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SOx Emissions

SOx Rules (global and local)

SOx Rules (global and local)

Timeline: MARPOL Annex VI (SOx related)

  • 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 limit.

Emissions of SOx 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 and most recently by Norway. 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 15-20 years through to 2020 or 2025 (the later date was an option that was not taken up by the IMO which in 2016 settled for the earlier date).

Since all of the relevant dates in the IMO’s international regulations have now passed, there are just two sulphur content levels allowed (0.1% in ECAs and 0.5% elsewhere).

Although the regulations have set sulphur content in fuel as the main criteria, the regulation allow the use of technology to reduce the SOx content of the exhaust gases to a level equivalent to that which would occur had no abatement technology been employed.

Secondly, MARPOL Annex VI allows the designation of certain areas as emission control areas (ECAs) the first of which 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.

Seven years later the first of two ECAs in the Americas was established followed by another two years later. In these two ECAs, both NOx and SOx are controlled. These four regions are currently the only places were SOx is controlled by international regulation but there is talk of a new ECA being established in the Mediterranean and there are other places around the globe where local regulations exist. In these areas, the permitted level of sulphur allowed in fuels or the level of SOx emitted is significantly reduced.

EU SOx regulations

The basic EU legislation for regulating sulphur emissions from ships was Directive 1999/32/

  1. 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 1 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 that 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 periods of less than one hour or 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 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 and regulations, such as requiring use of fuel with 0.5% m/m sulphur content or below.

Exhaust gas cleaning systems

Exhaust gas cleaning systems

Exhaust gas cleaning

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 needing to be to consider when used ashore but they are of much greater importance for ship operators so these profitable side effects will not be available.

When SOx limits were first adopted by the IMO, it was initially intended that the only means of compliance would be to use fuels that met the regulations. 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 treating the exhaust gas stream prior to discharge to the atmosphere.

Most cleaning systems use water to scrub out or remove the sulphur compounds from the exhaust.  The washwater used will become contaminated with oil, soot and other material which can be treated in a separator to allow some of the water to be discharged overboard and the residue stored on board. Each country party to Annex VI needs to ensure that its port and terminal facilities can accommodate residues from exhaust gas cleaning systems.

The wording or MARPOL means that the decision whether to allow scrubbers to meet the emission requirements rests with flag states and, although none has yet declared against scrubbers, it is possible that their use may not be available to every vessel. Where scrubbers are allowed, MARPOL rules permit their use by setting equivalent emission limits in regulations 14.1 and 14.4 of ANNEX VI. These limits are expressed as a ratio of SO2 (ppm)/CO2 (% v/v) and work out at approximately 43.3 for each 1% of sulphur content in the fuel.

Wet scrubbing technology

In a wet scrubber, the sulphur oxides in the exhaust are passed through a water stream reacting with it to form sulphuric acid. In this way, they are removed from the exhaust gas which then passes out of the system. Sulphuric acid is highly corrosive but when diluted with sufficient alkaline seawater it is neutralised and the washwater can be discharged into the open sea after being treated in a separator to remove any sludge.

The alkalinity of seawater varies due to a number of reasons. In estuaries and close to land it may be brackish and closer to neutral and in some areas where underwater volcanic activity takes place the water may naturally be slightly acidic.

In the shipping sector, wet scrubbers are divided into two types: open-loop and closed-loop. These technologies were developed separately but are now often combined into a hybrid system that can employ the most appropriate technology depending upon prevailing circumstances. There is one type of wet scrubber that has been developed which combines wet scrubbing with membrane technology.

In an open-loop scrubber, seawater is used as the scrubbing and neutralising medium and no additional chemicals are required. The exhaust gas from the engine or boiler passes into the scrubber and is treated with seawater. The volume of seawater will depend upon engine size and power output and to a lesser extent on the pH of the seawater but equates approximately to around 40m3 per MWh meaning a quite high pumping capability is required. The system is around 98% effective and even allowing for fuel oil with 3.5% sulphur should have no problem reaching the maximum 0.1% 2015 ECA level.

An open loop system can work perfectly satisfactorily only when the seawater used for scrubbing has sufficient alkalinity. Fresh water and brackish water are not effective, neither is seawater at high ambient temperature. For this reason, an open loop scrubber is not considered as suitable technology for areas such as the Baltic where salinity levels are not high. MARPOL regulations require the washwater to be monitored before discharge to ensure that the pH value is not too low, which would indicate that it is still acidic.

A closed-loop scrubber works on similar principles to an open-loop system but instead of seawater it uses fresh water treated with a chemical (usually sodium hydroxide but some systems others) as the scrubbing medium. This converts the SOx from the exhaust gas stream into harmless sodium sulphate. Unlike the flow-through method of open loop scrubbers, the washwater from a closed loop scrubber passes into a process tank where it is cleaned before being recirculated. The fresh water can either be carried in tanks or else produced on board if a freshwater generator is installed on the ship.

In order to prevent build-up of sodium sulphate in the system, a small amount of washwater is moved at regular intervals either over side or to a holding tank and new freshwater added.

The volume of washwater required in a closed loop system is around half that of the open loop version but more tanks are required. These are a process or buffer tank in the circulation system, a holding tank where discharge to sea is prohibited and a storage tank able to have a controlled temperature between 20º and 50ºC for the sodium hydroxide which is usually used as a 50% aqueous solution. There must also be storage space for the dry sodium hydroxide.

The hybrid system is a combination of both wet types that will operate as an open-loop system where water conditions and discharge regulations allow and as a closed-loop system at other times. Hybrid systems were proving to be the most popular because they can cope with every situation but a surge of scrubber sales through 2019 appears to have favoured open-loop systems.

The wet systems are not the most compact pieces of equipment and would take up considerable space if it were necessary to install them in under deck machinery spaces. Fortunately, they can be installed in the funnel casing and can in some cases replace part of the conventional exhaust system.

Membrane option

At least one system maker has modified the technology to produce a membrane scrubber. Essentially the membrane scrubber is a wet scrubber but instead of the exhaust coming into direct contact with the scrubbing water in a spray or cascade system, nanoporous ceramic membrane separation tubes are used to extract SOx from the engine exhaust.

These tubes form an array that is suspended in the exhaust stream and a manifold system circulates the absorbent solution through the membrane tubes. Exhaust gases pass over the membranes where the SOx is dissolved into the absorbent solution. The ceramic tubes have temperature limits exceeding 800°C and the use of stainless steel ensures the acidic nature of the sulphur oxides does not corrode the membrane modules.

The maker of system,  says one of the benefits of membrane scrubbing is the amount of effluent resulting from the system is significantly lower than typical closed loop scrubbers. The absorbent solution discharge rate is much lower than existing closed loop scrubbers and allows the Membrane Scrubber to store the absorbed effluent onboard for discharge ashore.

If sodium hydroxide is used as the absorbent fluid, the effluent can be regenerated for reuse with a sulphuric acid by-product. Other absorbents such as potassium carbonate are converted into potassium sulphate which has a commercial value equal to the base absorbent making the system cost neutral in terms of consumables.

It is claimed that due to the smaller volume of discharge water and the reduced amount of exhaust gas contaminants that are absorbed, the discharge water cleaning is much simpler. Removing the exhaust contaminants generates a small amount of sludge that must be stored onboard as part of the vessel’s oily water. The amount of sludge generated is less than 0.05tonnes/MWh. In the system, seawater pumps provide cooling water to the heat exchanger to cool the circulating absorbent solution but no seawater is used for scrubbing the exhaust gases.

The membranes require periodic cleaning to remove soot fouling on the membrane outer surfaces. The frequency of cleaning is dependent on operating conditions of the engines and the membranes are cleaned by circulating the absorbent solution under pressure to ‘back wash’ the membranes. The cleaning solution sludge is sent to the general sludge tanks and the amount of sludge produced is said to be typical of an economiser cleaning. 

Dry scrubbing technology

Dry scrubbing is used extensively in shore-based power plants and can produce valuable by-products. An additional benefit is that the high temperature in the scrubber burns off any soot and oily residues. The first dry scrubbers used for ships were large and did not have the advantage of being able to be housed within the stack. This system is potentially still available but an alternative has been developed.

The first dry system employed pellets of hydrated lime to absorb sulphur, transforming them to gypsum. Although spent pellets need to remain on board for discharge at ports, they are not considered as waste because they can be used for fertiliser and to produce plasterboard among other things. The dry system has a lower power consumption than wet systems as no pumps are required. However, the weight of the unit is much higher than wet systems.

The version developed by Andritz is quite different. In the process, sodium bicarbonate (NaHCO3), commonly known as baking soda, is injected as a dry powder into the existing exhaust pipe. Due to the prevailing high temperature and adequate residence time, the NaHCO3 particle is activated, which increases the reactive surface by many times. This activation is necessary for the NaHCO3 to react with the sulphur components.

Such a process requires a temperature of at least 150°C but if the temperature of the exhaust gas stream from the engines is higher than 250°C, a quench is connected upstream, which brings the exhaust gas to the desired temperature by means of evaporative cooling.

The dosed exhaust passes through a filter on which other particles such as soot and ash are also deposited in addition to the sodium bicarbonate. A filter cake builds up on the filter cloth, and this is where the decisive chemical reaction takes place. SO2 reacts with NaHCO3 to form Na2SO4 (sodium sulphate), which is also present as a powder.

After a defined period of time or due to the maximum allowed pressure loss, the dust filter is cleaned by means of a pulse-jet process. During operation, a short stream of air is introduced at high pressure into the bag filter, whereby the filter cake peels off and drops into a collecting funnel. From there, the product is carried off by compressed air and stored in a silo for disposal ashore. 

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.

Scrubber practicalities

While scrubbers can allow ships to continue to make use of lower cost fuels with high sulphur content, they do not suit every operating strategy.

When the global sulphur cap was 3.5%, for a ship entering an ECA or any other area where sulphur is limited only on very few occasions, the capital outlay on a scrubber may not have been recouped in a reasonable period. However, even though the global cap level dropped to 0.5% in 2020, a ship with a scrubber will be able to operate continuously on high-sulphur fuel.

All scrubber systems require a treatment bypass for when the ship is operating without the need to use the scrubber. This prevents damage to the scrubber and reduces maintenance.

Care needs to be taken to ensure that the scrubber is not causing backpressure to the engine as this could be damaging and will affect NOx reduction systems. Scrubbers are increasingly being fitted to newbuildings but the majority now in operation have been retrofits. The time for a retrofit is currently more than a typical scheduled drydocking, meaning that extra lost earning days add to the capital outlay. However, as experience with scrubber installation grows, so the time needed for installation is reducing.

In some instances, times of 11-14 days are being quoted, although this pre-supposes that surveying the installation site, measuring and prefabricating the units has been done before hand.

The capital cost of scrubbers is currently high at between $500,000 to $5M depending upon maker and vessel. Payback time for a scrubber depends upon three variables; the capital and installation cost of the system, annual fuel consumption and the price differential between distillate fuel and the normal fuel used on the vessel.

Take-up rates for scrubbers may be improved if flag states and others offered state aid or attractive financial deals. So far, aid has been limited to a small number of projects in Europe and some finance houses have begun offering schemes that assist with capital expenditure and which link repayments to savings made. Some shipowners have even gone so far as to take significant holdings in a scrubber manufacturer.

The number of ships fitted with scrubbers is growing and, with it, the number of scrubber manufacturers. At the time when the first commercial system was fitted in 2006, there were just a few organisations interested in the potential. Today the number of makers active in the field exceeds 20 and newcomers are appearing regularly.

There is however some resistance to their use from within and from outside the industry with some saying scrubbers are shifting pollution from the air to the sea. Some shipowners that disagree with this view have formed an alliance to promote scrubbers under the banner of the Clean Shipping Alliance 2020.

Proving scrubber performance

Flag states that decide to permit scrubbers on board ships will need to ensure that operators can prove compliance. Under MARPOL Annex VI Regulation 4, there are two schemes allowed for a system to be permitted that mirror the requirements for NOx compliance.

One demands that the performance of any scrubber is certified before use and, as with the NOx systems, providing it is always operated within approved parameters there is no need for continuous exhaust emission measurements on the ship.

Parameters that must be continuously recorded include scrubbing water pressure and flow rate at the scrubber inlet, exhaust pressure before the scrubber and the pressure drop, fuel oil combustion equipment load and exhaust gas temperature either side of the scrubber. A record of chemical consumption must also be maintained. Under the second scheme, the exhaust gas must be continuously monitored when the equipment is in use and there is no need for the system’s performance to be certified.

Under both schemes the condition of any washwater discharged to sea must be continuously monitored for acidity, turbidity and PAH (a measure of the harmful components of oil) and data logged against time and ship’s position. A test for nitrate content is also required at each renewal survey.

Washwater regulation

In May 2015 at MEPC 68 the meeting adopted amendments to the guidelines for exhaust gas cleaning systems which permits a calculation-based methodology for verification of washwater discharge criteria. The revision allows for calculation or modelling to verify the discharge of washwater pH at a point that is 4m from the point of discharge. This was to be reviewed after two years and, if necessary, changes made to the washwater discharge controls. However, any changes will apply only to new installations.

Wet scrubbers are good at removing particulate matter and soot which, although not currently regulated for specifically, are likely to be so in future. Typically, a scrubber will remove at least 500kg of particulate matter for every 100 tonnes of fuel oil burned and possibly more, depending on how much washwater is used.

These solids must be removed before the washwater is discharged overboard and to conserve space the system should have a separation phase included that removes as much of the water as possible before sending the sludge to be stored for later disposal ashore.

Despite being allowed under IMO regulation. scrubbers are a controversial solution to reducing SOx emissions. Open loop scrubbers are singled out as being the most polluting as the washwater is discharged direct to the sea. The argument is ongoing with both sides producing support for their position using scientific papers.

At the IMO, discussions are taking place within the MEPC and its PPR Sub-Committee. Most observers believe that the result will be a tightening of the guidelines and very likely the possibility that the guidelines will change from being voluntary to being mandated. Some form of monitoring equipment will then become compulsory and a small number of

companies are already producing monitoring systems. Rather than wait for the IMO and the scientific debate to be settled, some countries; notably Norway, Singapore and China, have introduced bans or restrictions on scrubber use in territorial waters. In addition, some port authorities have imposed their own restrictions.

Most P&I clubs maintain a list of current restrictions and these can usually be consulted by non-members. An example can be found here

Other SOx reduction techniques

Other SOx reduction techniques

Because the formation of SOx is solely dependent upon the sulphur level in the fuel being used, there are very limited options beyond exhaust gas cleaning for reducing SOx emissions. The most obvious option is to use a fuel with a low or zero sulphur content. 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 beyond what effect the regulation has on the price of fuels.

Only ships fitted with dual-fuel or pure gas 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.

A potential alternative to distillates is methanol which can be used by some existing engines with modification. There are a small number of ships that have been successfully running on methanol for some years and more are on order.

Methanol contains no sulphur so does not produce SOx although a small amount of pilot fuel – which can be HFO, MDO or MGO – is needed for ignition and these fuels may contain sulphur. Methanol has the added virtue of being a liquid at ambient temperature and can be stored and handled by most existing liquid fuel systems on ships. There are however, a number of safety issues around its toxicity that means careful thought must be given to its use. Research is currently progressing on other alternative sulphur-free fuels including ammonia, hydrogen and biofuels including a bio version of heavy fuel oil.

There are two factors that prevent owners from opting to run solely on oil 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. Before the 2020 changes were announced, only about 20% of all ship fuels were distillates the rest being HFO. Distillates for shipping compete with fuels for land use so increasing the quantity fivefold is a big ask.

Several economic analysts have forecast that the effect of the SOx regulation will extend to all areas of fuel use outside of shipping itself. This is because meeting the demand for distillate fuels will mean that refiners will either need to add desulphurisation plant to their

refineries or to refine much more crude oil. While some desulphurisation capacity has been  added the amount is considered well below that needed. If refiners are obliged to process more crude this will inevitably mean that a lot of residual fuel will be unsellable so the price of all distillates will have to rise. Furthermore, the distillates used by the shipping industry are also used by other forms of transport and power production thus competition will increase between the different industries driving up prices.

The disruption caused to shipping and industrial activity generally by the COVID-19 pandemic has meant that it has not been possible to test any of the forecasts made prior to 2020 with regards to fuel cost and availability.


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 could possibly become available at some point. These systems may be too large for shipboard use but could perhaps be used by bunker suppliers or by co-operatives of ship operators producing and storing compliant fuel ashore for use as needed.

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