Alternative marine Fuels
The most used fuel in international shipping today is HFO which accounts for approximately 77% of all fuel burned in marine engines today. This fuel is a residue from the refining industry, it has a very high energy density, a high carbon content and is relatively low in price. There is a need for alternative fuels in the shipping sector for various reasons: to deliver a reduction in local pollutants and comply with existing regulation; to mitigate climate change which is the main focus of the Maritime Industry Decarbonisation Council (MIDC). A broad variety of liquid and gaseous fuels is considered by MIDC.
The main alternative fuels that are currently available for marine use are primarily hydrogen, power to liquid fuels (the so-called e-fuels), various biofuels and LNG.
It is important in the assessment of alternative fuels that the emissions released over the full life-cycle are considered.
Classic marine fuels
Since the 1960’s HFO is the dominant marine fuel in the shipping sector. HFO is a high viscous fuel that produces a lot of pollutants when used in a marine engine. These pollutants are amongst others NOx , SOx and PM (Particulate Matter). With the latest regulations on SOx emissions there is an uptake of other marine fuels such as MGO and MDO. These fuels have a lower sulphur content in comparison with HFO but still produce as much GHG (greenhouse gas) emissions. For a marine fuel to be the fuel of the future it needs to comply with the “fuel triangle”.
The fuel triangle
To succeed in the face of competition from currently used fossil fuels, alternative fuels will need to meet the characteristics mentioned on the triangle. The first one is the “large energy density”. The energy density of a good alternative fuel will need to be comparable to the energy density of existing marine fuels. As a matter of fact, it is the large energy-density of HFO (and the low price) that has made this fuel so successful the past decades.
“The crucial element with regards to energy density is the energy content per volume. If said energy content is smaller than the energy content of current marine fuels, cargo capacity will be lost, as more space will be needed for fuel storage.”
The next very important characteristic is the availability and security of supply. Because shipping is a worldwide industry, alternative fuels need to be available all over the world, similar to current fuels. This requires the building of a worldwide infrastructure network meeting the demand for various alternative fuels. The regulatory framework on alternative fuels might raise hurdles on the path to develop said supply network.
Additionally future alternative fuels will also need to be GHG-neutral along the entire well-to-propeller chain. The MIDC is absolutely convinced that there will be a need to consider the greenhouse upstream greenhouse gas emissions. The matter is currently being studied in depth by the academic society. It is the only sensible way to tackle the decarbonisation of our society.
To determine which alternative fuels will become the shipping fuels of the future, the MIDC invites the various parties, i.e. the engine manufacturers, the fuel producers and the ports, to a joint consultation process.
The importance of hydrogen
Hydrogen the prime source of energy in all fuels, whether fossil fuels or alternative sustainable fuels. This section will look into the different production possibilities of hydrogen, storage of hydrogen and the potential of pure hydrogen as a marine fuel.
Production of hydrogen
Hydrogen can be produced from fossil fuels or from renewable energy sources. The main production processes to produce hydrogen from fossil fuels include steam methane reforming (SMR), catalytic decomposition of natural gas, partial oxidation of heavy oils, and coal gasification. The predominant production processes to produce hydrogen from renewable energy sources are water electrolysis, thermochemical water decomposition, photochemical, photoelectrochemical, and photobiological.
Steam methane reforming is the most used method of hydrogen production today. A green alternative for SMR is electrolysis with renewable energy.
Steam methane reforming
SMR is currently the cheapest way to produce hydrogen and uses lighthydrocarbons (e.g. methane and naphtha) as the source. The first step is synthesis gas generation, in which a desulfurized hydrocarbon is mixed with process steam over a nickel-based catalyst in the reformer. The second step is supplemental hydrogen generation, in which the synthesis gas enters the shift converter. Finally, the third step is gas purification, where the primary diluent, CO2, is removed in a scrubbing unit. The hydrogen produced typically has a purity of 97–98%. A big disadvantage of this method is the production of CO2 emissions during the process. Hydrogen produced with this method is also referred to as “brown hydrogen” if the carbon is not captured during the process and “blue hydrogen” if the carbon is captured. If we look into the well to propeller emissions of SMR produced hydrogen (brown) used onboard a ship, the emissions are comparable to the use of HFO as a marine fuel. The different “types” of hydrogen are illustrated below.
Water electrolysis involves the catalytic decomposition of water into hydrogen and oxygen using electricity.
“This process does not produce any emissions other than hydrogen and oxygen,”
The electricity used can be provided from renewable sources/energy. This results in very high quality hydrogen without any GHG emissions during the production process. This is called “green hydrogen”. If the entire merchant fleet would use green hydrogen heavy investments will be needed in renewable energy production to provide the electrolysers with energy.
Around 48% of the global demand for hydrogen is currently generated from natural gas, about 30% from oil/naphtha from refinery/chemical industrial off-gases, 18% from coal, 3.9% from water electrolysis, and the remaining 0.1% from other resources such as nuclear, biomass, wind, solar, geothermal, or hydroelectric energy. Only a small part of the produced hydrogen worldwide is used as a fuel. The main applications for hydrogen nowadays are: In the chemical industry it is used to make ammonia for agricultural fertiliser and cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals. It is also used to remove sulfur from fuels during the oil-refining process.
Hydrogen as a fuel
When talking about hydrogen fuel, fuel cells come to mind. Two types of hydrogen fuel use for ships need to be considered. Hydrogen (dual fuel) internal combustion engines can burn hydrogen with very low GHG emissions (MGO is used as pilot fuel). Fuel cell systems on the other hand use an electro-chemical reaction to generate electricity. The efficiency of fuel cells is relatively high, at around 45%, compared to internal combustion engines (roughly 20%). For the time being the use of an internal combustion engine seems to be more interesting compared to a fuel cell in a marine environment because of the following reasons:
No fundamental changes required to the main engine:
- When no hydrogen can be supplied, the engines run on MGO.
- If something fails in the hydrogen system, the system switches to pure MGO combustion.
- Co-combustion has almost no effect on the maintenance schedule.
Issues to consider with fuel cells:
- Salty environment of shipping and the large movements (up to 40° of banking) is probably too challenging for fuel cells.
- At high constant power output, the fuel cells have less efficiency compared to the co-combustion concept.
- Fuel cells produce electrical energy, but we need mechanical energy for the propulsion. The need for power electronics will make it very expensive.
Hydrogen can also be used as part of a compound, in which the hydrogen is bound to another molecule, e.g. methanol, where hydrogen is bound to CO2. Such fuels are also referred to as power-to-liquid or electro fuels (see further below).
How does Hydrogen score in the fuel triangle?
GREEN HYDROGEN scores very well regardingv the GHG emissions part in the fuel triangle because green hydrogen is GHG neutral on the overall supply chain. There is a global supply network for hydrogen for the different uses of hydrogen in the petrochemical industry. So for availability and security of supply hydrogen scores relatively well. This is however for BROWN HYDROGEN. For the worldwide availability of green hydrogen big investments will be needed in renewable hydrogen production infrastructure.
“A difficulty for hydrogen is its energy density. The energy density per unit volume of hydrogen at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher. If hydrogen is used as a marine fuel, large quantities of hydrogen will be needed to be stored onboard the ship. The two most promising techniques for now are compressed hydrogen in a pressure vessel or liquid hydrogen.”
Compressed hydrogen is a storage form where hydrogen gas is kept under pressures to increase the storage density. Compressed hydrogen is stored in tanks at 350 bar and 700 bar. Hydrogen is liquified by reducing its temperature to -253°C, similar to liquified natural gas (LNG) which is stored at -162°C. there is an efficiency loss of 12.79% due to the cooling of the hydrogen.
Power to liquid or electro fuels are produced by feeding hydrogen and CO2 into a synthesis reactor to form different types of energy carriers, see the figure further below. The most common types of energy carriers discussed are methane (Power-to-gas) and methanol. Small molecules, like methanol and methane, seems preferable since more complex molecules, like ethanol, require additional process steps, which lead to efficiency losses. High purity oxygen and heat are also produced during the production steps from electricity to fuel. High temperature and low temperature heat is produced in electrolysis and in the fuel synthesis reactor, respectively. The heat can, for example, be fed into a district heating system, and the oxygen can be used in other industrial processes.
The CO2 can come from many sources including various industrial processes giving rise to excess CO2, e.g. biofuel production facilities, natural gas processing, flue gases from fossil and biomass combustion plants, steel plants, oil refineries and other chemical plants, geothermal activity, air and seawater.
Producing electrofuels, in relation to conventional biofuel production processes, can increase the use of carbon atoms in the biomass. However, production of electrofuels is still in its infancy, and many challenges need to be overcome before these products are brought to market on a large scale. Several demonstration scale facilities of power-to-gas, or electrofuels, have been developed in Europe during the last decade.
The advantage of electrofuels is that the storage problem of hydrogen is partly solved. By creating liquid fuels the energy content per volume is bigger at ambient conditions. So the fuels can be stored easily in conventional fuel tanks as it is done for current marine fuels. A big disadvantage of some electrofuels is their CO2 content.
An alternative to this is amonia which is be produced from hydrogen and nitrogen without the addition of CO2.
By binding CO2 to hydrogen the CO2 will be released to the atmosphere when the fuels are burned in a marine engine. There are some technologies under development that can split the hydrogen and CO2 again onboard and so burning clean hydrogen in the engine while capturing the CO2. An other solution is capturing CO2 in the exhaust stream.
Biofuels are a possible alternative marine fuel because they have low GHG emissions over the well to propeller path and at the same time have low sulphur levels to comply with existing sulphur regulation. One of the challenges are the volumes that are required to supply the shipping sector.
“A single very large ship may consume the annual production from a single medium sized biofuel facility e.g. 100 mio. liters.”
The market entry for biofuels in the marine sector is therefore most favorable onboard smaller vessels for coastal waters or for use as auxiliary fuel in ports.
Of the different current biofuels commercially available, only biodiesel derived from plants or pulping residues and bioethanol are produced in volumes that can possibly supply a part of the marine industry. All the different types of biofuels can be seen in the flowchart below (IEABioenergy 2017). The current renewable diesel type fuels are mainly produced from plant based oils or products thereof e.g. used cooking oil (UCO), and the potential supply of sustainable renewable diesel with the current technology is an estimated 10-20 Mt.
A potential issue is that the plant oil based fuels are the main fuel type currently used at a significant scale for bio jet fuels, leading to competition for feedstocks between the shipping and aviation sectors.
Bioethanol can be sustainably produced from waste , with much higher supply potential, capable of replacing all fossil fuels in the shipping sector, but bioethanol is not compatible with current marine diesels, and cannot be used as a drop-in fuel.
The cost of biofuels is higher than the cost of fossil fuels and is expected to remain so in the short to medium term. Specific mandates on biofuels or carbon taxes will make biofuels economically more competitive. Alternatively low-carbon transport may be introduced as a business model, putting a value on lower CO2 emissions.
Both technical and logistic issues need to be resolved before biofuels can be introduced at a larger scale in the shipping sector, and a closer collaboration between biofuel producers, engine developers and ship owners is recommended as a path forward.
Liquefied natural gas (LNG) is predominantly methane that has been converted from a gas to liquid form to facilitate storage and transport. It takes up a lower volume than compressed natural gas (CNG), thus increasing its energy density, but on a volume basis it is still 60% that of diesel. Special cryogenic storage vessels have been designed to keep LNG at -162°C. By definition, LNG needs to contain at least 90% methane gas and needs to be treated to remove impurities (water, H2S, CO2) before storage at low temperature.
The introduction of natural gas as a fuel on ships has provided significant emissions reduction to the air. Especially natural gas engines have potential to lower exhaust gas emissions as NOx, SOx and PM (without any exhaust gas after treatment) which have an impact on local air quality. Compared to marine diesel operation, natural gas operation does not have the potential to significantly reduce the CO2 emissions. CO2 reduction from natural gas fuelled engines are mainly due to the lower carbon content in the fuel, but also due to higher thermal efficiency at high load of gas fuelled engines compared to diesel engines.
A phenomenon that occurs in gas fuelled engines is methane slip.
“Methane slip is the release of unburned methane from combustion in the atmosphere. This is concern because methane is a strong GHG gas with a global warming potentional factor 25 higher than CO2.”
There are two main reasons for unburned methane emitted form gas engines:
- Dead volume in form of crevices between cylinder unit components such as :
- Gasket area between cylinder head and cylinder liner
- Between piston top land and cylinder liner
- Behind anti-polishing ring
During the compression stroke the gas mixture is compressed into these crevices and hide away from the combustion. The Methane molecule is very stable and need high temperature to ignite/combust (above 600 degree C depending on air fuel ratio). In the expansion stroke the gas flows out from the crevices, but due to lower temperatures during expansion the methane molecules are to a large degree unburned, and comes out in the exhaust flow. This dead volume can be reduced to a minimum by design, but will always give a certain (significant) amount of unburned methane.
2. Uncomplete combustion in form of quenching at the coldest part of the combustion chamber is another reason for methane slip. Quenching occur when the mixture is too lean and cooled down along the cylinder liner. This will mainly be the case at low load operation. Quenching can be significantly reduced by improved process control by enriching the mixture closer to stoichiometric condition (lambda =1). Richer mixture will create more NOx so the control need to balance the trade-off between unburned methane and NOx.
Other sources of unburned methane could be blow through in the scavenging process and valve overlap. New engine design run with practically no valve overlap so hence unburned methane is consider neglect able.
It is a trade-off for NOx emissions and methane- and CO emissions. By running lean, NOx emissions will be reduced, and as leaner an engine run as lower will NOx emissions become. However, at a point the THC and CO emission starts to rise and at very lean mixtures the combustion process becomes poorer resulting exponential increase in THC and CO and significant reduction in engine efficiency. The following table gives an overview of the methane slip from different gas engines (Sintef 2017)
So far, the main strategy from engine suppliers seems to have been to apply primary measures as optimising engine components by design and engine control strategy. This has shown significant improvement on methane slip compared to first generation marine gas engines.
If stricter regulations should apply, which not could be handled by primary measures, a methane reduction catalyst would be required. Such catalyst need further development to achieve high methane conversion ratio and long term efficiency, and are not considered to be commercially available for ship application with low methane slip concentration.
Comparison of alternative marine fuels
The goal of alternative marine fuels is to reduce greenhouse gasses from shipping while complying with the regulations for local pollutants as SOx, NOx, PM and others. Otherwise, there is a risk of misleading the industry and policy on the true emission penalties of any alternative fuels.
It is important in the assessment of alternative fuels that the emissions released over the full life-cycle or considered and not just during fuel combustion.
“There is at present no (silver bullet) fuel that complies with existing regulation on local pollutants and at the same time delivers big reductions in greenhouse gas emissions and is economically viable“
Last years there is a big uptake in LNG. LNG works perfect for existing regulation on local polutants, but it is not a low GHG fuel.
Bio-derived fuels show potential, but only if it can be ensured that actual savings are realised; land-use change and other upstream emissions, for example from fertiliser use need to be accounted for.
The viability of hydrogen, or other synthetic fuels crucially depends on decarbonisation of the production process, through either grid decarbonisation or switching to renewable feedstocks. There are also other barriers, for example, regarding transport and storage of hydrogen and other alternative fuels.
As a result, while some unresolved issues relate more directly to shipping technology, others are not directly related to the shipping sector, or immediately amenable to regulation of the sector. Taken together, this has important implications considering the urgency to curtail greenhouse gas emissions.
“It is therefore important to ensure that any measure in the short-term does not diminish the potential for roll-out of low carbon fuels in the medium-term,”
in particular when taking into account the long life times of ships and fuel supply infrastructure. To meet the objective of reducing greenhouse gas emissions, whole life-cycle emissions need to be accounted for. For any promising option,
“significant efforts will be required first to demonstrate applicability in practice and subsequently to be scaled up to industrial level,”
with bunkering facilities available along major transport hubs. Aiming to ensure the medium-to long-term sustainability of the sector, action is needed across a range of sectors, and involving both industry and policy. A diverse set of challenges need resolving and any alternative fuel option must fulfil a range of criteria, including proper accounting for full life-cycle emissions.
“Otherwise, the sector could find itself addressing its near-term local pollutants targets at the expense of setting itself up to address its imminent longer-term carbon targets.”
P. Gilbert (Manchester University, 2018 – Assessment of full life-cycle air emissions for alternative shipping fuels.
R. Mcgill 2013 Alternative Fuels for Marine Applications
P. gilbert (2017) full lifecycle assessment alternative marine fuels
IAEBioenergy (2017) Biofuels for the marine sector
M. Taljegard (2015) Electrofuels – a possibility for shipping in a low carbon future?