Next-generation propulsion plant

Typical methods being examined for application to
next-generation plants are "dual fuel diesel engine,"
"combined gas and steam turbines (gas turbine engine +
steam turbine engine)" and "diesel engine with
reliquefaction plant."

DUAL FUEL DIESEL ENGINE
In the case of the dual fuel diesel engine, multi-fuel
firing of BOG and heavy oil is possible and combustion
efficiency is similar to that of conventional diesel engine.
However, this necessitates increasing the BOG pressure
for combustion as fuel and supplying a pilot fuel. Another
disadvantage of this method is that BOG cannot
be fired alone, and the operation is not flexible. In 
addition, diesel engine firing at high temperature increases
NOx emissions.


COMBINED GAS & STEAM TURBINE
In the case of the combined gas and steam turbines
(gas turbine engine + steam turbine engine), BOG is fired
in the gas turbine and the exhaust gas is used to generate
steam for driving the steam turbine. Similar to the
effect obtained from the operation of a cogeneration system
for power plants on land, fuel consumption efficiency
is improved when compared to ordinary steam turbines.
However, this method requires the supply of high-quality
petroleum fuel, though the exhaust gas is relatively
clean as in the case of steam turbines, and BOG cannot
be used for multi-fuel firing. They are not advantageous
for this method.

DIESEL ENGINE WITH RELIQUIFICATION PLANT
In the case of diesel engine with reliquefaction plant,
because BOG is reliquefied and returned directly to the
LNG tanks, the ship's propelling system can be separated
from the BOG processing system. This allows the
use of ordinary cargo ship engines, which helps to significantly
reduce fuel consumption. The additional
expenses of this method include the cost of reliquefaction
plant and extra fuel oil to generate the electric power
for its operation. However, the consumption of extra fuel
can be reduced by efficient utilization of the exhaust gas
from the diesel engines.


ECONOMIC CHOICE

The economical valuation of the above-mentioned
methods depends largely on the difference in price between
BOG and fuel oil. When the price of BOG is lower
than that of fuel oil, use of "steam turbine engine" will
be the most economical method. However, use of "dual
fuel diesel engine" may become advantageous if the price
of fuel oil rises. When viewed from an environmental
protection standpoint, "a combination of gas turbine and
steam turbine" may become more attractive. When the
value of BOG is estimated to be higher than others, use
of diesel engine with reliquefaction plant may become
advantageous.