Ideally, complete thermal decomposition of urea should produce only two products in active Selective Catalytic Reduction (SCR) systems: ammonia and carbon dioxide. In reality, urea thermal decomposition reaction includes the formation of isocyanic acid as an intermediate product. Being highly reactive, isocyanic acid can initiate the formation of larger molecular weight compounds such as cyanuric acid, biuret, melamine, ammeline, ammelide, and dicyandimide [1,2,3,4]. 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 such as melam, melem, and melon [5, 6] that are difficult to remove from exhaust systems. Deposits can affect the efficiency of the urea decomposition, and if large enough, can inhibit the exhaust flow. These deposits could also increase downstream Particulate Matter (PM) emissions, which could lead to an engine exceeding the regulated emission standards.
This paper presents a suitable analytical method adapted for quantification of urea and by-products of urea thermal decomposition. This method was able to quantify seven major urea-related soluble monomers, as well as fully dissociated insoluble oligomers to monomers.
Urea-related composition of deposits, including oligomers collected from a urea decomposition reactor under various exhaust conditions, are compared in the paper.
The paper also presents composition results of urea and urea-related by-products on PM filters collected downstream of the aftertreatment system of an off-road engine during certification test cycles.