Urea Thermolysis and Evaporation

As urea is the predominant NH3 precursor compound, its decomposition will be discussed in detail. In general, urea decomposes in the hot exhaust gas via a two-step reaction to yield the actual SCR reducing agent NH3 [9]. 

Urea thermolysis Isocyanic acid hydrolysis Overall reaction  

Before these chemical reactions occur, the dosed UWS aerosol is heated up by the surrounding exhaust gas and the contained water evaporates. The exact state of aggregation of urea during decomposition is still uncertain [26, 27]. Two recent theoretical studies [28, 29] relying on experimental data [30-33] point toward urea 

Urea thermolysis is usually considered a solely thermal reaction, whereas the intermediate isocyanic acid (HNCO) is stable in the gas phase but hydrolyzes on the SCR catalyst or on a dedicated hydrolysis catalyst [9]. Catalytic reactions wiU be discussed later this section is focusing on thermal decomposition. 

Back in 1966, Schmidt described a process to thermolyze urea into NH3 and HNCO gas on an industrial scale [35]. In this process, solid urea was blown into a fluidized bed of inert material, which was heated above 300 °C. The produced HNCO gas was used for melamine synthesis. 

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