1 introduction of nitrogen oxide waste gas
Nitrogen oxides refer to a series of compounds composed of nitrogen and oxygen, which are usually represented by the molecular formula nitrogen oxides. They mainly include N2O, NO, N2O3, NO2, N2O4 and N2O5. Nitrogen oxides in the atmosphere mainly exist in the form of NO and NO2.
Nitrogen oxides exist widely in nature. Any combustion process can make O2 and N2 in the air interact to generate NO, and NO is further oxidized to generate NO2. But in many industries, nitric acid is used for surface treatment and nitration, which will produce a lot of NO2.
2 Harm of nitrogen oxide waste gas
2. 1 Harm to living things
NO in nitrogen oxides (nitrogen oxides) has a strong affinity for hemoglobin in human body. After NO enters the blood, it replaces the position of oxygen in hemoglobin and firmly combines with hemoglobin, thus the ozone layer forms carcinogens, causing diseases such as bronchitis and emphysema, and causing damage to human respiratory system. It can also cause damage or even death to plants or animals.
2.2 the formation of chemical fumes
Catalyzed by sunlight, nitrogen oxides (nitrogen oxides) easily react with hydrocarbons to generate O3, resulting in photochemical smog. It causes serious pollution to the atmosphere, and even causes symptoms such as red eye, cough, sore throat, skin flushing, and severe heart and lung failure.
2.3 ozone layer destruction
N2O in nitrogen oxides (nitrogen oxides) can be converted into NO, destroying the ozone layer. Its production process can be expressed by an equation: NO+O3=NO2+O2, O+NO2=NO+O2. The overall reaction equation is O+O3=O2 (in which NO plays a catalytic role). The above reactions continue to circulate, so that the active O atoms in them are decomposed by photolysis, resulting in the destruction of the ozone layer.
2.4 NO in nitrogen oxides (nitrogen oxides) meets water to generate HNO3 and HNO2, which will form acid rain or acid fog with the rain reaching the ground; It can increase the incidence of chronic pharyngitis and bronchial asthma, reduce children's immune function, and increase the prevalence of eyes and respiratory tract in the elderly. Affected by acid rain, the crop yield is greatly reduced, and the output of soybeans and vegetables and the protein content are reduced.
3 nitrogen oxide waste gas treatment method
3.. 1 gas phase reaction method
3. 1. 1 reduction method
Reduction methods are divided into selective catalytic reduction and selective non-catalytic reduction. Selective catalytic reduction method is to use NH3 and C as reducing agents to selectively reduce NOX to harmless N2 and H3O under certain temperature and catalysis. Because this method has little impact on the atmosphere, it is the most mature and widely used denitration technology at present, with high denitration efficiency.
Selective non-catalytic reduction method refers to the selective reduction of NOX to harmless N2 and H3O by injecting reducing agents such as NH3 and C in a certain temperature range without catalyst. The main difference between them lies in the control of temperature and the presence or absence of catalyst. Because the selective non-catalytic reduction method has strict temperature control, urea is often used instead of NH3 as raw material agent at present, which can reduce NOx by 50% ~ 60%.
3. 1.2 low temperature plasma decomposition method
Low temperature plasma decomposition technology is a method using electron beam method and pulse corona method. High-energy active particles generated by discharge impact NOX molecules, generate free radicals, remove NOX and SO2 at the same time, and break chemical bonds into O2 and N2. Using low-temperature plasma technology is not only easy to realize, but also has a wide treatment range and good effect, saving energy and equipment and not causing secondary pollution. Therefore, the treatment of nitrogen oxides (NOX) has gradually attracted people's attention and has broad development prospects.
3. 1.3 electron beam irradiation method
Electron beam irradiation is a flue gas desulfurization and denitrification technology, which uses high-energy electron beam generated by electron accelerator to irradiate flue gas with a small amount of ammonia or methane gas to convert NOX and SO2 in flue gas into ammonium sulfate and ammonium nitrate. Electron beam irradiation technology is one of the effective methods to remove NOX from industrial flue gas. Its advantage is that it can remove SO2 and NOX, and the by-product (H4NO3) can be recycled without producing waste water, so it has a high removal rate.
3.2 liquid absorption method
There are many ways for liquids to absorb NOX, and they are also widely used, such as water, lye, dilute nitric acid and concentrated sulfuric acid.
Because NOX is extremely difficult to dissolve in water, water is used as absorbent, and the absorption efficiency is low. This method can only be used in places with small gas volume and low purification requirements, and cannot be applied to the treatment of nitrogen oxide waste gas in industrial enterprises. Using dilute nitric acid as absorbent, physical and chemical absorption of NOX can recover NOX, which has certain economic benefits, but it consumes a lot of energy and the utilization rate of industrial enterprises is low. NaOH is the best absorbent, but due to the influence of price, source and operation difficulty, Na2CO3 is used as absorbent instead of NaOH in industry.
Compared with other methods, liquid absorption method has the advantages of simple operation process and equipment, less investment and certain economic benefits, but its purification effect is poor.
3.3 adsorption method
The principle of adsorption method is that the amount of NOX adsorbed by adsorbent changes with the change of temperature or pressure. By changing the temperature or pressure in the reactor, the adsorption and desorption reactions of NOX are controlled, so as to separate NOX from the gas source. Common adsorbents include molecular sieve, activated carbon, natural zeolite, silica gel, peat and so on.
According to the different regeneration methods, adsorption methods can be divided into two categories: variable temperature adsorption method and pressure swing adsorption method. Some of them, such as silica gel, molecular sieve, activated carbon, etc. , which has catalytic performance, can catalyze the oxidation of NO in waste gas to NO2, and then it can be absorbed and recovered by water or alkali, which can promote the removal of NO, but it is not widely used because of its small adsorption capacity, large amount of adsorbent, huge equipment and frequent regeneration.
3.4 Microbial method
Microbial purification of nitrogen oxides is a new type of flue gas denitrification technology studied internationally in recent years, including nitrification and denitrification mechanisms. The biological purification process of waste gas is to remove NOX from waste gas by using the life activities of denitrifying bacteria. Suitable denitrifying bacteria can assimilate nitrogen oxides into organic nitrogen compounds and become a part of bacteria (anabolism) when there is an exogenous carbon source, and can also make denitrifying bacteria grow and reproduce themselves. However, through alienation and denitrification, NOX will eventually be converted into N2.
4 conclusion
China has entered a new era of energy conservation and emission reduction. In order to reduce the air pollution caused by nitrogen oxide exhaust gas from industrial enterprises, the research and development of new flue gas denitrification technology provides many new ways to further control NOX pollution, and various economical and effective high-tech flue gas denitrification methods will emerge continuously. However, at present, it is still necessary to choose the best governance mode according to China's national conditions, taking into account factors such as economic affordability and local resources. The development and perfection of these methods will make great contributions to the treatment of nitrogen oxide waste gas in industrial enterprises.
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