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Keywords:Waste gas treatment Dust removal equipment Bag-type strainer Water rotary dedusting equipment Smoke management

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Analysis and Discussion on medium and small dust removal facilities

Release time:2019-09-19  Click:448

Analysis and Discussion on medium and small dust removal facilities

April 19, 2017 Ningbo Changqing Environmental Protection Technology Co., Ltd

Ningbo Yuyao Zhonglv environmental protection technology center

Combined with many years of practical experience in dust control of small and medium-sized electric furnaces, some key problems of dust control system of electric furnaces are recognized and discussed.

1. The basic characteristics of electric furnace dust and the difficulties of dust removal

1.1 electrc furnace steelmaking process the main process of electric furnace steelmaking is to smelt and recover the waste steel in the electric furnace, and adjust the chemical composition of the metal by adding alloy elements. The energy used for smelting is mainly electric power, which can also be directly injected into the furnace oxygen and other fossil fuels to replace and supplement electric energy.

Electric furnace smelting generally consists of three smelting periods: melting, oxidation and reduction.

In the melting period, the oil combustible substances and metals in the scrap are gasified at high temperature, resulting in black brown smoke; in the oxidation period, the molten metals in the furnace are oxidized and decarburized violently due to oxygen blowing and ore adding, at this time, the smoke is reddish brown; in the reduction period, the oxygen and sulfur in the molten steel are removed, the chemical composition of the molten steel is adjusted, and slag making materials such as carbon powder or ferrosilicon are added, resulting in white or black smoke.

In the three smelting cycles, the amount of flue gas produced in oxidation period is the largest, the concentration of dust is the highest, the temperature of flue gas is the highest, and the particle size of dust is the smallest. Therefore, the flue gas in smelting period has become the key object of governance.

1.2 basic characteristics of flue gas and smoke of electric furnace

1.2.1 main technical parameters of electric furnace smoke and dust: the amount of smoke produced per ton of steel is 700-900m3, the temperature of smoke is 1250-1400 ℃, and the amount of dust produced per ton of steel is 12-16kg.

1.2.2 the composition of flue gas fluctuates greatly, which is affected by the oxygen blowing intensity, oxygen blowing time, charge type, additive and fuel type and quantity. It is difficult to deal with the volume fraction of each component in general oxidation stage. The temperature of electric furnace dust is high and its adhesiveness is strong. If the system design is unreasonable, the bag burning and high resistance will occur easily when the bag filter is used for treatment

3) Low operation efficiency. Because of the high resistance of the system, the fan works in the low efficiency area and the operation cost is high.

In view of the above three problems, in line with the design principles of low temperature and medium resistance, the following discussion is carried out.

2. At present, the most widely used dust removers are wet dust remover, electric dust remover and bag type dust remover. According to the characteristics of small and medium-sized electric furnaces, the adaptability of each furnace is analyzed as follows.

2.1 wet dedusting and wet dedusting have the characteristics of low investment, fast starting, low operation cost, simple operation and maintenance, but low dedusting efficiency, it is difficult to achieve the standard discharge, and there are secondary water pollution problems, which have been basically eliminated.

2.2 the electrostatic precipitator uses the function of high-voltage electric field to separate the dust containing gas from the gas. The dust collector has strict requirements on flue gas temperature. If it is too low, it will cause condensation and creepage. If it is too high, it will increase the specific resistance of dust, reduce the efficiency of dust collection. The plate is easy to be heated and deformed, which will affect the normal operation of the dust collector. In addition, the large one-time investment also limits the application of ESP in small and medium-sized electric furnaces.

2.3 as a kind of filter dedusting equipment, bag type deduster is used to catch non cohesive and non fibrous industrial dust. It has the advantages of high and stable dedusting efficiency, low equipment cost, simple management, convenient maintenance, etc. it is suitable to collect all kinds of dust, and can fully meet the environmental requirements of a reliable dedusting equipment. Bag type dedusting is currently used most in the dedusting of small and medium-sized electric furnaces. The dedusting efficiency of this method is high, generally more than 99%. It can stably discharge up to standard for a long time, and realize the production of smoke-free, dust-free and odorless.

3. Collection of smoke and dust

3.1 dust collection of dust cover is the most important part of the dust removal system of electric furnace. Whether it can effectively overcome the limitations of production process and operation of electric furnace and the influence of dust change in different stages of electric furnace smelting process is the key to select dust cover. At present, the commonly used dust hoods are as follows:

3.1.1 open type dust hood and roof dust hood this method is to hang the smoke hood on the roof frame of the plant, which is far away from the top of the furnace, and the smoke and dust will overflow freely from the furnace. The dust will be collected by the dust hood and enter the dust removal system by heat lifting.

Advantages: 1) no interference with the operation of the electric furnace; 2) simple design; 3) because the dust and cold air in the workshop are inhaled into the dust removal system together, it is not necessary to specially set up the flue gas cooling system.

Disadvantages: 1) low collection efficiency of smoke and dust; 2) large amount of smoke and large system volume.

Due to the lower collection efficiency is difficult to meet the increasingly strict environmental protection requirements and the high energy consumption brought by the large amount of flue gas, the open dust hood is not suitable for the dust removal of small and medium-sized electric furnaces.

3.1.2 closed cover or semi closed cover this method is to cover all or part of the electric furnace.

Advantages: 1) good collection efficiency; 2) sound insulation and heat insulation; 3) small air volume can be used to absorb most of the smoke and dust, with small fan power and low energy consumption.

Disadvantages: 1) due to the fact that almost no wild wind enters the dust removal system and the flue gas temperature is high, the requirements for the flue gas cooling system are high.

2) The fixing of dust cover affects the normal operation of electric furnace. The first problem that should be considered in the dedusting of electric furnace is that it does not affect the normal operation of electric furnace smelting, so the fixed closed cover or semi closed cover is not suitable for the dedusting of electric furnace. Moreover, for the high requirements of the closed hood on the flue gas cooling system, it is difficult for the small and medium-sized electric furnaces to achieve under the limited funds.

3.1.3 mobile semi airtight hood mobile semi airtight hood is suction

Advantages: 1) when charging and pouring molten steel, it can automatically move away without affecting the operation; in the smelting process, it is closed state, which effectively suppresses the strong diffusion effect of smoke and dust and achieves better dust collection effect.

2) Because the lower part is open and the upper part is closed, a part of cold air can be mixed to effectively reduce the flue gas temperature.

Disadvantages: the effect of dust collection is poor when feeding.

3.2 the advantages of mobile semi airtight hood are obvious, which is the commonly used way of dust collection at present. However, in order to strengthen the collection of dust during charging, a roof hood is usually attached to large electric furnace. In other words, the mobile semi airtight cover + roof cover form is used to collect smoke and dust, which can minimize the diffusion and overflow of smoke and dust. Of course, due to the low efficiency of the roof cover, the smoke volume inhaled by the roof cover is 2-3 times of that inhaled by the semi closed cover, that is to say, the total air volume of the system should be increased by 2-3 times. But this way is not suitable for medium and small electric furnaces. The generation of electric furnace dust is mainly concentrated in the oxidation stage, while the amount of dust produced in the feeding stage will not exceed 5% of the total smoke emission, and the emission concentration is also very low.

Due to the large emission base of large-scale electric furnace, although the proportion of smoke and dust emission in the charging stage is very small, the absolute amount of smoke and dust emission is also very large, so it must be collected and treated. If small and medium-sized electric furnaces want to collect smoke and dust with the emission and mass fraction not more than 5% in the charging stage, the cost is to increase the air volume of the system by 2-3 times. Considering the dedusting efficiency, energy consumption and equipment investment, it is obviously very uneconomical. Therefore, it is more practical and easy to popularize to use a single mobile semi closed hood on the small and medium-sized electric furnace than to use the mobile semi closed hood + roof hood.

4. Control of flue gas temperature

4.1 the influence of mixed air is the most important factor to reduce the flue gas temperature produced by the electric furnace itself to the applicable temperature of the dust remover. When the flue gas produced by the electric furnace is certain, according to the direct cooling formula of the suction air, the air that needs to enter the semi closed hood can be determined. According to the air volume, the relevant parameters of the semi closed hood can be determined by using the calculation formula of the suction volume of the semi closed hood.

Jiangxi Copper Group uses a set of electric furnace dedusting system designed and manufactured by the company. According to the cooling formula and the suction volume formula of the semi enclosed hood, the design has achieved good cooling and suction effect. The parameters of the electric furnace and dedusting system are shown in the table. After the flue gas temperature is effectively controlled, the problem to be solved is to prevent large particles Mars from entering the deduster. The following measures have been adopted for this purpose.

4.1 set combustion chamber. During the smelting process of electric furnace, there are combustible gases such as co entering the pipeline for continuous combustion. If the combustion is not completed in the pipeline, entering the dust collector will cause bag burning. The purpose of setting the combustion chamber is to make co burn completely before entering the dust remover, so as to prevent bag burning.

The combustion chamber is set behind the smoke hood, which can be made of heat-resistant materials. Its main functions are: 1) to remove the flying dust with large particles in the smoke (which plays the role of settling chamber); 2) to burn off the CO in the smoke to ensure the safe operation of the system; 3) to burn off the grease and some organic matters contained in the smoke.

4.2 set the air inlet baffle for the dust remover, and set the air inlet baffle at the air inlet of the dust remover, which can also prevent the burning of bags on Mars to a certain extent. If there is Mars entering the dust remover, it will collide with the baffle plate and change the movement direction so that it cannot contact the filter bag directly.

5. Control of system resistance

5.1 control of ash cleaning frequency. Generally, time is used to control the frequency of ash cleaning in the dust remover, so the system resistance is a variable. Therefore, when selecting the fan air pressure parameter at first, it is either too small or too large. If it is too small, the system cannot operate normally, and if it is too large, it will cause unnecessary energy consumption. This problem can be solved by using differential pressure to control the frequency of dust removal. The dust removal system always operates under a fixed pressure loss, which provides a reliable basis for the selection of fans, so that the dust removal system can operate stably, efficiently and energy saving.

5.2 the dust remover shall be set up in a targeted way. In normal operation, the three-state dust remover shall be used. When it is necessary to stop using the dust remover, the control system is set to clean the ash once after the main fan stops, so as to prevent the ash from hardening and prolong the service life of the bag.

6. If the design is improper, it is easy to cause ash deposition in the flue gas. If it is serious, 1 / 2 of the flow section of the flue can be blocked, which will affect the normal use of the dust removal system.

In order to prevent ash deposition, a small settling chamber can be set every other section. By using the principle of inertia, the velocity of flue gas is suddenly reduced and the flow direction is changed, so that the ash particles settle to the bottom of the settling chamber and are cleaned regularly.

On the horizontal pipeline, the flow rate of flue gas shall be controlled to be greater than 16m / s, so that the dust of small particles will be taken away with the flue gas and will not be deposited in the horizontal pipeline. For the convenience of ash cleaning, the ash cleaning door shall be set at the front and back of the horizontal pipeline, and the flue gas shall be cleaned regularly.

7. Conclusion

Through reasonable design, improve the working efficiency and stability of the dust removal system and effectively reduce the cost, will greatly improve the enthusiasm of enterprise owners for the dust removal of small and medium-sized electric furnaces, which is a great impetus to the cause of environmental protection in China.

 

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Office Address: 703, Building 1, Miao Huan Power Town, Qingfeng Road, Jiangbei High-tech Zone, Ningbo City, Zhejiang Province
Production Address: No. 3 Zhuhai Road, Chengdong Industrial Park, Xiangshan County, Ningbo City, Zhejiang Province
Sales Department:+86-574-87792453
+86 13081928795
+86 18257419819
Purchasing department: +86-574-87210884

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Fax: 0574-87211844
E-mail:nbxyhb@sina.com
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