Chemical Kinetic Calculations Research Articles

Mechanism Reduction of Hydrogen Production from Dimethyl Ether Partial Oxidation by Plasma Reforming

Chemical reaction kinetic model of hydrogen production from DME partial oxidation by plasma reforming was found. Mole fractions of main products of DME partial oxidation by spark plasma as the function of inlet gas flow rate were calculated at atmospheric pressure and ambient temperature. Comparing the results of calculation and experiment, the model was proved to be correct. The mechanism research was done by the method of sensitivity analysis and rate of production. The reduced mechanism which includes 16 species and 13 radical reactions was done. The calculation results of reduced mechanism and detailed mechanism were close. The result shows that the reduced mechanism can be used in chemical reaction kinetic calculation of hydrogen production from DME partial oxidation by spark plasma reforming.

An accelerated multi-zone model for engine cycle simulation of homogeneous charge compression ignition combustion

We have developed an accelerated multi-zone model for engine cycle simulation (AMECS) of homogeneous charge compression ignition (HCCI) combustion. This model incorporates chemical kinetics and is intended for use in system-level simulation software. A novel methodology to capture thermal stratification in the multi-zone model is proposed. The methodology calculates thermal stratification inside the cylinder based on a single computational fluid dynamics (CFD) calculation for motored conditions. CFD results are used for tuning zone heat loss multipliers that characterize wall heat loss from each individual engine zone based on the assumption that these heat loss multipliers can then be used at operating conditions different from those used in the single CFD run because the functional form of thermal stratification is more dependent on engine geometry than on operating conditions. The model is benchmarked against detailed CFD calculations and fully coupled HCCI CFD chemical kinetics calculations. The results indicate that the heat loss multiplier approach accurately predicts thermal stratification during the compression stroke and (therefore) HCCI combustion. The AMECS model with the thermal stratification methodology and reduced gasoline chemical kinetics shows good agreement with boosted gasoline HCCI experiments over a range of operating conditions, in terms of in-cylinder pressure and heat release rate predictions. The computational advantage of this method derives from the need for only a single motoring CFD run for a given engine, which makes the method very well suited for rapid HCCI calculations in system-level codes such as GT-Power, where it is often desirable to evaluate consecutive engine cycles.

Utilization of waste heat from a HCCI (homogeneous charge compression ignition) engine in a tri-generation system

The waste heat from exhaust gases and cooling water of Homogeneous charge compression ignition engines (HCCI) are utilized to drive an ammonia-water cogeneration cycle (AWCC) and some heating processes, respectively. The AWCC is a combination of the Rankine cycle and an absorption refrigeration cycle. Considering the chemical kinetic calculations, a single zone combustion model is developed to simulate the natural gas fueled HCCI engine. Also, the performance of AWCC is simulated using the Engineering Equation Solver software (EES). Through combining these two codes, a detailed thermodynamic analysis is performed for the proposed tri-generation system and the effects of some main parameters on the performances of both the AWCC and the tri-generation system are investigated in detail. The cycle performance is then optimized for the fuel energy saving ratio (FESR). The enhancement in the FESR could be up to 28.56%. Under optimized condition, the second law efficiency of proposed system is 5.19% higher than that of the HCCI engine while the reduction in CO2 emission is 4.067% as compared with the conventional separate thermodynamic systems. Moreover, the results indicate that the engine, in the tri-generation system and the absorber, in the bottoming cycle has the most contribution in exergy destruction.

Numerical simulation and validation of SI-CAI hybrid combustion in a CAI/HCCI gasoline engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In this study, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z). An ignition model is included to initiate the ECFM3Z calculation and induce the flame propagation. In order to precisely depict the subsequent auto-ignition process of the unburned fuel and air mixture independently after the initiation of flame propagation, the tabulated chemistry concept is adopted to describe the auto-ignition chemistry. The methodology for extracting tabulated parameters from the chemical kinetics calculations is developed so that both cool flame reactions and main auto-ignition combustion can be well captured under a wider range of thermodynamic conditions. The SI-CAI hybrid combustion model (HCM) is then applied in the three-dimensional computational fluid dynamics (3-D CFD) engine simulation. The simulation results are compared with the experimental data obtained from a single cylinder VVA engine. The detailed analysis of the simulations demonstrates that the SI-CAI hybrid combustion process is characterised with the early flame propagation and subsequent multi-site auto-ignition around the main flame front, which is consistent with the optical results reported by other researchers. Besides, the systematic study of the in-cylinder condition reveals the influence mechanism of the early flame propagation on the subsequent auto-ignition.

Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

The epoxidation reaction of cyclohexene is investigated for the catalytic system vanadyl acetylacetonate (VO(acac)2) with tert-butyl hydroperoxide (TBHP) as oxidant with the aim to identify the most active species for epoxidation and to retrieve insight into the most plausible epoxidation mechanism. The reaction mixture is composed of various inactive and active complexes in which vanadium may either have oxidation state+IV or+V. Inactive species are activated with TBHP to form active complexes. After reaction with cyclohexene, each active species transforms back into an inactive complex that may be reactivated again. The reaction mixture is quite complex containing hydroxyl, acetyl acetonate, acetate, or a tert-butoxide anion as ligands, and thus, various ligand exchange reactions may occur among active and inactive complexes. Also, radical decomposition reactions allow transforming V+IV to V+V species. To obtain insight into the most abundant active complexes, each of previous transformation steps has been modeled through thermodynamic equilibrium steps. To unravel the nature of the most plausible epoxidation mechanism, first principle chemical kinetics calculations have been performed on all proposed epoxidation pathways. Our results allow to conclude that the concerted Sharpless mechanism is the preferred reaction mechanism and that alkylperoxo species V+IVO(L)(OOtBu) and V+VO(L1)(L2)(OOtBu) species are most abundant. At the onset of the catalytic cycle, vanadium+IV species may play an active role, but as the reaction proceeds, reaction mechanisms that involve vanadium+V species are preferred as the acetyl acetonate is readily oxidized. Additionally, an experimental IR and kinetic study has been performed to give a qualitative composition of the reaction mixture and to obtain experimental kinetic data for comparison with our theoretical values. The agreement between theory and experiment is satisfactory.

The critical tube diameter and critical energy for direct initiation of detonation in C2H2/N2O/Ar mixtures

In this study, the dynamic detonation parameters, namely, the critical tube diameter and critical energy for direct initiation, of C2H2/N2O/Ar mixtures were measured at various initial conditions. Using chemical kinetics with the Konnov mechanism and experimental measured data, a simple correlation to evaluate the critical tube diameter of C2H2/N2O/Ar detonation is developed, given by dc=594.8φ0.623(1-XAr)0.2176(p/po)-0.0246ΔI which is applicable for stoichiometric mixtures with initial pressures ranging from 50 to 130kPa, equivalence ratios from 0.625 to 2.5 and stoichiometric mixtures with maximum percentage of argon dilution up to 50% at the atmospheric pressure. By combining this correlation function with a theoretical model based on a simple work done concept, the critical energy for direct blast initiation can be predicted; and the theoretical predictions are found to be in good agreement with the critical energy measured experimentally. To assess the detonation sensitivity, the critical energy results for direct initiation of detonation in C2H2/N2O/Ar mixtures are compared with those in H2/N2O/Ar mixtures [Zhang et al., Int. J. Hydrogen Energy 39 (2011) 5707–5716]. The results indicate that C2H2/N2O/Ar is more sensitive than H2/N2O/Ar at the same initial conditions. This is also supported qualitatively by the chemical kinetic calculation which shows that the ZND induction length scale for C2H2/N2O/Ar is relatively smaller than that of H2/N2O/Ar mixture. Lastly, the effect of the oxidizer on the detonation sensitivity is studied by comparing the critical energy between C2H2/N2O/Ar and C2H2/O2/Ar mixture. It is found that a higher energy is required to successfully initiate a spherical detonation in C2H2/N2O/Ar than in C2H2/O2/Ar mixtures and equivalently, the results agree qualitatively with the ZND induction length analysis.

Characteristics of oxy-fuel combustion in gas turbines

This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH4, O2, CO2 and H2O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas.

Analysis of the Trade-off between Soot and Nitrogen Oxides in Diesel-Like Combustion by Chemical Kinetic Calculation

Analysis of the Trade-off between Soot and Nitrogen Oxides in Diesel-Like Combustion by Chemical Kinetic Calculation

Application of Genetic Algorithm in Chemical Reaction Kinetics

In this paper, a summary of Genetic Algorithm methods developed recent years applied in chemical reaction kinetics was presented. The applications of the Genetic Algorithm in reduction of the chemical reaction kinetics, estimation of the chemical kinetic parameters and calculation of the chemical kinetic equations were expounded here. Eventually, the confronted problem and developing trend of the application of Genetic Algorithm methods in chemical kinetics were reviewed.

The Change in Composition of Inclusions in Molten Steel during Simultaneous Addition of Ca alloy and CaO–Al2O3 flux

In order to examine the effects of calcium addition method on inclusion composition, the variations of the inclusion composition were measured during simultaneous addition of calcium alloy and CaO–Al2O3 flux to the molten steel by 180 kg scale experiments. Fifty four grams CaSi alloy mixed with 13.5 g CaO–Al2O3 flux was added 10 times at the interval of 1 min and the samples of molten steel were sampled during the experiments. The composition of the molten steel and the composition of the inclusions were analyzed. From the experiments and chemical reaction kinetic calculation, the effects of CaO–Al2O3 composition in flux on the changes in the inclusion composition were examined. The results obtained are summarized as follows;(1) Calcium concentration in molten steel increased with increasing CaO concentration in CaO–Al2O3 flux.(2) The CaO concentration in inclusion at which CaS concentration began to increase was changed by the composition of CaO–Al2O3 flux.The results of the reaction rate calculation considering the reaction of calcium in molten steel and flux showed a good agreement with the experimental results.(3) The reaction path of the variation of inclusion concentration was changed as a result of the reaction among calcium in molten steel, inclusion and flux.(4) It was suggested that an inclusion composition could be optionally controlled by adding CaSi alloy and CaO–Al2O3 flux at the same time.

N<sub>2</sub>O Formation in Selective Non-catalytic NO<sub>x</sub> Reduction Processes

Nitrous oxide is not an environmentally regulated species in the U.S., but it does participate in the stratospheric ozone chemistry and contributes to the greenhouse effect. Nitrous oxide has been found to be a by-product of the selective non-catalytic reduction process. Chemical kinetic calculations demonstrated that the formation of nitrous oxide in the urea-based selective non-catalytic reduction process is linked to the conversion of NO by cyano species released from the process parent compounds. This conversion occurs within in temperature window between 850 and 1050℃. With urea injection, nitrous oxide emissions represent up to 20 percent conversion of the NOx reduced. The amount of nitrous oxide formed depends primarily on the process temperature, the amount of chemical injected, the initial NOx level, and the carbon monoxide level in the gas stream. These observations, which were based on the chemical kinetics of the process, should be considered in designing selective non-catalytic reduction systems to minimize nitrous oxide by- product formation.

The rates of bimolecular thermal reactions taking into account local medium fluctuations

The contribution of local medium fluctuations to the rates of thermal chemical reactions was calculated for the first time. It was shown for the “ideal gas” model that the fluctuation contribution to a rate constant was proportional to the specific volume of particles and amounted to 1–10% already at densities of about 20 atmosphere densities. For this reason, chemical kinetics calculations of reaction rates should be performed taking into account local medium fluctuations before taking into account specific volume in the equation of state.

A correlation for the laminar burning velocity for use in hydrogen spark ignition engine simulation

Hydrogen is an interesting fuel for internal combustion engines. It is a versatile fuel that enables high efficiencies and low emissions of oxides of nitrogen (NO x), throughout the load range. Computer simulations of hydrogen-fuelled spark ignition engines would facilitate the development of these engines. These necessitate the calculation of the turbulent combustion of hydrogen to track the flame propagation throughout the combustion chamber and resolve in-cylinder pressure and temperature. In order to do this, the laminar burning velocity of the in-cylinder mixture at the instantaneous pressure and temperature is needed. However, there is a scarcity of data in the literature, particularly at engine conditions. This is further complicated by the occurrence of flame instabilities at engine-like pressures, which compromises some of the existing data. This paper discusses the available experimental data and correlations for the laminar burning velocity of hydrogen mixtures, and their deficiencies. One-dimensional chemical kinetic calculations of the laminar burning velocity of mixtures of hydrogen, air and residuals, at engine-like pressures and temperatures are then reported. A correlation is derived for use in hydrogen engine codes and is compared to other correlations presented previously.

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH2O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 mu m and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH2O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH2O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH2O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH2O from the reaction zones to the preheat zone. The CH2O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH2O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O-2 and CO2 in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH2O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets

We used chemical equilibrium and chemical kinetic calculations to model chemistry of the volatiles released by heating different types of carbonaceous, ordinary and enstatite chondritic material as a function of temperature and pressure. Our results predict the composition of atmospheres formed by outgassing during accretion of the Earth and other terrestrial planets. Outgassing of CI and CM carbonaceous chondritic material produces H 2O-rich (steam) atmospheres in agreement with the results of impact experiments. However, outgassing of other types of chondritic material produces atmospheres dominated by other gases. Outgassing of ordinary ( H, L, LL) and high iron enstatite ( EH) chondritic material yields H 2-rich atmospheres with CO and H 2O being the second and third most abundant gases. Outgassing of low iron enstatite ( EL) chondritic material gives a CO-rich atmosphere with H 2, CO 2, and H 2O being the next most abundant gases. Outgassing of CV carbonaceous chondritic material gives a CO 2-rich atmosphere with H 2O being the second most abundant gas. Our results predict that the atmospheres formed during accretion of the Earth and Mars were probably H 2-rich unless the accreted material was dominantly CI and CM carbonaceous chondritic material. We also predict significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at high temperatures (1500–2500 K). Finally, our results may be useful for interpreting spectroscopic observations of accreting extrasolar terrestrial planets.

119 化学動力学に基づく3次元燃焼解析による直接噴射式ディーゼル機関のSoot排出量予測 : EGRの影響の検討(OS-2 ディーゼル(2))

119 化学動力学に基づく3次元燃焼解析による直接噴射式ディーゼル機関のSoot排出量予測 : EGRの影響の検討(OS-2 ディーゼル(2))

Prediction of Soot Emission from a Direct Injection Diesel Engine by Means of Three-dimensional Chemical Kinetics Calculation

Prediction of Soot Emission from a Direct Injection Diesel Engine by Means of Three-dimensional Chemical Kinetics Calculation

Estimated dynamic shear stress and frictional heat during the 1999 Taiwan Chi-Chi earthquake: A chemical kinetics approach with isothermal heating experiments

We performed isothermal heating experiments on samples from a breccia zone adjacent to the slip zone of the Taiwan Chelungpu fault that slipped during the 1999 Chi-Chi earthquake to determine the chemical kinetics function and parameters for thermal decomposition of carbonate minerals in the fault zone. We found that a zero-order reaction was the most likely reaction mechanism for thermal decomposition of these samples and determined an activation energy for the reaction of 137 kJ mol − 1 and a pre-exponential term of 1.15 × 10 4 s − 1 . We applied these parameters in chemical kinetics calculations, taking into account the temperature change over time caused by frictional heating and heat conduction, energy conservation, and energy taken up by the endothermic reaction, to estimate the dynamic shear stress and maximum temperature reached in the slip zone, and found them to be 6.62 MPa and 1079 °C, respectively.

Numerical evaluation of equivalence ratio measurement using OH ∗ and CH ∗ chemiluminescence in premixed and non-premixed methane–air flames

This work presents results from detailed chemical kinetics calculations of electronically excited OH (A 2 Σ, denoted as OH ∗) and CH (A 2 Δ, denoted as CH ∗) chemiluminescent species in laminar premixed and non-premixed counterflow methane–air flames, at atmospheric pressure. Eight different detailed chemistry mechanisms, with added elementary reactions that account for the formation and destruction of the chemiluminescent species OH ∗ and CH ∗, are studied. The effects of flow strain rate and equivalence ratio on the chemiluminescent intensities of OH ∗, CH ∗ and their ratio are studied and the results are compared to chemiluminescent intensity ratio measurements from premixed laminar counterflow natural gas–air flames. This is done in order to numerically evaluate the measurement of equivalence ratio using OH ∗ and CH ∗ chemiluminescence, an experimental practise that is used in the literature. The calculations reproduced the experimental observation that there is no effect of strain rate on the chemiluminescent intensity ratio of OH ∗ to CH ∗, and that the ratio is a monotonic function of equivalence ratio. In contrast, the strain rate was found to have an effect on both the OH ∗ and CH ∗ intensities, in agreement with experiment. The calculated OH ∗/CH ∗ values showed that only five out of the eight mechanisms studied were within the same order of magnitude with the experimental data. A new mechanism, proposed in this work, gave results that agreed with experiment within 30%. It was found that the location of maximum emitted intensity from the excited species OH ∗ and CH ∗ was displaced by less than 65 and 115 μm, respectively, away from the maximum of the heat release rate, in agreement with experiments, which is small relative to the spatial resolution of experimental methods applied to combustion applications, and, therefore, it is expected that intensity from the OH ∗ and CH ∗ excited radicals can be used to identify the location of the reaction zone. Calculations of the OH ∗/CH ∗ intensity ratio for strained non-premixed counterflow methane–air flames showed that the intensity ratio takes different values from those for premixed flames, and therefore has the potential to be used as a criterion to distinguish between premixed and non-premixed reaction in turbulent flames.

Comparison and Parametric Study of Flameless Oxidation in a Gas Turbine Using Two Kinetics Mechanisms

The so-called Oxidation is a novel combustion mode, in which combustion products are re-circulated and mixed into the fresh incoming fuel and air streams. This reduces the concentration of the reactants and thereby reducing the reaction rate through avoiding the formation of sharp high temperature zones in the combustion chamber. Flameless combustion has been acknowledged as one of the most interesting combustion technologies to meet both the targets of high energy efficiency and low pollutant emissions. This technology has already been successfully applied and exploited in industrial burners. The present investigation is concerned with the application of the flameless combustion mode to an adiabatic combustor, typically used in gas turbine engines. Detailed chemical kinetics calculations, by means of a specific zero-dimensional loop reactor model, have been performed to analyze its chemical aspects. The model simulates the combustor by a number of reactors that represent different zones in the combustion chamber. The main objective of this study is to increase the understanding of NOx formation from flameless mode, where currently very few experimental data are available. The investigation is focused on a comparison of the influence of pressure, residence time and temperature on the NOx and CO emissions, using two reaction mechanisms of the C/H/O/N system: the Miller-Bowman mechanism and the GRI_MECH3.0 mechanism. Simulation results clearly illustrate that even at high operating temperatures and pressures, NOx emissions could be reduced by flameless combustion to very low levels. A comparison between the predictions obtained by using the two chemical kinetics mechanisms is presented and discussed. It is shown that the predictions of the Miller-Bowman mechanism deviate from the predictions of the GRI3.0 mechanism in many aspects, especially as related to NOx emission results.