# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 3 Synergistic Effects on Benzene Formation
### Synergistic Effects between Hydrocarbons
\begin{table}
\begin{tabular}{c c c c} \hline
**Literature** & **Facility** & **Methods** & **Fuels** \\ \hline [PERSON] et al. [103] & Counterflow flames & PLII, LIF & ethylene/dimethyl ether \\ [PERSON] et al. [104] & Counterflow flames & LII, LIF & ethane/ethylene \\ [PERSON] et al. [105] & Counterflow flames & PLII, LIF & ethylene/propane/benzene \\ [PERSON] et al. [106] & Flow reactor, Laminar co-flow & FTIR b, GC, LII & methane/ethylene \\  & flame, Premixed laminar flame & FTIR b, & \\ [PERSON] et al. [107] & Premixed laminar flames, JSR & Numerical studies & toluene/n-heptane; toluene/iso-octane \\ [PERSON] et al. [108] & Counterflow flames & Numerical studies & ethylene/dimethyl ether \\ [PERSON] et al. [109] & Counterflow flames & Numerical studies & methane/dimethyl ether \\ [PERSON] et al. [110] & Premixed laminar flames & Numerical studies & ethylene/dimethyl ether \\ [PERSON] et al. [111] & Co-flow flames & Numerical studies & toluene/n-heptane; toluene/iso-octane \\ [PERSON] et al. [112] & Flow reactor & Numerical studies & 1,3 butadiene/propyne \\ [PERSON] et al. [113] & Counterflow flames & Numerical studies & toluene/n-heptane; toluene/iso-octane \\ [PERSON] et al. [114] & Laminar flames & Numerical studies & ethylene/methane \\ [PERSON] et al. [115] & Counterflow flames & Numerical studies & ethylene/propane/benzene \\ [PERSON] et al. [34] & Premixed laminar flames & Numerical studies & toluene/n-heptane; toluene/iso-octane \\ \hline \end{tabular}  
* SVUV-PIMS: synchrotron vacuum ultraviolet photoionization mass spectrometry, b GC: gas chromatography; GC-MS: gas chromatography-mass spectrometry, c LII: laser-induced incandescence, d LIF: laser-induced fluorescence, e PLIF: planar laser-induced fluorescence, f LS: laser scattering, g HPLC: high-performance liquid chromatography, h FTIR: Fourier-transform infrared spectrometer.  
\end{table} 
Table 1: Cont. carbon bond decomposition occurs, leading to an increase in methyl radical formation, subsequently enhancing propargyl radical formation via reactions (1 and 2). The synergistic effect induced by propane addition to an ethylene flame is dependent on the propane ratio in the mixture and temperature [115, 118]. [PERSON] et al. [118] discussed that at low temperatures (1660 K), the addition of propane to ethylene flame causes a decrease in benzene formation compared to pure ethylene flame due to a reduction in C4H6 species. However, an enhancement in benzene formation was observed at high temperatures (2020 K) in a propane-doped ethylene flame, as shown in Figure 14a. The increase in c-C5H6 due to propane addition led to the formation of benzyl radicals through hydrogen abstraction and acetylene addition, further facilitating the formation of toluene. Additionally, propane decomposition follows the pathway of C3H8\(\rightarrow\)iC3H7\(\rightarrow\)C3H6\(\rightarrow\)aC3H5\(\rightarrow\)aC3H4\(\rightarrow\)C3H3 for the formation of propargyl radical. Thus, ROP analysis revealed that the enhancement in C4H5-2, A1 CH3, and C3H3 at high temperatures converted into benzene via acetylene addition, methyl radical abstraction, and recombination reactions, respectively, as depicted in Figure 14b. This significant enhancement in benzene formation indicates the synergistic effect on benzene formation due to the addition of propane in ethylene diffusion flames at high temperatures compared to low-temperature flames. Figure 14: (**a**) Measured (symbols) and simulated (lines), benzene formation during propane doping to ethylene counter flow flames with different propane concentrations (0%: black, 5%: red and 10%: blue), low T (1660 K) (15%O2 + 85%N2) and high T (2020 K) (20%O2 + 80% N2). (**b**) ROP analysis for important reactions responsible for benzene formation in the ethylene flame (reprinted from [PERSON] et al., 2020) [118].