Study On Ammonium Bisulfate Formation Mechanism In SCR Flue Gas

Nitrogen oxides(NOx)are main atmospheric pollutants,threaten the environment and the human health in different degree.Selective catalytic reduction(SCR)is the most effective technology to reduce NOx in flue gas.With the decrease of the flue gas temperature,the unreacted reducing agent ammonia will easily react with sulfur trioxide(SO3)in flue gas,to produce ammonium bisulfate,a substance with strong stickiness and corrosiveness.Ammonium bisulfate condenses on the surface of air preheater,and captures the ash in flue gas,then leads to the deposition and corrosion.Nowadays,coal-fired units ultra-low pollutants emission technology is popularizing,and adding a layer of catalyze is the main measure to control NOx,with the aging of catalyze,the ammonia slip concentration will increase,and produce more ammonium bisulfate,leads to the deterioration of deposition.This thesis study the ammnium bisulfate formation mechanism to provide support of airpreheater designation and operation.In the perspective of chemical thermodynamics,the change of Gibbs free energy in NH3 and SO3 reaction was calculated and the direction of the reaction was determined.The results show that SO3 reacts quickly with H2O to form H2SO4,more SO3 in flue gas reacts in the form of H2SO4,SO3 and NH3 tend to form ammonium sulfate,rather than ammonium bisulfate.With experimental study of ammonium bisulfate deposition temperature,we find that NH3 and SO3 have the same effect on the deposition temperature.The results shows that there is a positive linear correlation between the deposition temperature and the logarithm of the reactant concentration product,with the concentration product Φ[NH3]·Φ[SO3]ranging from 20 to 1000 μL2·L-2,the deposition temperature changes in the range of 225-250 ℃.In the same concentration product,when the molar ratio of NH3/SO3 diverge 1:1,the deposition temperature of ammonium bisulfate slightly increases.As the flue gas temperature decreases,ammonium bisulfate and ammonium sulfate production is increasing.The increase of SO3 and NH3 concentration will both lead to the increase of ammonium bisulfate.The increase of NH3 concentration leads to the increase of ammonium sulfate production,while the increase of SO3 concentration leads to the decrease of ammonium sulfate production.Further analyze shows that the formation of ammonium bisulfate is controlled by the reaction equilibrium.Irrespective of the state of ammonium bisulfate,equilibrium constant Ki is well consistent.With the decrease of temperature,Ig Ki increases linearly in the range of 120～200 ℃ and 200-260 ℃.According to the hypothesis of complete condensation of ammonium bisulfate,the equilibrium constant K11 is consistent,and 1g K11 increases linearly with the temperature decreasing from 120 to 220 ℃.Both K1 and K11 have a strong practical value.According to the analysis,ammonium bisulfate reacts with NH3 to produce ammonium sulfate.