# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 2 Formation Mechanisms of Benzene and PAHs
### Formation Mechanisms of PAHs
#### 2.2.3 Formation Mechanism with Radicals
Dehydrocyclization occurred with a rise in temperature, leading to the transformation of thermally unstable phenyl radicals into stable condensed PAHs, as shown in Figure 8. [PERSON] et al. [62] proposed phenyl addition as a more efficient mechanism for PAH growth in circumstellar environments, particularly for triphenylene and  
Figure 7: Typical resonantly stabilized radicals for aromatic formation. fluoranthene. They revealed that the phenyl addition mechanism exhibits high efficiency and rapidly enhances PAH growth at high temperatures compared to traditional mechanisms. Although the phenyl addition mechanism cannot synthesize symmetrical PAHs such as coronene or corannulene, it is considered highly efficient when phenyl radicals are available, as it ensures an infinite expansion of PAHs by forming different triple fusing sites in each phase [32]. Furthermore, the presence of the phenyl addition process in astrochemical conditions is likely due to deep ultraviolet radiation penetration, leading to the generation of phenyl radicals and hydrogen atoms through the photodissociation of benzene. However, investigations were not conducted into larger PAH formations at low temperatures [81]. Subsequent analysis revealed that HACA, HVA, and PAC mechanisms have limitations and are unable to facilitate the formation of cyclopentane-fused rings such as acenaphhylene. [PERSON] and [PERSON] [82] proposed a mechanism involving hydrogen abstraction and vinyl radical additions (HAVA *) based on experimental work, as illustrated in Figure 7.0 They observed that the addition of C2 Hx species enhances PAH growth, with ethylene pyrolysis displaying mass spectra in 26 mass number sequences, indicating the role of C2H3 radicals in PAH formation. The addition of a vinyl radicals enhances the mass to 27 amu while hydrogen abstraction decreases, resulting in the production of styrene from benzene. Subsequently, styrene rapidly converts into phenylacetylene via hydrogen abstraction. The addition of vinyl radical to the ortho position of phenylacetylene facilitates the formation of naphthalene, which further leads to the formation of fluoranthene and corannulene via acenaphhylene through hydrogen abstraction and vinyl radical addition. Therefore, hydrogen abstraction and vinyl radical addition are considered more suitable for the formation of fluoranthene (C16H10) and corannulene (C20H10) via acenaphthylene compared to HACA in the presence of ethylene [82]. [PERSON] and [PERSON] [83] were the first to emphasize the significance of methyl radicals during the polymerization of methane in their experimental study. Subsequently, [PERSON] et al. [84], employing transition state theory, elucidated methyl addition to double and triple bonds between carbons. Additionally, [PERSON] et al. [85] emphasized the significance of methyl radical formation during toluene/acetone pyrolysis. Their findings revealed that the mass spectra of PAHs exhibit a 14 amu increment, signifying methyl addition increasing the mass to 15 amu, followed by hydrogen a  
Figure 7.1 Phenyl addition pathway for triphenylene formation [29,44]. Figure 7.2 HAVA * pathway for fluoranthene and corannulene. The dots on the species indicate the species are radicals. The solid and dashed arrows indicate single-step and multi-step reactions respectively [82]. mass by one amu [85]. Significantly, this observation indicated that the hydrogen abstraction reaction succeeds the methyl radical addition in the newly proposed methyl radical addition cyclization (MAC) for PAH formation [85]. The MAC mechanism entails the addition of multiple methyl radicals originating from ethyl chains onto the corresponding PAHs, followed by hydrogen elimination and cyclization to generate new aromatic rings, as depicted in Figure 7.3 Another essential aspect of the MAC mechanism is the formation of benzenoid rings through the expansion of cyclopentane-fused PAHs. [PERSON] et al. [61] elucidated the role of methyl radicals in pyrene formation from phenanthrene, as illustrated in Figure 7.4 [PERSON] et al.