Significance of droplet size when injecting aqueous urea into a Selective Catalytic Reduction after-treatment system in a light-duty Diesel exhaust

摘要

Application of SCR catalyst technology in a light-duty Diesel exhaust system requires injection of a reductant. Aqueous urea is injected with a spray unit directly into the exhaust upstream of an SCR catalyst. Ideally, the aqueous urea droplets must first evaporate and break down to ammonia and HCNO before reaching the catalyst. Multiple chemical reactions then occur on the catalyst reducing NOx. Droplet size is thus critical in this process. Laser PDPA measurements have been made in the laboratory of the diameter of water droplets from a commercially available urea spray unit in hot air flow to characterise the spray. These measurements have shown significant numbers of droplets with diameters greater than 120 microns. Measurements have also been made with a typical mixer unit downstream of the spray unit and the effect of this on the droplet size distribution is reported. A simple model of the warm up and evaporation of an aqueous urea droplet convecting in a Diesel exhaust has been developed. This can be run as a single droplet simulation programme but could be incorporated into a full CFD model. The latter is required for designing SCR injection systems without recourse to testing. The model is valid for the early part of the evaporation process where it is assumed that water leaves the aqueous urea droplet and also for the later stage where the urea leaves the droplet prior to breaking down by thermolysis. The model has been tested against data available in the literature. The effect of initial droplet size, droplet injection temperature and velocity, and of the exhaust temperature, vapour pressure and flow rate under typical operating conditions are demonstrated. The implications of the model are that the urea droplets must be very small, less than 50 microns, if they are to evaporate in the short distance between the injector and the catalyst in a typical light duty Diesel exhaust system.