New insights into the effect of molecular structure on stable intermediate formation during the pyrolysis of normal and branched alkanes – I: Multi-species time history measurements

Abstract

Decarbonization of energy systems that drive the transportation industry is imperative for effectively mitigating global greenhouse gas emissions. In this pursuit, the transition towards scalable and sustainable fuels will play a key role. These fuels are predominantly composed of a diverse array of hydrocarbons spanning various molecular classes. To better understand the complex chemistry of such fuels, we studied the impact of molecular structure on the distribution of pyrolysis products of C6-C12 hydrocarbons, encompassing both normal and branched alkanes, in a shock tube at combustion-relevant temperatures. An extensive suite of laser absorption spectroscopy-based diagnostics was used to simultaneously measure the time histories of key intermediates (methane, ethylene, and larger alkenes) and fuel during the pyrolysis of ten neat hydrocarbons: n-hexane, n-heptane, n-octane, n-decane, n-dodecane, 2-methylhexane, 2-methylheptane, 2-methylnonane, triptane (or 2,2,3-trimethylbutane), and iso-octane (or 2,2,4-trimethylpentane). These experiments were conducted behind reflected shock waves using 2% Fuel/Argon test mixtures at a nominal pressure of 2 atm over the temperature range of 1100–1500 K. Conducting these experiments under similar test conditions facilitated the observation of clear trends in the distribution of intermediates formed during the pyrolysis of these fuels. The current paper delineates the experimental strategy and presents the measured mole fraction time histories of the stable intermediates. A companion paper [1] delves into the observed trends and effects of molecular structure, in particular, the impact of increasing carbon number and the impact of varying degree of branching on the yields of these pyrolysis products. We also investigate the impact of fuel blending and provide insights into the role of inter-coupling on the distribution of stable intermediates. The authors anticipate that the trends and findings presented in these two papers will guide the design of sustainable fuels and play a pivotal role in modeling the combustion chemistry of the next generation of fuels. To the best of our knowledge, this work presents the first systematic, detailed pyrolysis study of several transportation-relevant hydrocarbon fuels, involving simultaneous measurements of multiple species in a shock tube using laser absorption spectroscopy.