Influence of n-Butanol Addition on C₃H₃ Formation in n-Butane Combustion

The effects of different n-butanol blending ratios (R_b) on the formation of propargyl radical (C₃H₃), an important benzene precursor, during the combustion of n-butanol/n-butane blends are studied. A detailed kinetic combustion model of n-butanol/n-butane is developed and the premixed n-butanol/n-butane flames are calculated at an equivalence ratio of 1.5, an initial pressure of 1.0 atm, and a temperature range from 800 to 2000 K in a perfectly stirred reactor (PSR), with R_b varying from 0 to 1.0. The results show that under the investigated conditions, the peak value of the mole fraction of C₃H₃ decreases non-linearly with the increase of R_b. Due to the interaction between combustion products of n-butane and n-butanol during the combustion process, the actual peak mole fraction of C₃H₃ is higher than the theoretical value. A rate of production (ROP) analysis reveals that the number of β-carbon atoms in the molecule of n-butane and n-butanol affects the efficiency of H-abstraction reactions in generating 2-butyl (sC₄H₉) and C₄H₈OH-3 (CH₃–*CH–CH₂–CH₂–OH), which are the two major original sources of C₃H₃. For both n-butane and n-butanol, the main pathway of forming C₃H₃ from propene (C₃H₆) is basically the same, which is C₃H₆ → C₃H₅-a (symmetric allyl radical) → C₃H₄-a (allene) → C₃H₄-p (propyne) → C₃H₃. When R_b ranges from 0.4 to 0.6, the deviation degrees of the peak mole fraction of the involved C₃ species reach a maximum, indicating that the interaction between the two fuels is the most significant. The non-linear decrease in the mole fraction of C₃H₃ can attribute to three reasons: (a) the increase of R_b promotes the increase of the conversion ratios of n-butane to sC₄H₉ and n-butanol to C₄H₈OH-3; (b) the contribution ratios of the reactions involved in the C₃H₅-a → C₃H₄-a → C₃H₄-p → C₃H₃ pathway decrease with increasing R_b; (c) C₃H₅-t (tertiary allyl radical) → C₃H₄-p → C₃H₃ is the secondary pathway for the formation of C₃H₃. With the increase of R_b, the dependence of C₃H₄-p on C₃H₅-t increases and the conversion ratio of C₃H₅-t to C₃H₄-p increases. This study investigates the non-linear decrease of the mole fraction of C₃H₃ by revealing the interactions between n-butanol and n-butane during the combustion, which can help better understand the effect of n-butanol on the formation of benzene.