Experimental And Kinetic Modeling Study Of N-Pentanol Pyrolysis And Combustion

The challenge of energy sustainability is caused by rapid consumption of fossil fuels. The concern on energy crisis drives people to develop and use renewable energy, especially biofuels. Biofuels have a lot of advantages on traditional fossil fuels, such as wide variety of sources, sustainability and environmental protection. The first generation biofuel, ethanol, has already been used as a pure fuel or an additive in internal engines. As prospective biofuels, long chain alcohols like n-butanol and n-pentanol have a lot of advantages over ethanol, such as higher energy density, better miscibility with fossil-derived transport fuels, lower water absorption, and higher suitability for conventional engines. Growing attentions have been paid to the novel production method of n-pentanol from biomass in recent years, since it has more similar physical and chemical properties to gasoline than n-butanol. Compared with the experimental studies of n-butanol combustion, only a limited number of experimental studies have been performed on n-pentanol combustion, most of which focused on the measurement of global combustion parameters such as ignition delay times and laminar flame speeds. Moreover, the models in literature were validated merely by few experimental data. It is recognized that the validation of n-pentanol models is limited due to the very few speciation information on n-pentanol combustion. Consequently, new experimental efforts on the speciation of n-pentanol pyrolysis and combustion, as well as a detailed kinetic model with comprehensive validation are desired.The flow reactor pyrolysis of n-pentanol at various pressures (30,150, and760Torr) and the low pressure (30Torr) laminar premixed flames of n-pentanol at lean and rich conditions (φ=0.7and1.8) are investigated using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis and flame species including stable species, unstable intermediates and radicals are detected by measuring the photoionization efficiency (PIE) spectra, and their mole fractions are evaluated. In both pyrolysis and premixed flames, olefins and CnH2nO species (including enols, aldehydes and2-en-ols) are observed to be the two major product families.A detailed model of n-pentanol consisting of314species and1602reactions is developed in this work. The C0-C4sub-mechanism used in the present model is mainly taken from our recently reported models of butanol isomers, and a new sub-mechanism of n-pentanol is constructed. The present n-pentanol model is also developed and validated on the new data measured in this work and the experimental data of n-pentanol combustion in literature, such as species profiles in the JSR oxidation at10atm and global combustion parameters including ignition delay times and laminar flame speeds at a variety of conditions.The rate of production (ROP) analysis and sensitivity analysis are performed at1330K for the30Torr pyrolysis and1130K for the760Torr pyrolysis to explore the key reactions in the decomposition of n-pentanol at low and atmospheric pressures. In general, the main decomposition pathways of n-pentanol are quite similar at30and760Torr, while the contributions of individual pathways change with pressures. For the primary decomposition of n-pentanol, the unimolecular decomposition reactions and the H-atom abstraction reactions are the two major types of reactions. The water elimination reaction producing1-pentene is the unimolecular decomposition reaction of n-pentanol with the lowest energy barrier. By comparing the concentration levels of water elimination products of n-pentanol and n-butanol, it is noticed that the maximum mole fractions of1-pentene in the pyrolysis of n-pentanol are much lower than those of1-butene in the pyrolysis of n-butanol. This phenomenon indicates the decreasing importance of the water elimination reaction in the pyrolysis of n-alcohols as the length of fuel carbon skeleton increases, which is closely related to its decreasing branching ratio in the unimolecular decomposition reactions of n-alcohols. Among the four types of bonds in n-pentanol, the C-C bonds have the lowest bond dissociation energies (BDEs). Therefore the C-C bond dissociation reactions play a much more important role in the primary decomposition of n-pentanol than the C-H, C-O and O-H bond dissociation reactions. The dissociation reactions of Cα-Cβ, Cβ-Cγ, and Cγ-Cδ bonds consume comparable portions of n-pentanol to the water elimination reaction. Besides the unimolecular decomposition reactions, the H-atom abstraction reactions of n-pentanol by H atom, OH radical, and CH3radical consume almost the rest of n-pentanol. The dominant products are five C5H10OH radicals, which mainly decompose through β-scission reactions, especially β-C-C and β-C-0scission reactions. The decomposition of aC5H10OH radical almost totally proceeds through the β-C-C scission reaction producing n-propyl radical (nC3H7) and ethenol (C2H3OH). bCsHioOH radical can mainly decompose through two pathways, i.e. the β-C-O scission reaction producing1-pentene+OH radical and the β-C-C scission reaction producing ethyl radial (C2H5)+ 2-propen-1-ol (C3H5OH). The decomposition of cC5H10OH radical is dominated by two β-C-C scission reactions producing1-butene+hydroxymethyl radical and3-buten-1-ol (C4H7OH)+methyl radical. The decomposition of both dC5H10OH and eC5H10OH radicals almost totally proceeds through the β-C-C scission reaction producing propene+hydroxyethyl radical (C2H4OH) and ethylene+hydroxylpropyl radical (eC3H6OH), respectively. Specific decomposition products are observed for most of C5H10OH radicals in this work, and the measurements of these decomposition products in this work provide useful experimental data for validating the H-atom abstraction reactions of n-pentanol.The ROP analysis indicates that due to the abundant production of radicals in flames, the H-atom abstraction reactions become more dominant to the consumption of n-pentanol in both flames than in the pyrolysis. Among different types of H-atom abstraction reactions of n-pentanol, the H-atom abstraction by OH radical plays the most important role in both flames. The second important one is the H-atom abstraction by H atom. The unimolecular decomposition reactions of n-pentanol behave quite differently in the lean and rich flames. These reactions have only negligible contributions to the decomposition of n-pentanol in the lean flame, while they become important in the rich flame and consume almost the rest of n-pentanol. The major difference in the consumption of C5H10OH radicals is that they can also react with O2to form enols and aldehydes in flames. In particular, the oxidation reactions of aC5H10OH radical are much more significant than β-scission reactions in the lean flame. For other C5H10OH radicals, the most favored pathways are still the β-scission reactions, while the oxidation reactions only have minor contributions. The experimental and simulated maximum mole fractions of1-butene are around2times as those of1-pentene. The ROP analysis indicates that1-pentene and1-butene are the most favored decomposition products of bC5H10OH and CC5H10OH radicals, respectively. Therefore the observation of much higher concentrations of1-butene than1-pentene exhibits the greater importance of γ-H-atom abstraction than β-H-atom abstraction in the consumption of n-pentanol.The jet-stirred reactor (JSR) oxidation experiments of n-pentanol at five different equivalence ratios are used to validate the present model. According to the ROP analysis, at all equivalence ratios, the leading consumption pathways of n-pentanol are the H-atom abstraction reactions producing five C5H10OH radicals, which is similar to the situation in the flames. Similar to the situations in the pyrolysis and flames, radical is the most dominant H-atom abstraction product. aC5H10OH radical is mainly consumed via the β-C-C scission reaction to produce n-propyl radical and ethenol and the reactions with O2. The experimental results of the ignition delay times of n-pentanol/air mixtures measured by using a shock tube and a rapid compression machine (RCM) are simulated to validate the present model. The sensitivity analysis of the ignition delay time experiments indicates that the reactions which control the ignition behaviors of n-pentanol are quite different at the high and low temperature region. The chain branching reactions that control the high temperature ignition behaviors of n-pentanol only have small sensitivities on the ignition delay time at the low temperature region. The laminar flame speeds of n-pentanol/air mixtures are also simulated in this work to validate the present model. The model can reasonably reproduce these laminar flame speed data at different conditions.