Investigation of Urea Derived Deposits Composition in SCR Systems

Scott  Eakle, Southwest Research Institute

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. 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 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.

Deposit formation in the decomposition reactor remains a significant challenge to aftertreatment system designers due to the complicity of the contributing factors such as temperature, flow rates and distribution, dosing rate, wall or mixer surfaces, droplet size and number distribution. To understand deposit formation in the design phase, Southwest Research Institute® (SwRI®) is working with industry partners to identify urea derived deposit formation pathways and model deposit formation in the emission control system.

Prior to this program, the capability to determine and quantify by-products of urea thermal decomposition in the diesel exhaust samples did not exist, thus making it impossible to validate the urea decomposition model. A suitable analytical method adapted for quantification of urea and by-products of urea thermal decomposition was developed and verified. This method was able to quantify seven major urea-related soluble monomers, as well as fully dissociated insoluble oligomers to monomers.

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