SCR Catalyst: NOx Reduction for Diesel Construction Equipment

Exhaust aftertreatment using AdBlue (urea solution) to reduce nitrogen oxides in construction equipment diesel engines.

SCR (Selective Catalytic Reduction) is an exhaust aftertreatment process in which nitrogen oxides (NOx) in the exhaust gas are converted into harmless nitrogen and water vapor through the injection of a urea solution (AdBlue/DEF) into the exhaust stream. SCR has been mandatory under EU Stage IIIB/IV for construction equipment over 56 kW and reduces NOx emissions by 80–95 %.

The system consists of an AdBlue tank (typically 10–15 % of diesel tank volume), a metering pump, a mixer, and the actual SCR catalyst in the exhaust line. The 32.5 % urea solution is injected into the hot exhaust stream, where it decomposes into ammonia — which reacts with nitrogen oxides in the catalyst.

AdBlue consumption is 3–6 % of diesel consumption. Empty AdBlue tanks result in engine power reduction or engine shutdown on Stage V machines — a critical logistical issue on remote construction sites. At temperatures below –11 °C, AdBlue crystallizes — heated tanks and lines are standard on winter-rated construction equipment.