NOx emissions from ships

Nothing has contributed to shipping’s ‘dirty’ image than the exhaust gases that come from engines burning heavy fuel oil. Burning residual fuels has allowed shipping to keep costs and therefore the price of commodities and goods at a lower level than would otherwise have been possible.

Whether or not that is an acceptable situation for the long term will depend upon politicians, electorates and regulators but it should be noted that shipowners and equipment makers have done their best to mitigate the environmental impact of ship operation.

The regulation of ship exhausts is one of the more recent aspects of MARPOL and is regulated by the last of the six annexes to the convention. It does not regulate all of the exhaust gases or products with NOx and SOx being the first to have limits set. CO2 is regulated indirectly under the Energy Efficiency Design Index (EEDI) rules and is not controlled in the way that NOx and SOx are. Ozone depleting substances are also regulated under Annex VI but this has mostly affected firefighting and refrigeration gases rather than normal ship operation.

Annex VI Prevention of Air Pollution from Ships was adopted in 1997 and entered into force 19 May 2005). The resulting regulation has been piecemeal and arguably flawed because controlling each of the different gases requires different treatment and controlling some can affect the production of others. It allows for a regulation of exhaust gases by setting limits on their emissions generally and with more stringent requirements in designated emission control areas (ECAs). These areas are established at the request of port states and after confirmation by the IMO. ECAs cover areas extending over the waters of several port states although It remains possible for individual states to set their own limits outside of MARPOL.

Norway is a good example of an individual state taking action against NOx. Although it does not place a limit on NOx production, since 2007 all ships with a main engine above 750kw which produce NOx are subject to a levy. The levy is not confined to shipping with air traffic and rail also subject to it.

Some of the proceeds of the levy were used to establish a fund from which certain projects aimed at NOx reduction could be granted financial assistance. Shipping has benefitted from this fund in many projects including the conversion of the Danish tanker Bit Viking to run on LNG and many offshore projects involving LNG and battery power.

Nitrogen oxides or NOx was the first aspect of exhaust gas to be regulated and is given particular attention because of the technical complexities involved with it. As a consequence, a large part of Annex VI is the NOx Technical Code 2008. The NOx Code is aimed at improving the environment by reducing the effect of greenhouse gases and so-called acid rain.

NOx is also implicated in some medical conditions. nitric oxide (NO) and nitrogen dioxide (NO2) are both implicated but as regards the greenhouse effect this is because they promote the formation of ozone in the troposphere. But the most potent greenhouse gas nitrous oxide (N20) is only a minute fraction (about 0.1%) of all the nitrogen products produced by combustion in the diesel engine. The NOx Code sets out three tiers of control that gradually became more stringent. The production of NOx is easier to control in some engine types than others and as a consequence the allowed limits for each stage of the IMO’s three stage roll out programme differ depending on engine speed with the low speed engines given the highest permissible output as shown below.

Tier I (all ships effective 19 May 2005)
Engine speed

< 130rpm 17.0g/kWh > 130–2,000rpm 45 × rpm(-0.2)g/kWh
> 2,000rpm 9.8g/kWh

Tier II (ships built from 1 January 2011)
Engine speed

< 130rpm 14.4g/kWh > 130–2,000rpm 44 × rpm(-0.23)g/kWh
> 2,000rpm 7.7g/kWh

Tier III (new ships built from 1 January 2016 operating in existing ECAs)
(new ships built from 1 January 2021 operating in North Sea and Baltic Sea ECAs
Engine speed

< 130rpm 3.4g/kWh > 130–2,000rpm 9 × rpm(-0.2)g/kWh
> 2,000rpm 2.0g/kWh

The position of dual-fuelled and gas burning engines with regard to the requirements of the NOx Code have been something of a grey area. In order to clarify this MEPC.258(67) redefines the term Marine Diesel Engine in MARPOL Annex VI. The definition of “marine diesel engine” in paragraph 14 is replaced by the following definition:

“Marine diesel engine means any reciprocating internal combustion engine operating on liquid or dual fuel, to which regulation 13 of this Annex applies, including booster/compound systems if applied. In addition, a gas fuelled engine installed on a ship constructed on or after 1 March 2016 or a gas fuelled additional or non-identical replacement engine installed on or after that date is also considered as a marine diesel engine.”

Timeline: MARPOL Annex VI (NOx related)

  • 26 Sept 1997 – Annex VI formally adopted
  • 1 Jan 2000 – Engine-makers begin building and certifying NOx Tier I engines
  • 19 May 2005 – Annex VI enters into force NOx Tier I
  • Oct 2008 – MEPC approves revised Annex VI and NOx Technical Code 2008
  • 17 Jul 2009 – MEPC approves proposed US/Canada ECA (SOx, NOx and PM)
  • 1 Jan 2011 – NOx Tier II
  • 1 July 2011 – MEPC approves proposed US Caribbean ECA (SOx, NOx and PM)
  • 1 Aug 2012 – Implementation of US/Canada ECA
  • 1 Jan 2014 – Implementation of US Caribbean ECA
  • 1 Jan 2016 – NOx Tier III (only applicable in existing ECAs)
  • 7 Jul 2017 – MEPC approves proposed North Sea and Baltic Sea NOx ECA
  • 1 Jan 2021 – NOx Tier III applicable to new ships operating in the new (NOx) North Sea and Baltic Sea ECAs.

If a ship’s engine(s) are replaced at any time with a new engine (as opposed to a used engine) the level at the date of replacement will apply.
NOx production and monitoring

In all internal combustion engines, boilers and incinerators, it is necessary to mix air with the fuel to allow combustion to take place. Air is mostly composed of nitrogen (about 78%) and oxygen (about 21%) with a few trace gases and water vapour. The fuels themselves are a complex mix of hydrocarbons with other components depending on their type. Even within the defined ISO 8217 fuel grades there are no fixed absolutes but minimum and maximum levels for constituents of the fuel.
Different fuel types burn best at different temperatures and this along with their chemical composition and the spray pattern into the combustion chamber is instrumental in determining the exhaust gases produced. The majority of engines are at their most efficient when cylinder pressures and temperature in the combustion chamber are high and when operating at an optimum loading.

When measured in the exhaust duct of a marine diesel engine, NOx emissions comprise about 95% nitric oxide (NO) and 5% nitrogen dioxide (NO2), which is formed as NO oxidises after the engine. The formation rate of the majority of nitric oxide is dependent on peak temperatures in the engine cylinders – above 1,200°C the formation is significant and above 1,500°C it becomes rapid.

A highly efficient engine will obviously reduce the amount of CO2 produced in relation to the power produced. However, such conditions are more likely to produce NOx when burning oil fuels. Reducing the temperature or pressure will reduce the amount of NOx produced but will inevitably result in a less efficient engine.

Ensuring engines meet the NOx limits is in the first instance down to the engine maker. The engine should come with a technical file and a certificate confirming the engine complies with the relevant limits. Thereafter, the owner has a choice of three methods of ensuring the engine continues to perform as required.
The first is the engine parameter check, under which it needs to be demonstrated that all those areas that influence NOx production remain in strict accordance with the engine maker’s original test bed condition as regards components, calibration, setting and operational parameters. Adopting this may mean that no change to engine settings can be made without it being accounted for in the technical file and it may mean that use of third-party spare parts is out of the question. The parts affected would probably include all those for the fuel injection system, camshaft, valves and valve timing, pistons, heads and liners, connecting rods and piston rods, charge air system and turbochargers, plus others depending on the engine type.

While some operators are quite happy to stick to OEM spares, others prefer cheaper pattern parts and for the latter there are two options to consider, namely the simplified measurement method or direct monitoring on board. Simplified measurement entails an effective repeat of the initial manufacturer’s test-bed certification procedure at every intermediate and special survey. This may involve specialist attendance. There is, however, no requirement that all parts on the engine need to be OEM parts.

Alternatively, direct measurement and monitoring is possible, using type-approved equipment available from a number of suppliers. Monitoring can either take the form of spot checks logged with other engine operating data on a regular basis and over the full range of engine operation, or monitoring can be continuous and the data stored.

A variety of technologies are used in the monitoring systems, most of which rely on traditional gas detection techniques. As is to be expected, each of the makers believes that its equipment (or the technology used in it) is the most appropriate. No system is perfect, however, and each of them could develop faults that would affect the accuracy of the test results. Probes and sensors can become clogged, affecting accuracy either way; leaks in the exhaust system and absorption of gases are also problems that have been identified.

To overcome this problem, the monitoring equipment needs to be calibrated on a regular basis to ensure that it is functioning correctly. The reliability of monitoring systems has improved over time as their use has expanded. When there was only a need to monitor NOx emissions most of the systems in use were set up to do just that. However, now that SOx scrubbers are becoming more common, so the makers of monitoring systems have enhanced their products to cover other regulated exhaust emissions.

The new breed of monitors come with other enhancements and at least one model on the market has a GPS input and can be programmed to send an alarm to the bridge when the vessel is close to a regulated emissions control zone in order that arrangements can be put in hand to ensure -compliance with the rules effective there.

It should be noted that the NOx limits apply to the engine and not the ship. A vessel which has replacement engines fitted will need to comply with the limits applicable at the time of the engine manufacture. There is also provision in the code for engines being obliged to comply with a higher Tier limits if the OEM produces means to make this possible. MAN Diesel & Turbo is one maker that has done this for a limited number of engine types. Meeting the NOx Code limits for Tier I and Tier II has been achieved without too much difficulty and for Tier III a number of options are being explored.