Combustion and Flame 204 (2019) 331–340 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustfame Structural effects on the growth of large polycyclic aromatic hydrocarbons by C 2 H 2 Abhijeet Raj Department of Chemical Engineering, The Petroleum Institute, Khalifa University, P. O. Box 2533, Abu Dhabi, United Arab Emirates a r t i c l e i n f o Article history: Received 10 January 2019 Revised 4 February 2019 Accepted 20 March 2019 Keywords: PAH Soot growth Kinetics Density functional theory Transition state theory a b s t r a c t Soot particles are composed of planar and curved polycyclic aromatic hydrocarbons with different types of reactive sites, where their growth and oxidation reactions occur. This study presents the effect of cur- vatures in PAHs present in soot on their growth in the flame environment. For this, six planar and curved model PAH molecules having five to nine aromatic rings with armchair and zigzag sites are selected. Den- sity functional theory (B3LYP functional and 6-311G (d,p) basis set) is used to study the reaction ener- getics for their growth through hydrogen-abstraction-C 2 H 2 -addition mechanism that is primarily used in all soot models for their chemical growth. The rate constants evaluated using transition state theory for the reactions involved in the mechanism are provided. Through energetics and kinetics comparison, the differences in the reactivity of planar and curved PAHs at different site types are observed. The growth at armchair site was found to be sensitive to the PAH structure, with the curved one having higher growth rate at all temperatures between 1000 and 2500 K studied in this work. The PAH curvature had less im- pact on the growth at zigzag sites of the model PAHs. The fast conversion of planar PAH to a curved one through ring addition and the high growth rate of curved PAHs at low temperatures suggests that tortuous PAHs can also be present in nascent soot. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Soot and polycyclic aromatic hydrocarbons (PAHs) are carbona- ceous materials formed from incomplete combustion or pyrolysis of hydrocarbon fuels, and are harmful to human health and the environment [1,2]. For instance, when inhaled, they can cause can- cer, and respiratory and cardiac problems [2–4]. Soot is also known to be the second largest contributor to global warming (after CO 2 ) [5]. Therefore, to curb their emissions, strict regulations have been imposed by the environmental protection agencies [6,7]. A variety of studies are present in the literature in order to un- derstand the mechanism of soot and PAH formation [8–14] and their physicochemical properties [15–18], to determine their con- centrations in different types of reactors through experiments and model development [19–22], and to find ways to reduce their emission from engines and other combustors [23–26]. The soot characterization experiments show the presence of PAHs in soot particles, which further indicates that soot particles are incepted by PAHs, though the exact processes of inception (physical addi- tion of PAHs or chemical bonding between them) and growth are not known with certainty. The high-resolution transmission elec- tron microscopy (HRTEM) images of soot particles show fringes E-mail address: abhijeet.raj@ku.ac.ae (representative of PAHs in soot), some of which are near-parallel, while the rest of them are randomly oriented. Moreover, the nu- merical analysis of such images indicates that most PAHs in soots from different fuels have curvatures in them with tortuosity value of more than 1 [16,27,28]. Such curvatures can arise from the pres- ence of an embedded five-membered ring in the PAH structure or due to steric effects. Thus, soot growth would take place through the growth of both planar and curved PAH structures present in soot, which makes it important to study the difference in the reactivity of planar and curved PAHs. In line with the above statement, some studies exist in the lit- erature, where the effect of PAH curvatures has been investigated. For example, in [29], the structural effect on the oxidative reac- tivity of PAHs present in soot particles was examined experimen- tally as well as theoretically using density functional theory. It was found that curved soot and PAH had much higher reactivity to- wards oxygen than the planar ones. In [30], an attempt to model PAH curvatures in a detailed soot model was made, where curved PAHs were formed through a bay-capping reaction with steady- state assumption that led to an embedded five-member ring in the PAH structure. The study presented a robust soot model that was able to satisfactorily predict the experimentally observed gauss curvature of PAHs in soot particles. The bay-capping reaction, used in their model, was taken from You et al. [31], where a kinetic Monte Carlo simulation was presented on PAH growth, and the https://doi.org/10.1016/j.combustflame.2019.03.027 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.