Formation of Polycyclic Aromatic Hydrocarbons in Fuel-Rich Oxidation of Toluene, Toluene/Dimethyl Ether Blend, and Toluene/Oxymethylene Ether‑3 Blend

Ethers are considered as a potential alternative or fuel additive to existing fuels due to their various advantages in reducing the level of CO₂ and pollutant emissions. Although the blending effects of ethers with the existing hydrocarbons on the formation of polycyclic aromatic hydrocarbons (PAHs) are crucial for their use as drop-in fuels, they have not been well understood. In this study, we employed two ethers, dimethyl ether (DME) and oxymethylene ether-3 (OME₃). The fuel-rich oxidation of neat toluene, toluene/DME, and toluene/OME₃ was investigated using an atmospheric flow reactor at temperatures of 1050–1350 K, an equivalence ratio of 9.0, and a residence time of 1.2 s. The identified and quantified PAHs with one to five aromatic rings were analyzed using gas chromatography mass spectrometry. PAH production was reduced by blending ethers at a temperature above 1250 K, while greater production of PAHs was observed in toluene/DME and toluene/OME₃ blends compared to neat toluene at a temperature below 1150 K. A chemical kinetic model that contains not only the PAH formation mechanism but also the oxidation reactions of DME, OME₃, and toluene was developed based on a combination of the existing models. The developed model could reasonably reproduce the experimental results of the PAHs obtained here. To investigate the temperature dependence of PAH formation, kinetic analyses using the proposed model were conducted to examine the formation pathways of PAHs in the three fuels, as well as the consumption pathways of DME and OME₃. The blending effects of DME and OME₃ to enhance PAH production were pronounced at a low temperature such as 1050 K, while they inhibited PAH formation with increasing temperature. At 1050 K, DME and OME₃ exhibited reactivity higher than that of toluene, resulting in the abundant production of reactive radicals and promoting toluene consumption. This led to larger PAH production in the blends at a low temperature. When comparing the mixing effects of DME and OEM₃, the reactivity of OME₃ was higher than that of DME, leading to a greater production of PAHs in the toluene/OME₃ blend compared to the toluene/DME blend at a low temperature.