A lean nitrogen oxides trap+selective catalytic reduction system for controlling the nitrogen oxides emissions from a diesel engine.

摘要

A Lean NOx Trap + Selective Catalytic Reduction (LNT+SCR) catalyst system with intermediate NH3 (or urea) injection is proposed for treating NOx from a diesel engine. To avoid rich purges of the LNT, hydrocarbon oxidation was used to promote NOx desorption under lean conditions. Novel techniques were developed to estimate the temperatures of the precious metal sites and NOx storage sites of the LNT during exothermic reactions.; The SCR alone was used below 400°C because of its low temperature NOx activity and NH3 storage capability, which simplified the NH3 injection control system. Above 400°C, where the NH 3 capacity was low, the LNT was used to store the NOx. Periodically, the stored NOx was desorbed with the lean hydrocarbon oxidation (LHO), and NH3 was injected so the SCR could reduce the released NOx. A LNT with 5.7% potassium and 3.5 g/liter platinum provided high NOx capacity and good hydrocarbon conversion for this application.; NOx stored in the rear of the LNT was not desorbed during LHO because the exotherm was limited to the front half. Without purging the rear, the indicated 60 second storage efficiency at 500°C and 25k hr -1 was only 40%. However, the exothermic heat was transported to the rear by convection. If the feedgas NOx remained off for 60 seconds, the increased temperature and absence of gas-phase NOx promoted the desorption of some NOx stored in the rear. This increased the average storage efficiency during a subsequent 60 second NO injection period to 80%. A dual LNT system with reversing flow satisfied the requirement for 60 second periods without NOx.; A LNT and SCR catalyst were evaluated with LHO and NH3 injection. NOx conversions of 90% were demonstrated when 1200 ppm NH 3 was injected before, during, and after 10 second hydrocarbon injections.; The average platinum site temperature on a 0.64 cm LNT sample was estimated during CO oxidation from the conversion of 200 ppm methane, propane, butane, or hexane. With 2% CO, the stabilized platinum temperature was approximately 80°C above the temperature of the gas exiting the catalyst.; The average NOx storage site temperature during propylene oxidation was estimated from the amount of NOx remaining on the LNT. With 3300 ppm propylene, the stabilized storage site temperature was approximately 20°C above the gas temperature.