What are the details of the ultra-low emission of sintering machine flue gas?

1. Research status of sintering machine flue gas desulfurization technology

The semi-dry and wet desulfurization technologies of sintering machine tail gas are currently relatively mature, and both can stably achieve sulfur dioxide emissions to ultra-low emission standards. The activated coke adsorption method is still in the stage of industrial application and promotion due to factors such as investment and operating costs. Semi-dry and wet desulfurization technologies have the same investment amount (wet includes wet electricity), and wet desulfurization has high operating costs. The difference between the two lies in the secondary products and their disposal.

1.1 Wet Desulfurization Technology

There are three secondary products in wet desulfurization technology: wastewater, gypsum and white smoke. Among them, gypsum can be used as a resource, which is no longer an environmental protection issue. The country has related policies that require zero discharge of desulfurization wastewater. The composition of desulfurization wastewater is complex, and its discharge is evaluated based on the concentration of chloride ions. Although conventional water treatment technologies can solve the problems, they have the problems of high cost, large area and high operating costs. For iron and steel enterprises, the desulfurization wastewater is mainly used as slag flushing water, which has certain hazards. In addition, since the sintering machine can consume most of the desulfurization wastewater in the first mix, the specific dosage needs to monitor the impact of chlorine, sodium and other elements on the quality of the sinter. The sulfate radicals in the wastewater will be decomposed into sulfur dioxide gas again during the sintering production process, but this part of the amount is very small compared to the total sulfur dioxide in the flue gas, and the impact on the operating pressure and cost of the desulfurization system can be ignored. There are many discussions on white smoke. For users equipped with high-efficiency wet electrostatic precipitators, most aerosol pollutants can be removed, and the white smoke is less harmful. For the use of cyclones and other mechanical defogging or wet electricity efficiency is low, the white smoke is relatively more harmful. At present, most sintering machines in Tangshan area use cooling, heating or a combination of two methods to eliminate white smoke, but the acidic condensate needs to be properly treated, and the overall return on investment is low in terms of environmental costs.

1.2 Semi-dry desulfurization technology

After 20 years of development and optimization, semi-dry desulfurization can meet the ultra-low emission requirements of sintering machine flue gas when the initial concentration of sulfur dioxide (recommended <1500mg/m3) is not high, and is suitable for most sintering machines. Semi-dry desulfurization only has a secondary emission product of desulfurization ash. Because its main components are calcium hydroxide, calcium sulfite, and calcium carbonate, resource utilization is difficult and not easy to dispose of. When equipped with a suitable ultra-clean bag filter, semi-dry desulfurization generally does not have the problem of excessive particulate emissions and white smoke. Another advantage of semi-dry desulfurization is that it can efficiently remove aerosol pollutants, such as typical sulfur trioxide, hydrogen chloride, etc. This has obvious advantages over wet desulfurization. The emission of sulfur trioxide is an important pollutant that cannot be ignored for the sintering machine after many actual measurements, and it makes a great contribution to regional air pollution.

1.3 Activated coke adsorption method

Activated coke desulfurization has high desulfurization efficiency, and the key technology and equipment of the process have also been made domestically. At present, the investment cost of the entire system is still relatively high and the activated coke industry is also affected by environmental protection policies, and the high operating cost has led to slow promotion. The product of the activated coke adsorption method is dilute sulfuric acid, which has relatively low added value and requires recycling in the plant. There is another technology that directly produces sulfur by carbothermic reduction. Although the product has high added value, there is no mature engineering application performance. At the same time, the active coke system, once there is a major error in design and operation control, the system still has safety risks.

From the above comparison, it can be seen that semi-dry desulfurization is currently a relatively advantageous desulfurization method, and it is recommended to use it when the desulfurization ash treatment method is well implemented.

2. Research progress of sintering machine tail gas denitration technology

Due to the very strict emission standards for nitrogen oxides, the current sintering machine head flue gas denitration is a difficult point in the implementation of ultra-low emissions, and it is difficult for companies to choose a technical route. At present, the selective catalytic oxidation method (SCR), oxidation method and activated coke method are widely used. The three methods currently have their own advantages and disadvantages, and it is difficult to solve the problem of denitrification and the secondary pollution caused by it.

2.1 Sintering machine tail gas reheating + SCR denitration technology

As the mainstream technology of denitration in coal-fired power plants, SCR has great advantages because its denitrification product is nitrogen and pollution-free, but there are the following limitations in the process of applying it to the flue gas treatment of sintering machines: ①The problem of temperature range. Even if a low-temperature catalyst is used, it is necessary to supplement the heat of the sintering flue gas and be equipped with GGH. Although this problem can be solved at present, the investment and land occupation have increased significantly; ②SCR placement problem. The disadvantage of SCR placed before desulfurization is that there are catalyst clogging, poisoning, SO2 being oxidized to SO3, etc., which exacerbates the clogging problem of the catalyst; the advantage is that the presence of sulfur dioxide can fix ammonia, which alleviates the problem of ammonia escape to a certain extent. , The higher flue gas temperature before desulfurization also saves the amount of heat source and reduces the volume of GGH. The SCR placed after desulfurization can effectively avoid catalyst poisoning and blockage, but because the flue gas is directly discharged into the atmosphere, the ammonia escape problem cannot be solved, and the flue gas temperature after desulfurization is low, and the GGH temperature difference is large, which leads to the volume, cost, and operating cost of the reheating equipment. Substantial Increase. ③Ammonia escape problem. SCR applications still require fine design and optimization to solve the problems of denitrification efficiency and ammonia escape under the very stable working conditions of the power plant. The sintering production process determines the instability of its flue gas working conditions. The initial concentration of nitrogen oxides fluctuates widely, fast and unpredictably. Under the condition that the denitrification efficiency is so high, the amount of ammonia can be controlled in real time. It is difficult to avoid this problem. Under the fluctuating operating conditions of nitrogen oxides, the amount of ammonia escaped is even much higher than that of nitrogen oxides, and the environmental cost is high. If the NOx standard is relaxed to 80~100mg/m3, the ammonia escape problem can be greatly alleviated or even solved. ④The catalyst life issue. The nominal catalyst is generally 2 to 3 years, and the waste catalyst is hazardous waste, and the treatment cost is relatively high. Under the current ultra-low emission standards, the use of reheat + SCR denitrification technology requires careful consideration.

2.2 Oxidative denitration technology

Oxidation is not widely used as a newly developed denitration technology, and many technical schools have emerged. At present, the technical level of oxidation denitrification is uneven. The most common problem is yellow smoke. Yellow smoke is the color of low-concentration NO2. When the concentration of NO2 is not fully absorbed, a clear yellowish color can be observed when the concentration reaches 30mg/m3, and it is more obvious when the air humidity is high and the pressure is low. At present, many places have forced CEMS to install NO2 converters or directly install NO2 detection modules to facilitate supervision of fake denitrification problems. Perfect oxidation denitrification, especially in the case of wet desulfurization, should adopt peroxidation method to oxidize NO to N2O5, which can solve the problem of low NO2 absorption efficiency of conventional desulfurization system and actual discharge exceeding standard. At present, to achieve the goal of peroxidation, most of the oxidants of choice are ozone. When the initial concentration of nitrogen oxides is low (<150mg/m3), the route of preliminary oxidation to NO2 can be used for the oxidation denitration with semi-dry desulfurization. If the absorption efficiency is low and cannot meet the requirements, a dedicated denitrification agent can be used. Because peroxide has obvious operating cost advantages, under these conditions, cheaper oxidants (such as chlorine dioxide, sodium chlorite, etc.) can be considered to reduce construction and operating costs. ②Disposal of nitrate and nitrite produced by absorption after oxidation. The product has a positive effect on the application of desulfurized gypsum in the building materials industry, and improves the material’s thermal insulation ability. It needs to be studied to determine whether there is secondary pollution when used in other occasions; the nitrate and nitrite in the desulfurization wastewater will be improperly disposed of For polluting water bodies, sintering machine desulfurization wastewater using this type of denitrification technology should not be used for slag flushing, and can be added to the first mix for disposal and utilization. From the analysis of the thermogravimetric mechanism in the laboratory and the actual engineering application, most of the decomposition products of nitrate and nitrite carried in the wastewater are nitrogen during the production process of the sintering machine, but the specific amount of wastewater needs to be controlled. ③ is a supplementary explanation for atmospheric ozone pollution. The ozone in the low-altitude environment of the atmosphere mainly comes from photochemical reactions and has nothing to do with denitrification by oxidation. The biggest escape risk of ozone lies in the ozone generator workshop rather than in the flue gas. The ozone that does not participate in the reaction will quickly decompose into oxygen in the high humidity and reducing atmosphere in the desulfurization tower, and theoretically no secondary pollution will occur, unless the desulfurization tower is designed There is serious flue gas drift due to irregularities, so the problem of large fluctuations in flue gas parameters of the sintering machine can be effectively solved under the condition that excessive ozone dosing does not cause secondary pollution. After actual measurement of several peroxide demonstration projects, the ozone level at the chimney position was not detectable at the ppm level. Ozone has been used in my country's water treatment industry for many years, and the equipment is domestically made, and there are also perfect preventive measures for workshop escape. Ozone oxidation denitrification has strict requirements for flow field design, especially when combined with wet desulfurization, it is necessary to avoid secondary pollution caused by the oxidation of sulfur dioxide caused by excessive local ozone concentration. When combined with semi-dry desulfurization, the requirements are relatively loose. Generally speaking, the oxidation method is more suitable for situations where the initial concentration is not high (<200mg/m3).

2.3 Integrated technology of activated coke desulfurization and denitrification

A practical problem that restricts the large-scale application of the activated coke method is that the construction and operation costs are relatively high. From the perspective of engineering application, it is generally difficult to stably meet the standard for single tower operation, and the situation of double towers in series is relatively stable, and the cost and operation are relatively stable. The cost is also higher.

From the above situation, the activated coke method is more suitable for companies with strong capital and rich construction land. The oxidation method is suitable for situations where the initial concentration is not high, and is affected by policies, which may affect the rating of steel companies (this point makes the author very puzzled, and the evaluation is based on the technical route rather than the actual effect). Although the SCR method has wider applicability and high policy support, the problem of ammonia escape will lead to greater risks in the future. If ammonia escape is strictly regulated and nitrogen oxide emission standards are relaxed, SCR is still the best technical route. Under the current circumstances, the choice of denitration technology route is still a difficult problem to solve.

3. Research progress of sintering machine exhaust dust removal technology

At present, sintering machine head flue gas dust removal and desulfurization are similar, and both have stable and reliable technology and equipment. When equipped with semi-dry desulfurization, the ultra-low bag filter is generally used; when equipped with wet desulfurization, the wet electrostatic demister is generally used. The technical route of wet desulfurization with mechanical demisters is not recommended. On the one hand, mechanical demisters have poor aerosol removal ability. Although the cost and operating cost are low, their contribution to environmental pollution will increase. In addition, it is important to note that if the integrated activated coke desulfurization and denitrification technology is not equipped with tail dust removal equipment, it is difficult to achieve stable and ultra-low particulate emissions. If a wet electrostatic defogger is used, the dry electrostatic precipitator can be discharged before the main extraction. The concentration requirements are relatively loose (<50mg/m3).

4. Other pollutant control methods and research progress

Comparing the situation of foreign steel companies and the nature of the flue gas from the sintering machine head, the next policy may continue to tighten the emissions of dioxins, sulfur trioxide, VOCs, carbon monoxide and carbon dioxide. Some technical schools of existing pollutant control systems can produce a strong collaborative control ability for certain pollutants, which can be used as a reference.

4.1 Dioxins

As the most toxic substance known, theoretically, it should be controlled first. However, due to the complexity and high cost of its detection equipment, the current supervision is difficult, so it is difficult to supervise except for some special industries. The activated coke method has a high adsorption efficiency for dioxin, and the problem of secondary pollution in the desorption process can be solved to meet future dioxin emission standards. The SCR catalyst also has the ability to adsorb dioxin, but its degradation ability is poor. After the adsorption is saturated, the emission of dioxin will increase, and there is a great risk in meeting the dioxin emission standard in the future. Oxidation denitrification also has a high dioxin degradation ability and no secondary pollution, which can meet future dioxin emission standards. The removal of VOCs is similar.

4.2 Sulfur trioxide

SO3 is a regional pollutant and it is difficult to diffuse remotely in the atmosphere. It has a particularly large impact on the ambient air quality in cities where heavy industries are concentrated. It is not included in the standard due to the difficulty of detection. With the improvement of detection methods and equipment, it is expected to be included in the scope of supervision in the future . At present, the conventional equipment with the ability to remove sulfur trioxide is activated coke adsorption, and the other is semi-dry desulfurization. The wet electrostatic defogger also has a higher damage and removal effect on sulfur trioxide aerosol, but it is slightly less efficient than the previous two technologies. Sulfur trioxide also has a greater impact on the blockage of the SCR catalyst. However, sulfur trioxide can be efficiently removed by spraying slaked lime powder in the front flue of the dust collector, and the future risk is small.

4.3 Carbon monoxide

The CO concentration emitted by the sintering machine is quite high, and it may be regulated in the standard in the future. The emission of carbon monoxide causes energy waste on the one hand, and environmental pollution on the other. Catalytic oxidation is generally used in the chemical industry with similar situations, and the steel industry can learn from it. Especially in the case of SCR denitrification, the addition of carbon monoxide catalyst at the front end of the denitrification catalyst can effectively remove carbon monoxide and use the calorific value of its oxidation reaction to increase the flue gas temperature (about 10°C) to save or even eliminate the use of heating flue gas For gas, many of this technology are currently in the pilot stage. In addition, there is carbon monoxide denitrification technology, but it needs to use a huge rotary reactor, which is tried and applied in biomass boiler plants, but it has not been seen in the application of sintering machine.

4.4 Carbon dioxide

The steel industry’s carbon dioxide emissions are second only to the power industry. Under the requirements of 30-year carbon peak and 60-year carbon neutrality, the emission pattern is very severe. For steel companies, before conducting carbon verification, it is recommended to conduct carbon trace analysis to facilitate the discovery and implementation of breakthrough points and feasible routes for carbon emission reduction. The advantages of iron and steel enterprises are energy-saving transformation and carbon fixation with steel slag. Energy saving means emission reduction, which can be used as a carbon emission reduction indicator. At present, iron and steel enterprises still have a lot of room for waste heat utilization; steel slag can be considered in wind quenching or air quenching due to its alkaline characteristics. Coupled with carbon fixation during water quenching. The quicklime used in large quantities by iron and steel enterprises can also be used in a closed recycling process to greatly reduce the calcination of limestone.

5. in conclusion

The above analysis conclusions stem from the use of standardized design and construction data in various technical schools that the author has mastered. Generally speaking, the choice of the ultra-low emission route of the sintering machine head flue gas is mainly the problem of denitration. Although standard design and construction of various desulfurization and dust removal equipment have their own advantages and disadvantages, they all meet the needs of ultra-low emissions and can be selected according to local conditions. Only denitrification technology requires comprehensive consideration and careful selection. The best routes under different conditions and demands are different, and the technical capabilities and construction levels of construction manufacturers are also quite high.