The Tri-fuel engine technology is already present in the marine market and is mainly used for LNG ships with marine diesel oil, heavy fuel oil, and liquefied natural gas (LNG) used together as marine fuels. This concept was introduced by Wartsila and MAN for gas carrier ships.
The same technology is going through research and development for using the tri fuel engine in container ships. The first implementation of such engine is under process on a 7300 TEU container ship undertaken by Wartsila. The new gas ignition auxiliary engines will be installed in this ship which will run on tri fuel technology.
The main aim behind gas fuel for container ship is to counter the high cost of cold ironing and Marine Gas Oil for European ports and to reduce emission as required by emission control area.
LNG for Auxiliary engines
Container ships don’t carry gas cargo in bulk hence unlike gas tankers; they don’t have enough supply of gas fuel. But a small tank for LNG fuel storage can supply enough gas fuel in port to auxiliary engine of a container ship.
The auxiliary engine of such system is provided by conventional fuel supply i.e. MDO and HFO along with LNG supply when in port to comply for the emission norms. During Normal Sea voyage, MGO or HFO fuel is used to generate power.
Just as used in duel fuel engine of MAN B&W, safety precautions are also taken to supply gas to the injection system of the engine. Pilot fuel injection i.e. MDO is used for initial ignition as auto ignition temperature of natural gas is very high i.e. 580 degree centigrade.
The gas must be stored on ship so that it can be supplied to the generators when in port. Two basic methods can be adopted for storing LNG-
1) Gas cylinder containers:
LNG fuel containers can be loaded along with the cargo and quantity should be decided as per the voyage period. A forty feet LNG container can store 31.5 m3 of gas fuel.
The container can be unloaded when empty and exchanged with a filled gas fuel container when the ship is at port.
2) LNG storage Tank
Such tanks can be installed in the ship itself to store LNG fuel along with all required bunkering arrangements and pipelines.
The tank must be a pressure vessel approved by administration with two skins- inner and outer skin.
The inner skin vessel is designed for internal pressure and is normally made up of stainless steel. The outer skin vessel acts as a secondary barrier made up of stainless or carbon steel.
The space between inner and outer skin is filled with insulation like perlite or vacuum.
Advantages of Tri Fuel Engines
The LNG fuel for container ship has following advantages:
- Use of expensive shore power can be eliminated.
- LNG is cheaper and cleaner fuel than Marine Gas oil.
- Ships generator sets can be used to remove voltage and frequency constraints of shore supply.
- Shore supply is not available globally
- No need to install additional and costly exhaust gas cleaning system.
- The Operating and investment cost is less as compare to other means of reducing emissions.