A Waste Heat Recovery System allows large vessels to save energy and reduce CO2
emissions. However, the IMO is putting strict regulation in place regarding NOx and SOx
emissions inside ECAs. A way to reach these emissions is to implement an Exhaust Gas
Recirculation system. Whether these two systems can work together has been investigated.
Fuel composition is evaluated from the lower heating value using the statistical method.
A mixture, with similar LHV and atomic composition, of three known lighter fuels, was
used to simulate combustion with the Glassman mechanism. The excess air ratio has been
taken as given by MAN with no cross-over considered.
EGR is applied, re-introducing a part of the exhaust gas back into the combustion
chamber. This reduces the concentration of O2 and decreases the adiabatic flame
temperature. The production of NOx is highly dependent on the temperature of the
combustion. Lowering this temperature lowers the formation of NOx. By applying EGR,
the Tier III limitation can be reached.
The WHRS converts part of the thermal energy in the exhaust gas to electricity through
one or more Rankine cycles. Water is evaporated and superheated, and is then sent
through a condensation turbine. Higher temperatures and higher pressures at the turbine
inlet is found to increase the system efficiency. This is in accordance with previous
investigations. A WHRS with 2 cycles is set up to utilize available heat sources to reach
the highest possible combination of pressure and temperature.
The system design is optimized using a genetic solver, with an embedded Hessian-based
solver to optimize operation. The system is found capable of producing from 400 to
1900 kW, with a weighed average power relative to the consumption profile of 958 kW.
The consumption profile is found to significantly influence the weighed average power,
where the Tier II/Tier III operation distribution have a much smaller influence. It is
furthermore found that the optimum low pressure is generally between 3.5 and 4 bars,
while the optimal high pressure goes as high as 12.4 bar.
By increasing the efficiency of the overall system, the CO2 emissions can be reduced and
therefore the EEDI can be improved. Taking an average heat recovery value, the CO2
emissions can be reduced by around 5 000 tons/year, corresponding to a 3.5% reduction
in EEDI.
The addition of a third cycle, used only in Tier III is investigated. While increasing the
total heat exchanger areas by approximately 40%, the cycle is found to increase the power
production in Tier III operation up to almost 3000kW, corresponding to an increase of up
to 50%.