How are nitrogen oxides formed?

Nitrogen oxides can be produced through three pathways: 1. Lightning storms in nature generate a small amount of them in the air; 2. Artificial sources such as fertilizer factories also produce nitrogen oxides; 3. The largest source is biological nitrogen fixation, carried out by rhizobia in leguminous plants. This seems to be mentioned in high school and junior high school chemistry textbooks. . .

During lightning, N2 is converted into NO. Rhizobia in leguminous crops convert N2 into combined nitrogen. Industrially, ammonia is synthesized from N2 and H2, and then nitrogen oxides are produced as a result of this reaction

Nitrogen oxides in the flue gases generated by combustion equipment [((NOx)), mainly referring to NO, NO2, N2O, N2O3, N2O4, N2O5, etc., with NO being the predominant component, accounting for over 90% of the total NOx, followed by NO2]. In the atmosphere, NO is further oxidized to NO2, causing its concentration to increase rapidly. Although nitrous oxide N2O is present in small amounts, it is one of the factors that contribute to the greenhouse effect and the depletion of the ozone layer ； NOx in flue gas is also one of the causes of acid rain. There are many factors that influence the formation of NOx in flue gas, and the main factors affecting its formation can be summarized into three aspects. On the one hand, it is related to the properties of the fuel; the more nitrogen in the fuel, the easier it is for NOx to be generated ； The more volatile matter in the fuel, the higher the content of nitrogen compounds such as NH, NH2, NH3, CN, HCN, etc., within that volatile matter. These volatiles tend to form NOx when burned ； The more moisture in the fuel, the lower its calorific value; this results in a larger volume of flue gases, which increases the likelihood of NOx formation. On the other hand, it is also related to certain technical parameters during the combustion process – for instance, when the combustion temperature reaches a certain critical level, the concentration of NOx increases exponentially with temperature. Furthermore, the higher the excess air coefficient (oxygen concentration O), the greater the NOx content in the flue gases. Furthermore, the longer the flue gas stays in the high-temperature area, the more NO is generated. The third aspect relates to the type of combustion equipment and burner. For coal-fired boilers in power plants, when liquid slag removal is used (with an excess O2 level of 5%) and a direct-flow burner is employed, the NOx emission levels range from 9 to 1300 mg/m3; when liquid slag removal is used along with a swirl burner, the levels are between 1300 and 2000 mg/m3. For solid slag removal (with an O2 level of 6%) using a direct-flow burner, the emission levels are 500 to 1100 mg/m3, while with a swirl burner they range from 800 to 1700 mg/m3. For oil-fired boilers, the emission levels are 380 to 820 mg/m3 (at an O2 level of 3%). For stratified combustion boilers, the levels are 300 to 800 mg/m3 (at an O2 level of 7%), and for fluidized bed boilers, they are 200 to 700 mg/m3 (at an O2 level of 9%).

Nitrogen oxides are produced through three pathways: 1. Lightning storms in nature generate a small amount of them in the air; 2. Artificial sources such as fertilizer factories also produce nitrogen oxides; 3. The largest source is biological nitrogen fixation (rhizobia in leguminous plants, vehicle exhaust, and plants like trees), which, over millions of years, contributes to the formation of petroleum and coal

There are four ways in which nitrogen oxides are generated: 1. Nitrogen oxides are produced when the nitrogen contained in fossil fuels (coal, oil, natural gas, etc.) is completely burned； 2. Nitrogen and oxygen in the air are ionized by lightning to form free radicals, which then combine with each other to produce nitrogen oxides ； 3. Emissions of nitrogen oxides from factory activities in the industrial production of *ao acid ； 4. Generated by other biomass activities such as the decomposition (or combustion) of animal and plant carcasses.