Abstract: At present, denitrification devices are widely used in the iron and steel industry, and sintering flue gas desulfurization and denitration technology has also made great development and progress, effectively reducing the discharge of heavy metal pollutants. Therefore, it is necessary to discuss the main sintering flue gas desulfurization and denitrification technologies, analyze their main advantages and disadvantages, discuss effective sintering flue gas desulfurization and denitrification technologies, and continuously improve the flue gas treatment and emission reduction work in the iron and steel metallurgical industry, and create for people A healthier ecological environment.
In June 2017, the Ministry of Environmental Protection issued the “Emission Standards for Air Pollutants from the Iron and Steel Sintering and Pelletizing Industry”, which adjusted the particulate matter limit for the special emission limit of sintering machines and pellet roasting equipment to 20mg/m3, and the sulfur dioxide limit to 50mg/m3 and the nitrogen oxide limit is adjusted to 100mg/m3. The vast majority of iron and steel enterprises will face the problem of excessive sulfur dioxide and nitrogen oxide emissions. And with the widespread application of denitrification technology in thermal power plants during the “Twelfth Five-Year Plan” period, steel The proportion of nitrogen oxide emissions from the industry will increase, and the country will pay more attention to the reduction of nitrogen oxides in sintering flue gas.
1. Characteristics of sintering flue gas
In the production process of the iron and steel industry, sintering is a production process in which various powdery and iron-containing raw materials are mixed with suitable fuels and solvents into the sintering equipment to be ignited and burned. After studying the production process, it can be found that during the sintering process, the combustion of the fuel will produce a series of chemical reactions. With the development of production, some liquid phases such as softening and melting appear on the surface of the particles of the mixture. The phase will wet other ore particles that have not yet melted, and once cooling occurs, the liquid phase will reconnect the ore particles and form a new sinter. The sintering flue gas appears during the calcination of this sinter. After the material is ignited, sintering flue gas is generated with the operation of the trolley. Generally speaking, the pollution of sintering flue gas is large, and it has the following characteristics:
(1) Large amount of smoke. In the current production process of the iron and steel industry, because the air leakage rate is in the range of 40-50%, and the solid material circulation rate is relatively high, this will result in a part of the air that cannot directly participate in the production process through the sintered material layer during the production process. , Finally increase the amount of sintering flue gas. At this stage, in the steel production process, 1t of sinter produces about 5000~5500m3 of flue gas.
(2) The flue gas temperature fluctuates greatly. The temperature of the sintering flue gas is generally below 180°C, but based on changes in the production process, the temperature of the sintering flue gas can be as low as 100°C. Under such temperature conditions, if heat exchange or heating measures are not implemented, it is difficult to implement SCR denitration technology.
(3) The concentration of smoke dust is high. The dust in the sintering flue gas is mostly iron and its compounds, and depending on the combustion raw materials, there will be a certain amount of heavy metals in it.
2. Current status and development trend of sintering flue gas desulfurization and denitrification technology
(1) Activated carbon fiber method for flue gas desulfurization technology
In the removal of sulfur dioxide in the flue gas, this technology uses a desulfurized activated carbon fiber catalyst, while recycling sulfur resources for the production of sulfuric acid or sulfate. Its desulfurization rate exceeds 95%, the process and operation are relatively simple, and the equipment is less, and it can also realize the recovery and utilization of sulfur resources. It is mainly used in power plant boiler flue gas, non-ferrous smelting flue gas, steel plant sintering flue gas, large and medium-sized The flue gas pollution control of industrial boilers. The process and principle of the activated carbon fiber method for flue gas denitration technology are relatively complicated, but when this process is used to remove SO2 from flue gas, it has great advantages. Activated carbon has the characteristics of adsorption, which is very effective for NOX in flue gas. Pollutants such as, dioxin and mercury can be effectively removed, and it has the function of removing multiple pollutants. After the activated carbon is catalytically reduced to remove nitrogen oxides, a denitration rate of 60%-70% can be achieved, and the removal rate of dioxins can reach 95%. It can be used to adsorb and oxidize to remove heavy metals, and the removal rate can also be as high as 90%.
(2) Circulating fluidized bed desulfurization + SCR denitration process
The absorbent of this process is dry slaked lime powder. Of course, other dry powders or slurries that have the ability to absorb sulfur dioxide can also be used.
Usually the flue gas discharged from the boiler will enter from the bottom of the absorption tower (ie fluidized bed) with a Venturi device. At this time, its speed will increase. After mixing with the fine absorbent powder, there will be violent occurrences between particles, gas and particles. Friction, at this time the sprayed water mist should be uniform, and reduce the smoke temperature, it will react to produce CaSO3 and CaSO4. The flue gas after desulfurization will carry a large amount of solid particles, which will be discharged through the top of the absorption tower and processed and discharged by the recirculation dust collector. This process has a relatively mature denitration technology, a wide range of applications, relatively high pollutant removal efficiency, the most stringent pollutant discharge can use this technology, and the total project investment and operating costs are moderate. At present, sintered pellet enterprises that have built desulfurization equipment can continue to construct the denitration part without the problem of repeated construction, and its maintenance and operation are convenient. However, this process also produces many by-products, and there is no optimal application path and resource recovery value, so it can only be used for waste treatment.
(3) Activated coke integrated desulfurization and denitration technology
The key to the integrated activated coke desulfurization and denitrification technology is to use cylindrical carbon adsorbents with a size of 5-9mm in the activated coke to adsorb heavy metals and other pollutants in the sintering flue gas. From the effect point of view, activated coke is a raw material with good physical properties. It has the advantages of pressure resistance and abrasion resistance. Compared with activated carbon, activated coke has a smaller specific surface area, so it has a better effect in desulfurization and denitration. Significantly. Through the use of activated coke materials, it is not easy to reduce the adsorption performance in the desulfurization and denitrification process, especially in the heating temperature state, the adsorption effect will also increase, so in the iron and steel industry sintering flue gas desulfurization and denitrification process, the use of activated coke technology Helps to further improve the treatment effect.
The integrated activated coke desulfurization and denitrification technology based on activated coke technology is mainly composed of adsorption, desorption and sulfur recovery. In this technical system, when the flue gas enters the moving bed absorption tower rich in activated coke, the sintering The temperature of the flue gas will be reduced to a certain extent, reaching 110~150℃ (but when using this integrated technology in some areas, the temperature can be appropriately increased by the air preheater). The temperature treatment effect at this level is the most ideal. A better desulfurization and denitration effect can be obtained. Afterwards, the absorption tower in the integrated technology is mainly divided into two parts. After the activated 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 flue gas After the sulfur dioxide is absorbed, it will horizontally pass through the first stage of the absorption tower; after entering the second stage, ammonia gas is sprayed into the sintering flue gas to remove nitrogen oxides in the sintering flue gas. At the regeneration stage, the saturated adsorbent will be sent to the regenerator for heating. When the heating temperature exceeds 450°C, the activated coke can desorb the concentrated sulfur dioxide gas, and the activated coke after the final regeneration is passed through the circulation device. Transfer to the reactor. From the effect point of view, the integrated technology of activated coke desulfurization and denitrification fully reflects the advanced nature of activated coke, and can simultaneously complete desulfurization and denitrification in the same temperature range, reducing the temperature requirements of sintering flue gas treatment, and therefore it is advanced. At the same time, compared with the semi-dry desulfurization + SCR denitrification technology, this technology also has the advantages of simple operation and management, small footprint, etc., and the desulfurization and denitration effects between the two are not much different, and both can remove the sintering flue gas at the same time. Pollutants such as heavy metals achieve the purpose of protecting the environment. At the same time, according to the above introduction, the method can also obtain highly concentrated sulfur dioxide, which can be sold as a by-product, which increases the efficiency of the enterprise to a certain extent.
(4) Wet desulfurization + SCR combined denitration process
This process technology is mature and the denitration efficiency is relatively good, but it is still less used in sintering flue gas treatment. Because the flue gas temperature after wet desulfurization is generally between 50-80 ℃, and SCR technology can only react at a temperature of 320-450 ℃. A higher reaction temperature will cause sintering or crystallization of the catalyst, but if 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 denitrification process is used before the desulfurization process, the sintering flue gas needs to be heated by a heating device, first denitrify and then use a heat exchange device for cooling treatment, at this time the flue gas is purified.
The desulfurization and denitrification of sintering flue gas in the iron and steel industry has become a hot issue of concern to the whole society. For the relevant staff, they should fully understand the characteristics of the above-mentioned desulfurization and denitrification technologies in future work, and make scientific choices according to the specific requirements of the enterprise. Desulfurization and denitrification technology will ultimately lay the foundation for the realization of green production.
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