Study On The Chemical Kinetic Mechanism For Multi-component Surrogate Fuel Composed Of N-butanol/gasoline

In face of the stress about environmental pollution and energy shortage,it is urgent to develop new alternative fuels for internal combustion engines（ICE）.Compared with methanol and ethanol,n-butanol is regarded as a potential alternative fuel for traditional gasoline because of its excellent combustion performance.In addition,the physical and chemical properties of n-butanol are similar to those of gasoline,so the butanol/gasoline blends can be used directly in the gasoline engines.Therefore,the research on butanol/gasoline is of strategic importance to achieve energy saving and emission reduction in ICE.In the present,the most researches about butanol/gasoline are focused on the practical application on ICE,while there are few studies involving chemical kinetic model and the understanding on the chemical mechanism of butanol/gasoline is still unclear.It is essential to develop reliable chemical kinetic model of butanol/gasoline for further analysis about it.In this paper,a comprehensive study of butanol/gasoline in terms of chemical kinetics was presented,a reduced mechanism was constructed.It is expected to provide prediction and theoretical guidance for the subsequent application of butanol/gasoline blends.Firstly,the combustion reaction pathway of n-butanol was analyzed in detail utilizing Chemkin-Pro software based on detailed butanol isomers mechanism.It was found that the dehydrogenation of n-butanol is the main way to consume n-butanol.There were different branching reactions because of different dehydrogenation products（C₄H₈OH-1,C₄H₈OH-2,C₄H₈OH-3,C₄H₈OH-4）.Among them,C₄H₈OH-1 was oxygenated and the n-butyraldehyde（n C₃H₇CHO）which has higher activation energy was generated in the subsequent chain propagation,so that the branching reaction was terminated.The other products continued to undergo oxygenation,isomerization and cracking reactions until final stable products formed.In addition,the effect of different combustion parameters such as temperature,pressure and equivalence ratio on the combustion process of n-butanol was investigated.The results showed that the combustion process of n-butanol was significantly influenced by temperature.As the temperature increased,the combustion pathway of n-butanol changed a lot and active free radicals increased.What’s more,the changes in pressure and equivalence ratio did not affect the combustion pathway of n-butanol.The key reactions of n-butanol were summarized and the detailed mechanism was reduced by combining various skeleton-simplification strategies such as the direct relationship diagram method（DRG）and sensitivity analysis method（SA）.The reduced n-butanol mechanism which verified by different basic experimental data was consisted of 59species and 244 reactions.Secondly,n-heptane,iso-octane and toluene were selected to represent toluene reference fuel（TRF）.The reaction pathways of above components were summarized by analyzing the detailed sub-mechanisms.The results showed that the reaction pathways of iso-octane and n-heptane at low temperature were dominated by reactions such as dehydrogenation of alkanes,alkylperoxy isomerization,decomposition of ketohydroperoxides,etc.Both of n-heptane and iso-octane had NTC（Negative Temperature Coefficient）phenomenon at medium temperature,which mainly caused by cracking of alkanes.As an aromatic hydrocarbon,toluene does not have NTC phenomenon.However,the reactions involved were complex,and the decomposition of toluene was the most important reaction.On this basis,a reduced TRF mechanism with 79 species and316 reactions was obtained combining the idea of‘decoupling’.During the process of constructing the reduced mechanism,the H₂/CO kernel mechanism and the detailed mechanism of C₁-C₄were quoted,while the key reactions of n-heptane、iso-octane and toluene were added into it.Thirdly,the n-butanol mechanism was coupled with TRF mechanism to obtain the reduced mechanism for multi-component surrogate composed of butanol/gasoline blends,which included110 species and 445 reactions.In view of the prediction error of ignition delay time of butanol/gasoline,the constructed mechanism was optimized based on the experimental data and the sensitivity analysis of ignition delay time.In order to verify the accuracy of the optimized mechanism,the ignition delay time,laminar flame velocity and key species concentration predicted by the mechanism were verified under wide range of operating conditions using different reactors in Chemkin-Pro.Finally,the combustion processes of different butanol-gasoline blends were further analyzed.It was found that the reaction activity and the ignition delay time of the blends changed with the increase of n-butanol.The blended fuel with 20%n-butanol content had the strongest reaction activity at a low temperature of 700 K,and the ignition delay time of the blending fuels at low temperature was prolonged as n-butanol increased.In addition,toluene played as an important‘intermediate’role in the reaction system.It enhanced the activity of relevant reactions which promote ignition.Simultaneously,the oxidation reaction of toluene absorbs a large amount of reactive radicals,balancing the overall reaction rate of the blending fuels.