Experimental And Kinetic Modeling Studies Of Biodiesel Surrogate Fuels

Energy,which is the core power and important safeguard for our social development and economic growth,has become more and more important.Since the first Industrial Revolution,fossil fuels have been demonstrated on historic stage.Over the past decades,the combustion of fossil fuels have contributed a very large scale to energy consumption(about 88%).However,a series of accompanying problems have been gradually visible such as environmental pollution and energy crisis.Thus,a great deal of public and scientific attention has been transferred into the clean,eco-friendly and regenerative biofuels,in particular,biodiesel fuels.Biodiesel,the first-generation biofuel,can be synthesized from a variety of renewable feedstocks such as animal fats,waste cooking grease,or vegetable oils,through an important process of transesterification with a simple monohydric alcohol(such as methanol and ethanol).Virtually,typical biodiesel is a mixture of alkyl esters with large molecules,or rather,saturated and unsaturated fatty acid methyl esters(FAMEs),e.g.,methyl palmitate,methyl oleate and methyl linoleate.Due to its complicated components,it is pretty hard to study the biodiesel combustion experimentally and computationally,the kinetics measurements and model simulations in particular.In order to overcome these difficulties,the viable solution is to seek for representative surrogates with shorter chain lengths to study the combustion chemistry of real biodiesels.Herein,smaller surrogate fuels with similar chemical functional groups and properties are considered to be preferable to represent the real fuel.With a long enough C4 and C10 hydrocarbon chain and a methyl ester group,methyl butanoate(MB),methyl crotonate(MC)and methyl decanoate(MD)are chosen in this dissertation to limit the number of pyrolysis products and the kinetic mechanism to a manageable level.Present work was carried out in two ways containing experiments and kinetic models.Firstly,the pyrolysis of biodiesel surrogates were investigated in a flow reactor.MB,MC,MD and the related pyrolysis species(i.e.major species,stable intermediates and free radicals)were detected and identified by using gas chromatography-mass spectrometry(GC/GC-MS)and synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS).The mole fraction profiles of fuel and some important species as a function of temperature were constructed.Secondly,the new comprehensive kinetic model of MB and MC as well as the pyrolysis kinetic model of MD were developed and validated by the pyrolysis results.Furthermore,the new comprehensive kinetic model of MB and MC was also validated against the previous experimental data from literatures.The related simulations were performed by the CHEMKIN-PRO software with different modules.Modeling analyses,which include sensitivity analysis and rate of product(ROP)analysis,were also utilized to reveal the combustion characteristics of fuel and releated species.In this dissertation,the pyrolysis of MB,MC and MD was conducted in a flow reactor at various pressures.For MB and MC,the flow reactor pyrolysis experiments at various pressures(30,150 and 780 Torr)were studied by using GC/GC-MS.A number of pyrolysis species including alkanes,olefins and aromatics,etc.were measured and identified.Their mole fractions as a function of temperature were also obtained.A comprehensive kinetic model for MB and MC,which includes 304 species and 1790 reactions,was developed to verify our present experimental results to explore the role of C=C bond in combustion process.The ROP analysis shows that the unimolecular dissociation and H-abstraction reactions play a vital role in MB and MC decomposition.Besides,due to the existence of C=C bond in MC,H-addition reactions are also momentous for MC consumption except unimolecular dissociation and H-abstraction reactions.During the pyrolysis process,we found that the peak mole fractions of benzene and other unsaturated hydrocarbons like acetylene(C2H2),allene(aC3H4)and propyne(pC3H4)in MC pyrolysis are higher than those in MB pyrolysis.The ROP analysis shows that substantial radicals like BAOJ2D(CH3CH=CHCOO),MB2DMJ(CH3CH2CH2COOCH2)and sC3H5CO(CH3CH=CHCO)are the secondary decomposition products of MC,which can be further decomposed to form the previously mentioned unsaturated species,such as C2H2,aC3H4,pC3H4 and benzene with higher concentration levels in the whole thermal decomposition process of MC.However,during the pyrolysis process of MB,the formation pathways of these unsaturated species would suffer a series of sequential reactions,which may limit these products' mole fraction concentrations.Furthermore,previous experimental data such as laminar premixed flame,oxidation in jet-stirred reactor and pyrolysis in shock tube were also applied to verify our combustion kinetic model.Simulations shows a good agreement between the experimental data and the simulated results.For MD,the pyrolysis experiments at 30 and 760 Torr and the temperature ranging from 773 to 1198 K were investigated in a flow reactor.In our work,SVUV-PIMS was employed to observe and identify a great variety of pyrolysis products such as free radicals,n-alkanes,1-alkenes,alkynes,unsaturated esters and aromatics.Anew kinetic model for MD pyrolysis,which includes 286 species and 2165 reactions,was constructed and applied to validate the experimental data.The analysis results show that the decomposition of MD is determined by the C-C bond unimolecular dissociation reactions and H-abstraction reactions via H atom and CH3 radical during the whole pyrolysis process,whereas the contributions of H-abstraction reactions are enhanced as the pressure elevates.C4-C9 unsaturated esters are principally yielded from the ?-scission of ester radicals;while the ?-scission reactions of MD radicals are responsible for the formation of C5-C91-alkenes.In addition,1-alkenes can be further decomposed to form small radicals and molecules.Through the combination reactions such as the reaction routes of C3 + C3,C4 + C2 and C5 + C2,these radicals and molecules can be transformed into benzene and benzyl radical,which are demonstrated as the crucial precursors of polycyclic aromatic hydrocarbons.