Practice of desulfurization and denitrification process for sintering flue gas of Taiyuan

1 project background

SO2 is one of the main pollutants in the atmosphere, and its emissions seriously affect the living environment and economic development of human beings. Currently, SO2 emissions in the steel industry is second only to the power industry, the second highest in the country emissions. In the steel industry, the sintering process is the main source of SO2 production in steel production processes.

Sintering flue gas has the following characteristics:

1) The amount of smoke is large;

2) Due to the influence of the raw material structure of the sintering machine, the composition of the sintering flue gas fluctuates greatly and the temperature fluctuates greatly;

3) The concentration of SO2 in the flue gas is relatively low. The concentration of SO2 emitted by the general power plant is about 5000mg/Nm3, while the concentration of SO2 in the flue gas is generally lower than 1000mg/Nm3.

4) The composition of the flue gas is complicated. Because the sintering process uses a variety of raw fuels, the composition of the sintering flue gas is complicated compared with the boiler of the power station. In addition to SO2, the flue gas contains various harmful gaseous pollutants such as NOx and HF, and iron. dust, heavy metals and other solid contaminants;

5) The oxygen content in the flue gas is relatively high. Generally, the oxygen content in the flue gas emitted by the power plant is about 8%, and the oxygen content in the flue gas is about 15%.

It is precisely because of the above characteristics of sintering flue gas that the desulfurization of sintering flue gas cannot fully refer to the flue gas desulfurization technology of power plants, and it is necessary to find a desulfurization process technology suitable for its own development needs.

There are many kinds of flue gas desulfurization methods, which are generally classified into wet method, semi-dry method and dry method. Since the 1970s, the sintering flue gas desulfurization technology has gradually entered industrial application in some developed countries in Japan and Europe. Due to the differences in environmental policies and laws and regulations of various governments, sintering desulfurization technologies with local characteristics have been formed around the world. In Japan, the early limestone - gypsum and magnesium oxide method (wet) based, in recent years, sintered construction dry flue gas desulfurization activated places mainly in Europe mainly in CFB. Since the end of the 20th century, China has paid attention to the problem of SO2 pollution in sintering flue gas. Through years of introduction and absorption and continuous independent research and development, it presents a pattern of blooming flowers. At present, the sintering flue gas desulfurization technologies adopted by various domestic steel enterprises mainly include: limestone-gypsum method, ammonia method, double alkali method, circulating fluidized bed method and the like.

Taigang 450m2 sintering machine was completed and put into use in 2006, with a flue gas volume of 1.4 million Nm3/h, annual emission of SO2 of about 9800t, NOx of 3800t and dust of 1200t. After more than three years of tracking the flue gas desulfurization technology in the same industry at home and abroad, Investigate and compare, Taigang finally believes that the activated carbon desulfurization and denitrification and acid synthesis integrated device is the optimal solution for sintering flue gas desulfurization and denitrification treatment.

2 Taigang Sintering Flue Gas Desulfurization and Denitration Process System Composition

The Taigang Sintering Flue Gas Desulfurization and Denitrification Process System consists of a flue gas system, a desulfurization system, a denitration system, and corresponding electrical and instrument control (including monitoring devices). The process flow is shown in Figure 1.

The flue gas system mainly includes a flue gas system and a booster fan system.

The desulfurization system mainly includes an adsorption system, a desorption system, a delivery system of activated carbon, a recharge of activated carbon, a hot air circulation system, and a cold air circulation system.

The denitration system mainly includes an ammonia system (including liquid ammonia storage, transportation, evaporation, mixed injection, etc.).

2.1 Flue gas system

The total resistance of the flue gas system is considered at 8000Pa.

Booster fan parameters:

1) Flow rate: 3059760m3/h (working conditions);

2) Full pressure: 8000Pa;

3) Power: 8500kW;

4) Fan speed: 745r/min;

5) Rated voltage: 10kV.

2.2 Desulfurization system

The desulfurization system is divided into: adsorption system, desorption system, activated carbon transportation system, activated carbon replenishment system, dedusting system and hot air circulation system, and cold air circulation system.

2.2.1 adsorption system

The adsorption system is the most important system in the whole project. The main equipment consists of absorption tower and NH3 addition system. An inlet and outlet perforated plate is arranged in the absorption tower to make the flue gas flow rate uniform and improve the purification efficiency. Three layers of activated carbon moving layer are arranged in the absorption tower to facilitate efficient desulfurization.

2.2.2 Desorption system

The activated carbon adsorbed by the sulfide is sent to the desorption tower through the conveyor. Here, the activated carbon runs from the top to the bottom, and is first heated through the heating section to be heated above 450 ° C to desorb the material adsorbed by the activated carbon. Sulfur-rich sulfur dioxide (SRG) is discharged to a post-treatment facility to produce sulfuric acid. The desorbed activated carbon is cooled to below 150 ° C in the cooling section, and then sent to the adsorption tower through the conveyor for recycling.

2.2.3 Activated carbon delivery system

Activated carbon recycling is carried out through two chain conveyors to ensure that the activated carbon is recycled between the adsorption column and the desorption column.

The No.2 AC chain conveyor is located in the lower part of the absorption tower and transports the activated carbon adsorbing SO2 in the flue gas to the desorption tower.

The No.1 AC chain conveyor is located in the lower part of the desorption tower, and the desorbed activated carbon is sent to the adsorption tower for reuse.

2.2.4 Activated carbon replenishment system

In the desulfurization process, the activated carbon will be damaged and the particle size will be reduced. In order to ensure the desulfurization efficiency, the small particles of carbon powder need to be discharged, which requires constant addition of new activated carbon. The consumption of activated carbon is 400 kg/h.

In this system, the purchased activated carbon is transported to the activated carbon storage tank through a belt, and the storage tank specification is Φ3.6m×16.5m, which is equivalent to 7 days usage.

2.2.5 hot air system

The hot air system mainly provides hot air for desorbing activated carbon. In this system, coal gas generator by air heated to 450 ℃, to the heating section by the circulation fan.

2.2.6 Cold air system

The deactivated activated carbon is cooled to below 150 ° C in the cooling section.

2.3 Main equipment for desulfurization

2.3.1 Absorption tower

In this project, the absorption tower consists of six identical modules.

Tower size: length: 7m × 6m, width: 9.28m, height: 41.12m.

One of the absorption tower modules is composed of two mutually symmetrical panels, each of which is composed of a plurality of cells of an activated carbon bed. By selecting the appropriate absorber module and the number of cells, it is possible to handle a certain amount of exhaust gas. (The standard capacity of an absorber module to treat exhaust gases is 150000-250000 Nm3/h). Exhaust gas is distributed into each absorber module through an inlet conduit, and the gas is purified as it passes through the left and right activated carbon bed panels.

The activated carbon bed consists of an inlet and outlet grid and a separator. These grids are specially designed to prevent being filled with large particles and carbon powder. The absorption tower consists of three beds divided into a front bed ("FB"), an intermediate bed ("MB") and a back bed ("RB"). Each bed has a roller unloader to control the amount of activated carbon discharged.

The characteristics of the roller unloader are as follows:

1) Controlling the falling speed of activated carbon ensures the highest performance of decontaminants (such as SOx, NOx, dust and others).

2) By controlling the falling speed of the activated carbon, it is possible to prevent the pressure drop of the absorption tower from rising.

2.3.2 Desorption tower

The desorption tower is mainly composed of a heater and a cooler, and the heater and the cooler are multi-tube heat exchangers. In the heater, the activated carbon is heated to above 400 ° C, and the substance adsorbed by the activated carbon is discharged after desorption, and the gas discharged here is called sulfur-rich gas. The deactivated activated carbon is cooled to below 150 ° C in the cooling section. The activated carbon discharged from the desorption tower is sieved by a vibrating sieve, and the sieved material is transported back to the absorption tower by a No. 1 AC chain conveyor.

In order to ensure the balance of the amount of activated carbon falling, a roller unloader is placed in the lower part of the desorption column.

In order to ensure that harmful gases are not leaked, a double-layer rotary discharge valve is installed in the upper and lower parts of the desorption tower.

Characteristics of activated carbon: Activated carbon (AC) itself is a flammable substance. Especially during the first three months of use, since the adsorption of activated carbon is an exothermic reaction, the temperature of the activated carbon will be about 5 ° C higher than the temperature of the flue gas because the new activated carbon is more susceptible to oxidation.

When the flue gas system is operating normally, the heat of oxidation of the activated carbon will be carried away by the flue gas. However, when the flue gas system fails, such as a booster fan failure, the heat cannot be removed at this time, and the temperature of the activated carbon in the absorption tower will continue to increase. When the temperature of the activated carbon exceeds 165 ° C, the shut-off valves of the inlet and outlet need to be closed, and nitrogen gas is sprayed into the interior of the absorption tower to prevent fire. At this time, the activated carbon continues to fall and is sent to the desorption tower, and the desorption tower is filled with nitrogen gas to extinguish the fire. In order to ensure that the activated carbon does not burn, the activated carbon will have to be circulated once from the absorption tower to the desorption column to the absorption column (approximately one week). Therefore, in the first three months, the temperature of the flue gas is controlled to be about 120 °C.

2.4 Denitration system

The denitration system mainly includes an ammonia gas supply system, unloading of liquid ammonia, evaporation, pressure regulation, and mixing with air to supply to the absorption tower.

The ammonia gas supply system includes a liquid ammonia storage tank, an ammonia gas evaporator, a compressor, an ammonia gas dilution tank, an ammonia gas pressure regulating device, an ammonia gas and air mixing device, and an auxiliary pipeline system and a control device. The purchased liquid ammonia is transported to the user area by the tank truck, discharged to the liquid ammonia storage tank by the compressor, vaporized by the evaporator, and then sent to the mixing unit by the pressure regulating device to the user pressure. The mixing unit is provided with a gas quantity and pressure for controlling the valve adjustment, and a spark trap is provided to prevent explosion and tempering, and is mixed with air heated to 130 ° C after being pressurized, and then supplied to the process system for use.

3 Environmental effects and by-products

3.1 Environmental effects

After the project is put into operation, the annual SO2 emission will be reduced from 6821t to 341t, and the annual emission of SO2 6480t will be reduced. The desulfurization efficiency will be 95%. The annual dust discharge will be reduced from 1050t to 210t, and the efflux dust will be reduced by 840t per year. The dust removal efficiency is 80%. The annual NOx emission is reduced from 2774t to 1858t, and the NOx emissions are reduced by 916t per year, and the denitration efficiency is 33%.

3.2 by-products

The concentrated SO2 waste gas in this project is prepared by a waste gas purification system and a sulfuric acid production system to prepare 98% (concentration) concentrated sulfuric acid, and the output is 9500 t/a (calculated by 8400 h per year).

4 Investment

The preliminary design of the flue gas desulfurization and denitrification project of 450m2 sintering machine of Taiyuan Iron and Steel Works, the estimated investment of the project is 33,508,700 yuan, including static investment of 32,320,570 yuan and construction loan interest of 11.88 million yuan.

5 performance test results

The performance test results are shown in Table 1.

Table 1 Performance test results

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Project guaranteed value desulfurization test results

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SO2 ≤41mg/Nm3 (dry) 7.5mg/Nm3 (dry)

Desulfurization rate ≧95% 98%

NOx ≤ 213mg/Nm3 (dry) 101mg/Nm3 (dry)

Denitration rate ≧33% 50%

Dust ≤ 20mg / Nm3 (dry) 17.1mg / Nm3 (dry)

PCDD/F ≤0.2ng/Nm3-TEQ (dry) 0.15ng/Nm3-TEQ (dry)

NH3 escape ≤ 39.5ppm (dry) 0.3ppm (dry)

Acidic sulfuric acid 98% first-class first-class products meet the standard

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6 Conclusion

In the past year, Taigang's sintering flue gas activated carbon desulfurization and denitrification and acid production system have achieved an operating rate of over 95%, a desulfurization rate of over 95%, and a denitrification rate of over 40%. According to the Taiyuan Environmental Monitoring Center, the SO2 concentration of exhaust gas is 7.53mg/Nm3, the NOx concentration is 101.33mg/Nm3, and the dust concentration is 17.13mg/Nm3. The environmental protection index is significantly improved. With an annual output of 9000 tons of concentrated sulfuric acid, it is widely used in the pickling process and coking sulphur ammonia production of Taigang, turning waste into treasure, providing a successful example for the realization of circular economy industrial chain in the field of metallurgical sintering. Sintering flue gas activated carbon desulfurization and denitrification and acid production technology is worthy of promotion and application in the national metallurgical industry.

Plastic Parts

Water-assisted injection molding: Water-assisted injection molding technology is an advanced injection molding process in which part of the melt is injected into the mold cavity and high-pressure water is injected into the melt through the equipment to finally shape the workpiece.

Due to the incompressibility of water, the water front creates a solid interface, squeezing the inner wall of the product into a cavity, and the water front also acts as a rapid cooling. Therefore, the water supplement has many advantages over the gas supplement. Research and application show that the water supplement can produce thinner and more uniform cavity wall, and the inner surface of the flow channel is very smooth. Especially for thick wall workpiece, the cooling time can be greatly reduced compared with gas auxiliary.