Experimental And Kinetic Modeling Studies Of Butanol Combustion

It is suggested that the use of biofuels has the potential to relieve the energy crisis and reduce the greenhouse gas emissions from combustion of fossil fuels. Bio-butanols including four isomers are typical biofuels, which have several advantages over bio-ethanol, such as higher energy density, better miscibility with practical fuels, lower water absorption, and higher suitability for conventional engines. Thus bio-butanols are considered to be a kind of promising biofuels. Investigating combustion chemistry of butanols will help us to understand their combustion behaviors and assess pollutant formation. In this work, the pyrolysis of four butanol isomers at various pressures was studied using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). With the help of high level theoretical calculations, a universal kinetic model was developed to understand the combustion chemistry of four butanol isomers and validated by comprehensive experimental results from this work and literature studies.The pyrolysis of four butanol isomers were studied at800-1600K in a flow reactor. The pressure in the pyrolysis chamber varied from5to760Torr in order to investigate the fall-off effect of unimolecular decomposition reactions of butanols. The photoionization efficiency (PIE) spectra of all observed mass peaks were measured to identify the pyrolysis species. Approximately20-30species, including radicals and enols, were detected and identified in each butanol pyrolysis. The signals were measured at various temperatures with several photon energies to evaluate the mole fraction profiles of the pyrolysis species. The temperature distributions along the centerline of the flow tube were measured and the pressure distributions were calculated to accurately describe the physical model of the flow reactor. Furthermore, the low-pressure premixed flames of n-butanol with the equivalence ratio ranging from0.7to1.8were also studied by SVUV-PIMS. Flame species were identified by the measured PIE spectra, and their mole fraction profiles varying with the axial direction of the burner were measured.Based on our previously developed butene model, a new butanol model with186species and1314reactions was developed to simulate the pyrolysis, oxidation and combustion of four butanol isomers in this work. Compared with previous butanol models, the present model has three primary contributions. First, the unimolecular decomposition reactions of butanols are key reactions in the sub-mechanism of butanols. Available knowledge on the kinetics of these reactions is very limited. In the present thesis, the unimolecular decomposition pathways of four butanol isomers were calculated with high level quantum chemical methods. Then the temperature-and pressure-dependent rate constants of these reactions were calculated by using the RRKM/Master equation method within the range of800-2000K and5-76000Torr. Secondly, the H-abstraction reactions of butanols lead to the production of C4H8OH radicals which are crucial intermediates in combustion of butanols. The kinetics of the p-scission reactions of C4H8OH radicals and their temperature and pressure dependence were also studied theoretically, which helps to optimize the sub-mechanism of C4H8OH radicals. Thirdly, enols are important intermediates in combustion of butanols. The sub-mechanism of enols were developed and optimized based on recent experimental and theoretical studies on elementary reactions of enols.The present model was fully validated and optimized by using the experimental data measured in this work and the literature data from shock tube pyrolysis, laminar premixed flames, jet-stirred reactor (JSR) oxidation, ignition delay times and laminar flame speeds. By using the Chemkin-PRO software, the simulation, rate of production (ROP) analysis and sensitivity analysis were performed to do this work. Furthermore, the simulations with enough accuracy enable us to deeply investigate the combustion chemistry of butanols.The butanols pyrolysis experiments performed in both flow reactor (this work) and shock tube (Hanson’s work) were simulated by the present model. Generally, the present model predicts most pyrolysis species within the experimental uncertainties. It is conclude that the mole fractions of pyrolysis species have high sensitivities to the rate constants of unimolecular decomposition reactions of butanols. In a word, the accuracy of the calculated rate constants in this work is well validated by the comparison between the experimental and simulated results. On the other hand, we also simulated the pyrolysis experiments by previous models, as a result, significant discrepancies on the simulated mole fractions of important pyrolysis intermediates were found by using various models. Significant simulation errors were found for most previous models comparing with our measurements. The ROP analysis and the comparison of the rate constants of unimolecular decomposition reactions in various models indicates that the discrepancies were mainly caused by the rough estimations on these rate constants in the previous models.It is found that the pyrolysis of butanols at all investigated pressures was mainly induced by unimolecular decomposition reactions (including H2O elimination and C-C bond fission) and H-abstraction reactions by H, OH and other small radicals. The dominant decomposition channels change with pressures. Unimolecular decomposition reactions play significant roles at low pressure, while H-abstraction reactions become more and more important as the pressure increases. Significant deviations on the simulated results were observed by changing the rate constants of unimolecular decomposition reaction by a factor of2. It indicates that the pressure-dependent pyrolysis experiment in the flow reactor is very sensitive to the unimolecular decomposition reactions of the studied fuels, especially at low pressure. Consequently it is very suitable to validate the sub-mechanisms of fuels and primary products. The simulated results become much more sensitive to the unimolecular decomposition reactions at low pressure due to the decrease of resident time and molecular density. The H2O elimination reactions have great contributions to the decomposition of all four butanols, and their kinetics clearly reflects the isomeric effects of butanols. In this thesis, we found that the contribution of H2O elimination was highest in tert-butanol pyrolysis, while it was lowest in iso-butanol pyrolysis. The reason is that tert-butanol has nine p-H atoms, while iso-butanol only has one. As seen from the structures of the four butanol isomers, the H2O elimination of n-butanol forms1-butene, and that of sec-butanol produces1-butene and2-butene, while those of iso-butanol and tert-butanol generate i-butene. Our measurements were exactly in accordance with this prediction. Furthermore, the maximum mole fraction of i-butene in tert-butanol was very much higher than those in the pyrolysis of other three isomers.JSR oxidation at atmospheric pressure and higher pressures and low-pressure laminar premixed flames were also simulated by the present model. The simulated mole fractions of most species agree well with the experimental results. The ROP analysis shows that H-abstraction reactions are the most important reactions for the decomposition of butanols under oxidation and flame conditions due to the large amount of H, OH and other small radicals. Thus these oxidation and flame experiments were used to validate the H-abstraction reactions of butanols and the further decomposition reactions of their C4H8OH products. Besides, the low-temperature oxidation chemistry was also validated by the JSR oxidation experiments. The simulation indicates that enols play crucial roles in the decomposition of butanols and the production of aldehyde and ketone pollutants under oxidation and flame conditions. Therefore, the role of enols must be considered to solve the trade-off problem of soot emissions and emissions of aldehyde and ketone pollutants in butanols combustion. Finally, the ignition delay times and laminar flame speeds were used to test the performance of the present model in predicting global combustion parameters.It is concluded that the present model is suitable for combustion simulations under wide range conditions with the temperatures ranging from800to2000K, the pressures varying from5to7600Torr and the equivalence ratios of0.5to∞. It has better performance than previous models on the simulation of the concentrations of pyrolysis/flame species and global combustion parameters such as ignition delay times and laminar flame speeds. This model presents much better accuracy and applicability comparing with previous models, which provides great opportunities to numerically simulate the combustion behaviors of bio-butanols in engines...