Concentrations Of Stable Species Research Articles

Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy.

High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) is becoming a common technique for improving the sensitivity of solid-state nuclear magnetic resonance (SSNMR) by the hyperpolarization of nuclear spins. Recently, we have shown that gamma irradiation is capable of creating long-lived free radicals that are amenable to MAS DNP in quartz and a variety of organic solids. Here, we demonstrate that ball milling is able to generate millimolar concentrations of stable radical species in diverse materials such as polystyrene, cellulose, borosilicate glass, and fused quartz. High-field electron paramagnetic resonance (EPR) was used to obtain further insight into the nature of the radicals formed in ball milled quartz and borosilicate glass. We further show that radicals generated in quartz by ball milling can be used for solid-effect DNP. We obtained 29Si DNP enhancements of approximately 114 and 33 at 110 K and room temperature, respectively, from a sample of ball milled quartz.

Oxidation kinetics of methyl decanoate in a motored engine

Low-temperature oxidation of methyl decanoate was experimentally investigated in a motored engine. The engine was operated in a premixed mode at a fixed speed of 700 rpm and an intake temperature of 125 °C. Compression ratio was varied between 5.5 and 9.0 for an equivalence ratio of 0.25. The study analyzes heat release and stable species concentrations in the oxidation products at each studied compression ratio. A gas chromatograph-mass spectrometer was used to identify and quantify the composition of the exhaust gas. More than sixty stable intermediate species were detected. Quantitative concentration measurements were carried out for selected alkenes, aldehydes, ketones, ethers, and alcohols. A Fourier transform infrared spectrometer was used to detect and quantify carbon monoxide and dioxide. The oxidation process shares features in common with alkane oxidation, with differences related to methyl ester intermediates formation specific to methyl decanoate oxidation. Specifically, the unsaturated ester methyl prop-2-enoate was generated in considerable amounts. The heat release and speciation trends show two-stage ignition and negative temperature coefficient features. The experimental data are also compared to numerical results calculated based on an existing reaction mechanism for methyl decanoate.

Atmospheric pressure plasma activation of water droplets

Low temperature plasma treatment of water is being investigated due to its use in pollution abatement, wound treatment and agriculture. Plasma produced reactive oxygen and nitrogen species (RONS) are formed in the gas phase and solvate into the liquid. Activation of the liquid is often limited by transport of these RONS to the liquid surface. Micrometer scale droplets immersed in the plasma have a large surface to volume ratio, which increases the interaction area for a given volume of water, and can increase the rates of transport from the gas to liquid. In this paper, results from 0 and 2D modeling of air-plasma activation of water micro-droplets are discussed. The solvation dynamics are sensitive to the Henry’s law constant (h) of each species, which describes its hydrophobicity (low h) or hydrophilicity (high h). The liquid densities of stable species with high h values (e.g. H2O2, HNOx) are sensitive to droplet diameter. For large droplets, hydrophilic species may deplete the gas-phase inventory of RONS before liquid-phase saturation is reached, limiting the total in-liquid density for species with high h. For smaller droplets, higher average in-droplet densities of these species can be produced. Liquid concentrations of stable species with low h (e.g. O3, N2O, H2) had a weak dependence on droplet size as droplets are quickly saturated and solvation does not deplete the gas phase. An analysis of this behavior is discussed using the well-stirred reactor (0D) approximation. Spatial non-uniformity of the plasma also has an impact on the solvation rates and kinetics. Gas phase depletion of high-h species leads to a decrease in solvation rates. Low-h species that saturate the surface of the droplets during plasma-on periods can quickly de-solvate in the afterglow.

Analysis of kinetic models for rich to ultra-rich premixed CH4/air weak flames using a micro flow reactor with a controlled temperature profile

Aromatic formation in methane combustion is still not well understood. The first step to build kinetic models for aromatic formation is to understand fuel-rich oxidation and formation of aromatic precursors like acetylene and ethylene in varied temperature and equivalence ratio conditions. This understanding is also useful to optimize partial oxidation of methane in fuel-rich conditions. With this view, the current work presents investigation of fuel-rich but non-sooting premixed methane/air flames for equivalence ratios between 1.7 and 6.0 using a micro flow reactor with a prescribed temperature gradient from 300 K up to maximum temperature of 1200 K. Weak flames which represent the ignition behavior of fuel were studied. Concentrations of major stable species, CH4, CO, CO2, C2H6, C2H4 and C2H2 were measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). Computations with 1-D reactive flow model using existing kinetic models, GRI 3.0, SD 2016 (San Diego mech), KAUST (USC II) and ARAMCO 1.3 were performed. Change in weak flame structure with change in equivalence ratio was studied. KAUST (USC II) predicted the weak flame positions very well at both equivalence ratios of 2.0 and 6.0, but species mole fractions were not so well predicted. It was found that models with higher rates of CH3+O2 <=> CH2O+OH (SD 2016) and CH3+HO2 <=> CH3O+OH (GRI 3.0) predicted upstream weak flame positions and thus higher reactivity. However, SD 2016 and GRI 3.0 predicted species mole fractions better, particularly at higher equivalence ratios. All the mechanisms used in the current work underpredicted the mole fractions of C2H2 by factors of four to five at all equivalence ratios addressed here. Reaction path analysis showed significant differences among kinetic models. Particularly, consumption pathways of CH3 vary greatly among kinetic models leading to different reactivity. Flexibility and applicability of existing mechanisms to predict the current fuel-rich to ultra-fuel-rich CH4 combustion was discussed. HO2 radicals, which are generally important at high pressures are found to be significant in current fuel-rich conditions.

In-line Process Control in the Active Screen Plasma Nitrocarburizing Using a Combined Approach Based on Infrared Laser Absorption Spectroscopy and Bias Power Management*

Abstract The active screen plasma nitrocarburizing (ASPNC) technology is an extension of the ASPN method in that carbon-bearing gases such as CH4 are admixed to the N2-H2 process gas. Besides process gas composition and working pressure, a bias power applied to the component surface is an important parameter in the nitrocarburizing process governing the competing uptake between nitrogen and carbon on the metallic surface. A variation of the bias power during the treatment provides the possibility of the dynamic control of both the nitriding and the carburizing potential without change of process gas composition. A two-stage process was implemented to improve the quality of the compound layer. The application of advanced in-situ and non-intrusive spectroscopic method of laser absorption spectroscopy in the mid infrared spectral range provides the valuable information about the plasma chemical reactions in the afterglow plasma, supplying the absolute concentrations of stable and transient molecular species monitored direct in process. The amount of hydrogen cyanide (HCN) measured in-situ by QCLAS diagnostics was found to be a reliable parameter to control the carburizing potential of the nitrocarburizing atmosphere.

Extinction limits and flame structures of ETBE, DIPE and TAME non-premixed flames

In this study, extinction limits measurements for Ethyl tert-butyl ether (ETBE), Diisopropyl ether (DIPE) and tert-Amyl methyl ether (TAME) non-premixed flames were carried out by using a counterflow burner. Studies were carried out by supplying a fuel diluted with nitrogen from one duct and an oxidizer stream made up of oxygen and nitrogen from the other duct. Flame structures were also measured. Concentrations of stable species were measured by removing gas samples from the reaction zone by using a quartz microprobe and by analyzing them in a gas chromatograph. Temperature profiles were measured by using a thermocouple. Numerical calculations were performed under the same conditions with experiments to validate chemical kinetics and clarify the peculiarity of ether flames. Experimental results for ETBE, DIPE and TAME flames showed almost the same strain rate at extinction, a2,E, at same stoichiometric mixture fraction, Zst, and an equilibrium temperature. Comparisons among ether flames having a different carbon number showed that the larger the number of carbon atoms is, the smaller the extinction limit is. a2,E for ether flames decrease once with an increase of Zst at low Zst conditions. After that, they start to increase. Zst expressing the minimum value of a2,E becomes smaller with an increase of carbon atom number. The measured profiles of the temperature and major species showed a similar structure for each fuel. Calculated profiles agreed well with experimental data except for minor species. The reproducibility of chemical kinetic computations and variations in extinction limits with Zst were discussed.

Effect of Co3O4 and Co3O4/CeO2 Infiltration on the Catalytic and Electro-catalytic Activity of LSM15/CGO10 Porous Cells Stacks for Oxidation of Propene

The objective of this work was to study the effect of Co3O4 and Co3O4/CeO2 infiltration on the propene oxidation catalytic activity of a La0.85Sr0.15MnO3/Ce0.9Gd0.1O1.95 electrochemical porous cell stack (11 layers, 5 single cells in series). The effect of the infiltration of Co3O4 and Co3O4/CeO2 on the electrochemical properties of the porous cell stack was also investigated by electrochemical impedance spectroscopy (EIS).Co3O4 and Co3O4/CeO2 exhibited high catalytic activity for propene oxidation. The increase of propene oxidation rate with +4V (0.8V/cell) polarization reached 10% for the Co3O4 infiltrated reactor and 48% of efficiency at 300°C.The Co3O4/CeO2 co-infiltration decreased the reactor polarization resistance, while Co3O4 infiltration had negligible effect on reactor electrochemical performance. The beneficial effect of CeO2 on the electrode activity was attributed to the increased concentration of stable oxygen species on the electrode surface.

An experimental and modeling study of n-octanol combustion

This study presents the first investigation on the combustion chemistry of n-octanol, a long chain alcohol. Ignition delay times were determined experimentally in a high-pressure shock tube, and stable species concentration profiles were obtained in a jet stirred reactor for a range of initial conditions. A detailed kinetic model was developed to describe the oxidation of n-octanol at both low and high temperatures, and the model shows good agreement with the present dataset. The fuel’s combustion characteristics are compared to those of n-alkanes and to short chain alcohols to illustrate the effects of the hydroxyl moiety and the carbon chain length on important combustion properties. Finally, the results are discussed in detail.

Experimental and kinetic modeling study of trans-2-butene oxidation in a jet-stirred reactor and a combustion bomb

Butenes are intermediates ubiquitously formed by decomposition and oxidation of larger hydrocarbons (e.g. alkanes) or alcohols present in conventional or reformulated fuels. This study provides new complementary data for the oxidation of trans-2-butene. The oxidation of trans-2-butene was studied for measuring stable species concentration profiles during the oxidation of the fuel at atmospheric pressure, over a range of equivalence ratios (0.5⩽φ⩽2), and temperatures (900–1450K). A combustion bomb apparatus was used to determine laminar flame velocities of trans-2-butene in air at 1atm, 300K, and for equivalence ratios of 0.8–1.4. The oxidation of trans-2-butene was simulated under these experimental conditions using an extended detail chemical kinetic reaction mechanism (201 species involved in 1788 reactions). This mechanism is based on a previously proposed scheme for the oxidation of hydrocarbons. Good agreement with experimental data presented in this article was obtained which significantly improves kinetic modeling ability. The structure of a trans-2-butene premixed low pressure flat flame recently published was also successfully modeled. Sensitivity and reaction pathways analyses were performed to get insights into the processes involved in the oxidation of trans-2-butene.

Extinction limits and flame structures of 1-butanol and Diethyl Ether non-premixed flames

Extinction limits measurements on non-premixed flames were carried out by using a counterflow burner, for 1-butanol flames and Diethyl Ether (DEE) flames. Studies were carried out by injecting a fuel stream made up of fuel and nitrogen from one duct and an oxidizer stream made up of oxygen and nitrogen from the other duct. The flame structure was also measured. Concentrations of stable species were measured by removing gas samples from the reaction zone using a quartz microprobe and analyzing them in a gas chromatograph. The temperature profile was measured by using a thermocouple. Numerical calculations were also performed by using detailed chemistry at conditions corresponding to those used in the experiments to validate chemical kinetics. Experimental results showed that DEE flames have almost the same value of the extinction limits as 1-butanol flames, at same stoichiometric mixture fraction, Zst, and equilibrium temperature. Extinction limits of 1-butanol flames increased monotonically with increasing Zst, suggesting that flames become stronger and more difficult to extinguish, monotonically. On the other hand, extinction limits of DEE flames decreased once with increase in Zst at low Zst range. After that, they started to increase. Measured profiles of the temperature and major species showed a similar structure for both fuels. Calculated profiles agreed well with experimental data. By contrast, profiles of minor species showed differences between fuels. Furthermore, calculated results for minor species did not match with experimental results. The reproducibility of chemical kinetic computations and variations in extinction limits for Zst was discussed.

Experimental Study of Tetralin Oxidation and Kinetic Modeling of Its Pyrolysis and Oxidation

Tetralin is the simplest polycyclic naphtheno-aromatic hydrocarbon found in liquid fuels (e.g., jet fuels, diesel). It is also produced by the pyrolysis and oxidation of decalin. To get a better understanding of tetralin combustion characteristics, new oxidation experiments were performed in a jet-stirred reactor, and the results are reported here. For the first time, stable species concentration profiles were measured at 1 and 10 atm over a range of equivalence ratios (φ = 0.5–1.5) and temperatures (790–1400 K). The oxidation of tetralin under these conditions was simulated using a semidetailed kinetic reaction scheme (∼10 000 reactions and ∼400 species) deriving from a chemical kinetic model proposed earlier for the oxidation of decalin over a wide range of conditions (jet-stirred reactor, plug-flow reactor, and shock-tube). The proposed kinetic model shows reasonable agreement with the present measurements. It can also be used to represent tetralin pyrolysis based on a variety of results available in t...

Experimental realization and characterization of unstretched planar one-dimensional diffusion flames

A burner configuration that allows the creation of nearly unstretched one-dimensional diffusion flames has recently been introduced. This paper presents the first detailed characterization of flames produced in such a burner to assess how well they approach the idealized one-dimensional configuration, which is used extensively for the development of theoretical models for diffusion flame stability. Particular emphasis is directed at quantifying the remaining inhomogeneities in the burner, such as residual flame stretch or variations in the boundary conditions, and identifying possible means to minimize them. Measurements made include the velocity field, stable species concentration profiles and temperature distribution throughout the burning chamber. Additionally, the flame position in the burner is determined as a function of mixture strength, reactant transport properties and bulk flow velocity. All the measurements are found to be in good agreement with a simplified one-dimensional flame model. However, the boundary conditions and their physical location have to be determined experimentally on both sides of the flame, which complicates comparison with results from theoretical and numerical models. Despite these limitations, we conclude that this new burner type provides a good experimental approximation of the one-dimensional idealized construct.

Effect of ethanol on the chemistry of formation of precursors of polyaromatic hydrocarbons in a fuel-rich ethylene flame at atmospheric pressure

The effect of the addition of ethanol (EtOH) to the initial combustible mixture on the concentration of various compounds, in particular, those preceding the formation of polyaromatic hydrocarbons in a fuel-rich (equivalence ratio of fuel ϕ = 1.7) flat premixed ethylene/oxygen/argon flame at atmospheric pressure was studied experimentally and by numerical modeling using a detailed mechanism of chemical reactions. Concentrations of various stable and labile species, including reactants, major combustion products, and intermediates in C2H4/O2/Ar and C2H4/EtOH/O2/Ar flames were measured along the height above the burner using molecular beam mass spectrometry. Experimental mole fraction profiles were compared with those calculated using the previously proposed mechanisms of chemical reactions. This mechanism was analyzed to determine the cause of the ethanol effect on the flame concentration of propargyl, the main precursor of polyaromatic hydrocarbons.

Jet-stirred reactor and flame studies of propanal oxidation

There is a strong drive towards utilizing oxygenated biofuels in blends with existing fossil fuels. Improving the kinetic modeling of the oxidation of these bio-derived oxygenates requires further investigation of their key stable intermediates such as the aldehydes. In this study, an experimental and chemical kinetic modeling investigation of propanal oxidation was carried out. Experiments were conducted in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and ambient pressures. Stable species concentration profiles were measured in the jet stirred reactor and laminar flame speeds were measured. A detailed chemical kinetic reaction model was validated using the present experimental results and existing literature data. The model was used also to provide insight into the controlling reaction pathways for propanal oxidation in both the low- and high-temperature kinetic regimes.

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 0.1s and 0.5s at respectively 1 and 10atm, over a range of equivalence ratios (ϕ=0.5–1.5) and temperatures (750–1350K). 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.

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).

Experimental and Detailed Kinetic Modeling Study of 1-Hexanol Oxidation in a Pressurized Jet-Stirred Reactor and a Combustion Bomb

1-Hexanol is among the promising renewable long-chain alcohols usable as an alternative to petrol-derived gasoline and diesel fuels. To better understand the combustion characteristics of 1-hexanol, new experimental data for the oxidation and combustion of 1-hexanol were obtained. Stable species concentration profiles were measured in a jet stirred reactor (JSR) at 10 atm over a range of equivalence ratios and temperatures. Burning velocities of 1-hexanol/air mixtures were measured at 1−10 bar and 423 K, over a range of equivalence ratios. The effect of total pressure on flame speed was determined. The oxidation of 1-hexanol in these experimental conditions was modeled using an extended detailed chemical kinetic reaction mechanism (2977 reactions involving 600 species). The proposed mechanism shows good agreement with the present experimental data. Reaction path analyses and sensitivity analyses were performed to interpret the results.

Experimental and detailed kinetic modeling study of 1-pentanol oxidation in a JSR and combustion in a bomb

1-Pentanol is among the possible alcohols usable as an alternative to conventional gasoline and diesel fuels. In order to better understand the combustion characteristics of 1-pentanol, this study presents new experimental data for 1-pentanol in two complementary experimental configurations. Stable species concentration profiles were measured in a jet-stirred reactor (JSR) at 10atm over a range of equivalence ratios and temperatures. Laminar flame speeds of 1-pentanol-air premixed laminar flame were measured at 1atm and 423K for equivalence ratios of 0.7–1.4. The oxidation of 1-pentanol in these experimental configurations was modeled using an extended detailed chemical kinetic reaction mechanism (2099 reactions involving 261 species) derived from a previously proposed scheme for the oxidation of methanol-, ethanol-, and 1-butanol-based fuels. The proposed mechanism shows good agreement with the present experimental data. Reaction path analyses were conducted to interpret the JSR results.

Formation and consumption of NO in H 2 + O 2 + N 2 flames doped with NO or NH 3 at atmospheric pressure

Flat premixed burner-stabilized H 2 + O 2 + N 2 flames, neat or doped with 300–1000 ppm of NO or NH 3, were studied experimentally using molecular-beam mass-spectrometry and simulated numerically. Spatial profiles of temperature and concentrations of stable species, H 2, O 2, H 2O, NO, NH 3, and of H and OH radicals obtained at atmospheric pressure in lean ( ϕ = 0.47), near-stoichiometric ( ϕ = 1.1) and rich ( ϕ = 2.0) flames are reported. Good agreement between measured and calculated structure of lean and near-stoichiometric flames was found. Significant discrepancy between simulated and measured profiles of NO concentration was observed in the rich flames. Sensitivity and reaction path analyses revealed reactions responsible for the discrepancy. Modification to the model was proposed to improve an overall agreement with the experiment.

Soot surface growth and oxidation at pressures up to 8.0 atm in laminar nonpremixed and partially premixed flames

The flame structure and soot particle surface reaction properties, including growth and oxidation, of laminar jet nonpremixed flames were studied experimentally at pressures of 1.0–8.0 atm. Ethylene–helium mixtures were used in an oxygen/helium coflow at normal temperature (300 K) in order to minimize the effects of buoyancy. The following properties along the axis of flames were measured as a function of distance from the burner exit: soot concentrations by laser extinction, soot temperatures by multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radical species (H, OH, O) by Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local flame properties in order to find soot surface growth and oxidation rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures involving laminar premixed and diffusion flames fueled with a variety of hydrocarbons. The growth rates from all the available flames were in good agreement with each other and with existing hydrogen-abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature. Measurements of early soot surface oxidation rates at pressures of 1.0–8.0 atm (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures. The oxidation rates of up to 8 atm in flame environment could be explained by reaction with OH, having a collision efficiency of 0.12.