Combustion and Flame, Vol.225, 120-135, 2021
Effects of adding cyclohexane, n-hexane, ethanol, and 2,5-dimethylfuran to fuel on soot formation in laminar coflow n-heptane/iso-octane diffusion flame
The molecular structure strongly affects the fuel sooting propensity. This study aims to investigate the effects of oxygen-free and oxygen-containing aliphatic and aromatic hydrocarbons on soot formation. Experiments were carried out in laminar coflow diffusion flames of a n-heptane and iso-octane mixture doped sequentially with cyclohexane, n-hexane, ethanol, and 2,5-dimethylfuran by 15%. Soot samples were collected for characterization using field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-Ray diffraction (XRD), Raman spectroscopy, and thermal gravimetric analysis. Peculiar structures consisting of ultrafine particles were observed in the deposit collected in all the flames except the one doped with ethanol. According to the surface morphology of soot samples, a model of the peculiar structure of soot deposit was proposed: oxidation free radical attack-deposit and edge defect carbon oxidation model. Based on the results of TEM image, XRD pattern, Raman spectroscopy signal, and thermogravimetric analysis, the reasons for the formation of this peculiar structure were analyzed. The results show that the aged soot particles formed in the flame doped with 15% ethanol (E15) have smaller interlayer spacing, larger polycyclic aromatic hydrocarbon stacking thickness (L-c), smaller grain width (L-a), smaller crystal stacking layer (N), smaller size, and a higher apparent activation energy. These characteristics were consistent with the fairly flat surface of the E15 soot deposits. Soot produced in other flames, especially doped with cycloalkanes, had a large grain width and crystal stacking layers, a lower apparent activation energy, higher oxidation reactivity, and displayed large gaps on the surface and was easily attacked by oxidizing free radicals. Moreover, the large gaps between the crystal layers of the shell structure of single soot particles, and the existence of edge polycarbonate made the crystal edge easy to be activated to produce defects. These soot had high oxidation activity and were prone to secondary combustion, leading to smaller soot particles and forming the peculiar structure on the deposit surface. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.