Study on decomposition and by-products formation mechanism of urea solution impinging on heated plate in SCR system

Selective catalytic reduction technology is one of the most effective methods to control NOx emissions from diesel engines. However, the generation of solid urea and by-products, which will lead to the incomplete decomposition of urea and reduce NOx reduction efficiency. The accumulation of deposits will also affect the back pressure and even block the catalyst, causing a vicious cycle. In this paper, a numerical model coupled with a detailed decomposition mechanism of urea was established. Based on this model, four heat transfer methods and chemical reactions in the film were calculated. The results show that the main heat transfer methods between the film and the plate are conduction and evaporation, both of which have maximum values higher than 10⁷ W/m². The mass of the film varies with heat flux can be divided into four stages. In the first stage, the film forms rapidly, and intense heat exchange occurs. In the second stage, the film increases slowly and the decomposition mass of urea increases. The main component of the film is solid urea. In the third stage, the solid urea mass begins to decrease. In the fourth stage, film mass decreases, and a large amount of urea is decomposed. The main byproduct of urea decomposition is biuret in the first three stages and cyanuric acid in the fourth stage. The solid urea and byproducts are mainly distributed in regions with low heat flux. With an increase in the initial plate temperature, the heat flux increases, the film and solid urea masses decrease, the byproducts appear earlier and their mass increases faster. When the temperature is 523 K, 528 K, and 533 K, the maximums of biuret mass occur at about 3.1s, 1.4s and 0.5s, respectively. The present work provides guidance understanding the relationship between heat transfer characteristics and byproducts formation of urea decomposition.