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The different types of fuels for ships

Oil Fuels – ISO 8217

Oil Fuels – ISO 8217

Rudolf Diesel intended the engines that bear his name to run on a wide variety of fuels and in fact his first two engines were fuelled by coal dust but were unsuccessful and Diesel turned instead to oil. Less than 20 years after Diesel patented his engine, the first ships powered by them were brought into service. The fuel used then was a variant of modern MDO with heavier oils coming later.

Modern shipowners enjoy a choice of fuels that their predecessors could only dream of with the available fuel mix today including all the standard types of mineral oils such as HFO, MDO and MGO together with vegetable and animal-based bio-fuels, LNG, ethane, methanol and to a lesser extent battery power. Hydrogen is tipped as the next fuel type – although maybe not in an internal combustion engine – and ammonia is also being touted as a future fuel.

Whatever the future of shipping may hold with regard to fuel and propulsion systems, it is generally accepted that fossil fuels will be the main source of power for the next 30 years at least. Oil fuels exist in several varieties and although it is possible for there to be an infinite number of different composition fuels, in practice the use of standard fuels is the norm. There is an ISO standard for marine fuels which is updated at regular intervals.

Work on the fourth edition of ISO 8217 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. Up to and including the 2012 version of ISO 8217, all of the fuels were fossil fuels, The changes in the 2017 version are far-reaching, not least since the main change is the addition of a new set of distillate grades containing bio-fuels.

These new fuels are distinguished as DFA, DFZ and DFB and as can be seen from the table below, they share most characteristics with their fossil fuel counterparts but have an additional entry for fatty acids.

The ISO 8217 standard for distillate fuels lists several grades as can be seen from the tables. Whilst these fuel types will be readily recognisable to bunker professionals and engineers, they may be unfamiliar to other shipping practitioners. For all practical purposes the fuel generally referred to as Marine Diesel Oil (MDO) is listed as DMB in the ISO standard and Marine Gas Oil (MGO) as DMA.

For residual fuels, the terms Heavy Fuel Oil (HFO) and Intermediate Fuel Oil (IFO) are often used interchangeably. This can be confusing as both terms are technical correct. HFO is a residual fuel incurred during the distillation of crude oil. The quality of the residual fuel depends on the quality of the crude oil. To achieve various specifications and quality levels, these residual fuels are blended with lighter fuels such as marine gasoil or marine diesel oil. The resulting blends are also referred to as intermediate fuel oils (IFO) or marine diesel oil. They are classified and named according to their viscosity, IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively.

In the MARPOL Marine Convention of 1973, heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C. Heavy fuel oils have large percentages of heavy molecules such as long-chain hydrocarbons and aromatics with long-branched side chains.

The ISO 8217 international standard divides marine fuels into distillate marine fuels and residual marine fuels (RMA). The latter are collectively called heavy fuel oils. An exception is the lowest viscous quality level, RMA 10, which is no longer referred to as an HFO, as its proportion of heavy fuel oil is so small. ISO 8217 stipulates that residual fuels, and therefore all heavy fuel oils, may not contain old oil or lubricating oils.



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, most other vessel types including cruise ships, tankers, container ships, car carriers, ferries and offshore have also been built with or had retrofitted engines that run on LNG.

The coming into force of the IMO’s IGF Code, which sets out rules for gas-fuelled ships with regard to both systems and crew training, appears to have removed a long-standing obstacle to greater take-up of LNG. However, the number of ships is still relatively small and the percentage of newbuildings specified as running on LNG remains in single figures.

Another obstacle has been the lack of supply infrastructure, which is being addressed and new facilities are now in place in most of the world’s major bunkering centres. It needs to be understood by those planning to run ships on LNG that it 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%) so 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. Engine makers have for some time now been building oil-fuelled engines that have the ability to be converted to enable them to run on gas. Several projects to do this have now been completed and more are in the pipeline.

In scientific terms, methane is an alkane; one of the simplest forms of hydrocarbons. It is the first in a series of similar products that are already used as fuels and which includes ethane, propane and butane.

Higher up the list of alkanes are more complex hydrocarbons which are found in crude oils and known collectively as volatile organic compounds (VOCs). It has long been the practice in tanker operation for the VOCs, which naturally vent from the cargo during transportation, to be either allowed to diffuse or more recently to be collected and returned to the cargo because the VOCs are considered as environmentally damaging. Some countries have enacted regulations to restrict VOC release.

All three of the main dual-fuel engine builders (MAN Energy Solutions, Wartsila and WinGD) have developed systems for collection and liquefying VOCs released from crude cargoes and for the liquid VOCs to be mixed with LNG for use in the engine. There are operational matters to be addressed when doing this as the addition of VOCs reduces the purity of the LNG and could cause ‘knocking’ but the operating parameters of engines can be adjusted to take account of this and the use of VOCs will reduce the consumption of the LNG fuel used on board. Two owners – Teekay and AET – have recent new vessels equipped with a VOC recovery and liquefaction plant on board.



Methanol is a liquid at ambient temperatures and is considered as a ‘drop-in replacement’ for oil because it needs no special storage. It 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 of 12°C and therefore its 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.

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 a ship’s flag state.

Methanol has already been selected as a fuel by a small number of owners with the first newbuild ships beginning operations in 2016. Mostly the ships are methanol carriers but the ro-pax Stena Germanica, which converted to run on methanol in 2015, highlights that application to other ships is perfectly feasible.



Propane and butane are two gases more commonly together called LPG or liquefied petroleum gas. Ethane and LPG are alkanes as LNG is and have both been added recently to the growing list of marine fuels.

Dual-fuel engines of ships using these fuels need to incorporate some detailed design changes to accommodate the higher pressure needed for their operation. These include redesigned fuel valves, control block and piping as well as some material changes.



Research and indeed use of ammonia in an internal combustion engine goes back many years but it is only since the IMO embarked on its decarbonisation programme that it has come to be discussed as a potential fuel for ships.

Ammonia is liquid at normal temperatures making storage a simple matter. It has the chemical formula NH3 which would suggest that the exhaust gases from its combustion will be water vapour and nitrogen compounds. These could be nitrous oxide (N2O) which is a highly potent greenhouse gas or NOx. Although if the ignition temperature can be kept low enough, NOx will not form and nitrogen will be emitted instead. Both nitrous oxide and NOx could be removed by SCR.

There are engines which can and do run on ammonia and there is research being undertaken by engine makers with an interest in marine applications. In 2008, Caterpillar filed a patent (US Patent US20100019506A1) which described an ammonia-fuelled engine and ancillary systems. In the patent description, Caterpillar specifically mentioned the search for zero carbon fuels, but it also described some of the problems relating to using ammonia as a fuel.

Caterpillar said the characteristics of ammonia fuel, such as zero CO2 emissions, relatively high energy density, well-established production infrastructure, and competitive cost, have made ammonia an attractive alternative fuel for combustion engines.

On the negative side, when ammonia is combusted, the combustion produces a flame with a relatively low propagation speed. In other words, the combustion rate of ammonia is low. This low combustion rate of ammonia causes combustion to be inconsistent under low engine load and/or high engine speed operating conditions.

Most existing combustion engines that use ammonia as engine fuel typically require a combustion promoter (ie, a second fuel such as gasoline, hydrogen or diesel) for ignition, operation at low engine loads and/or high engine speed. However, the requirement for the combustion promoter fuel fluctuates with varying engine loads and engine speed which can cause control issues.

To eliminate the use of combustion promoter fuel, combustion engines that burn ammonia alone as engine fuel have been explored. Both Wärtsilä and MAN Energy Solutions see a significant role for ammonia in future in combination with internal combustion engines. Wärtsilä is researching its use in four-strokes while MAN favours its use in two-stroke engines particularly it ME-GI models.

Ammonia does not present any particular problems with regard to storage and use in a dual-fuel engine as it can be stored under similar conditions to LNG or LPG when carried as a cargo. It does have an issue with toxicity which needs to be managed carefully.

While ammonia is seen as a good potential fuel, its global production is already fully utilised so there will need to be an upscaling. There are projects and research involving producing sufficient quantities of ammonia for it to be able to be used on a large scale as a fuel for marine and shore situations. Some of these involve manufacturing ammonia by combining hydrogen produced by electrolysis of water using surplus renewable energy with nitrogen from the air. Whether this is economically viable remains to be determined.

Producing ammonia is one aspect but solving the problem of poor combustion under changeable load conditions and engine speed has still to be tackled. One idea that could overcome this may be worthy of consideration. Hybrid ships using batteries are now accepted technology. It would surely not be beyond the abilities of engineers to devise a system whereby the ammonia powered engine runs as a genset at optimal conditions as regards load and speed, which would eliminate its combustion problems. The electrical power produced could be used to charge batteries which would in turn power electric propulsion motors without problems associated with changing load demands.



Hydrogen has been described as the perfect fuel for all industrial uses as its waste products are generally only considered as being water. Its main disadvantage is that hydrogen in its elemental state does not exist on earth. All hydrogen is bound up in other compounds notably fossil fuels and water.

Obtaining hydrogen is expensive and requires considerable energy. In theory it is possible to use renewable energy to electrolyse water into hydrogen and oxygen but once transport and storage is taken into account the economics once again appear negative.

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. After some projects with fuel cells around 2011/12, interest seemed to wane, and many companies ceased research. However, there has been a reawakening of interest in hydrogen, with cruise ships, ferries and inland craft all being constructed with fuel cells as either the primary source of power or as a complement to traditional engines.

The usual description of a fuel cell is a device for combining hydrogen with oxygen in an electrochemical reaction with electrical energy being produced for power utilisation and water and heat as by products. What is often omitted is that there are many more components needed to reform the fuel if it is not pure hydrogen. These can include heaters, pumps and other items that equate to the fuel conditioning and delivery systems, starting air compressors and exhausts of a conventional diesel engine. These ancillary systems require their own power source which, depending upon the type of fuel cell, may produce undesirable by-products and pollutants.

In its simplest form as a proton exchange membrane (PEM) fuel cell, the only requirements are a constant feed of hydrogen to the anode and oxygen via air to the cathode. The hydrogen is fed to the anode of the fuel cell where the electrons in the hydrogen are separated to pass as an electric current to the motor or other system. The electrons continue on to the cathode to meet the hydrogen ions (protons) which have passed across the membrane and are combined with oxygen from the air to form water.

Using hydrogen as the fuel presents problems for ships that may be unsurmountable for large vessels making long voyages. Hydrogen in gaseous form is probably the least energy dense fuel possible so must be refrigerated and kept under pressure as a liquid for a PEM fuel cell to be viable for marine use.

Although it is the hydrogen that is important for a fuel cell to function, it is not necessary for it to be kept as pure hydrogen. Any fuel that contains hydrogen, including diesel, LNG, methanol and many more, can be used instead. However, except for a very few fuel types, the fuel has to be reformed to extract the hydrogen before it can be used in the cell.

As an example, methane as LNG (CH4) can be reformed by using steam (H20) to produce hydrogen and carbon monoxide. The hydrogen is then used to power the fuel cell while the carbon monoxide reacts further with some of the oxygen making the waste products of the fuel cell water and carbon dioxide.

In another type of fuel cell, methanol (CH3OH) can be used directly as a liquid alone or mixed with water with all of the hydrogen atom electrons producing the current and again water and CO2 are the waste products. The production of CO2 as waste products would appear to negate the benefits claimed for hydrogen.

Hydrogen can also be burned in an internal combustion engine but in gaseous form it is a very low energy density fuel and long-term contact with the gas can cause some metals to become brittle. Nevertheless BeHydro, a joint venture between Belgian ship operator CMB and compatriot engine maker ABC has launched a range of engines based upon ABC’s dual-fuel DZD engine range.

Two of these engines have been ordered for installation in a tug ordered by the Port of Antwerp for delivery in 2021/2

Bio and synthetic fuels

Bio and synthetic fuels

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. On the marine front it has been used as the sole fuel for a dredger which was operated on the plant-based fuel for over 2,000 hours.

Some adverse effects of biodiesel have been described based upon experience in the leisure sector. These include:

  • Biodiesel is not a fuel with long-term stability, it can oxidise 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.

As well as bio-diesel, there is also biomethane that could be used in ships with dual-fuel engines. Biomethane is produced by the natural breakdown of organic material: green waste, household waste, agricultural waste, food industry waste and even industrial waste. The process of breaking down this material in an oxygen-free environment produces biogas, which is then purified to become biomethane which has the same characteristics as natural gas.

There are at least three methods of producing biomethane; firstly there is the anaerobic decomposition of organic waste, secondly there is gasification from ligno-cellulosic biomass (wood and straw), using a thermochemical conversion process and finally the direct transformation of micro-algae cultivated in high-yield photosynthetic reactors using natural light, water and minerals. This is an emerging technology expected to reach industrial scale use within the next decade.

Cultivating algae has the added advantage of recycling CO2 produced as a consequence of other power production rather than needing to resort to carbon capture and storage. Algae can also be processed to form oil fuel equivalents.

Making use of waste products to manufacture biofuels 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 biofuels made their use economically questionable. Given the decline in crude oil prices from the peak the cost may have become.

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