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Natural gas is an important source of energy and the trade for natural gas is carried out through specially designed ships known as LNG carriers, which in turn are a special type of gas carriers. Now we know that liquefied gas always has some vapour on top of it and this is known as the BOG or boil-off-gas.
Since ships store cargo in large volumes in their cargo tanks, the amount of vapours are also quite substantial. Since it is essential to take care of these vapours for the safety of the ship, they need to be disposed off in some form or the other. So engineers thought they would use these vapours to supply a portion of the ship’s propulsive power which gave rise to special types of ships having dual fuel systems as we shall see below.
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Dual Fuel Systems
There are several ways in which the boil-off-gas can be handled. The very first way is to use the boiler to burn it off. Of course this is a very simple method but the only disadvantage of it is that the energy which is contained in the gas simply goes waste without solving any useful purpose.
Another alternative which is attractive for the customers or owners of the cargo is that this vaporized gas is liquefied again and put back to the cargo tanks in the liquid form. This is of course a great idea but the only disadvantage it has is that the cost of equipment and paraphernalia associated with the re-liquefaction of this gas is quite large. This also increases the fuel consumption of the ship. At the end of the day this cost would have to be passed on to the consumers. So they may not end up gaining by way of gas savings, as they might have to pay sufficiently extra for the facility. Hence from the business point of view, this option is not very attractive either for the ship operator or the cargo owner.
Last but not least, there is the option of utilizing the energy of this BOG by burning it. But this time it is not burnt unnecessarily without any utilization but used to supply at least a part of the power required for the engines of the ship. This certainly seems to be an interesting idea since the ship is using power from a part of the cargo which it is supposed to carry. But then of course they are just using a part of the cargo which even otherwise would have gone waste and not stealing cargo or something.
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Engines Burning Dual Fuel
As you can intuitively imagine, this requires special types of dual fuel engines which can burn normal fuel such as diesel or heavy oil, as well as these gaseous vapours. When the gas supply is not available these engines can burn normal diesel oil and work based on the diesel cycle. On the other hand when the gas is available these turn into dual mode working on the otto cycle.
It must be noted that the transition between dual mode to single mode is done automatically by using appropriate equipment and ideally it should not affect the output power or speed in any appreciable manner. We will learn more about such dual fuel engines and their actual working procedures and principles in our later articles.