Evaluating SCR Outlet NH_3/NO_x Sensor Measurement Performance Using CFD

摘要

As per emission regulations, the emission byproducts of the combustion process from stationary and mobile sources needs to be treated before it released to atmosphere. Exhaust aftertreatment components such as DOC, DPF and SCR integrate into diesel systems to control the PM and NO_x emissions. Among many NO_x abatement technologies, SCR is one of the prominent technology that is widely used. In this technology, DEF is injected into the exhaust system, upstream of the SCR, to generate NH_3 through thermolysis and hydrolysis process. And in the SCR catalyst, NH_3 reacts with NO_x to convert into N_2. In an ideal system, all the DEF should be completely evaporated to NH_3 and evenly distributed across the SCR inlet face. But based on the design and flow conditions, the SCR may have a nonuniform NH_3 distribution that results in NH_3 slip and NO_x not being converted after the SCR. It is important for the system to measure how much of the species escaped and to notify the engine control unit to adjust the dosing strategies. As this process is crucial for the robust dosing control, the sensor should measure the species accurately. In this work, a CFD methodology is proposed on how to evaluate the sensor measurement performance, which is then used to optimize the SCR outlet mixer designs. In the first phase, SCR inlet NH_3 is calculated using spray modeling and with basic assumptions, SCR outlet NO_x and NH_3 are determined. In the second phase, the predicted species are mapped to an SCR outlet domain to determine the sensor measurement performance.