Research On Selective Removal Of Sulfur Trioxide

Coal is China's most important primary energy source.The combustion of coal produces a variety of atmospheric pollutants,of which the production of SO3 is small,but the damage is huge.SO3 is easily combined with water vapor to form H2SO4,which increases the acid dew point of the flue gas.This can cause low temperature corrosion of the air preheater and the back of the equipment contact surface,and affect the safe operation of the unit.SO3 reacts with ammonia in the selective catalytic reduction(SCR)denitration system to form a more viscous NH4HSO4.This can result in a decrease in catalyst activity and block the air preheater.In a power plant that adsorbs mercury by activated carbon,SO3 in the flue gas will compete with mercury for adsorption on the surface of activated carbon,which will seriously reduce the efficiency of mercury removal.Therefore,it is important to develop an efficient and economical SO3 control technology.Alkaline absorbent spray technology is widely used for removing SO3 from coal-fired flue gas.The principle of this technique is to spray the alkaline absorbent into the flue in the form of a slurry or a solid powder,and to effectively remove SO3 by an acid-base reaction.Absorbents which are commonly used include alkali metals such as sodium and potassium,and alkaline earth metal absorbents such as calcium and magnesium.However,since coal-fired flue gas contains both SO2 and SO3,the currently widely used alkaline absorbent reacts with SO3 and also reacts with SO2 in the flue gas which is much larger than the SO3 content.This will cause the absorbent to be quickly depleted.In practical applications,the selectivity of the absorbent for SO3 directly affects the daily consumption of the absorbent.Therefore,it has been important to develop an absorbent which can selectively remove SO3 efficiently.In order to solve this problem,the removal efficiency and selectivity of SO3 from seven absorbents including NaHSO3,Na2SO3,NasCO3,Ca(OH)2,CaCO3,Mg(OH)2 and CaO,were investigated and NaHSO3 was screened out as the optimal one.The effect of flue gas conditions on the removal efficiency and selectivity of SO3 was studied with NaHSO3.The results showed that with the increase of temperature,space velocity,and SO3 concentration,the cumulative removal amount of SO3 was increased at the same time.The NaHSO3 before and after the reaction was characterized by electron microscopy(SEM)to study the change of the absorbent NaHSO3 before and after SO3 removal.At the same time,in order to further improve the selective removal ability of the absorbent NaHSO3 to SO3,an additive was tried to modify it.The addition of other absorbents as an additive by a wet preparation method is carried out to improve the selective removal effect on SO3.Under the same working conditions,the removal rale and selectivity of SO3 in flue gas were compared with different modified absorbents.The results indicated that NaHCO3 was the best modified absorbent for selective removal of SO3.Under the mass ratio of NaHSO3 to NaHCO3 of 10:1,the removal efficiency of SO3 was about 91%and the selectivity was 100%after continuous reaction for 1 h at flue gas temperature of 300? and space velocity of 60000h-1.Furthermore,the experimental results were fitted and analyzed by intraparticle diffusion model,Elovich kinetic model and the pseudo-second order kinetic model.It was found that intraparticle diffusion was not the only factor controlling the removal process,and chemisorption might be the main way for NaHSO3 and NaHCO3 modified NaHSO3 absorbent to remove SO3.