*2.1. Control of NOx Emissions*

Nitrogen oxides (NOx) are formed from nitrogen and oxygen at high temperatures of combustion in the cylinder. NOx emissions are considered carcinogenic compounds and contribute to the formation of photochemical smog and acid rain.

A global approach to the control of NOx emissions has been undertaken by the IMO through Annex VI to MARPOL 73/78.

Annex VI applies to engines with power over 130 kW installed on new ships built after 1 January 2000 (the date the keel was laid) and pre-built engines that are subject to significant technical changes.

The starting NOx emission level recommended by the IMO (dependent on the rotational speed of the engine crankshaft—*n*) is as follows:


Amendments agreed by IMO in 2008 will set progressively tighter NOx emission standards for new engines, depending on the date of their installation (see also Table 1).


**Table 1.** The maximum content of NOx by MARPOL Annex VI.

\* The maximum content of NOx in areas of special control. In areas of common border with the values of 2011.

Tier I applies to diesel engines installed on ships constructed on or after 1 January 2000 and prior to 1 January 2011, and represents the 17 g/kWh NOx emissions standard stipulated in the original Annex VI.

Tier II covers engines installed in a ship constructed on or after 1 January 2011, and reduces the NOx emission limit to 14.4 g/kWh.

Tier III, covering engines installed in a ship constructed on or after 1 January 2016, reduces the NOx emissions limit to 3.4 g/kWh when the ship is operating in a designated ECA. Outside such an area, Tier II limits will apply.

Much tougher curbs on NOx and other emissions are set by regional authorities, such as California's Air Resources Board, and Sweden has introduced a system of differentiated ports and fairway dues, making ships with higher NOx emissions pay higher fees than more environment-friendly tonnage of a similar size.

To show compliance, an engine has to be certified according to the NOx technical code and delivered with an Engine International Air Pollution Prevention (EIAPP) certificate of compliance. The certification process includes NOx measurement for the engine type concerned, stamping of components that affect NOx formation, and a technical file, which is delivered with the engine.

**Figure 2.** Exhaust gas components of a modern two-stroke marine diesel engine (based on [4]).

NOx technical code-certified engines have a technical file, which includes the applicable survey regime, termed the onboard NOx verification procedure. The associated parameter check method effectively stipulates the engine components and range of settings to be adopted to ensure that NOx emissions from the given engine, under reference conditions, will be maintained within the certified value.

### *2.2. The Overall Amount of Global Sulphur Oxide Emissions at the Sea and in the Port Areas*

Studies on sulphur pollution showed that in 1990, SOx emissions from ships contributed around 4% to the total in Europe. In 2001, such emissions represented around 12% of the total and could rise to as high as 18%.

The simplest approach to reducing SOx emissions is to burn bunkers with a low sulphur content. A global heavy fuel oil sulphur content cap of 4.5% and a fuel sulphur limit of 1.5% in certain designated sulphur emission control areas (SECAs), such as the Baltic Sea, North Sea, and English Channel, are currently mandated by the International Maritime Organization (IMO) to reduce SOx pollution at the sea and in the port areas. In 2008, the IMO approved further amendments to curb SOx emissions (see also Figure 3):


**Figure 3.** The maximum sulphur content of marine fuel by the MARPOL Convention (author's drawing based on the data with [6]).
