Experimental and kinetic modelling studies of 3,4-dimethylhexane pyrolysis: Effect of methyl side chain positions on dimethylhexane pyrolysis

Fischer-Tropsch diesel fuel represents a promising alternative energy source in the context of the global energy crisis and environmental pollution. Dimethyl-substituted alkanes are important components of Fischer-Tropsch diesel. It has been reported that the branch chains have significant effects on the combustion performance of Fischer-Tropsch diesel. Therefore, investigating the effects of dimethylhexanes on the combustion performance is crucial for determining the ideal composition of Fischer-Tropsch diesel fuel. This work investigated 3,4-dimethylhexane (C₈H₁₈-34) pyrolysis experimentally and theoretically in a jet-stirred reactor (JSR) at temperatures ranging from 800 to 1200 K and at atmospheric pressure. The influence of the methyl side chain positions on dimethylhexane pyrolysis was also investigated. Over twenty main pyrolysis products were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry, including acetylene, ethylene, allene/propyne, propene, 1,3-butadiyne, vinyl acetylene, 1,3-butadiene, 1-butene, 2-butene, 1,3-cyclopentadiene, benzene, toluene, styrene etc. A detailed kinetic model for C₈H₁₈-34 pyrolysis was constructed based on previous reports. The model was validated against the experimental data obtained in this work. Rate of production (ROP) analysis revealed that C₈H₁₈-34 was mainly consumed through unimolecular decomposition to generate 2-butyl radical and through H-abstraction reactions to generate 3,4-dimethylhex-3-yl radical. The comparison of the pyrolysis of dimethylhexanes (2,3-dimethylhexane, 2,5-dimethylhexane and C₈H₁₈-34) with that of linear n-octane shows that the presence of methyl side chains enhances the pyrolysis reactivity. Positions of the methyl side chains have a marginal impact on pyrolysis reactivity but significantly influence the distributions of C2 – C4 species.