If they operate in the North Sea and Baltic emission control zones, all new vessels with diesel engines on board and their keels laid out after 1 January 2021 will need to comply with IMO Tier III requirements (ECAs).

Their pollution limits would be similar to new tugs operating in the US that must comply with the Tier 4 requirements of the US Environmental Protection Agency (EPA).

Nitrogen oxides (NOx) are expected by both to be almost eliminated from diesel engine emissions. For installed marine diesel engines of over 130 kW output capacity, NOx control specifications apply. Inside ECAs, a selective catalytic reduction (SCR) exhaust gas after-treatment device or another removal technology will need to be installed for tugs.

Via a catalytic reaction using a reductant, typically a urea solution, SCR systems process exhaust gases, removing particulates and NOx. This is pumped into the exhaust and funnelled to a reactor containing a catalyst that allows for a series of chemical reactions via a mixing chamber.

Urea is converted into ammonia and carbon dioxide, and ammonia eventually reacts to create nitrogen and water with nitrogen oxides.

If there is ample space in the engine room to accommodate storage tanks, SCR after-treatment technology can be retrofitted for tugs. Urea tanks, dosing pumps, piping made of stainless steel, mixing chambers and SCR reactors are required.

A part of the exhaust gas is cooled and combined with fresh compressed and cooled combustion air depending on the engine load, and ultimately routed into the cylinders.

This allows the combustion temperature in the cylinder to be decreased, which minimizes the formation of NOx.

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