SOx reduction in exhaust emissions

Malcolm Latarche
Malcolm Latarche

11 November 2017

After NOx, the next family of exhaust gases to attract regulatory attention was sulphur oxides or SOx. Emissions are covered by the same MARPOL Annex VI as NOx but the rules and the timelines involved are quite different. As with NOx, individual states are free to impose different regulations on vessels entering their ports or waters and this has been done in several areas of the world, notably the EU and China.

The Regulation of SOx began when Annex VI became effective in 2005. This was done in two ways. Firstly, the level of sulphur permitted in fuels was regulated according to a reducing level over a period of 15 to twenty years through to 2020 or 2025. The permitted levels are as follows:

Secondly, MARPOL Annex VI allows the designation of certain areas as emission control areas (ECAs) the first of these was the Baltic Sea which was designated as a SECA (sulphur emission control area) in 2005 with the rules applying from 2006. A second SECA – The North Sea SECA was established the following year. In these areas, the permitted level of sulphur allowed in fuels is significantly reduced.

The timeline for SOx regulation covered in Annex VI is as follows:

  • 26 Sept 1997 - Annex VI formally adopted
  • 19 May 2005 - Annex VI enters into force Sulphur level in fuel set at – 4.5% global, 1.5% ECA
  • 19 May 2006 - Baltic Sea SECA established
  • 11 Aug 2007 - EU implements North Sea SECA prematurely
  • 21 Nov 2007 - Official IMO date for North Sea SECA
  • 17 Jul 2009 - MEPC approves proposed US/Canada ECA (SOx, NOx and PM)
  • 1 July 2010 - SOx 1.0% ECA
  • 1 July 2011 - MEPC approves proposed US Caribbean ECA (SOx, NOx and PM)
  • 1 Jan 2012 - SOx 3.5% global
  • 1 Aug 2012 - Implementation of US/Canada ECA
  • 1 Jan 2014 - Implementation of US Caribbean ECA
  • 1 Jan 2015 - SOx 0.1% ECA
  • October 2016 – MEPC 70 Completes of review into availability of low-sulphur fuel
  • 1 Jan 2020 - SOx 0.5% global

EU SOX Regulations

The basic EU legislation for regulating sulphur emissions from ships was Directive 1999/32/EC. This was amended by Directive 2005/33/EC, which designated the Baltic Sea, the North Sea and the English Channel as sulphur emission control areas (SECAs) approved and adopted by the IMO and limited the maximum sulphur content of the fuels used by ships operating in these sea areas to 1.5%. The same fuel standards also applied to passenger ships operating on regular service in EU waters outside of the controlled areas.

From 1st January 2010 when the 2005 directive came into force, as well as reinforcing the limits of sulphur for vessels operating in SECAs and limiting the sulphur content of fuels used ashore in the EU, it also introduced legislation governing the maximum sulphur content of fuels used by inland waterway vessels and ships at berth in Ports which are part of the European Community. The limit for ships in ports was set at 0.1% Sulphur, which is the maximum sulphur content of Gas Oil under ISO standards.

The rules permit some leeway in that they allow ‘sufficient time’ for the crew to complete any necessary fuel changeover operation as soon as possible after arrival at the berth and as late as possible before departure. Ships in port for period less than one hour of those that connect to shore electricity supplies are exempt from the requirement.

Considering the IMO timeline for sulphur levels, the EU is now only out of step in applying the 0.1% limit to ports outside of the two SECA zones.

Just as the EU was premature in imposing regulation in the two European SECAs, it has also done this in respect of reducing the global cap from 3.5% to 0.5%. Although the IMO only took that decision in 2016, EU Directive 2012/33/EU had already laid down a maximum sulphur content of 0.5% for fuel used outside the European ECAs in the territorial waters and exclusive economic zones of EU member countries for the year 2020.

Chinese Regulation

China is another country that has imposed its own regulation with the Air Pollution Control (Marine Light Diesel) Regulation 01/04/2014. This introduced a new sulphur content cap of 0.05% for the locally supplied marine light diesel (MLD). In addition, Hong Kong’s Environmental Protection Department has required all ocean-going vessels to use low-sulphur fuel, defined in the new legislation as fuel with sulphur content not exceeding 0.5% by weight, when at berth in Hong Kong waters. All such ships must initiate fuel switch upon arrival at berth, complete the switch to low sulphur fuel within one hour, then use low sulphur fuel throughout the berthing period until one hour after departure.

On 4 December 2015, China announced the establishment of further ship ECAs in the Pearl River Delta, the Yangtze River Delta and the Bohai Bay rim area. The regulation applies to all merchant ships navigating, anchored or under operation in the waters of the control areas. With effect from 1 January 2016, ships were required to follow the requirements current international conventions or local laws/regulations (whichever is stricter) on the emission control of SOx, particulates and NOx. If the port condition allows, ports within control areas may implement stricter requirements than current conventions, regulations such as requiring use of fuel with 0.5% m/m Sulphur content or below.
The implementation schedule for the Chinese requirements is:

  • 1 January 2016, some ports (if the port condition allows) within the control areas may implement the requirement for use of fuel with 0.5% m/m sulphur content or below when ships are alongside or at anchor. Note that this is for any port within the control area, not just the key/core ports;
  • 1 January 2017, key/core ports of control areas implement the requirements for use of fuel with 0.5% m/m sulphur content or below when ships are alongside or at anchor;
  • 1 January 2018, all ports within control areas shall implement requirements for use of fuel with 0.5% m/m sulphur content or below when ships are alongside or at anchor; and
  • 1 January 2019, ships entering into control areas shall use fuel with 0.5% m/m sulphur content or below. It should be noted that following an assessment of the effects of the above actions China will possibly implement requirements for use of fuel with 0.1% m/m Sulphur content or below after 31 December 2019. The requirements for ships at berth or at anchor are applicable from one hour after ships are berthed to one hour before departure. Ships may use other alternative measures to reduce emissions, such as shore power, clean energy systems or scrubbers.

A costly choice for shipping

The regulations concerning SOx reduction in exhaust emissions is potentially the most expensive regulation that shipping has ever had to meet. More to the point, the effect of SOx regulations are felt by almost every vessel afloat regardless of age. This is because, unlike the NOx rules which apply to the engine rather than the ship, SOx rules fall on the ship itself.

As things stand the only step in the MARPOL Annex VI timetable still to be taken is the final reduction in the global cap from 3.5% to 0.5%. The impact of this and the controversy surrounding the IMO’s acceptance of the 2016 review has been covered in a special ShipInsight Report on the 2020 Cap which can be accessed here. This will be a particularly expensive exercise for the shipping industry as none of the options for meeting the global cap are cheap. They include making use of expensive fuels or installing exhaust gas cleaning systems.

Ways to SOx compliance

Fiel choice

MARPOL sets limits by mass for the sulphur content of fuels as the primary means for controlling SOx emissions from ships. Reducing SOx levels in exhaust emissions can come about in one of two ways. Either the sulphur level in fuel has to be reduced or abatement technology – commonly referred to as scrubbing – has to be employed. Unlike with NOx, there are no adjustments that engine manufacturers can make but the use of low sulphur fuel requires additional precautions that need to be taken in the choice of engine lubricants.

Because it is purely a product of the combustion process, SOx is only an issue for vessels burning residual fuels either in diesel engines or in boilers. Ships that operate purely on low-sulphur distillates, LNG or any of the newer gas fuels that do not contain sulphur are not affected by any of the regulations controlling SOx and are saved the additional expense of complying with the requirements of MARPOL.

Only ships fitted with dual-fuel engines can use LNG as an alternative to oil fuels. For all other vessels, the only fuels that allow compliance without some form of treatment are distillates. Almost all diesel engines can run on distillates without modification and indeed it is quite normal to do so when manoeuvring in ports in any case.

There are two factors that prevent owners from opting to run solely on distillate fuels, the first is price and the second is availability. Distillate typically cost between 50% and 100% more than residual fuels making them an expensive option when bunker fuel already accounts for between 30% and 50% of running costs. Availability is not a problem when the normal quantities of distillates used by ships are considered but increasing availability could be a huge problem. Only about 20% of all ship fuels are distillates the rest being HFO. Distillates for shipping compete with fuels for land use so increasing the quantity five-fold is a big ask can almost certainly not achievable by current refineries.


One of the questions most commonly asked about fuels is whether the sulphur can be extracted from the fuel before use. This is possible in refineries but it is an expensive process and the equipment is not available in all refineries. The refining sector is under no obligation to shipping to provide compliant fuels but operators must become compliant when the global cap comes into effect.

Small scale desulphurisation plant is in the process of being developed and will likely become available before the 2020 date arrives. Some of the systems being developed were covered in an Article in the ShipInsight Journal (link to Innovations piece from last journal). These systems may be too large for shipboard use but could be used by bunker suppliers or by co-operatives of operators producing and storing compliant fuel ashore for use as needed.

Exhaust gas cleaning

It was initially intended that the only means of compliance would be to use fuels that met the regulations but under pressure from ship operators it was agreed that abatement technology would also be permitted and in 2009, the MEPC.184(59) guidelines for Exhaust Gas Cleaning Systems (EGCS) were adopted. These guidelines enable a ship to achieve low-sulphur requirements by water washing the exhaust gas stream prior to discharge to the atmosphere. Each country party to Annex VI needs to ensure that its port and terminal facilities can accommodate residues from exhaust gas cleaning systems.

Long before SOx was considered an issue for the shipping industry it was being regulated around the globe in connection with the use of large diesel engines in power production and other shore-based industries. As a consequence, exhaust gas cleaning or scrubbing technology is already a long established reality in shore-based situations cleaning up emissions from oil and coal-based power plants.

When fuel oil containing sulphur is burned in the presence of air, the sulphur in the fuel combines with oxygen to form sulphur oxides. In a scrubber, the sulphur oxides in the exhaust are removed from the exhaust gas which then passes out of the system.

The technology falls into two distinct basic categories – wet and dry. In shore-based scenarios, scrubbers not only remove SOx from exhaust gases but in doing so the by-products are used in the production of plasterboards for use in the construction industry. Space limitations and power consumption of the scrubbing equipment are rarely factors to consider when used ashore but they are of much greater importance for ship operators so these profitable side effects will not be available.

There are several makers of marine scrubbing systems, most of whom are members of a trade body known as the Exhaust Gas Cleaning Systems Association (EGCSA). Their products operate on similar lines to shore-based systems although the use of dry systems is limited to a choice of one or two.