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Explaining the types of fuel used on ships

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There has never been a time in shipping’s history when there have been more fuel types available than there are today. Although coal-powered ships are now almost entirely heritage vessels, the available fuel mix today includes all the standard types of mineral oils, vegetable and animal based bio-fuels, LNG, ethane, methanol and to a lesser extent hydrogen and battery power.

Describing a battery as a fuel type is technically incorrect since it is rather a means of storing electrical power that could be generated in a number of ways. The battery powered ferries trading in Norway for example, recharge their batteries from the shore grid and in Norway most electrical power is from hydro.

A high number of ships that are now or will in the future be equipped with batteries will be using them as a means of peak shaving, storing excess power when possible and making use of it at times of high demand instead of bringing a further generator on line. There are other possible means of energy storage such as flywheels but these are currently not considered as anything other than experimental in commercial shipping circles.

It is anticipated that environmental regulation will at some point see fuel oils displaced in favour of supposedly cleaner fuels such as LNG and methanol although that view has held sway for almost a decade and has not yet proved to be correct.

At some future point – perhaps following the 2020 global cap on sulphur the variety of fuel available may well be much wider than it is today but most ships today and more than 90% of those on order are intended to burn one or more grades of oil refined from crude oil.

The process of refining involves heating the crude oil with or without the use of a catalyst. As the oil is heated fuels are drawn off at different temperatures. From the point of view of marine fuels, the first types to be of use are marine gas oil (MGO) and marine diesel oil (MDO). These are the distillate fuels and used mostly in high and medium speed engines and gensets. They have a low viscosity and flash point and a lower energy content measured by volume (by weight they have a higher energy content) than more viscous fuels but are generally cleaner and produce less polluting emissions.

The heavier fuels that remain after all other useable fuel types have been drawn off are the residual fuels and are highly viscous. The fuels are used only in low and medium speed engines but require heating in order to reduce their viscosity allowing them to be pumped along the fuel system and injected into the combustion chamber. Such fuels have a higher energy content by volume but can also retain many more of the pollutants from the crude oil. When refined using the catalytic cracking process there may also be catalytic or cat fines present. Cat fines are particles of aluminium and silicon oxides that are remnants of the catalyst used. They are hard and extremely abrasive and damaging to engines.

The current International Standard for an acceptable level of cat fines is set at a maximum of 60mg/kg, or parts per million (ppm). Most engine manufacturers would expect levels to be around 15ppm for their machinery to operate without unusual wear and tear but it is reported that average levels are around 22ppm meaning removal during the fuel treatment process on board is highly desirable.

There is an ISO standard for marine fuels which is updated at regular intervals. Work on the fourth edition began in March 2008, about the same time that the IMO requested ISO to prepare a specification for marine fuels to coincide with the implementation of the Revised MARPOL Annex VI on 1st July 2010. The next version appeared in 2012 (5th edition) and the most recent in March 2017 (6th edition). While these standards exist there is no obligation on freely contracting parties to accept only the latest or indeed any standard whatsoever. It is still a fact that the vast majority of bunker supplies are made in accordance with the earlier 8217:2005 standard. It is important when ordering fuels to stipulate exactly which standard should apply.


Bio-diesel is a catch all term for a wide variety of products. It is possible to produce a bio-diesel from plant material, animal material and various combinations of both. Often a small quantity of bio-diesel can be added to mineral diesel to produce a more stable fuel. There are few cases of biodiesel being used on a commercial scale in large marine engines but its use in leisure engines is more widespread.

Likely the largest use of bio-diesel in commercial marine is by the Meriaura Group of companies based in Finland. The group has shipping interests and has a small number of 5,000dwt dry cargo vessels that make use of the EcoFuel produced by another subsidiary VG Marine.

Using a process developed by another Finnish company – Sybimar – the waste stream of fish and aquaculture plants is converted into EcoFuel which being sulphur-free is ideal for use by the ships operating in the Baltic and North Sea SECAs. EcoFuel can be used directly as heavy marine fuel or in a blend with fossil diesel, or it can be processed to light marine fuel. It is also used as environmentally friendly heating oil. The raw materials of VG Marine EcoFuel are recycled vegetable oils and fish processing residues.

Some trials of another bio-diesels have been carried out on ships, with Maersk Line in particular being an enthusiastic pioneer. Fuels derived from algae may have some potential as might fuels derived from lignin a vegetable product that remains after other useful products have been extracted. Presently lignin is used mostly as a soil improver.

Volvo Penta has some comments on the use of bio-diesel in leisure engines which could in some cases apply equally to larger marine engines. The comments are:

  • The biodiesel must be of good quality, which means that it must comply with the EU’s EN14214 fuel standard.
  • Biodiesel is an efficient solvent that can, when first used, dissolve constituents in the fuel system. The fuel filter should therefore be changed after a short period of usage.
  • Biodiesel is not a fuel with long-term stability, it can oxidize in the fuel system. The entire fuel system must be emptied and operated on normal diesel before any extended period of still standing, such as during winter storage.
  • Biodiesel has a negative effect on many rubber and plastic materials. Rubber hoses and plastic components in the fuel system must be checked regularly and changed at more frequent intervals than usual to avoid leakage.
  • Biodiesel impairs the lubricating capacity of oil due to its higher boiling point. The intervals for changing lubricating oils and oil filters must be halved compared with normal.

Making use of waste products to manufacture bio-fuels may be acceptable but with a growing world population it may be difficult to make a case for diverting food grade oils or using crop land for fuel production. In addition, even when crude oil prices were at their peak, the cost of producing potential bio fuels made their use economically questionable. Given the decline in crude oil prices by 60% from the peak makes the cost even more unrealistic.

Low-sulphur choices

The IMO has decided that the final reduction of permitted sulphur levels in fuels currently regulated under MARPOL Annex VI will take place in 2020. It is not certain if the refining industry will accommodate that date by producing low-sulphur fuels in the needed quantity and if it does not, then the quantity of distillates, which will be the only option to ships without scrubbers or able to run on LNG, may also be well below what is needed for the shipping industry to function.

The report on which the IMO based its decision to opt for a 2020 date, included data that showed the shipping industry used 228 million tonnes of heavy fuel oil in 2012 compared to just 65 million tonnes of marine grade distillates. Some major upgrades to refineries will be needed if that 228 million tonne figure is to be switched to low sulphur fuel oils or distillates – and it should not be forgotten that other uses for refinery products are also increasing demand.

The matter of actual availability rather than predicted availability is something that will be made clearer as the 2020 date draws closer. If there is a shortage then the IMO will need to rethink the regulation. In the meantime, ships operating in ECAs and some other regions where sulphur levels are limited must already meet a level of 0.1% which is below the 0.5% of the global cap.

There are some low sulphur fuel oils (LSFO) available today although he quantities available are not high and there are some newer ultra-low sulphur fuel oils (ULSFO) sometimes referred to as hybrid fuels developed to meet the 2015 reduction to 0.1% in ECAs. Early pioneers in developing ULSFOs were ExxonMobil (HDME 50), Shell (ULSFO), BP, SK Energy (SK ULSFO), Bunkers International, Chemoil and Lukoil.

The first generation of low-sulphur fuels was quickly followed by newer products. Bunkers International and partner CI Vanoil have a 0.1%S fuel oil-based product in Cartagena, Colombia. According to product specifications, the fuel has viscosity of 15-30 CST, 70º C flash point and a pour point of -3ºC.

In March 2015, ExxonMobil launched ExxonMobil Premium AFME 200 Marine Fuel joining the older ExxonMobil Premium HDME 50. Exxon-Mobil describes Premium AFME 200 as an advanced fuel oil formulated using a proprietary refining process that removes sulphur, metals and other contaminants. This enables the fuel to comply with the ECA sulphur cap and also helps to optimise the performance of engines and extend component life. The viscosity of ExxonMobil Premium AFME 200 is comparable to heavy fuel oils (HFOs) enabling similar storage and handling practices for both fuels on board vessels. Both fuels require preheating, therefore reducing the risk of thermal shock to engine components during switchovers to comply with the ECA sulphur cap. Thermal shock may result in fuel pump seizures and engine shutdowns and has the potential to occur when switching from heated HFO to marine gas oil (MGO) at ambient temperatures.

Generally LSFO is available in viscosities of 380csts and 180csts it is considered to have a sulphur content of 1.0%. This is too high for use in an ECA but ULSFO with a sulphur content of 0.1% meeting ECA requirements is available at some ports.

ULSFOs are specially formulated fuels developed by major bunker suppliers and typically have a premium price around 150% higher than standard IFO 380 fuels, the premium over IFO 180 is slightly less but still significant. At the present time, ULSFO fuels are used only in ECAs and special precautions are needed during switchovers.
This will be less of a problem if fuels with a sulphur content of 0.5% do become readily available as the switchover will be between fuels that are potentially much more similar in properties. Potential compatibility issues may occur between fuels from different suppliers and this is one of the issues identified during the IMO discussions that is not yet resolved satisfactorily.

The fuels can be very different in characteristics from conventional fuel oil and this has led numerous organisations to issue guidance to operators on their use. Lloyd’s Register issued the following advice in its publication Using hybrid fuels for ECA-SOx compliance.

Most of the new hybrid fuels are blended products and have some characteristics of distillate products. This means they may exert a ‘cleaning’ action, mobilising previously deposited asphaltenic material, potentially leading to increased filter loading and other operational issues. It is therefore recommended that fuel tanks which will carry these new fuel types are cleaned or at least cleared of the ‘unpumpables’ at the tank bottom.

Despite their distillate characteristics, most of these hybrid fuels are particularly waxy in nature, as exhibited by their pour point (the lowest temperature at which a fuel will continue to flow). The exact pour point may vary from product to product, but the usual rule is to maintain any fuel oil no lower than 7°C above its tested pour point. These fuels therefore need to be stored and handled in systems with heating arrangements.

These types of fuels should not be stored in tanks which are subject to low external temperatures, such as a ship’s side tanks. Even in tanks with heating coils that maintain the bulk of the fuel as liquid, the formation and then breakaway of material at the cold interface could result in operational problems.

These fuels will also need to be purified, taking into account their density (gravity disc selection) and viscosity for optimised preheat. Based on the tested viscosity and density of the fuels, the purifier manufacturer’s recommendations should be followed for the correct operational adjustments.

Advice has also been issued by class societies, P&I Clubs, engine makers and the USCG on safe switch over procedures when entering ECAs. Much of the advice is a repeat of that needed some years ago when the EU imposed a 0.1% sulphur cap on fuel used during port stays within the EU but with many more ships and owners now affected, repeating it is probably a wise precaution.

Distillate fuels

Distillate fuels such as DMA and DMB usually referred to as MGO and MDO respectively are frequently used in the main engines of most ships not running on ULSFO or fitted with scrubbers and operating in ECAs and by smaller ship types as a normal fuel of choice. Distillates also power most auxiliary engines on all ship types although some larger vessels will use IFO when possible.

They are available in standard and low sulphur versions with the former currently averaging 1 – 1.5% sulphur and the low sulphur version being ECA compliant at 0.1%. Of the two main types mentioned, MGO is the lightest and contains least sulphur. MDO is effectively MGO with a small proportion of residuals and is likely to have a higher sulphur content.

Because they can be used in main engines normally run on HFO, distillates represent the easiest means of meeting the 0.5% global cap if availability is the main criteria. However, although readily available, distillates currently account for less than 25% of all marine fuels used. They are also heavily used in many non-marine sectors in far greater quantities.

An increased use of distillates as a means to meet the 0.5% sulphur cap will therefore bring the shipping industry into competition with other users with no guarantee that sufficient supplies will be available.

Increased use of distillate fuels for shipping generally will also badly impact those ships that have been specifically designed to operate with them and which are mostly employed in short sea trades and for local passenger and cargo ferries. The current cost of distillates is around 5-10% higher than ULSFO in major bunkering centres such as Rotterdam. In the past MGO prices have been as much as double that of IFO380.

Emulsified fuels

Water in fuels can be a problem and most engine makers traditionally recommend that water in HFO should be removed entirely by separation before entering the engine. This is mostly due to the fact that cat fines are more easily transported in water and sea water in the fuel oil is a major source of sodium. Sodium, along with ash and vanadium is to be avoided where possible because compounds of the chemicals tend to promote mechanical wear, high temperature corrosion and the formation of deposits in the turbocharger and on the exhaust valves.

However, controlled use of water such as humid air, direct water injection and emulsion fuels can be beneficial in reducing levels of NOx and SOx. While the first two options are for engine makers to research and develop, the last option is receiving attention from some specialists in the fuel sector.

Emulsified fuels work by using a quantity of water in the fuel which has the effect of reducing the size of oil droplets compared to conventional fuels. This results in more complete combustion of the oil and so increasing the energy delivered from a given quantity of fuel. Because the oil droplets surround a water core, the heat in the combustion chamber also causes the water to vaporize which breaks the oil down into even finer droplets. The water vapour itself adds energy much as it would in a steam engine.

Although the governor of an engine running on such fuel may open up more to meet the speed demand set by the bridge, the volume of water contained in the emulsion will more than cover the extra amount of emulsified fuel injected. Therefore, the fuel saving will be equal to the volume of water contained in the fuel less the extra percentage of emulsified fuel injected.

One of the fuel specialist in this area is UK-based Quadrise Fuels International, Trials of the company’s Multiphase Superfine Atomized Residue (MSAR) fuel have been taking place in conjunction with AP Møller Maersk for a number of years and although the falling crude oil price has resulted in some negative sentiment by investors, the company claims MSAR would be commercially viable even at crude prices of around $40 per barrel.

To date, Wärtsilä and MAN Diesel have both participated in the programme and the fuel has been tested on some of their two-stroke engines. Quadrise’s focus since mid-2014 has been on preparations for the “Letters of no Objection” (LONO) programme, issued by the engine manufacturers. These manufacturers require operating data and a range of technical results from an extended and continuous use of the fuel, in normal commercial operations, to provide the basis on which to issue the LONO.

The LONO qualification assessment will involve inspecting various key components in the engine periodically to confirm that the performance results are compatible with those experienced in earlier MSAR fuels trials and that nothing unexpected or problematic is evident. The LONO assessment period is typically 4,000 hours of operational service or circa nine months, although periodic assessments may reduce or lengthen this duration depending on results.

To meet the LONO programme requirements, MSAR must be produced in higher quantities than has so far been the case. To cater for a multi-vessel roll-out programme, Quadrise considers it preferable and more economic to find a refining partner with both production and logistics economics which would optimise the roll-out programme. In September 2015 Quadrise reported it had reached agreement with refiner group CEPSA to supply, install and commission a manufacturing unit to produce MSAR emulsion fuel at the CEPSA San Rogue refinery close to Gilbraltar and that extended trials with Maersk began in early 2016.

In what must be considered a case of severe bad fortune, the vessel on which the trials were being carried out was involved in a collision in March 2017 and had to be sent for repairs. The vessel was then diverted to another service meaning the trial could not be completed.

Although the premature end to the trial has meant no LONO is likely to be forthcoming, all parties were apparently satisfied with the way the trail was progressing. A new trial is apparently planned but no details have yet been announced.

Quadrise and Maersk are not the only organisations conducting trials with emulsified fuels. In New Zealand it was announced in May 2015 that as part of KiwiRail’s commitment to greater sustainability, the Interislander ferry Arahura was trialling Fuel Oil Emulsion (FOE) technology in one of the ship’s auxiliary engines. The technology is supplied by Blended Fuel Solutions NZ and is one that is already accepted.

The trial took place over three months and follows a two-day trial in 2013 that showed reduced fuel consumption and emissions. It was independently monitored and included a month running on normal fuel to give a base line and pre and post internal engine inspections.

Although the trail was a success, falling fuel prices and a lack of environmental regulation in New Zealand for domestic ships have combined against a longer term use of the emulsified fuel.

Singapore-based Blue Ocean is another pioneer in this area and can point to several references involving the likes of APL, Regent Seven Seas Cruises and NCL. The systems installed date back to 2011 and have demonstrated fuel savings of between 3-5%. The savings have been verified by leading classification societies and approved by engine makers.

Interest is growing in emulsified fuels and one new entrant to the arena is SulNOx which has developed emulsifying equipment and is attempting to enter the marine market. Company reports by SulNOx filed last year indicate that the fall in bunker prices has had a negative effect on the company’s plans and it is now attempting to find strategic partners to take the project forward.

A factor that could limit the acceptance of emulsified fuels produced on board is that extra equipment is needed and the water used should be either fresh water supplied from shore which is an expense in itself or water produced onboard using a fresh water generator.

Non-oil fuels

Currently the only alternative fuel to oils that is used in any quantity is liquefied natural gas (LNG). It is formed by cooling natural gas to a very low temperature (-162°C) until it condenses into a cryogenic liquid. In this state it has significantly higher energy content per volume – 1 litre of LNG contains approximately 600 litres of natural gas.
LNG carriers have been using LNG boil off from the cargo to run steam turbines for many years and in the last decade also as fuel for burning in dual-fuel diesel engines. In addition to LNG carriers, a small number of ferries and offshore vessels have also been built with or had retro-fitted engines that run on LNG.

In Europe in particular, LNG has been heavily promoted as an alternative to oil for vessels operating in the Baltic and North Sea ECAs but despite one or two high profile projects, take-up has been minimal. Clearly some of the resistance stems from the fact that there is a notable lack of storage and supply infrastructure in place. This seems to be changing slowly but will hit an inevitable barrier if usage by ship operators fails to accelerate.

LNG can also be used with gas turbines as well as burned in steam turbines, dual-fuel or pure gas burning engines. What is less well understood is that LNG is not a single-grade fuel. Its combustion properties and energy content vary with the amount of methane contained in it. LNG offered for fuel might contain anything from 80%-95%. The LNG used for dual-fuel operation should contain high levels of methane (preferably 95%). If LNG is ever used in significant number of ships it seems likely that some form of grading system as used for oil fuels will need to be established.
Shipowners wishing to use LNG as fuel currently have two options. They can install a dual-fuel engine or fit a pure gas burning engine. However, engine makers are now building engines that have the ability to be converted to enable them to run on gas. MAN Diesel and Turbo exhibited such an engine at SMM in 2012. The engine in question was a 6L32/44 CR engine in the process of conversion to a 6L35/44DF. Another maker to offer conversion ready engines is Caterpillar with its M43C models. Caterpillar says that 450 ships are now using the M43Ctype engine, which can be converted to an LNG-fuelled M46 DF engine.

The majority of these engines are installed on ships with German owners. About 190 of these ships are less than six years old and therefore in principle suitable for conversion to LNG. Most of the ships are container feeders of similar design. There is thus the potential for a standardised, cost-effective retrofit of a large number of ships.

Closely related to LNG, ethane is another liquid gas that can be used as a fuel. It has been tested in a Wärtsilä 50DF engine and MAN Diesel has secured a contract for three of its ME-LGI engines to be installed in a series of ethylene gas carriers for a Norwegian owner. The engines need to incorporate some detailed design changes to accommodate the higher pressure needed for ethane operation. These include redesigned fuel valves, control block and piping as well as some material changes.

In addition, both methanol and hydrogen have been suggested as possible fuels for future use but little experience has yet been gained with either. Methanol can be used in a diesel engine but hydrogen is more suited to powering fuel cells. Methanol is produced from a variety of sources chiefly natural gas although China produces large quantities from coal. It can even be manufactured by high pressure hydrogenation of CO2. On the emissions side, there are reductions of around 99% SOx, 60% NOx and 95%PM. Importantly as regards future EEDI regulations CO2 can be reduced by 25% compared to oil fuels. Another promising characteristic for use as a marine fuel is that it is not considered polluting and could therefore be stored in unprotected locations on board including in double bottoms.
However, it does have a low flashpoint – 12°C and therefore storage tanks require inerting for safety reasons. This coupled with the fact that methanol burns with a very low flame temperature and a difficult to see light blue flame makes fire detection difficult. A product of methanol combustion is formaldehyde which is highly toxic at certain levels.

Both LR and DNV GL have draft rules on the use of methanol as a fuel. As a liquid, Methanol is not covered by the IMO’s IGF Code but it is expected that work will soon begin on drafting a Methanol Code based on the IGF Code. Until an internationally agreed Code has been adopted, any plans to use methanol will require a case-by-case approval from the flag state.

In early 2015, the Stena Line ferry Stena Germanica had its Wärtsilä engine modified to run on methanol and has been operating on the fuel on its route between Sweden and Germany. According to Stena, emissions from methanol are roughly the same as for LNG, but the fuel is easier to process and does not place the same demands on infrastructure.

More vessels were added in 2016 as MAN Diesel had orders for seven methanol-fuelled two-stroke engines to power methanol carriers. The requirement to inert the fuel tanks in those vessels did not involve much in the way of extra expense for the vessels since the cargo is already required to be treated in this way and the equipment is therefore already on board. MAN’s engines are the ME-LGI type and operate on the Diesel principle which is seen as being more effective than the Otto cycle. The first ME-LGI engine was successfully demonstrated in March 2015.

Hydrogen is further away from commercialisation although some small craft running on it do exist. It is generally believed that hydrogen’s best chance of acceptance is in conjunction with fuel cells for which much was promised a decade ago. Considering that companies which have been experimenting with fuel cells for marine use seem to be diminishing in number, it has to be said that use of the technology in the short term is unlikely and is most likely further down the line than was once envisaged. Even so it has its proponents and is regularly discussed as a pollution free alternative to more conventional fuels.

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