# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 2 Formation Mechanisms of Benzene and PAHs
### Formation Mechanisms of PAHs
#### 2.2.1 Hydrogen Abstraction and Acetylene Addition Mechanism  
[PERSON] et al. [23; 24] proposed a two-step mechanism for the process of PAH formation involving hydrogen abstraction and acetylene addition to the radical site, as depicted in Figure 5. This mechanism was termed HACA, as proposed by [PERSON] and [PERSON] et al. [25]. Initially proposed for pyrene formation, the HACA route was later modified to explain naphthalene formation starting from benzene [26]. Subsequently, it became a fundamental pathway for elucidating PAH formation in various hydrocarbons, such as ethane, acetylene, and ethylene [25]. Over the past few decades, HACA has emerged as a central mechanism for PAH formation [25; 27; 28; 29; 30; 31]. Experimental and computational studies conducted by [PERSON] et al. [31] and [PERSON] et al. [30] further validated HACA, particularly focusing on pressure and time-dependent naphthalene formation using time-resolved molecular beam mass spectrometry. [PERSON] et al. [31] observed that branching to loss H on the C\({}_{8}\)H\({}_{7}\) potential energy surface led to the consumption of phenyl radical with significant impacts due to temperature variation on C\({}_{8}\)H\({}_{9}\)-1 production. The high hydrogen reaction affinity and low reversibility of the acetylene addition reaction in HACA contribute to the acceleration of benzenoid PAH formation, elucidating soot formation growth. The intrinsic properties of HACA, including reaction reversibility and kinetic driving force, significantly influence its mechanism. Notably, the reaction steps of HACA exhibit high exothermic  
Figure 4: Schematic diagram for PAH formation through different reaction pathways. properties and low reaction barriers, facilitating the overcoming of energy barriers during the HACA reactions [32]. Typically, the estimated barriers for acetylene addition to PAH radicals range from 8-30 kJ/mole, with an exothermic energy of 120 kJ/mole. Additionally, the ring closure stages typically entail barriers of 8-20 kJ/mole and exothermic heat of 125-200 kJ/mole [27]. The kinetics of HACA reactions are crucial in PAH formation. Some experimental results suggest that the HACA reaction proceeds relatively slowly compared to the fast processes involved in PAH formation [34]. This is because, during HACA, the addition of acetylene and hydrogen abstraction increases the mass of product species by 24 amu, leading to the formation of naphthalene, acenaphthylene, phenanthrene, and pyrene. [PERSON]'s [33] process proposed a similar pathway to the HACA reaction pathway but differs in that the second acetylene molecule is added to the first to produce the C4H4 chain, which then undergoes ring closure to form an additional ring, as depicted in Figure 5. However, investigations such as those by [PERSON] et al. [35] have shown the limited probability of [PERSON]'s [33] reaction compared to HACA in naphthalene formation due to the presence of C8H7 radicals in flames at high temperatures. In the combustion of fuels like methane, ethane, propane, ethylene, butane, heptane, benzene, toluene, phenylacetylene, and dimethyl ether, the formation of acetylene has been identified as a precursor leading to enhanced PAHs via the HACA pathway [36; 37; 38; 39; 40; 41]. During HACA, the molecular structure of PAHs comprises both aliphatic and aromatic components, indicating a combination of aromatic and aliphatic systems. This combination is observed in various astrophysical settings, suggesting the potential presence of HACA in such environments. [PERSON] et al. [42] investigated the site effect on PAH formation during the HACA mechanism reactions, revealing that hydrogen abstraction and acetylene addition reactions are kinetically favored at specific sites, while being kinetically adverse at ortho. [PERSON] et al. [43] investigated acetylene addition to 1- and 2-naphthyl radicals in the combustion process and identified acenaphthylene formation via the HACA reaction route. [PERSON] et al.