Analysis of technical issues in the dust removal system for smelting electric furnaces

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Analysis of Technical Issues in the Dust Removal System for Smelting Electric Furnaces Author: Jiangyin Xingcheng Special Steel Company, Li Peiquan, Lu Yuqiao, Yu Jiazhi Abstract: This paper analyzes three key factors that affect the efficiency of the dust removal system in electric furnaces as well as their operational costs: the quality of the filter media used in the dust collectors, the dust cleaning techniques, and the temperature of the flue gas. Keywords: dust collector ； Filter media quality ； Dust removal technology ； Flue gas temperature: With the improvement of environmental protection regulations in China and the rising awareness of environmental protection among the public, when introducing foreign electric furnace smelting technologies and equipment or upgrading existing ones, it is necessary to consider adopting world-class electric furnace dust removal systems that complement them. In recent years, process improvements such as the installation of gas (oil) oxygen nozzles and hot-charging of molten iron in electric furnaces have led to an accelerated production pace, reduced smelting time, and a corresponding increase in output. As a result, the dust removal systems became insufficient in capacity and their effectiveness declined. Various factories then conducted technical evaluations and took measures to meet the **specified emission standards. In this regard, three key questions are raised regarding the electric furnace dust removal system for analysis. I. Process Flow of the Dust Removal System 1. Commonly Used Process Flow: A dry dust removal method that combines primary smoke extraction from inside the electric furnace, secondary smoke extraction from the furnace’s enclosure and the roof of the factory building, as well as smoke extraction from the refining furnace and the feeding system; this is combined with fan-driven pressure to send the smoke into the main ducts of the furnace’s dust removal system, along with filter bag filtration. It is a process that operates under negative pressure, as shown in Figure 1. Figure 1 Ordinary process flow for dust removal in electric furnaces 2. PLC automatic control (1) Pulse (set time or pressure difference) reverse blowing for dust cleaning. (2) Automatically adjust the air volume, air pressure, and fan power at each dust collection point according to the smelting conditions. II. Key Technical Issues 1. Quality of the filter media in dust collectors The choice of filter media quality determines the operating resistance and operational costs as well as the investment costs of the dust collector, and it has a direct impact on the emission concentration of dust and fumes. 2. Dust removal technology: The quality of dust removal performance directly affects the filtration efficiency and the lifespan of the filter bags. 3. Flue gas temperature: The control of flue gas temperature has a direct impact on the exhaust efficiency of the enclosures and the roof of the factory building, the creation of a slight negative pressure inside the furnace, and the lifespan of the filter bags. III. Technical Analysis 1. Bag Filter Media: The commonly used bag filter media is polyester needle-punched felt, and its filtration principle relies on the primary dust layer (dust cake) formed on the surface of the bag to carry out filtration. Characteristics of electric furnace dust: light, fine, highly dispersible, and poor in fluidity ； The filter bags can become easily clogged, a condition known as “bag clogging”. The main components in the flue gas and the dust components (see Tables 1 and 2). FeO, FeO/Fe2O3, and CaO tend to crystallize into lumps on the surface of the filter bags, covering the pores in the filter media. Additionally, dust penetrates into the inner layers of the filter media, eventually blocking these pores; as a result, the gas flow rate through the filter bags decreases, pressure loss increases, and the operating resistance of the dust collector rises (Figure 2). Each dust collection point is ineffective at capturing smoke and dust, allowing it to spread and pollute the environment ； The back-blow cleaning cycle for the bag filters becomes shorter, or back-blowing occurs continuously, which reduces the fatigue life of the bags and leads to their rupture, resulting in excessive dust emissions (above 100 mg/Nm3). To meet environmental emission requirements, it is necessary to replace the bag filters, which increases operating costs. Additionally, the inevitable accident water in the system will also be a source of moisture in the exhaust gas. Therefore, it is recommended that traditional polyester needle felt be specially treated as the filter media for cloth bags, in order to meet the following requirements: a. Hydrophobicity – water does not bind to the cloth bag, and dust does not stick to it; instead, dust and water settle in the ash bin. b. Fiber break strength of bag filter media: it is recommended that the transverse tensile strength be >120 kg/5 cm ； Longitudinal splitting strength > 70 kg/5 cm. Figure 2 Operating resistance of the dust collector 2, dust cleaning technology (1) Dust cleaning tools use a pressurizable medium gas. In the initial stage of use, the bag filter media exhibits high filtration efficiency, with mild blockage of the bags; therefore, the backwashing pressure for cleaning can be maintained at 3–4 kg. Avoid premature fatigue of the bag due to prolonged operation under high pressure ； As the bag filter media are used over time, the degree of blockage of the bags increases, and the operating load keeps rising. Low-pressure backwashing is insufficient for cleaning the dust from the bags; at this point, a pressure of 5–7 kg can be used for cleaning, thereby minimizing the filtration pressure drop and maintaining an effective dust-cleaning performance, which in turn helps to extend the service life of the bags. But at this time, a venturi tube should be added to prevent high-pressure gas from causing the bag to rupture. (2) The medium gas used for dust removal should be free of water and oil; nitrogen is ideal for steel mills. Because a certain amount of moisture is present in the compression control under specific pressure and temperature conditions (see Figure 3). Figure 3 Saturation moisture content in compressed air at different pressures (3) After the dust collector stops operating, backwashing for two more cycles should be carried out to prevent water in the air from combining with the dust on the surface of the filter bags, resulting in caking and blockage of the bags. 3. Flue gas temperature: (1) If the temperature (T) of the flue gas entering the dust collector is too high, the bag filter media contracts and deforms, resulting in an increase in operating resistance. If the flue gas temperature (T1) exceeds the softening temperature of the filter media, it will cause the bag filters to fail or burn out. Therefore, before entering the dust collector, there must be a mixing mechanism with accident protection to ensure that outside ambient air mixes thoroughly with the flue gas and cools it, thereby guaranteeing that the flue gas enters the dust collector below the softening temperature of the bag filter media. (2) If flue gas continuously enters the dust collector in an abnormal condition, it will inevitably cause a \"short circuit\" in the entire piping network of the dust collection system, resulting in a significant reduction in the efficiency of dust collection at various suction points ； The T2 temperature is too high; water cooling is used, and the sliding sleeve automatically slides away from the flue gas elbow on the furnace lid, making it difficult to create a slight negative pressure inside the furnace. The flue gas pipeline is required to have sufficient cooling capacity ； If the water-cooled flue is long enough or has a sufficient diameter, and the mechanical cooler has a sufficient cooling area. IV. Conclusion: Choosing oil- and water-resistant polyester needle-punched felt, adopting appropriate dust removal techniques, and controlling the flue gas temperature in the electric furnace dust removal system will result in lower operating resistance, better dust removal efficiency, a longer lifespan for the filter bags, and reduced operating costs. An organic and rational combination of the three elements, based solely on domestic solutions, is sufficient to ensure that the electric furnace dust removal system meets the emission standards set by environmental regulations.