Smoke elimination and nitrogen removal
It can be divided into dry method and wet method. Dry methods mainly include catalytic reduction method and adsorption method. Catalytic reduction method: suitable for treating nitrogen oxides discharged from various pollution sources. Can be divided into non-selective reduction method and selective reduction method. Non-selective reduction method uses reducing gases such as carbon monoxide, hydrogen and methane as reducing agents and oxides of metals such as platinum, palladium or cobalt, nickel, copper, chromium and manganese as catalysts to reduce nitrogen oxides to nitrogen at 400 ~ 800℃. At the same time, some reducing agents react with excess oxygen in flue gas to generate water and carbon dioxide, and give off a lot of heat. This method has high efficiency, but consumes a lot of reducing agent. In the selective reduction method, platinum or oxides of copper, iron, cobalt and vanadium are used as catalysts and ammonia (NH3) or hydrogen sulfide (H2S) as reducing agents, and the reaction temperature is 200 ~ 450℃ (120 ~ 150℃ when NH3 is used as reducing agent). The reducing agent consumption of this method is only 1/5 ~ 1/4 of that of the non-selective reduction method. China uses molybdenum, copper chromium and iron chromium as catalysts, and the temperature range is 100 ~ 120℃. Adsorption method: using molecular sieve and other adsorbents to adsorb nitrogen oxides in nitric acid tail gas. Molecular sieves such as hydrogen mercerized fluorite, 13X, silica gel, peat and activated carbon are good adsorbents for nitrogen oxides. In the presence of oxygen, molecular sieves can not only adsorb nitrogen oxides, but also oxidize NO to NO2. Introducing hot air (or a mixture of hot air and steam) for desorption can recover nitric acid (HNO3) or NO2. After adsorption treatment of nitric acid tail gas, nitrogen oxides can be controlled below 50ppm. Adsorption method can also be used to treat other low concentration nitrogen oxide waste gas. Wet method includes direct absorption method, oxidation absorption method, redox absorption method, liquid phase absorption reduction method and complex absorption method. Direct absorption method: water absorption method, nitric acid absorption method, alkali solution (alkaline liquid such as sodium hydroxide, sodium carbonate and ammonia water) absorption method, concentrated sulfuric acid absorption method and other methods. Bleached dilute nitric acid (1.5 ~ 30%) is used as absorption liquid in China. Under the conditions of gauge pressure of 2kg/cm2, absorption temperature of 20℃, gas-liquid ratio of 290: 1 and empty tower velocity of 0.52m/s, the content of nitrogen oxides in tail gas can be reduced below the national emission standard. NO can be directly absorbed by low pressure bleaching dilute nitric acid. For example, the solubility coefficient (β) of NO in 12% bleaching nitric acid is 4.2. When NO is directly absorbed by water, the β value is only 0.04 1. When the molar ratio of NO:NO2 is 1, the absorption rate is accelerated. In order to oxidize a part of NO to NO2 and keep the molar ratio at 1, measures such as pressurization, cooling, catalytic oxidation and increasing the capacity of absorption tower are generally taken. Direct absorption of NO with bleached dilute nitric acid can not only reduce pollution, but also increase nitric acid output. Bleached dilute nitric acid after absorbing nitrogen oxides can be blown off (bleached) by gas. The blown nitrogen oxides are sent to the absorption tower for recovery. This method can recover 80 ~ 90% nitrogen oxides from tail gas. Alkaline solution absorption method is to obtain nitrate and nitrite with 30% NaOH solution or corresponding concentration of ammonia water. Ammonium nitrate and ammonium nitrite absorbed by ammonia water can be used as farmland fertilizers. Absorption with concentrated sulfuric acid can not only remove nitrogen oxides, but also remove SO2 from flue gas, which is still in the laboratory research stage.
Oxidation absorption method
Under the action of oxidant and catalyst, NO is oxidized into NO2 and N2O3 (nitrogen trioxide) with high solubility, and then nitrogen is absorbed by water or alkali solution, which is widely used in wet flue gas denitrification process. The oxidant can be ozone (O3), chlorine dioxide (ClOx), sodium chlorite (NaClO2), sodium hypochlorite (NaClO), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), chlorine gas (Cl2) and nitric acid (HNO3). According to different oxidation methods, it can be divided into catalytic oxidation absorption method, gas phase oxidation absorption method and liquid phase oxidation absorption method. Catalytic oxidation absorption method is to oxidize NO into NO2 under the action of catalyst, and then absorb it with alkali solution. The oxidant is excess oxygen in the flue gas. The catalyst is vanadium, tungsten, titanium and rare earth metal oxides with activated carbon, alumina and silica as carriers. This method has been used to purify the flue gas of glass furnace, and the denitrification rate is over 90%. The concentration of nitrogen oxides in the purified flue gas is less than 60ppm. The advantage of this method is that closed cycle can be used; Can simultaneously remove pollutants such as SO2 and dust particles; No additional oxidant is needed; Injecting 5% steam into flue gas can improve the efficiency and life of catalyst; The equipment investment and operation cost are low. The gas phase oxidation absorption method uses O3 and ClO2 as strong oxidants to oxidize NO into NO2 and N2O3 which are easily absorbed by water, acid and alkali liquor. Dilute nitric acid can be recovered by absorption with water. This method has been used to purify boiler flue gas with liquefied natural gas as fuel, and the denitrification rate is over 90%. This method has simple purification process and reliable operation, does not affect the normal operation of the boiler, and can recover high-grade nitric acid. However, due to the large amount of O3, it takes a long time for NO to be oxidized into N2O3, and the oxidation tower is correspondingly huge. Liquid-phase oxidation absorption method is to oxidize NO with liquid-phase oxidant and then absorb it with alkali absorption method. When potassium permanganate and sodium hypochlorite are used as liquid oxidants, the denitrification rate can reach 90 ~ 95%. Redox absorption method: NO is oxidized by strong oxidants such as O3 and ClO2 in gas phase to easily absorbed nitrogen oxides and N2O3, then absorbed by dilute HNO3 or nitrate solution, and then reduced to N2 by reducing agents such as sodium sulfite (Na2SO3), sodium sulfide (Na2S), sodium thiosulfate (Na2SO3) and urea (NH2)2CO in liquid phase. This method has been applied to the purification of flue gas in heating furnace. SOx and nitrogen oxides in flue gas can be simultaneously removed in the same tower, and the desulfurization rate is 99% and the denitrification rate is over 90%.