Experimental Study of SCR in a Light-Duty Diesel Exhaust to Provide Data for Validation of a CFD Model Using the Porous Medium Approach 2010-01-1177

Removal of NOx from a light-duty diesel automotive exhaust system can be achieved by SCR reactions using aqueous urea spray as the reductant. Measurements of emissions from such a system are necessary to provide data for CFD model validation. A test exhaust system was designed that featured an expansion can, nozzle and diffuser arrangement to give a controlled flow profile to define an inlet boundary for a CFD model and to approximate to one-dimensional flow. Experiments were carried out on the test exhaust using injection of either ammonia gas in nitrogen or aqueous urea spray. Measurements were made of NO, NO₂ and NH₃ at inlet to and exit from the SCR using a CLD analyzer. The NO and NO₂ profiles within the bricks were found by measuring at the exit from different length bricks. The spray and gas measurements were compared, and insights into the behavior of the droplets upstream and within the bricks were obtained. Approximately half to three-quarters of the droplets from the spray remained as droplets at entry to the first brick. Approximately 200 ppm of ammonia was released from the droplets to react in the first SCR brick. Between 10 and 100 ppm of potential ammonia passed through the first brick as droplets under conditions ranging from NOx-matched spray input to excess spray. The CFD model for the gas cases gave reasonable predictions for long bricks. For short bricks, the space velocity was high and there was breakthrough of all species. Nevertheless, the reaction kinetics used, based on a scheme published in the open literature, were shown to have some ability to describe the species profiles within the bricks.