RATIONALE:The compositional and structural information of the soot particles is essential for a better understanding of the chemistry and mechanism during the combustion. The aim of the present study was to develop a method to analyze such soot particulate samples with high complexity and poor solubility. METHODS:The solvent-free sample preparation MALDI technique was combined with the ultrahigh resolution FT ICR mass spectrometry for the characterization of solid soot particulates. Moreover, a modified iso-abundance plot (Carbon Number vs. Hydrogen Number vs. Abundance) was introduced to visualize the distributions of various chemical species, and to examine the agreement between the hydrogen- abstraction- carbon- addition (HACA) mechanism and the polycyclic aromatic hydrocarbon growth in the investigated flame system. RESULTS:This solvent-free MALDI method enabled the effective ionization of the solid soot particulates without any dissolving procedure. With the accurate m/z ratios from FT ICR MS, a unique chemical formula was assigned to each of the recorded mass signals. The combustion products were proven to be mainly large polycyclic aromatic hydrocarbons, together with a small amount (<5%) of oxidized hydrocarbons. CONCLUSIONS:The developed method provides a new approach for the molecular characterization of soot particulates like carbonaceous materials. The investigated soot particulates are mainly polycyclic aromatic hydrocarbons (PAHs) with no or very short aliphatic chains. The PAHs growth mechanism during combustion can be examined against the classic HACA mechanism.

Synergistic octane blending behavior of ethanol with gasoline and its surrogates has been observed by many researchers. The nonlinear octane boosting tendency is observed at mid and high molar blends of ethanol in primary reference fuels. The present work aims to provide chemical insight into this nonlinear blending behavior of ignition processes when ethanol is blended with primary reference fuels. To this end, ignition delay time (IDT) calculations, using a well-validated mechanism, were performed for several fuel blends of iso-octane, n-heptane, and ethanol. Temperature and pressure values were found, correlating experimentally measured octane numbers and simulated homogenous batch reactor IDTs. The temperature and pressure conditions obtained, were then used to study the evolution of heat release and reactivity before the onset of auto-ignition in a homogeneous premixed reactor. Markers of low- and high-temperature reactivity (OH and HO2) were analyzed for various molar blends of n-heptane with ethanol-iso-octane. Ethanol was observed to be better at radical scavenging than iso-octane at a higher mole fraction. A computational singular perturbation analysis was conducted for a selection of blends to clarify the reactions responsible for the synergistic blending behavior of ethanol in n-heptane. The role of the H-abstraction reactions was highlighted during the first ignition stage; reactions related to n-heptane were found to compete with the H-abstraction reactions of iso-octane or ethanol. Notably, the H-abstraction path of ethanol was more favored than that of the iso-octane, as a result of the smaller activation energies of the related reactions in ethanol. The competition of the H-abstraction paths resulted in a smaller radical pool in the n-heptane-iso-octane-air case, and an even smaller pool in n-heptane-ethanol-air. In all the cases considered, the second stage was dominated mainly by hydrogen-related reactions, regardless of the initial mixture, with H2O2 (+M) → 2OH (+M) and H + O2 → O + OH playing the most important roles. This work employed a novel approach to examine specific reactions responsible for auto-ignition in ethanol blends, which can be used for fuel design, primarily around the generation/consumption of radical pool intermediates by interaction with fuel components.

2,2,3-Trimethylbutane (i.e., triptane) is a potential gasoline octane booster with a research octane number (RON) of 112. Recent studies showed that it can be catalytically produced with high selectivity from methanol (CH3OH) and dimethyl ether (DME), which presents a promising route for utilizing biomass derivatives as transportation fuels. Understanding the ignition properties of triptane at engine relevant conditions is crucial for its further evaluation. In this work, a detailed kinetic model for triptane combustion is developed and validated. The rate rules for the low-temperature oxidation reactions are evaluated based on quantum chemistry calculations from literature, and thermochemical properties of all the species are assessed based on new thermodynamic group values with careful treatment of gauche interactions. In addition, alternative isomerization pathways for peroxy-alkylhydroperoxide species (ȮOQOOH) are incorporated in the model. The model is validated against new ignition delay data from facilities at King Abdullah University of Science and Technology (KAUST): rapid compression machine (RCM) experiments at pressures of 20 and 40 bar, equivalence ratios of 0.5 and 1 and across a temperature range of 620 to 1015 K, and shock tube experiments at 2 and 5 bar, 0.5 and 1 equivalence ratio and over 1000–1400 K. Moreover, the model prediction of various species is compared against species profiles from jet stirred reactor experiments at three equivalence ratios (0.5, 1 and 2) at atmospheric pressure. Finally, triptane is compared with its less branched isomers, n-heptane and 2-methylhexane, to evaluate the effect of branching on fuel reactivity and importance of alternative isomerization pathway.

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.

The introduction of high cetane components has enabled the use of low cetane base gasoline in compression ignition engines. This study provides an understanding of the autoignition characteristics of various ethers blended with light distillates. The spontaneous ignition of mixtures was herein studied both experimentally [ignition quality tester (IQT)] and computationally, allowing the impacts of distillate composition, ether structure, and reaction progress on key ignition pathways to be determined. Various multicomponent base fuel surrogates were formulated to closely match actual fuel composition, thereby accurately simulating the interplay between distillates and oxygenates. Despite its lower cetane number, di-n-butyl ether (DNBE) was found to promote a more vigorous ignition than diethylether. However, OH radical scavenging by p-xylene counteracts the DNBE effect. Two preignition phases may be distinguished, namely, oxidation initiation by ether and subsequent chemical runaway involving simultaneously fuel and ether. According to the present kinetic mechanism, direct cross-reactions between ether radicals and light distillate components have little impact on the ignition delay time under the IQT operating conditions. As ignition progress increases, ether contribution to OH production decreases and oxidation paths related to aliphatic and cyclic alkanes become dominant. In the case of polyoxymethylene methyl ethers, the extra production of formaldehyde during the ignition phase does not impair the overall reactivity. The respective effects of OME1 and OME3 on ignition may be explained by the emergence of a new OH production path from OME3 oxidation products, while methyl formate production from OME1 acts as an OH radical sink. Even though locally lean zones of the IQT reactor may favor specifically neopentane oxidation at the expense of n-hexane, the new OH production path remains active over a wide range of conditions. Overall, the present detailed model qualitatively captures the nonlinear impact of various ethers on autoignition over the 15–30 DCN range, which makes it attractive for optimizing low cetane fuel formulation.

Asphaltenes are n-alkane insoluble compounds found in crude oils and heavy fuels (high and non-boiling petroleum fractions). Asphaltene molecular structure has not been fully elucidated, and their presence in fuels is a source of concern. They reduce combustion efficiency and are responsible for particulate matter emissions. Removing the asphaltenes, or deasphalting, is a way of upgrading the fuel to improve its quality. This study reports the removal of asphaltenes from heavy fuel oil (HFO) using a solvent extraction method, and the detailed molecular characterization of the deasphalted oil (DAO) using positive ion atmospheric pressure photo ionization Fourier transform-ion cyclotron resonance mass spectrometry (APPI (+) FT-ICR/MS) and 1H and 13C Nuclear magnetic resonance (NMR) spectroscopy. Approximately 8.2 mass % of asphaltenes were removed from HFO using n-heptane as solvent. This resulted in significant improvements in the HFO’s physical properties. The resulting DAO was five times less viscous and contained significantly less heavy metals (e.g., Ni and V). There was also a slight reduction in the sulfur content from 3.3 to 3.1 mass %. 52,753 and 46,315 ions with a mass to charge ratio (m/z) ranging from 154 to 1200 were detected in HFO and DAO samples, respectively, using APPI FT-ICR/MS. Amongst them, 6729 (HFO) and 6030 (DAO) ions were resolved into their underlying elemental compositions (C, H, O, N and S) and a unique chemical formula was assigned to each mass. The resolved masses were then further classified based on their molecular class and were analyzed as a function of double bond equivalent (DBE) vs carbon number. 1H and 13C NMR analyses of HFO and DAO were performed and the results indicate the total aromatic groups in HFO (1H 7.7 mol %, 13C 37.6 mol %) are more compared to DAO (1H 4.7 mol %, 13C 32.6 mol %). The average molecular parameters (AMPs) of HFO and DAO were also calculated from the 1H and 13C NMR spectra and compared. A surrogate molecule that visualizes the average molecular structure of the entire fuel was developed for both HFO and DAO using the data from the above analytical techniques. Understanding the molecular chemistry of these fuels provides valuable data to develop better desulfurization techniques for these sulfur laden fuels and help predict fuel properties using structure-property relationships.

The prospect of blending gasoline fuel with ethanol is being investigated as a potential way to improve the knock residence of the base gasoline. However, one of the drawbacks is a lack of proper understanding of the reason for the non-linear response of blending ethanol and gasoline. This non-linearity could be better understood by an improved knowledge of the interactions of these fuel components at a molecular level. This study proposed a highly reduced four-component (toluene/n-heptane/iso-octane/ethanol) gasoline surrogate model containing 59 species and 270 reactions. The model was reduced using the direct relation graph with expert knowledge (DRG-X) (Lu and Law, 20015; Lu et al., 2011) and isomer lumping method. The computational singular perturbation (CSP) analysis were performed to reduce the potential stiffness issues by accordingly adjusting the Arrhenius coefficients of the proper reactions. The model has been comprehensively validated against wide range of ignition delay times (IDT) and flame speed (FS) measurement data as well as compared against two representative literature models from Liu et al. (2013) and Wang et al. (2015). Overall, good agreements were observed between model predictions and experimental data across the entire research octane number (RON), equivalence ratio, pressure and temperature range. In addition, the model has also been coupled with the computational fluid dynamic (CFD) models to simulate the experimental data of constant volume reacting spray of a low-octane gasoline (Haltermann straight-run naphtha), and in-cylinder pressures and temperatures of a high-octane gasoline (Haltermann Gasoline) combustion in a heavy duty compression ignition engine. The coupled model can qualitatively predict the experimentally obtained data with an improved performance for PRF, TPRF, and TPRF-ethanol surrogates.

The effect of specific cathode surface area (SCSA; 2, 4 and 8 m2/m3) on biogas production and biofouling control was investigated in anaerobic electrochemical membrane bioreactors (AnEMBRs). Nickel-based hollow fibers were used as both a cathode for hydrogen evolution and a membrane for the filtration of treated water. Varying the SCSA was found to influence the current density, biogas quantity and composition, and trans-membrane pressure (TMP). In situ gas generation was monitored using a high-speed camera; where the 8 m2/m3 AnEMBRs were found to mainly evolve smaller sized bubbles (45–114 µm) compared to the 2 and 4 m2/m3 AnEMBRs. This correlated with the lowest TMP (0.37 bars) compared to the other reactors (4 m2/m3, 0.76 bars; 2 m2/m3, 1.18 bars). The cathodic microbial community varied with SCSA, with Acetobacterium dominating at a relative abundance that reached 62% (4 m2/m3), while hydrogenotrophic methanogens (represented by Methanocorpusculum) comprised up to 2.5% of the total community. This community variability apparently influenced both the hydrogen (QH2) and methane (QCH4) production rates, reaching a maximum QH2 of 0.416 m3/m3/day (SCSA 8 m2/m3). Overall, this study discloses the cumulative effect of QH2, QCH4, bubble distribution and frequency on biofouling with SCSA variation in AnEMBRs, demonstrating that with higher SCSA the in situ gas scouring effect is more pronounced leading to a lower fouling propensity.

The growth of polycyclic aromatic hydrocarbons (PAH) can proceed via multiple chemical mechanisms. The mechanism of naphthyl radical and vinylacetylene (CH) addition reaction has been systematically investigated in this computational study. A combination of DFT/B3LYP/6-311+G(d,p), CCSD/6-311+G(d,p) and CBS-QB3 methods were performed to calculate the potential energy surfaces. It revealed that the products, including phenanthrene, anthracene, a PAH with a five-membered ring structure, and PAH with a CH radical substitution, can be formed in A-1 (1-naphthyl)+CH and A-2 (2-naphthyl) +CH reaction networks. The reaction rate constants at 0.1-100 atm were evaluated by RRKM theory by solving the master equation in the temperature range of 800–2500 K, which showed that the rate constants of reactions A-1 (A-2)+CH→product+H are highly temperature-dependent but nearly pressure-independent. The distribution of products was investigated in a 0-D batch reactor, wherein the initial reactant concentrations were taken from experimental measurements. The results showed that adduct intermediates were the main products at low temperature (T < 1000 K), and the phenanthrene and PAH with CH radical substitution became the dominant products at temperatures where PAHs and soot form in flames (T > 1000 K). It was observed that a significant amount of phenanthrene is formed from PAH with a CH radical substitution with the assistance of H atom. Reaction pathway sensitivity analysis for the PAH radical+CH reaction system was performed and showed that the new benzene rings are more likely to be generated near the zig-zag edge surface site instead of the free edge. For the development of a PAH mechanism, the analogous treatment of rate constants for larger PAH radical + CH reaction system are discussed. The formation rate of naphthalene from the reaction of phenyl+CH was found to be very close to that of phenanthrene from the reaction of naphthyl+CH, suggesting that the analogous treatment of the rates is reasonable in PAH mechanisms.