SCR removes NOx from exhaust gas by a catalytic chemical reduction. An aqueous solution of urea (a 40% (or 32/40%) urea solution in fresh water) is sprayed into the hot exhaust gas upstream of a catalytic reactor. The heat of the gas decomposes the urea into ammonia (NH3) and CO2. In the reactor, the mixture passes over a catalyst (typically vanadium pentoxide/titanium dioxide, or zeolite), on which the ammonia reacts with the NOx (NO and NO2) to form nitrogen and water:

4NO + 4NH3 + O2 => 4N2 + 6H2O

6NO2 + 8NH3 => 7N2 + 12H2O

The catalyst provides the active surface and requires the gas to be above a minimum temperature (about 250 to 300 deg C for the vanadium-based, up to 350 in some) for effective conversion and to avoid ammonium-salt deposition.

Components: (1) urea storage tank; (2) urea supply/ dosing pump; (3) urea injection/dosing unit (nozzle/air-assisted injector with controlled metering) fitted in the exhaust duct; (4) atomising air supply and a mix/static mixer to distribute urea evenly; (5) the SCR reactor (catalyst modules, often arranged in layers, with a by-pass/soot cleaning arrangement); (6) temperature sensors, NOx analyzer, pressure sensors, flow meters and a control unit which meters urea injection proportional to engine load and NOx; (7) downstream a reductant-complete (ammonia slip) sensor/treatment, and on HPSCR there may be a cleaning/soot blow arrangement. The layout is either upstream (HPSCR) of the turbocharger or downstream (LPSCR).

1. Pre-check: confirm the urea tank level, urea quality, dosing pump ready, air supply available, and the exhaust temperatures are within the SCR operating window.
2. When the engine reaches a defined load the exhaust temperature is checked to be above the minimum for the SCR (e.g. >280 deg C); if it is too low, the engine may be operated so the temperature rises or a reheat/bypass used.
3. The control calculates the required urea flow from the engine load/fuel flow and the measured NOx (feed-forward with feedback trim).
4. The dosing pump delivers urea to the injection nozzle where it is atomised by air and sprayed into the exhaust; the urea evaporates/decomposes to ammonia and is mixed by the static mixer into the gas.
5. The exhaust gas passes through the catalyst layers where the NOx is reduced to N2 and H2O.
6. The system monitors outlet NOx and ammonia slip; the control trims the urea flow to maintain target NOx below the limit without excess ammonia slip.
7. It operates throughout the engine load range; if temperature falls out of window, the SCR is bypassed (or dosing stopped) to avoid catalyst fouling; on shutdown the system is purged to prevent urea crystallization in the injector nozzles.

1. Temperature window: at low engine load the exhaust temperature may be below the minimum; the catalyst is ineffective and deposits of ammonium bisulphate/sulphate can form, reducing activity; needs reheat or limiting the operating window.
2. Catalyst fouling/poisoning: soot, ash and sulphur deposit on the catalyst, causing gradual loss of activity; the catalyst must be cleaned (soot blowing) or regenerated; certain fuels (high ash, vanadium, silicon) poison it.
3. Urea-related problems: urea quality/contamination, crystallization blocking nozzles and lines, and dosing pump/air system faults; urea freezing at low temperature (must be kept warm).
4. Ammonia slip: if too much urea is dosed, excess ammonia leaves in the exhaust - an environmental/regulatory issue; needs precise control.
5. Control and sensors: NOx analyzers, temperature and pressure sensors and the control need frequent checking/calibration; demand-based control is sensitive to engine load changes.
6. Space and back-pressure: extra back pressure in the exhaust and space for the reactor; on HPSCR the added load and stress on the turbocharger; on LPSCR reheat costs energy; deposits can affect the turbocharger if upstream.