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, Wärtsilä 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.
New kid on the block
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 will be 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.