Experimental And Kinetic Modeling Study Of 1-hexene Combustion

Alkenes are an important component family in gasoline. As the octane numbers of alkenes are higher than those of n-alkanes with the same carbon number, alkenes are also known to be good anti-knock boosters and can be used as surrogate components for gasoline. Alkenes are also important intermediates in the pyrolysis and combustion of large long-chain alkanes, cycloalkanes and alcohols. Thus the sub-mechanism of alkenes plays an important role in the combustion of large long-chain alkanes, cycloalkanes and alcohols. Besides, the research of alkenes combustion can provide specific insight into the chemistry of C-C double bond which also widely exists in unsaturated fatty acid-ester molecules in biodiesel. Compared to the studies on alkanes and C4 smaller alkenes, the researches in long-chain alkenes are still limited. The database of fundamental experiment studies on 1-hexene combustion is not perfect, and the conditions of existing combustion experiments for kinetic model validation are limited.In this work, experimental investigations on typically long-chain alkene 1-hexene were performed in flow reactor pyrolysis and the measurement of laminar flame speeds at various pressures. In the pyrolysis experiments, two methods of diagnostic analysis were used. One method was to use synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) to study 1-hexene pyrolysis at low pressure (0.04 atm). It measured the photoionization efficiency (PIE) spectra for species identification and got the mole fraction profiles of pyrolysis species by varying temperatures. About 20 pyrolysis species were detected in the SVUV-PIMS experiments including the reactants 1-hexene (C6H12), active free radicals such as propargyl (C3H3) and allyl (aC3H5), stable species and intermediates eg. hydrogen (H2), methane (CH4), acetylene (C2H2), ethane (C2H4), allene (aC3H4) and propyne (PC3H4), propene (C3H6),1,3-butadiene (C4H6),1-butene (1-C4H8),2-butene (2-C4H8),1-pentene (1-C5H10),2-pentene (2-C5H10), benzene (C6H6), etc. Another method was to investigate 1-hexene pyrolysis at various pressures (0.04,0.2 and 1 atm) using gas chromatography combined with mass spectrometry (GC/GC-MS). In the quantitative analysis, the gas chromatography combined with hydrogen flame ion detector (FID) was used to determine the mole fraction profiles of pyrolysis species with varying temperatures. In the qualitative analysis, the gas chromatography combined with mass spectrometry was used to identify pyrolysis species. In addition, the laminar flame speeds of 1-hexene/air mixtures (?=0.7-1.5) at various pressures of 1-10 atm were also measured using a home-made single-chamber constant-volume combustion vessel from our research group for measuring flame speed of high-boiling-point fuels at conditions of wide pressure range.According to previou studies on theoretical calculation of 1-hexene decomposition pathways, experiments and kinetic models, a kinetic model of 1-hexene combustion with 122 species and 919 reactions was developed based on the alkane and butanol kinetic models from our research group. It was validated and optimized on different types of experimental data from this work (species profiles in pyrolysis and laminar flame speed) and previous literatures (species profiles in jet-stirred reactor oxidation and laminar premixed flame, ignition delay times) over a wide range of experimental conditions, such as low to high pressures, intermediate to high temperatures, and pyrolysis to oxidation circumstances. In general, this model can reproduce the experimental results satisfactorily. According to the rate of production analysis and the sensitivity analysis, discussion about the primary decomposition pathway of 1-hexene, the formation and consumption pathways of C2-C5 alkenes and benzene in the pyrolysis and premixed flames, and the consumption of 1-hexene in jet-stirred reactor oxidation were conducted. It discovered that the dominant consumption pathway of 1-hexene is the allylic C-C bond dissociation in pyrolysis, while the H-atom abstraction reactions and the allylic C-C bond dissociation reaction are the main decomposition reactions in the oxidation circumstances. ROP and sensitivity analysis were also prefomed to understand the elementary reaction and its kinetic effects on flame propagation and ignition delay time. The presence of double C-C bond in 1-hexene molecule leads to the enhanced formation of resonantly stabilized radicals (e.g. aC3H5) and unsaturated intermediates (e.g.1,3-butadiene) and the reactions of these radicals and intermediates are also influential to the flame propagation. These results and analysis also highlight the vital role of allylic reactions (C-C, C-H) in 1-hexene combustion.In general, this work enriches the fundamental experiment database of 1-hexene combustion, and develops a 1-hexene combustion kinetic model of which the applicability has been demonstrated over a wide range of experimental conditions, such as low to high pressures, intermediate to high temperatures, and pyrolysis to oxidation circumstances. Besides, the chemical kinetic characteristics in 1-hexene combustion are analysed and discussed. This model can be used in the development of kinetic models for gasoline alternative fuels as well as long-chain alkenes, alkanes, alcohols and esters.