Present situation and development trend of flue gas desulfurization and denitration technology

Abstract: at present, denitration devices are widely used in the iron and steel industry, sintering flue gas desulfurization and denitration technology has also made great development and progress, effectively reducing the emission of heavy metal pollutants. Therefore, it is necessary to explore the main sintering flue gas desulfurization and denitrification technology, analyze its main advantages and disadvantages, explore effective sintering flue gas desulfurization and denitrification technology, continuously improve the flue gas treatment and emission reduction of iron and steel metallurgy industry, and create a more healthy ecological environment for people.

introduction
In June 2017, the Ministry of environmental protection issued the emission standard of air pollutants for iron and steel sintering and pelletizing industry, which adjusted the special emission limits of sintering machine and pelletizing roasting equipment to 20mg / m3 for particulate matter, 50mg / m3 for sulfur dioxide and 100mg / m3 for nitrogen oxides. Most iron and steel enterprises will face the problem of excessive emission of sulfur dioxide and nitrogen oxides, And with the extensive application of denitration technology in thermal power plants during the 12th Five Year Plan period, the proportion of nitrogen oxide emissions in the iron and steel industry will increase, and the state will pay more attention to the reduction of nitrogen oxide emissions in sintering flue gas.
1. Characteristics of sintering flue gas
In the production process of iron and steel industry, sintering is a production process in which all kinds of powdery raw materials containing iron are put into sintering equipment for ignition and combustion by mixing suitable fuel and solvent. Through the study of this production process, it can be found that in the sintering process, the combustion of fuel will produce a series of chemical reactions. With the development of production, some liquid phases such as softening and melting appear on the particle surface of the mixture, and the liquid phase will wet other ore particles that have not yet melted, and once cooling occurs, the liquid phase will be used to wet the ore particles, The liquid phase will reconnect the ore powder particles and form a new sinter. The sintering flue gas appears in the sintering process of this kind of sinter. After the mixture is ignited, the sintering flue gas is generated with the trolley running. Generally speaking, the sintering flue gas has the following characteristics: high pollution and high pollution
(1) The amount of flue gas is large. In the current production process of iron and steel industry, because the air leakage rate is in the range of 40 ~ 50%, and the circulation rate of solid material is relatively high, a part of air can not directly participate in the production process through the sintering material layer in the production process, which eventually increases the amount of sintering flue gas. At present, in the process of iron and steel production, 1 t sinter produces about 5000 ~ 5500 m3 of flue gas.
(2) The temperature of flue gas fluctuates greatly. The temperature of sintering flue gas is generally below 180 ℃, but based on the change of production process, the temperature of sintering flue gas can be as low as 100 ℃. Under such temperature, it is difficult to implement SCR denitrification technology without heat exchange or heating measures.
(3) The concentration of flue gas dust is high. Most of the dust in sintering flue gas is iron and its compounds, and there are some trace heavy metals in it due to different combustion materials.
2. Present situation and development trend of sintering flue gas desulfurization and denitration technology
(1) Activated carbon fiber flue gas desulfurization technology
In the removal of sulfur dioxide in flue gas, this technology uses desulfurization activated carbon fiber catalyst, and at the same time, the sulfur resource is recycled for the production of sulfuric acid or sulfate. The desulfurization rate is more than 95%, the process and operation are relatively simple, and the equipment is few. It can also realize the recovery and utilization of sulfur resources. It is mainly used in the flue gas pollution control of power plant boiler, non-ferrous smelting flue gas, steel plant sintering flue gas, large and medium-sized industrial boilers. The process and principle of flue gas denitration technology with activated carbon fiber method are relatively complex, but when using this process to remove SO2 in flue gas, it has great advantages. Activated carbon has the characteristics of adsorption, which can effectively remove NOx, dioxin, mercury and other pollutants in flue gas, and has the function of removing a variety of pollutants. After the catalytic reduction of nitrogen oxides by activated carbon, the denitration rate can reach 60% – 70%, and the removal rate of dioxins can reach 95%. The removal rate of heavy metals by adsorption and oxidation can also reach 90%.
(2) Circulating fluidized bed desulfurization + SCR denitration process
The absorbent of this process is dry hydrated lime powder. Of course, other dry powder or slurry with absorption capacity for sulfur dioxide can also be used.
Usually, the flue gas discharged from the boiler will enter from the bottom of the absorption tower (i.e. fluidized bed) with venturi device. At this time, its speed is accelerated. After mixing with fine absorbent powder, there will be severe friction between particles and between gas and particles. At this time, the water mist should be injected evenly and the flue gas temperature should be reduced, and CaSO3 and CaSO4 will be generated. The desulfurized flue gas will carry a large number of solid particles, which will be discharged through the top of the absorption tower and treated by the recycling dust collector. This process has mature denitration technology, wide application range, high pollutant removal efficiency, the most stringent pollutant emission can use this technology, and the total investment and operation cost of the project is moderate. At present, sintering pelletizing enterprises which have built desulfurization device can continue to build denitration part, which will not have the problem of repeated construction, and its maintenance and operation are convenient. However, this process also produces a lot of by-products, there is no best way of application and resource recovery value, only waste treatment.

(3) Integrated desulfurization and denitrification technology of activated coke
The key to the integrated technology of desulfurization and denitrification of activated coke is the adsorption of heavy metals and other pollutants in sintering flue gas by cylindrical carbonaceous adsorption materials with the size of 5 ~ 9mm. From the effect point of view, activated coke is a kind of raw material with good physical properties, with the advantages of pressure resistance, wear resistance, etc., and compared with activated carbon, the specific surface area of activated coke is smaller, so the effect of desulfurization and denitrification is better. Through the use of activated coke material, it is not easy to reduce the adsorption performance in the process of desulfurization and denitrification, especially in the heating temperature state, its adsorption effect will increase. Therefore, in the process of sintering flue gas desulfurization and denitrification in iron and steel industry, the use of activated coke technology is helpful to further improve the treatment effect.
The integrated technology of desulfurization and denitrification based on activated coke technology is mainly composed of adsorption, desorption and sulfur recovery. In this technology system, when the flue gas enters the moving bed absorption tower rich in activated coke, the temperature of sintering flue gas will be reduced to a certain extent, reaching 110 ~ 150 ℃. However, when the integrated technology is used in some areas, the temperature of sintering flue gas will be reduced, The air preheater can be used to increase the temperature properly), and this level of temperature treatment effect is the best, which can obtain better desulfurization and denitrification effect. After that, the absorption tower in the integrated technology is mainly divided into two parts. After the active coke vertically absorbs the pollutants in the sintering flue gas, it will drop from the top to the bottom under the action of gravity. In this process, the sulfur dioxide in the flue gas is absorbed, and then it will pass through the first section of the absorption tower horizontally; After entering the second stage, the nitrogen oxide in the sintering flue gas is removed by injecting ammonia into the sintering flue gas. In the regeneration stage, the saturated adsorbent will be transferred to the regenerator for heating. When the heating temperature exceeds 450 ℃, the activated coke can desorb the concentrated sulfur dioxide gas. Finally, the regenerated activated coke will be transferred to the reactor through the circulation device. From the results, the integrated technology of desulfurization and denitration of activated coke fully embodies the advanced nature of activated coke, and can simultaneously finish desulfurization and denitration in the same temperature area, reducing the requirements of sintering flue gas treatment on temperature, so it has advanced nature. At the same time, compared with the semi dry desulfurization + SCR denitration technology, the technology also has the advantages of simple operation and management, small floor area, etc., and the desulfurization and denitration effects between the two technologies are similar, which can remove heavy metals and other pollutants in sintering flue gas at the same time, so as to achieve the purpose of environmental protection. At the same time, according to the introduction above, this method can also obtain highly concentrated sulfur dioxide, which can be sold as a by-product, increasing the efficiency of the enterprise to a certain extent.
(4) Wet desulfurization + SCR combined Denitration Process
This technology is mature and has good denitration efficiency, but it is seldom used in sintering flue gas treatment. Because the flue gas temperature after wet desulfurization is generally between 50-80 ℃, while SCR technology can only react at 320-450 ℃. When the reaction temperature is high, the catalyst will be sintered or crystallized, but when the reaction temperature is too low, ammonium sulfate on the surface of the catalyst will condense and block the micropores of the catalyst, which will greatly reduce the activity of the catalyst. If the flue gas denitration process is used before the desulfurization process, the sintering flue gas needs to be heated by the heating device, denitration first, and then the heat exchange device is used for cooling treatment, then the flue gas is purified.
epilogue
The desulfurization and denitrification of sintering flue gas in iron and steel industry has become a hot issue of the whole society. For the relevant staff, in the future work, they should fully understand the characteristics of the above desulfurization and denitrification technologies, and scientifically select the desulfurization and denitrification technologies according to the specific requirements of the enterprise, so as to lay the foundation for the realization of green production.
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