scavenging
Introduction:
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.
DATA
Main Engine
Model 12K98ME/MC Mk6
Manufacturer Man B&W- Denmark
Nominal MCR 68640 KW @ 94 rpm ( guaranteed upto tropical conditions)
BMEP 18.2 bar
SFOC 171 g/Kwh (Nox compliant) @ ISO conditions
Bore 980 mm
Stroke 2660 mm
Lube oil 1480 m3/hr, 4.8 bar, 70degC max, 12900 Kgs
Cooling sea wtr 2140 m3/hr, 2.5 bar, 50degC max
Cooling fresh wtr 550 m3/hr, 3 bar, 100 degC max
Exhaust gas flow 179.6 Kgs/s, 245 degC at nominal MCR (tropical)
Air flow 176.4 Kgs/s (98% w/w)
Dry Weight 2190 tone
Turbocharger
Model TPL 91B x 3 units
Manufacturer IHI-ABB Japan
Max air flow 55.7 m3/s per unit
overall (max) 74%
turbine (max) 85%
compr (max) 85%
compr (max) 4.0
Shaft power 10,450 KW/unit @ 12000 rpm
Dry weight 14.5 tones/unit
Coges Unit + Exhaust Gas Economizer
Manufacturer Peter Brotherhood Ltd - UK
MCR 7000 KWe
Steam 7 Bar, 270 degC
Condenser 0.06 bar
Feed water 135 degC, hf= 570 Kj/Kg
Turbo generator 6750 rpm
Power turbine 12000 rpm
Generator 1800 rpm
gear box 0.97
generator 0.95
effectiveness 0.70 (E.G.E)
Heat Balance for Main Engine
Heat Balance of a standard (18.2 bar) engine at ISO reference conditions & 100% SMCR
Input
(Fuel power) = = 171 x 42700 x 68640 / 1000 x 3600 = 139,219 KW,
Output
To SMCR = 68640 KW (49.3%)
To Lube oil = = 1480 x 900 x 0.75 x (60-45) / 3600 = 4163KW (2.98%)
To Jkt Wtr = = 550 x 1000 x 4.186 x (80-68) / 3600 = 7674 KW (5.51%)
To Exhaust Gas = = 194 x 1.005 x (215-25) = 37050 KW (26.5%)
(Exhaust gas flow & temperature is corrected for ISO conditions as per project guide instructions)
To Scavenge air = = 194 x 0.98 x 1.005 x (175-45)
= 24840 KW (17.8%)
Power turbine Calculations
After Calculations we have
Work Done = 194 x 0.12 x 1.005 x (706-568)
= 3229 KW Electrical power = 3229 x 0.97 x 0.95 = 2975 KWe (4.33%)
Exhaust Gas Economizer
Heat Recoverable = 194 x 1.005 x (295-165) x 0.70 = 17745 KW
Steam Generation
Steam generation is as follows:
For turbo generator = 20500/3600 x (2997-570) = 13820 KW
For feed wtr heating = 4000/3600 x (2764-570) = 2438 KW
For Domestic purposes = 2500/3600 x (2764-570) = 1524 KW
Turbo generator power = 13820 x 0.3 = 4146 KW = 4146 x 0.97 x 0.95 = 3820 KWe (5.56%)
Economical considerations
Assumptions:
a) Increase in SFOC of engine is avoided using a higher BMEP
b) All electric power produced onboard is utilized.
c) SFOC of diesel generators are at same level as main engine.
d) Annual Savings in maintenance/lube oil costs of 2 gensets is $100000
Now total electric production from COGES unit = 5.56 + 4.33 = 9.9% of SMCR
When operating at 100% SMCR for 280 days/year at ISO conditions:
Annual Fuel costs = 280 x 24 x 0.00017 x 68640 x 355($/ton) = $27,837,089
Annual Fuel Savings = 27,837,089 x 0.099 = $2,755,871
Total Annual Savings = 2,755,871 + 100000 = $2,855,871
Cost of COGES = $4,600,000
S.P.P = 4,600,000/2,855,871 = 1.6 years.
Practical Considerations and other ECO
On the face of it CHP with COGES looks very attractive & is being promoted by both wartsila and man b&w.
However if one reads between the lines there are many other considerations involved
Engines would be operating with much reduced mass purity of gases. With the present fuel specs (ash, carbon residue, sodium and vanadium contents) this would certainly involve much greater fouling in the hot gas circuit resulting in reduced maintenance time between overhauls of all the associated equipments.
Reduced thermal loading with increased exhaust temperatures would lead to greater hot corrosion of exhaust valves.
Increased back pressure from E.G.E would decrease the surge margin and affect both the turbocharger and power turbine performance.
Due to increased firing pressures the engines would have to be derated at sub zero conditions
Apart from the capital cost the installation cost would be very high as this is a non standard piece of equipment in a merchant vessel.
As engines are normally optimized for continuous operations below 85% of nominal MCRs, the actual SFOC is reduced by around 9g/KWh. Besides engines normally operate between 50 to 85% of SMCR. This would result in reduced saving potential.
Diesel gensets would have to be operated in parallel with this COGES unit during narrow passages and rough weather conditions due to their reliability and independent nature. This would reduce the saving potential.
It is very unlikely to absorb 10% of SMCR on a vessel as such electrical loads do not exist.
One idea which is coming up is a turbocharger with an integral motor/generator drive. At lower loads when the T/C is unable to meet the engine requirement the motor drive would feed the additional energy. At higher loads when T/C does not need all the energy of exhaust gases this extra energy could be abstracted by the integral auxiliary drive working as a generator.
This system would simplify engine design. Auxiliary blowers and scavenge air valves could be omitted. This would result in better combustion at part load operations with resultant lower thermal loads & smoother acceleration. Surplus electric power would also be available at service loads.
Introduction:
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.
DATA
Main Engine
Model 12K98ME/MC Mk6
Manufacturer Man B&W- Denmark
Nominal MCR 68640 KW @ 94 rpm ( guaranteed upto tropical conditions)
BMEP 18.2 bar
SFOC 171 g/Kwh (Nox compliant) @ ISO conditions
Bore 980 mm
Stroke 2660 mm
Lube oil 1480 m3/hr, 4.8 bar, 70degC max, 12900 Kgs
Cooling sea wtr 2140 m3/hr, 2.5 bar, 50degC max
Cooling fresh wtr 550 m3/hr, 3 bar, 100 degC max
Exhaust gas flow 179.6 Kgs/s, 245 degC at nominal MCR (tropical)
Air flow 176.4 Kgs/s (98% w/w)
Dry Weight 2190 tone
Turbocharger
Model TPL 91B x 3 units
Manufacturer IHI-ABB Japan
Max air flow 55.7 m3/s per unit
overall (max) 74%
turbine (max) 85%
compr (max) 85%
compr (max) 4.0
Shaft power 10,450 KW/unit @ 12000 rpm
Dry weight 14.5 tones/unit
Coges Unit + Exhaust Gas Economizer
Manufacturer Peter Brotherhood Ltd - UK
MCR 7000 KWe
Steam 7 Bar, 270 degC
Condenser 0.06 bar
Feed water 135 degC, hf= 570 Kj/Kg
Turbo generator 6750 rpm
Power turbine 12000 rpm
Generator 1800 rpm
gear box 0.97
generator 0.95
effectiveness 0.70 (E.G.E)
Heat Balance for Main Engine
Heat Balance of a standard (18.2 bar) engine at ISO reference conditions & 100% SMCR
Input
(Fuel power) = = 171 x 42700 x 68640 / 1000 x 3600 = 139,219 KW,
Output
To SMCR = 68640 KW (49.3%)
To Lube oil = = 1480 x 900 x 0.75 x (60-45) / 3600 = 4163KW (2.98%)
To Jkt Wtr = = 550 x 1000 x 4.186 x (80-68) / 3600 = 7674 KW (5.51%)
To Exhaust Gas = = 194 x 1.005 x (215-25) = 37050 KW (26.5%)
(Exhaust gas flow & temperature is corrected for ISO conditions as per project guide instructions)
To Scavenge air = = 194 x 0.98 x 1.005 x (175-45)
= 24840 KW (17.8%)
Power turbine Calculations
After Calculations we have
Work Done = 194 x 0.12 x 1.005 x (706-568)
= 3229 KW Electrical power = 3229 x 0.97 x 0.95 = 2975 KWe (4.33%)
Exhaust Gas Economizer
Heat Recoverable = 194 x 1.005 x (295-165) x 0.70 = 17745 KW
Steam Generation
Steam generation is as follows:
For turbo generator = 20500/3600 x (2997-570) = 13820 KW
For feed wtr heating = 4000/3600 x (2764-570) = 2438 KW
For Domestic purposes = 2500/3600 x (2764-570) = 1524 KW
Turbo generator power = 13820 x 0.3 = 4146 KW = 4146 x 0.97 x 0.95 = 3820 KWe (5.56%)
Economical considerations
Assumptions:
a) Increase in SFOC of engine is avoided using a higher BMEP
b) All electric power produced onboard is utilized.
c) SFOC of diesel generators are at same level as main engine.
d) Annual Savings in maintenance/lube oil costs of 2 gensets is $100000
Now total electric production from COGES unit = 5.56 + 4.33 = 9.9% of SMCR
When operating at 100% SMCR for 280 days/year at ISO conditions:
Annual Fuel costs = 280 x 24 x 0.00017 x 68640 x 355($/ton) = $27,837,089
Annual Fuel Savings = 27,837,089 x 0.099 = $2,755,871
Total Annual Savings = 2,755,871 + 100000 = $2,855,871
Cost of COGES = $4,600,000
S.P.P = 4,600,000/2,855,871 = 1.6 years.
Practical Considerations and other ECO
On the face of it CHP with COGES looks very attractive & is being promoted by both wartsila and man b&w.
However if one reads between the lines there are many other considerations involved
Engines would be operating with much reduced mass purity of gases. With the present fuel specs (ash, carbon residue, sodium and vanadium contents) this would certainly involve much greater fouling in the hot gas circuit resulting in reduced maintenance time between overhauls of all the associated equipments.
Reduced thermal loading with increased exhaust temperatures would lead to greater hot corrosion of exhaust valves.
Increased back pressure from E.G.E would decrease the surge margin and affect both the turbocharger and power turbine performance.
Due to increased firing pressures the engines would have to be derated at sub zero conditions
Apart from the capital cost the installation cost would be very high as this is a non standard piece of equipment in a merchant vessel.
As engines are normally optimized for continuous operations below 85% of nominal MCRs, the actual SFOC is reduced by around 9g/KWh. Besides engines normally operate between 50 to 85% of SMCR. This would result in reduced saving potential.
Diesel gensets would have to be operated in parallel with this COGES unit during narrow passages and rough weather conditions due to their reliability and independent nature. This would reduce the saving potential.
It is very unlikely to absorb 10% of SMCR on a vessel as such electrical loads do not exist.
One idea which is coming up is a turbocharger with an integral motor/generator drive. At lower loads when the T/C is unable to meet the engine requirement the motor drive would feed the additional energy. At higher loads when T/C does not need all the energy of exhaust gases this extra energy could be abstracted by the integral auxiliary drive working as a generator.
This system would simplify engine design. Auxiliary blowers and scavenge air valves could be omitted. This would result in better combustion at part load operations with resultant lower thermal loads & smoother acceleration. Surplus electric power would also be available at service loads.
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