New insights into the effect of molecular structure on stable intermediate formation during the pyrolysis of normal and branched alkanes − II: Impact of carbon number and degree of branching

Abstract

Transitioning to scalable and sustainable fuels produced from renewable feedstocks will be essential for decarbonizing the transportation sector. As these fuels typically encompass a wide range of molecular classes, it is important to understand the impact of varying molecular structure on their overall combustion behavior. To that end, experiments were conducted to investigate the thermal decomposition of ten neat hydrocarbons with varying carbon numbers and degree of branching in a shock tube at engine-relevant temperatures. A multi-wavelength, laser absorption spectroscopy-based approach was used to measure key stable intermediates formed during the pyrolysis process. The experiments revealed clear trends in intermediate formation and distribution, influenced by the molecular structure of each fuel. Additional experiments were conducted with a 2% composite fuel (consisting of a 50% n-heptane/50% iso-octane mixture)/Ar mixture to assess the impact of fuel blending on the pyrolysis process. A companion paper (Boddapati et al., 2024) delineates the experimental strategy and provides comprehensive mole fraction time histories of the major products, along with thorough analysis of carbon atom recovery. This paper establishes empirical trends in the measured intermediate yields with varying carbon number and degree of branching, and provides a discussion on the chemical phenomena governing the thermal decomposition of these fuels. In addition, this paper also lays the foundation for developing compact, robust chemical kinetic models for conventional and alternative fuels based on compositional information obtained through infrared spectra. The insights gained from these studies have the potential to guide sustainable fuel design and contribute to modeling the combustion chemistry of next-generation fuels.