Kinetic Reaction Mechanism Research Articles

Modeling Tar Recirculation in Biomass Fluidized Bed Gasification

A biomass gasification model is proposed and applied to investigate the benefits of tar recirculation within a gasification plant. In the model, tar is represented by the four species phenol, toluene, naphthalene, and benzene. The model is spatially one-dimensional, assuming plug flow for the gaseous compounds and perfect mixing of the fuel particles in the bed; it includes fuel particle heating and drying, bed fluidization, pyrolysis, a kinetic reaction mechanism, entrainment of char particles from the fluidized bed into the freeboard, the motion of char particles in the freeboard and in pipes, and the particle and gas flow within a cyclone. An uncertainty analysis identifies the most critical parameters of the model that severely affect the results of simulation. The model is validated against experimental data obtained at a pilot gasifier plant. A detailed parameter study suggests that an efficiency rise can be expected, the tar and soot content in the produced gas will be lowered, the heating value of...

Analysis of Chemical Reaction Kinetics Behavior of Nitrogen Oxide During Air-staged Combustion in Pulverized Boiler

Because the air-staged combustion technology is one of the key technologies with low investment running costs and high emission reduction efficiency for the pulverized boiler, it is important to reveal the chemical reaction kinetics mechanism for developing various technologies of nitrogen oxide reduction emissions. At the present work, a three-dimensional mesh model of the large-scale four corner tangentially fired boiler furnace is established with the GAMBIT pre-processing of the FLUENT software. The partial turbulent premixed and diffusion flame was simulated for the air-staged combustion processing. Parameters distributions for the air-staged and no the air-staged were obtained, including in-furnace flow field, temperature field and nitrogen oxide concentration field. The results show that the air-staged has more regular velocity field, higher velocity of flue gas, higher turbulence intensity and more uniform temperature of flue gas. In addition, a lower negative pressure zone and lower O2 concentration zone is formed in the main combustion zone, which is conducive to the NO of fuel type reduced to N2, enhanced the effect of NOx reduction.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane.

Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

Study of the Ignition Characteristics of Light Fractions of Crude Oil and Their Surrogate Fuels under a Range of Low-to-Medium Temperatures

To improve the self-ignition capability of oil reservoirs and build a detailed reaction kinetics mechanism of in situ combustion (ISC), ignition and reaction characteristics of light fractions of crude oil (boiling point of <100 °C, 100–125 °C, 125–150 °C, 150–175 °C, 175–200 °C, and 200–225 °C) were investigated on a rapid compression machine at compressed pressures of 20 bar and a compressed temperature range of 660–900 K, with a stoichiometric ratio of 1.0. According to our analysis with a gas chromatography–mass spectrometry instrument (GC-MS), cyclohexane, methyl-cyclohexane, p-xylene, and mesitylene were selected as surrogate fuels and were tested under the same conditions on a rapid compression machine (RCM). The experimental results show that three groups of light fractions, <100 °C, 100–125 °C, and 175–200 °C, display strong reactions at low and high temperatures and negative temperature coefficient (NTC) behavior. These phenomena imply that these reactive components can be used as possible promoters to accelerate the development of an ISC project and to improve the reaction speed of the entire process. Two groups of 125–150 °C and 200–225 °C fractions are difficult to ignite in the low-to-medium temperature range under the compressed pressure of 20 bar, but they can be ignited at relatively higher top dead center (TDC) temperature ranges under the compressed pressure of 30 bar. Moreover, the logarithm of ignition delays and the reciprocal temperature clearly show a linear relationship, meaning that the reactivity of these two groups is poor. The results obtained with these surrogate fuels match well with the experimental results of each light fraction.

TÉCNICAS MATEMÁTICAS NA REDUÇÃO DE MECANISMOS CINÉTICOS COM ILUSTRAÇÃO PARA CHAMAS DO METANO

The computational treatment of detailed kinetic reaction mechanisms of combustion is very expensive, especially when the objective is the implementation of CFD (Computational Fluid Dynamics) models; therefore, major effort in searching for methods of mechanisms reduction has been observed. In the pursuit of efficient techniques, various ideas and theories have emerged in recent years. Sensitivity analysis, lumping and DRG (Directed Relation Graph) are some of these techniques, which seem to be powerful tools that can assist in this challenge. This paper shows these strategies, objectively describing their applications on reduction of reaction mechanisms. A reduced mechanism for methane flames consisting of six-step among nine reactive species is obtained from the skeletal mechanism, which consists of 25 reactions.

Probing the Buried Metal-Organic Coating Interfacial Reaction Kinetic Mechanisms by a Hydrogen Permeation Based Potentiometric Approach

Corrosion driven delamination of coatings is crucially determined by the rate of the cathodic oxygen reduction reaction at the buried metal-organic coating interface. Quantitative measurement of this rate at such interfaces by conventional techniques is impeded due to the blocking of ion transport by the coating. A new approach where hydrogen permeation is used as a tool to measure the oxygen reduction rate underneath coatings has been recently introduced. This permeation based potentiometry approach measures the rate of the oxygen reduction reaction by correlating the open circuit potential established as a consequence of the dynamic electrochemical equilibrium between hydrogen oxidation and oxygen reduction reactions on the coated exit side with the hydrogen uptake rate on the entry side. In this work, the interfacial reaction kinetics of the oxygen reduction reaction causing delamination of the coating are investigated by this hydrogen permeation based potentiometric approach. Moreover, thus performed prolonged cathodic polarizations of the palladium/coating interface have been found to destroy the interface, just like it is the case in the cathodic delamination process. Initial results obtained on ultra-thin films of iron on palladium are also presented, showing that the technique is applicable also on technically more relevant metal surfaces.

Chemistry acceleration with tabulated dynamic adaptive chemistry in a realistic engine with a primary reference fuel

Detailed kinetic reaction mechanisms are necessary for accurate prediction of combustion characteristics such as ignition and emissions in realistic engines. However, the calculation of chemically reacting flows with detailed chemistry is computational expensive due to the large number of species and reactions involved. In this study, a combined approach of dynamic adaptive chemistry (DAC) and in situ adaptive tabulation (ISAT) for efficient chemistry calculations has been implemented into a three-dimensional flow solver to simulate a homogenous charged compression ignition (HCCI) engine with a primary reference fuel (PRF). In the combined method, ISAT speeds up the chemistry calculation by reducing the number of integrations of ordinary differential equations (ODEs) governing chemical kinetics through tabulating and re-using the ODE solutions. At the meantime, DAC accelerates the necessary ODE integrations via the use of locally valid reduced mechanisms, which are obtained using the direct relation graph (DRG) method. The study shows that ISAT–DAC can achieve a speedup factor of about three with accurate prediction of composition and heat release even for the pressure-varying transient engine simulation with significant composition inhomogeneity resulting from wall heat loss. A detailed analysis reveals that the combined method effectively reduces the computational cost through taking advantage of the respective acceleration characteristics of DAC and ISAT at different combustion stages. In the low temperature combustion stage between about 650K and 1000K, even though the reduction in the mechanism size and consequently the ODE integration of the chemical kinetics by DAC is not significant, the combined method can still reach a speed-up factor of more than 100 due to the fact that the tabulated entries can effectively be reused. For the high temperature region, even though the tabulated entries cannot be reused due to the rapid change of the pressure and large composition inhomogeneity resulting from the active combustion and heat loss, DAC can effectively reduce the size of the needed ODE integrations by freezing a significant number of unimportant species and reactions. The study further quantifies the effect of composition inhomogeneity on the computation efficiency in detailed chemistry calculations in realistic engine simulations. For the case considered, the temperature inhomogeneity due to the wall heat loss obviously increases at around top dead center (TDC), reaching a level of 350K difference in temperature within the combustion chamber. Consequently, the overall computational efficiency in chemistry calculation by the combined method has been reduced by 40% compared to the case without heat loss.

A new improvement on a chemical kinetic model of primary reference fuel for multi-dimensional CFD simulation

In the present study, for the multi-dimensional CFD (computational fluid dynamics) combustion simulations of internal combustion engines, a new optimized chemical kinetic reaction mechanism for the oxidation of PRF (primary reference fuel) instead of gasoline has been developed. In order to carry out the in-depth research for combustion phenomenon of internal combustion engines, an optimized reduced PRF mechanism including more intermediate species and radicals was developed. The developed mechanism contains of iso-octane (C8H18) and n-heptane (C7H16) surrogates, which contains of 51-species and 193 reactions. Compared with many other mechanisms of PRF, more reactions of C0–C1 oxidation (100 reactions) are added in the present mechanism. In order to improve the performances of the model, the developed mechanism focused on the improvement through the prediction of the ignition delay time. The developed mechanism has been validated against various experimental and simulation data including shock tube data, laminar flame speed data and HCCI (homogeneous charge compression ignition) engine data. The results showed that the developed PRF mechanism was agreements with the experimental data and other approved reduced mechanisms, and it could be applied to the multi-dimensional CFD simulations for internal combustion engines.

Kinetic study of antibiotic ciprofloxacin ozonation by MWCNT/MnO2 using Monte Carlo simulation

Kinetic Monte Carlo simulation was used to investigate kinetics of antibiotic ciprofloxacin degradation by direct and heterogeneous catalytic (MnO2 and carbon nano-tube loaded with MnO2) ozonation. The reaction kinetic mechanisms of each system have been obtained. The rate constant values for the each step of the reaction mechanisms were attained as adjustable parameters by kinetic Monte Carlo simulation. The carbon nano-tube loaded with MnO2 plays important role as catalyst in the ciprofloxacin ozonation by increasing reactivity of ozone and ciprofloxacin drug on the surface of carbon nano-tube. Optimized amount of ozone and catalysts were obtained via studying the effect of inlet ozone concentration and initial amount of catalyst on the rate of ciprofloxacin degradation using Monte Carlo simulation. The simulation results of this study have reasonably agreement with the present experimental data for the ozonation of ciprofloxacin drug.

The Kinetic Reaction Mechanism of the Vibrio cholerae Sodium-dependent NADH Dehydrogenase

The sodium-dependent NADH dehydrogenase (Na(+)-NQR) is the main ion transporter in Vibrio cholerae. Its activity is linked to the operation of the respiratory chain and is essential for the development of the pathogenic phenotype. Previous studies have described different aspects of the enzyme, including the electron transfer pathways, sodium pumping structures, cofactor and subunit composition, among others. However, the mechanism of the enzyme remains to be completely elucidated. In this work, we have studied the kinetic mechanism of Na(+)-NQR with the use of steady state kinetics and stopped flow analysis. Na(+)-NQR follows a hexa-uni ping-pong mechanism, in which NADH acts as the first substrate, reacts with the enzyme, and the oxidized NAD leaves the catalytic site. In this conformation, the enzyme is able to capture two sodium ions and transport them to the external side of the membrane. In the last step, ubiquinone is bound and reduced, and ubiquinol is released. Our data also demonstrate that the catalytic cycle involves two redox states, the three- and five-electron reduced forms. A model that gathers all available information is proposed to explain the kinetic mechanism of Na(+)-NQR. This model provides a background to understand the current structural and functional information.

Experimental and Kinetic Modeling Study of 2-Methyl-2-Butene: Allylic Hydrocarbon Kinetics.

Two experimental studies have been carried out on the oxidation of 2-methyl-2-butene, one measuring ignition delay times behind reflected shock waves in a stainless steel shock tube, and the other measuring fuel, intermediate, and product species mole fractions in a jet-stirred reactor (JSR). The shock tube ignition experiments were carried out at three different pressures, approximately 1.7, 11.2, and 31 atm, and at each pressure, fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0), and fuel-rich (ϕ = 2.0) mixtures were examined, with each fuel/oxygen mixture diluted in 99% Ar, for initial postshock temperatures between 1330 and 1730 K. The JSR experiments were performed at nearly atmospheric pressure (800 Torr), with stoichiometric fuel/oxygen mixtures with 0.01 mole fraction of 2M2B fuel, a residence time in the reactor of 1.5 s, and mole fractions of 36 different chemical species were measured over a temperature range from 600 to 1150 K. These JSR experiments represent the first such study reporting detailed species measurements for an unsaturated, branched hydrocarbon fuel larger than iso-butene. A detailed chemical kinetic reaction mechanism was developed to study the important reaction pathways in these experiments, with particular attention on the role played by allylic C-H bonds and allylic pentenyl radicals. The results show that, at high temperatures, this olefinic fuel reacts rapidly, similar to related alkane fuels, but the pronounced thermal stability of the allylic pentenyl species inhibits low temperature reactivity, so 2M2B does not produce "cool flames" or negative temperature coefficient behavior. The connections between olefin hydrocarbon fuels, resulting allylic fuel radicals, the resulting lack of low-temperature reactivity, and the gasoline engine concept of octane sensitivity are discussed.

Modeling hydrogen inhibition in gasification surface reactions

The gasification of carbonaceous char into synthesis gas species presents a viable thermochemical conversion pathway of both fossil fuel-based and biomass-based solid fuels. Observations of these synthesis gas species—particularly hydrogen—inhibiting gasification rates have been documented for decades. Finite-rate surface kinetic reaction mechanisms that can be applied to a variety of reactor models and conditions are needed, particularly those which describe gasification inhibition. To quantify the effects of hydrogen inhibition reactions in gasification conditions, existing surface kinetic mechanisms were employed in simulations and compared with experimental data available in the literature. A computationally inexpensive method to model the surface chemistry in simple gasification reactors is utilized in this study—where appropriate, these experiments in the literature were simulated as perfectly stirred reactors. When used in comparison with kinetically controlled experiments, this method can readily facilitate the identification of missing reaction steps in surface mechanisms. Results reveal the inability of current surface mechanisms in predicting experimentally observed inhibition trends. Modeling the inhibition with an adsorption–desorption pair of reactions based on a combination of theoretical and empirical considerations was found to improve the overall model when these reactions were appended to a current surface mechanism. Reasonable agreement with experimental data is found with regard to the kinetics of carbon gasification reactions in the inhibiting presence of hydrogen.

Effect of the Methyl Substitution on the Combustion of Two Methylheptane Isomers: Flame Chemistry Using Vacuum-Ultraviolet (VUV) Photoionization Mass Spectrometry

Alkanes with one or more methyl substitutions are commonly found in liquid transportation fuels, so a fundamental investigation of their combustion chemistry is warranted. In the present work, stoichiometric low-pressure (20 Torr) burner-stabilized flat flames of 2-methylheptane and 3-methylheptane were investigated. Flame species were measured via time-of-flight molecular-beam mass spectrometry, with vacuum-ultraviolet (VUV) synchrotron radiation as the ionization source. Mole fractions of major end-products and intermediate species (e.g., alkanes, alkenes, alkynes, aldehydes, and dienes) were quantified axially above the burner surface. Mole fractions of several free radicals were also measured (e.g., CH3, HCO, C2H3, C3H3, and C3H5). Isomers of different species were identified within the reaction pool by an energy scan between 8 and 12 eV at a distance of 2.5 mm away from the burner surface. The role of methyl substitution location on the alkane chain was determined via comparisons of similar species trends obtained from both flames. The results revealed that the change in CH3 position imposed major differences on the combustion of both fuels. Comparison with numerical simulations was performed for kinetic model testing. The results provide a comprehensive set of data about the combustion of both flames, which can enhance the erudition of both fuels combustion chemistry and also improve their chemical kinetic reaction mechanisms.

Design of a dinuclear ruthenium based catalyst with a rigid xanthene bridge for catalytic water oxidation

A new dinuclear ruthenium complex 1 (L1[Ru(bda)2(picoline)]2) based on Ru–bda (H2bda=2,2′-bipyridine-6,6′-dicarboxylic acid) and dipyridyl xanthene (L1=4,5-dipyridyl-2,7-di-tert-butyl-9,9-dimethyl xanthene) ligand was synthesized to probe the catalytic oxidation of water. An oxygen evolution experiment displays a low catalytic water oxidation activity with a first-order reaction kinetic mechanism. The result indicates that the OO bond formation of the dinuclear catalyst 1 follows a water nucleophilic attack pathway rather than a radical coupling pathway. The most plausible interpretation is that the steric hindrance effects of the L1 and bda ligands lead to a disadvantage in forming the face-to-face configuration of the two active sites in a one dimer molecule.

Quantitative biophysical characterization of intrinsically disordered proteins.

Intrinsically disordered proteins (IDPs) are broadly defined as protein regions that do not cooperatively fold into a spatially or temporally stable structure. Recent research strongly supports the hypothesis that a conserved functional role for structural disorder renders IDPs uniquely capable of functioning in biological processes such as cellular signaling and transcription. Recently, the frequency of application of rigorous mechanistic biochemistry and quantitative biophysics to disordered systems has increased dramatically. For example, the launch of the Protein Ensemble Database (pE-DB) demonstrates that the potential now exists to refine models for the native state structure of IDPs using experimental data. However, rigorous assessment of which observables place the strongest and least biased constraints on those ensembles is now needed. Most importantly, the past few years have seen strong growth in the number of biochemical and biophysical studies attempting to connect structural disorder with function. From the perspective of equilibrium thermodynamics, there is a clear need to assess the relative significance of hydrophobic versus electrostatic forces in IDP interactions, if it is possible to generalize at all. Finally, kinetic mechanisms that invoke conformational selection and/or induced fit are often used to characterize coupled IDP folding and binding, although application of these models is typically built upon thermodynamic observations. Recently, the reaction rates and kinetic mechanisms of more intrinsically disordered systems have been tested through rigorous kinetic experiments. Motivated by these exciting advances, here we provide a review and prospectus for the quantitative study of IDP structure, thermodynamics, and kinetics.

Characteristics of regulation of the activity of glucokinase from rat liver

A simple method for isolation of glucokinase from a soluble fraction of rat liver was proposed, making it possible to obtain the enzyme preparation characterized by high specific activity, native cooperative characteristics of the enzyme, and actually complete absence of other molecular forms of hexokinase. Dependence of the enzyme activity on the glucose concentration shows a pronounced sigmoidal behavior: with in the whole range of substrate concentrations at 25 and 2°C, the Hill coefficients are equal to 1.76 and 1.35, correspondingly, while the S0.5 values of glucose are 6.90 and 20 mM, respectively. The role of the kinetic reaction mechanism in the regulation of glucokinase activity in liver is under discussion.

Non-Isothermal Degradation of Vitamin C by Simultaneous Thermogravimetric and Differential Thermal Analysis

The non-isothermal degradation process of Vitamin C (Galenika, Serbia) was investigated by simultaneous thermogravimetric and differential thermal analysis (TG-DTA), in the temperature range from an ambient one up to 650°C. The scope of presented work is toward stability of active pharmaceutical substance – ascorbic acid and calculation of all relevants kinetics parameters: activation energy (Ea), pre-exponential factor (A) and half-time (t1/2) for stability assesment in explored formulation. It was established that the degradation proceeds through five degradation stages (designated as I, II, III, IV and V), which involve: the dehydration, the melting of ascorbic acid, as well as the decomposition of excipients present: microcrystalline cellulose, D-glucose, carboxymethylcellulose, magnesium stearate , lactose and corn starch. During thermal degradation, all excipients increase their thermal stability and some kind of solid-solid and/or solid-gas interaction occur. The kinetic parameters and reaction mechanism was determined for the II stage, where decomposition of ascorbic acid happened using powerfull kinetic software KINETICS05. Two approaches were used: isoconversional and modell fitting method. It was concluded that degradation of ascorbic acid is best described with N-th order model with Ea= 139.550kJ/mol, A= 1.9689∙1013s-1 and n=1.2132.

A Study on the Kinetic Model of Biodiesel Produced from Palm Oil with Assistant of CaO-La2O3

By using palm oil as feedstock, methanol as an esterifying agent, and CaO-La2O3 as the catalyst, effects of the reaction condition on concentration of methyl esters were researched. The best reaction time, amount of catalyst, and ratio of methanol and palm oil were and 3.5 h, 2.5%, and 15:1, respectively. According to the guiding principles of reaction kinetic model mechanism, reaction kinetics model of biodiesel is pointed out. Model simulated data are close to experimental data.

A Single Pulse Shock Tube Study on the Pyrolysis of 2,5-Dimethylfuran

Abstract The pyrolysis of 2,5-dimethylfuran has been studied in a single pulse shock tube equipped with fast probing device at temperatures between 1175 K and 1450 K and pressures of 8.0±0.5 bar. The initial concentration of 2,5-dimethylfuran diluted in argon (500 ppm) was much lower than in previous studies reported in the literature. Sixteen different product species were quantified by gas chromatography. The product distribution pattern was compared with the prediction of two comprehensive chemical kinetic reaction mechanisms taken from the literature. In general, the predictions of the mechanisms fit the results of the experiments; however, the comparison reveals some differences between the two mechanisms as well as between simulations and experiments.

Kinetic modeling of methane dehydroaromatization chemistry on Mo/Zeolite catalysts in packed-bed reactors

This paper develops a detailed chemical kinetics reaction mechanism to represent methane dehydroaromatization (MDA) chemistry on bi-functional Mo/zeolite catalysts. The model is validated using a range of previously published results from packed-bed experiments. The reaction mechanism consists of four methane activation reactions on Mo2C sites. The resulting gas-phase hydrogen and ethylene continue to react on Brønsted acid sites within the zeolite structure using 46 reaction steps. In addition to the desired benzene formation, the model also predicts the formation of toluene, naphthalene, and other side products. In addition to reaction kinetics on the catalyst surfaces, the packed-bed model incorporates a Dusty-Gas model that accommodates ordinary and Knudsen diffusion as well as pressure-driven advection. Because the model represents essentially all published reports of MDA performance, it is reasonably expected that the model can be applied as a predictive tool to support reactor and process development.