Eakle, S., Kroll, S., and Henry, C., "Investigation of Urea Derived Deposits Composition in SCR Systems," SAE Technical Paper 2016-01-2327, 2016, doi:10.4271/2016-01-2327.
Ideally, complete decomposition of urea should produce only two products in active Selective Catalytic Reduction (SCR) systems: ammonia and carbon dioxide. In reality, urea decomposition reaction is a two-step process that includes the formation of ammonia and isocyanic acid as intermediate products via thermolysis. Being highly reactive, isocyanic acid can initiate the formation of larger molecular weight compounds such as cyanuric acid (CYN), biuret (BIU), melamine (MEL), ammeline (AML), ammelide (AMD), and dicyandimide (DICY). These compounds can be responsible for the formation of deposits on the walls of the decomposition reactor in urea SCR systems. Composition of these deposits varies with temperature exposure, and under certain conditions can create oligomers that are difficult to remove from exhaust pipes. Deposits can affect efficiency of the urea decomposition, and if large enough, can inhibit the exhaust flow and negatively impact ammonia distribution on the SCR catalyst. This paper presents results of investigation of the deposits collected at various gas temperatures for quantification of urea and by-products of urea thermal decomposition and for their trace elements. Urea related compounds, including oligomers and elemental composition of deposits collected from a urea decomposition reactor under various exhaust conditions, are compared in the paper.