不同升温速率下尿素沉积物热重试验对比分析

Comparativeanalysis of thermogravimetric test of urea deposits under different heating rates

尿素选择性催化还原系统(Urea-SCR)还原氮氧化物(NO x)应用中的一个主要问题是生成冷凝尿素和尿素分解副产物组成的固体沉积物,造成排气管堵塞、背压升高等问题,影响发动机正常运行。采用热重分析法对尿素沉积物及重要组分进行升温速率为5℃/min、10℃/min、20℃/min热重试验研究。尿素、缩二脲热解过程受到反应时间和反应温度的影响,而三聚氰酸及三聚氰酸同系物热解过程受温度的影响更为明显。起始分解温度按尿素<缩二脲<三聚氰胺<三聚氰酸<三聚氰酸一酰胺<三聚氰酸二酰胺顺序依次升高。沉积物含量占比按三聚氰酸(52%~53%)>三聚氰酸一酰胺与三聚氰酸二酰胺(27%)>缩二脲(16%~17%)>尿素(4%)顺序依次降低。温度低于265℃时沉积物主要由尿素、缩二脲和三聚氰酸组成,温度高于265℃时可产生三聚氰酸一酰胺和三聚氰酸二酰胺,温度高于500℃时沉积物完全分解。可见,柴油机低温瞬态工况时、SCR系统尿素喷射管段局部低温处易生成沉积物,而不同的反应温度可能导致沉积物各组份占比的不同。提高排气温度、延长尿素水溶液反应时间和利用沉积物相关组分催化热解有利于尿素沉积物的减少。

A major problem in the application of the urea selective catalytic reduction system(Urea-SCR)to reduce nitrogen oxides(NO x)is the formation of solid deposits composed of condensed urea and urea decomposition by-products,causing problems such as blockage of exhaust pipe and increased back pressure,which affects the normal operation of the engine.The thermogravimetric analysis method was used to conduct thermogravimetric test research on urea deposits and components at heating rates of 5℃/min,10℃/min and 20℃/min.The pyrolysis process of urea and biuret was affected by reaction time and temperature,while the pyrolysis process of cyanuric acid and cyanuric acid homologues was more obviously affected by temperature.The initial decomposition temperature increased in the order of urea<biuret<melamine<cyanuric acid<ammelide<ammeline,while the proportion of deposit content was sequentially decreased in the order of cyanuric acid(52%~53%),ammelide and ammeline(27%),biuret(16%~17%),and urea(4%).When the temperature was lower than 265℃,the deposits were mainly composed of urea,biuret and cyanuric acid;as it was higher than 265℃,ammelide and ammeline were produced;while it was over 500℃,those deposits were completely decomposed.It can be seen that deposits are easily formed at the local low temperature of the urea injection pipe section of the SCR system in the transient operating conditions of the diesel engine at low temperature,and different reaction temperatures may lead to different proportions of the deposits.Increasing the exhaust gas temperature,extending the reaction time of the urea aqueous solution,and using deposit-related components to catalyze pyrolysis are beneficial to the reduction of urea deposits.