A gas turbine engine is also called the combustion turbine engine due to combustion of gases taking place inside the system. It is like any other internal-combustion engine in which gas acts as a working fluid used to turn a turbine. A gas turbine engine mainly consists of a compressor, a combustion chamber and a turbine. The major components of the gas-turbine engines are compressor, combustion chamber, turbine ,control and start-up.
Operation of gas-turbine engines:-
A source of power is required to rotate the compressor. Atmospheric air is taken from the atmosphere. When this air passes through the compressor (radial or axial), it is highly compressed at very high pressure. It is then fed into the combustion chamber where fuel and ignition is provided Here combustion of fuel takes place at a constant pressure with a portion of air. This leads to an enormous expansion of volume of hot combustion gas available. Now, if another fan or turbine is placed in the path of rapidly expanding gases, the blades of the turbine start moving at very high speed. This moving turbine, if attached to any target, can be used as a means of doing mechanical work, eg to rotate the compressor. A gas-turbine engine differs from the turbo-jet engine in a way that all the events intake, compression, ignition, combustion and exhaust take place in a separate chamber, at same time without any interruption. Gas-turbine engines produce power through the controlled heating of compressed working fluid. For more efficient combustion of the fuel, the combustor is comprised of number of combustion chambers. The most important application of gas turbine engines is in the field of aviation where they provide motive power for jet propulsion. One more area of application is marine propulsion. It has several advantages over steam and diesel –driven plants because it is light weight and compact. In one of the experiment conducted by U.S naval forces in early 1970s, it has been seen that a gas turbine which can generate maximum 20,000 horsepower power can continuously work for more than 5,000 hours.
Below is the clear picture of a primitive type of gas-turbine engine.
The conventional gas-turbine engine has relatively low thermal efficiency as compared to turbo-charged diesel engine or compound diesel engine. This drawback is due to comparatively low temperature and pressure at which combustion takes place. To solve this problem, several advancements have been made and different modes of operation of gas turbine has been adopted to improve its performance.
An abrupt increase in number of ships being launched has resulted in an acute shortage of propulsion system operators, who need to be highly skilled to carry on the process. This has led to the development of new techniques such as boil-off gas (BOG) re-liquifaction plant and duel fuel gas engine.
The S/S LNG Jamal, built at MHI’s Nagqsaki Shipyard has adopted the conventional type turbine as its propulsion system. Cheap heavy fuel oil is used for its operation and saving of BOG is done by re-liquifaction on voyage with cargo loaded. The re-liquifaction process runs on the principle of “intermediate cooling system” using the Brayton cycle in which nitrogen is used as a refrigerant. In this process BOG is liquefied and subcooled at high pressure and is returned back to the tank. Nitrogen is cheap and is easily available in nature. For BOG compression, a system of two centrifugal, single-stage compressors is used for the purpose of sharing boiler supply. For nitrogen compression, a system of three-stage centrifugal compressors has been adopted in which first and second stage is driven by steam turbine and third stage is driven by nitrogen expansion turbine so that power could be recovered during expansion process. Discovery of this re-liquifaction system has lead to automatic(unmanned) operation during normal navigation. This application has dramatically reduced the workload and high skill of operators.
Another discovery is Duel Fuel Engine (DFL). It is a four stroke internal combustion engine. It can attain high thermal efficiency (48%) and low NOx emission rate ( approx. 3kg/kwh on burning gas fuel) It was adopted as the main engine of an electrical propulsion in 2004. It has the capability of being operated both in Gas Mode (follows Otto-cycle in which fuel gas is the main fuel and diesel oil is the pilot fuel) as well as Oil Mode( diesel cycle), therefore it is called the dual fuel engine. Since this engine develops an average of 5 bar gas pressure which is much less than that developed in high-pressure gas injection type diesel engine along with having excellent safety features, it is suitable for being used as a marine engine.
Paxman oil engines are based on four-stroke design. The four-stroke engine is more economical to run, more reliable and less costly to maintain The consumption of power by it is around 20% less than the equivalent two-stroke engine. The reason for its high efficiency is that the exhaust gases are fully scavenged before the inlet stroke and full charge of fresh air leads to complete combustion of fuel.
One more factor is that post combustion, the exhaust valves remain closed for some time, maintaining the maximum possible pressure on the piston to extract the useful energy. Other features of four-stroke engine are utilization of lower lubricating oil, perfect combustion, cooler running and clean exhaust which makes it suitable to operate in those areas where smoke and dirt is unacceptable. It requires very low maintenance cost. Low speed diesel engine when coupled to a fixed-pitch propeller can be popularly used in large-scale merchant ships because of its excellent performance at high power and its wide power range. Here, operators security is not the problem. It requires frequent servicing and maintenance. However, the high-pressure gas injection type low-speed diesel engine is not preferred for marine engines because of reliability, operating results and high-pressure gas handling. Hence, it is used in combination with BOG re-liquifaction plant.
Nowadays, an electrical propulsion system is commonly used as the engine for passenger ships because it is highly reliable and produce less vibrations. But, it can’t be used in large-scale merchant ships because of its expensive expenditure.
The Hybrid propulsion system is a recently discovered system in which diesel engine propulsion and auxiliary electrical propulsion as well as re-liquifaction system and gas combustion system are combined in the propulsion plant and BOG treating plant, respectively. Hybrid propulsion plant, for the first time put into service in June 2004. It is considered to be a suitable system for high- speed vessels and high-powered ships, for eg, for next-generation high-speed container ships. It can generate high power and requires a very less expenditure on fuel. Hybrid Bog treatment plant is a system in which BOG is treated in a safe and efficient manner by adjusting the amount of BOG to be re-liquified and that to be fired. The waste heat generated by the main engine and by partial combustion of BOG is utilized to supply electric power to POD and liquefying power. This system, when combined with advanced marine techniques, can be used in heavy ships. Its advantage is that the demand of skilled crew members is greatly reduced, as compared to other propulsion systems.
Modern Closed-cycle turbine engines use helium as a working-fluid rather than air. The gas is heated by a large sophisticated gas heater. Super cooled helium is used as a cooling agent and system is designed in a complex manner. Due to havig a confined amount of gas, the closed cycle gas turbine is nit called as an internal combustion engine. In this system, normal combustor is replaced with a second heat-exchanger to heat the compressed air before it enters the turbine. The heat is supplied by the nuclear reactors.. The heat produced is utilized in the cycle as a source of providing power to compressors The efficiency of closed cycle gas turbine is double of that of the open cycle. Modern gas turbine engines aims at generation of more power in comparably small machine. Eg. More than 50 MW power is produced in each engine suspended from the aircraft wing. They have been proposed for missions to Mars and other space applications.
Environmental protection measures are necessary to be taken care of while using gas turbine engines. It is required that low-sulfur fuel should be used and NOx emission from the system should be reduced, especially in the coastal areas of Europe and U.S.A. LNG (liquid nitrogen gas) has replaced other fuels I terms of cost, sulfur content. Therefore, HYBRID LNG system are operated by diesel engines during navigation at sea.
The concept of All Electric Ship (AES) was given in 1993. Some of them are
· IEP (Integrated Electric Propulsion): Here a common power source is incorporated for both ship services and propulsion system which is purely electric. Examples are T45, AO and LPD (R).
· IFEP( Integrated Full Electric Propulsion) : It has some advanced power electronics and energy storage system , incorporated into the architecture to give further cost and operational benefits.
· Electric Ship: Advanced prime movers and widespread electrification of auxiliaries are added into IFEP architecture.
Electric Warship: High power weapons and sensors are incorporated.
In France, USA and United Kingdom, several programs have been launched to develop highly torque dense propulsion motors. As a result, radial flux, transverse flux and axial flux came into being. In one of the experiment, French Electric Ship incorporated with AIM (Alstom Advanced Induction Motor) which provide propulsion drives for the “Type 45 Destroyer” has been tested in US IPS program. This technique is used for USA high-temperature super-conducting motor development. Its success has opened the way for making more advanced engines based on excellent design and technology. Current analysis showed that more than 61% efficiency ca be achieved by using available technology. All major manufacturing companies (of large-size gas turbine) aim at achieving at least 60% electrical efficiency in combined cycle application
Conclusion: The evolution in the field of gas turbine engines has lead to the technological advancement in marine turbine propulsion. Those engines which have reduced smoke emission and improved fuel economy with proper safety conditions are best suited to be incorporated as the marine turbine propulsion engine. However, more innovation is still required to develop high torque density propulsion motors so that electric propulsion can be installed in all the ships. More work is required to be done on system design and analysis to maximize operational benefits.
Referred to sites
http://en.wikipedia.org/wiki/Gas_turbine/ Gas turbine
http://www.patentstorm.us/patents/6014855-description.html/ Description of Gas Turbine Engines.
http://www.sciencedirect.com/science/ Lifing methods and safety criteria in aero gas turbines
http://www.allstar.fiu.edu/aero/turbine3.html/ Aircraft Propulsion – Level 3/ Gas Turbine Operation and Design Requirements
http://www.files.hodgenet.org/ sThe AES – Where Next/ C G Hodge FREng BSc MSc CEng
Dated 25th June 2007.