Study On Biodiesel Chemical Kinetics And Optimization Of Its Skeletal Surrogate Mechanism

As the problems caused by greenhouse effect,increasing consumption of fossil fuels and energy crisis become serious,exploring and using new energy is the only way for human to embark on a new era.In recent years,biodiesel has been regarded as the most potential alternative fuel due to its environmental protection,renewability,use in engines without any modification and superior emission performance than diesel.However,biodiesel is complex and composed of a variety of long-chain fatty acid methyl esters.The current research on its chemical reaction kinetics still has a long way to go.Considering of the insufficient research on the reaction kinetics and mechanism of biodiesel,firstly the experiments of biodiesel combustion and pyrolysis were carried out by using the thermo-gravimetric analyzer and Fourier transform infrared spectroscopy.The activation energy,pre-exponential factor and mechanism function were calculated by using the multivariate nonlinear regression method and they were validated by F-test.Secondly,a skeletal mechanism of biodiesel surrogate was constructed,the ignition delay time and main species concentration were validated under wide conditions,and the mechanism was optimized for predicting the early CO2 formation.Finally,the skeletal mechanism of biodiesel surrogate including ozone sub-mechanism was constructed,and the promoted effects of ozone on the ignition and combustion were studied.A reasonable prediction of the combustion process of biodiesel with ozone addition in HCCI engines could be provided by using the mechanism.The results show that the TG-DTG curve of combustion and pyrolysis of biodiesel at different heating rates are consistent.The pyrolysis process is a single-step reaction,and the combustion process is a two-step continuous reaction.The activation energy obtained by the multiple nonlinear regression method is close to the activation energy obtained by the model-free method.The reliability of the obtained mechanism function is verified by the F-test method and the correlation coefficient R2,which is consistent with the prediction of pyrolysis and combustion mechanism.The optimized mechanisms of biodiesel surrogate have been validated well,the ignition delay time in the shock tube and main species concentrations in the jet stirred reactor show good agreements with the experimental value.The capability for prediction of early CO2 formation has been improved obviously;because the newly added reaction path promotes the CO2 rates of production and dominates the early CO2 formation.The ignition process of biodiesel in shock tube and HCCI engine with different ozone concentrations can be predicted well by constructed skeletal mechanism.The biodiesel combustion in HCCI engine with ozone addition was analyzed under the conditions of different initial temperature,equivalent ratio and biodiesel surrogates content,it is found that the effect on combustion phasing by ozone is less sensitive to initial temperature but more sensitive to equivalent ratio.However,as the equivalent ratio and initial temperature rise,the promoting effect of ozone will increase.In addition,the combustion phasing moves along the direction where n-heptane content is high.Based on sensitivity analysis,it reveals that the influence of ozone on multi-component fuels depends on their respective content and sensitivity to ozone.Ozone addition could be used as a useful method to control combustion phasing in HCCI engine for biodiesel.Furthermore,a suitable initial temperature,equivalence ratio and blended fuel that is sensitive to ozone also could be considered as auxiliary adjustment for ozone assisting to control combustion phasing.