Exhaust emissions from marine diesel engines mainly comprise nitrogen, oxygen, carbon dioxide (CO2) and water vapour, plus smaller quantities of nitrogen oxides, sulphur oxides, carbon monoxide, various hydrocarbons at different states of combustion and complex particulate matter (PM).

It is these smaller quantities, together with CO2, that are of most concern to human health and the environment. Adverse effects are experienced at local, regional and global levels.

Further information about these impacts, including contribution to climate change through increasing concentrations of CO2 in the atmosphere; respiratory damage; cancers and genetic mutation; and damage to the natural and built environment is available from Lloyd’s Register Marine.

The regulatory framework

In response to these impacts, the International Maritime Organization (IMO), through its Marine Environment Protection Committee (MEPC), introduced regulations for the prevention of air pollution under Annex VI of the MARPOL Convention.

Annex VI imposes a framework of mandatory limits on emissions of sulphur oxides (SOx) and nitrogen oxides (NOx) both globally and within designated sea areas, known as Emission Control Areas (ECAs). These are regions where neighbouring states have shown that emissions to air have particular impacts on human health and the environment. In parallel with Annex VI, a number of regional, national and local regulators have introduced their own controls, leading to a patchwork of regulatory requirements. Later in this article is an overview of MARPOL Annex VI.

Compliance options

As emission limits become more stringent, compliance becomes more challenging and costly. There are a number of ways to comply, each of which presents different technical and operational challenges. This guidance provides an understanding of the different compliance options and the practical challenges of implementing them on board ships.

Primary and secondary techniques

Within this guidance, we divide compliance options into primary and secondary techniques. Primary compliance techniques (covered in Sections 3 and 4) work by lowering SOx and NOx emissions at source. Examples include using low-sulphur fuel to reduce SOx or manipulating the combustion process to reduce NOx. Secondary compliance techniques lower SOx and NOx emission levels by treating the exhaust gas after combustion. In this guidance, we refer to them as exhaust gas treatment systems (EGTS). Treatment systems include ‘scrubbers’ for SOx and selective catalytic reduction (SCR) for NOx.

We have structured the guidance in this way for two reasons:

•  Feedback from the industry indicates that many operators are interested in primary options because they avoid the need to fit secondary compliance technologies

•  There are common challenges associated with secondary techniques for NOx and SOx

SOx compliance options

The MARPOL regulations are clear in that the default means of SOx compliance is to use low-sulphur fuel. This is the primary compliance option. There is a wide variety of low-sulphur fuels, including low-sulphur distillate oil, ‘hybrid’ fuel oil, liquefied natural gas (LNG), liquefied petroleum gas (LPG), biofuels, dimethyl ether (DME), ethane and methanol. It is also possible to de-sulphurise residual fuel. This is technically straightforward, although the energy demand and costs are high if using traditional techniques. There are technologies which can de-sulphurise high-sulphur fuels as they are delivered to the ship, or even on board the ship by washing the sulphur out of the fuel as it is transferred between the storage and service tanks.

The secondary compliance option for SOx is to use an exhaust gas treatment system, such as a scrubber, to de-sulphurise the exhaust gas and reduce SOx emissions to a level equivalent to the required fuel sulphur content. This offers the flexibility to operate on either low-sulphur fuels or higher sulphur fuels.

The choice of SOx compliance option depends on a number of factors, including the cost of compliant low-sulphur fuels, the capital expenditure (CAPEX) and operating expenditure (OPEX) of installing an alternative compliance method such as a scrubber, and the amount of time that the ship is expected to spend inside ECA-SOx.

NOx compliance options

Unlike SOx emissions, which are a product of fuel sulphur content, NOx emissions are created during the combustion process. How much NOx is created depends on the fuel being used; some fuels, such as LNG and methanol, have lower NOx emissions than traditional marine fuel oils. As with SOx, there are primary compliance techniques which achieve the required NOx emission levels and there are secondary compliance techniques that denitrify the flue gas. Ships built after 1 January 2016 will need to meet the more stringent Tier III NOx emission limits inside ECAs, designated to control NOx emissions (ECA-NOx).

As with any emerging technology, there is a steady stream of new innovations and developments. However, this guidance covers only techniques that have demonstrated compliance with emissions regulations. At the time of publishing this guidance, every effort was made to ensure that it reflects the current status of emissions compliance technologies and emission regulations. It will be updated regularly, and to download the latest version visit www.lr.org/eca

Air pollution

International, regional, national and local instruments regulate emissions of SOx, NOx and particulate matter from ships. In response to greater concern about air quality, the extent and complexity of regulations have increased while emissions limits have become tougher. Annex VI of the MARPOL Convention applies to all ships trading internationally and has been used as the basis for many other regional, national and local regulations.

Even the lowest limits for SOx and NOx emissions, either already in force or planned for ships engaged in international trade, are higher than equivalent emission limits for land-based industry located in areas where there are ECAs, when considered on the basis of sulphur content of fuel consumed or an engine’s NOx emissions in g/kWh. However, when considered on the basis of unit of emission per unit of transport work delivered (e.g. SOx per teu•km) the emissions of ships are generally lower than other forms of transportation due to shipping’s significantly higher transport efficiency.


With MARPOL Annex VI, the IMO has prioritised reduction of NOx and SOx emissions to air on the basis of their environmental and public health impacts. Shipping emits other environmentally damaging pollutants to air from machinery (such as carbon monoxide and particulate matter) but only NOx and SOx are subject to quantified emission limits. PM is also included within Annex VI, but is not quantified. There are industrial standards for PM emissions but these have not been adopted by the marine industry.

MARPOL applies to ships engaged in international trade; it does not apply to ships which do not trade internationally. Ships which trade entirely within the waters of a single state will be subject to the state’s national laws and regulations, or in some cases they will be subject to regional requirements which are outside the scope of MARPOL.

SOx emission regulations

MARPOL Annex VI Regulation 14

Regulation 14 limits fuel sulphur content to restrict SOx and PM emissions, and applies to all ships in service. The regulation specifies different limits for operating inside and outside ECAs for SOx (ECA-SOx) and these follow a stepped reduction over time. Four ECA-SOx are currently in force: in the Baltic; the North Sea (which includes the English Channel); the North American; and the US Caribbean.

MARPOL Annex VI Regulation 4

Regulation 4 allows flag administrations to approve alternative means of compliance with Regulation 14 that are at least as effective in terms of emissions reduction as the prescribed sulphur limits. This means that a ship may operate on fuel with a higher sulphur content than that allowed by the regulations, providing that SOx emissions are controlled to a level which is no higher than the levels emitted if using compliant fuel. If an alternative means of compliance such as a scrubber is fitted then it must be approved by the flag administration and this approval must be notified to the IMO.

Approval of ‘wet’ scrubbers as an alternative compliance method must generally be in accordance with the IMO Exhaust Gas Cleaning Systems Guidelines (MEPC 184(59) – 2009 Guidelines for Exhaust Gas Cleaning Systems).

The IMO MEPC 184(59) Guidelines specify two testing, survey, certification and verification schemes:

•  Scheme A: initial approval and certification of performance followed by in-service continuous monitoring of operating parameters plus daily spot checks of the SO2 / CO2 emission ratio

•  Scheme B: continuous monitoring of SO2 / CO2 emission ratio using an approved system with in-service daily spot checks of operating parameters

In either case, any washwater discharged to sea must also be continuously monitored. Interestingly, it is washwater discharge that has tended to be the most problematic part of scrubber approvals. There are no IMO guidelines for other alternative means of SOx compliance such as fuel washing, non-thermal plasma or even dry scrubbing. Therefore, approvals are considered on a case-by-case basis by flag administrations and their recognised organisations (ROs).

NOx emission regulations

MARPOL Annex VI Regulation 13

Regulation 13 limits NOx emissions from marine diesel engines. The limits are divided into three ‘tiers’. How these tiers apply depends on the ship’s construction date (or the installation date of additional or non-identical replacement engines) and the engine’s rated speed (n), as shown in Figure 1.

Tier I and Tier II limits apply to engines installed on ships built on or after 1 January 2000, and 1 January 2011, respectively. Tier III limits will apply to ships built on or after 1 January 2016, if operating within the North American and US Caribbean ECA-NOx. To date these are the only two declared ECA-NOx. Within any new ECA-NOx, Tier III will apply to ships built on or after the date that the ECA is circulated for adoption. Alternatively, the IMO may specify a later date when designating the ECA.

NOx Technical Code

The NOx Technical Code 2008 is a mandatory IMO code, which provides detailed requirements for the certification of engines with respect to NOx emissions.

Primary compliance techniques for SOx

MARPOL Annex VI Regulation 14 limits fuel sulphur content. Several low-sulphur fuels are available including low-sulphur distillate oil, ‘hybrid’ fuel oil, liquefied natural gas (LNG), liquefied petroleum gas (LPG), biofuels, dimethyl ether (DME), ethane and methanol. Residual fuel oil (RFO) can also be de-sulphurised. These are primary compliance techniques as they lower SOx emissions at source.

The science of SOx

SOx derives directly from fuel sulphur content. The sulphur is oxidised in the combustion chamber, forming principally sulphur dioxide (SO2) and sulphur trioxide (SO3), typically in a ratio of 15:1. The use of alkaline lubricants to protect the engine surfaces from corrosion converts a small (and relatively insignificant) proportion of the SOx to calcium sulphate. The sulphur emissions from the engine are essentially proportional to the sulphur content of the fuel.

Low-sulphur distillate oil

Due to the nature of crude oil and refinery operations, RFO meeting the 0.10% sulphur limit is not expected to be widely available. So it is anticipated that low-sulphur distillate oil (LSDO) will generally be used to comply. It is also the simplest way to comply. LSDO will normally consist of marine diesel oil (MDO) or marine gas oil (MGO); the terms MGO and MDO have no precise definition other than that both are distillates and therefore do not require heating before injection, whereas RFO, whatever the grade, does require heating.

Alternative low-sulphur fuels

Hybrid fuels

To meet the increasing demand for 0.10% sulphur marine fuel, several suppliers have developed new low-sulphur ‘hybrid’ fuels that are claimed to be more cost-effective than conventional distillate fuels like MGO. They are called hybrids because they combine properties of distillate and residual marine fuels. Typically, they have lower viscosity and density, and better ignition and combustion properties, than conventional RFO.

Due to the wide range of products in development and the relatively early stage of their development, this guidance doesn’t cover hybrid fuels in detail, but some factors to consider include:

•  Compatibility with other fuels, particularly residual fuels

•  Fuel segregation

•  Pour point, viscosity and heating requirements

•  Low density variations that may require separator plant adjustment

Positive characteristics of hybrid fuels include their improved combustion characteristics and low levels of metals and ash, in particular abrasive catalytic fines.


There are also biofuels that are low in sulphur; the most common are Fatty Acid Methyl Ester (FAME) types derived from vegetable oils. Second and third generation biofuels are expected to address some of the societal concerns relating to the supply of FAME-type fuels. Biofuels are generally very similar to petroleum distillate oils. There are materials compatibility and storage issues related to the use of biofuels.


LNG is low in sulphur and easily combusted in engines and boilers using mature and reliable technology. Gas engines are widely used in land-based industry and have been used in LNG carriers for many years. The IMO is developing a new code for gas as fuel – the IGF Code – but until this enters into force there is uncertainty over the legal framework for operators and designers to work within.

Converting existing ships to alternative fuels such as LNG is possible, and there is a lot of interest in this area in the North American market. However, conversions are expensive and technically challenging. Challenges include installing the fuel tank and containment systems, gas zoning and engine conversion.

Primary compliance techniques for NOx

All current marine engines can easily achieve Tier II compliance. Tier III compliance, however, will require significant changes to the engine using either the complex primary techniques discussed in this section or by using secondary exhaust gas treatment systems.

The science of NOx

The formation of NOx is complex. NOx is the collective term for Nitrogen dioxide (NO2) and Nitrous Oxide (NO). Nitrous Oxide is not NOx. Nitrogen is a natural element in the atmosphere and is also found in the chemical structure of some fuels. During the fuel combustion process, NOx is formed in the cylinder in three ways:

•  Thermal formation, as a result of the reaction between atmospheric nitrogen and oxygen at high temperatures

•  Fuel formation, as a result of the reaction between nitrogen in the fuel and oxygen

•  Prompt formation, as a result of complex reactions with hydrocarbons and atmospheric nitrogen

NOx is formed both at the initial stage of combustion in very high temperatures and later in the combustion process after a longer dwell time in the combustion chamber. Therefore, the formation of NOx requires both high temperatures and exposure time. The major component of NOx on exit from the ship is nitric oxide, which readily oxidises in the atmosphere.

The proportion of nitric oxide attributable to thermal and fuel formation depends on the combustion conditions, which in turn are determined by the combustion unit type, configuration and operation, together with the fuel’s grade and composition. Prompt formation can exceed thermal formation under certain conditions where combustion temperatures are low, residence time is short and combustion conditions are fuel-rich.

Lower combustion temperatures and lower combustion, atmospheric oxygen and nitrogen levels are the main approaches to reducing NOx emissions.

Gas as fuel

Fuel type is critical to engine NOx emissions performance. While the difference in NOx emissions between residual and distillate fuels is not dramatic, some of the alternative fuels listed in 3.3 can reduce NOx to a level where Tier III compliance can be achieved. Fuels such as natural gas (stored on-board as LNG or potentially compressed natural gas) can achieve Tier III NOx levels. However, this depends on the engine design; not all gas engines can achieve Tier III. Some of these design considerations include: Thermal cycle, methane slip, and dual fuel engines.

Exhaust gas recirculation

Exhaust gas recirculation (EGR) is a mature technology, in which exhaust gas is fed back into the cylinder air intake, lowering oxygen and increasing CO2. This slows combustion and reduces temperature, lowering NOx.

Advanced fuel injection

In the longer term, the most promising primary NOx compliance option is advanced fuel injection, using split or pulsed injection to control placement of fuel and the combustion characteristics within the cylinder. This technology is not available for marine engines yet but indicates future possibilities; it is already widely used in automotive engines.

Advanced fuel injection allows better control of cylinder pressures and heat release rates, establishing separate heat release stages so that cylinder temperatures and NOx formation are reduced. It may be used with a simple EGR system to achieve Tier III compliance. It is also technically possible that advanced fuel injection could achieve Tier III with no EGR or secondary exhaust treatment device.

Fuel emulsification

Fuel emulsification has been recognised as an effective way of reducing NOx emissions for many years. Forming a stable and homogeneous emulsion can be challenging (particularly with distillate fuels) but it can be done. While achieving Tier III compliance using emulsification alone is proving to be challenging, it could be used with other techniques such as mild EGR or high-pressure super charging to achieve Tier III.

High-pressure supercharging

Since combustion temperatures are related to the compression ratio of internal combustion engines, reducing this compression ratio can lower temperature and reduce NOx. This can be achieved by high-pressure supercharging using multi-stage turbochargers and by applying the Miller thermal cycle.

In a Miller engine, the air inlet valves remain open for much longer than in a Diesel or Otto engine, with the result that typically only 70-80% of the upward piston stroke is compressing the charge air or pre-mixed charge air and fuel. While this is unlikely to achieve Tier III emissions compliance by itself, it can be used in conjunction with other techniques.


Looking ahead there two key years: 2016 and 2018. Ships built after 1 January 2016 will need to comply with NOx ‘Tier III’ limits when trading to the US or Canada and we explore some of the technological compliance options. Other ECAs for NOx may be introduced in the future, but these will only affect newbuild ships.

In 2018, the IMO will publish a fuel availability study, determining whether the global 0.50% sulphur limit will enter into force in 2020 or 2025. If 2020, the implications will be widespread: a possible rapid uptake of scrubber technology (with a question mark over whether supply will meet demand) and a dramatic increase in operational costs for those who choose to operate on distillate fuels. Early adopters of LNG-as-fuel could start seeing a real return on their investment and any ‘LNG-ready’ ships may start converting to LNG, with a parallel drive towards bunkering infrastructure development.

The time for decisions is fast approaching. In 2012, the industry needed to start considering their options; today, in 2015, this time is running out. The compliance options are clear. Ship operators need to evaluate their compliance strategies based on their ships’ specific operational and risk criteria. This evaluation needs to be unbiased and separate from any vested interests.

Published: 16th Sep 2015 in AWE International