Joule (Constant Pressure) Cycle
Shown above is a simple gas turbine layout. Filtered air is drawn into the system and compressed to about 6 bar. It passes through to the combustion chamber where fuel is introduced and combustion occurs. The hot gas passes through the turbine through to the exhaust. In passing through the turbine it provides motive force for driving the compressor and external requirements.
Advantages and disadvantages
Most modern gas turbine installations are based on aero industry design with suitable marinisation. This takes the form substantially of material changes with respect to the poorer quality fuel used and increased sulphur content.
Specific fuel consumption of 360g/kWh are possible with simple installations. Heat recuperation from the exhaust and gas inlet temperatures of 650'C can improve this to 280g/kWh.
Where inlet temperatures can be increased to 1200'C this falls further to 200g/kW/h. Very special materials are required to make this possible with ceramic and cooled metallic blades being fitted This compares to the 140g/kWh possible in some large slow speed diesel installations. The advantage of the gas turbine falls mainly around its compact size, low weight and reduced maintenance requirements Gas turbine plant was very much out of favour for a long time due to its poor fuel consumption. Emergency generators and fire pumps saw some applications. With improved performance and more flexible plant design- controllable pitch propellers and electrical drives- the much reduced engine size and high power to weight has seen some operators specify this plant especially
for high speed ferries
The above shows a typical layout of a marine gas turbine with thermal efficiency of 38-40%. The NOx output is about 1/10 that of a marine slow speed diesel. The turbine inlet temperature is up to 1200'C and requires critical blade design and material choice.
Shown is a single combuster, in reality there will be several equally spaced around the assembly.
Specific fuel consumption is about 257 g/kWh
Intercooling and Recuperation
ICR technology promises significantly higher efficiencies , flatter fuel consumption curves and improved power to weight ratios for gas turbine propulsion plants.
The system will initially be fitted to warships.
A 30% fuel saving over current simple cycle marine turbines is claimed. Lower manpower requirement, enhanced reliability, reduced exhaust emissions and low airbourne noise are also pointed out.
Components of proven reliability are used such as the RR RB211 and Trent turbines.
An ICR cycle features the following process; Intake air compressed in a low pressure compressor is cooled by rejecting heat via an on-engine intercooler before entering the high pressure compressor. This reduces the work required to compress the air, improving HP spool efficiency and raising net output power. Intercooling also serves to reduce the HP compressor discharge temperature which increases the effectiveness of the recuperator. The recuperator preheats the combustion air by recovering waster energy from the exhaust, thus improving the overall cycle efficiency. The result is reduced fuel consumption over the whole power range.
Low power efficiency is further improved by the use of the power turbines variable nozzles. These maintain a constant power turbine entry temperature which, in turn, maintains recuperator gas side entry conditions and improves recuperator effectiveness as power reduced
CO-GEN WITH A MARINE PROPULSION ENGINE
Reproduced from original work by
Large 2-stroke, direct reversible, turbocharged diesel engines are the dominant prime movers for the world's deep sea shipping. Large 4-stroke engines are generally used on smaller vessels or for diesel-electric propulsion in cruise vessels due to space restrictions and power concentration required. Steam turbines remain only in the niche of LNG carriers while gas turbines have made a very marginal entry in cruise vessel propulsion due to their advantages in size, weight, lower NOx emissions and low noise levels.
C/S view of a large 2-stroke Diesel Engine
Current 2-stroke diesel engines are operating with overall thermal efficiency of 50%, with very low exhaust temperatures after T/C & NOx levels at their limiting values (IMO). This temperature is just sufficient to generate the domestic L.P. steam requirement in an E.G.E. The liquid enthalpy of its cooling water is used for fresh water production in a L.P. Evaporator.
These engines employ uniflow scavenging with constant pressure turbo charging. Electrically driven auxiliary blowers supplement the scavenge air requirement at low loads (30% & lower).
Currently 100MW engines are in service driving a single propeller below 100 rpm.
As fuel prices are at a historical high, it is imperative to reduce fuel consumption. However any further reduction in SFOC will involve a natural increase in NOx emissions.
This COGES plant integrated with a marine propulsion diesel engine is a practical path forward towards reduced operating costs & lower CO2 emissions.
PRINCIPLE OF OPERATION:
By adapting the engine for ambient air intake by changing its timings & rematching of its turbochargers, exhaust gas energy level can be increased & also 10% flow can bypass the turbochargers & feed the power turbine of COGES unit without increasing thermal loading of the engine. Infact as shown below the thermal loading of main engine decreases. This is due to the full utilization of the available turbocharger efficiency.
Engine thermal loading v/s SMCR(12RTA96 Wartsila)
This adapted tuning however incurs a penalty of about 1% increase in fuel consumption, but the gain in recovered energy more than compensates for the loss in efficiency from higher fuel consumption.
However the brake mean effective pressure would normally be increased as compared to a standard engine & thereby an increase in specific fuel oil consumption can be avoided.
As this engine would be operating at elevated firing pressures either it would be derated for minus ambient temperatures or a waste gate would be incorporated to prevent any excess built up of scavenge pressures from ISO limits.
The higher exhaust gas temperatures are used in a natural draft, closed fin exhaust gas economizer to generate larger mass flow of superheated low pressure steam which operates a turbo generator & low mass flow of saturated low pressure steam for domestic heating services.
The power turbine is able to generate about 40% of COGES output power.
DESCRIPTION OF COGES UNIT:
Schematic view of the COGES unit
This system consists of an exhaust gas fired boiler, multistage condensing steam turbine (turbo generator), a single stage exhaust gas turbine (power turbine) and a common generator for electric power production. The turbines & generator are placed on a common bedplate.
The power turbine operates between 50 to 100% SMCR of main engine only, as below this load the efficiency of main turbochargers drop significantly. Due to this bypass arrangement the mixed exhaust gas temperatures rise by around 50 degC. Power output from power turbine is fed to turbo generator via a reduction gearing & overspeed clutch which protects the power turbine from over speeding in case the electric generator drops out due to overload.
The steam turbine feeds its generated power to generator via another set of reduction gearing. In general, when producing excess power the surplus steam to turbo generator can be dumped to a vacuum condenser by the speed control governor via a single throttle valve. While operating in parallel with other diesel generators, the governor operates in a regular way to give correct load sharing.
Arrangement of COGES unit as proposed by Peter Brotherhood Ltd.
A more complicated arrangement also incorporates a tail shaft motor/generator set. This unit is able to generate & feed power to the grid while sailing in low load requirements & vice-versa able to motor the main engine in high load/torque requirements.
This Coges unit claims to deliver upto 10% SMCR KWe at full load.
Heat Balance for Main Engine
Heat Balance of a standard (18.2 bar) engine at ISO reference conditions & 100% SMCR