Research On The Compound Combustion Kinetic Mechanism Of Ammonia/High Reactivity Fuel And The Combustion Optimization

China has established the strategic goal of low-carbon development of its energy structure.The development and utilization of ammonia,a zero-carbon fuel,is an important way to promote low-carbon energy transformation.Due to the low reactivity of ammonia,the combustion of ammonia needs to be stabilized by high reactivity fuels.In this study,based on the National Natural Science Foundation of China and the Jilin Province Natural Science Foundation,the compound combustion characteristics and combustion mechanism of ammonia/high reactivity fuels were investigated for two combustion assisting fuels with different reactivities,namely,fossilized diesel（hereafter called as diesel）and coal-based synthetic diesel（coal-to-liquid,CTL）.The main reaction paths of ammonia/diesel and ammonia/CTL compound combustion under typical in-cylinder conditions of compression ignition engine,and key species and reactions during the combustion process were studied by means of reaction kinetic analysis.Thermodynamic engine tests were conducted to investigate the combustion and emission characteristics of ammonia/diesel and ammonia/CTL compound combustion.Besides,the experimental study revealed the type of combustion assisting fuel for realizing efficient and clean combustion under high ammonia energy fractions,as well as the potentials for further improving the indicated thermal efficiency and reducing pollutant emissions by adjusting fuel injection parameters.Optical diagnosis and three-dimensional numerical simulation were used to study the flame development,in-cylinder concentration field,temperature field,and the variations of key species and reaction rates during the compound combustion of ammonia/diesel and ammonia/CTL,revealing the intrinsic effects of ammonia energy fraction,combustion assisting fuel and injection parameters on the compound combustion of ammonia/high reactivity fuel.Based on experiment and simulation results,the optimization strategy for injection parameters were proposed.The main research contents and conclusions are as follows:1.The CTL surrogate fuel consisting of 70.56%n-hexadecane and 29.44%iso-hexadecane by mole were constructed based on the practical composition,cetane number,density and lower heating value of CTL.Setting a wide-distillation fuel reduced mechanism as the base model,the ammonia/small-molecule carbon-based fuel mechanism was added.The species of the base model was expanded to include iso-hexadecane andα-methylnaphthalene by adding related sub-mechanisms,and the polycyclic aromatic hydrocarbons（PAHs）and NO_x blocks were updated,resulting in the reduced ammonia/high reactivity fuel compound mechanism which contains 227 species and 937 reactions.Based on the non-dominated sorting genetic algorithm II（NSGA-II）,multi-objective optimization of the pre-exponent factors of the ammonia/interactions between small molecule N-based and C-based species/NO_x sub-mechanism was carried out.The optimized mechanism was validated against various combustion characteristics of pure ammonia and ammonia/various combustion assisting fuels.The validation results indicated the optimized mechanism has good accuracy.2.In order to reveal the coupling effects between ammonia and high reactivity fuels during the ammonia compound combustion process,reaction kinetic analyses on ammonia/diesel and ammonia/CTL compound combustion were carried out.The results showed that during the compound combustion,the start of ammonia combustion is triggered by the exothermic reaction describing the conversion from high reactivity combustion assisting fuel to CO and CO₂,and the concentrations of small-molecule reactive radicals after ignition get lower as the ammonia energy fraction increases.The ammonia reaction pathway of the ammonia compound combustion mainly consists of the oxidation,dehydrogenation,self-bonding reactions of ammonia and its intermediate species,NO production,reduction,and the conversion from NO to NO₂ and N₂O.Under homogeneous conditions,the proportion of self-bonding reactions in various consumption pathways of NH₂ and NH increase with the increase of ammonia energy fraction,while the oxygenation reaction pathway is weakened,and the importance of NO reduction reactions is enhanced.For ammonia compound combustion with different combustion assisting fuels,the effects of combustion assisting fuels on the ammonia reaction pathways and NO_x reaction rates are not significant,and the use of CTL as a combustion assisting fuel leads to a significant reduction in the conversion from fuel to large polycyclic aromatic soot precursors.In addition,the interactions between N-based and C-based species do not have a significant effect on the compound combustion of ammonia/combustion assisting fuel.3.By using research methods including thermodynamic engine experiment,optical diagnostic and three-dimensional numerical simulation,the effect of ammonia energy fraction on ammonia/diesel compound combustion was investigated.The results showed that 20%-40%ammonia energy substitution ratio helps to increase the peak heat release rate and indicated thermal efficiency compared with pure diesel combustion.With further increase in the ammonia energy fraction,the flame brightness,flame area,combustion temperature and OH concentration decrease significantly,the unburned ammonia mass increases and the indicated thermal efficiency decreases substantially.The ammonia energy fraction has a multifaceted effect on NO_x generation.With the increase of ammonia energy fraction,on the one hand,the fuel NO generation is promoted,on the other hand,the reaction rates of NH_i（i=0,1,2）self-bonding reaction（which avoid massive formation of NO）and NO reduction reaction increase,meanwhile,the thermal NO generation is suppressed.Therefore,as the ammonia energy fraction increases,NO emission decreases firstly and then increases,the emission of NO₂ as a subsequent product of NO gradually decreases,but N₂O emission increases due to lower combustion temperature.As the ammonia energy fraction increases,CO emission first increases and then decreases,and total particulate mass emission decreases significantly.4.In order to clarify the effects of combustion assisting fuels with different reactivities on ammonia compound combustion,the combustion characteristics and combustion evolution processes of ammonia/diesel and ammonia/CTL compound combustion were comparatively analyzed.The results indicate that compared with ammonia/diesel combustion mode,ammonia/CTL combustion mode realize higher indicated thermal efficiency when the ammonia energy fraction is 60%and 70%.The use of CTL as a combustion assisting fuel is more conducive to the expansion of ammonia energy substitution ratio while ensuring there is no significant reduction in the indicated thermal efficiency.When the ammonia combustion assisting fuel is changed from diesel to CTL,the flame brightness increases and remains essentially unchanged at 20%and 60%ammonia energy fractions,respectively,while the combustion temperature and OH concentration decrease and the unburned ammonia emission increases.Compared with the ammonia/diesel compound combustion,in the ammonia/CTL compound combustion mode,the NH_i self-bonding reaction and NO reduction reaction are promoted,whereas the fuel NO and thermal NO generation are suppressed,leading to a significant decrease in NO and NO₂ emissions.Ammonia/CTL compound combustion tends to obtain lower CO,soot,soot precursors and particulate emissions,but results in higher N₂O emission.5.Concentrating on the ammonia/CTL compound combustion and its problem of high unburned ammonia emission,the influence and improvement potential of injection timing and injection pressure on combustion and emission characteristics were investigated.The results indicate that as the injection timing gets advanced from-5°crank angle after top dead center（CA ATDC）to-13°CA ATDC,the peak heat release rate of ammonia/CTL compound combustion does not change much,the center of gravity of combustion gets closer to the top dead center,and the peak cylinder pressure and the indicated thermal efficiency increase.At earlier injection timings,the combustion temperature and OH concentration are significantly higher in the later stage of combustion,resulting in a decrease in unburned ammonia emission.As the injection pressure increases from 80MPa to 140MPa,the fuel-gas mixing process of ammonia/CTL compound combustion is improved,combustion volumetricity,combustion temperature,OH radical concentration and indicated thermal efficiency are increased,and the unburned ammonia mass decrease.The advance of the injection timing or the increasement of the injection pressure accelerate fuel NO production,lead to an increase in NO and NO₂emissions,and reduce CO,total particulate mass concentration,soot mass and N₂O emissions.6.Based on experimental and simulation results,the optimization study of injection parameters for ammonia/CTL compound combustion was carried out,and the optimization strategy for the injection parameters was proposed.The results show that when the ammonia energy fraction is 60%,compared with the injection strategy with injection timing of-9°CA ATDC and injection pressure of 100MPa,keeping the injection timing unchanged and increasing the injection pressure to 140MPa can further improve the indicated thermal efficiency,reduce the unburned ammonia mass,while avoid the significant increase in NO emission.After optimization of the injection parameters,the indicated thermal efficiency increases by 1.66%,the unburned ammonia mass decreases by 62.48%and the NO emission increases by 18.68%.