Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review

Abstract

Aromatics, especially polycyclic aromatic hydrocarbons (PAHs), are important combustion pollutants known to be carcinogenic and mutagenic and are also precursors of soot and, consequently, combustion-generated particulate matters that can significantly threaten environmental security and human health. In engine combustion, the multi-component and broad-source feature of transportation fuels makes synergistic effects commonly exist and greatly enhances the formation of aromatics and soot. Understanding the synergistic effects of different fuel components on aromatic formation facilitates concrete guidance for controlling soot emissions. This review focuses specifically on the synergistic effects of aromatics formation, including benzene, indene, naphthalene, and larger PAHs, in combustion among hydrocarbon blends and hydrocarbons blended with oxygenated fuels. Progresses in experimental measurements, theoretical calculations of critical reactions, and kinetic modeling are reviewed in detail. Special attention is paid to blends of aromatics and linear fuels, which show pronounced synergistic effects in PAH formation. Furthermore, some prospects for future research on synergistic effects in aromatic formation are provided.

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 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 Li et al. [11], pure toluene and pure acetylene can only produce indene with a maximum mole fraction of 5.2 × 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 × 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 × 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.

2. Formation Mechanisms of Benzene and PAHs

2.1. Formation Mechanisms of Benzene

In the formation of polycyclic aromatic hydrocarbons (PAHs) and soot from aliphatic hydrocarbons, the initial step of forming the first aromatic ring is often considered the rate-controlling step [5]. Benzene, a fundamental aromatic compound, is typically produced through several reaction pathways, including the C₂ + C₁, C₃ + C₃, C₄ + C_2, and C₅ + C₁ pathways. These pathways illustrate the sequential combination of carbon-containing species to yield benzene, as depicted in Figure 3.

The mechanism of benzene (A1) formation varies depending on the fuel type, composition, and combustion conditions. Numerous studies have investigated benzene formation across different fuels and combustion setups. Woods et al. [19] explored benzene formation during the reaction of acetylene with formyl cation under high temperatures and ionization rates. Their findings revealed an efficient mechanism of benzene formation facilitated by the abundance of acetylene (C₂H₂) and formyl cations in the system. Stein et al. [22] observed benzene formation in the acetylene flame and pyrolysis process under low-pressure conditions, emphasizing the significant impact of propargyl recombination reactions on benzene and fulvene formation. Buras et al. [17] explored potential benzene formation pathways via vinyl addition to 1,3-butadiene using experimental studies in MIT laser photolysis, predicting rate coefficients for this reaction across various kinetic models. Zhang et al. [21] conducted molecular beam experiments and RRKM calculations on the reaction of diatomic carbon with 1,3-butadiene, where the signal m/z = 77 indicated the formation of phenyl radical, subsequently converted to benzene through the hydrogen addition reaction. They also discussed the formation of the first aromatic ring during the experimental study of acetylene. The kinetic mechanism revealed benzene formation during the reaction of acetylene with the 1,3-butadienyl radical and, subsequently, hydrogen abstraction [23]. Additionally, Wang et al. [20] theoretically reviewed the pathway to benzene formation via the reaction of 1,3-butadienyl with propargyl radical. Branko et al. [18] employed density functional theory to calculate the energy barrier for benzene formation via the reaction of cyclopentadiene and ethylene, revealing a significantly lower energy barrier for this reaction pathway. Furthermore, the decomposition of carbon–carbon bonds during alkane combustion facilitates the generation of methyl radicals (CH₃), consequently augmenting propargyl radical (C₃H₃) formation through reactions (1) and (2). The involvement of produced propargyl radicals in reactions with propyne (pC₃H₄) (3) and allene (aC₃H₄) (4) further contributed to the formation of benzene.

C₂H₂ + CH₃ = pC₃H₄ + H

(1)

pC₃H₄ + H = C₃H₃ + H₂

(2)

pC₃H₄ + C₃H₃ = A1 + H

(3)

aC₃H₄ + C₃H₃ = A1 + H

(4)

2.2. Formation Mechanisms of PAHs

Over the past two decades, significant efforts have been directed towards elucidating PAH formation mechanisms. These efforts have resulted in the development of various reaction kinetic pathways, which have been rigorously validated against data obtained from diverse combustion environments to better understand PAH formation. The formation mechanisms include the hydrogen abstraction and acetylene addition (HACA) mechanism, the hydrogen abstraction and vinyl acetylene addition (HAVA) mechanism, formation mechanisms with radicals, and the hydrogen abstraction and phenylacetylene addition (HAPaA) mechanism. Among these, the HACA mechanism stands as a classical and widely utilized pathway for PAH formation. However, in fuel blends where vinyl acetylene is abundant, the hydrogen abstraction and vinyl acetylene addition (HAVA) mechanism may prevail over HACA due to favorable reaction kinetics. Among formation mechanisms with radicals, methyl addition cyclization (MAC), although not competitive with the HACA mechanism, plays a crucial role in augmenting the PAH chain. Similarly, the addition of phenyl radicals (PAC) contributes to the enhancement of 6-ring membered aromatics within the polycyclic aromatic chain. Furthermore, the presence of vinyl radicals, originating from ethylene, promotes the hydrogen abstraction and vinyl radical addition (HAVA *) mechanisms, particularly under moderate temperature conditions, facilitating the formation of naphthalene and larger PAHs such as fluoranthene. However, it is crucial to acknowledge that all these pathways exhibit limitations in the formation of pre-condensed aromatic hydrocarbons. Consequently, the recently proposed HAPaA mechanism has emerged to address this challenge, specifically targeting the formation of pre-condensed aromatic hydrocarbons. A summarized overview of the diverse pathways involved in PAH formation is provided in the schematic diagram presented in Figure 4.

2.2.1. Hydrogen Abstraction and Acetylene Addition Mechanism

Frenklach et al. [23,24] proposed a two-step mechanism for the process of PAH formation involving hydrogen abstraction and acetylene addition to the radical site, as depicted in Figure 5. This mechanism was termed HACA, as proposed by Frenklash and Wang et al. [25]. Initially proposed for pyrene formation, the HACA route was later modified to explain naphthalene formation starting from benzene [26]. Subsequently, it became a fundamental pathway for elucidating PAH formation in various hydrocarbons, such as ethane, acetylene, and ethylene [25]. Over the past few decades, HACA has emerged as a central mechanism for PAH formation [25,27,28,29,30,31]. Experimental and computational studies conducted by Chu et al. [31] and Kislov et al. [30] further validated HACA, particularly focusing on pressure and time-dependent naphthalene formation using time-resolved molecular beam mass spectrometry. Chu et al. [31] observed that branching to loss H on the C₈H₇ potential energy surface led to the consumption of phenyl radical with significant impacts due to temperature variation on C₈H₉-1 production. The high hydrogen reaction affinity and low reversibility of the acetylene addition reaction in HACA contribute to the acceleration of benzenoid PAH formation, elucidating soot formation growth. The intrinsic properties of HACA, including reaction reversibility and kinetic driving force, significantly influence its mechanism. Notably, the reaction steps of HACA exhibit high exothermic properties and low reaction barriers, facilitating the overcoming of energy barriers during the HACA reactions [32]. Typically, the estimated barriers for acetylene addition to PAH radicals range from 8–30 kJ/mole, with an exothermic energy of 120 kJ/mole. Additionally, the ring closure stages typically entail barriers of 8–20 kJ/mole and exothermic heat of 125–200 kJ/mole [27].

The kinetics of HACA reactions are crucial in PAH formation. Some experimental results suggest that the HACA reaction proceeds relatively slowly compared to the fast processes involved in PAH formation [34]. This is because, during HACA, the addition of acetylene and hydrogen abstraction increases the mass of product species by 24 amu, leading to the formation of naphthalene, acenaphthylene, phenanthrene, and pyrene. Bittner–Howard’s [33] process proposed a similar pathway to the HACA reaction pathway but differs in that the second acetylene molecule is added to the first to produce the C₄H₄ chain, which then undergoes ring closure to form an additional ring, as depicted in Figure 5. However, investigations such as those by Mebel et al. [35] have shown the limited probability of Bittner–Howard’s [33] reaction compared to HACA in naphthalene formation due to the presence of C₈H₇ radicals in flames at high temperatures.

In the combustion of fuels like methane, ethane, propane, ethylene, butane, heptane, benzene, toluene, phenylacetylene, and dimethyl ether, the formation of acetylene has been identified as a precursor leading to enhanced PAHs via the HACA pathway [34,36,37,38,39,40,41]. During HACA, the molecular structure of PAHs comprises both aliphatic and aromatic components, indicating a combination of aromatic and aliphatic systems. This combination is observed in various astrophysical settings, suggesting the potential presence of HACA in such environments. Peng et al. [42] investigated the site effect on PAH formation during the HACA mechanism reactions, revealing that hydrogen abstraction and acetylene addition reactions are kinetically favored at specific sites, while being kinetically adverse at ortho. Parker et al. [43] investigated acetylene addition to 1- and 2-naphthyl radicals in the combustion process and identified acenaphthylene formation via the HACA reaction route. Kislov et al. [27] analyzed the dominance of five-membered ring PAH growth over six-membered ring growth in the HACA mechanism. Shukla and Koshi [44] revealed that during acetylene pyrolysis, HACA products mainly consist of polyacetylenes and cyclopentafused PAHs. Furthermore, in the toluene/acetylene pyrolysis [11], the reaction between benzyl radicals (A1CH₂) and acetylene (5) significantly contributes to the enhancement of indene (C₉H₈) formation. In acetylene/benzene blends, the HACA reaction pathway involves the reaction of phenyl radicals with acetylene, producing A1C₂H radicals (6), which subsequently react with acetylene, thereby enhancing the formation of naphthyl radicals (A2-) (7) [45].

A1CH₂ + C₂H₂ = C₉H₈ + H

(5)

A1- + C₂H₂ = A1C₂H + H

(6)

A1C₂H + C₂H₂ = A2- + H

(7)

2.2.2. Hydrogen Abstraction and Vinylacetylene Addition Mechanism

Badger et al. [46] investigated the formation of tars and emphasized the significance of vinyl acetylene as an intermediate species. They demonstrated that the presence of vinyl acetylene influences the reactions leading to PAH formation. It was established that PAH formation primarily occurs through vinyl acetylene addition, followed by hydrogen abstraction. In various combustion flame types, the abundance of vinyl acetylene increases the probability of PAH formation via the HAVA mechanism, eliminating the requirement for subsequent benzene ring formation [47,48,49].

During the HAVA mechanism, the formation of PAH radicals leads to the generation of a new ring in two steps: the initial addition of vinyl acetylene, followed by cyclization. Experimental studies on the formation of naphthalene and phenanthrene at elevated temperatures provided insight into this mechanism. Kinetic simulations further demonstrated that the presence of double and triple bonds enhances the activity of the carbon atom, facilitating their attack on carbon atoms in PAH molecules [48,50]. This mechanism has been observed in various environments, including the interstellar medium, where PAH formation occurs both at high temperatures during chemical vapor deposition processes and in cold molecular clouds. In these conditions, the formation of naphthalene has been observed via a barrierless route of the HAVA mechanism, suggesting it is an alternative pathway to HACA [21,51,52].

Further, Zhao et al. [53] proposed the formation of higher and more complex PAH via the HAVA reaction pathway, which involves the photodissociation of phenanthrene and anthracene. These fragments then react with vinylacetylene to form six-ring PAHs. Additionally, Liu et al. [52] found that benzene ring formation was favored at the zigzag edge surface site rather than the free edge during the formation of phenanthrene through the reaction of PAH radicals and vinylacetylene. Similar pathways were observed for the formation of naphthalene and phenanthrene through the reaction of phenyl radicals with vinylacetylene and naphthyl radicals with vinylacetylene, respectively. Overall, the HACA mechanism exhibits versatility in different reaction environments and conditions for PAH formation [54,55,56,57], as illustrated in Figure 6.

2.2.3. Formation Mechanism with Radicals

Radicals have a vital role in enhancing molecular weight, thereby facilitating the formation of aromatic compounds during the combustion process [59,60,61,62]. Interactions between radicals and between radicals and other hydrocarbons can lead to the formation of polycyclic aromatic hydrocarbons [59,60,61,62]. This section delves into the contributions of two types of radicals in PAH formation: resonantly stabilized radicals and non-resonantly stabilized radicals, which participate in additional pathways that contribute to PAH formation.

Resonantly stabilized radicals, such as propargyl, cyclopentadienyl, benzyl, indenyl, and allyl radicals, possess the same nuclear structure but exhibit different electronic systems [63,64,65,66,67,68,69,70,71,72,73,74]. For instance, propargyl and allyl radicals have two electronic structures with identical numbers of carbon and hydrogen atoms but different arrangements of radical sites. Aromatic hydrocarbons, owing to the presence of a benzenoid ring, exhibit several resonance structures, exemplified by the benzyl radical, as depicted in Figure 7. The first two structures are analogous to benzene, demonstrating how different structures can be generated based on the radical site. Thermodynamically, resonantly stabilized radicals are more stable than non-resonant radicals. This thermodynamic stability has significant implications; for example, aromatic hydrocarbons tend to decompose into resonantly stabilized radicals rather than non-resonant radicals. Theoretical studies by Daniel et al. [75] have shown that allyl radicals are 3 kcal/mole more stable than benzyl radicals. The resonantly stabilized radicals also exhibit slower dissociation and reaction with oxygen compared to non-resonant radicals [76,77]. Additionally, the recombination reactions of propargyl and its reaction with allyl contribute to benzene formation. Moreover, the reaction of the propargyl radical with A2CH₂ leads to the formation of phenanthrene (8). Similarly, the recombination reaction of resonantly stabilized cyclopentadienyl (9) and their reaction with methyl radicals lead to the formation of naphthalene and benzene, respectively. The hydrogen abstraction reaction of toluene led to the formation of benzyl radicals. Subsequently, the recombination reaction of benzyl radicals, along with their reaction with other resonantly stabilized radicals, facilitates the enhanced formation of PAHs such as indene, naphthalene (10), phenanthrene, or higher polyaromatic hydrocarbons. For instance, indenyl radicals are generated through the hydrogen abstraction reaction of indene, and their reaction with cyclopentadienyl and A2CH₂ radicals leads to the formation of phenanthrene (11, 12), and the recombination reaction of indenyl radicals leads to the formation of pyrene (13). This discussion emphasizes that the high production rate and slow dissociation of resonantly stabilized radicals contribute to increased concentrations, which are crucial for the formation of aromatic and PAHs [73,78,79,80].

A2CH₂ + C₃H₃ = A3 + 2H

(8)

2C₅H₅ = A2 + 2H

(9)

A1CH₂ + C₃H₃ = A2 + H₂

(10)

C₉H₇ + C₅H₅ = A3 + 2H

(11)

A2CH₂ + C₉H₇ = A4

(12)

2C₉H₇ = A4 + H₂

(13)

Shukla et al. [29] investigated toluene pyrolysis and analyzed various PAH species with a mass difference of 76 amu. Based on this difference, they proposed a phenyl addition/cyclization (PAC) mechanism followed by hydrogen abstraction. With the addition of phenyl radical, the mass increased to 77 amu, followed by hydrogen abstraction, resulting in a decrease of 1 amu. Dehydrocyclization occurred with a rise in temperature, leading to the transformation of thermally unstable phenyl radicals into stable condensed PAHs, as shown in Figure 8. Zhao et al. [62] proposed phenyl addition as a more efficient mechanism for PAH growth in circumstellar environments, particularly for triphenylene and fluoranthene. They revealed that the phenyl addition mechanism exhibits high efficiency and rapidly enhances PAH growth at high temperatures compared to traditional mechanisms. Although the phenyl addition mechanism cannot synthesize symmetrical PAHs such as coronene or corannulene, it is considered highly efficient when phenyl radicals are available, as it ensures an infinite expansion of PAHs by forming different triple fusing sites in each phase [32]. Furthermore, the presence of the phenyl addition process in astrochemical conditions is likely due to deep ultraviolet radiation penetration, leading to the generation of phenyl radicals and hydrogen atoms through the photodissociation of benzene. However, investigations were not conducted into larger PAH formations at low temperatures [81].

Subsequent analysis revealed that HACA, HAVA, and PAC mechanisms have limitations and are unable to facilitate the formation of cyclopentane-fused rings such as acenaphthylene. Shukla and Koshi [82] proposed a mechanism involving hydrogen abstraction and vinyl radical additions (HAVA *) based on experimental work, as illustrated in Figure 9. They observed that the addition of C₂Hx species enhances PAH growth, with ethylene pyrolysis displaying mass spectra in 26 mass number sequences, indicating the role of C₂H₃ radicals in PAH formation. The addition of a vinyl radicals enhances the mass to 27 amu while hydrogen abstraction decreases, resulting in the production of styrene from benzene. Subsequently, styrene rapidly converts into phenylacetylene via hydrogen abstraction. The addition of vinyl radical to the ortho position of phenylacetylene facilitates the formation of naphthalene, which further leads to the formation of fluoranthene and corannulene via acenaphthylene through hydrogen abstraction and vinyl radical addition. Therefore, hydrogen abstraction and vinyl radical addition are considered more suitable for the formation of fluoranthene (C₁₆H₁₀) and corannulene (C₂₀H₁₀) via acenaphthylene compared to HACA in the presence of ethylene [82].

Weissman and Benson [83] were the first to emphasize the significance of methyl radicals during the polymerization of methane in their experimental study. Subsequently, Balderas et al. [84], employing transition state theory, elucidated methyl addition to double and triple bonds between carbons. Additionally, Shukla et al. [85] emphasized the significance of methyl radical formation during toluene/acetone pyrolysis. Their findings revealed that the mass spectra of PAHs exhibit a 14 amu increment, signifying methyl addition increasing the mass to 15 amu, followed by hydrogen abstraction, reducing the mass by one amu [85]. Significantly, this observation indicated that the hydrogen abstraction reaction succeeds the methyl radical addition in the newly proposed methyl radical addition cyclization (MAC) for PAH formation [85]. The MAC mechanism entails the addition of multiple methyl radicals originating from ethyl chains onto the corresponding PAHs, followed by hydrogen elimination and cyclization to generate new aromatic rings, as depicted in Figure 10. Another essential aspect of the MAC mechanism is the formation of benzenoid rings through the expansion of cyclopentane-fused PAHs. Georgiana et al. [61] elucidated the role of methyl radicals in pyrene formation from phenanthrene, as illustrated in Figure 10. Zhao et al. [69] investigated the reaction of the indenyl radical with acetylene and vinylacetylene. They found that when 1-indenyl reacted with methyl radicals, it resulted in naphthalene formation instead of following the HACA and HAVA mechanisms. This discussion explained that the CH₃ radical exhibits slower kinetics in occupying the phenanthrene armchair position compared to C₂H₂, revealing that the HACA mechanism outcompetes MAC in the presence of acetylene.

In the interstellar medium, characterized by a carbon-rich environment, ethynyl formation has been observed in the decomposition of acetylene, in contrast to the diffused interstellar medium, rendering the ethynyl addition mechanism more feasible [86,87]. Marsh et al. [88] conducted a semi-empirical quantum theoretical investigation to explore potential energy surfaces for chemical pathways. Their study revealed that PAH formed through ethynyl addition exhibits the lowest energy. Mebel et al. [87] proposed a mechanism for ethynyl addition as a viable alternative to HACA for the formation of PAH at low temperatures, particularly if ethynyl addition occurs due to photoinduction. Additionally, a mechanism involving hydrogen abstraction and ethynyl radical addition was suggested for the formation of benzo(a)pyrene from chrysene and benzo(a)anthracene, as illustrated in Figure 11 [89].

2.2.4. Hydrogen Abstraction Phenylacetylene Addition (HAPaA) Mechanism

HACA and HAVA mechanisms have demonstrated limitations in describing the formation of pre-condensed aromatic hydrocarbons (PCAHs) across different temperatures. Jin et al. [90] proposed the HAPaA mechanism while conducting their experimental and theoretical investigation of the reaction between phenylacetylene and 1-naphthyl/4-phenanthryl radicals. Their findings emphasized the efficiency of this pathway in PCAH formation. Experimental studies were conducted using synchrotron vacuum ultraviolet photoionization and examined the reactions of 1-naphthyl with phenylacetylene and phenanthryl with phenylacetylene. Additionally, quantum chemical calculations elucidated the pathway for PAH formation in both interstellar and combustion mediums, as depicted in Figure 12.

3. Synergistic Effects on Benzene Formation

The synergistic effect resulting from the interaction between process parameters influences the behavior of the blends. This phenomenon is demonstrated by the higher concentration of MAH and PAHs formed in blends compared to their corresponding pure fuels, attributable to species crosslinking arising from various fuel component interactions. Some authors have reported the total synergistic effect on aromatic formation across different systems, as summarized in

Table 1. Literature on synergistic effect on benzene and PAH formation.

Literature	Facility	Methods	Fuels
Liu et al. [91]	Flow reactor	SVUV-PIMS a	toluene/propyne; toluene/allenes
Sun et al. [92]	Shock tube	GC b/GC–MS	toluene/propylene; toluene/propyne
Sun et al. [93]	Shock tube	GC/GC–MS	toluene/ethylene; toluene/acetylene
Li et al. [11]	Flow reactor	SVUV-PIMS	toluene/acetylene
Hamadi et al. [93]	Shock tube	GC/GC–MS	benzene/ethylene; benzene/acetylene; benzene/propyne; benzene/allenes
Shao et al. [67]	JSR	GC/GC–MS	toluene/n-heptane
Yan et al. [31]	Counterflow flames	LII c, LIF d	ethylene/ethanol
Sirignano et al. [94]	Laminar premixed flames	LII, LIF	benzene/ethylene
Liu et al. [95]	Laminar diffusion flame	PLIF e	toluene/n-heptane; toluene/iso-octane
Li et al. [96]	Counterflow flames	LS f	ethylene/dimethyl ether
Park et al. [97]	Counterflow flame	LII, LIF	toluene/n-heptane; toluene/iso-octane
Park et al. [98]	Counterflow flames	LII	ethylene/propane
Choi et al. [99]	Counterflow flames	LIF	n-heptane/ethylene, ethanol/ethylene, benzene/ethylene, toluene/ethylene
Poddar et al. [12]	Flow reactor	GC	1,3 butadiene/propyne
Liu et al. [100]	Co-flow flames	LII	ethylene/dimethyl ether
Choi et al. [101]	Counterflow flame	LII, LIF	toluene/n-heptane; toluene/iso-octane
Poddar et al. [102]	Flow reactor	HPLC g, GC	catechol/propyne
Yoon et al. [103]	Counterflow flames	PLII, LIF	ethylene/dimethyl ether
Yoon et al. [104]	Counterflow flames	LII, LIF	ethane/ethylene
Lee et al. [105]	Counterflow flames	PLII, LIF	ethylene/propane/benzene
Roesler et al. [106]	Flow reactor, Laminar co-flow flame, Premixed laminar flame	FTIR h, GC, LII	methane/ethylene
Xue et al. [107]	Premixed laminar flames, JSR	Numerical studies	toluene/n-heptane; toluene/iso-octane
Zhang et al. [108]	Counterflow flames	Numerical studies	ethylene/dimethyl ether
Bhattacharya et al. [109]	Counterflow flames	Numerical studies	methane/dimethyl ether
Kang et al. [110]	Premixed laminar flames	Numerical studies	ethylene/dimethyl ether
Consalvi et al. [111]	Co-flow flames	Numerical studies	toluene/n-heptane; toluene/iso-octane
Li et al. [112]	Flow reactor	Numerical studies	1,3 butadiene/propyne
An et al. [113]	Counterflow flames	Numerical studies	toluene/n-heptane; toluene/iso-octane
Cuoci et al. [114]	Laminar flames	Numerical studies	ethylene/methane
Wang et al. [115]	Counterflow flames	Numerical studies	ethylene/propane/benzene
Raj et al. [34]	Premixed laminar flames	Numerical studies	toluene/n-heptane; toluene/iso-octane

^a 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.

. Both diagnostic methods and numerical methods have been adopted to investigate the synergistic effects among hydrocarbons and hydrocarbons with oxygenated fuels.

3.1. Synergistic Effects between Hydrocarbons

Li et al. [112] and Poddar et al. [12] conducted studies on the synergistic effect observed during the co-pyrolysis of 1,3-butadiene and propyne, focusing on benzene formation. Poddar et al. [12] experimentally investigated the co-pyrolysis of 1,3-butadiene and propyne in an isothermal laminar flow reactor at 973 K to 1273 K and a residence time of 0.3 s. Subsequently, Li et al. [112] developed a kinetic model for the co-pyrolysis of 1,3-butadiene and propyne. Figure 13 illustrates the substantial increase in benzene yield as a function of temperature during propyne pyrolysis, 1,3-butadiene pyrolysis, and their co-pyrolysis. In Figure 13, the weighted sum denotes the expected yield from the 1,3-butadiene/propyne co-pyrolysis experiment if the two fuels behaved as a linear combination of their individual contributions. In contrast, the CO-PY denotes the actual concentrations observed in the 1,3-butadiene/propyne co-pyrolysis. As shown in Figure 13, the benzene concentration in the CO-PY is higher than the weighted sum, indicating synergistic effects between 1,3-butadiene and propyne. Kinetic analysis revealed the synergistic effect of propyne addition on 1,3-butadiene, leading to enhanced formation of intermediate species and ultimately promoting benzene formation. The primary sources of benzene during the 1,3-butadiene pyrolysis were attributed to acetylene addition to the butadienyl radical. Sensitivity analysis involves increasing and decreasing each reaction rate constant by a factor of two and calculating their effect on the simulation benzene concentrations. Sensitivity and ROP analysis of co-pyrolysis at 1173 K revealed that the isomerization reaction of 1,3-butadiene exhibited the highest sensitivity to the formation of propargyl radicals through 1,2-butadiene [112]. The reaction of propargyl radical with propyne (3) predominantly yielded benzene, accounting for 57% of the benzene production. In comparison, the reaction of propargyl radical with allene (4) contributed to approximately 17% of benzene formation during the CO-PY and played a crucial role in the synergistic effect observed in benzene formation.

A synergistic effect has been elucidated in fuel blends comprising methane, ethane, propane, and propene with ethylene in both co-flow flame and counter-flow diffusion flame [103,104,114,116,117,118]. The addition of methane to ethylene flames leads to an increase in methyl radicals, subsequently enhancing the formation of propargyl radicals (3, 4). Notably, propargyl radicals play a crucial role in benzene formation. Therefore, these studies have demonstrated that methane addition to ethylene co-flow flames, laminar diffusion flames, and counter-flow diffusion flames induces a synergistic effect on benzene formation [103,114,116]. Researchers have observed a synergistic effect resulting from the addition of ethane and propane to ethylene flames [103,115,118]. In ethane and propane, 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].

Wang 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→iC₃H₇→C₃H₆→aC₃H₅→aC₃H₄→C₃H₃ for the formation of propargyl radical. Thus, ROP analysis revealed that the enhancement in C₄H₅-2, A1CH₃, 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.

3.2. 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 [9,31,39,96,100,103,108,119,120,121,122,123,124,125]. 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. Li 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 CH₃ originating from DME and C₂ species from ethylene. The pathway for benzene formation involves DME decomposition and hydrogen abstraction reactions (DME→CH₃(H)→C₂H₂(C₃H₃)→A1). Zhang 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 [10,31,121,123,125]. 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 C₂ species to increase the production of propargyl radicals and C₄ species. The resulting propargyl radical undergoes self-recombination, as shown in Figure 3b, and reacts with C₂H₂ 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 C₃H₃ formation due to a reduced rate of C₂ species. Moreover, the formation of C₄H₆ from CH₃ + C₂H₄ decreases with a decreasing percentage of C₂H₄ during the flame. Thus, ethanol concentration is crucial for the synergistic effect on benzene and soot formation during ethylene flames.

4. Synergistic Effects on PAHs Formation

4.1. Synergistic Effect on Indene Formation

Indene (C₉H₈), a PAH composed of one-, five-, and six-membered aromatic rings, is of significant concern for its carcinogenic, mutagenic, and environmentally harmful properties. Research studies have focused on understanding the formation mechanism of indene under various experimental conditions. Mebel et al. [48] described several pathways for indene formation, including the reaction of allene with phenyl radical followed by hydrogen abstraction, the reaction of propargyl radical with phenyl radical and benzene followed by hydrogen abstraction, acetylene addition to benzyl radical, and the addition of propene to phenyl radicals [48]. Experimental studies by Parker et al. [127] in the interstellar medium demonstrated that the addition of acetylene to the benzyl radical resulted in the formation of indene. These findings contribute to our understanding of the complex mechanisms involved in the formation of indene.

Several studies have investigated the synergistic effect on indene formation across different experimental setups, such as shock tubes, jet flow reactors, and plug flow reactors. One such study focused on the experimental and kinetic analysis of indene formation during co-pyrolysis of toluene with C₃H₄ isomers in a flow reactor [91], with its sensitivity analysis illustrated in Figure 17. The sensitivity analysis emphasized the significant impact of the reaction between acetylene and benzyl radical (5) on indene formation. Additionally, the reaction of propyne and H played a crucial role in producing ethylene, which is essential for indene formation in the presence of toluene. Interestingly, ethylene formation is favored at higher pressures, indicating the influence of pressure-sensitive reactions on the synergistic effect of indene formation rather than the molecular structure of the C₃H₄ isomer. This highlights the complex interaction between reaction pathways and operating conditions in determining the rate of indene formation in various experimental environments.

Recently, Li et al. [11] focused on the co-pyrolysis of toluene and acetylene at 1 atm pressure in a plug flow reactor and observed the synergistic effect on indene formation with the addition of acetylene, as shown in Figure 18a. They employed synchrotron vacuum ultraviolet (VUV) light to ionize the pyrolysis products and subsequently analyzed the ions utilizing a reflection time-of-flight mass spectrometer. Products such as benzene, toluene, indene, and bibenzyl were identified and measured. The addition of acetylene enhances the decomposition of toluene, revealing the synergistic effect during co-pyrolysis. Among the key reactions contributing to this effect are the thermal decomposition reaction of toluene, hydrogen abstraction from toluene, and the reaction of methyl radicals with toluene. These pathways are significant as they contribute to the consumption of toluene and the generation of intermediate species that are essential for subsequent reactions leading to indene formation. Furthermore, the presence of acetylene influences the reaction pathways of benzyl radicals, favoring the acetylene addition reaction (5) over recombination reactions, thus promoting indene formation synergistically, as illustrated in Figure 18b. The synergistic effect of the n-heptane/toluene mixture on indene formation was analyzed in a jet flow reactor, a counterflow diffusion flame, and a flow reactor [67,97,128]. These studies highlighted the crucial role of the interaction between benzyl radical and acetylene, produced due to heptane, in observing the synergistic effect between n-heptane and toluene. The synthesis of indene is governed by the concentrations of acetylene and benzyl radicals in an n-heptane/toluene fuel mixture, thus corroborating the nonlinear correlation between indene formation and toluene concentration, alongside the synergistic effect between n-heptane/toluene fuel mixture components [67]. Hamadi et al. [45] investigated the interaction of C₃ species with benzene during co-pyrolysis in a shock tube. They utilized an Agilent gas chromatograph, specifically the 7890B series, connected in series with a Thermo Trace GC Ultra, which was coupled to a Thermo DSQ MS for the detection and analysis of pyrolysis products. Their kinetic study revealed that the isomerization reaction of C₉ species, the benzyl radical reaction with acetylene, and the reaction of propargyl with benzene could enhance the indene formation. Furthermore, the addition of C₃ to benzene was found to enhance indene formation. In summary, the formation of indene during the benzyl radical and acetylene reactions in the co-pyrolysis of toluene and acetylene exhibits a strong synergistic effect compared to all the blends described.

4.2. 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. Parker 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 [15,34,45,67,95,97,107,113,128]. Shao 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.

Choi 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 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 Liu et al. [95] on the counterflow diffusion flame using two previously reported mechanisms by An et al. [113] and Wang et al. [129], it was observed that the mechanism proposed by An et al. [113] highlighted the significance of reaction (14) in naphthalene formation due to the addition of toluene in the isooctane toluene blend, followed by the formation of C₁₀H₉ through benzyl radical. Conversely, the mechanism proposed by Wang et al. [129] emphasized that naphthalene formation during the isooctane/toluene flame study primarily occurred through the recombination reaction of a resonantly stabilized pentadienyl radical (9) and the reaction of benzene with iC₄H₅.

A2- + H₂ = A2 + H

(14)

Hamadi et al. [45] conducted experiments on benzene/ethylene and benzene/acetylene blend pyrolysis in a single pulse shock tube at high pressure, with product composition detected through the gas chromatography-mass spectrometry technique. Their study revealed a synergistic effect in naphthalene formation during the pyrolysis of the blend at high pressure, favoring the formation of larger PAH, as illustrated in Figure 21a. Interestingly, the ethylene/benzene blend was found to lower the temperature of naphthalene formation compared to the pure benzene and benzene/acetylene blends. Figure 21a indicates a negligible difference between the acetylene/benzene blend and pure benzene in terms of naphthalene formation, suggesting that the HACA pathway is less efficient in the presence of acetylene. The kinetic study and rate of production analysis elucidated those dominant reactions during pyrolysis, including hydrogen abstraction and benzene decomposition. Furthermore, the HACA reaction pathway through A1C₂H enhanced the formation of the naphthyl radical, which then underwent hydrogen abstraction, while adding ethylene enhanced the formation of naphthalene via 6, 7 and 15. Additionally, Choi et al. [99] investigated the addition of various fuel components, such as n-heptane, ethanol, benzene, and toluene, to ethylene in a diffusion flame. Their study concluded that adding liquid fuel components, particularly benzene and toluene, to ethylene exhibited a high synergistic effect on naphthalene compared to all mentioned fuel blends with ethylene and pure ethylene diffusion flame, as depicted in Figure 21b.

A2- + C₂H₄ = A2 + C₂H₃

(15)

4.3. 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, Hussain 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 A2CH₂, while pathway 2 entails the self-addition reaction of A1CH₂. 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].

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 Figure 24. The addition of benzene to the ethylene/propane mixture flame enhanced the formation of larger species, reducing the synergistic effect on benzene formation. LIF signals revealed weaker signals at 330 nm compared to others for longer-wavelength detection. Additionally, non-monotonic characteristics of odd-carbon species and resonantly stabled radicals such as C₅H₅, C₆H₅CH₂, A2CH₂, and C₉H_7, alongside C₄H₄ and H atoms, played a crucial role in elucidating the synergistic effect on the formation of larger PAH. Wang et al. [115] illustrated the synergistic effect of these species, emphasizing their role in the formation of larger PAH through various reaction routes, including reactions (10 and 11) for the synergistic effect of A3 formation and the self-addition reaction of C₉H₇ (12 and 13) significantly impacting the synergistic effect on A4 formation, as depicted in Figure 24.

5. Conclusions and Perspectives

Carcinogenic polycyclic aromatic hydrocarbons are produced during the combustion process., with their formation contingent upon the fuel type and reaction conditions. Fuel blends play a pivotal role in shaping the composition of PAH formation. Given the diverse and intricate nature of PAHs, numerous reactions have been delineated and consolidated in this review, delineating different pathways for PAH formation. However, ongoing research may uncover new mechanisms that contribute to a deeper understanding of PAH formation.

The review offers a comprehensive analysis of the synergistic effect on both mono-aromatic and PAH formation during the combustion process, drawing from a combination of experimental and simulation studies. It emphasized the significant influence of temperature and pressure on the interaction between fuel components, such as 1,3-butadiene/propyne, propane/ethylene, and ethane/ethylene, leading to the formation of mono-aromatic hydrocarbons. Conversely, the addition of dimethyl ether and ethanol to ethylene exhibited a synergistic effect at low concentrations but diminished at higher concentrations of oxygenated fuel components. Furthermore, the review summarizes findings related to synergistic effects in the formation of indene, naphthalene, and higher polyaromatic hydrocarbon formation in various mixtures. Mechanisms such as hydrogen abstraction and acetylene addition (HACA) were found to significantly influence indene and naphthalene formation, while reactions involving resonantly stabilized indenyl radicals and cyclopentadienyl radical reactions contributed to phenanthrene and pyrene formation.

A further key development for understanding the synergistic effects on aromatic formation is the identification of more critical reactions, especially for larger PAHs. Additional reaction pathways have been uncovered, contributing to PAH formation in various contexts, such as sustainable aviation fuel combustion and the interstellar medium. These advancements encompass more comprehensive measurements of PAHs in both premixed and non-premixed flames, flow reactors, and jet-stirred reactors, as well as quantum chemical computational calculations aimed at enhancing our understanding of the key reactions involved in synergistic effects on aromatic formation. Furthermore, the understanding of synergistic effects on soot formation will be facilitated by the use of diagnostic methods and numerical simulations.