Study On The Removal Of SO₃ By Sodium-based Absorbent Alkali Injection

With the increasingly stringent emission regulations of pollutants in China,selective catalytic reduction(SCR)technology is widely used in coal-fired power plants to control the emission of NOx.While removing NOx,SO3 concentration in flue gas increases.SO3 will cause blockage of air preheater and deactivation of catalyst,affect the safe and stable operation of power plant,and also damage the environment and human health.SO3 control technology of coal-fired power plants has gradually became a research hotspot.The alkali injection at middle temperature can achieve the efficient removal of SO3.It is widely used in high sulfur coal-fired power plants abroad.The core of this technology is the atomization and evaporation of alkali solution and its reaction characteristics with SO3.But,there is no systematic study.In order to obtain the key technology of SO3 removal,a small-scale hot cocurrent bed test rig is built to study the characteristics of SO3 removal by sodium-based sorbent alkali injection.Quantitative stabilization of SO3 is the precondition for the study of SO3 control technology.In this paper,SO2 catalytic oxidation is used to realize the quantitative preparation of SO3.The conversion of SO3 is affected by the reaction temperature,SO2 concentration and space velocity.Temperature is the key factor affecting the catalytic effect.The conversion rate increases significantly at 485?-505?.Further increase of temperature will inhibit the reaction and affect the activity of the catalyst.The concentration of SO2 has little effect on the conversion,and the space velocity decreases.The residence time of SO2 in the catalyst layer becomes longer,which promotes the oxidation reaction.The particle size,uniformity and spray range of alkali droplets directly affect the removal efficiency of SO3.Therefore,this paper uses the two-fluid nozzle used in the experiment to explore the performance of alkali atomization.The results show that the spray angle is 19.80°-35.49° when the amount of atomizing gas is 3-6L/min,the ratio of gas to liquid increases and the spray angle decreases gradually with the increase of the amount of atomizing gas,and the phenomenon of spray wall can be avoided under the action of flue gas beam.With the constant pressure,the control of the average particle size(SMD)of the atomized droplets can be realized by changing the gas-liquid ratio.The distribution of SMD with different gas-liquid ratios ranged from 70 to 320 microns at 3 L/min,12.35 to 15.42 microns at 5 L/min,and 11.26 to 13.67 microns at 6 L/min.The effects of absorbent types,process parameters and operating conditions on SO3 removal characteristics are studied on a hot cocurrent bed test rig.The experimental results show that the droplet size decreases and the uniformity of droplet size is better with the increase of atomized gas volume,and the contact area between absorbent and SO3 gas increases,resulting in a significant increase in absorption efficiency.With the increase of SO3 concentration,the removal rate of SO3 increased obviously,the molar ratio was 1:1,the concentration of SO3 increased from 50mg/m3 to 150mg/m3,and the removal rate increased from 31%to 46%.With the increase of molar ratio,the removal efficiency of SO3 increased gradually,but the growth rate showed a downward trend.The removal rate increased from 67%to 73%with molar ratio of 3:1 to 4:1.When the reaction time is less than four seconds,the removal rate increases rapidly with the extension of the reaction time;when the reaction time is greater than four seconds,the removal rate increases slowly,so the effective residence time should be longer than four seconds.When the temperature is lower than 250?,the main controlling factor of the reaction is the chemical reaction rate.With the increase of temperature,the chemical reaction rate increases,and the removal rate increases rapidly,reaching the highest value at 250?,while with the increase of temperature,the change of the removal rate is not obvious.Sodium carbonate,sodium bicarbonate and sodium hydroxide also affect SO3 removal.Under the same working conditions and operating conditions,the removal efficiency of SO3 is in the order of sodium hydroxide,sodium bicarbonate and sodium carbonate.Among the three absorbents,sodium hydroxide has the stronger alkalinity and the higher reaction rate with SO3.Sodium bicarbonate decomposes at high temperature,releases C02 and HO2,forms porous sodium carbonate,increases the specific surface area of the absorbent and improves the absorption rate.There is a competitive reaction between S02 and SO3 in flue gas,which affects the absorption of SO3.Among the three absorbents,sodium hydroxide has the most obvious effect,while sodium bicarbonate and sodium carbonate have less effect.The experiment of alkali evaporation and drying followed by reaction shows that the gas-solid reaction is the dominant one in the process of SO3 removal by alkali injection,and the gas-liquid reaction can be neglected.