Selective Catalytic Reduction (SCR) systems for Tier III NOx compliance - various types:
HPSCR: The reactor is placed upstream of the turbocharger (between the engine exhaust and the turbine), where the exhaust gas is at high pressure and high temperature. The urea is injected into the hot gas and the NOx is reduced on the catalyst. Advantages: the high temperature (above 300 deg C) gives efficient conversion even at low load, no reheating is needed, and the turbocharger operates on the cleaned gas. Disadvantages: the reactor must withstand high pressure and vibration, it is large and heavy, and the catalyst is exposed to soot/deposits.
SCR with static mixers: A static mixer is a passive device (baffles/vanes) placed in the exhaust duct downstream of the urea injection point to thoroughly mix the urea/ammonia with the exhaust gas before it reaches the catalyst. This ensures an even distribution of the reductant across the catalyst, improving conversion efficiency and reducing ammonia slip and urea consumption. Static mixers are used in both HP and LP systems to improve the mixing.
LPSCR: The reactor is placed downstream of the turbocharger, in the low-pressure exhaust line (near atmospheric pressure). The urea is injected and the NOx is reduced on the catalyst. Advantages: the system is lighter, cheaper, and easier to retrofit and maintain, and standard exhaust piping can be used. Disadvantages: at low load the exhaust temperature after the turbine may be too low (below about 280-300 deg C) for effective conversion, so the gas must be reheated or the load range limited; the catalyst may be fouled by soot.
Upstream (HPSCR): The reactor is before the turbine. Advantages: high gas temperature gives efficient low-load operation, no reheating, and the turbocharger is protected from soot. Disadvantages: high mechanical/thermal loading, complex engine-top layout, and difficulty cleaning a large high-mounted reactor.
Downstream (LPSCR): The reactor is after the turbine. Advantages: simpler, cheaper, easier to retrofit and maintain. Disadvantages: low temperature at part load requires reheating or load limiting, and the catalyst may be fouled.
The choice between HP and LP SCR depends on the engine, the space, the cost, and the required operating range; both can achieve Tier III compliance in their intended load range.