In order to prevent excessive NOx pollution of the environment after coal combustion in the boiler, coal should be denitrified. At present, there are many denitrification processes for flue gas denitrification circulating pumps, and there are probably the following types: flue gas denitrification, scr denitrification technology, and dilute nitric acid absorption Method, alkaline solution absorption method, etc., now we will introduce them one by one.
1. Flue gas denitration
Flue gas denitrification refers to reducing the generated NOX to N₂, thereby removing NOX in the flue gas. According to the treatment process, it can be divided into wet denitrification and dry denitrification. Some scientific researchers at home and abroad have also developed methods to treat NOX exhaust gas with microorganisms.
Since more than 90% of the NOx in the flue gas discharged from the combustion system is NO, and NO is hardly soluble in water, a simple washing method cannot be used for the wet treatment of NOx.
The principle of flue gas denitrification is to oxidize NO to NO₂ with an oxidant, and then absorb the generated NO₂ with water or alkaline solution to achieve denitrification. O₃ oxidation absorption method uses O₃ to oxidize NO to NO₂, and then absorb it with water. The HNO₃ liquid produced by this method needs to be concentrated, and O3 needs to be produced with high voltage, and the initial investment and operating costs are high.
ClO₂ redox method ClO₂ oxidizes NO to NO₂, and then uses Na₂SO₃ aqueous solution to reduce NO₂ to N₂. This method can be used in combination with wet desulfurization technology that uses NaOH as a desulfurizer, and the desulfurization reaction product Na₂SO₃ can also be used as a NO₂ reducing agent. The denitration rate of the ClO₂ method can reach 95%, and it can desulfurize at the same time, but the price of ClO₂ and NaOH is higher, and the operating cost increases.
2. Selective catalytic reduction denitrification scr denitrification technology
Selective catalytic reduction SCR denitration is to use ammonia, CO or hydrocarbons as reducing agents in the presence of a catalyst to reduce NO in the flue gas to N₂ in the presence of oxygen. NH₃, CO, H₂, methane, ethylene, propane, propylene, etc. can be used as reducing agents for SCR reaction. When ammonia is used as the reducing gas, the NO removal efficiency that can be obtained is the highest. The SCR reaction is a redox reaction, so it follows the redox mechanism or Mars-van Krevelen-type mechanism. At present, foreign scholars have reached an agreement that the reactant of the SCR reaction is NO instead of NO₂, and O₂ has participated in the reaction.
3. Dilute nitric acid absorption method
Since the solubility of NO and NO₂ in nitric acid is much greater than that in water (for example, the solubility of NO in nitric acid with a concentration of 12% is 12 times greater than that in water), the dilute nitric acid absorption method is used to improve the NOX removal rate The technology is widely used.
As the concentration of nitric acid increases, its absorption efficiency is significantly improved. However, considering factors such as practical industrial application and cost, the concentration of nitric acid used in actual operations is generally controlled within the range of 15% to 20%. The efficiency of dilute nitric acid to absorb NOX is not only related to its own concentration, but also related to absorption temperature and pressure. Low temperature and high pressure are beneficial to the absorption of NOX.
4. Alkaline solution absorption method
This method uses alkaline solutions such as NaOH, KOH, Na₂CO₃, NH₃·H₂O as absorbents to chemically absorb NOX, and ammonia (NH₃·H₂O) has the highest absorption rate. In order to further improve the absorption efficiency of NOX, a two-stage ammonia-alkali solution absorption has been developed: firstly, ammonia reacts completely with NOX and water vapor in the gas phase to generate ammonium nitrate white smoke.
An alkaline solution is then used to further absorb the unreacted NOX. Nitrate and nitrite are formed, and NH₄NO₃ and NH₄NO₂ will also be dissolved in the alkaline solution. After the absorption liquid is circulated many times, after the lye is exhausted, the solution containing nitrate and nitrite is concentrated and crystallized, which can be used as fertilizer.
