Overview of marine fuel chemistry
Updated 11 Oct 2019
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.
Most of the newer fuel types available have come about as a result of emission regulation. LNG would likely not have taken off for ship types other than LNG carriers were it not for the Norwegian NOx tax, even if today it is seen as being a solution to SOx and CO2 regulation as well as NOx. In advance of the 2020 cap on sulphur content being reduced from its current 3.5% to 0.5%, the latest trend by the oil majors is towards developing a new range of oil fuels with ultra-low sulphur content.
The adoption at MEPC 72 of an ambitious strategy that would aim for a 50% reduction in so-called greenhouse gases (GHG) from shipping by 2050 followed by a complete decarbonisation policy thereafter would, on the face of it, 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.
In many minds, the term ‘greenhouse gases’ refers almost exclusively to CO2 but the IPCC (the UN body overseeing the debate on climate change) lists a great many more. The IPCC lists water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) as the primary greenhouse gases in the Earth’s atmosphere and the IMO itself in its latest GHG strategy recognises that methane slip from LNG-fuelled power systems is a problem that needs to be dealt with.
The inclusion of water vapour as a GHG may come as a surprise to most but many studies recognise it as the most common of the GHG and highly potent if short-lived. In the era of fossil fuels the attention has been focussed on CO2, but if there were to be a big switch to fuel cells and hydrogen in internal combustion engines then it is possible that something as innocuous as water vapour will be a matter requiring attention. Burning one tonne of a typical fuel oil produces around 3.5 tonnes of CO2 but burning the same amount of hydrogen would produce 18 tonnes of water or water vapour.
The common thread that connects all of the fuels used by or touted for ship use is that they contain the easily combustible elements of carbon and hydrogen in varying ratios. Oil fuels usually have twice as many hydrogen atoms as they have carbon atoms but because the atomic weights of the elements are so different an oil fuel is typically 82% by mass of carbon and 12% by mass of hydrogen – the remainder being other elements and compounds. LNG as
pure methane has one carbon atom and four hydrogen atoms. Ethane and propane have a higher carbon content than methane but are still less carbon rich than oil.
Hydrogen and ammonia both contain no carbon. Hydrogen is of course an element so contains only hydrogen and a single molecule of ammonia has one nitrogen atom and three hydrogen atoms. Hydrogen and ammonia can both be used in internal combustion engines but there are difficulties associated with their use in this way.
Oil fuels – the mainstay of shipping
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 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.
The table below details fuel types under ISO 8217-2010. This version table is included in this guide because this older standard is still the main choice for bunker contracts despite there being newer versions available. 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.
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.
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 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 makes the cost even more unrealistic.
It is not certain if the refining industry will accommodate the 1 January 2020 low-sulphur deadline by producing low-sulphur fuels in the needed quantity. 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 228M tonnes of heavy fuel oil in 2012 compared to just 65M tonnes of marine grade distillates. Some major upgrades to refineries will be needed if that 228M 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 – rather than predicted – availability is something that will be made clearer in the coming months. 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 the quantities available are not high and some newer ultra-low sulphur fuel oils (ULSFO) – sometimes referred to as hybrid fuels – have been developed to meet the 2015 reduction to 0.1% in ECAs. The first generation of low-sulphur fuels was quickly followed by newer products and some refiners are working on newer versions still to meet the 2020 rules. Some are already becoming available.
At the present time, ULSFO fuels are used mostly in ECAs and special precautions are needed during switchovers. This will be less of a problem when 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. More information on this is included in Resolution MEPC 320(74).
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 points (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 other class societies, P&I clubs, engine makers and the USCG on safe switchover 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.
In the run up to 2020, many of the oil majors have developed new bunker fuels with a sulphur content of 0.5% or below. There has been much speculation as to the miscibility of fuels from different suppliers and although the suppliers have tried to allay fears, concerns continue to exist and will only be satisfied once more experience is gained.
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 criterion. 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 including in power production, road, rail and off-road plant, agriculture and many other industries.
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.
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. This, along with ash and vanadium are 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 increases 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 vaporise 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.
Alkanes as alternatives to oil
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 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. There are now many more ship types – including container carriers, bulkers, tankers and car carriers – fitted with dual-fuel engines and running on 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 is 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. However, engine makers are now building 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. The latter two gases are more commonly together called LPG or liquefied petroleum gas. Ethane and LPG have both been added recently to the growing list of marine fuels and 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.
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 – are currently having vessels built that will have a VOC recovery and liquefaction plant on board.
Other non-oil fuels
In addition, methanol, ammonia and hydrogen have been suggested as possible fuels for future use, but little experience has yet been gained with ammonia or hydrogen and only a small number of ships can currently run on methanol. Methanol can be used in a diesel engine, but hydrogen is more suited to powering fuel cells. Both LR and DNV GL have draft rules on the use of methanol as a fuel.
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.
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. Hydrogen is increasingly being talked about as a pollution-free alternative to more conventional fuels.
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.
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.
In the last few years, battery systems making use of energy from land-based sources or from excess energy produced on board have been heavily promoted as a non-polluting power supply for ships in some circumstances. 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. 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. Batteries are clearly not a solution as the sole power supply for deepsea ships; even for local domestic service vessels they make sense only if the production of electricity can be done more cleanly than from an onboard power source.
Ammonia as a potential marine fuel
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 NOx although if the ignition temperature can be kept low enough, NOx will not form and nitrogen will be emitted instead.
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.
Furthermore, the use of dual fuels generally requires dual-fuel storage systems, dual delivery systems and dual injection systems, thus adding additional weight, complexity, and cost to the engine system. To eliminate the use of combustion promoter fuel, combustion engines that burn ammonia alone as engine fuel have been explored.
Another engine maker with an interest in ammonia is MAN Energy Solutions which has been conducting research in co-operation with Alfa Laval with particular regard to LPG fuel conditioning. In April 2019 Alfa Laval issued a press release describing the testing of a MAN B&W ME-LGIP engine running on propane and the Alfa Laval Fuel Conditioning Module (FCM). The set-up was also evaluated for use with ammonia. Apparently, propane has some of the same problems as ammonia when used in a two-stroke engine. A full
technical paper about the Alfa Laval FCM LPG and the results achieved at the MAN test facility was presented at the CIMAC Conference 2019, taking place in Vancouver,Canada, 10-14 June.
There are also 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.