Selective catalytic reduction of nitrogen oxides is achieved through the combined use of a catalyst and a reductant. The most widely used reductant is a urea-water solution at a concentration of 32.5% or 40%.

The urea is collected from a tank by a dosing pump triggered by an ‘engine run’ signal in combination with minimum and maximum exhaust gas temperatures. The temperature limits depend on the fuel quality and the chosen catalyst, and generally fall between 250 ºC and 525 ºC.

The amount of urea injected into the exhaust stream depends on the base levels of nitrogen oxide emitted by the engine and the desired reduction.

Once the urea has been injected into the exhaust stream by a special misting nozzle, a series of static mixers and homogenisers convert the urea into ammonia. The exhaust gasses and ammonia are then fed past catalytic elements that convert almost all of the nitrogen oxide into nitrogen (N2) and water vapor (H2O).’

A nitrogen oxide sensor positioned after the catalytic elements provides feedback on the actual emission levels. This allows the controller to check and, if necessary, adjust the urea dosing rate.

System performance is determined by the control parameters, the degree of urea atomisation and mixing with the exhaust gas, and the catalytic surface area. Depending on performance requirements, the system can also be configured with an integrated ammonia slip catalytic converter to eliminate ammonia slip.

At the end of the process, the purified exhaust gasses are sent to the boiler or condenser or other equipment, or discharged into the atmosphere.