Selective Catalytic Reduction (SCR) removes NOx from exhaust gas by injecting a reductant (aqueous urea, which decomposes to ammonia) into the gas stream and passing it over a catalytic reactor, where NOx is reduced to nitrogen and water:
4NO + 4NH3 + O2 => 4N2 + 6H2O
The three basic SCR system types differ mainly in where the reactor is placed relative to the turbocharger and the engine.
In an HPSCR system the reactor and urea injection are located between the engine exhaust outlet and the turbocharger inlet (upstream of the turbine), where exhaust gas pressure and temperature are high. Because the gas is hot (usually above 300 to 350 deg C), no reheating is required, and the catalyst works efficiently even at low engine loads. Disadvantages: the reactor and injection grid must withstand high pressure and vibration, the space and structure around the engine top must accommodate a large reactor, and the catalyst is exposed to soot and deposits which reduce life and require more frequent cleaning. The turbocharger operates on the cleaned gas, which reduces blade fouling.
In an LPSCR system the reactor is placed downstream of the turbocharger, in the low-pressure (near atmospheric) exhaust line. The system is lighter, cheaper and easier to retrofit, and standard marine exhaust piping can be used. The main drawback is that at low load the exhaust temperature after the turbine can be too low (below about 280 to 300 deg C) for effective reduction, so the gas must be reheated or the temperature maintained by engine management, which consumes extra energy and demands additional measures.
A static mixer is a passive device placed in the exhaust duct immediately downstream of the urea injection point. It consists of baffles, vanes or grids that create turbulence and thoroughly mix the injected urea/ammonia vapour with the exhaust gas. This ensures an even distribution of reductant across the catalyst face, avoiding both ammonia slip (excess ammonia leaving the system) and areas of high NOx leakage due to poor mixing. Static mixers improve conversion efficiency and reduce the amount of urea required. No moving parts make them robust and reliable.
Upstream installation (HPSCR) places the reactor before the turbine, utilising high gas temperature and providing efficient low-load operation and turbocharger protection. The disadvantages are high mechanical and thermal loading, more complex engine top layout and difficulty of cleaning a large high-mounted reactor.
Downstream installation (LPSCR) places the reactor after the turbine in the low-pressure exhaust. It is simpler, cheaper and easier to maintain and retrofit. Its principal drawback is the low temperature at part load, which must be managed by gas reheating or by limiting the load range in which the SCR is effective. In practice both configurations satisfy Tier III in their intended load range, and the choice is a trade-off between cost, space, temperature and maintenance.