Study On The SO₃ Formation Characteristics On Different Denitration Catalysts

Selective catalytic reduction technology is currently the most widely used denitration technology for industrial fixed-source NOx treatment,and catalyst is the core of this technology.When burning high-alkali coal,V2O5-WO3/TiO2 catalyst is arranged in high dust area and is easily deactivated by sodium salts in the flue gas;the most common low temperature catalyst is Mn/ZSM-5 catalyst.Increasing the proportion of NO2 in the flue gas will cause a fast SCR reaction,which accelerates the process of catalytic reduction reaction.However,the presence of sodium salts and NO2 have a significant effect on the SO3 formation.Most scholars focus on the denitration performance of alkali metal poisoning catalysts,but neglect the SO3 formation and the effect of NO2 on SO3 formation and cannot comprehensively evaluate the catalyst performance.The effect of different Na2SO4 and Na2S2O7 loadings on the SO3 formation characteristics on V2O5-WO3/TiO2 catalysts was firstly studied,and characterization methods were used to study the physicochemical properties of the catalysts and the SO3 formation mechanism.Experiment results showed that the SO3 formation rate on the catalyst significantly increased after the sodium salts were loaded.Loading the Na2S2O7 content of 3.6%had the strongest effect on SO3 formation on the catalyst,the catalyst achieved a maximum SO3 formation rate of 1.44%at 410℃.The denitration efficiency on the catalyst decreased after the sodium salts were loaded.The catalyst deactivation was the most severely after loading the Na2SO4 content of 3.6%,and the denitration efficiency was only 50.97%at 350℃.Characterization results showed that sodium salts consumed the active ingredients and reduced the V4+/V5+ ratio,inhibiting the denitration performance.However,sodium salts increased the chemically adsorbed oxygen content and the vibration peak intensity of the V5+=O bonds,resulting in an increase in SO3 formation.Secondly,the SO3 formation characteristics on different Fe loadings modified Mn/ZSM-5 catalysts were studied.The situation of SO3 formation on the catalyst in the presence of NO2 was also investigated.The physicochemical properties of the catalyst and the SO3 formation mechanism were studied using characterization methods.Experiment results showed that the 9Mn9Fe/ZSM-5 catalyst had the highest SO3 formation rate and NOx conversion rate,and the addition of Fe enhanced the catalyst’s SO2 resistance.In fast SCR reaction,the NOx conversion rate on 9Mn9Fe/ZSM-5 reached a maximum of 99.44%at 250℃,but the SO3 formation rate also reached a maximum of 0.85%at 300℃.When NO2 is not contained in the flue gas,the NOx conversion rate on 9Mn9Fe/ZSM-5 reached a maximum of only 83.68%at 350℃,and the maximum of SO3 formation rate at 300℃ also decreased to 0.75%.This was due to NO2 caused the formation of nitrate ions on the catalyst surface in the fast SCR reaction.These nitrate ions re-oxidized the active sites more rapidly than O2 at low temperatures,thereby promoting the redox reaction.Characterization results also indicated that optimal Fe loading amount improved the dispersibility of the active compositions on the catalyst surface and the redox performance of the catalyst,resulting in an increase in SO3 formation.Through the above studies,it was found that after the sulfur-containing sodium salts was loaded on the V2O5-WO3/TiO2 catalyst,SO3 formation rate increased significantly.In the fast SCR reaction,SO3 formation rate on the Fe-modified Mn/ZSM-5 catalyst also increased significantly.Therefore,when burning high-sodium coal in power plants,it is necessary to pay attention to the problems of blockage of the air preheater caused by the formation of SO3,and to control the load of Fe used in the modified Mn/ZSM-5 catalyst to a reasonable range.