NOx emissions

Updated 11 Oct 2019

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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 regulation

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 two main elements in fossil fuels, be they oil or gases such as LNG ethane or methane, are carbon and hydrogen. These combine with oxygen in the air to form mostly carbon dioxide, carbon monoxide and water vapour. The main element in air (approximately 79%) is nitrogen and this will also combine with oxygen to form NOx compounds. The amount of NOx is dependent on the temperature in the combustion chamber. Because oil fuels burn at a higher temperature than LNG for example, more NOx is formed with oil than with LNG.

The regulation of ship exhausts is one of the more recent aspects of MARPOL and is addressed by the last of the six annexes to the convention. It does not regulate all of the exhaust gases or products; NOx and SOx are 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 on 19 May 2005. The resulting regulation has been piecemeal and arguably flawed because each of the different gases requires different treatment and controlling some can affect the production of others. It allows for regulation of exhaust gases by setting limits on their emissions generally, 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 that produces NOx are subject to a levy. The levy is not confined to shipping; air and rail traffic are 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, such as the conversion of the Danish tanker Bit Viking to run on LNG and many offshore projects involving LNG and battery power.

The US basically follows IMO regulations for NOx but domestic ships there have also to comply with local EPA regulations. China is a recent addition to the list of states with local rules. It issued a regulation effective as of 1 September 2018 with validity of five years, that requires both international and national carriers operating within Chinese domestic waterways to comply, as a minimum, with IMO Tier II NOx requirements. This is mainly in effort to prevent old Tier I ships being bought and for operation in China.

The NOx Code
Nitrogen oxides, known collectively as NOx, formed the first class of exhaust gas to be regulated and is given particular attention because of the technical complexities involved with it. As a consequence, the largest part of Annex VI is the NOx Technical Code 2008, or ‘NOx Code’. This 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 so the allowed limits for each stage of the IMO’s 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, a ship 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 and 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 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 paragraph 14 in these words:

“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 ga- 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.

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, instead of fixed absolutes there are 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. Oil fuels burn at a higher temperature than LNG and so a greater quantity of NOx is produced from oil engines.

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 while 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 NOx 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, cylinder 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, which 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 these problems, the monitoring equipment needs to be calibrated on a regular basis to ensure that it is functioning correctly. Its reliability has improved over time as its use has expanded. For example, 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, 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 some have 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 that has replacement engines fitted will need to comply with the limits applicable at the time of the engine’s manufacture.

There is also provision in the code for engines being obliged to comply with a higher tier’s limits if the OEM offers a way to make this possible; MAN Energy Solutions 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 have been explored.

Methods for meeting the NOx Tier III targets

Engine makers have explored many avenues to find ways to meet the increasingly stringent NOx regulations. While there may be more yet to come, there are effectively five means that are attracting the majority of attention. These include:

  • Engine Tuning (Miller timing);
  • Fuel water emulsions or Direct Water Injection;
  • Air humidification;
  • Exhaust Gas Recirculation (EGR); and
  • Selective Catalytic Reduction (SCR) – up to 95% reduction – more difficult but achievable on low-speed diesels due to lower exhaust gas temperature – allows engine to be tuned for minimum fuel consumption.

In addition to these methods LNG-fuelled engines can achieve Tier III levels without treatment.

The first four options are under the control of engine manufacturers and will doubtless be incorporated into future engine models. Several makers have already announced Tier III-compliant engines but that does not mean that other methods will not also be made use of – not least because, with some of the options, there are drawbacks such as increased fuel consumption or sub-optimal operation.

Engine tuning
Engine tuning in a camshaft-controlled four-stroke marine diesel engine works by closing the inlet valve before the piston reaches bottom dead centre. This is called Miller timing and has the effect of lowering the cylinder pressure as the piston continues downwards and dropping the temperature of the air.

Although the engine is still doing work as the piston is descending on the inlet stroke, there is a saving in work during the compression stroke and the maximum air temperature and pressure is reduced on compression. In camshaftless engines with variable valve timing, Miller timing is easier to achieve.

However, the earlier closing of the inlet valve reduces the amount of air in the cylinder and this clearly affects engine efficiency. This can be overcome with the addition of two-stage turbocharging, which can achieve compression ratios twice that of single-stage turbocharging. In a two-stage turbocharger, the exhaust gas passes first through the turbine of a high-pressure turbocharger before being led to a low-pressure turbocharger. The charge air enters the system through the low-pressure turbocharger, passes through a cooler and is then further compressed in the high-pressure turbocharger before being again cooled and taken into the combustion chamber.

A variation of Miller timing can also be used for two-stroke engines with variable valve timing and two-stage turbocharging. In some engines it may be possible to meet the NOx Tier III requirements through the use of Miller timing alone but in some others an additional means may be needed.

Water cooling
Reducing the combustion temperature is also the concept behind adding water to the fuel. This can be done in any of three ways: as an emulsion, direction injection or by humid air. Emulsified fuels have a further benefit in that the injected fuel is present in smaller agglomerations than when pure fuel is injected. This means that there is more complete combustion and the expansion of the water present adds to the power generated. This can result in a fuel saving which some claim could be as high as 5%.

Emulsified fuels do sometimes generate a white plume in the exhaust which gives the appearance of smoke but is in fact harmless water vapour. The vapour also softens any soot in the exhaust and helps to reduce PM emissions.

One of the companies pioneering emulsified fuel equipment says that the reduction in NOx satisfies Tier II requirements but is insufficient to meet the more challenging Tier III requirements that came into force in January 2016. However, the company concerned does say that when combined with SCR, the reduction in NOx in the exhaust places less demand on the SCR equipment reducing its operating and maintenance costs. A similar effect is expected if a SOx scrubber is installed since the prior removal of PMs makes that equipment more effective.

Exhaust gas recirculation
The in-engine technique of EGR has been common in smaller road engines for some time and is now being rapidly adopted into marine engines. By re-circulating exhaust gas into the charge air, the oxygen content in the cylinder is reduced and the specific heat capacity increased. Both cause lower combustion temperatures and therefore fewer NOx emissions. If high-sulphur fuel is to be used, EGR can also be combined with an exhaust gas scrubber after the main engine exhaust receiver to achieve full SOx compliance in an ECA.

In a typical EGR system, a proportion of around 40% of the exhaust gas from the main engine exhaust receiver, instead of being directed to the turbocharger, is passed through a dedicated closed loop scrubber which removes PM and SOx which could cause engine damage and cools the exhaust gas to be re-circulated. The re-circulated gases cause oxygen as O2 in the scavenge air to be replaced with CO2 which has a higher heat capacity and so helps reduce peak temperatures in the cylinder.

The reduced O2 content in the scavenge air also reduces the combustion speed, which further reduces peak temperatures in the cylinder which reduces the formation of NO and therefore NOx, helping towards Tier III compliance.

Tier III only applies when vessels subject to the rules are operating in ECAs that limit NOx emissions. When outside of such areas, engines need only meet Tier II standards and this makes SCR a possibly attractive option. Presently, EGR is a slightly more expensive option than SCR, especially for smaller and mid-size engines. The contaminated water from the scrubber must also be cleaned and the sludge generated disposed of ashore which usually involves an additional extra cost.

At MEPC 73, new guidelines for EGR washwater were adopted. However, the increased use of SOx scrubbers has generated more interest in the composition of washwater and MEPC 75 in March 2020 will revisit the issue.

Selective catalytic reduction
SCR systems are arguably the more conventional way of reducing NOx as systems have been in place for several years on some vessels operating in northern Europe, particularly Norway where the NOx levy is in place.

In an SCR system a reducing agent (usually gaseous ammonia, aqueous ammonia or aqueous urea solution) is added into the stream of exhaust gas. The exhaust gases and reducing agent at a temperature of 300°C to 400ºC are absorbed onto a catalytic surface, upon which the nitrogen oxides are transformed into nitrogen (N2) and water (H2O). When urea is used, CO2 is also formed during the process. This was not an issue when the main purpose was to reduce NOx but it does become problematical when attempting to meet the EEDI requirements aimed at reducing CO2.

SCR is capable of removing up to 99% of the NOx, which is comfortably in excess of the 80% reduction from Tier I levels required under Tier III. But SCR systems are not foolproof: if the exhaust gas temperature is too high, the ammonia burns rather than forming a compound with nitric oxide. If it is too low, it forms ammonium hydrogen sulphate and gradually blocks the catalytic converter. The same happens if the sulphur content of the exhaust gas is too high. The minimum temperature required depends on the fuel’s sulphur content.

The catalyst in an SCR system consists of a ceramic carrier with the active catalyst an oxide of a metal such as tungsten or vanadium.

SCR systems are separate from the engine and, although leading engine makers are involved in their development, there are also independent suppliers. Some of the companies producing SCR systems have formed a trade body known as the International Association for Catalytic Control of Ship Emissions to Air (IACCSEA).

SCR systems do have a relatively high capital cost and annual running costs to take into account. The catalyst will need replacing at intervals of around four to five years but because the catalysts are arranged in a layered system which allows for only damaged catalysts to be identified, removed and exchanged it is not necessary to replace the entire catalyst at the same time.

A limiting factor in the take-up of SCR – beyond the fact that the need for them is really only just beginning – has been the size and weight of the systems and the need to carry sufficient supplies of ammonia (normally in the form of urea). Even on the smallest ship type, the reagent storage tanks would likely need to be 5m³ and on a large tanker, bulker or container ship possibly ten times larger than that. As regards the requirements of the NOx Technical Code, a ship fitted with an SCR system will need to also be fitted with continuous monitoring equipment to prove compliance.

Continual development to improve technology
The NOx Tier III levels came into effect for ships built from the beginning of 2016 meaning some ships have already needed to be compliant. However, this does not mean that the technologies employed are now mature and indeed some would argue that is very far from being the case.

The methods being used are in their early production stages and the intention is to refine them to reduce size, cost implications and robustness and reliability. In addition, the impending global reduction in fuel sulphur levels due in 2020 will mean that exhaust gas cleaning systems capable of dealing with both NOx and SOx will need to be production ready in the not too distant future.

And as has already been mentioned, NOx reduction almost always results in increased CO2 production. Even with an SCR system that does not need a lower engine operating temperature and so does not reduce efficiency in that connection, extra weight and pumping systems are needed and lead to an increased power demand.

Alternative fuels
All of the technologies previously discussed are based on ships continuing to burn oil fuels. It is possible to use fuels that do not produce NOx or do so in much smaller quantities. LNG is often proposed as the ideal solution to reduce NOx emissions and while it is true that the level of NOx from a gas burning engine is very low, it is only a solution for ships equipped with pure gas or dual-fuel engines and for steam turbine powered LNG carriers.

While LNG has been used as a marine fuel since the early days of LNG carriers, it is mainly in connection with steam turbine systems where the boil-off gas from the cargo provides the fuel for the steam turbine boilers. Several LNG carriers have now been built with dual-fuel diesel engines and the number of other ship types has expanded quite rapidly. The take-up of LNG is still behind early predictions but the advent of the IMO’s IGF Code, which lays down international rules for gas-fuelled vessels, has meant that fuel storage and delivery systems are accepted universally rather than by specific flag states. The take-up is being further accelerated as an LNG bunkering infrastructure takes shape.

Aside from LNG, a series of large ethane carriers have been built with dual fuel engines able to burn ethane also supplied as a boil off gas from the cargo. This is very much a niche fuel but there is also interest and pioneering developments in using methanol and ethanol, the liquid alcohol derivatives of methane and ethane, as marine fuels. There are issues to be overcome with these fuels if they are intended for wider commercial use because of fuel density issues which make them best suited for short voyages.

It should be noted that although the term LNG is equated with methane, the actual composition of LNG cargoes and potentially commercially-supplied bunkers is variable and it is possible for an LNG cargo to contain high quantities of nitrogen and other chemicals. Because nitrogen has a much lower boil-off point than methane, it is likely that most nitrogen in a cargo will boil off early in the voyage and so NOx emissions will reduce as the voyage progresses.

As experience with LNG engines grows, it may be that some problems come to light. Some believe that in order to achieve maximum efficiency from a gas-burning engine it will be necessary to constantly monitor the fuel composition and adjust engine parameters accordingly. Equipment that can do this is not yet commercially available but is under development.

It is possible for some existing diesel engines to be converted to run on LNG but this is a major conversion and one that would have to be evaluated weighing up many factors. Only a few conversions have been completed or are in the pipeline but the advent of the 2020 sulphur cap may encourage more owners with suitable candidates to consider conversions.

In anticipation of operators wishing to convert engines to dual-fuel versions, leading engine makers have for some time been developing engines with a high degree of modularisation. Most dual-fuel engines have been based upon an existing diesel engine and it is therefore easier to convert one of the base diesel engines to a gas burner because of this. Conversion of older engines is probably not economically viable. It needs to be borne in mind that a converted engine will also need new fuel storage and conditioning systems to be installed.

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