In recent years, Guangdong, Shandong, Zhejiang, Henan, Hebei, Shanxi and other provinces have formulated and tightened the ultra-low emission standards for the cement industry according to their own actual conditions. For example, Shandong issued a document requiring that the hourly average value of nitrogen oxide emission concentration should not be higher than 50mg/m ³ under the condition of 10% reference oxygen content in the flue gas of cement kiln and kiln tail waste heat utilization system. After the vigorous introduction
of ultra-low emission in the cement industry, especially after the introduction of differentiated peak-staggering production, as some provinces and municipalities stipulate that ultra-low emission can be achieved without the implementation of peak-staggering production or the reduction of peak-staggering production days, the control of air pollutant emissions in cement enterprises is also continuing to increase.
Who knows that behind the promotion of ultra-low emission is "black box operation". In order to make the original denitrification device meet the requirements of ultra-low emission control, some cement enterprises adopt excessive ammonia injection, resulting in a large number of ammonia escape, which seems to reduce the emission of nitrogen oxides in cement plants, and the actual environmental problems may be more serious.
Previously, in the process of implementing ultra-low emissions in a certain place in China, there was a strange situation of "one ton of ammonia is hard to find" and even competing with agriculture for ammonia, which to some extent also shows that the practice of reducing nitrogen oxide emissions by excessive ammonia spraying does exist. Ammon
ia overspray has a huge negative impact on the environment. On the one hand, over-spraying of ammonia water will lead to a significant increase in ammonia escape, resulting in serious environmental problems. Data show that ammonia is actually an important factor causing haze, the increase of ammonia escape not only can not alleviate environmental pollution problems, but also may aggravate the already fragile atmospheric environmental quality, causing more serious haze problems; On the other hand, the preparation of ammonia itself is a process with high energy consumption and high pollution. As a reducing agent of nitrogen oxides, ammonia should be used to improve the reaction efficiency as much as possible, so that as much ammonia as possible can participate in the reaction process, thereby reducing the consumption of ammonia. Over-spraying of ammonia will lead to a sharp increase in ammonia consumption and bring greater pollution to the source of ammonia preparation.
Ammonia escape control is an unavoidable problem in the treatment of nitrogen oxides in cement plants. How to achieve ultra-low emissions in the face of this situation? It is understood that at present, the main ways to deal with denitrification in cement enterprises are SNCR (non-selective catalytic reduction), SCR selective catalytic reduction technology, LCR denitrification technology and thermal carbon catalytic reduction composite denitrification technology.
In recent years, SNCR (non-selective catalytic reduction) has been widely used in the cement industry because of its relatively low cost, but its technology also has some drawbacks, that is, the denitrification rate is usually between 40% and 60%, which means that in the process of achieving ultra-low emissions. The SNCR devices of existing cement enterprises may be inadequate.
In this regard, industry experts believe that under the premise of the existing SNCR denitrification device, if the ultra-low emission of nitrogen oxides is to be achieved in the cement production process, the cement production system can be optimized and upgraded, the front-end control efforts can be strengthened, and the generation of nitrogen oxides can be reduced, such as low nitrogen combustion, graded combustion, intelligent control, etc. And then the SNCR technology is used for processing to achieve the ultra-low emission of nitrogen oxides; or the ammonia water injection position of the nozzle is intelligently optimized to achieve the ultra-low emission of nitrogen oxides.
SCR selective catalytic reduction technology is the main denitrification technology in the world at present. Ammonia water or urea is used as denitration agent, and catalytic selective absorption is carried out under the action of catalyst in the absorption tower, and the denitration rate can reach more than 90%. The SCR denitrification project of Henan Hongchang Cement Co., Ltd. has passed the verification of the expert group. It is known that the project can achieve the emission of nitrogen oxides less than 50 mg/m3 , which is the first in China's cement industry. Although the denitrification rate of SCR denitrification technology is as high as more than 90%, due to its high investment cost, it will cause certain economic pressure on the existing cement production line in terms of operation, but the advantage is that the utilization rate of ammonia water is high, ammonia escape is less, and the emission concentration can be reduced to a lower level, laying the foundation for the possible implementation of near-zero emission in the future cement industry.
LCR denitration technology refers to the use of high-efficiency denitration tower for the overall treatment of nitrogen oxides contained in flue gas emissions. In the temperature range of 15-280 C, the high-efficiency denitration tower uses the catalytic reaction of liquid denitration agent to treat nitrogen oxides. LCR denitration integrates absorption, catalytic reaction and other technologies. Nitrogen oxides are converted into H2O and N2 without secondary pollution. The technology has the characteristics of medium and low temperature and high efficiency. The denitrification efficiency is 95%, and the emission of nitrogen oxides can be reduced to less than 10 mg/m ³. At the same time, desulfurization, denitrification and dust removal can be integrated to achieve ultra-low emission of dust, sulfur, nitrate and ammonia.
Of course, there are other denitrification methods in the industry, including thermal carbon catalytic reduction composite denitrification technology , which can also achieve ultra-low emissions of nitrogen oxides.
Generally speaking, the ultra-low emission of cement industry is imperative, but the problem of ammonia escape must be paid enough attention to by the industry. If the reduction of one pollutant leads to the production of another pollutant in large quantities, then such environmental governance is tantamount to robbing Peter to pay Paul, which has limited significance for improving environmental quality and promoting the construction of ecological civilization.
In this context, China Cement Network will be on April 24-25, 2023. The " China Cement Industry Green Development Summit Forum and Ultra-clean Emission Technology Exchange Conference " and the " Fourth China Cement Intelligent Summit Forum " were held in Changsha, Hunan Province, inviting industry experts, cement and related enterprises. Deeply discuss the comprehensive treatment solutions to reduce ammonia escape, and jointly contribute to the high-quality development of the cement industry! After the meeting, we will visit Sinoma Zhuzhou Cement Co., Ltd. and the first set of international "one-box SCR denitrification project of dust and nitrate".
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