Experimental Study On The Removal Of SO₃ From Coal-Fired Flue Gas By Alkaline Sorbent Injection

Under the guidance of the principle of "green development",the emission of atmospheric pollutants from coal-fired power plants is strictly controlled,and coal-fired power plants successively upgrade the selective catalytic reduction denitration system(SCR),resulting in the efficient removal of NOx.However,the SO3 concentration is increased.SO3 not only pollutes the atmosphere,but also poses serious hazards to the operation of the power plant,such as the formation of ammonium bisulfite to poison the SCR catalyst and block the air preheater.For high-sulfur coal power plants,the problems caused by SO3 are more serious.The control of coal-fired flue gas SO3 has.gradually attracted attention and become a research hotspot.At present,coal-fired power plants mainly use low-low-temperature electrostatic precipitators,wet electrostatic precipitators,and wet desulfurization towers and other pollutant removal equipment to cooperatively control SO3.Although these methods can reduce SO3 emissions to some extent,they cannot reduce the production of ammonium bisulfite.Therefore,this paper proposes a method to remove SO3 before the SCR system by alkaline sorbent injection,and conducts laboratory experiments and pilot experiments.First,the absorption performance of alkaline sorbents with SO3 was studied.The SO3 absorption performance of three representative alkaline sorbent:Ca(OH)2 in calcium-based sorbents,Mg(OH)2 in magnesium-based sorbents,Na2CO3 in sodium-based sorbents were compared.The absorption performance of SO3 in the range of 300-400? was found to be:Na2CO3>Ca(OH)2?Mg(OH)2.Absorption performance of Na2CO3 was superior to Ca(OH)2 and Mg(OH)2 for two main reasons.On the one hand,the activation energy required for Na2CO3 to react with SO3 was smaller,so at the same temperature,the gas-solid interface reaction rate was faster.On the other hand,CO2 was continuously released during the reaction of Na2CO3 with SO3,and the product layer was a porous structure.The growth of the product layer caused less resistance to gas diffusion and help the diffusion of SO3 to the unreacted absorbent surface.However,the product of Ca(OH)2 and Mg(OH)2 reacting with SO3 was denser and hinder the further diffusion of SO3.SO2 absorption performance of alkaline sorbents was also studied.Compared with Ca(OH)2 and Mg(OH)2,the absorption performance of Na2CO3 on SO3 was significantly greater than that of SO2,which can reduce the consumption of absorbent by SO2.Second,the effect of temperature,SO3 concentration,particle size,CO2 concentration,and water vapor on the absorption performance of SO3 was studied for the preferred alkaline sorbent sodium carbonate.It was found that within the temperature range of 150-300?,the SO3 absorption of sodium carbonate increased with the increase of temperature.Above 300?,the absorption was insensitive to the temperature change.At this time,the absorption was limited by the diffusion rate of SO3.Increasing the SO3 concentration significantly increased the SO3 absorption and increaseds the SO3 concentration to more than 1200 ppm,and the absorption can continue to increase.When the particle size of sodium carbonate was reduced,the absorption of SO3 increased.When the average particle size was less than 50 ?m,the absorption no longer increased significantly while the particle size continued to decrease.The presence of CO2 in the flue gas reduced the SO3 absorption,but the effect was small.It was due to the release of CO2 after Na2CO3 absorbed SO3,and the CO2 in the flue gas would cause some resistance to the diffusion of CO2 in the product into the main gas flow.The addition of water vapor to the flue gas increased the absorption of SO3 when the temperature was lower than 300?.It may be that the presence of water vapor promoted the conversion of SO3 to H2SO4 and the reaction mechanism was changed.Above 300?,the effect of water vapor absorption on SO3 was not significant.Through the above studies,it was found that,above 300?,the absorption efficiency of SO3 by Na2CO3 was mainly limited by external diffusion,which was determined by the high reactivity of Na2CO3 with SO3 and the lowconcentration of SO3 in the flue gas.An externaldiffusion control model for SO3 absorption rate was established and the mass transfer coefficient of SO3 was obtained.The key methods for enhancing the removal of SO3,such as increasing the number concentration of the absorbent in the flue gas and increasing the dispersion uniformity of the absorbent,were proposed.Finally,a pilot experimental platform for SO3 removal by injection of Na2CO3 solution was established.The operation parameters that influence the removal efficiency of SO3 such as sodium carbonate particle size distribution,Na2CO3:SO3 molar ratio,residence time,flue gas temperature,and SO3 concentration were studied.The pilot test results showed that the smoke temperature above 300?,the sodium carbonate solution was rapidly evaporated after spraying,the particle size distribution was concentrated at about 2 ?m,the particles below 10 ?m were above 99%,greatly reduced the diffusion resistance of SO3 to the surface of the particle,and increased the removal efficiency of SO3 and the utilization of absorbent.SO3 removal efficiency increased with increasing Na2CO3:SO3 molar ratio,residence time and SO3 concentration and was insensitive to temperature changes.When the molar ratio of Na2CO3 to SO3 was 4:1 and the residence time was 3.66 s,the removal efficiency of SO3 was 96.4%.The SO3 efficiency model was established by using the main operating parameters of alkaline sorbent injection as variables,which has a good degree of fitting to the experimental results and can be used to guide engineering applications.