Flue gas denitrification refers to the reduction of NOX produced to N2, thereby removing NOX from flue gas. According to the treatment process, it can be divided into wet denitration and dry denitration. Some researchers at home and abroad have also developed methods for treating NOX waste gas with microorganisms.
Since 90% or more of NOx in the flue gas discharged from the combustion system is NO, and NO is hardly soluble in water, the wet treatment of NOx cannot be performed by a simple washing method. The principle of flue gas denitration is to oxidize NO to NO2 with an oxidant, and the generated NO2 is absorbed by water or an alkaline solution to achieve denitration. The O3 oxidation absorption method oxidizes NO to NO2 with O3 and then absorbs it with water. The HNO3 liquid of the method needs to be concentrated, and O3 requires high voltage production, and the initial investment and operating cost are high. ClO2 redox method ClO2 oxidizes NO to NO2, and then reduces NO2 to N2 with an aqueous solution of Na2SO3. The method can be combined with a wet desulfurization technology using NaOH as a desulfurizing agent, and the desulfurization reaction product Na2SO3 can be used as a reducing agent for NO2. The dechlorination rate of the ClO2 method can reach 95%, and the sulfur can be simultaneously desulfurized, but the price of ClO2 and NaOH is higher, and the running cost is increased.
Wet flue gas denitration technology:
Wet flue gas denitrification is the principle of using liquid absorbent to dissolve NOX to purify coal-fired flue gas. The biggest obstacle is that NO is difficult to dissolve in water, and it is often required to first oxidize NO to NO2. For this reason, NO is generally first reacted with an oxidant O3, ClO2 or KMnO4 to form NO2, and then NO2 is absorbed by water or an alkaline solution to achieve flue gas denitration.
(1) Dilute nitric acid absorption method
Since the solubility of NO and NO2 in nitric acid is much greater than in water (for example, the solubility of NO in 12% nitric acid is 12 times greater than that in water), dilute nitric acid absorption is used to increase NOX removal rate. The technology is widely used. As the concentration of nitric acid increases, the absorption efficiency increases remarkably. However, considering the practical application and cost of the industry, the concentration of nitric acid used in actual operation is generally controlled within the range of 15% to 20%. The efficiency of NOx absorption by dilute nitric acid is related to the absorption temperature and pressure, and the low temperature and high pressure is beneficial to the absorption of NOX.
(2) Alkaline solution absorption method
The method adopts an alkaline solution such as NaOH, KOH, Na2CO3, NH3·H2O as an absorbent to chemically absorb NOX, wherein ammonia (NH3·H2O) has the highest absorption rate. In order to further improve the absorption efficiency of NOX, two-stage absorption of ammonia-alkali solution was developed: first, ammonia was completely gas-phased with NOx and water vapor to form ammonium nitrate white smoke; then unreacted NOX was further absorbed by an alkaline solution. Nitrate and nitrite are formed, and NH4NO3 and NH4NO2 will also be dissolved in the alkaline solution. After the absorption liquid is circulated a plurality of times and the alkali solution is exhausted, the solution containing the nitrate and the nitrite is concentrated and crystallized, and can be used as a fertilizer.