Environmental Technology

Two decades of NOx reduction

Malcolm Latarche
Malcolm Latarche

08 November 2018

Two decades of NOx reduction

Whenever a substance is burned as a fuel in an internal combustion engine, a string of chemical reactions take place. Some of these reactions occur in the combustion chamber itself and others as the exhaust gases are released into the atmosphere. The reactions and their resulting products depend upon the chemical composition of the fuel, the amount of air available to support the combustion process and the temperature of the fuel and the air.

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. It should be noted that the date of the engine’s manufacture is the deciding factor so that for example ships built prior to 2011 only ever needs to comply with the Tier I limits so long as it retains its original engines and they are not modified.

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 Tier III limits may well be extended to other areas in the future at the request of port states but the applicable dates will be dependent on when the ECA is approved.

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.”

In addition, MEPC.1/Circ.854 gives further guidance on how the different methods of ignition for dual-fuel and gas engines should be assessed under the NOx Code.

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.