Reaction Flow Analysis Research Articles

Thermodynamic model: steam and oxidative reforming of methane over nickel catalyst

In this paper, we have used a thermodynamic model for the first time to investigate the steam and oxidative reforming of methane over a nickel catalyst in a wide temperature range, i.e., 400–1200 K. The available literature focus on the kinetic models and hence, thermodynamic models require attention to understand the behaviour of the thermochemistry of the species involved in the mechanism. This study presents the comparison between the species concentration produced using the thermodynamic model against the available kinetic model to validate the results. The investigation is further extended, firstly, to perform the sensitivity analysis of the reactions involved in a thermodynamic model to figure out the most influential reactions at various temperatures and pressures. This allows us to compare the most influencing reactions in reforming process for kinetic and thermodynamic model to optimize the processes. Secondly, the reaction flow analysis is carried out for the thermodynamic model to comprehend the effect of the thermochemistry of the species and the major difference in the reaction pathways for both the models are noted.

Computational Analysis on Combustion Control of Diesel Engines Fueled with Ester Alcohol Diesel

As the urgency for environmental sustainability escalates globally, the exploration of alternative fuels for diesel engines becomes a crucial endeavor. By combining chemical reaction kinetics and three-dimensional simulation software, the combustion and emission characteristics of a diesel engine fueled with two oxygenated fuels, hydrogenated biodiesel and ethanol, and adopting a multi-stage injection strategy were studied. The combustion mechanism of hydrogenated biodiesel ethanol diesel hybrid fuel was established, and the reaction activity of ester alcohol diesel with different mixing ratios was studied through reaction flow analysis at high and low OH temperatures. The established mechanism was coupled with CFD 2021 three-dimensional simulation software to compare the combustion and emission performance of diesel engines fueled with different ratios of ester alcohol diesel. The results show that as the proportion of ester alcohol mixture increases, at low temperatures, the OH generation rate decreases, the consumption rate increases, and the reaction activity decreases, which is not conducive to the promotion of combustion reaction; at high temperatures, the generation rate of OH increases, the consumption rate decreases, and the reaction activity increases, which is conducive to the promotion of combustion reactions. Compared to diesel, the reaction system activity of mixed fuel is enhanced, and the main peak values of cylinder pressure and instantaneous heat release rate are higher than that of diesel. The diffusion of oil and gas in the cylinder is improved. As the proportion of ester alcohol diesel mixture increases, the oxygen content increases, nitrogen oxides emissions increase compared to diesel, and soot emissions decrease compared to diesel. Soot emissions are mainly distributed in areas with a high equivalence ratio and high temperature, which is consistent with the distribution area of C2H2, the precursor of soot generation.

Hydrogen Production and Carbon Capture by Gas-Phase Methane Pyrolysis: A Feasibility Study.

Using natural gas and sustainable biogas as feed, high-temperature pyrolysis represents a potential technology for large-scale hydrogen production and simultaneous carbon capture. Further utilization of solid carbon accruing during the process (i. e., in battery industry or for metallurgy) increases the process's economic chances. This study demonstrated the feasibility of gas-phase methane pyrolysis for hydrogen production and carbon capture in an electrically heated high-temperature reactor operated between 1200 and 1600 °C under industrially relevant conditions. While hydrogen addition controlled methane conversion and suppressed the formation of undesired byproducts, an increasing residence time decreased the amount of byproducts and benefited high hydrogen yields. A temperature of 1400 °C ensured almost full methane conversion, moderate byproduct formation, and high hydrogen yield. A reaction flow analysis of the gas-phase kinetics revealed acetylene, ethylene, and benzene as the main intermediate products and precursors of carbon formation.

Laminar flame speed of methane/air stratified flames under elevated temperature and pressure

Flame propagation under mixture stratification is relevant to a wide range of applications including gas turbine combustors and internal combustion engines. One of the local stratification effects is known as the back-support effect, where the laminar flame speed is modified when a premixed flame propagates into gradually richer or leaner mixtures. A majority of previous studies have focused on the propagation of methane/air stratified flames under standard temperature and pressure. However, stratified combustion often occurs under elevated temperature and pressure in practical applications, which may influence the characteristics of the back support effect through modified reaction pathways. This study performs numerical simulations of stratified laminar counterflow flames under an Atmospheric Temperature and Pressure (ATP) condition and an Elevated Temperature and Pressure (ETP) condition and examines the influence of elevated temperature and pressure on the back-support effect. Reaction flow analyses were extensively conducted to elucidate the difference in the primary reaction pathway between the two conditions. When scaled by the stratification Damköhler number, the back-support effect on the rich-to-lean stratified flame is weaker under the ETP condition than the ATP condition in the stoichiometric to lean region. This is due to increased contribution from reactions involved with OH radicals under the ETP condition, which leads to lower H2 reproduction in the reaction zone than under the ATP condition. The contribution from OH radicals is increased under the ETP condition because the conversion of H into OH is enhanced. These results suggest that the back-support effect may become negligibly small in practical combustors operating under elevated temperature and pressure due to (1) the flame being less sensitive to stratification because of the thinner flame, and (2) the lower H2 reproduction that deteriorates the radical production that drives the back-support effect.

Exploring the interaction kinetics of butene isomers and NOx at low temperatures and diluted conditions

The oxidation of 1-butene and i-butene with and without addition of 1000 ppm NO was experimentally and numerically studied primarily at fuel-rich (ϕ = 2.0) conditions under high dilution (96% Ar) in a flow reactor operated at atmospheric pressure in the low temperature range of approximately 600-1200 K. Numerous intermediate species were detected and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). An elementary-step reaction mechanism consisting of 3996 reactions among 682 species, based on literature and this work, was established to describe the reactions and interaction kinetics of the butene isomers with oxygen and nitrogenous components. Model predictions were compared with the experimental results to gain insight into the low- and high-temperature fuel consumption without and with NO addition and thus the respective interaction chemistry. This investigation firstly contributes a consistent set of temperature-dependent concentration profiles for these two butene isomers under conditions relevant for engine exhaust gases. Secondly, the observed oxidation kinetics is significantly altered with the addition of NO. Specifically, NO promotes fuel consumption and introduces for i-butene a low-temperature behavior featuring a negative temperature coefficient (NTC) region. The present model shows reasonable agreement with the experimental results for major products and intermediate species, and it is capable to explain the promoting effect of NO that is initiated by its contribution to the radical pool. Further, it can describe the observed NTC region for the i-butene/NO mixture as a result of the competition of chain propagation and chain terminating reactions that were identified by reaction flow and sensitivity analyses.

Methane thermal decomposition in regenerative heat exchanger reactor: Experimental and modeling study

In this work, thermal decomposition of methane (TDM) was experimentally studied at nominal gas temperatures of 1070 K–1450 K in a non-catalytic laboratory test reactor. The purpose was to use a simple kinetic mechanism to describe the TDM reaction, which could be applied in industrial reactor design. The experimental data was utilized to optimize global kinetic parameters describing the TDM reaction in the test reactor. For comparison, a 37-step reaction mechanism for TDM was adopted from the literature. When analyzing experimental datasets from the literature, the optimized global kinetics provided better agreement with the experimental data than the 37-step mechanism when the reactor temperature profiles were defined in detail. Since the 37-step mechanism was not able to predict the solid carbon formation well enough, the mechanism was slightly adjusted according to a reaction flow and sensitivity analysis. Additionally, it was suggested that the 37-step mechanism can be improved by optimizing the reaction mechanism by using a detailed experimental data of hydrocarbon formation in TDM achieved in an environment where the temperature profiles are fully defined.

A compact skeletal mechanism for n-dodecane with optimized semi-global low-temperature chemistry for diesel engine simulations

A skeletal mechanism with 54 species and 269 reactions was developed to predict pyrolysis and oxidation of n-dodecane as a diesel fuel surrogate involving both high-temperature (high-T) and low-temperature (low-T) conditions. The skeletal mechanism was developed from a semi-detailed mechanism developed at the University of Southern California (USC). Species and reactions for high-T pyrolysis and oxidation of C5-C12 were reduced by using reaction flow analysis (RFA), isomer lumping, and then merged into a skeletal C0-C4 core to form a high-T sub-mechanism. Species and lumped semi-global reactions for low-T chemistry were then added to the high-T sub-mechanism and a 54-species skeletal mechanism is obtained. The rate parameters of the low-T reactions were tuned against a detailed mechanism by the Lawrence Livermore National Laboratory (LLNL), as well as the Spray A flame experimental data, to improve the prediction of ignition delay at low-T conditions, while the high-T chemistry remained unchanged. The skeletal mechanism was validated for auto-ignition, perfectly stirred reactors (PSR), flow reactors and laminar premixed flames over a wide range of flame conditions. The skeletal mechanism was then employed to simulate three-dimensional turbulent spray flames at compression ignition engine conditions and validated against experimental data from the Engine Combustion Network (ECN).

Numerical optimization and reaction flow analysis of syngas production via partial oxidation of natural gas in internal combustion engines

The production of hydrogen is studied numerically under uncatalyzed partial oxidation and homogeneous charge compression ignition (HCCI) conditions in an internal combustion engine fueled by natural gas. The HCCI process is modeled by a single-zone variable volume reactor using a global heat transfer model and elementary-step reaction mechanisms. Numerical optimization is applied to maximize the hydrogen yield at the end of the expansion stroke by varying the equivalence ratio, engine speed and initial pressure for a fixed initial temperature. Suitable constraints were defined, including peak pressure and bounds to the optimization variables. From these results, maximum hydrogen yield profiles and the associated operating parameter profiles as functions of initial temperature were obtained. The profiles exhibit strong linear dependency with initial temperature. Reaction flow analysis was performed to gain detailed insight into the chemical processes involved when the engine is run under optimal conditions for a maximum hydrogen yield. The integral reaction flow analysis shows that substantial amounts of hydrogen are produced from precursors, which are also valuable products, such as methanol, formaldehyde and ethylene.

1-, 2- and 3-Pentanol combustion in laminar hydrogen flames – A comparative experimental and modeling study

Oxidation pathways of three different pentanol isomers 1-, 2- and 3-pentanol are investigated using the electron–ionization molecular-beam mass spectrometry (EI-MBMS) technique. New experimental speciation data was obtained from laminar, flat, low-pressure H2/O2/Ar base flames seeded with equal amounts of pre-vaporized 1-, 2- or 3-pentanol. The experimental investigation is supported by kinetic modeling. Here, one single detailed reaction mechanism for the three linear pentanol isomers has been constructed and compared against new and existing experimental data. The model itself was obtained by an open-source reaction model generation software (RMG) and tested against existing ignition delay times, flame speeds and new quantitative experimental results for mole fraction profiles of major and intermediate species.The overall discussion of individual species profiles is guided by the model-based reaction flow analysis focusing on the initial steps of the fuel destruction paths for the three pentanols down to C3-hydrocarbon species. Therefore, the reaction pathways for the initial fuel destruction steps are shown and analyzed using combined experimental and predicted results. Analysis is performed by means of secondary decay products for each of the three pentanols. The kinetic reaction model was successfully tested against available ignition delay time and laminar flame speed data. Comparisons against 23 new quantitative species profiles are presented for each of the pentanol doped flames. In general, a good predictive capability of the detailed model can be noted for all three investigated straight-chain pentanols regarding the mole fraction profiles of major and intermediate species.

Development of a reduced n -butanol/biodiesel mechanism for a dual fuel engine

A reduced chemical kinetic mechanism of the n-butanol/biodiesel blend was developed for dual fuel engine simulations. The reaction flow analysis reduction method was adopted to lump and remove the unimportant species and the related reactions. The reduced mechanism of n-butanol contains 71 species and 349 reactions. The reduced mechanism of n-butanol was merged into a reduced mechanism of biodiesel to construct a combined mechanism of n-butanol/biodiesel with 171 species and 765 reactions. The combined mechanism was validated against the n-butanol experimental data including ignition delays in shock tubes and the mole fractions of species in a jet-stirred reactor. The n-butanol/biodiesel mechanism was further validated against the engine experiments fuelled with the n-butanol/biodiesel dual fuel under multiple operating conditions. The predicted pressure and the heat release rate profiles, as well as CO, HC, NOx, and soot emissions under various conditions agreed well with the experimental data. Analysis of the interaction of n-butanol/biodiesel during the ignition process shows that the biodiesel produces OH radicals at the low in-cylinder temperature, and the produced OH radicals are consumed by n-butanol to produce HO2 radicals after a consecutive reaction. HO2 radicals are then transformed to H2O2, which dissociate to OH radicals at higher in-cylinder temperature. OH radicals in turn facilitate further consumption of n-butanol/biodiesel dual fuel. Analysis of the evolution of oxygen contained in the fuel molecules indicates that the O atom in the n-butanol molecule ends up in CH2O, which is turned into CO and the O atoms in the biodiesel may also give rise to CO2 directly.

A model-based understanding of solid-oxide electrolysis cells (SOECs) for syngas production by H2O/CO2 co-electrolysis

High temperature co-electrolysis of H2O and CO2 offers a promising route for syngas (H2, CO) production via efficient use of heat and electricity. The performance of a SOEC during co-electrolysis is investigated by focusing on the interactions between transport processes and electrochemical parameters. Electrochemistry at the three-phase boundary is modeled by a modified Butler–Volmer approach that considers H2O electrolysis and CO2 electrolysis, individually, as electrochemically active charge transfer pathways. The model is independent of the geometrical structure. A 42-step elementary heterogeneous reaction mechanism for the thermo-catalytic chemistry in the fuel electrode, the dusty gas model (DGM) to account for multi-component diffusion through porous media, and a plug flow model for flow through the channels are used in the model. Two sets of experimental data are reproduced by the simulations, in order to deduce parameters of the electrochemical model. The influence of micro-structural properties, inlet cathode gas velocity, and temperature are discussed. Reaction flow analysis is performed, at OCV, to study methane production characteristics and kinetics during co-electrolysis. Simulations are carried out for configurations ranging from simple one-dimensional electrochemical button cells to quasi-two-dimensional co-flow planar cells, to demonstrate the effectiveness of the computational tool for performance and design optimization.

Automatic Generation of Kinetic Skeletal Mechanisms for Biomass Combustion

We present in this paper simplified chemical mechanisms for gas phase biomass combustion based on automatic reduction of detailed and comprehensive kinetics. The reduction method that has been employed is a combined reaction flow and sensitivity analysis well-known to combustion, resulting in a necessity index ranking all chemical species for automatic reduction. The objective is to obtain more compact chemical models, so-called skeletal mechanisms, for implementation into computational fluid dynamics, CFD, in order to reduce computational time. In the current work, the physical system used for the development and validation of the chemical models is that of a tubular reactor, or plug flow reactor, with operating conditions typically found in biomass reactors. The focus has been on gas phase reactions only, and the fuel composition is based on experimental values from biomass and coal gasification. Emphasis has been on the reliability of the simplified models and the correct prediction of important emission parameters such as NOx and important intermediate species. The original chemical model, consisting of several sub models for important reaction paths known in biomass combustion, contained 81 species and 1401 reactions. This was successfully reduced down to 36 species, providing a compact and reliable chemical model for implementation into CFD. The model still contains the reaction paths of C2 species, allowing for more realistic fuel gas compositions. The model has been experimentally validated for a wide range of temperatures including low temperature chemistry and reducing conditions for NOx. The computational time saved using the simplified models was significant with over 80% reduction in CPU time.

PAHs Formation Routes in the n-Heptane Laminar Flow Premixed Flame

A detailed reaction mechanism is adopted to simulate n-heptane laminar premixed flame in this paper. The reaction mechanism comprises 108 components and 572 elementary reactions. The main reactants (O2,n-C7H16), reaction products (CO2,CO,H2,H2O), intermediate products (CH4, C2H4, C2H2, C3Hx) and the concentration distribution of PAHs in the flame are analyzed in the numerical method. The numerical results are in good agreement with the experimental results, which states that the mechanism can predict PAHs in the n-heptane flame. In this paper, the sensitivity and reaction flow analysis method is used to analyze the numerical results and the PAHs formation routes in n-heptane laminar flow premixed flame are obtained.

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography – Part III: 2,5-Dimethylfuran

This work is the third part of a study focusing on the combustion chemistry and flame structure of furan and selected alkylated derivatives, i.e. furan in Part I, 2-methylfuran (MF) in Part II, and 2,5-dimethylfuran (DMF) in the present work. Two premixed low-pressure (20 and 40mbar) flat argon-diluted (50%) flames of DMF were studied with electron–ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) under two equivalence ratios (ϕ=1.0 and 1.7). Mole fractions of reactants, products, and stable and radical intermediates were measured as a function of the distance to the burner. Kinetic modeling was performed using a reaction mechanism that was further developed in the present series, including Part I and Part II. A reasonable agreement between the present experimental results and the simulation is observed. The main reaction pathways of DMF consumption were derived from a reaction flow analysis. Also, a comparison of the key features for the three flames is presented, as well as a comparison between these flames of furanic compounds and those of other fuels. An a priori surprising ability of DMF to form soot precursors (e.g. 1,3-cyclopentadiene or benzene) compared to less substituted furans and to other fuels has been experimentally observed and is well explained in the model.

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography – Part II: 2-Methylfuran

This is Part II of a series of three papers which jointly address the combustion chemistry of furan and its alkylated derivatives 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) under premixed low-pressure flame conditions. Some of them are considered to be promising biofuels. With furan as a common basis studied in Part I of this series, the present paper addresses two laminar premixed low-pressure (20 and 40mbar) flat argon-diluted (50%) flames of MF which were studied with electron–ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) for equivalence ratios ϕ=1.0 and 1.7, identical conditions to those for the previously reported furan flames. Mole fractions of reactants, products as well as stable and reactive intermediates were measured as a function of the distance above the burner. Kinetic modeling was performed using a comprehensive reaction mechanism for all three fuels given in Part I and described in the three parts of this series. A comparison of the experimental results and the simulation shows reasonable agreement, as also seen for the furan flames in Part I before. This set of experiments is thus considered to be a valuable additional basis for the validation of the model. The main reaction pathways of MF consumption have been derived from reaction flow analyses, and differences to furan combustion chemistry under the same conditions are discussed.

Detailed kinetic modeling of the combustion of the four butanol isomers in premixed low-pressure flames

With regard to the importance of butanol as a potential replacement or additive to fossil transportation fuels, a detailed understanding of butanol combustion chemistry is desirable. Routes to different isomers of butanol from biomass are becoming available, and it is known that the structure of fuel molecules can be of crucial importance with respect to the intermediate species pool and the nature and amount of potential pollutants, including regulated air toxics. Quantitative major and intermediate species profiles for the combustion of the four butanol isomers under low-pressure premixed flame conditions, measured with two different in situ mass spectrometric instruments have recently been reported (P. Oßwald et al., Combust. Flame 158 (2011) 2–15), and this large consistent dataset has motivated us to further develop a detailed and comprehensive chemical kinetic model which was used here to complement the experimental results with numerical simulations, including reaction flow analyses. The major differences in the overall chemical pathways in these flames will be discussed, especially with respect to the formation of undesired emissions, and from agreement and differences between experiment and model, suggestions for further work will be given.

Reduction of large kinetic mechanisms with a new approach to the necessity analysis method

A new approach to the necessity analysis method using simultaneously combined sensitivity and reaction flow analysis through each reaction is proposed, validated and applied in this paper. Principles of the necessity analysis method and developments in the new approach are given in detail. Validity of the new approach is investigated by the reduction of considerably small reaction mechanism, i.e., methane mechanism, for the CPOX (catalytic partial oxidation) for the homogenous gas phase in the DETCHEMBATCH code. After the validation, it is applied for the CPOX of iso-octane, which was developed by the Lawrence Livermore National Laboratory (LLNL), for the homogenous gas phase for the same conditions. It is seen that necessity analysis method allows deleting up to 53–56% of the species, from the detailed mechanism without much loss information, for the considered conditions (C/O=1.6 and C/O=1.2). The new approach allows deleting 63% of the species for the same cases.

Automatic generation of skeletal mechanisms for ignition combustion based on level of importance analysis

A level of importance (LOI) selection parameter is employed in order to identify species with general low importance to the overall accuracy of a chemical model. This enables elimination of the minor reaction paths in which these species are involved. The generation of such skeletal mechanisms is performed automatically in a pre-processing step ranking species according to their level of importance. This selection criterion is a combined parameter based on a time scale and sensitivity analysis, identifying both short lived species and species with respect to which the observable of interest has low sensitivity. In this work a careful element flux analysis demonstrates that such species do not interact in major reaction paths. Employing the LOI procedure replaces the previous method of identifying redundant species through a two step procedure involving a reaction flow analysis followed by a sensitivity analysis. The flux analysis is performed using DARS © , a digital analysis tool modelling reactive systems. Simplified chemical models are generated based on a detailed ethylene mechanism involving 111 species and 784 reactions (1566 forward and backward reactions) proposed by Wang et al. Eliminating species from detailed mechanisms introduces errors in the predicted combustion parameters. In the present work these errors are systematically studied for a wide range of conditions, including temperature, pressure and mixtures. Results show that the accuracy of simplified models is particularly lowered when the initial temperatures are close to the transition between low- and high-temperature chemistry. A speed-up factor of 5 is observed when using a simplified model containing only 27% of the original species and 19% of the original reactions.

Pyrolysis of Acetylene for Vacuum Carburizing of Steel: Modeling with Detailed Kinetics

Acetylene was pyrolyzed in a thermogravimetric reactor system under the vacuum carburizing conditions of steel. The products of pyrolysis were collected and analyzed by gas chromatography. The reactor was numerically simulated by using a detailed reaction mechanism. Sensitivity analysis and reaction flow analysis was performed under the investigated conditions to identify important reaction steps. The comparison of experimental and modeling results is discussed in this paper.

A comprehensive skeletal mechanism for the oxidation of n-heptane generated by chemistry-guided reduction

Applied to the primary reference fuel n-heptane, we present the chemistry-guided reduction (CGR) formalism for generating kinetic hydrocarbon oxidation models. The approach is based on chemical lumping and species removal with the necessity analysis method, a combined reaction flow and sensitivity analysis. Independent of the fuel size, the CGR formalism generates very compact submodels for the alkane low-temperature oxidation and provides a general concept for the development of compact oxidation models for large model fuel components such as n-decane and n-tetradecane. A defined sequence of simplification steps, consisting of the compilation of a compact detailed chemical model, the application of linear chemical lumping, and finally species removal based on species necessity values, allows a significantly increased degree of reduction compared to the simple application of the necessity analysis, previously published species, or reaction removal methods. The skeletal model derived by this procedure consists of 110 species and 1170 forward and backward reactions and is validated against the full range of combustion conditions including low and high temperatures, fuel-lean and fuel-rich mixtures, pressures between 1 and 40 bar, and local (species concentration profiles in flames, plug flow and jet-stirred reactors, and reaction sensitivity coefficients) and global parameters (ignition delay times in shock tube experiments, ignition timing in a HCCI engine, and flame speeds). The species removal is based on calculations using a minimum number of parameter configurations, but complemented by a very broad parameter variation in the process of compiling the kinetic input data. We further demonstrate that the inclusion of sensitivity coefficients in the validation process allows efficient control of the reduction process. Additionally, a compact high-temperature n-heptane oxidation model of 47 species and 468 reactions was generated by the application of necessity analysis to the skeletal mechanism.