Although there was significant advancement on polycyclic aromatic hydrocarbon (PAH) formation, current mechanisms are still limited in providing an integrated and accurate scheme of PAH yield in combustion conditions; thus, a more detailed and comprehensive understanding is necessary. This work provides a systematic investigation of PAH growth by phenylacetylene addition. A combination of the density functional theory (DFT/B3LYP/6-311+G(d,p)) and the complete basis set method (CBS-QB3) is utilized to calculate the potential energy surfaces. The reaction system is initiated by the H elimination reaction of phenylacetylene + H → o-ethynylphenyl + H2, and then, the addition reaction of phenylacetylene and o-ethynylphenyl can produce PAHs with one, two, three, and four rings. The temperature- and pressure-dependent reaction rate coefficients are calculated via a combination of conventional transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory with solving the master equation in the temperature range of 500-2500 K and at the pressure range of 0.01-10 atm. There are 263 species and 65 reactions in this reaction system. It shows that the rate constants of this reaction system are highly temperature-dependent and slightly sensitive to the pressure at temperatures lower than 1300 K. To evaluate the yield distributions of various PAH products in the whole reaction network, a closed 0-D batch reactor model in Chemkin is used to calculate the C6H5C2H-C2H2-H-Ar reaction system. The results showed that the prevailing products of this system are three-ring PAHs with side chain structures. Compared with the traditional HACA pathways, the investigated reaction system presents higher efficiency in large PAH formations, which could subsequently promote the formation of soot particles. The phenylacetylene and o-ethynylphenyl reaction network emphasizes the importance of species with side chains, and it enriches current PAH growth pathways aside from the addition of small species such as C2H2.

Oxygenated polycyclic aromatic hydrocarbons (OPAH) have received increasing attention due to their toxic effect on human health. This study comprehensively investigates the evolution of OPAH chemistry at flame temperatures. Jet-stirred reactor (JSR) experiments with benzene/phenol/C2H2/N2 and benzene/C2H2/O2/N2 revealed that OPAH with oxygenated heterocycle can be formed by the addition of C2H2 at 1400 K. To further clarify the evolution of OPAH chemistry in soot systems, OPAH formation and decomposition reaction pathways and kinetic parameters have been theoretically investigated. The potential energy surfaces of 1-naphtholate and 2-naphtholate growth, and thermal decomposition reactions, were calculated by combining the density functional theory B3LYP/6–311+G(d,p) and CCSD(T)/cc-pvdz methods. The reaction rate coefficients in the temperature range of 800–2500 K and pressure range of 0.1–100 atm were calculated using RRKM theory by solving the master equations. The potential energy surface of C2H2+1-naphtholate and C2H2+2-naphtholate growth reactions showed that the O atom could be locked in a naphthofuran molecule with the formation of a C[sbnd]O[sbnd]C oxygenated heterocycle; and the reaction rates were determined by adding the C2H2 elementary step with the energy barrier of 26.0 and 19.9 kcal/mol, respectively. Thermal decomposition reactions of 1-naphtholate and 2-naphtholate yielded an indenyl radical and CO. The thermal decomposition reaction rates were significantly sensitive to the zig-zag site structure next to the C[dbnd]O bond. The decomposition rate of 1-naphtholate at 1500 K, with a zig-zag site near the C[dbnd]O bond, was 14.8 times lower than that of 2-naphtholate with no zig-zag site near the C[dbnd]O bond. Rate comparison results indicate that the C[dbnd]O functional group rapidly converts to a C[sbnd]O[sbnd]C functional group with the addition of C2H2. The formation, growth and thermal decomposition reactions of 1-naphtholate and 2-naphtholate were added to a detailed PAH mechanism to check the effect of OPAH reactions on PAH formation chemistry. The concentration profile of naphthalene predicted by the updated PAH mechanism was lower than current PAH mechanism predictions by 29%, indicating that the OPAH reactions had a significant effect on PAH formation chemistry, and should be included in the PAH mechanism. However, due to the relatively low concentration of OPAH compared to PAH, it is possible to ignore the correlation between OPAH and soot nucleation at flame temperatures; therefore an OPAH evolution pathway (PAH → incipient soot → OPAH formation on soot particle → selective thermal decomposition of OPAH), is proposed to explain the high content of OPAH molecules (e.g., 9,10-anthraquinone, benz(a)anthracene-7,12-dione, and benzanthrone) adsorbed on the soot particle.

Tailoring fuel properties to maximize the efficiency of internal combustion engines is a way towards achieving cleaner combustion systems. In this work, the ignition properties along with the chemical composition (expressed as functional groups) of various light distillate (e.g., gasoline) cuts were analyzed to better understand the properties of full boiling range fuels. Various distillation cuts were obtained with a spinning band distillation system, which were then tested in an ignition quality tester (IQT) to obtain their global chemical reactivity (i.e., ignition delay time (IDT)). The distillates were further analyzed with 1H nuclear magnetic resonance (NMR) spectroscopy to identify and quantify various functional groups present in them. Various gasolines of research grade with specific target properties set forth by the Coordinating Research Council (CRC) that are known as FACE (fuels for advanced combustion engines) gasolines were distilled. When fuels with low aromatic content were distilled, the higher boiling point (BP) range (i.e., higher molecular weight) fractions exhibited lower IDT. However, distilled fractions of fuels with high aromatic content showed an initial decrease in IDT with increasing BP, followed by drastic increase in IDT primarily due to increasing aromatic groups. This study provides an understanding of the contribution of various volatile fractions to the IDTs of a multicomponent fuel, which is of relevance to fuel stratification utilized in gasoline compression ignition (GCI) engines to tailor heat release rates.

Many practical soot-emitting combustion systems such as diesel and jet engines rely on diffusion flames for efficient and reliable operation. Efforts to mitigate soot emissions from these systems are dependent on fundamental understanding of the physicochemical pathways leading from fuel to soot in laminar diffusion flames. Existing diffusion flame−based soot studies focused primarily on over-ventilated coflow flame where the fuel gas (or vapor) issues from a cylindrical tube into a co-flowing oxidizer, and counterflow flame, where a reacting zone is established between two opposing streams of fuel and oxidizer. As a canonical diffusion flame configuration, laminar counterflow diffusion flames have been widely used as a highly controllable environment for soot research, enabling significant progress in the understanding of soot formation for several decades. In view of the possibility of fuel/oxidizer premixing in practical systems, counterflow partially premixed flames have also been studied. In the present work we intend to provide a comprehensive review of the researches on various aspects of soot formation utilizing counterflow flames. Major processes of soot formation (formation of gas phase soot precursors, soot inception and surface reactions, as well as particle-particle interactions) are examined first, with focus on the most recent (post-2010) research progress. Experimental techniques and associated challenges for the measurement of soot-related properties (some knowledge of which is helpful for full appreciation of the experimental data to be reviewed) are then introduced. This is followed by a detailed description of soot evolution in counterflow flames, which is complemented by a discussion on the similarity and differences of the sooting structures between counterflow and coflow diffusion flames. Parametric studies of the effects of fuel molecular structure, fuel additive, partial-premixing, pressure, temperature, stoichiometric mixture fraction, and residence time on soot formation in counterflow flames will then be addressed in detail. This review concludes with a summary of the knowledge and challenges gathered and demonstrated through decades of research, and an outlook on opportunities for future counterflow flame−based soot research towards a more complete understanding of soot formation and the development of novel techniques for soot mitigation in practical combustion devices.

The ignition characteristics of isooctane and n-heptane in an ignition quality tester (IQT) were simulated using a two-stage Lagrangian (TSL) model, which is a zero-dimensional (0-D) reactor network method. The TSL model was also used to simulate the ignition delay of n-dodecane and n-heptane in a constant volume combustion chamber (CVCC), which is archived in the engine combustion network (ECN) library (http://www.ca.sandia.gov/ecn). A detailed chemical kinetic model for gasoline surrogates from the Lawrence Livermore National Laboratory (LLNL) was utilized for the simulation of n-heptane and isooctane. Additional simulations were performed using an optimized gasoline surrogate mechanism from RWTH Aachen University. Validations of the simulated data were also performed with experimental results from an IQT at KAUST. For simulation of n-dodecane in the CVCC, two n-dodecane kinetic models from the literature were utilized. The primary aim of this study is to test the ability of TSL to replicate ignition timings in the IQT and the CVCC. The agreement between the model and the experiment is acceptable except for isooctane in the IQT and n-heptane and n-dodecane in the CVCC. The ability of the simulations to replicate observable trends in ignition delay times with regard to changes in ambient temperature and pressure allows the model to provide insights into the reactions contributing towards ignition. Thus, the TSL model was further employed to investigate the physical and chemical processes responsible for controlling the overall ignition under various conditions. The effects of exothermicity, ambient pressure, and ambient oxygen concentration on first stage ignition were also studied. Increasing ambient pressure and oxygen concentration was found to shorten the overall ignition delay time, but does not affect the timing of the first stage ignition. Additionally, the temperature at the end of the first stage ignition was found to increase at higher ambient pressure and oxygen concentration. Sensitivity analysis was performed using the TSL model to elucidate the reactions that control the overall ignition process. The present TSL modeling approach demonstrates the suitability of using detailed chemical kinetic models to provide insights into spray combustion processes. © 2016 Elsevier Ltd

As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40. atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270. K. Results at temperatures above 900. K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900. K demonstrate a stronger negative temperature coefficient behavior for FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two- and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition. © 2016 The Combustion Institute.

Heavy fuel oil (HFO) is primarily used as fuel in marine engines and in boilers to generate electricity. Nuclear Magnetic Resonance (NMR) is a powerful analytical tool for structure elucidation and in this study, 1H NMR and 13C NMR spectroscopy were used for the structural characterization of 2 HFO samples. The NMR data was combined with elemental analysis and average molecular weight to quantify average molecular parameters (AMPs), such as the number of paraffinic carbons, naphthenic carbons, aromatic hydrogens, olefinic hydrogens, etc. in the HFO samples. Recent formulae published in the literature were used for calculating various derived AMPs like aromaticity factor 〖(f〗_a), C/H ratio, average paraffinic chain length (¯n), naphthenic ring number 〖(R〗_N), aromatic ring number〖 (R〗_A), total ring number〖 (R〗_T), aromatic condensation index (φ) and aromatic condensation degree (Ω). These derived AMPs help in understanding the overall structure of the fuel. A total of 19 functional groups were defined to represent the HFO samples, and their respective concentrations were calculated by formulating balance equations that equate the concentration of the functional groups with the concentration of the AMPs. Heteroatoms like sulfur, nitrogen, and oxygen were also included in the functional groups. Surrogate molecules were finally constructed to represent the average structure of the molecules present in the HFO samples. This surrogate molecule can be used for property estimation of the HFO samples and also serve as a surrogate to represent the molecular structure for use in kinetic studies.

Heavy fuel oil (HFO) as a fuel in industrial and power generation plants ensures the availability of energy at economy. Coke and cenosphere emissions from HFO combustion need to be controlled by particulate control equipment such as electrostatic precipitators, and collection effectiveness is impacted by the properties of these particulates. The cenosphere formation is a function of HFO composition, which varies depending on the source of the HFO. Numerical modelling of the cenosphere formation mechanism presented in this paper is an economical method of characterising cenosphere formation potential for HFO in comparison to experimental analysis of individual HFO samples, leading to better control and collection. In the present work, a novel numerical model is developed for understanding the global cenosphere formation mechanism. The critical diameter of the cenosphere is modelled based on the balance between two pressures developed in an HFO droplet. First is the pressure (Prpf) developed at the interface of the liquid surface and the inner surface of the accumulated coke due to the flow restriction of volatile components from the interior of the droplet. Second is the pressure due to the outer shell strength (PrC) gained from van der Walls energy of the coke layers and surface energy. In this present study it is considered that when PrC ≥ Prpf the outer shell starts to harden. The internal motion in the shell layer ceases and the outer diameter (DSOut) of the shell is then fixed. The entire process of cenosphere formation in this study is analysed in three phases: regression, shell formation and hardening, and post shell hardening. Variations in pressures during shell formation are analysed. Shell (cenosphere) dimensions are evaluated at the completion of droplet evaporation. The rate of fuel evaporation, rate of coke formation and coke accumulation are analysed. The model predicts shell outer diameters of 650, 860 and 1040 µm, and inner diameters are 360, 410 and 430 µm respectively, for 700, 900 and 1100 µm HFO droplets. The present numerical model is validated with experimental results available from the literature. Total variation between computational and experimental results is in the range of 3–7%.

Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4 (e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2 addition, intramolecular isomerization, and OH release; C8H14O4 species are proposed to result from subsequent reactions of C8H16O5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances. © 2015 The Combustion Institute.

Gasoline is the most widely used fuel for light duty automobile transportation, but its molecular complexity makes it intractable to experimentally and computationally study the fundamental combustion properties. Therefore, surrogate fuels with a simpler molecular composition that represent real fuel behavior in one or more aspects are needed to enable repeatable experimental and computational combustion investigations. This study presents a novel computational methodology for formulating surrogates for FACE (fuels for advanced combustion engines) gasolines A and C by combining regression modeling with physical and chemical kinetics simulations. The computational methodology integrates simulation tools executed across different software platforms. Initially, the palette of surrogate species and carbon types for the target fuels were determined from a detailed hydrocarbon analysis (DHA). A regression algorithm implemented in MATLAB was linked to REFPROP for simulation of distillation curves and calculation of physical properties of surrogate compositions. The MATLAB code generates a surrogate composition at each iteration, which is then used to automatically generate CHEMKIN input files that are submitted to homogeneous batch reactor simulations for prediction of research octane number (RON). The regression algorithm determines the optimal surrogate composition to match the fuel properties of FACE A and C gasoline, specifically hydrogen/carbon (H/C) ratio, density, distillation characteristics, carbon types, and RON. The optimal surrogate fuel compositions obtained using the present computational approach was compared to the real fuel properties, as well as with surrogate compositions available in the literature. Experiments were conducted within a Cooperative Fuels Research (CFR) engine operating under controlled autoignition (CAI) mode to compare the formulated surrogates against the real fuels. Carbon monoxide measurements indicated that the proposed surrogates accurately reproduced the global reactivity of the real fuels across various combustion regimes.