Semi dry dedusting process of flue gas from the ho

2022-09-25
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Converter flue gas semi dry dedusting process

at present, there are more than 550 converters in China, of which more than 10 are dry dedusting, and the rest are wet dedusting. Both wet and dry methods have many problems, such as high emissions, high energy consumption, high water consumption, large maintenance and so on. Therefore, Shanghai Waigaoqiao spray system company applied for a semi dry dedusting process of converter flue gas that is more suitable for the current national conditions, and obtained the authorization of Chinese invention patent in September, 2006

some performance indexes of semi dry dedusting, dry dedusting and wet dedusting of converter flue gas are compared as follows:

at present, the semi dry evaporative cooling tower technology of this process has been used in the reconstruction or new construction projects of more than 10 converters in China, with significant environmental protection, energy saving, water saving, reducing maintenance, increasing production and reducing costs, and significantly reducing construction investment The positive effect of floor space and shortening the transformation time has been confirmed and recognized by more and more users

1. Semi dry high-efficiency spray cooling dedusting tower

the temperature of converter flue gas after being cooled by vaporization cooling flue is usually 800 ~ 1000 ℃. Evaporative cooling tower is used for dry method and Yiwen is used for wet method. In this process, single or multiple hollow semi-dry high-efficiency spray cooling dedusting towers are used for cooling, that is, dry evaporative cooling technology is adopted; The difference is that spray dedusting is still used for dedusting, and the generated sewage is still treated by water flushing

the practical application results show that the most significant effect of replacing the traditional one by using the semi dry high-efficiency spray cooling dedusting tower is that the system resistance is reduced by 90%, from 3 ~ 5 kPa to about 300 Pa. this effect can be brought into play from the following aspects:

environmental protection: when only one text is transformed into a tower, the saved differential pressure can be added to the second text, Thus, the dust concentration of the discharged flue gas is reduced to less than 100 or 50 mg/s-m3 (depending on the fan capacity), that is, the resistance of the system is redistributed

improve equipment capacity: it can solve the problems of insufficient fan capacity and smoke from chimney and furnace mouth caused by converter expansion, expansion and increase of oxygen supply intensity with very low investment, and improve the processing capacity of the system

energy saving: if the dust concentration has reached the standard, the fan speed can be reduced. Only transforming Yiwen into a tower can save about 1 ~ 2kwh/T of steel

water saving: the circulating water volume is reduced by at least 50%, and there is no circulating water even after dry ash collection, which solves the problems of insufficient water treatment capacity, poor water quality and high water treatment operation cost

2. Wet electrostatic precipitator

fine dust removal of converter flue gas at present, dry electrostatic precipitator is used in dry method, and the main problems are as follows: first, the dust content cannot be stably controlled below the designed 10 mg/standard cubic meter; Second, the anode plate/cathode line is easy to corrode, deform and scale due to long service time, and the amount of maintenance is particularly large; Third, the electrostatic precipitator must adopt reliable anti explosion measures

the main problems of wet process are: first, the emission concentration is high, and the recovered gas needs to be cleaned by wet electrostatic precipitator before use to ensure normal use; Second, the required differential pressure is particularly large, and the system energy consumption is high; Third, the circulating water volume is particularly large

this process adopts wet electrostatic precipitator (horizontal or vertical) for fine dust removal. The main advantages are:

the outlet temperature is low, and the recovered gas can directly enter the gas holder without cooling like the dry method

the purification effect is the best, which can ensure that the dust concentration is less than 10 mg/standard cubic meter or less

50% energy saving per ton of steel (~ 4kwh/T steel), the same as dry process

the circulating water is reduced by 50 ~ 90% compared with the two wet methods. Basically, there is no circulating water, and only a small amount of circulating water is used for equipment cleaning

washing the electrode with water can also eliminate the problems of secondary dust and ash deposition on the electrode plate/cathode line of the dry process

3. Spray at the furnace mouth

the optimal micro differential pressure between the converter mouth and the movable hood is ± 10 Pa, which means that the system should not smoke outward, but also try to reduce the amount of air inhaled by the system and improve the calorific value of gas. At present, the dry method adopts AC variable frequency speed regulation of primary air fan, and the wet method adopts the way of adjusting the pressure difference at the throat of Erwen to control the micro differential pressure at the furnace mouth. The actual micro differential pressure of a considerable number of converters is not automatically controlled or poorly controlled, resulting in a large amount of flue gas overflow, or a large amount of inhaled air, resulting in a decrease in the CO content of gas and a sharp increase in the amount of flue gas processed by the system

at the same time, there are also quite common problems such as slag sticking at the furnace mouth, water leakage of the smoke hood, and a large amount of maintenance

this process uses the spray system at the furnace mouth to supplement the control of micro differential pressure after obtaining the certificate. Its main functions are: when the furnace mouth pressure is high, it can play the role of dust removal and cooling; When the pressure at the furnace mouth is low, it can play a soft sealing role, reducing the carbon monoxide combustion, dust particle refinement and cooling caused by the inhaled air; At the same time, it can cool the smoke hood and furnace mouth equipment to reduce slag sticking and water leakage; The system can also be extended to the secondary dedusting during molten iron mixing, scrap adding and tapping, so as to replace or reduce the processing capacity of the secondary dedusting system. Since the atomized nozzle is used, there is no potential danger of explosion caused by mechanical water

4. AC variable frequency speed regulation of fan

after adopting wet electrostatic precipitator, there is no dual voltage regulation function of wet method; At the same time, because the flue gas flow changes greatly during the smelting cycle of the converter, in order to ensure the micro differential pressure at the furnace mouth and save energy, the fan must adopt AC variable frequency speed regulation; Other pressure regulating methods can also be used to maintain the micro differential pressure at the furnace mouth

5. Water flushing ash and sewage treatment

some or all of the dust removed by spray cooling dedusting and wet electrostatic dedusting is washed by continuous or intermittent water flushing of the original system and transported to the sewage sedimentation tank for sludge separation and sewage treatment. The treated water is recycled, and a small amount of industrial water is regularly supplemented

6. Spray gun atomizing gas

the atomizing medium required by the same gas atomizing nozzle used for evaporative cooling in this process can be superheated steam or saturated steam, or nitrogen. There is an automatic switching mechanism between the two, and the system can give priority to steam or nitrogen atomization according to the production conditions of the steel plant. Domestic production practice shows that steam atomization should be preferred in the case of excess steam emission; If the steam has been fully utilized and there is no surplus, it is more economical to atomize with nitrogen, and its operating cost is only less than 30% of that with steam

7. Mechanism of reducing circulating water by semi dry method

as we all know, there are two different cooling mechanisms for flue gas cooling by direct water spraying: the first is the so-called saturation cooling, which is to absorb the heat of flue gas through water heating to realize flue gas cooling. Theoretically, the heat absorption per kilogram of water is only 50 kcal

the second is evaporative cooling, that is, using the latent heat of water evaporation to absorb the heat of flue gas to realize the cooling of flue gas, and continuously improve the proportion of new materials in the raw material industry. Theoretically, the evaporation latent heat per kilogram of water is more than 500 kcal, which is exactly 10 times that of saturation cooling, so the water flow required to cool the same flue gas is 1/10 of that of saturation cooling. Like dry method, semi dry method also applies evaporative cooling mechanism, so not only the flow of circulating water can be greatly reduced, but also there is no requirement for water temperature, and even circulating hot water can be used directly

8. Mechanism of energy saving and improving system capacity by semi dry method

the total system resistance of semi dry dedusting process is the same as that of dry method, only 6 ~ 8 kPa; In contrast, the total resistance of the wet system is 22 ~ 28 kPa. The mechanism of saving energy and improving the processing capacity of the system is analyzed as follows:

n = QH/102 η (1)

where:

n = power of primary air fan

q = air volume under working condition

h = total pressure of fan

η= Comprehensive efficiency of fan

assuming that the air volume and the comprehensive efficiency of the fan under working conditions remain unchanged, formula (2) can be derived, which shows that the proportion of the reduction of the total resistance of the system is equal to the percentage of the reduction of the power required by the fan

Δ N/N1= Δ H/H1 (2)

where:

Δ N = the difference between the required power of the fan before and after the transformation ~ 70%

n1 = required power of fan before transformation

Δ H = the difference between the resistance required by the fan before and after the transformation ~ 15 ~ 18 kPa

h1 = total system resistance before transformation

similarly, assuming that the fan power and comprehensive efficiency remain unchanged, formula (3) can be derived, which shows that the change of air volume handled by the fan is inversely proportional to the change of system resistance, so the semi dry method can significantly improve the processing capacity of the system. Generally, if only one article is modified, and other equipment is not modified, the processing air volume of the fan can be increased by 15% - 20%; If the wet electrostatic precipitator is used, the original fan can be considered; It is better to replace the fresh air fan with a total pressure of about 8 kPa, which is more economical

Δ Q/Q1=H1/ Δ H (3)

conclusion

the semi dry dedusting process of converter flue gas, which has obtained the Chinese invention patent, is suitable for new converter projects, especially for the construction of Shandong Hongwang project, which makes up for the blank of North China stainless steel tandem cold rolling production line and the reconstruction of the existing converter wet dedusting system. Its main characteristics are:

1. Short transformation time: the shutdown time required for the actual transformation has reached the minimum of 5 days

2. Most of the current technology and equipment can be used directly, which can greatly reduce the investment compared with the imported technology and equipment

3. it can solve some problems of imported process technology and equipment, such as excessive circulating water flow of saturated spray cooling tower technology

it can be expected that the further popularization, application and continuous improvement of FL lower limit frequency (Hz) process technology will certainly play a certain role in promoting the environmental protection and emission reduction of dust/wastewater, energy and water conservation, improving the comprehensive treatment capacity and the level of circular economy of converter dust removal in China. (end)

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