Experimental and modeling study on the pyrolysis and oxidation of n-decane and n-dodecane

High pressure n-decane and n-dodecane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, n-decane or n-dodecane represent the normal paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on both n-alkanes was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on both n-decane and n-dodecane oxidation and pyrolysis were obtained for temperatures from 867 to 1739 K, pressures from 19 to 74 atm, reaction times from 1.15 to 3.47 ms, and equivalence ratios from 0.46 to 2.05, and ∞. Both n-decane and n-dodecane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at the conditions studied. This observation prompted an experimental and modeling study of n-decane and n-dodecane pyrolysis using a recently submitted revised n-decane/iso-octane/toluene surrogate model. The surrogate model was extended to n-dodecane in order to facilitate the study of the species and the 1-olefin species quantified during the pyrolysis of n-dodecane and n-decane were revised with additional reactions and reaction rate constants modified with rate constants taken from literature. When compared against a recently published generalized n-alkane model and the original and revised surrogate models, the revised (based on our experimental work) and extended surrogate model showed improvements in predicting 1-olefin species profiles from pyrolytic and oxidative n-decane and n-dodecane experiments. The revised and extended model when compared to the published generalized n-alkane and surrogate models also showed improvements in predicting species profiles from flow reactor n-decane oxidation experiments, but similarly predicted n-decane and n-dodecane ignition delay times.     

Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions

Decalin is the simplest polycyclic alkane (polynaphtenic hydrocarbon) found in liquid fuels (jet fuels, Diesel). In order to better understand the combustion characteristics of decalin, this study provides new experimental data for its oxidation in a jet-stirred reactor. For the first time, stable species concentration profiles were measured in a jet-stirred reactor at a constant mean residence time of 0.1 s and 0.5 s at respectively 1 and 10 atm, over a range of equivalence ratios (? = 0.5–1.5) and temperatures (750–1350 K). The oxidation of decalin under these experimental conditions was modeled using a semi-detailed chemical kinetic reaction mechanism (11,000 reactions involving 360 species) derived from a previously proposed scheme for the ignition of the same fuel in a shock-tube. The proposed mechanism that includes both low- and high-temperature chemistry shows reasonably good agreement with the present experimental data set. It can also represent well decalin pyrolysis and oxidation data available in the literature. Reaction path analyses and sensitivity analyses were conducted to interpret the results.     

Multi-species time-history measurements during high-temperature acetone and 2-butanone pyrolysis

High-temperature acetone and 2-butanone pyrolysis studies were conducted behind reflected shock waves using five species time-history measurements (ketone, CO, CH₃, CH₄ and C₂H₄). Experimental conditions covered temperatures of 1100–1600 K at 1.6 atm, for mixtures of 0.25–1.5% ketone in argon. During acetone pyrolysis, the CO concentration time-history was found to be strongly sensitive to the acetone dissociation rate constant k₁ (CH₃COCH₃ → CH₃ + CH₃CO), and this could be directly determined from the CO time-histories, yielding k₁(1.6 atm) = 2.46 × 10¹⁴ exp(?69.3 [kcal/mol]/RT) s^?1 with an uncertainty of ±25%. This rate constant is in good agreement with previous shock tube studies from Sato and Hidaka (2000) [3] and Saxena et al. (2009) [4] (within 30%) at temperatures above 1450 K, but is at least three times faster than the evaluation from Sato and Hidaka at temperatures below 1250 K. Using this revised k₁ value with the recent mechanism of Pichon et al. (2009) [5], the simulated profiles during acetone pyrolysis show excellent agreement with all five species time-history measurements. Similarly, the overall 2-butanone decomposition rate constant k_tot was inferred from measured 2-butanone time-histories, yielding k_tot(1.5 atm) = 6.08 × 10¹³ exp(?63.1 [kcal/mol]/RT) s^?1 with an uncertainty of ±35%. This rate constant is approximately 30% faster than that proposed by Serinyel et al. (2010) [11] at 1119 K, and approximately 100% faster at 1412 K. Using the measured 2-butanone and CO time-histories and an O-atom balance analysis, a missing removal pathway for methyl ketene was identified. The rate constant for the decomposition of methyl ketene was assumed to be the same as the value for the ketene decomposition reaction. Using the revised k_tot value and adding the methyl ketene decomposition reaction to the Serinyel et al. mechanism, the simulated profiles during 2-butanone pyrolysis show good agreement with the measurements for all five species.     

An experimental and modeling study of the shock tube ignition of a mixture of n-heptane and n-propylbenzene as a surrogate for a large alkyl benzene

Alkyl aromatics are an important chemical class in gasoline, jet and diesel fuels. In the present work, an n-propylbenzene and n-heptane mixture is studied as a possible surrogate for large alkyl benzenes contained in diesel fuels. To evaluate it as a surrogate, ignition delay times have been measured in a heated high pressure shock tube (HPST) for a mixture of 57% n-propylbenzene/43% n-heptane in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.49, 0.98 and 1.95 and compressed pressures of 1, 10 and 30 atm over a temperature range of 1000–1600 K. The effects of reflected-shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. A combined n-propylbenzene and n-heptane reaction mechanism was assembled and simulations of the shock tube experiments were carried out. The simulation results showed very good agreement with the experimental data for ignition delay times. Sensitivity and reaction pathway analyses have been performed to reveal the important reactions responsible for fuel oxidation under the shock tube conditions studied. It was found that at 1000 K, the main consumption pathways for n-propylbenzene are abstraction reactions on the alkyl chain, with particular selectivity to the allylic site. In comparison at 1500 K, the unimolecular decomposition of the fuel is the main consumption pathway.     

Modeling of two- and three-ring aromatics formation in the pyrolysis of toluene

A detailed chemical kinetic model for the pyrolysis of toluene and subsequent formation of polycyclic aromatic hydrocarbons (PAHs) was constructed for the low pressure (～10 Torr) experimental condition of Shukla et al. [J. Phys. Chem. A 11 (1) (2007) 8308–8324; J. Phys. Chem. A 112 (2008) 2362–2369]. The kinetic model for PAHs formation in premixed flames proposed by Richter et al. [Proc. Combust. Inst. 30 (2005) 1397–1405] was used as a starting-point model and was revised for a number of reactions. Computational and empirical estimates of the rate constants were performed for important reactions involving resonance stabilized hydrocarbon radicals. The resultant model consists of 289 species and 947 reactions. The numerical simulation with the proposed model satisfactory reproduced the formation of two- and three-ring aromatic hydrocarbons observed in the mass spectrometric experiments for the low-pressure flow-reactor pyrolysis of toluene and toluene/acetylene and toluene/benzene mixtures at temperatures 1136–1507 K by Shukla et al. The sensitivity and flux analyses indicated the importance of the reactions of resonance stabilized radicals such as benzyl, fulvenallenyl and propargyl radicals for the PAHs formation processes.     

High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels

Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p₅ = 10, 20, and 40 bar) for equivalence ratios ? = 0.5 and 1.0 and over a wide temperature range 690 K < T₅ < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, ? = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated.     

Piloted ignition of solid fuels at low ambient pressure and varying igniter location

In order to investigate the effects of ambient pressure and igniter location on piloted ignition of solid fuels, the ignition mass flux of PMMA was experimentally determined for locations of the igniter between 6 and 70 mm above the solid surface, under two external heat fluxes of 21.2 and 25.4 kW/m². The experimental results show that the ignition mass flux decreases as the igniter approached the solid surface until it reached a minimum, and then the ignition mass flux remains nearly constant followed by a slight increase with a further decrease of the igniter location. In addition, in another series of experiments the ignition mass flux for elm wood decreases by a factor 0.6 at reduced pressure 0.67 (Tibet 0.67 atm) compared to the ignition mass flux at normal pressure (Hefei, 1.0 atm). The results of this work are explained well by a numerical piloted ignition model which also explains recent observations on the ignition mass flux at reduced pressures in a forced-flow ignition and flame spread apparatus.     

Modeling ammonia oxidation over a Pt (533) surface

We present a new reaction model for ammonia oxidation on a Pt (533) surface and perform numerical simulations using mean field equations. Kinetic parameters were taken from experiments and Density Functional Theory (DFT) calculations. The model is based on an oxygen-activated ammonia decomposition and includes NHx (x = 0, 1, 2) intermediates. Reaction rates and coverages obtained from calculations show semiquantitative agreement with values from kinetic and in-situ XPS measurements up to 0.1 mbar pressures. Pathways for ammonia oxidation were analyzed by varying kinetic parameters in the model, which provides new insights into the relative importance of different reaction steps.     

Evolution of flame to surface heat flux during upward flame spread on poly(methyl methacrylate)

The heat feedback profile across 5 cm wide and 15 cm tall samples of poly(methyl methacrylate) was studied from ignition until total sample involvement as a flame spread vertically upward. Incident heat flux to a water-cooled gauge was measured at 1 cm intervals. At 6–15 cm above the bottom edge of the flame, the maximum heat flux value was found to be on the order of 35 kW m^?2. Lower in the sample, 2–5 cm above the flame bottom, where the flame is thinner and thus closer to the sample’s surface, the maximum heat flux is slightly higher, about 40 kW m^?2. Using these results and finely resolved measurements of sample burning rate recorded throughout the length of experiments, an analytical model that accurately predicts the measured heat flux profile along the vertical dimension of samples solely as a function of the burning rate was developed. Coupling this model with an accurate pyrolysis solver, which predicts material burning rate based on incident heat flux, is expected to enable highly accurate simulations of the flame spread dynamics.     

Ignition behavior of pure and blended methyl octanoate,n-nonane,and methylcyclohexane

Excited-state species profiles and ignition delay times were obtained under dilute conditions (99% Ar) using a heated shock tube for methyl octanoate (C₉H₁₈O₂), n-nonane (n-C₉H₂₀), and methylcyclohexane (MCH) over a broad range of temperature and equivalence ratio (? = 0.5, 1.0, 2.0) at pressures near 1 and 10 atm. Measurements were then extended to include two ternary blends of the fuels using 5% and 20% (vol.) of the methyl ester under stoichiometric conditions. Using three independently validated chemical kinetics mechanisms, a model was compiled to assess the influence of methyl ester concentration on ignition delay times of the ternary blends. Under near-atmospheric pressure, ignition delay times were of the following order for the pure fuels: methyl octanoate < n-nonane < methylcyclohexane. Experimental results indicate that the ignition behavior of the higher-order methyl ester approaches that of the higher-order linear alkane with increased pressure regardless of equivalence ratio. Methyl octanoate also displayed significantly lower pressure dependence relative to the linear alkane and the cycloalkane species. Both of these results are supported by model calculations. Blending of methyl octanoate with n-nonane and methylcyclohexane impacted ignition delay time results more strongly at 1.5 atm, yet had only a small effect near 10 atm for temperatures above 1400 K. The study provides the first shock-tube data for a ternary blend of a linear alkane, a cycloalkane, and a methyl ester. Ignition delay time measurements for the C₉:0 methyl ester were also measured for the first time.     

Phase stability and oxygen non-stoichiometry of SrCo0.8Fe0.2O3−δ measured by in situ neutron diffraction

The phase stability, oxygen stoichiometry and expansion properties of SrCo_0.8Fe_0.2O_3−δ (SCF) were determined by in situ neutron diffraction between 873 and 1173 K and oxygen partial pressures of 5 × 10^− 4 to 1 atm. At a pO₂ of 1 atm, SCF adopts a cubic perovskite structure, space group Pm3¯m, across the whole temperature range investigated. At a pO₂ of 10^− 1 atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr₂Co_1.6Fe_0.4O₅, space group Icmm. A pure brownmillerite phase is present at pO₂ of 10^− 2 and 5 × 10^− 4 atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a two-phase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO₆ (B = Co, Fe) octahedra is present in the brownmillerite structure with apical bond lengths of 2.2974(4) Å and equatorial bond lengths of 1.9737(3) Å at 1021 K and a pO₂ of 10^− 2 atm. SCF is highly oxygen deficient with a maximum oxygen stoichiometry, 3 − δ, measured in this study of 2.58(2) at 873 K and a pO₂ of 1 atm and a minimum of 2.33(2) at 1173 K and a pO₂ of 5 × 10^− 4 atm. Significant differences in lattice volume and expansion behavior between the brownmillerite and cubic perovskite phases suggest potential difficulties in thermal cycling of SrCo_0.8Fe_0.2O_3−δ membranes.     

An experimental and theoretical study of pyrrolidine pyrolysis at low pressure

Pyrrolidine serves as a model substance for a series of saturated nitrogenated heterocycles in plants, including certain amino acids such as proline and hydroxyproline. Thus the pyrolysis of this compound was investigated regarding the increasing interest on biofuels. The pyrolysis of pyrrolidine diluted with 95% argon was studied in a flow reactor at 40 mbar over the temperature range from 950 to 1450 K. Isomer-specific assignment and quantification was performed using molecular-beam mass spectrometry and ionization with tunable synchrotron vacuum ultraviolet radiation. The prominent decomposition pathways were analyzed based on the quantified mole fractions of pyrolysis species. Computations including an ab initio calculation and kinetic modeling for the primary fuel decomposition were used to refine the analysis and reveal the combustion chemistry of this saturated heterocyclic compound. Based on the theoretical calculation, a new pathway for the isomerization reaction pyrrolidine → CH₂CHCH₂CH₂NH₂ via a diradical intermediate was proposed. The rate constant calculations showed that this channel has a large contribution to pyrrolidine pyrolysis.     

Slow pyrolysis of wood particles: Characterization of volatiles by Laser-Induced Fluorescence

For a better understanding of the complex processes involved in slow pyrolysis of thermally thick wood particles it is essential to know the composition of the gaseous mixture leaving the pyrolyzing particle. In this work the volatiles in the pyrolysis product gas are characterized by means of in situ Laser-Induced Fluorescence. The results indicate that there are secondary heterogeneous cracking reactions of the primary and secondary tar species occurring in spherical beech wood particles with 25 mm diameter at conversions higher than approximately 0.6 and at heating rates of 0.3 K/s and 0.2 K/s. For pyrolysis of smaller beech wood particles of 0.5–1 mm size complete conversion was achieved without indications of secondary reactions at a heating rate of 0.3 K/s. In order to correctly model the composition of the gas mixture leaving a pyrolyzing wood particle, it may therefore be necessary to consider intra-particle heterogeneous cracking reactions.     

Surface modification of Ni and Co metal nanowires through MeV high energy ion irradiation

Nickel (Ni) and cobalt (Co) metal nanowires were fabricated by using an electrochemical deposition method based on an anodic alumina oxide (Al₂O₃) nanoporous template. The electrolyte consisted of NiSO₄ · 6H₂O and H₃BO₃ in distilled water for the fabrication of Ni nanowires, and of CoSO₄ · 7H₂O with H₃BO₃ in distilled water for the fabrication of the Co ones. From SEM and TEM images, the diameter and length of both the Ni and Co nanowires were measured to be ∼ 200 nm and 5–10 μm, respectively. We observed the oxidation layers in nanometer scale on the surface of the Ni and Co nanowires through HR–TEM images. The 3 MeV Cl²⁺ ions were irradiated onto the Ni and Co nanowires with a dose of 1 × 10¹⁵ ions/cm². The surface morphologies of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were compared by means of SEM, AFM, and HR–TEM experiments. The atomic concentrations of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were investigated through XPS experiments. From the results of the HR–TEM and XPS experiments, we observed that the oxidation layers on the surface of the Ni and Co nanowires were reduced through 3 MeV Cl²⁺ ion irradiation.     

Ion and electron conduction in SrFe1−xScxO3−δ

The oxygen ion and electron transport in SrFe_1−xSc_xO_3−δ  (x = 0.1–0.3) system at 700–950 °C were studied analyzing the total conductivity dependencies on the oxygen partial pressure, pO₂. The conductivity measurements were performed both under reducing conditions (10^− 19 ≤ pO₂ ≤ 10^− 8 atm) comprising the electron-hole equilibrium point, and in oxidizing atmospheres (10^− 5 ≤ pO₂ ≤ 0.5 atm) which are characterized by extensive variations of the oxygen content studied by coulometric titration technique. The incorporation of 10% Sc³⁺ cations into the iron sublattice suppresses transition of the cubic perovskite phase into vacancy-ordered brownmillerite, thus improving ion conduction at temperatures below 850 °C. When scandium content increases, the ion conductivity becomes considerably lower. The hole mobility is thermally-activated and varies in the range of 0.001 to 0.05 cm² V^− 1 s^− 1, increasing with oxygen concentration and decreasing on Sc doping.     

Efficient H2O2/CH3COOH oxidative desulfurization/denitrification of liquid fuels in sonochemical flow-reactors

The oxidative desulfurization/denitrification of liquid fuels has been widely investigated as an alternative or complement to common catalytic hydrorefining. In this process, all oxidation reactions occur in the heterogeneous phase (the oil and the polar phase containing the oxidant) and therefore the optimization of mass and heat transfer is of crucial importance to enhancing the oxidation rate. This goal can be achieved by performing the reaction in suitable ultrasound (US) reactors. In fact, flow and loop US reactors stand out above classic batch US reactors thanks to their greater efficiency and flexibility as well as lower energy consumption. This paper describes an efficient sonochemical oxidation with H₂O₂/CH₃COOH at flow rates ranging from 60 to 800 ml/min of both a model compound, dibenzotiophene (DBT), and of a mild hydro-treated diesel feedstock. Four different commercially available US loop reactors (single and multi-probe) were tested, two of which were developed in the authors’ laboratory. Full DBT oxidation and efficient diesel feedstock desulfurization/denitrification were observed after the separation of the polar oxidized S/N-containing compounds (S ⩽ 5 ppmw, N ⩽ 1 ppmw). Our studies confirm that high-throughput US applications benefit greatly from flow-reactors.     

Acoustic cavitation in 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide based ionic liquid

In this work, a comparison between the temperatures/pressures within acoustic cavitation bubble in an imidazolium-based room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide ([BMIM][NTf₂]), and in water has been made for a wide range of cavitation parameters including frequency (140–1000 kHz), acoustic intensity (0.5–1 W cm⁻²), liquid temperature (20–50 °C) and external static pressure (0.7–1.5 atm). The used cavitation model takes into account the liquid compressibility as well as the surface tension and the viscosity of the medium. It was found that the bubble temperatures and pressures were always much higher in the ionic liquid compared to those predicted in water. The valuable effect of [BMIM][NTf₂] on the bubble temperature was more pronounced at higher acoustic intensity and liquid temperature and lower frequency and external static pressure. However, confrontation between the predicted and the experimental estimated temperatures in ionic liquids showed an opposite trend as the temperatures measured in some pure ionic liquids are of the same order as those observed in water. The injection of liquid droplets into cavitation bubbles, the pyrolysis of ionic liquids at the bubble-solution interface as well as the lower number of collapsing bubbles in the ionic liquid may be the responsible for the lower measured bubble temperatures in ionic liquids, as compared with water.     

Autoignition of gasoline and its surrogates in a rapid compression machine

The analysis and interpretation of the combustion chemistry is greatly simplified by using simple mixtures of pure components, referred to as surrogates, in lieu of fully-blended transportation fuels, such as gasoline. Recognizing that the ability to model autoignition chemistry is critical to understanding the operation of Homogeneous Charged Compression Ignition engines, this work is an attempt to experimentally and computationally assess the autoignition responses of research grade gasoline and two of its proposed surrogates reported in the literature using a rapid compression machine (RCM), for the low-to-intermediate temperature range and at high pressures. The first surrogate studied is a three-component mixture of iso-octane, n-heptane, and toluene. The second is a four-component mixture that includes an olefin (2-pentene), in addition to the ones noted above. Ignition delay times of stoichiometric mixtures, for gasoline and the two surrogates in air, are measured using an RCM for pressures of 20 and 40 bar, and in the temperature range of 650–900 K. The four-component surrogate is found to emulate the ignition delay times of gasoline more closely when compared to the three-component surrogate. Additionally, the experimental data are compared against the computed results from a recently developed surrogate model for gasoline combustion. A good agreement between the experimental and computed results is observed, while discrepancies are also identified and discussed.     

Water vapour solubility and conductivity study of the proton conductor BaCe(0.9 − x)ZrxY0.1O(3 − δ)

The perovskite BaCe_(0.9 ? x)Zr_xY_0.1O_(3 ? δ) has been prepared by solid state reaction at 1400 °C and conventional sintering at 1700 °C. Water uptake experiments performed between 400 and 600 °C, at a water vapour pressure of 0.02 atm, provide data on the concentration of protons incorporated in the sample. The direct current conductivity has been measured as a function of oxygen partial pressure, at a water vapour partial pressure of 0.015 atm. The total conductivity has been resolved into a p-type and an ionic component using a fitting procedure appropriate to the assumed defect model. An estimation of the protonic component was made by assuming a conductivity isotope effect between 1.4 and 1.8. The total conductivity, obtained using impedance spectroscopy has been measured as a function of temperature in the water and heavy water exchanged states. The activation energy has been found to be 0.56 eV to 0.59 eV in the water exchanged state with values 0.03 to 0.04 eV higher in the heavy water exchanged state. Impedance spectra measured at 200 °C showed a reduction in grain boundary resistivity with increasing cerium content. The stability of the compounds to carbon dioxide has been studied by thermogravimetry.