Large ships need powerful engines to move heavy cargo, research equipment, or people across oceans. Many older vessels burn heavy fuel oil or marine diesel, which can release carbon dioxide, nitrogen oxides, sulfur oxides, and soot. Liquefied natural gas, or LNG, is used as a cleaner ship fuel because it contains mostly methane and almost no sulfur.
Dual-fuel engines let a ship use LNG when available while still keeping conventional marine fuel as a backup.
Understanding Ships and Submarines: LNG and Dual-Fuel Engines
LNG changes the fuel system as much as it changes the engine. It is kept in heavily insulated cryogenic tanks, often shaped like cylinders or spheres to handle pressure safely. Heat slowly enters even a well insulated tank.
A small amount of liquid then turns back into gas. This is called boil off gas. Ships can send this gas to the engines, keep it under controlled pressure, or manage it with special equipment.
Tank location matters because LNG tanks take up more space than oil tanks holding the same useful energy. Designers must balance cargo space, fuel range, stability, and access for inspections.
Before methane reaches an engine, it passes through pipes, valves, filters, and control systems. Its pressure and temperature must be adjusted carefully. In many large dual fuel engines, a tiny amount of diesel starts combustion.
The burning diesel provides a hot ignition source for the gas mixed with air. Other engine designs use different ways to ignite or inject the gas. The control system changes the fuel flow whenever engine load changes.
This matters during manoeuvres near a port, when a ship may need to speed up or slow down quickly. If the gas and air mixture is wrong, the engine can lose power, knock, or produce more unburned fuel.
LNG can improve air quality near busy shipping routes because it avoids most sulfur pollution and greatly cuts soot. That can matter for people living near ports, where ship exhaust mixes with road traffic and factory emissions. Its climate effect needs careful study, however.
Burning methane makes carbon dioxide, though usually less carbon dioxide per unit of energy than many oil fuels. Methane that escapes without burning is a serious concern because it traps much more heat than carbon dioxide over a shorter time period. This escape is called methane slip.
It can happen in engines, fuel pipes, tanks, or during refuelling. A good environmental comparison includes fuel production, transport, storage, and use rather than only the exhaust leaving the funnel.
Refuelling with LNG is called bunkering. Crews follow strict procedures because the fuel is extremely cold and the gas can form a flammable mixture with air. They check connections, ground equipment to reduce static electricity, watch for leaks, and keep ignition sources away.
Gas detectors and emergency shutoff valves are important parts of the system. Students learning this topic should separate two ideas. One is local air pollution, such as soot and sulfur oxides.
The other is global warming caused by greenhouse gases. A fuel can perform well in one area while still having limits in the other. This is why engineers measure emissions carefully and continue developing engines that reduce methane slip.
Key Facts
- LNG is natural gas cooled to about -162 degrees C so it becomes a liquid.
- Liquefaction reduces natural gas volume by about 600 times, making it practical to store on a ship.
- Main methane combustion reaction: CH4 + 2O2 = CO2 + 2H2O + energy.
- Dual-fuel engines can burn LNG with a small pilot injection of diesel or switch fully to marine diesel depending on design.
- Using LNG can greatly reduce SOx and particulate emissions because methane contains almost no sulfur and produces little soot.
- Carbon dioxide from combustion can be estimated by moles: 1 mol CH4 produces 1 mol CO2.
Vocabulary
- LNG
- Liquefied natural gas is mostly methane that has been cooled into a liquid for compact storage and transport.
- Dual-fuel engine
- A dual-fuel engine is a marine engine that can operate on LNG and also use conventional liquid fuel such as diesel or heavy fuel oil.
- Cryogenic tank
- A cryogenic tank is an insulated storage tank designed to keep very cold liquids such as LNG at low temperature.
- Methane slip
- Methane slip is unburned methane that escapes from an engine or fuel system into the atmosphere.
- Emissions
- Emissions are gases and particles released into the air from combustion or other processes.
Common Mistakes to Avoid
- Thinking LNG is stored as a high-pressure gas, which is wrong for most marine LNG systems because it is mainly stored as a very cold liquid in insulated tanks.
- Assuming LNG has zero emissions, which is wrong because burning methane still produces CO2 and water vapor, and unburned methane can escape as methane slip.
- Confusing dual-fuel with hybrid electric propulsion, which is wrong because a dual-fuel engine switches between fuels rather than between an engine and a battery motor.
- Ignoring the pilot fuel in many LNG engines, which is wrong because some dual-fuel engines need a small diesel injection to ignite the natural gas mixture.
Practice Questions
- 1 A ship has 1200 m3 of LNG. If liquefaction reduces natural gas volume by a factor of 600, what volume of natural gas would this become at normal gas volume?
- 2 Using CH4 + 2O2 = CO2 + 2H2O, how many moles of CO2 are produced when 250 moles of methane burn completely?
- 3 A cargo ship can run on LNG near a coast but switches to marine diesel when LNG is unavailable. Explain one environmental advantage and one engineering tradeoff of using a dual-fuel engine.