Selective Catalytic Reduction

The Selective Catalytic Reduction (SCR) uses a reducing agent injected into the flue gases upstream of a catalytic reactor to effect chemical reduction of NOx to nitrogen.

Reducing reagents used in Selective Catalytic reduction (SCR) are typically ammonia/urea based. Reduction of NOx to nitrogen and water occurs on the catalyst. The optimum temperature range for this system is 300 to 400°C and is mildly exothermic. It is variously fitted downstream of an economiser and/or air heater and, in some cases, a fly ash dust collector. On some applications gas fired reheat is used to maintain optimum temperature for the catalyst bed.

As duct on plant utilising SCR are large, injection is via a multipoint grid assembly placed upstream of the catalyst bed to allow good mixing of the ammonia/urea solution with the flue gas.

Selective Catalytic reduction systems have been developed for gas, oil and coal firing, with the catalyst type and configuration being selected as a function of fuel type, flue gas analysis and dust burden. They need to be robust enough to withstand thermal cycling, attack/poisoning by sulphur and halogens and be able to withstand ash fouling.

Catalyst are based on a number of materials, the most popular being titanium, or valadium, dioxide based and are typically located in units between the economiser and preheater. The catalyst unit may take the form of plate, honeycomb, grid cylinder or pellet reactor mounted in a carbon steel housing.