高负载铜基SSZ-13分子筛催化剂SCR模型研究

Modeling of Highly Loaded Copper-Based SSZ-13 Molecular Sieve Catalysts for Selective Catalytic Reduction

随着日益严格的排放法规出台,降低重型柴油机的氮氧化物(NO_(x))排放已成为研究的重点.为提高SSZ-13分子筛催化剂在低温下的二氧化氮(NO_(2))转化效率,通过数值仿真的方法研究了高负载铜基的小孔径分子筛催化剂在不同工况下的最佳NO_(2)占比情况.利用GT-SUITE软件建立了一维反应器模型以及催化反应动力学模型,采用数值模拟计算的方法对选择催化还原(selective catalytic reduction,SCR)系统内化学反应进行了研究.模型考虑了423~673 K的温度范围内,氨气(NH_(3))的吸附、脱附和氧化反应、一氧化氮(NO)氧化反应、NO_(x)的还原反应以及硝酸铵(NH_(4) NO_(3))的分解反应,并且在排出气体中监测重要的温室气体——氧化亚氮(N_(2)O)的排放量.结果表明:在高负载铜基的SSZ-13分子筛催化剂作用下,NO_(2)与NH_(3)反应生成的NH_(4)NO_(3)在573 K以下的低温段结晶,晶体堵塞孔道抑制反应进行;而随着温度升高,NH_(4)NO_(3)逐渐分解,反应得以正常进行,因此NO_(2)的含量对NO_(x)的转化效率产生影响,即不同温度段的最高NO_(x)转化效率对应的NO_(2)进气量不同,当温度升高,最高NO_(x)转化效率对应的NO_(2)占比先升高后降低,最高NO_(2)占比不超过40%,并且对应温度区间内排出的N_(2)O含量低于6×10^(-6).改变氨氮比、空速、催化剂活性位点密度等工况探究氮氧化物转化效率最高对应的NO_(2)占比,得到的NO_(2)占比均为动态变化值.定义氮氧化物转化效率超过80%的温度范围为最佳温度窗口,变化工况发现最佳温度窗口均为503~673 K.

With the increasingly stringent emission legislation,reducing NO_(x) emissions from heavy-duty diesel engines has gained extensive research attention.Aiming to improve the NO_(2) conversion efficiency of the SSZ-13 molecular sieve catalysts at low temperatures,the optimal NO_(2) percentage of small-pore-size molecular sieve catalysts with high copper loading under different operating conditions was investigated using numerical simulations.A onedimensional model and a kinetics model of the catalytic reaction were developed using the GT-SUITE software,and the chemical reactions within the selective catalytic reduction(SCR)system were investigated using numerical simulations.The model contains the adsorption,desorption,and oxidation of NH_(3),oxidation of NO,reduction reactions of NO_(x),and decomposition reactions of NH_(4)NO_(3) over a temperature range of 423—673 K and monitors the emissions of an important greenhouse gas,N_(2)O,in the discharge gas.The results indicated that the NH_(4)NO_(3) produced by the reaction of NO_(2) with NH_(3) crystallized in the low-temperature interval below 573 K under the action of an SSZ-13 molecular sieve catalyst with high copper loading.The crystals blocked the pores and suppressed the reaction,while NH_(4)NO_(3) gradually decomposed as temperature increased,enabling the reaction to proceed normally.Therefore,the NO_(2) concentration impacted the NO_(x) conversion efficiency,i.e.,the highest NO_(x) conversion efficiencies in different temperature intervals corresponded to different NO_(2) intake volumes.As temperature increased,the NO_(2) content corresponding to the highest NO_(x) conversion efficiency increased and subsequently decreased.The maximum NO_(2) percentage did not exceed 40%,and the N_(2)O concentration emitted in the corresponding temperature interval was below 6×10^(-6).By varying the ammonia to nitrogen ratio,air velocity,catalyst active site density and other operating conditions to explore the NO_(2) percentage corresponding to the highest NO_(x) conversion efficiency,the NO_(2) percentage obtained are all dynamically varying values.The temperature range with the NO_(x) conversion above 80%was defined as the optimal temperature window,which was found within 503—673 K under all operating conditions.