The Role of Methyl Radical in Soot Formation

ABSTRACT

It was noted a significant increase in soot formation with the addition of methane in the pyrolysis of acetylene in shock tubes. Qualitative kinetic analysis revealed an increase of propargyl formation due to methyl/methylene and acetylene recombination. Here, we study the role of methyl radicals in the soot formation process more carefully. Mixtures of acetylene with various sources of methyl radicals – methane, diacetyl and dimethyl ether (DME) – were studied under shock tube pyrolysis conditions over a temperature range of 1650–2250 K and a pressure range of 3.5–4.5 bar. Laser light extinction was used for soot volume fraction and induction time of the particle inception measurements. A more than threefold increase in soot volume fraction with the addition of 1% of methane and 2% of acetylene diluted in argon was observed. Additions of diacetyl and DME resulted in less significant increases in soot volume fraction. All additions of methane, diacetyl and DME reduced the induction time of particle inception. The numerical modeling of the initial gas phase part of the process confirmed our hypothesis that in all cases the increase in the soot yield was proportional to the appearance of an additional amount of CH₃ radicals, which promoted the formation of propargyl, benzene and pyrene.

KEYWORDS:

• Soot formation
• acetylene pyrolysis
• methyl radical
• soot kinetics
• diacetyl pyrolysis
• DME pyrolysis

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Funding

This work was supported by the German Research Foundation (DFG) [grant SCHU 1369/24-1] and the Russian Foundation for Basic Research (RFBR) [grant RFBR-16-58-12014].