in Table 1.
Four, enterprise practical application case analysis
Take the dust removal process transformation of a blast furnace pit process section of a steel group in Hebei Province as an example. The company originally used a bag filter to remove dust from the exhaust gas generated in the blast furnace pit section. However, it was discovered during use that it would cause confusion due to working conditions. The bag problem. At the same time, due to the poor dust removal effect of the filter bag, the exhaust gas emissions of this section cannot stably meet the requirements of ultra-low emission standards. Considering the condition of reaching the standard and the capital investment to replace the filter bag, the company decided to transform the dust removal process and replace the bag filter with a filter cartridge filter. The parameters and effect comparison before and after the transformation are shown in Table 2.
According to the online monitoring data before and after the transformation, the particulate emission concentration of the exhaust gas in this section has been greatly reduced, and it can stably reach within 10 mg/m3, meeting the requirements of ultra-low emission standards. Compared with before the transformation, after using the filter cartridge dust collector, the problem of easy wear and leakage of the filter bag is avoided, basically it can be used for a long time without maintenance, even if the filter cartridge is removed and replaced, it is very convenient, and it is enlarged in a limited space. The effective filter area is reduced, the pressure difference is small, and the dust removal effect is relatively stable. But after replacing the filter cartridge dust collector, there are also some shortcomings.
Through communicating with the internal staff of the company, the author learned that the equipment after the transformation is more complex than before, and the company needs to have a high level of equipment dispatching, installation and maintenance management. In addition, the selectivity of the filter cartridge dust collector for dry dust types is not as good as expected, and it does not have high dust removal efficiency for all dust types. If you want to apply it to all processes, it still needs to be in-depth research and development. In general, under the increasingly severe environmental protection situation, based on the consideration of environmental compliance, the effect of the replacement is still very significant.
Five, summary suggestions
1. Suggestions for process selection
At present, without considering wet dust removal, the best selection of dust removal technology in the ultra-low emission situation should be cartridge dust collector and bag filter. The two types of dust collectors have their own advantages and disadvantages. For the ultra-low particulate emission transformation of steel enterprises, it is recommended that enterprises can choose dust removal technology according to actual conditions and their own needs. If the original bag dust removal process is still unable to achieve stable emission standards, the first step can be Consider replacing the PTFE microporous membrane and ultra-fine fiber surface layer gradient filter material. Secondly, consider replacing the filter cartridge dust removal process to complete the ultra-low emission transformation and achieve the standard emission.
2. Engineering design suggestions
In order to help companies meet the relevant requirements of the “Opinions” and provide references for engineering design and construction, in January 2020, China Environmental Protection Industry Association issued the “Technical Guidelines for Ultra-low Emission Reconstruction of Iron and Steel Enterprises”, in which the high-efficiency bag dust removal process and filtration The drum dust removal process proposes a series of technical parameter reference values, and it is recommended that enterprises can refer to them in the process of ultra-low emission transformation based on their actual conditions. Taking the bag filter as an example, it is recommended that when the company makes a contract, the filter wind speed should be designed to be less than 0.8 m/min. The filter wind speed here should be the full filter wind speed. The full filtration wind speed is the theoretically calculated filtration wind speed. When the off-line dust collector cleans the dust, one of the bins will be closed and the actual filtration wind speed will increase. This is also the time when emissions are most likely to exceed the standard, so the requirement is the full filtration wind speed ; It is recommended that the dust collector is designed with a deflector to control the airflow distribution. If the deflector is not selected, the filter bag or filter cartridge will be washed by the airflow and reduce the service life.
The “Blue Sky Defense” has entered the final stage of tackling tough problems. As the absolute main battlefield for air pollution prevention and control, the steel industry is imperative for ultra-low emission transformation. Iron and steel companies must actively respond, clarify environmental pollution control ideas, and promote environmental quality improvement and industrial transformation upgrade.