Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction (SCR) was originally introduced on stationary power plants and engines and then large engines such as those on ships, but it is now fitted to most new heavy-duty (i.e. truck and bus) diesel engines in Europe. It is also available as retrofit device for NOx emissions reduction, possibly combined with a DPF.

In the SCR system, ammonia is used as a selective reductant, in the presence of excess oxygen, to convert over 70% (up to 95%) of NO and NO₂ to nitrogen over a special catalyst system. Different precursors of ammonia can be used; one of the most common option is a solution of urea in water (e.g. AdBlue^®) carefully metered from a separate tank and sprayed into the exhaust system ahead of the SCR catalyst.

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AdBlue^® is a stable, non-flammable, colorless fluid containing 32.5% urea which is not classified as hazardous to health and does not require any special handling precautions. It is made to internationally-recognized standards. Urea is used as an artificial fertilizer and is found in products such as cosmetics. The consumption of AdBlue^® depends on the amount of NOx that needs to be converted. For example, a Euro III engine, emitting 5g/kWh NOx, has been brought to Euro V (2g/kWh NOx) with a retrofit SCR system using ~2% urea compared to the fuel consumption. Systems on-board the vehicle alert the driver when it is time to top up with AdBlue^®. A special website www.findadblue.com is available to show facilities where AdBlue^® is available.