| The investigation of bio-oil high temperature oxidation mechanism is of great importance for rational utilization of bio-oil.The detection of intermediates of bio-oil combustion experimentally is still challenging due to the complexity of bio-oil composition and the high temperature feature of combustion procedure.Quantum mechanics(QM)is a commonly employed method for combustion mechanism investigation.It can accurately calculate the thermaldynamic parameters for species involved in an elementary reaction.Kinetic parameters of the elementary reaction can be obtained with QM approach when combined with proper reaction rate theory.However,the atom number of a molecular system investigated with QM is often limited less than or around 100 due to the very expensive computational cost,which makes it difficult to fully characterize complex systems such as bio-oil.Meanwhile,priori knowledge of reaction pathway is necessary when employing quantum mechanics to investigate reaction mechanism,which makes it a more difficult task to study the complex combustion reactions in bio-oil.Reactive Force Field is a reactive force field based on bond order,which becomes an useful method for investigating chemical reactions of larger scale systems when combined with molecular dynamics.ReaxFF MD updates the charge of every atoms at each simulation time step employing Electronegativity Equilibrium Method,which guarantee a better consideration of polarization.The method can also reproduce the potential energy of chemical reactions calculated by Density Functional' Method.Particularly none of priori knowledge on reaction pathways is required in ReaxFF MD simulations,which is an important feature for its application in investigating the reaction mechanism of complex systems.ReaxFF MD has been applied investigation of complex systems such as pyrolysis,combustion,detonation and catalysis with reasonable results obtained.These features of ReaxFF MD makes it potential for investigating complex systems like bio-oil and provides an alternative in studying bio-oil oxidation mechanism at high temperature.To explore the simulation strategy of complex combustion system,the relatively well studied RP-3 high temperature oxidation was investigated first.Temporal evolution data of major reactants(fuel and O2 molecules),product(C2H4)including radical(·CH3)at different temperatures are obtained,which are found to be in the same magnitude of CHEMKIN predictions under the same temperature and initial pressure conditions.Systematic analysis of chemical reactions during the simulations was done by employing VARxMD.Detailed chemical structures and reactions obtained through VARxMD analysis are in broad agreement with literaures.Homolysis and H-abstractions are found to be the 2 main initiation reactions while the former one dominates.The statistics of different initiation pathways shows the possibility of reactions under simulation conditions.Oxygen related reactions are further analyzed,and it is found that O2 mainly react with small molecules of C1-C3,which might provide insights for mechanism reduction.Reaction network of n-decane,one component of the RP-3 surrogate,in initial stage of oxidation at high temperature was revealed based on the analysis of reaction pathways.The high temperature reaction pathways of bio-oil oxidation was then investigated by simulations of a 24-components bio-oil model using ReaxFF molecular dynamics.Construction of the 24-components bio-oil model was based on the GC-MS analysis result of bio-oil yielded from fast pyrolysis of Pterocarpus Indicus.Evolution profiles of fuel,O2 and major products including radicals with time and temperature during the initial stage of bio-oil oxidation were obtained.Major products generated during the simulations are consistent with observations reported in the literature.A kinetic model obtained from the simulated bio-oil oxidation is able to predict long time evolution trend of fuel consumption in ReaxFF MD simulations.With the aid of VARxMD,reaction networks of 5 representative components of the bio-oil model were revealed.The bio-oil oxidation is initiated by series of homolysis and H-abstraction.Propagation reactions involve H-shift,H-abstraction and p-scission.Oxidation of unsaturated C-C bond,ring reduction of phenolic radical and abscission of-CO moister(decarbonylation)appears frequently.Reaction pathways obtained from the comprehensive observations of simulation results are in broad agreement with literature.5 surrogate bio-oil models each containing 6 components were formulated according to the chemical constituent and functional groups in the 24-component bio-oil model constructed in this work.ReaxFF MD simulations are performed on each of the 6-component surrogates and the 24-component oil model(considered as parent fuel model)under the same simulation conditions.In addition to element ratio comparison,the surrogate models are validated by comparing the simulated temporal evolution of major reactants and products including radicals using the 24-component model as a baseline.Analysis shows that all the surrogate models have similar element composition that is very close to the parent fuel model.By comparing the simulation results,it is shown that model#3 can better reproduce the evolution of fuel and O2;Model#4 has better reproducibility for evolution profiles of CH2O,C2H2O and ·CH3;CO evolution is best reproduced by model#2,while H2O by model#5.The 24-component bio-oil model constructed can have a better representing the composition and chemical structure diversity of bio-oil.What obtained from oxidation simulations of the 4-component surrogate of RP-3 jet fuel and a 24-components bio-oil model in this work demonstrates a methodology for investigating the high temperature oxidation pathways of complex fuel systems with ReaxFF MD simulations permitted by the comprehensive reaction analysis capbility of VARxMD.It is also a new attempt to have the formulated bio-oil surrogate models evaluated with ReaxFF MD simulation using the 24-component bio-oil model. |
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