Implementation of Exhaust Gas Recirculation for Double Stage Waste Heat Recovery System on arge Container Ship

Abstract

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%.

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Implementation of Exhaust Gas Recirculation for Double Stage Waste Heat Recovery System on arge Container Ship