Studies On Biodiesel Physical Property Prediction And Combustion Reaction Kinetics Of Unsaturated Fuels

Biodiesel is a mixture of long chain fatty acid alkyl esters,which are primarily made from vegetable oils,animal fat and other oil-bearing sources via the reaction of transesterification with alcohol.It is a bio-fuel featured with clean-burning,safe-handling,renewability,etc.,and biodiesel has been received as renewable alternative fuel for conventional petroleum based diesel fuel?petro diesel?.Biodiesel,however,has different physical and chemical properties compared with petro diesel,and these properties lead to its different spray and combustion characteristics in engines from those of petro diesel.Studies on prediction models of biodiesel physical property and combustion kinetics underlie the investigations on biodiesel spray and combustion processes,and these two aspects are the main objective of this study.Since it is unrealistic to provide the physical property data required for spray combustion modeling by experimental measurement,predictions models for the density,the viscosity and the vapor pressure of biodiesel were investigated in this study.Firstly,a biodiesel density prediction model named“Rackett-Revised”was proposed with the reference compressibility factor determined by the proposed linear regression method.It was validated against the densities from room temperature up to523 K of three biodiesels derived from canola,jatropha and soapnut oils,in comparison with three GCVOL group contribution density models?GCVOL-Elbro,GCVOL-Ihmels and GCVOL-Pratas?and three other density models based on modified Rackett equation?Rackett-Yamada,Rackett-Soave and Rackett-Yuan?.Results showed that the proposed Rackett-Revised model had the best prediction accuracy over the whole temperature range investigated with an overall average relative deviation of 0.42%,and the temperature dependency of biodiesel density was also well reproduced,which could provide the densities from room temperature to critical temperature for biodiesel spray simulation study.In addition,this study also indicated that the GCVOL group contribution density models are not applicable to temperatures above 373 K.Secondly,an artificial neural network?ANN?model was developed to predict the biodiesel kinematic viscosity at 313 K.The ANN model only requires the mass fractions of fatty acid methyl esters?FAMEs?,without the need of the viscosities of the individual FAMEs required by the Knothe–Steidley model and the Ramírez-Verduzco model.Moreover,the ANN model could learn to account for the interactions between the individual components during the training process;thereby showing better prediction accuracy compared with the other two models,and has the lowest mean square error?0.0099?and the highest correlation coefficient?0.9774?.Moreover,the proposed ANN model has the capability to account for more FAMEs.Thirdly,taking the Riedel equation as the temperature dependency equation to describe the whole vapor pressure curve and introducing of the CH₂?ester?group to distinguish fatty acid methyl,ethyl,propyl and butyl esters,this study developed a group contribution model for the vapor pressure prediction of fatty acid esters with4687 vapor pressure data.Evaluated with 2584 test experimental data,the group contribution model developed in this study showed better reliability and accuracy than the Ceriani group contribution model with accuracy improved by a factor of two.Moreover,the group contribution model of this study was also better in predicting the vapor pressure of biodiesels with the overall average relative deviation of 5.49%,and the temperature dependency of vapor pressure was also well reproduced,which could provide more accurate vapor pressure data for the vaporization simulation study.In terms of combustion kinetics,because unsaturated fatty acid alkyl esters are usually the major component of biodiesel,and alkenes are important intermediate products during the combustion of fatty acid alkyl esters,numerical and experimental studies were firstly carried out for 1-hexene and 1-octene combustion,respectively,based on the recent theoretical progress in reactions of alkenes.Experiments were performed to investigate the low temperature combustion of methyl 10-undecenoate with both a double bond and an ester function.To begin with,a detailed combustion kinetic mechanism for 1-hexene comprising 1113 species and 4794 reactions was developed and validated against experimental data obtained by jet-stirred reactors?JSRs?of atmospheric pressure and high pressure,rapid compression engine,and shock tubes.Among the updated or added reactions in this study,the reaction of addition of hydroxyl radical??H?to1-hexene is the primary fuel consumption pathway of low temperature oxidation.The Waddington mechanism inhibits the low temperature reactivity of 1-hexene.The concurrent reaction channel competing with the Waddington mechanism is the formation of hydroxycyclic ethers.The reactions of allyl radicals with the hydroperoxyl?H?₂?radical show profoundly promoting the low temperature reactivity of 1-hexene,and are the main pathways for consuming allyl radicals and the formation of unsaturated aldehydes.The?-hydroxyalkyl radical oxidation had an inhibiting effect on fuel consumption and contributes to the most production of hexanal and 2-hexanone.Furthermore,the low temperature oxidation of 1-octene was experimentally studied in a JSR at atmospheric pressure?reaction temperatures of 500–1100 K,equivalence ratios of 0.25–2.0?,and a detailed combustion kinetic mechanism comprising 2337 species and 10028 reactions was developed.During the experiment,the formation of 3-ethylcyclohexene and 4-ethylcyclohexene indicates that the endo-cycloaddtion reaction is one of the consumption pathways of alkenyl radicals of1-octene,competing with the isomerization,addition of oxygen molecule and other reactions,which is important reference data for the experimental and numerical studies on the low temperature combustion of long chain unsaturated fuels.Finally,the low temperature oxidation of methyl 10-undecenoate was experimentally studied in a JSR at atmospheric pressure?reaction temperatures of500–1100 K,equivalence ratios of 0.5–2.0?.Experimental results showed an obvious negative temperature coefficient behavior,and an increase of equivalence ratio would inhibit the low temperature reactivity?<800 K?.The formation of methyl10-oxoundecanoate indicates that the oxidation of?-hydroxyalkyl radical bearing an ester function also plays an important role in the low temperature oxidation,and would inhibit the reactivity.Allyl radicals with an ester function could react with H?₂to produce an unsaturated aldehyde with an ester function,i.e.methyl11-oxo-9-undecenoate,and the competing pathway is the formation of acrolein,a typical toxic non-regulated emission.The existence of the ester function decreases the bond dissociation energy of the C-C bond beta to it,which leads to more production of 1,8-nonadiene than 1,9-decadiene.Unsaturated esters can be primarily produced from?-scissions of radicals bearing an ester function,and the retro-ene reaction of unsaturated esters.The detections of oxirane,tetrohydrofuran,and hydroxycyclic ethers with an ester group indicate the additions of H?₂,H and?H are important reaction pathways of low temperature oxidation of methyl 10-undecenoate.Moreover,the detection of cyclohexene bearing an ester function proves that the radicals generated from the loss of an H atom of the long chain unsaturated esters can go through endo-cycloaddition reactions to form cycloalkenes.In terms of low temperature reactivity,methyl 10-undecenoate is lower than the saturated methyl decanoate and methyl stearate,and lies between methyl oleate and methyl linoleate.The experimental data and reaction pathway analyses are important data and references for the development and validation of biodiesel combustion kinetic mechanism.