# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 1 Introduction  
Aromatics are widely produced during the combustion of fossil transport fuels, biofuels, e-fuels, and sustainable aviation fuels [1, 2], which have attracted significant attention [3, 4, 5] as they are precursors of soot and combustion-derived particulate matter. Both physical and chemical processes have been proposed to elucidate soot formation [6], as illustrated in Figure 1. Understanding the formation mechanism of aromatics, especially polycyclic aromatic hydrocarbons (PAHs), is crucial in directing endeavors to reduce soot emissions. In the realm of transportation fuels, a wide range of elements exist, classified into four prominent families: alkanes, alkenes, cycloalkanes, and aromatics, each comprising  
Figure 1: The schematic diagram for soot formation (reprinted from [PERSON] et al., 2021) [7]. numerous constituents. The composition of these components can vary significantly depending on factors such as geographical origin and production methods. Given the intricate nature of these fuel mixtures, interactions among their components are inevitable. For a fuel mixture composed of fuel A and fuel B, the blending effect means that the aromatic concentration follows a linear relationship as the proportion of each component changes. In contrast, the synergistic effect means that the aromatic concentration exhibits an enhanced nonlinear relationship, which can be attributed to the interactions between fuel A and fuel B [8; 9; 10; 11]. For example, according to the investigation by [PERSON] et al. [11], pure toluene and pure acetylene can only produce indene with a maximum mole fraction of 5.2 \(\times\) 10\({}^{-4}\) and an almost negligible mole fraction in their individual pyrolysis, respectively. If indene formation follows the blending effect, its maximum mole fraction in the co-pyrolysis of 50% toluene/50% acetylene should be 2.5 \(\times\) 10\({}^{-5}\). However, the experimental results from the co-pyrolysis of 50% toluene/50% acetylene show that indene has a maximum mole fraction of 5.2 \(\times\) 10\({}^{-4}\), which is 20 times higher than the blending effect, as shown in Figure 2. Similar examples can also be found in other studies, such as Ref. [12]. The complex multi-component and wide-ranging sources of transportation fuels contribute to the complexity of these synergistic effects, warranting a comprehensive understanding of the interactions between various fuel components. Moreover, in response to the imperative to reduce carbon dioxide emissions, oxygenated fuels with carbon-neutral attributes, such as bioethanol and dimethyl ethers, have been implemented as blending agents in transportation fuels [13; 14; 15]. The growing adoption of this blending strategy highlights the significance of investigating the interactions involved in aromatic formation between oxygenated fuels and hydrocarbons. Over the past two decades, significant efforts have been made to elucidate the synergistic effects of aromatic formation, contributing to a deeper understanding of the aromatic formation mechanism. This review aims to provide comprehensive, cutting-edge progress on the synergistic effects of aromatic formation, drawing from experimental studies, theoretical calculations, and kinetic modeling. Special emphasis will be placed on examining blends comprising both aromatics and linear fuels, as these have been shown to exhibit pronounced synergistic effects in the formation of PAHs. Additionally, previous endeavors to reveal key reaction pathways responsible for the synergistic effects will be highlighted, providing additional insight into the complex mechanisms that drive the formation of aromatic compounds. This review first briefly introduces the main formation mechanisms for benzene and PAHs. Then, it focuses on the synergistic effects observed in benzene and PAH formation, including indene, naphthalene, and larger PAHs. Finally, future research directions will be discussed to guide further exploration of the complex dynamics of aromatic formation in the combustion process.