Design And Performance Study Of Combined Power Cycle For Waste Heat Recovery Of The Marine Diesel Engine Coupled With Exhaust Gas Recirculation

Exhaust gas recirculation(EGR)and waste heat recovery(WHR)bottoming power cycle technology are effective ways to satisfy the Tier Ⅲ emission regulation of nitrogen oxides(NOx)and energy efficiency design index(EEDI)proposed by the International Maritime Organization(IMO).The high pressure EGR(HP-EGR)system with great WHR potential is compact and easy to be installed,nevertheless has a noticeable impact on the performance of two-stroke diesel engines.To make the diesel engines reach the best operation efficiency as much as possible,it is necessary to adjust the HP-EGR system according to different emission requirements.Considering different emission operation modes,the WHR bottoming power cycle system design as well as the improvement of the engine fuel consumption remains to be studied.Therefore,this paper focuses on the study of the combined power cycle technology of WHR coupled with EGR by adopting the numerical simulation and thermodynamic analysis methods,and the main research contents and related conclusions are as follows:(1)The GT-Power simulation model of the marine two-stroke diesel engine coupled with the HP-EGR system was established in this paper first.Then the influence of the HP-EGR system on the in-cylinder combustion and scavenging flow characteristics of the two-stroke diesel engine was analyzed by simulation,and the optimization matching scheme of the HPEGR system for two-stroke diesel engines was proposed.Based on this,the optimal operation scheme of the diesel engine considering IMO Tier Ⅱ and Tier Ⅲ modes was obtained through DoE optimization.The results showed that,based on the optimal operation scheme,the scavenging pressure and scavenging mass flow of the engine in IMO Tier Ⅱ and Tier Ⅲ modes are restored to the original state.The fuel consumption in IMO Tier Ⅲ mode is reduced by 5～11 g/kW·h compared to that before optimization,and the fuel consumption in IMO Tier Ⅱmode is 0～3g/kW·h less than that of the original engine.(2)Based on the related parameters of the engine under the optimal operation scheme,an analysis of waste heat characteristics of the diesel engine coupled with the HP-EGR system using the energy and exergy analysis methods was conducted.The results showed that waste heat characteristics of the engine are related to operation mode and are load-dependent.The waste heat energy accounts for about 49～53%in IMO Tier Ⅱ operation mode,and the amount of the waste heat exergy is about 337～1469kW.On this basis,the proportion of waste heat energy in IMO Tier Ⅲ operation mode is increased by 2～5%,and the waste heat exergy is increased by 77～232k W.The increase of waste heat energy and exergy in IMO Tier Ⅲ operation mode is mainly reflected in the high-temperature heat source,which is beneficial to improve the thermal efficiency of the engine coupled with the EGR system by applying WHR bottoming power cycles.Considering the multiple waste heat sources,off-design performance,and IMO Tier Ⅱ and Tier Ⅲ operation modes,a dual-pressure steam Rankine cycle WHR system design was proposed in the present study.With the energy and exergy analysis methods,a thermodynamic calculation model of the dual-pressure steam Rankine cycle was built in MATLAB,and the model was verified under variable load conditions.(3)With the dual-pressure steam Rankine cycle thermodynamic model,a detailed parametric study of design condition and analysis of the system off-design performance were conducted,followed by the analysis of the improvement of fuel consumption of the marine diesel engine brought by the WHR bottoming power cycle.The results showed that LP and HP evaporation pressures have significant effects on the energy and exergy distributions of the dual-pressure steam Rankine cycle.Increasing the HP superheated temperature can improve the net power output of the bottoming power cycle and ensure the reliable operation of steam turbines.The heat source temperatures determine the steam Rankine cycle thermal efficiency(8.1～15.4%),and the net power output is also load-dependent.The application of the dualpressure steam Rankine cycle can improve the fuel consumption of the marine diesel engine by 3.5%(full steaming)and 2.2%(slow steaming)in IMO Tier Ⅱ operation mode,by 5.7%(full steaming)and 4.4%(slow steaming)in IMO Tier Ⅲ operation mode.With sailing time in ECAs accounting for 12.5%,the annual fuel economic cost of the marine diesel engine can be reduced by 4.1%(full steaming)and 2.9%(slow steaming).