Investigation on n-pentylbenzene combustion at various pressures: Insight into effects of side-chain length on alkylbenzene combustion

This work investigated the flow reactor pyrolysis of n-pentylbenzene using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 0.04 and 1 atm, and measured the laminar burning velocities (LBVs) of n-pentylbenzene in a high-pressure constant-volume cylindrical combustion vessel at 473 K, 1–10 atm and equivalence ratios (ϕ) of 0.7–1.5. A kinetic model of n-pentylbenzene combustion was developed from our previous models of n-butylbenzene and validated against the present experimental data. The unimolecular decomposition reactions and H-abstraction reactions are the dominant consumption pathways of n-pentylbenzene under pyrolysis conditions. Aromatic intermediates with unsaturated side-chains play crucial roles in indene and naphthalene formation, while phenyl and benzyl are precursors of unfused polycyclic aromatic hydrocarbons (PAHs) which in turn convert to fused tricyclic PAHs. In the laminar flame propagation, small species reactions are crucial under lean conditions, while chain-termination reactions, especially fuel-specific reactions, show important inhibition effects on the laminar flame propagation under rich conditions. Furthermore, the effects of side-chain length on the pyrolysis and laminar flame propagation of alkylbenzenes were also investigated for toluene, ethylbenzene, n-propylbenzene, n-butylbenzene and n-pentylbenzene. It is concluded that alkylbenzene smaller than n-butylbenzene tends to decompose at low temperature as the side-chain length increases, while n-butylbenzene and n-pentylbenzene have very close decomposition temperature regions. For indene and naphthalene formation, decomposition/cyclization reaction sequences of C₉ and C₁₀ alkenylbenzenes have rapidly increasing importance in the pyrolysis of large alkylbenzenes. Benzyl is concluded to play an important role in inhibiting laminar flame propagation of alkylbenzenes, which is an important reason for the observations that toluene and ethylbenzene have the lowest and highest LBVs, respectively.