Modeling and optimization of integrated exhaust gas recirculation and multi-stage waste heat recovery in marine engines

Waste heat recovery combined with exhaust gas recirculation is a promising technology that can address both the issue of NOx (nitrogen oxides) reduction and fuel savings by including a pressurized boiler. In the present study, a theoretical optimization of the performance of two different configurations of steam Rankine cycles, with integrated exhaust gas recirculation for a marine diesel engine, is presented. The first configuration employs two pressure levels and the second is configured with three-pressure levels. The models are developed in MATLAB based on the typical data of a large two-stroke marine diesel engine. A turbocharger model together with a blower, a pre-scrubber and a cooler for the exhaust gas recirculation line, are included. The steam turbine, depending on the configuration, is modeled as either a dual or triple pressure level turbine. The condensation and pre-heating process is optimized to utilize the maximum waste heat recovery. The Genetic algorithm and fmincon active-set algorithm are used to optimize the design and operation parameters for the two steam cycles. The optimization aims to find the theoretically optimal combination of the pressure levels and pinch-point temperatures to maximize the power production. Results show that the two-pressure level steam cycle produces 1577 kW of net power; whereas the three-pressure level cycle produces 1641 kW at full load operation. The optimum pressure levels for the two-pressure level configuration are found to be 33.4/4.7 bara. For the three-pressure level configuration, the optimum pressure levels are found to be 33.5/10.5/4.7 bara. The amount of waste heat recovery from the pressurized boiler is significantly higher than from the main boiler for both cycles. It is, therefore, concluded that the three-pressure level steam cycle (configuration 2) is more efficient than the two pressure level cycle (configuration 1). At the same time, the engine equipped with waste heat recovery with a three-pressure level steam cycle is simpler to operate in Tier II operation. However, the two-pressure level steam cycle is a simpler configuration.