Selective Catalytic Reduction (SCR)

• The principle of SCR is that flue gas, when mixed with a reagent – ideally Ammonia (NH4) – and passed over a catalyst will reduce the NOx components to N2, water and O2.
• SCR is the technology of choice for NOx reductions, and is planned for the next generation of road and off road diesel engine trucks and equipment
• SCR technology is being used on a number of ships to successfully reduce NOx emissions, mainly for auxiliary engines
• SCR can in principle be integrated with other abatement technologies, with, for example, the exhaust being further scrubbed to remove SOx.
• To read more about companies using SCR equipment see

INDEPTH
There are complexities that make SCR a tricky technology to install and get to work reliably:

• Ammonia is a nasty material to handle, and it is more common to use urea – a readily available industrial chemical – which largely converts to ammonia when injected into the hot exhaust stream.
• Effectiveness of conversion depends on good control of the urea injection. Too much or too little for the exhaust flows damages efficiency and effectiveness.
• The catalysts, often embedded on ceramic substrates, create backpressures with adverse impacts on engine performance. The greater the reduction required, the greater the backpressure.
• The catalysts are poisoned by sulphate salts, and so deteriorate faster the more sulphates are present. This can be countered in two ways: by having no sulphur compounds in the exhaust stream or by operating at temperatures high enough to prevent the sulphate salt formation.
• The catalysts have quite short life and need renewing every few years. It is not yet clear how the spent catalysts can best be disposed of safely.

Ammonia escaping after SCR processes is dangerous, so rigorous controls are needed
Urea (or ammonia) is consumed, so a urea supply chain must exist.