Shock-tube study on the influence of oxygenated co-reactants on ethylene decomposition under pyrolytic conditions

Abstract

The influence of various oxygenated co-reactants on soot formation was addressed by studying the pyrolysis of gas mixtures using ethylene as a base fuel in the absence of molecular oxygen. Alcohols (methanol, ethanol, and n-butanol) and ethers (diethyl ether, dimethoxymethane, furan, and tetrahydrofuran) were selected as oxygenated co-reactants. The pyrolysis process was studied behind reflected shock waves at around 3 bar. Laser extinction at 633 nm was used to determine soot-inception times and the related optical densities as a function of reaction time. The temporal variation in temperature was measured via time-resolved two-color CO absorption using two quantum-cascade lasers at 4.73 and 4.56 µm. Soot particle sizes were determined by time-resolved laser-induced incandescence using a Nd:YAG laser at a wavelength of 1064 nm. The major finding is that based on C2H4 as soot precursor, only CH3OH does not have a soot-promoting effect, whereas the addition of all other oxygenated hydrocarbons results in increased soot yields. The measured temperatures were compared with simulations based on a detailed chemical kinetics mechanism from the CRECK Modeling Group (Pejpichestakul et al. Proc. Combust. Inst. (2019)). In addition, kinetics modeling of hydrocarbon pyrolysis, PAH formation and soot growth was performed in OpenSMOKE++ software to give qualitative insight in the experimental results using the CRECK mechanism and an assembled mechanism composed of the CRECK and the recent PAH sub-mechanism (Sun et al. Proc. Combust. Inst. (2021)). The simulation reveals that the observed promotion of soot formation in presence of oxygenated co-reactants is associated with the release of CH3 and C3H3 during the thermal decomposition resulting in acceleration of C6H6 ring formation.