Multi-mode operation of a novel dual-pressure steam rankine cycle system recovering multi-grade waste heat from a marine two-stroke engine equipped with the high-pressure exhaust gas recirculation system

Abstract

As a promising technology for marine two-stroke engines satisfying the IMO Tier Ⅲ NOx regulation, the high-pressure exhaust gas recirculation (HP-EGR) systems in multi-mode operation significantly redistribute the waste heat, thereby complicating the design and control of the waste heat recovery (WHR) systems. In this paper, a novel dual-pressure steam Rankine cycle (SRC) is proposed to recover multi-grade waste heat from marine engines in multi-mode operation. The HP-EGR system is designed on the 6EX340 engine, followed by the evaluation of waste heat distribution in multi-modes. Detailed thermodynamic and economic analyses of the designed SRC and the combined cycle power plant are then conducted. The results show that the thermal efficiency of the designed SRC is 8.1 %–13.3 % in Tier II mode, increasing to 10.1 %–15.7 % in Tier III mode, attributable to a 16 %–42 % increase in waste heat exergy. Considering typical propulsion profiles, the reduction in weighted average fuel consumption for the combined cycle power plant reaches up to 3.4 % in Tier II operation and 6.1 % in Tier III operation. When 12.5 % of sailing time is spent within ECAs, the combined cycle power plant is expected to yield up to 4 % in annual fuel cost savings.