The key principle is simple: do not judge a fuel only by what comes out of the funnel. The climate impact depends on its lifecycle, from producing the energy carrier through transport, bunkering and use on board. The IMO’s lifecycle GHG-intensity framework reflects this “well-to-wake” perspective. A fuel that has no carbon in the ship’s exhaust may still have substantial upstream emissions if it is made with fossil energy.
Why shipping needs alternative fuels
International shipping needs deep emissions reductions while continuing to move essential goods. The IMO’s 2023 GHG Strategy aims for net-zero GHG emissions from international shipping by or around 2050, with indicative checkpoints and a 2030 ambition for zero- or near-zero GHG technologies, fuels and energy sources to represent at least 5%, striving for 10%, of energy used by international shipping. Energy efficiency remains essential, but efficiency alone cannot decarbonise a sector that still relies on carbon-intensive fuel.
Alternative fuels are not “drop-in” in the simple sense. They affect the entire vessel and supply chain: tank design, fire protection, ventilation, hazardous-area classification, engine choice, crew competence, bunkering procedures, port permits, emergency response, fuel sampling, certification and commercial contracts. A good fuel strategy therefore begins with a route and a vessel mission, not a fashionable fuel name.
LNG: mature infrastructure, but not a zero-GHG solution
LNG is natural gas cooled to approximately minus 162°C so that it becomes a liquid. Its lower volume compared with compressed gas makes it practical for marine storage, and dual-fuel engines and bunkering systems are well established in several ship segments. LNG can reduce sulphur oxides and particulate emissions substantially when compared with high-sulphur residual fuel, and it can reduce some local air-pollutant emissions. It has consequently been used to meet air-quality requirements in emission-control areas.
However, LNG is principally methane. Methane is a powerful greenhouse gas, and methane slip—unburned methane emitted from engines or across the supply chain—can materially affect lifecycle climate performance. Fossil LNG is therefore not a long-term zero-emission fuel. Biomethane and synthetic methane may offer lower lifecycle emissions if credibly produced and certified, but supply is limited and claims must be assessed against robust lifecycle data. LNG safety also demands cryogenic protection, gas detection, ventilation, hazardous-area controls and trained crews.
Methanol: liquid handling with carbon considerations
Methanol is a liquid alcohol that can be stored at ambient temperature and pressure, making its shipboard handling simpler than cryogenic fuels in some respects. Dual-fuel methanol engines are commercially available, and a growing number of container ships and other vessels have been ordered with methanol capability. Because methanol is a liquid, it can use tank, pump and transfer concepts familiar to the shipping industry, although its toxicity, low flashpoint and material compatibility require specific safeguards.
Conventional methanol made from fossil feedstock is not a climate solution by itself. Renewable methanol may be produced from sustainable biomass or from captured carbon combined with renewable hydrogen. Its value depends on the origin of the carbon and hydrogen, the electricity used, land-use impacts and transparent certification. The right question is not “is methanol green?” but “what is the verified lifecycle GHG intensity of this particular methanol supply?”
Ammonia: carbon-free at use, demanding in safety and supply
Ammonia contains no carbon, so burning it or using it in a suitable energy-conversion system does not produce carbon dioxide at the point of use. This makes it a prominent candidate for deep decarbonisation, especially for ships that need long range and high onboard energy storage. Ammonia is already traded globally as a chemical commodity, giving it an existing industrial base that hydrogen does not yet have at the same scale.
Its challenges are substantial. Ammonia is toxic, corrosive to certain materials and hazardous to people and the environment if released. Fuel systems must prevent exposure, detect leaks, manage ventilation and provide emergency response. Combustion can produce nitrogen oxides and potentially nitrous oxide unless the engine and after-treatment system are carefully controlled. “Green ammonia” requires renewable hydrogen and low-carbon nitrogen production; “blue ammonia” depends on credible capture and storage performance. The fuel can be important, but it is not simple.
Hydrogen: versatile energy carrier with a storage challenge
Hydrogen can be used in fuel cells or combustion engines. At the point of use, a fuel cell produces electricity, heat and water. Hydrogen is attractive for smaller vessels, short-sea routes and applications where frequent bunkering is possible. It can also be a building block for e-methanol, e-ammonia and synthetic methane.
The main difficulty is volumetric energy density. Compressed hydrogen requires high-pressure tanks; liquid hydrogen requires very low cryogenic temperatures; chemical carriers create their own conversion and safety issues. Storage volume can reduce cargo capacity or operational range. Hydrogen has a wide flammability range and small molecules that can leak easily, so ventilation, detection and ignition control are fundamental. Like all alternative fuels, the climate benefit depends on how the hydrogen is produced: renewable electricity-based hydrogen differs greatly from hydrogen made from fossil gas without robust emissions control.
Biofuels: an immediate option with sustainability limits
Sustainable biofuels can be attractive because some blends may be used with existing or adapted engines and fuel systems. Hydrotreated vegetable oil, fatty-acid methyl ester blends and advanced waste-derived fuels are often discussed. They can help operators lower lifecycle emissions without waiting for a completely new ship or global bunkering network.
But “bio” does not automatically mean sustainable. Feedstock origin, indirect land-use change, biodiversity impacts, traceability, water use and competition with food production must be considered. Fuel quality, storage stability and compatibility also matter. A credible biofuel programme uses recognised sustainability criteria, reliable chain-of-custody records and engine-maker guidance. It avoids treating a paper certificate as a substitute for a sound physical supply chain.
Batteries, shore power and wind assistance
Not every solution is a liquid or gaseous fuel. Batteries are effective where the route is short, charging is predictable and power demand can be managed—ferries, harbour craft, inland vessels and some offshore operations are prominent examples. Hybrid systems can combine batteries with engines to reduce generator running at low load, absorb peaks and allow zero-emission operation in port. Shore power can similarly reduce local emissions and noise while a ship is alongside, provided the vessel and terminal are compatible and electricity is available.
Wind-assisted propulsion includes rotors, rigid sails, suction wings and kites. These devices do not eliminate the need for an engine on most commercial routes, but they can reduce fuel consumption by using a freely available energy source. Their performance depends on route wind patterns, deck space, cargo operations, air-draft limits and crew procedures. Efficiency technologies are not a distraction from alternative fuels; they reduce the amount of expensive new fuel a ship will need.
Safety and crew competence
Fuel transition is a human-element transition. Engineers and deck officers need to understand new hazards, operating envelopes, emergency shutdowns, bunkering sequences, personal protective equipment, gas detection, ventilation and incident reporting. Ports need trained bunker operators, pilots, fire services and emergency planners. Equipment design should make the safe action the easy action: clear segregation, reliable alarms, remote isolation, effective drainage, safe escape routes and realistic drills.
Training must be specific to the fuel and ship system. A generic course cannot teach the precise sequence for every bunker station or engine-room arrangement. Operators should combine statutory training, maker manuals, familiarisation, tabletop exercises, simulator practice and drills. Safety culture matters as much as hardware: people must be able to stop an operation when they see an abnormal condition.
How to compare fuel options responsibly
- Define the vessel’s route, duty cycle, speed, port time and required range.
- Calculate energy demand after realistic efficiency measures.
- Compare verified lifecycle GHG intensity, not only tank-to-wake emissions.
- Check fuel availability and bunkering reliability at the ports actually used.
- Assess tank volume, payload effect, machinery maturity and maintenance needs.
- Complete a structured safety and operability assessment with crew input.
- Plan training, emergency response, insurance and regulatory approval early.
Final perspective
Alternative fuels are not a contest with one universal winner. LNG may suit some transitional applications; methanol offers practical liquid handling; ammonia and hydrogen could support deep decarbonisation where their safety and supply challenges are managed; biofuels can provide near-term reductions if genuinely sustainable; batteries, shore power and wind can reduce energy demand. The future belongs to operators who measure lifecycle emissions honestly, invest in efficiency first and treat safety and crew competence as design requirements.