A super-low smoke emission standard for a steel sintering machine
Minister Li Ganjie stated at the National Environmental Protection Working Conference in 2018 that China will start ultra-low emission reform of the steel industry in 2018. At present, only coal-fired boilers in thermal power plants in China have undergone ultra-low emission reforms, which means that coal-fired boilers in thermal power plants adopt a multi-pollutant high-efficiency collaborative collaborative removal and integration system technology, making the concentration of atmospheric pollutants basically meet the emission limits of gas-fired units. The value is that under the condition of a reference oxygen content of 6%, SO2 does not exceed 35 mg/m3, NOx does not exceed 50 mg/m3, and soot does not exceed 10 mg/m3.
Unlike coal-fired boilers in thermal power plants, the steel industry has complex production processes and a large number of pollution sources. What should be included in the ultra-low emissions of the steel industry? The atmospheric pollutant emissions from the iron and steel industry have two most typical characteristics. First, the proportion of smoke emissions from sintering machine heads is large, and particulate matter, SO2, and NOx emissions account for 40% and 70% of the total emissions of steel plants. More than 50%; Second, the issue of unorganized emissions of particulate matter in the steel production process is outstanding. Therefore, to achieve ultra-low emissions, the steel industry must at least simultaneously achieve ultra-low emission of sinter head smoke and ultra-low emissions of unorganized particulates.
Among them, the ultra-low emissions of the steel sintering machine head has received the most attention. In August 2017, the Ministry of Environmental Protection's “Letter to Solicit the Opinions of the 20 National Pollutant Emission Standard Revisions (Draft for Soliciting Opinions)†including the “Steel Iron and Steel Sintering, Pellet Industry Air Pollutant Emission Standards†issued a special note on the sinter flue gas. Emission limit value, that is, under the condition of reference oxygen content of 16%, particulate matter less than 20mg/m3, SO2 less than 50mg/m3, NOx less than 100mg/m3, then whether the special emission limit modification bill can be used as the head of steel sintering machine Gas ultra-low emission standards? We believe that ultra-low emission standards should be more stringent because ultra-low emissions represent the most advanced and most efficient air pollution control technologies available, and special limit modification limits can be achieved with some conventional treatment technologies.
Compared with the ultra-low emission standards of thermal power plants, there is still a large gap between the revision limits of the special exhaust limits for the steel sintering machines. However, in terms of the concentration of the original flue gas pollutants and the difficulty of dust removal, desulfurization, and denitrification treatment, the steel sintering flue gas is not difficult to be used in thermal power coal-fired power plant flue gas. Therefore, it is entirely possible to refer to the thermal power coal-fired power plants to formulate ultra-low emissions standards for steel sintering flue gas, that is, particulate matter less than 10 mg/m3, SO2 less than 35 mg/m3, and NOx less than 50 mg/m3 at a reference oxygen content of 16%.
Analysis on Hot Spots of Ultra-low Emissions of Flue Gas from No.2 Steel Sintering Machine
Since 2017, many new viewpoints have emerged in the field of iron and steel sintering flue gas treatment, such as "wet method to change the dry method", "whitening", "fully promoting the use of activated carbon" and so on, which have aroused heated debate in the industry. In order to solve the current difficulties in the treatment of steel sintering flue gas, it is good for everyone to think collectively, but some viewpoints are not thorough enough to analyze the difficulties in the treatment of steel sintering flue gas, and even mislead the local air pollution control work. Therefore, it is necessary to conduct in-depth analysis of relevant hot issues.
(a) The refractory disease in the treatment of sintering flue gas is not wet desulfurization, but instead, tofu dregs construction
In recent years, some experts have targeted the treatment of steel sintering flue gas to wet desulfurization. It is believed that the wet smoke discharged after sintering wet desulfurization contains a large amount of particulate matter and causes haze, as long as the wet process is changed to (half) ) Dry process, haze problems can be solved. However, this view only saw the appearance of the problem. A considerable number of sintering wet desulfurization towers have tailing phenomenon. This is not due to the adoption of the wet desulfurization process, but the construction of dust collectors and wet desulfurization towers to cut corners. Low quality and low price not only did not play the role of dust removal and desulfurization, but also caused secondary pollution. If high standards and strict requirements cannot be achieved in the process of engineering design, construction, and material selection, the smoke emitted by the (semi-)dry process is also difficult to reach, and even the desulfurization system cannot operate stably. However, because the visual perception is not obvious, this exceeds the standard. Emissions are more subtle.
The efficiency and reliability of the wet desulfurization process have been fully demonstrated in flue gas treatment of thermal coal-fired boilers. Japan's wet desulfurization accounts for 98%, the United States accounts for 92%, Germany accounts for 90%, and the world average accounts for 85%. Therefore, we do not have to doubt the effect of wet desulfurization on the prevention and control of atmospheric pollution. Instead, we should make great efforts to carry out special rectification of steel sintering flue gas desulphurization facilities, and follow the requirements of ultra-low emissions of thermal power coal-fired boilers to benchmark the sintering flue gas desulphurization facilities. Low-level construction of desulphurization facilities for rectification.
(B) Sintering machine head flue gas denitrification process route selection
The flue gas dust removal and desulfurization processes of the sintering machine head are all very mature and form a complete set of technical routes. However, there are fewer examples of denitrification applications. At present, the flue gas denitrification process of the sintering head has the following types: oxidation denitrification, low-temperature SCR denitrification, high-temperature SCR denitrification, and activated carbon denitrification. According to the characteristics of the sintering flue gas, it is suggested that the high-temperature SCR denitrification and activated carbon denitrification are feasible technologies for the denitration of the sintering flue gas.
The principle of denitrification by oxidation method is to use ozone, hydrogen peroxide and other oxidants to oxidize NO in flue gas to NO2 and other substances, and then enter the desulfurization tower to absorb with a basic absorbent. This method can be achieved from the principle of denitrification, but there are two key issues. First, there is a conflict between the denitration process and the nitrogen oxide monitoring method. At present, the monitoring method of nitrogen oxides is to monitor the concentration of NO in flue gas and convert it into NOx. Therefore, even though the NO in the flue gas is oxidized to NO2, although the monitoring result shows that it is up to standard, NO2 may not be absorbed but directly discharged into the atmosphere and there is a regulatory loophole. Second, the absorption process of the denitration process is carried out at the same time as the desulfurization process, resulting in an increase in the amount of the alkaline absorbent and the amount of by-product generated, and the byproduct is a mixture containing nitrite and nitrate, which is easily soluble in water. Comprehensive utilization is extremely difficult.
Low temperature SCR denitrification, the reaction temperature range below 200 °C, compared with the high temperature SCR denitrification, is closer to the steel sintering flue gas temperature. However, at present, there are still four key problems that need to be solved in the application of low-temperature SCR denitrification to sinter flue gas. One is that the low-temperature SCR denitration catalyst is poor in anti-toxicity and susceptible to sulfur oxides, water, heavy metals and other substances in the flue gas, so the low-temperature SCR denitrification device can only be arranged in the rear of the dust removal and desulfurization tower; the second is the sintering smoke. The gas temperature, especially the flue gas temperature after desulfurization, cannot reach the temperature range of the low-temperature SCR denitrification reaction, and the flue gas reheating still needs to be performed. Thirdly, compared with the medium-high temperature SCR denitration catalyst, the cost and operation of the low-temperature SCR denitration catalyst The cost is higher; Fourth, the low-temperature SCR catalyst has no effect on the removal of di-vanadate in the sintering flue gas.
High-temperature SCR denitrification is a denitrification process that is very mature in flue gas denitrification of thermal coal-fired boilers and can be completely transplanted to the sintering flue gas. The key is after the flue gas heating system and SCR denitrification device in front of the SCR denitration device. The design of the flue gas heat exchange system reuses the heat recovered by the flue gas heat exchanger for heating the flue gas at the front end and reduces the energy consumption. When the medium-high temperature SCR denitration device is started, it is necessary to heat the smoke around 150°C to 280°C or more, which consumes a large heat source; during normal operation, the heat is recovered by the heat exchanger and reused, and only extra supplement is needed. 30~50°C can warm up. In addition, in the high-temperature SCR denitrification also need to pay attention to control the reaction temperature range below 300 °C, to avoid the synthesis of two VA Ying after decomposition.
Activated carbon denitrification is currently recognized as a viable technology for the application of sinter flue gas denitrification. However, a two-stage design should be used in the design of the system. After the front-end desulfurization reaction is completed, ammonia is used for denitrification to increase the denitration efficiency. It is also necessary to add an efficient bag filter after the activated carbon device to ensure that the nitrogen oxides can be achieved. Ultra low emissions.
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