# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 4 Synergistic Effects on PAHs Formation  
### Synergistic Effect on Indene Formation  
Indene (C\({}_{9}\)H\({}_{8}\)), a PAH composed of one-, five-, and six-membered aromatic rings, is of significant concern for its carcinogenic, mutagenic, and environmentally harmful properties. Research studies have focused on understanding the formation mechanism of indene under various experimental conditions. [PERSON] et al. [48] described several pathways for indene formation, including the reaction of allene with phenyl radical followed by hydrogen abstraction, the reaction of propargyl radical with phenyl radical and benzene followed by hydrogen abstraction, acetylene addition to benzyl radical, and the addition of propene to phenyl radicals [48]. Experimental studies by [PERSON] et al. [127] in the interstellar medium demonstrated that the addition of acetylene to the benzyl radical resulted in the formation of indene. These findings contribute to our understanding of the complex mechanisms involved in the formation of indene. Several studies have investigated the synergistic effect on indene formation across different experimental setups, such as shock tubes, jet flow reactors, and plug flow reactors. One such study focused on the experimental and kinetic analysis of indene formation during co-pyrolysis of toluene with C\({}_{3}\)H\({}_{4}\) isomers in a flow reactor [91], with its sensitivity analysis illustrated in Figure 17. The sensitivity analysis emphasized the significant impact of the reaction between acetylene and benzyl radical (5) on indene formation. Additionally, the reaction of proparg and H played a crucial role in producing ethylene, which is essential for indene formation in the presence of toluene. Interestingly, ethylene formation is favored at higher pressures, indicating the influence of pressure-sensitive reactions on the synergistic effect of indene formation rather than the molecular structure of the C\({}_{3}\)H\({}_{4}\) isomer. This highlights the complex interaction between reaction pathways and operating conditions in determining the rate of indene formation in various experimental environments. Recently, [PERSON] et al. [11] focused on the co-pyrolysis of toluene and acetylene at 1 atm pressure in a plug flow reactor and observed the synergistic effect on indene formation with the addition of acetylene, as shown in Figure 18a. They employed synchrotron vacuum ultraviolet (VUV) light to ionize the pyrolysis products and subsequently analyzed the ions utilizing a reflection time-of-flight mass spectrometer. Products such as benzene, toluene, indene, and biphenyl were identified and measured. The addition of acetylene enhances the decomposition of toluene, revealing the synergistic effect during co-pyrolysis. Among the key reactions contributing to this effect are the thermal decomposition reaction of toluene, hydrogen abstraction from toluene, and the reaction of methyl radicals with  
Figure 16: (**a**) Simulated benzene mole fractions; (**b**) contribution of different reactions to benzene (A1) production in ethanol-doped ethylene counterflow flames with various ethanol doping ratios of 0% to 25% (reprinted from [PERSON] et al., 2019) [31]. toluene. These pathways are significant as they contribute to the consumption of toluene and the generation of intermediate species that are essential for subsequent reactions leading to indene formation. Furthermore, the presence of acetylene influences the reaction pathways of benzyl radicals, favoring the acetylene addition reaction (5) over recombination reactions, thus promoting indene formation synergistically, as illustrated in Figure 18b. The synergistic effect of the n-heptane/toluene mixture on indene formation was analyzed in a jet flow reactor, a counterflow diffusion flame, and a flow reactor [67, 97, 128]. These studies highlighted the crucial role of the interaction between benzyl radical and acetylene, produced due to heptane, in observing the synergistic effect between n-heptane and toluene.