Implementation of Exhaust Gas Recirculation for Double Stage Waste Heat Recovery System on Large Container Vessel

摘要

Concerned to push ships to have a lower impact on the environment, the International Maritime Organization are implementing stricter regulation of NOx and SOx emissions, called Tier III, within emission control areas (ECAs). Waste Heat Recovery Systems (WHRS) on container ships consist of recovering some of the waste heat from the exhaust gas. This heat is converted into electrical energy used on-board instead of using auxiliary engines. Exhaust Gas Recirculation (EGR) systems, are recirculating a part of the exhaust gas through the engine combustion chamber to reduce emissions. WHRS combined with EGR is a potential way to improve system efficiency while reducing emissions. This paper investigates the feasibility of combining the two systems. EGR dilutes the fuel, lowering the combustion temperature and thereby the formation of NOx, to reach Tier III limitation. A double stage WHRS is set up to reach the highest possible combination of pressure and temperature, and adapted to Tier III by introducing two alternative superheaters. The system design is optimized and found capable of producing from 400 to 1900 kW, with a weighted average power of 958 kW. The consumption profile is found to significantly impact the weighted average power, while the operation distribution between Tier III and Tier II (outside ECAs) has a much smaller influence. Furthermore, it is found that the low pressure should be kept near minimum, while the optimum high pressure increases from 7 to 12 bar with the load. By increasing the efficiency of the overall system, the CO2 emissions can be reduced. 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 by an average of 15%, and up to 50% at full load.