NOx reduction; the SCR technology
With lean burn (diesel or gas) engines, Selective Catalytic Reduction (SCR) can almost eliminate the emissions of NOx. In the SCR process a reductant is used in combination with a catalyst. The most widely used reductant is a urea-water solution at a concentration of 32,5% or 40% concentration. A feed dosing pump feeds the urea from a tank. The pump is triggered by an “engine run” signal in combination with a minimum and a maximum exhaust gas temperature. The temperature limits depend on the fuel quality and the selected type of catalyst. However, they will rarely be lower than 250 ºC or higher than 525 ºC. A special nozzle, installed in the exhaust system, injects a defined amount of urea into the exhaust gas. The amount injected is directly related to the base line NOx level and the required reduction. The injected urea and the exhaust gasses are led through a series of static mixers and homogenisers to ensure that the urea is optimally converted to ammonia. The gasses are then led through the suitably charged catalytic elements which almost completely convert NOx into nitrogen (N2) and water vapour (H2O). A NOx sensor positioned after the catalytic elements provides information about the actual emission level and allows the controller to compute and, if necessary, adjust the urea dosing rate. Depending on the required performance, the system may be enhanced with a clean-up catalyst to minimise the risk of ammonia slip. Finally, the purified exhaust gasses are either fed to the boiler, condenser or other equipment, or discharged into the atmosphere. The performance of the system is determined by the control strategy, the degree of urea atomisation and mixing with the exhaust gas, and the catalytic volume.