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How can the cement industry achieve ultra-low emission measures

Abstract: In order to achieve ultra-low emissions in the cement industry, in accordance with the cement clinker production process and exhaust gas limits, combined with the advantages of the SNCR denitrification system and bag filter, the continuous development and progress of new materials and new technologies have been used in a targeted manner. Three methods are used for ultra-low emission transformation, and the technical transformation of denitrification treatment and dust collection treatment is briefly discussed.

Introduction
In March 2020, Hebei Province issued the local standard of "Ultra Low Emission Standard of Air Pollutants for Cement Industry", which controls the emission of air pollutants from cement kiln and waste heat utilization system of kiln tail. The emission limits of particulate matter, sulfur dioxide, and nitrogen oxide are respectively 10mg/m3, 30mg/m3, 100mg/m3; In April 2020, Henan Province issued the "Implementation Plan for Ultra-low Emission Reconstruction of Henan Cement Industry (Draft for Comment)". The draft opinion requires that by the end of 2020, the province's cement enterprises complete the transformation of organized and unorganized ultra-low emission. Cement kiln and kiln waste heat utilization system exhaust gas baseline oxygen content of 10%, the hourly average emission concentration of particulate matter, sulfur dioxide, nitrogen oxide, fluoride, mercury and their compounds shall not be higher than 10, 35, 100, 3, 0.05 respectively Mg/m3; encourage enterprises to adopt new technologies and strive to control the hourly average concentration of particulate matter, sulfur dioxide, and nitrogen oxide emissions below 10, 20, and 50 mg/m3. The escaped ammonia concentration of ammonia denitrification is not higher than 8 mg/m3. It can be seen that the subsequent environmental protection trends in the Beijing-Tianjin-Hebei region and the surrounding areas will move towards higher standards, and in accordance with the national environmental protection requirements for differentiated management and control of pollution prevention and control companies, actively carry out new technological exploration and reserves, and implement more stringent For cement companies, this standard is a stricter screening and a better opportunity.

The content of organized flue gas management in the cement industry

In the cement industry, rotary kiln tail gas is the most important and most critical source of exhaust gas emissions. Take most cement companies in BBMG Jidong as an example. The rotary kiln has an alkaline environment and the raw material is limestone. The calcium oxide obtained from the decomposition reacts with the sulfur dioxide emitted from coal combustion in a large amount. Under normal operating conditions of the rotary kiln, the sulfur dioxide emission concentration at the kiln tail is very low. Therefore, the kiln tail flue gas does not require end desulfurization measures, so the flue gas of the cement clinker production line Governance mainly focuses on two important links: denitrification and dust removal.

At present, commonly used denitration technologies for flue gas include low-nitrogen combustion technology, selective non-catalytic reduction technology (SNCR), selective catalytic reduction technology (SCR), and thermal carbon catalytic reduction composite denitration technology; dust removal technology is bag filter, electric Dust collector and electric bag compound dust collector. Now we discuss the use of low-nitrogen combustion technology, selective non-catalytic reduction technology (SNCR) and bag filter for ultra-low emission of nitrogen oxides and dust.

(1) Low-nitrogen combustion technology

Low-NOx combustion technology is treated from the source, mainly including low-nitrogen burner and decomposition furnace staged combustion technology. The low-nitrogen burner increases the burner air duct and reduces the primary air ratio, so that the pulverized coal is burnt in stages. The staged combustion technology of the calciner uses the staged combustion air or the staged addition of fuel to form an oxygen-lean atmosphere or a fuel-rich area to produce a certain amount of reducing agents such as CO, CH4, H2, HCN, and fixed carbon. These reducing agents react with the NOx generated in the kiln to reduce the NOx to non-polluting inert gas such as N2. In addition, the combustion of pulverized coal under oxygen-deficient conditions also inhibits the production of its own fuel-type NOx, which greatly reduces the NOx content out of the calciner, thereby achieving the goal of reducing NOx in the cement production process. The reaction formula is as follows:
 2CO+2NO=N2+2CO2
 NH+NH=N2+H2
 2H2+2NO=N2+2H2O

Therefore, when using the staged combustion technology to reduce nitrogen oxides, in order to control the atmosphere in the kiln, firstly, according to the type of fuel used, combined with the sulfur and alkali content in the raw meal, etc. The installation of a good and easy-to-adjust burner is conducive to the generation of CO from the pulverized coal in the oxygen-poor zone. At the same time, because the wind speed in the cone area is larger, it is conducive to the dispersion of the pulverized coal and avoids the generation of local high temperature zones; secondly, it can be appropriately used The air pipe is moved upward to maintain the micro-oxidizing atmosphere in the kiln and extend the time for the reducing agent to reduce NOx. , So that nitrogen oxides can be minimized.


(2) Selective non-catalytic reduction technology (SNCR)

The selective non-catalytic reduction method (SNCR) is treated from the end, in the absence of a catalyst, the reducing agent such as ammonia or urea is sprayed into the high temperature zone of the furnace by mechanical atomization and carrying air to reduce nitrogen oxides to N2 and H2O Methods. The reducing agent is sprayed into the proper temperature range (generally between 850℃-1050℃) at the end of the kiln (or the middle and lower part of the decomposition furnace), and the denitration efficiency is 30%-70%. The reaction formula is:
4NH3+4NO+O2→4N2+6H20
6NO2+8NH3→7N2+12H2O

Therefore, when using SNCR denitrification technology to reduce nitrogen oxides, choose the injection position, generally choose the C5 inlet position, and inject the reducing agent into the most effective temperature window area in the furnace as much as possible to ensure the maximum reaction between the reducing agent and NOX , Improve the utilization rate of reducing agent, reduce the amount of ammonia water and ammonia escape. To achieve ultra-low emission standards, special consideration must be given to the fine distribution of the reducing agent in the corresponding temperature window area. Therefore, it is very important to choose a spray gun with good atomization effect. For example, a dual-fluid spray gun has good atomization effect and anti-clogging function. Secondly, during the debugging and optimization period, the atomization performance and flow rate of each spray gun must be reasonably adjusted, the spray coverage area must be expanded, and a scientific and reasonable NH3/NOx molar ratio must be set for the characteristics of the cement clinker production line, so that the ammonia escape does not exceed the standard. , In order to achieve the most complete reaction and the best denitration effect.


(3) Bag dust collector

At present, most of the cement industry uses air box pulse bag dust collectors, but in comparison, spray pulse bag dust collectors are more reasonable in space utilization and layout than air box pulse bag dust collectors, and the air inlet and outlet methods are more flexible. The emerging high-performance filter bag and dust collector structure technology can completely achieve the dust emission concentration ≤ 10mg/m3, meeting the ultra-low emission standard. Therefore, to achieve ultra-low emission of organized dust, it is more effective to replace or upgrade the original air box pulse bag dust collector to a jet pulse bag dust collector.

The content of unorganized dust control in the cement industry

The links in the cement industry that may produce unorganized dust emissions include ore mining, raw and auxiliary materials and finished products storage, raw and auxiliary materials and finished products transportation, and technological processes. Therefore, the management of fugitive dust needs to comprehensively strengthen the control of fugitive emissions from blasting mining, material storage, transportation and production processes, promote the clean transportation of bulk materials, and strengthen the comprehensive management of fugitive dust. Taking the measures taken by BBMG Jidong Cement Company in the treatment of unorganized dust as an example, ultra-low emission of unorganized dust in the cement industry can be achieved.

Ore blasting mining: rational use of blasting dust suppression vehicles during blasting, mining and use of drilling rigs with built-in dust collection facilities, such as Atlas drilling rigs, the mining surface is covered with fog cannon and dust suppression vehicles in all directions, so as to achieve superfluous dust suppression in the blast mining process. Low emission.

Storage of raw and auxiliary materials and finished products: all storage in a closed storage warehouse or a fully enclosed material shed. Dust suppression measures such as spraying during material loading and unloading. The material shed is equipped with dust collection facilities and practical infrared sensor stacking doors, and the material outlet is installed with spray dust suppression. Equipment and vehicle washing devices, all sheds are equipped with micro-monitoring equipment to carry out real-time fugitive emission monitoring and video monitoring. These measures can ensure strict control of fugitive dust emissions in the raw and auxiliary material storage links, and realize the storage of raw and auxiliary materials Ultra-low emission of unorganized dust.

Transportation of raw materials and finished products: all materials are transferred and transported by fully enclosed belts, enclosed corridors, tubular belt conveyors and enclosed carriages, vacuum tankers, pneumatic conveying and other transportation methods, which can maximize the unorganized dust during the transportation process Limited reuse and control. Taking into account various factors such as capital investment and geographical location in the actual situation, most companies cannot realize transportation methods such as pipeline transportation and pneumatic transportation, and still use vehicle transportation for the transfer of raw and auxiliary materials. Therefore, the production and transportation roads need to be fully hardened and exposed to earth and stone. All are greened and equipped with high-efficiency sweepers, vacuum cleaners, and sprinklers. Wheel washing devices are installed on the roads when entering and leaving the factory. The material transportation vehicles adopt new energy vehicles or national VI emission standard vehicles. The transportation vehicles are fully enclosed. Achieve ultra-low emission of unorganized dust in the transportation of raw materials and finished products.

Process: The feeding device before the mill is airtight, the discharge port of the mill tail and the dust outlet of the dust collector are equipped with air-locking devices, and the rotary kiln system maintains a slight negative pressure, and there is no air leakage or material leakage. Organize ultra-low emissions of dust.

In conclusion
Under the new policy of ecological civilization construction of "green water and green mountains are golden mountains and silver mountains", from the perspective of the government's strengthening of environmental protection and people's pursuit of a better life, the ultra-low emission limit of the cement industry in various places is gradually lowering, and it has become a new It is the basis for the revision of environmental protection standards. At the same time, local government departments strengthen the control of ultra-low pollutant emissions during the autumn and winter heating seasons, guide industrial enterprises to carry out standardized construction, encourage enterprises to upgrade and upgrade, and evaluate differentiated management levels in accordance with regulations. Gradually drive local enterprises to improve their environmental awareness and level, and the resulting environmental and economic benefits go hand in hand. The ultra-low emission transformation of the cement industry is imperative, so only by learning advanced experience, striving to become a green garden enterprise, and fully winning the battle against the blue sky, is the real way for cement enterprises.

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