Evolution of structure and oxidation reactivity from early-stage soot to mature soot sampled from a laminar coflow diffusion flame of ethylene

This work investigated the structure and oxidation reactivity of soot sampled from a laminar coflow dif-fusion flame of ethylene. A capillary-nozzle-hybrid sampling method was developed to extract soot from five sampling positions along flame axis, covering both early-stage and mature soot samples. The results reveal that residence time plays an important role in modifying surface functional groups. Oxygenated and aliphatic groups gradually disappear, soot structure becomes more organized. As a consequence, the rate of mass losses is impaired during thermo-chemical conversion. The derivative thermogravime-try (DTG) results show that oxidation of early-stage soot can be separated into low-temperature (low-T) conversion and carbonaceous substances oxidation processes. The former process including both volatile organic fraction (VOF) releasing and early oxidation reactions generates the first maximum mass loss rate at about 510 degrees C, while the latter forms the second maximum mass loss rate at about 600 degrees C. Recognizing that the two processes are partially merged, the distributed activation energy model (DAEM) was intro-duced to decouple the bimodal behavior of DTG curves. The DAEM results reveal that with increased degree of soot maturity, relative contribution from low-T conversion process decreases abruptly, and DTG curve eventually becomes unimodal and can be well simulated by considering only carbonaceous sub-stances oxidation process. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved