Improved NOx Reduction Using Wiremesh Thermolysis Mixer in Urea SCR System 2008-01-2636
A compact, knitted, crimped wiremesh mixer disposed in the exhaust system of an internal combustion engine, between the reductant injection and the urea SCR unit, increases the uniformity of the reductant in the exhaust stream by the time the stream reaches the SCR catalysis unit. Wiremesh mixer enhances thermolysis of urea into ammonia and iso-cyanic acid (HNCO). Computational Fluid Dynamics (CFD) modeling shows improved uniformity index from 0.94 to 0.99 within 35 mm travel length due to longitudinal and radial flow of the exhaust gas through the body of the wiremesh mixer. The higher thermolysis and rapid warm-up nature of the wiremesh provides enhanced ammonia production from urea thermolysis. Wiremesh physical attributes such as material composition, geometry and structure, wire diameter, mesh crimp pitch, crimp depth, crimp angle and the contour are optimized for minimum back pressure and maximum mixing efficiency. The modified geometry and channel dimensions of the wiremesh provide zigzag path for the gas flow to obtain efficient mixing. NOx reduction performance was improved in steady state engine tests. About 10-30 % improvement in NOx reduction is observed using wiremesh mixer. CFD model shows the concomitant improvement in ammonia uniformity index. Very minimum (less than 0.5%) urea crystallization is observed in the wiremesh mixer. Wiremesh layers are brazed together to avoid telescoping and to provide structural integrity and durability. The brazing strength is optimized by careful selection of filler and binder and verified by deflection test. The tolerances of the mixer contours are established by controlled rolling, brazing and heat treatment. The concentricity of the contours is optimized and process capability established. Thermal shock, water splash and hot vibration tests are performed to confirm the durability of the brazed wiremesh mixer.