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.
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.