Numerical Investigation Of High Efficiency And Ultra-low Emission On A Large Marine Low-speed Dual Fuel Engine

The goal of carbon peaking and carbon neutrality and the proposition of near-zero emissions promote development of carbon-free/low-carbon fuels.Natural gas is one of the most popular low-carbon fuels,but the low compression ratio limits the improvement of thermal efficiency on natural gas engines,which have also some problems related to methane slip and NOx emissions.Fuel reforming can improve combustion in cylinder on natural gas engines,and exhaust gas recirculation system can reduce methane slip effectively.Therefore,the 5RT-flex50 DF low-speed two-stroke dual fuel engine was taken as research target,using fuel reforming,EGR and fuel cell to solve the problem mentioned above.There are several works being carried out in this paper:The process of fuel reforming reaction is simulated and analyzed based on Chemkin software.The steam reforming and dry reforming of methane were analyzed to study the effects of temperature and reactant equivalent ratio.In addition,the effect of the addition of n-heptane of the reaction was analyzed.The simulation results show that adding n-heptane can reduce the reaction activation energy and the reaction temperature required.Finally,based on the hydrogen generation,energy consumption and carbon monoxide generation,the case on fuel reforming were selected.Based on the three-dimensional simulation model,the in-cylinder combustion was analyzed.The effects of high pressure EGR and fuel reforming on combustion and emissions were investigated.The results show that the increase of EGR ratio can reduce the methane slip and NOx emission,but reduce the thermal efficiency,while the increase of fuel reforming ratio can increase the thermal efficiency but also increase the emission of NOx.Otherwise,the results show that the maximum thermal efficiency can be obtained when the NOx emission meets Tier III with 5% fuel regeneration ratio on 20% EGR engine.The NOx emission was 2.93 g/k Wh,the engine thermal efficiency was 49.3%,which was 2% higher than the original engine,and methane escape was reduced by about 20%.A combined system of solid oxide fuel cell and low-speed dual fuel engine with fuel reforming was proposed.The fuel cell matching fuel reforming was optimized,and the temperature of fuel reforming and fuel cell were selected.Secondly,the fuel cell was used to replace part of power from the engine in order to a higher efficiency.The results show that when the power of fuel cell was about 25%,the total NOx emission can be reduced to 0.97 g/k Wh,and the thermal efficiency can be increased to 49.8%,2.5% higher than the original engine.