# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 4 Synergistic Effects on PAHs Formation
### Synergistic Effect on Naphthalene Formation  
Naphthalene (A2) is the simplest PAH, comprising two fused six-member rings. Numerous experimental and theoretical investigations have focused on understanding the formation of naphthalene. [PERSON] et al. [51] theoretically discussed the formation of naphthalene at low temperatures through barrierless phenyl radical and vinylacetylene exoergic reactions. Several studies have explored the synergistic effect of naphthalene formation due to n-heptane and toluene [15; 34; 45; 67; 95; 107; 113; 128]. [PERSON] et al. [67] conducted an experimental and kinetic modeling study on the synergistic effect of naphthalene formation in a jet-stirred reactor connected with two online GCs and the Agilent Refinery Gas Analysis for the analysis of products for heptane and toluene mixtures. Benzene, ethylbenzene, styrene, indene, and naphthalene were thus detected as aromatic products. This synergistic effect may stem from the reaction of benzyl radicals with propargyl radicals (10) or acetylene. While the enhancement of benzyl radicals is attributed to toluene, the concentration of propargyl radicals, or acetylene, is boosted by n-heptane. The formation of benzyl radicals is elevated in blends rich in toluene. Conversely, the concentrations of acetylene and propargyl, stemming from the decomposition of heptane, were anticipated to be diminished in toluene-rich blends. Consequently, fuel mixtures containing high proportions of either toluene or heptane might not facilitate the formation of naphthalene optimally. Blending both toluene and n-heptane exhibits a synergistic effect on naphthalene formation at an intermediate concentration level, as exemplified by fuel 3, as depicted in Figure 19a. Hence, the observed synergistic effect on naphthalene formation is attributed to the inclusion of n-heptane [67]. Additionally, Figure 19b illustrates the impact of the percentage of n-heptane in the blend on naphthalene formation, indicating that up to 40% n-heptane presence favors an increase in naphthalene formation. However, a further increase in the percentage of n-heptane would result in a reduction in the naphthalene percentage. Thus, this comprehensive study revealed the significant synergistic effect of n-heptane and toluene compositions on naphthalene. [PERSON] et al. [101] adopted LII and LIF techniques in counterflow diffusion flame experiments to investigate the blends of toluene with n-heptane and isooctane, respectively, with various kinetic studies reported on the resulting PAH and soot formation [34; 97; 107; 113]. According to the authors, the measured LIF data is expected to be contributed primarily by A2 and A4, while the measured LII data is expected to be A5. The authors revealed that, among the n-heptane/toluene and isooctane/toluene mixtures, the latter exhibited a relatively strong synergistic effect on the PAH formation, particularly the naphthalene formation, as depicted in Figure 20. The formation of resonantly stabilized radicals, such as benzyl and propargyl radicals, contributed to the naphthalene formation. The ratio  
Figure 19: (**a**) Measured (symbols) and simulated (lines) mole fractions of naphthalene in the JSR pyrolysis of toluene/n-heptane mixtures with different blending ratios (fuel 1: 2075 ppm of toluene and 460 ppm of n-heptane; fuel 2: 1380 ppm of toluene and 1130 ppm of n-heptane. ; fuel 3: 680 ppm of toluene and 1840 ppm of n-heptane.). (**b**) Effect of n-heptane concentration on naphthalene formation (symbols: measurements; lines: simulations) during co-pyrolysis of toluene and n-heptane in JSR at 1 atm and residence time of 1.0 sec (reprinted from [PERSON] et al., 2020) [67]. of blend components plays a vital role in this synergistic effect, with an increase in the toluene ratio correlating with enhanced naphthalene formation, as shown in Figure 20. In a chemical kinetic analysis conducted by [PERSON] et al. [95] on the counterflow diffusion flame using two previously reported mechanisms by [PERSON] et al. [113] and [PERSON] et al.