Abstract
To explore the potential interactions between toluene/benzyl and the common C3 combustion intermediates, toluene-propylene and toluene-propyne co-pyrolysis is studied in the current work by taking neat toluene pyrolysis as a reference. Experiments are carried out at a nominal pressure of 20 bar over a temperature range of 1050−1700 K, using a single-pulse shock tube facility coupled to the gas chromatography-mass spectrometry speciation diagnostic technique. Temperature-dependent mole fraction profiles are obtained for numerous species ranging from small-molecule products to three-ring polycyclic aromatic hydrocarbons (PAHs). A kinetic model, which has been under development in our serial works, is extended by including the interplays between toluene/benzyl and propylene/propyne chemistry. The updated model can satisfactorily predict the measurements, regarding the absolute mole fractions as well as the variation trends brought by different initial fuel compositions. Increased reactivity is observed in the conversion of toluene with the presence of propylene or propyne, while the consumption rates of the studied C3 fuels are barely influenced by toluene. Benzene formation is facilitated by the C3+C3 reactions introduced by the C3 fuels. The pyrolysis of propylene (or propyne) significantly enhances the level of C1−C3 molecules/radicals that further react with aromatic species. For instance, the reactions of benzyl+propyne result in much higher mole fractions and lower speciation temperature windows of indene and naphthalene in toluene-propylene (or propyne) co-pyrolysis. The reactions with small hydrocarbons result in reduced levels of benzyl and other C7 radicals in the reaction system in toluene- propylene (or propyne) co-pyrolysis. Consequently, for the PAHs which are mainly formed through C7+C7 reactions, such as bibenzyl and phenanthrene, the mole fractions are lowered by the addition of propylene (or propyne). Propyne has more obvious influences on the species pool of toluene pyrolysis than propylene, because the effective C7C3 interactions are mostly through the reactions between toluene/benzyl and propyne/propargyl in both cases of toluene-propylene and toluene-propyne co-pyrolysis.
Highlights
Towards a predictive kinetic model for the formation of polycyclic aromatic hydrocarbons (PAHs) in the combustion of practical and surrogate fuels, a good characterization of toluene/benzyl chemistry is an indispensable building block
Co-pyrolysis of toluene and propylene, is studied in this work through shock tube experiments and detailed kinetic modeling over a temperature range of 1050–1700 K and at a nominal pressure of 20 bar
The addition of propylene promotes the formation of benzene, mainly through propargyl self-recombination, and more diverse and abundant monocyclic aromatic hydrocarbon (MAH) are observed in toluene-C3 co-pyrolysis
Summary
Towards a predictive kinetic model for the formation of polycyclic aromatic hydrocarbons (PAHs) in the combustion of practical and surrogate fuels, a good characterization of toluene/benzyl chemistry is an indispensable building block. Kinetic modeling analyses suggested that the build-up of PAHs in C3 fuel pyrolysis largely depends on the reaction channels starting from benzene/phenyl [28] It is worth exploring the potential interactions between toluene and C3 fuels, which is scarcely addressed in literature [24], even though specific C3 species, such as propyne and propargyl, are present among the species pool of toluene pyrolysis [11,12]. Postshock gas mixtures are sampled for chemical composition analyses, yielding temperature-dependent mole fraction profiles for species ranging from small hydrocarbons to three-ring PAHs. Updates are made to an ongoing PAH formation kinetic model that has been developed in our recent works [12,22,28,29,30], mainly by supplementing cross-linkage reactions of toluene/benzyl and C3 species. The responsible reaction processes will be identified and the differences of propylene and propyne addition will be unraveled
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