Mole Fractions Of Species Research Articles

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.

Effect of devolatilization model on flame structure of pulverized coal combustion in a jet-burner system

A numerical simulation was performed with the two competing model in the devolatilization process for a pulverized coal combustion jet flame by means of LES. The target was a simple jet burner flame in CRIEPI. To solve the LES equations, a CFD code FFR. Comb was employed with the dynamic Smagorinsky SGS turbulent model. A simple global kinetic mechanism was implemented to predict combustion of the gas and solid phase combustion. The interactions between the two phases were calculated by PSI-Cell model while the reaction rate in turbulent flow was established by SSFRRM. The simulation was validated by comparing results to the experimental data in terms of particle dispersion and velocity as well as gaseous velocity. The flame structure was discussed with temperature, mole fraction of major species. In addition, the effect of the devolatilization model was investigated simultaneously by comparing to another simulation that employed the single first order reaction model because the devolatilization was one of the major processes in coal combustion and it had an influence on the flame structure from all reactive regions. The release rate was calculated by two different parameter sets in the Arrhenius rate equation for the two competing model that were corresponding different temperature regions whereas the released rate was determined by only one fixed parameter set in the single first order reaction model. From the simulation, it was revealed that the main reactions took place at the upstream and the first fuel oxidation was stronger at the inner reaction zone comparing to the far side combustion. It was confirmed as well that the two competing model could capture the quick devolatilization faster than the single first order reaction model though the dominant part appeared later than the single first order reaction model.

Effect of Ion Valency on the Properties of the Carbon Dioxide-Methane-Brine System.

Molecular dynamics simulations and theoretical analysis were carried out to study the bulk and interfacial properties of carbon dioxide-methane-water and carbon dioxide-methane-brine systems under geological conditions. The density gradient theory with the bulk phase properties estimated using the cubic-plus-association (CPA) equation of state (EoS) can well describe the increase in the interfacial tension (IFT) of the CO2-water system in the presence of methane. The theoretical estimates of species mole fractions in the carbon dioxide-methane-water system are in good quantitative agreement with the experimental results. Furthermore, simulations of carbon dioxide-methane-brine system show that the IFT of the CaCl2 case is generally higher than that of the NaCl case. This is probably due to the stronger hydration of Ca2+ ions and their stronger repulsion from the interface compared to Na+. While the overall shape of the ionic profiles is not much affected by the ion type, the water profiles show a local enrichment at the interface in the system with CaCl2. In contrast to the case of NaCl, the slopes of the plots of IFT vs CaCl2 concentration are dependent on temperature. Species mole fractions in the carbon dioxide-methane-brine system predicted by combining the CPA EoS with the Debye-Hückel electrostatic term are in good agreement with simulation results.

Multi-wavelength speciation of high-temperature 1-butene pyrolysis

Species time-history measurements provide important kinetics targets for the development and validation of detailed reaction models. Here, a multi-wavelength, multi-species laser absorption strategy is demonstrated that provides high-bandwidth species time-histories during 1-butene pyrolysis behind reflected shock waves. Measured shock tube absorbance traces at nine wavelengths were used to resolve nine species mole fractions in the pyrolysis of 5% 1-butene in argon at 1300 K, 1.8 atm. In addition to existing sensors for 1-butene, methane, ethylene, 1,3-butadiene, propene, allene, benzene, and cyclopentadiene, a new laser absorption sensor for ethane at 3.3519 µm was developed by measuring cross-sections of ethane and of expected major interferers. Additional measurements to complete a square absorption cross-section matrix were also performed that enabled the simultaneous solution of species mole fractions from absorbance time-histories under appropriate physical system constraints. Measured time-histories of the nine product species were also compared with existing flow-reactor-based kinetic mechanisms. The demonstrated method has further application potential in the speciation of larger, more complex fuels.

Modeling of plasma-enhanced chemical vapor deposition growth of graphene on cobalt substrates

Two complementary modeling approaches (1D and 2D) are developed to explain the temperature-dependent growth kinetics of PECVD graphene formation on cobalt substrate. The gas temperature and the species concentrations are predicted for different process conditions by involving gas-phase and surface reaction mechanisms which consist of 15 gas species, 43 gas reactions, 10 surface species and 34 surface reactions. The influence of the growth temperature, the microwave power and the methane flow rate, affecting the gas species mole fractions in the reactor and the surface coverage is evaluated. The numerical results clearly indicate that the hydrogen atoms play an important role in the graphene growth in microwave plasma systems. A global sensitivity analysis is then performed in order to understand the basic mechanisms that lead to the transport of reactants by gas diffusion from the main gas stream through the boundary layer providing insight regarding the growth of graphene on the cobalt substrate.

Experimental assessment of the Tenti S6 model for combustion-relevant gases and filtered Rayleigh scattering applications

Accurate descriptions of Rayleigh–Brillouin scattering (RBS) spectra for gas-phase species are important in light scattering measurement techniques including filtered Rayleigh scattering (FRS). The current manuscript targets evaluation of the well-known Tenti S6 model for calculating the RBS spectra for combustion-relevant species over a broad range of temperatures with relevance towards FRS applications in reacting flows. In this work, testing of the Tenti S6 model was performed by comparing measured FRS signals to synthetic FRS signals generated through the combination of the Tenti S6 model and an experimentally verified I2 absorption model. First, temperature-dependent FRS signals were measured for a number of individual gases including Ar, N2, O2, CH4, H2, CO, and CO2 from 300 to 1400 K. Comparisons between the measurements and synthetic FRS signals show excellent agreement (< 4% average difference) over the full temperature range. For pure CO2, rotational Raman scattering effects must be taken into account when comparing measured and synthetic FRS signals. FRS measurements in binary mixtures were performed to assess the commonly used (but not verified) assumption that the total FRS signal from a mixture can be treated as the mole fraction-weighted average of the FRS signals from each component. Measured FRS signals in mixtures with large variations in both molecular weight and Rayleigh scattering cross section show a linear relationship with constituent mole fraction, indicating that this assumption is valid within the kinetic regime. Finally, FRS measurements were performed in near-adiabatic H2/air and CH4/air flames. Comparisons between measured and synthetic FRS signals show excellent agreement over a broad range of equivalence ratios ( $$\phi$$ ), which includes a temperature range of 1100 < T(K) < 2400 and large relative changes in species mole fractions. Overall, the results indicate that the predicted RBS lineshapes calculated using the Tenti S6 model are sufficiently accurate in the context of FRS measurements for the species and temperatures evaluated.

Intercomparison of atmospheric trace gas dispersion models: Barnett Shale case study.

Greenhouse gas emissions mitigation requires understanding the dominant processes controlling fluxes of these trace gases at increasingly finer spatial and temporal scales. Trace gas fluxes can be estimated using a variety of approaches that translate observed atmospheric species mole fractions into fluxes or emission rates, often identifying the spatial and temporal characteristics of the emission sources as well. Meteorological models are commonly combined with tracer dispersion models to estimate fluxes using an inverse approach that optimizes emissions to best fit the trace gas mole fraction observations. One way to evaluate the accuracy of atmospheric flux estimation methods is to compare results from independent methods, including approaches in which different meteorological and tracer dispersion models are used. In this work, we use a rich data set of atmospheric methane observations collected during an intensive airborne campaign to compare different methane emissions estimates from the Barnett Shale oil and natural gas production basin in Texas, USA. We estimate emissions based on a variety of different meteorological and dispersion models. Previous estimates of methane emissions from this region relied on a simple model (a mass balance analysis) as well as on ground-based measurements and statistical data analysis (an inventory). We find that in addition to meteorological model choice, the choice of tracer dispersion model also has a significant impact on the predicted down-wind methane concentrations given the same emissions field. The dispersion models tested often underpredicted the observed methane enhancements with significant variability (up to a factor of 3) between different models and between different days. We examine possible causes for this result and find that the models differ in their simulation of vertical dispersion, indicating that additional work is needed to evaluate and improve vertical mixing in the tracer dispersion models commonly used in regional trace gas flux inversions.

고압에서 Methyl decanoate/Air 예혼합 화염의 연구

Numerical simulations of freely propagating flames burning methyl decanoate(MD)/air mixture are performed under the condition of initial temperature 650 K and pressure 1~10 atm in order to understand the pressure effect. The revised chemical kinetic mechanism is used, which involves 118 species and 1,674 forward reactions. The calculations about the flame speeds at high pressure and the mole fraction of species at atmospheric pressure are compared with those obtained from the experiments performed by other researcher, the results of which are in good agreement. The flame structure of a premixed methyl decanoate/air flame has a tendency similar to that of hydrocarbon fuel. Decomposition of methyl decanoate occurs mainly through reactions with hydrogen atoms. At high temperature the unimolecular reaction, MD→C₃HSUB5/SUBO₂ + CSUB8/SUBHSUB17/SUB, plays an important role to decompose methyl decanoate. Increasing the pressure leads to an increase in the net production rate of carbon dioxide.

Coherent Anti-Stokes Raman Spectroscopy of a Hydrogen Diffusion Flame in a Ramjet

Dual-pump coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the mole fractions of major species as well as the rotational and vibrational temperatures of molecular nitrogen in a hydrogen-fueled dual-mode scramjet flowpath operated in the “ram” mode. Developments in CARS methods and uncertainties are described, including a detailed analysis of the effects of spatial averaging. The mean and standard deviation of the turbulent fluctuations of the temperature and mole fractions at multiple planes in the flowpath and scatter plots of vibrational and rotational temperature are presented. The flame is stabilized downstream of the ramp and grows under the influence of turbulence and rollup of the counterrotating vortex pair formed at the ramp; combustion continues in accelerating flow approaching the end of the measurement domain. Thermal nonequilibrium is observed in the mixing of air with the molecular hydrogen jet close to the injector. Spatial averaging errors are significant at some locations; new methods to correct these errors are described.

Large eddy simulation/probability density function simulations of the Cambridge turbulent stratified flame series

The LES/PDF methodology is applied to the Cambridge/Sandia turbulent stratified flame series. The methane chemistry is represented by the 16-species reduced ARM1 mechanism, and the in situ adaptive tabulation method is adopted to accelerate the chemistry calculations. Differential diffusion effects are taken into account. The simulations are performed for premixed (SwB1), and moderately and highly stratified (SwB5 and SwB9, respectively) cases under non-swirling conditions. The results from LES/PDF simulations are compared with the experimental measurements and with previous calculations. The calculated length of the recirculation zone, the mean and r.m.s. profiles of velocity, temperature, equivalence ratio and mass fractions of species are in very good agreement with the measurements. In the stratified cases, the CO profiles are underestimated within the recirculation zone, close to the bluff body. Scatter plots of species mole fractions and temperature are presented and compared with the experimental data. Conditional means of species mass fractions demonstrate overall good consistency with the measurements. A parametric study is then performed to examine the effect of differential diffusion and the effect of the parameter controlling the scalar mixing rate. It is found that differential diffusion has a negligible effect on the mean and r.m.s. results, whereas, the mixing rate parameter has a considerable effect on the flow structure. Finally, the effect of stratification is investigated and characterized by scatter plots of OH mass fraction and heat release rate (HRR) in the equivalence ratio space.

Three-dimensional multiphysics model of a planar solid oxide fuel cell using computational fluid dynamics approach

A multiphysics model of a planar solid oxide fuel cell has been developed based on computational fluid dynamics approach and validated by cell level experiment in the present article. The three-dimensional model of multiphysics nature includes the full-field computational fluid dynamics solutions coupled with the electrochemical model for a planar type of solid oxide fuel cell developed in the Technical University of Denmark. The software COMSOL Multiphysics was used to solve the equations in three-dimensions. With the employment of appropriate boundary conditions at respective parts and through solving the fluid dynamics, heat transfer and electrochemical equations, pressure, velocity, temperature and current density fields were established for a given cell voltage. It is shown that the spatial variation of mole fractions of species are determined by the rate of electrochemical reactions, while that of hydrogen reaching maximum at locations beneath the interconnect ribs and that of oxygen reducing to the fractional level of 2.3×10−4 within the active cathode layer due to the mass flow resistance. The variation of temperature increases as the flow proceeds along the main flow direction due to the electrochemical reactions as well as the ohmic and activation overpotentials. It was shown that the exchange current density field for the anode is determined by the temperature distribution caused by the highly exothermic process of formation of water, and also by the partial pressures of hydrogen and water. It was further established that the variation of overpotential at the anode/electrolyte interface are justified by the mechanisms of irreversible ohmic and activation losses taking place within the cell, a high electronic conductivity in certain locations and the relatively higher ohmic overpotential in respective regions.

An experimental and modeling study of the oxidation of 3-pentanol at high pressure

High pressure oxidation of 3-pentanol is investigated in a jet-stirred reactor and in a shock tube. Experiments in the reactor were carried out at 10 atm, between 730 and 1180 K, for equivalence ratios of 0.35, 0.5, 1, 2, 4 and 1000 ppm fuel, at a constant residence time of 0.7 s. Reactant, product and intermediate species mole fractions were recorded using Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC). Ignition delay times were measured for 3-pentanol/O2 mixtures in argon in a shock tube at 20 and 40 bar, in a temperature range of 1000–1470 K and for equivalence ratios of 0.5, 1 and 2. The fuel did not show any low-temperature reactivity under these conditions in neither experimental set-up and produced various aldehydes and ketones as well as the olefin 2-pentene as intermediates. A kinetic sub-mechanism is developed in order to represent the present data and analyze the reaction pathways.

Experimental and theoretical study on acetone pyrolysis in a jet-stirred reactor

Pyrolysis of acetone was investigated in a jet-stirred reactor coupled with on-line GC and GC–MS in the temperature range 700–1136 K at atmospheric pressure. Six species, namely 1-C4H8, iC4H8, nC4H10, iC4H10, C5H6 and C6H6, were newly identified and quantified. A kinetic mechanism involving 296 species and 1836 reactions, including subsets of acetone and 2-butanone, was developed with reasonable predictions against the experimental data. C–C bond cleavage releasing CH3CO and CH3 radicals is the chain-initiation and the most intensively promoting reaction for the consumption of acetone. Its high-pressure limit rate constant was recalculated as k1 = 1.12 × 106·T3.84·exp(−78.6 kcal mol−1/RT) at the CBS-QB3 level. Besides this step, H- and CH3-abstractions of acetone by CH3 radical, with the calculated high-pressure limit rate constant as k4 = 1.38·T3.49·exp(−8.58 kcal mol−1/RT) and k7 = 5.70·T3.96·exp(−41.7 kcal mol−1/RT), respectively, govern the major consumption routes of acetone. CH3 radicals contribute as the key species to the production of the major hydrocarbons such as CH4, C2H4 and C2H6, and minor branches to benzene and C2H5COCH3. Moreover, the present kinetic mechanism could predict fairly the mole fractions of the gas species in the high-temperature pyrolysis of acetone and the ignition delay times of acetone at different temperature and pressure region, which indicates the applicability of this model in a wide-range condition.

Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH3 bond in anisole (C6H5OCH3) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C6H5O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C5H6) and cyclopentadienyl radicals (C5H5). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10−3–10−2) are CH3, CH4, C2H2, C2H4, C2H6, CH2O, C5H5 (cyclopentadienyl radical), C5H6 (cyclopentadiene), C6H6 (benzene), C6H5OH (phenol), and C6H5CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C6H5CHO), (2) formation of phenol (C6H5OH), and (3) unimolecular decomposition of anisole to phenoxy (C6H5O) and CH3 radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C5H5 and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.

Simple and Complex Polymer Electrolyte Fuel Cell Stack Models: A Comparison

The High Temperature Polymer Electrolyte Fuel Cell (HT-PEFC) converts chemical energy to electricity and heat. Operating at around 160°C, the heart of a HT-PEFC is a phosphoric-acid-doped polybenzimidazole membrane, which exhibits good protonic conductivity. HT-PEFCs may readily operate with either hydrogen or reformate as fuel. As is the case with most fuel cells, HT-PEFCs are operated in stacks in order to increase the overall electric potential. However, there are few comprehensive models of HT-PEFC stack performance. The results of this research program are among the first to obtain performance calculations for HT-PEFC at the stack scale with experimental validation. HT-PEFC stacks designed at the Forschungszentrum Jülich are actively cooled with polyalkyline glycol liquid (oil) coolant, flowing in internal passages within the solid bi-polar plates, upon the surfaces of which a complex pattern consisting of straight and serpentine passages have been machined, in order to supply the air and fuel to the cell. Because of the elevated temperature, liquid water is not generally found in the gas channels or porous layers. Considerable experimental data have been previously gathered, in-house, for a 5-cell stack based on this design. Two high temperature polymer electrolyte fuel cell (HT-PEFC) models of a HT-PEFC stack are described in detail: (i) A detailed cell-level model where a set of conformal meshes are body-fitted to all the different parts of the stack. The code typically runs, in parallel, on 1000-2000 cores at the Jülich Supercomputer Centre; and, (ii) A coarse-grid stack model based on a ‘distributed resistance analogy’ whereby rate equations supplant local diffusion terms in selected locations and directions to reduce computational requirements. A multiply-shared space (MUSES) method is employed to obtain simultaneous solutions for concentration, heat and momentum for the different phases occupying the ‘same’ global space. Both (i) detailed and (ii) MUSES methods are implemented by instantiating 5 distinct meshes corresponding to (i) air, (ii) fuel, (iii) oil (fluid), and (iv) membrane electrode assembly, (v) bipolar plate (solid) regions, and obtaining solutions of the governing equations on these meshes. For both methods a solution is obtained simultaneously for the stack manifolds, cell manifolds (entrance regions) as well as the ‘core’ of the fuel cell stack. Thermal equilibrium between the 3 fluid phases is not presumed, a priori. In both methods, the electric field potential is expressed as the ideal (Nernst) potential less activation, ohmic, and transport losses. Individual cell potentiostatic boundary conditions are iteratively corrected until the desired overall galvanostatic condition is obtained for all cells in the stack. Both codes are developed in the modern open-source object-oriented library, OpenFOAM. The results of the two models are compared with each other in terms of local current density and species partial pressure distributions. Both models are then compared with experimental data. The results show that the coarse-grid stack model (ii) agrees well, both quantitatively and qualitatively, with the detailed model (i) and experimental results and calculations are performed in two orders of magnitude less computation time. However, neither the experimental results nor the stack model (ii) are able to resolve local extrema in the current density and species mole fraction values that are observed with the detailed model (i). These are due to complex flow regimes associated with the meandering channels and flow bypassing in the porous transport layers. Importantly, these lead to significant local extrema in the local current density and other parameters of significance. These are among the first fully-comprehensive physicochemicohydrodynamic models of a HT-PEFC stack which allows calculations to be performed simultaneously for all the fluid and solids in the manifolds, entrance regions and stack core, together with experimental validation.

Knowledge generation through data research: New validation targets for the refinement of kinetic mechanisms

Abstract Chemical structures of low-pressure premixed flames are routinely used as species-resolved validation targets for the development of chemical kinetic combustion mechanisms. Species identities, for example based on flame-sampling mass spectrometry, and species-specific mole fractions as function of distance to the burner are especially important targets. This work, which is based on a compilation of the chemical structures of 55 low-pressure premixed flames from various laboratories, unravels previously unexplored validation targets. It is shown here that such a compiled data set allows for the extraction of fuel-specific chemistry and can be used to identify inconsistent data, thus potentially improving the accuracy of the validation set. Most importantly, this large data set of flame structures was essential to discover that, despite the different fuels and flame conditions used in the individual flame studies, the maximum mole fractions of some intermediate species are correlated to each other in a systematic way across a large number of flames. Some examples, including the correlations between maximum mole fractions of benzene (C6H6) vs. toluene (C6H5CH3) and the correlation between the isomeric allene (CH2CCH2) and propyne (CH3CCH), are discussed and it is shown that these obtained correlations can be used as targets for model refinement.

Composition effect on thermophobicity of ternary mixtures: An enhanced molecular dynamics method.

Thermodiffusion or the Ludwig-Soret effect is known as the cross effect between the temperature gradient and induced separation of mixture species in multicomponent mixtures. The performance of the boundary driven non-equilibrium molecular dynamics enhanced heat exchange (eHEX) algorithm was validated by evaluating the sign and magnitude of the thermodiffusion process in methane/n-butane/n-dodecane (nC1-nC4-nC12) ternary mixtures. The eHEX algorithm consists of an extended version of the HEX algorithm with an improved energy conservation property. In addition to this, the transferable potentials for phase equilibria-united atom augmented force field was employed in all molecular dynamics (MD) simulations to accurately represent molecular interactions in the fluid. Our newly employed MD algorithm was capable to appropriately reflect the thermophobicity concept and the coupled effect of relative density and mole fraction of the mixture species on the thermodiffusion process. The separation ratio of the ternary mixture for five different compositions (at 333.15 K and 35 MPa) showed good agreement with experimental data and better accuracy in predicting the sign change of the intermediate component (nC4) as its concentration in the mixture increases, when compared to other MD models.

Hetero-/homogeneous combustion of fuel-lean CH4/O2/N2 mixtures over PdO at elevated pressures

The heterogeneous and homogeneous combustion of fuel-lean CH4/O2/N2 mixtures over PdO was investigated experimentally and numerically at equivalence ratios φ = 0.27–0.44, pressures 1–12 bar and surface temperatures 710–1075 K. In situ Raman measurements of major gas-phase species concentrations across the boundary layer of a channel-flow catalytic reactor assessed the heterogeneous reactivity, while planar laser induced fluorescence (LIF) of the OH radical monitored homogeneous combustion. Simulations were performed using a 2-D code with detailed heterogeneous and homogeneous reaction mechanisms. Comparisons between Raman-measured and predicted transverse profiles of major species mole fractions attested the atmospheric-pressure suitability of a detailed surface mechanism and allowed for the construction of a global catalytic step valid in the range 1–12 bar. The methane catalytic reaction rate exhibited an overall pressure dependence ∼p1−n where the exponent n was itself a monotonically increasing function of pressure, rising from 0.58 at 3 bar to 1.02 at 12 bar. This resulted in a non-monotonic pressure dependence of the catalytic reaction rate in the range 1–12 bar, a behavior in stark contrast to other noble metals (Pt and Rh) where the methane reaction rates always increased with rising pressure. Surface temperatures remained well-below the PdO decomposition temperature at each corresponding pressure, owning largely to the “self-regulating” temperature effect of PdO, and this in turn mitigated homogeneous ignition. Simulations using the PdO decomposition temperatures as boundary conditions for the wall temperatures were further performed for practical CH4/air catalytic reactors in power generation systems. It was shown that for p < 7 bar (a range relevant to microreactors) homogeneous ignition was altogether suppressed. For higher pressures relevant to gas-turbine burners, however, gaseous combustion ought to be considered in the reactor design.

Experimental and kinetic studies of the effect of CO2 dilution on laminar premixed n-heptane/air flames

In the present study, experimental and kinetic studies of the effect of CO2 dilution on laminar premixed n-heptane/air flames were performed via the outwardly propagating spherical flames and the simulation of one-dimensional planar flames. The laminar flame speeds and Markstein lengths of n-heptane/air/CO2 mixtures at different CO2 dilution rates were measured and the chemical effect of CO2 was separated from its total effects by introducing a type of fictitious carbon dioxide (FCO2) in the simulation. Results show that the laminar flame speeds of n-heptane/air/CO2 mixtures nonlinearly decrease with the increase of CO2 dilution rate. Moreover, as CO2 dilution rate increases, the instability of laminar n-heptane/air/CO2 flames decreases due to the decrease of the hydro-dynamic instability. The results of kinetic analysis show the increase of CO2 concentration in the n-heptane/air flames decreases the peaks of mole fractions of the main radical species (OH, O, H, CH3 and HO2), net reaction rates of the main elementary reactions and flame temperature. Both the physical and chemical effects of CO2 suppress the laminar flame speeds and flame temperatures and inhibit the productions of the main radical species and net reaction rates of the main elementary reactions, and the former dominates. The chemical effect of CO2 remarkably promotes the production of CO via shifting the equilibrium of R24 CO + OH = CO2 + H, and the third-body effect of CO2 slightly increases the production of H2O shifting the equilibrium of R8 H2O + M = H + OH + M. The backward reactions of R24 and R8 enhanced by CO2 addition compete with the important chain branching reaction R1 H + O2 = O + OH for H atoms, which plays the chemical role in reducing the laminar flame speed and flame temperature.

Photoelectron Photoion Coincidence Spectroscopy to Unveil Reaction Mechanisms by Isomer-selective Detection of Elusive Molecules: From Combustion to Catalysis.

Elusive and reactive intermediates, such as radicals, play a central role in reaction mechanisms. Photoelectron photoion coincidence spectroscopy with tunable vacuum ultraviolet synchrotron radiation offers a multiplexed, sensitive, mass- and isomer-selective way to identify and, in some cases, determine mole fractions of reactive species. It thus helps to unveil the missing link(s) between reactants and products. After a brief overview of the technique, we review two systems in three different reactive environments. First, the unimolecular decomposition mechanism of ortho-xylyl radicals is revealed in pyrolysis experiments. Second, the insights gained are used to analyze a fuel-rich meta-xylene flame, which suggests that important xylyl isomerization reactions are currently missing in combustion models. Third, photoion mass-selected threshold photoelectron spectra identify the fulvenone ketene as the crucial intermediate in the catalytic fast pyrolysis of a lignin model compound and help map heterogeneous catalysis mechanisms.