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NOx-reduction-options

Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction involves injection of a urea solution or ammonia into the exhaust gas at a temperature of 290–350°C, necessitating a prior warm-up period. A catalyst is installed in the exhaust gas channel, where the reducing agents react with the nitrogen oxides to form nitrogen and water. Use of SCR requires no changes to the basic engine design. One disadvantage of SCR is the occurrence of ammonia slip, which can cause corrosion in the exhaust channel.

 

SCR can reduce NOx emissions by 90-99%. Some sources also cite a of 25-40% reduction in PM emissions. Operation of SCR leads to increased fuel consumption and CO2 emissions. The technology can be applied on new-build vessels as well as retrofitted on existing ships.

 

Over the past decade, approximately 1,250 SCR systems have been installed and used on marine vessels. Use of SCR in combination with a scrubber is still not sufficiently tested.

 

Exhaust Gas Recirculation (EGR)

As the name suggests, EGR involves filtering off part of the exhaust gas and recirculating it to the combustion chamber after cooling. This leads in two ways to reduced formation of NOx: the main exhaust components have a higher specific heat capacity than that of air, and there is a reduced supply of oxygen. A drawback of the technology is the particulate matter in the recirculated air, which can lead to deposits in the engine and contamination of the lubricating oil. Because of the presence of (gaseous) sulphur, there may also be corrosion due to sulphur acids formation. No more than 15-20% of the exhaust gas can be recirculated. Experiments carried out by MAN Diesel in cooperation with Maersk have verified IMO Tier III compliance by measuring an 85% reduction of NOx. EGR also results in limited additional fuel consumption and thus higher CO2 emissions.

 

Further reading

85-100% less pollutants

LNG as a shipping fuel can help significantly reduce the environmental impacts of maritime transport, most likely without increasing costs. With this fuel, NOx, SOx and particulate emissions can be reduced by 85-100% in comparison with HFO.