Thermal and Fluid Dynamic Considerations in Aftertreatment System Design for SCR Solid Deposit Mitigation 2012-01-1287
Selective Catalytic Reduction (SCR) of oxides of nitrogen (NOx) with ammonia gas has established itself as an effective diesel aftertreatment technology to meet stringent emission standards enforced by worldwide regulatory bodies. Typically, in this technology, aqueous urea solution of eutectic composition - known as Diesel Exhaust Fluid (DEF) - is injected into hot exhaust gases leading to a series of thermal, fluid dynamic and reactive processes that eventually produces the ammonia necessary for NOx reduction reactions within monolithic catalytic substrates. Incomplete decomposition of the injected urea can lead to formation of solid deposits that adversely affect system performance by increasing the engine back pressure, reducing de-NOx efficiency, and lowering the overall fuel economy. In the present study, fundamental thermal and fluid dynamic factors that affect deposit formation and removal, and their relation to design and operating considerations of aftertreatment systems were explored using both simulations and experiments. Lagrangian Drop - Eulerian Fluid spray models were used to assess the deposit formation propensity, and to arrive at strategies that minimize the probability of formation of solid deposits, while an in-house developed Eulerian phase change model was used to investigate effects of critical parameters such as exhaust flow rate and temperature on deposit removal rate. A combination of experimental techniques including spray characterization by optical techniques, on-engine deposit testing, transient temperature measurements and thermogravimetric analyses were used to complement simulations and to aid in deposit root cause detection.