Integrated Oxidative Removal Of CO,NO_x And SO₂ From Flue Gas By Micro-Nano Bubbles

Air pollution has become one of the environmental problems of common concern all over the world.Global warming,frequent haze and photochemical smog,and rampant acid rain are taking a serious damage on the Earth’s environment and human health.Carbon monoxide,as an indirect greenhouse gas,has an emission concentration limit of 80mg/m3（24h average）in the "Pollution Control Standard for Domestic Waste Incineration",while many industrial exhausts with CO up to tens of thousands of milligrams per cubic meter are emitted directly into the atmosphere,which clearly runs counter to the initiative to combat global warming.In terms of desulfurization and denitrification,most conventional purification processes are tandem,with large floor space and high infrastructure costs,making it difficult to cope with the increasingly stringent atmospheric emission standards.Therefore,it is extremely urgent to study a fast and efficient,low energy consumption,integrated flue gas multi-pollutant removal method.Micro-nano bubbles（MNBs）have physicochemical properties that conventional bubbles do not possess,such as long residence time,large specific surface area,high mass transfer efficiency,high ζ-potential,self-pressurization dissolution,and strong oxidizing properties,which have attracted much attention in environmental and other fields.In this paper,we investigated the oxidative removal performance of CO,NO_x and SO₂ in flue gas by micro-nano bubbles,and examined the effects of different parameters on the removal of pollutants in direct and indirect removal modes,respectively.The oxidation capacity of the system was enhanced by using micro-nano bubbles coupled with Fenton’s reagent to find the optimal process parameters,and four different aeration modes were compared.In addition,this paper also focuses on the analysis of the oxidation products of CO,NO_x and SO₂ pollutants,while the mechanism of pollutant removal by micro-nano bubbles was investigated based on hydroxyl radical quenching experiments and trapping experiments,and the results are as follows.（1）The direct catalytic oxidation of CO in flue gas using an air micro-nano bubble system,the effects of initial temperature,number of cycles,initial p H,SDS concentration,Fe²⁺ concentration,Mn²⁺ concentration and NH⁴⁺ concentration on the removal of CO from the system were investigated by single-factor experiments,and it was found that the highest removal rate of CO was achieved at a volume fraction of 0.15%,the optimal reaction temperature was 25°C.The acidic conditions promoted the oxidative removal of CO and the alkaline conditions inhibited the removal of CO.Certain concentrations of SDS,NH⁴⁺,Fe²⁺ and Mn²⁺ all played a positive role in the oxidation of CO,among which Fe²⁺ and Mn²⁺ played a catalytic role in the system,and the catalytic effect Fe²⁺ was better than Mn²⁺.（2）Indirect catalytic oxidation of CO,NO_x and SO₂ in flue gas using air micro-nano bubble system,it was found that with the increase of CO concentration,the removal of CO was the first to increase and then decrease,and the removal of NO_x and SO₂ remained basically stable.The highest removal rate of CO was achieved when the volume fraction of CO was 0.10% by keeping the NO_x concentration at 0.85 g/m3（NO: NO₂ ≈ 1: 3.5）and SO₂ volume fraction at 0.085% constant.The oxidation removal rate of CO is strongly influenced by the initial temperature,initial p H,SDS concentration,and Fe²⁺ concentration of the solution,while NO_x and SO₂,which are easily soluble in water,are less influenced by the above factors.Under the acidic conditions,the oxidation removal rate of CO in the system of Fenton reagent coupled with micro-nano bubbles was greatly increased.The removal efficiency of CO enhanced with the increase of H₂O₂ concentration and then decreased,and improved with the increase of Fe²⁺ concentration and then remained stable.Under the optimal conditions,the removal rates of CO,NO_x and SO₂ reached 63.26%,98.67% and 99.76%,respectively,when the absorption solution was tap water.The micro-nano bubble coupled Fenton reagent method is expected to achieve ultra-low emission of low concentration CO flue gas.（3）The removal mechanism of pollutants was investigated,and the same concentrations of CO,NO_x and SO₂ were treated by common aeration,micro-nano bubble method,common aeration with Fenton system and micro-nano bubble coupled Fenton reagent method,and the experimental results were compared,it showed that: micro-nano bubble coupled Fenton reagent > micro-nano bubble ≈ common aeration of Fenton system > common aeration.Under the optimal conditions,the micronano bubble coupled with Fenton’s reagent method can remove 1.5 times more CO than the micronano bubble method and 4.8 times more than the normal aeration method when treating multiple pollutants in flue gas.In addition,the free radical trapping and bursting experiments demonstrated that ·OH played a key role in the oxidation process of CO,NO_x and SO₂.The reaction products analysis proved that the micro-nano bubbles oxidized CO to CO₂,NO_x to NO₂-and NO^3-,and SO₂ to SO₃^2- and SO₄^2-.