# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 3 Synergistic Effects on Benzene Formation
### Synergistic Effects between Hydrocarbons and Oxygenated Fuels  
Research has investigated the addition of oxygenated fuel, such as dimethyl ether (DME) and ethanol, to ethylene flames to analyze the synergistic effect [9; 31; 39; 100; 103; 108; 119; 120; 121; 122; 123; 124; 125]. The primary focus has been on analyzing the synergistic effect of soot formation, which mainly depends on the formation of the first aromatic hydrocarbon. The synergistic effect due to DME addition to ethylene flames depends on the pressure and DME mixing ratio. [PERSON] et al. [96] discussed the impact of pressure on the synergistic effect of benzene formation due to DME addition to the ethylene counterflow flames. Figure 15a illustrates the mole fraction of benzene as a function of the DME mixing ratio. The DME mixing ratio is defined as moles of DME relative to the total moles of the DME/ethylene fuel mixture. The results revealed that elevated pressure has a significant synergistic effect on benzene formation. At a high pressure of 5 atm, benzene formation is enhanced below a 0.2 mixing ratio of DME compared to low pressure. The observed synergistic effect resulted from the interactions between CH\({}_{3}\) originating from DME and C\({}_{2}\) species from ethylene. The pathway for benzene formation involves DME decomposition and hydrogen abstraction reactions (DME\(\rightarrow\)CH\({}_{3}\)(H)\(\rightarrow\)C\({}_{2}\)H\({}_{2}\)(C\({}_{3}\)H\({}_{3}\))\(\rightarrow\)A1). [PERSON] et al. [108] discussed cases based on various mole fractions of DME in the mixture. They found that adding a small amount of DME can enhance the benzene (A1) mole fraction produced in the product mixture of DME doped ethylene counterflow flames, as shown in Figure 15b. DME enhances the formation of methyl radicals, leading to the formation of propargyl radicals. The self-combination reaction of propargyl radicals significantly impacts benzene formation, as shown in Figure 3b. The analysis revealed that DME additions of up to 20% enhanced the benzene formation, leading to a synergistic effect. However, further increases in the mixing ratio of DME cause suppression of benzene formation, decreasing the synergistic effect. A synergistic effect on benzene formation was revealed during the doping of ethanol into ethylene flames, including counterflow diffusion flames, laminar premixed flames, and no-premixed flames [10; 31; 123; 125; 121]. This synergistic effect depends on the amount of ethanol in the mixture. Studies revealed that ethanol doping with ethylene flames exhibits a synergistic effect when the ethanol composition is below 30% in the blend, as depicted in Figure 16a [31; 126]. The analysis revealed that a higher percentage of ethanol led to less acetylene production, which suppressed benzene formation. Conversely, a lower doping ratio of ethanol enhanced the methyl radical pool, promoting interactions with C\({}_{2}\) species to increase the production of propargyl radicals and C\({}_{4}\) species. The resulting propargyl radical undergoes self-recombination, as shown in Figure 3b, and reacts with C\({}_{2}\)H\({}_{2}\) for benzene formation during the ethanol-doped ethylene flames, as illustrated in Figure 16b [31]. These reactions contribute to the synergistic effect when ethanol concentrations are low, whereas an increase in ethanol concentration suppresses  
Figure 15: (**a**) Simulated benzene concentration at pressures of 1–5 atm in DME-doped ethylene flames (reprinted from [PERSON] et al., 2018) [96]. (**b**) Simulated benzene concentration in DME-doped ethylene counterflow flames with different DME ratios (cases 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 denote 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 100%, respectively) at 1 atm pressure (reprinted from [PERSON] et al., 2019) [108]. C\({}_{3}\)H\({}_{3}\) formation due to a reduced rate of C\({}_{2}\) species. Moreover, the formation of C\({}_{4}\)H\({}_{6}\) from CH\({}_{3}\) + C\({}_{2}\)H\({}_{4}\) decreases with a decreasing percentage of C\({}_{2}\)H\({}_{4}\) during the flame.