CO-GEN WITH A MARINE PROPULSION ENGINE

Introduction:

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:

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:

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

Heat Balance for Main Engine

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

• 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.