(a) Describe, with the aid of a sketch, a waste heat recovery system for electrical generation using main engine exhaust gas in combined gas/steam turbine systems. (8)
(b) Describe the operation of the waste heat recovery system described in part (a) whilst the associated main engine is running. (8)
Structured for DG Shipping MEO Class II examination scoring criteria.
At the heart of the system is the exhaust gas boiler situated in the uptake. This crucial component generates superheated steam by harnessing the heat from the main engine's exhaust gas. This superheated steam serves as the primary energy source for the downstream steam turbine. The main engine exhaust gas, including outlets from turbochargers and the gas turbine, is directed into the exhaust gas boiler, facilitating efficient heat transfer.
The gas turbine, clutched and geared to a power turbine, forms an integral part of the system. This power turbine is directly linked to a generator, contributing to electrical power generation. Simultaneously, a steam turbine is connected to the gas turbine, providing an additional means of harnessing energy from the superheated steam generated in the exhaust gas boiler.
A control valve regulates the excess steam flow, directing it to a dump condenser. The system is further complemented by seawater-cooled vacuum condensers, which receive the exhaust from the steam turbine. This comprehensive waste heat recovery system ensures that the thermal energy in the main engine exhaust gas is efficiently converted into electrical power through the combined operation of gas and steam turbines.
Critical to the system's efficiency is the generation of sufficient steam by the exhaust gas boiler, a key requirement met at around 41% engine load. The generator is then synchronized to the main engine switchboard by the power management system, which not only oversees synchronization but also monitors and manages the generated power. The power management system plays a crucial role in the dynamic control of the overall electrical power distribution.
The operation also incorporates the start and stop functionality of auxiliary diesel generators, driven by the power management system to ensure a balance in the electrical power supply. Meanwhile, the main engine control system takes charge of the power (gas) turbine, initiating start and stop procedures and adjusting exhaust gas bypass when the gas turbine is not in operation. This careful design ensures that the gas turbine comes into play at around 53% engine load, contributing additional power through the steam turbine when needed. Importantly, the system is designed to cut out the power turbine when the engine load drops to 47% for a specified duration.
To maintain optimal conditions, especially as the gas turbine reduces available exhaust gas for the main engine turbochargers, the control system monitors scavenge pressure and makes necessary adjustments to the gas flow. This intricate operational scheme ensures not only the efficient generation of electrical power but also the safety and stability of the waste heat recovery system in conjunction with the main engine's operation.