Fuel Processing Technology xxx (xxxx) xxx Please cite this article as: Yoichiro Araki, Fuel Processing Technology, https://doi.org/10.1016/j.fuproc.2020.106673 0378-3820/© 2020 Elsevier B.V. All rights reserved. Research article Effects of carrier gas on the properties of soot produced by ethylene pyrolysis Yoichiro Araki a , Yoshiya Matsukawa a, * , Yasuhiro Saito a , Yohsuke Matsushita a , Hideyuki Aoki a , Koki Era a, b , Takayuki Aoki b a Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b Asahi Carbon Co., Ltd., 2 Kamomejima-cho, Higashi-ku, Niigata 950-0883, Japan A R T I C L E INFO Keywords: Soot properties Carrier gas effect Pyrolysis Particle size distribution Crystallites Primary particle diameter ABSTRACT Herein, we investigate the mechanism of soot formation produced by controlled-temperature pyrolysis of ethylene in Ar, He, or N 2 and analyze the size distributions of soot particles to understand soot aggregation and nucleation behavior. Additionally, we probe the reactivity of soot, revealing that soot particles were highly unlikely to react with molecules of the above carrier gases. The use of Ar is demonstrated to promote soot production, facilitate crystalline growth, and increase the diameter of primary soot particles, while the use of He has an opposite effect, which is ascribed to the large size of the former molecules and the small size of the latter ones. Consequently, the soot generation mechanism is found to be affected by carrier gas type. 1. Introduction Air pollution, one of the most important environmental issues facing mankind, largely results from the emission of particulate aerosols such as soot into the atmosphere. In particular, soot particles can very effectively absorb sunlight and convert it into heat, thereby signifcantly contributing to global warming and harming the environment [1]. Furthermore, the genesis of soot particles is believed to involve the formation of strongly carcinogenic polycyclic aromatic hydrocarbons (PAHs) [2]. Consequently, the crystallites, morphology, and genesis of soot particles and generation mechanisms of PAHs have been exten- sively investigated to minimize soot production and emission [3–19]. On the other hand, soot is an important industrial product (carbon black, CB) widely used for tire and ink fabrication. The morphology of CB strongly affects the energy loss and wear resistance of tires, which makes the development of highly accurate techniques for CB morphology and properties control a task of high practical signifcance [20–25]. Currently, the above goal is mostly achieved by trial-and-error, since CB particles are generated in a very short period of time at high tem- perature. Transcending the empirical control method and controlling the morphology with theoretical one based on the CB generation mechanism will make it possible to control its morphology with suff- cient accuracy. Therefore, to deeply understand factors affecting CB morphology, one needs to investigate CB generation mechanism. The mechanism of CB generation resembles that of soot generation and can be divided into the stages of macromolecular nuclei formation and aggregation. The former stage features feedstock pyrolysis to generate PAHs that are further decomposed to produce soot nuclei. Shukla et al. analyzed chemical species generated by pyrolysis of various feedstocks by time-of-fight mass spectrometry and demonstrated that PAH formation involved hydrogen abstraction – carbon addition, phenyl addition/cyclization, and methyl addition/cyclization as the three main mechanisms [26–29], while Ono et al. showed that the addition- cyclization mechanism is particularly important for the generation of large-molecular-weight PAHs [30]. After PAH generation, soot nuclei fuse with each other to afford spherical soot particles. In the subsequent aggregation stage, these spherical particles undergo Brownian move- ment and continuously collide with each other to form soot aggregates. Additionally, Ono et al. pointed out that CB formation involves the reduction of soot particle surface area via sintering [22]. Despite the above insights, the extremely short time scale of CB production and the multitude of involved processes complicate further mechanistic investigations. In general, CB is industrially manufactured via continuous thermal decomposition of feedstocks using the heat supplied by fuel combustion [31]. In this process, CB formation takes place in the presence of large amounts of nitrogen and other atmospheric (carrier) gases, the mole- cules of which are believed to act as third bodies in radical reactions * Corresponding author. E-mail address: matsukawa@tohoku.ac.jp (Y. Matsukawa). Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc https://doi.org/10.1016/j.fuproc.2020.106673 Received 25 June 2020; Received in revised form 15 October 2020; Accepted 8 November 2020