# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 4 Synergistic Effects on PAHs Formation
### Synergistic Effect on Larger PAHs Formation  
As discussed in the preceding sections, the combustion process can yield larger PAHs beyond indene and naphthalene. This discussion now delves into the impact of various fuel blends on PAH formation, which can result in either a positive or negative synergistic effect. A negative synergistic effect would entail a decrease in PAH formation upon the addition of certain fuel components. Several research studies have explored this phenomenon across different experimental setups, including shock tubes and counter-flow diffusion flames [45; 99; 115]. Combining benzene with acetylene and ethylene in the shock tube experiments demonstrated a synergistic effect on PAH formation. Co-pyrolysis of benzene and acetylene at temperatures ranging from 1400-1500 K revealed a synergistic effect attributed to the formation of phenylacetylene (6) [45]. The hydrogen atom produced alongside phenylacetylene plays a vital role in promoting the consumption of acetylene and benzene. Moreover, the formation of phenylacetylene via acetylene addition enhanced the addition elimination reaction between phenyl acetylene and phenyl radicals, resulting in the formation of PAH isomers such as phenanthrene and anthracene, as illustrated in Figure 22. The addition of acetylene to the fuel mixture enhanced the HACA route for PAH formation. Furthermore, [PERSON] et al. [130] examined the co-pyrolysis of benzene/acetylene and benzene/acetylene with dimethyl ether addition. Their findings revealed that the addition of acetylene and DME enhanced the formation of PAH, including phenanthrene and pyrene, due to an enhanced radical pool during co-pyrolysis. However, the influence of adding dimethyl ether was less significant under identical fuel conversion conditions, albeit improving PAH formation at a relatively lower temperature. The study emphasized the propensity of the HACA reaction pathway to favor phenanthrene and pyrene formation. Figure 23 illustrates the reaction pathways P1-P5 involved in the synergistic effect of the phenanthrene formation. Pathway 1 involves the interaction of C3H5 with A2 CH2, while pathway 2 entails the self-addition reaction of A1 CH2. Pathway 3 involves the replacement of a methyl radical from methyl phenanthrene, while pathway 4 includes C3H2 addition to methyl naphthalene. Pathway 5 depicts the biphenyl radical reaction with acetylene. These pathways play a significant role in phenanthrene formation, with pathways P2-P4 particularly crucial for the synergistic effect compared to P1 and P5 during the isooctane/toluene flame study [34]. Figure 21: (**a**) Measured (symbols) and simulated (lines) mole fractions of naphthalene in the shock tube pyrolysis of benzene (108 ppm), benzene (108 ppm)/acetylene (500 ppm), and benzene (108 ppm)/ethylene (532 ppm) at 20 atm (reprinted from [PERSON] et al., 2022) [45]. (**b**) Simulated naphthalene mole fractions in the ethylene counterflow flames doped by n-heptane, ethanol, benzene, and toluene (reprinted from [PERSON] et al., 2015) [99]. The synergistic effect was observed in the counterflow diffusion flame's experimental and simulation setups [98; 105; 115], particularly concerning the ethylene/propane mixture flame and the ethylene/propane mixture doped with benzene. Experimental findings indicated that the ethylene/propane mixture flame exhibits a synergistic effect on PAH formation, with comparisons of LIF intensity at various wavelengths corroborating simulated results for other PAHs. Notably, larger PAHs showed a more pronounced synergistic effect, indicating that additional variables beyond incipient ring production influence PAH formation rates. Compared to pure propane, ethylene leads to more PAH and soot formation, with PAH formation depending on the propane concentration in the mixture, as shown in Fig. 23.  
Figure 23: Reactions leading to synergistic effects on phenanthrene (reprinted from [PERSON] et al., 2012) [34].