Detailed Reaction Scheme Research Articles

Decomposition mechanism of furfural under supercritical water gasification conditions using ReaxFF simulations

The supercritical water gasification (SCWG) is a convenient and efficient method to utilize hydrogen stored in biomass. Furfural is an important platform during the conversion of biomass thus it is of great significance to understand the microscopic mechanism of the SCWG of furfural. In this work, the SCWG of furfural is investigated using reactive molecular dynamic (RMD) simulations under the temperature range from 1800 K to 2800 K. Based on the detailed reaction scheme of furfural pyrolysis and SCWG, it is found pyrolysis dominates the conversion of furfural under low temperature such as 2400 K and produces only a small amount of H2; while under higher temperature, H2O participates in SCWG as reactant and actively reacts with furfural and C3 fragments in forms of H and OH radicals, which promotes the production of H2. In addition, the combination of water and C3 fragments also effectively inhibits the coke formation. This work could provide a further insight for the conversion of biomass to hydrogen energy at the atomic level.

Investigating the Mukaiyama-type epoxidation reaction of alkenes with Cr(III) salophen catalysts: A confrontation between experiments and DFT calculations

This study aimed at the complete elucidation of the mechanism of the Mukaiyama-type alkene epoxidation reaction using the [(salophen)CrIIICl] precatalyst, by combining spectroscopic (UV–visible and EPR) studies, catalytic reactivity and quantum chemical modeling. These studies performed in parallel allowed us to propose a detailed reaction scheme, in which two consecutive active species incorporating both {salophen(CrV = O)} moieties were formed in situ, resulting in two distinct catalytic regimes.

Family of Skeletal Reaction Mechanisms for Methane–Oxygen Combustion in Rocket Propulsion

Analyzing methane–oxygen rocket propellant combinations requires suitable modeling of the major chemical reaction processes. Although several detailed kinetic mechanisms for methane oxidation in air exist, most do not reproduce the reaction pathways of high-pressure methane–oxygen combustion, typical of liquid rocket engines. Moreover, when large-scale computational fluid dynamics simulations are pursued, detailed reaction schemes are not computationally viable. In the present study, we identify a reliable detailed kinetic scheme for liquid rocket applications, and then we perform a wide reduction campaign leveraging computational singular perturbation theory. Enforcing various reduction targets, we obtain a family of seven skeletal schemes, including 11–39 species. Each mechanism targets different combustion modes, namely, homogeneous ignition, complex flows and flame extinction, premixed burning, reaction processes under intense turbulent mixing, and largely off-stoichiometric mixtures, typical of rocket engine preburners. We test the skeletal mechanisms against meaningful validation targets, attaining appreciable predictive accuracy compared with the detailed parent scheme. We expect the proposed family of skeletal schemes to offer a wide and flexible range of solutions—in terms of size, accuracy, and dominant combustion mode—for performing large-scale yet cost-affordable computational fluid dynamics of methane–oxygen flames under rocket-engine-relevant conditions.

Mathematical Modeling and Experiments on Pyrolysis of Walnut Shells Using a Fixed-Bed Reactor

Pyrolysis is a low-emission and sustainable thermochemical technique used in the production of biofuels, which can be used as an alternative to fossil fuels. Understanding the kinetic characterization of biomass pyrolysis is essential for process upscaling and optimization. There is no accepted model that can predict pyrolysis kinetics over a wide range of pyrolysis conditions and biomass types. This study investigates whether or not the classical lumped kinetic model with a three-competitive reaction scheme can accurately predict the walnut shell pyrolysis product yields. The experimental data were obtained from walnut shell pyrolysis experiments at different temperatures (300–600 °C) using a fixed-bed reactor. The chosen reaction scheme was in good agreement with our experimental data for low temperatures, where the primary degradation of biomass occurred (300 and 400 °C). However, at higher temperatures, there was less agreement with the model, indicating that some other reactions may occur at such temperatures. Hence, further studies are needed to investigate the use of detailed reaction schemes to accurately predict the char, tar, and gas yields for all types of biomass pyrolysis.

Microscopic pyrolysis mechanism of tert-butyl hydroperoxide via ReaxFF molecular dynamics

Tert-butyl hydroperoxide (TBHP) is a common curing agent and free radical initiator in petrochemical industries, but may trigger fires and explosions once it decomposes. Therefore, it is imperative to elucidate the pyrolysis mechanism. Herein, the pyrolysis behaviors of TBHP are investigated via ReaxFF molecular dynamics simulations with a reliable force field validated by the density functional theory (DFT) method. The main pyrolysis products of water, acetone, methanol, oxygen, isobutene, methane, tert-butyl alcohol, formaldehyde, and propene, are all detected in the reported experiments. It is found that the peroxy bond scission and the hydrogen abstraction of TBHP by the radicals are two main initial reactions. In the subsequent reactions, the decomposition pathways of two main large radicals of (CH3)3CO· and (CH3)3COO· are tracked, followed by the main consumption and generation channels of the small reactive species of ·OH, ·CH3, and O2. A detailed reaction scheme of the TBHP pyrolysis is thus proposed. In addition, the apparent activation energy is calculated as 20.8 kcal/mol, which reasonably agrees with the experimental value. This work will provide scientific guidance for the process safety of organic peroxides and the development of suppression technologies.

Simulation tool for the analysis of in-situ combustion experiments that considers complex kinetic schemes and detailed mass transfer- theoretical analysis of the gas phase CO oxidation reaction

A simulation tool was designed for analyzing various experimental setups that include the ability to model detailed chemical reaction schemes for in-situ combustion (ISC) analysis.,. The simulation tool was illustrated with a theoretical example to the extent of CO oxidation in a gaseous phase takes place during ISC. The models in the simulation tool are based on fundamental conservation laws, physical correlations for porous media properties, and property databases available in literature. Emphasis is made on the analysis of chemical reactions in the gas phase, a characteristic that may be useful when temperatures are above 700°C and oxygen, unburned hydrocarbons, and CO coexist. The three modules of the simulation tool: (i) Kinetic cell, (ii) One-dimensional reactor, and (iii) Combustion tube, can be used to represent in detail the processes taking place in the typical laboratory-scale equipment used to characterize ISC. Tools for the analysis of transport phenomena and multiphase reactions, present in all three models, can support the process of finding chemical kinetic parameters for an easier calculation of device-independent kinetic constants. Four applications have the simulator scope: (i) Analysis of reactions in the gas phase, (ii) Axial gradients in a kinetic cell, (iii) Pressure build-up in a combustion tube, and (iv) Ignition in a combustion tube. These examples highlight the importance that homogeneous reactions may have in these systems and the existence, under certain conditions, of concentration gradients that are normally neglected, and can affect the interpretation of ISC experiments.

An open source fluid catalytic cracker - fractionator model to support the development and benchmarking of process control, machine learning and operation strategies

The present study describes the capabilities of an open source integrated framework for the dynamic simulation and control of a refining operation, comprising a fluidized bed catalytic cracker (FCC) and a fractionator. A detailed cracking reaction scheme that yields realistic predictions of product distributions is used. In addition, the fractionator is rigorously modeled and solved using the dynamic stagewise adiabatic flash algorithm. A regulatory control layer is also included.Two simulation examples are shown for set point tracking and FCC feed quality disturbance rejection, respectively. We expect that the model can serve as a platform for multiple research efforts, including algorithm development and benchmarking in fault detection, control and process operations. The model can be downloaded using the following link: https://github.com/Baldea-Group/FCC-Fractionator.

Understanding the mechanism of large-scale template elimination during calcination of Mcm-41

An in-depth study of the thermal detemplation of the MCM-41 was carried out in a designed setup running with a high catalyst loading. Mapping the temperature along the catalyst bed during detemplation provides an understanding of the heat transfer phenomena involved under (an)aerobic conditions. Comparison between the results obtained at low (TG/DTA study) and high sample loadings (fixed-bed reactor) suggest that oxygen presence modifies the detemplation mechanism and high flows increase the local temperature in the bed due to the exothermic nature of hydrocarbon oxidation. Thermal cracking products detected during calcination may be considered a benchmark to deduce the local conditions in the catalyst bed. Despite the differences in detemplation processes, both environments lead to solids with similar structural properties. The use of advanced analytical techniques, such as 2D GC, led to the proposal of a detailed reaction scheme for the detemplation step.

CFD–DEM modeling of autothermal pyrolysis of corn stover with a coupled particle- and reactor-scale framework

Autothermal operation of fast pyrolysis is an efficient process-intensification technique wherein exothermic oxidation reactions are used to overcome the heat-transfer bottleneck of conventional pyrolysis. The development of accurate, reliable modeling toolsets is imperative to generating a deeper understanding of biomass autothermal pyrolysis systems to support scale-up and industrial deployment. This modeling effort describes the development of single-particle and reactor models which incorporate detailed reaction schemes and simultaneous exothermic oxidation reactions. The particle-scale model was parameterized for corn stover feedstock with particle morphology, density, ash content, and biopolymer composition, all of which impact the emergent conversion characteristics during pyrolysis. Results were then used to parameterize a reactor-scale autothermal pyrolysis model, which was developed using a coarse-grained computational fluid dynamic–discrete element method. The simulation results compared well with experimental results, with the predicted bio-oil, light gas, and biochar yield within 3.0 wt% of the experimental yields. Further analyses were performed to test the influence of equivalence ratio, biomass injection position, and particle size distribution on autothermal pyrolysis. The analysis of the physio-chemical properties of the fluid and solid phase inside the reactor and at the reactor outlet help reveal important process interactions of autothermal pyrolysis.

Numerical investigation of the effect of rotation on non-premixed hydrogen combustion in developing turbulent mixing layers

This study was aimed at examining the influence of the system rotation as an external action on the development of vortical structures and combustion. Specifically, three-dimensional direct numerical simulations of compressible mixing layers with non-premixed /air combustion were performed using a detailed chemical reaction scheme. The relationship between the developing vortical structures and chemical reactions in the flow field with the rotation was investigated. The development of combustion changed depending on the vortical structures, and the presence of roller vortices promoted the combustion phenomena. The influence of the vortical structures on the elementary reactions, which contribute to the heat release rate, was small. During the anticyclonic rotation, the roller vortices collapsed and suppressed the combustion. In contrast, the cyclonic rotation resulted in the generation of quasi-2D roller vortices, which enlarged the high-heat-release-rate regions and promoted the combustion. Overall, the vortical structures induced by the rotation can change the development of combustion even though the elementary reactions that contribute to the heat release rate remain unchanged. The presented findings can guide the prediction and control of turbulent combustion in practical situations involving fluid machinery.

Modelling laminar diffusion flames using a fast convergence three-dimensional CVFEM code

Laminar diffusion flames have been employed extensively to investigate the effects of physical and chemical-related parameters on combustion phenomena. Attention has been given to improve combustion models while keeping the two-dimensional (2D) axisymmetric approach and simplified boundary conditions. The flow structure due to the discrete jet nature of the reactants claims a more realistic inlet boundary condition for the burner plate which can only be accomplished if the solution domain is 3D. In this work, we present a novel 3D simulation code for the analysis of non-premixed laminar flames of methane and air, considering detailed inlet boundary conditions. The code is based on the control volume finite element method, written in MATLAB environment. To accomplish that a novel flow-oriented upwind scheme was implemented for the advection terms of the conservation equations. Combustion modelling can be carried out by either an enhanced flame sheet model in which an enthalpy equation is integrated in combination with the mixture fraction equation or detailed reaction schemes. Radiative heat losses were accounted for in the added energy equation. Three-dimensional numerical predictions, based on the flame sheet model, were able to simulate the near-wall gas temperature and inflow velocity field of a set of perforated plate burner with different porosities. It was found that the discrete nature of the injection of fuel and oxidiser, inflow boundary condition, plays a significant role on flame shape and length. Much better agreement between published experimental data and numerical predictions was obtained, particularly regarding flame structure, species mass fractions and temperature distribution of non-premixed laminar flames regardless of the adopted simplified combustion model. It was observed that flame length decreased as the burner porosity increased, up to a certain limit. The modified flow-oriented scheme greatly improved solution stability and computational time of the reacting system predictions.

High-temperature pyrolysis of isoprenoid hydrocarbon p-menthane using ReaxFF molecular dynamics simulation

The pyrolysis chemistry of p-menthane, a promising “drop-in” fuel of bio-derived isoprenoid hydrocarbon, has not been well understood especially under high temperatures. In this work, the pyrolysis of p-menthane is further investigated using molecular dynamics simulations with the reactive force-field interatomic potential (ReaxFF) under 2600 K. It is found that the scission of isopropyl and the ring-open reaction in the site adjacent to the isopropyl are two predominant initiations of p-menthane decomposition in our simulations. The main generation and consumption channels of important products, such as methane and ethylene, are also tracked by the straightforward scrutinize on simulation trajectories. In addition, a detailed reaction scheme of the high-temperature pyrolysis is proposed. The apparent activation energy calculated from our reactive molecular dynamics simulations in the temperature range from 2200 K to 3000 K is 232 kJ mol−1, which is reasonably consistent with our experimental result of 225 ± 5 kJ mol−1. In summary, the simulation results provided by ReaxFF are helpful to explain pyrolysis processes readily, which is the effective verification and extension of previous experimental studies.

Mechanisms and kinetics studies of the atmospheric oxidation of eugenol by hydroxyl radicals and ozone molecules

Eugenol is a representative methoxyphenol derived from the pyrolysis of lignin containing a branched alkene group. Its concentration in the atmosphere is equivalent to guaiacol and syringol. In this present paper, the gas phase reaction mechanisms and kinetic parameters of eugenol with hydroxyl radicals (OH) and ozone molecules (O3) were calculated at the M06-2×/6-311+G(3df,2p)//M06-2×/6-311+G(d,p) level. There are two distinct reaction types between eugenol and OH. In particular, Path2 is most favorable in the OH additions, whereas IM16 is most advantageous in H atom abstraction pathways. OH additions have more advantages than H abstraction reactions. Thus, the comprehensive and detailed reaction schemes for the further reactions of IM2 were presented. The main products generated by IM2 are methyl (Z)-3-(2-formylpenta-1,4-dien-1-yl)-2-hydroxyoxirane-2-carboxylate (P2B-4), 2-methoxy-2-oxoacetic acid (P2B-10), 2-allylmalealdehyde (P2B-11) and other carbonyl or carboxyl compounds. As for the reaction of eugenol with O3, the cycloaddition reactions and subsequent oxidative degradation processes were also explored, which yielded the most dominant product 2-(4-hydroxy-3-methoxyphenyl) acetaldehyde (P8-1). The reaction constants of the primary reactions for eugenol with OH and O3 under the temperature range of 225– 375 K were successively calculated by POLYRATE and MESMER program. At 298 K and 1 atm, the respective rate coefficients are 5.91 × 10−11 and 5.48 × 10−16 cm3 molecule−1 s−1 and the corresponding atmospheric lifetimes are 4.70 h and 0.72 h. The short lifetimes suggest that once eugenol enters the atmosphere, it is likely to be rapidly degraded. This work aims to provide theoretical guidance for the photochemical reaction mechanisms of eugenol with OH and O3, and present a reference for more experimental researches.

Electrolyte Oxidation Pathways in Lithium-Ion Batteries.

The mitigation of decomposition reactions of lithium-ion battery electrolyte solutions is of critical importance in controlling device lifetime and performance. However, due to the complexity of the system, exacerbated by the diverse set of electrolyte compositions, electrode materials, and operating parameters, a clear understanding of the key chemical mechanisms remains elusive. In this work, operando pressure measurements, solution NMR, and electrochemical methods were combined to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment LiCoO2/Li cells were cycled with a lithium-ion conducting glass-ceramic separator so that the species formed at each electrode could be identified separately and further reactions of these species at the opposite electrode prevented. One principal finding is that chemical oxidation (with an onset voltage of ∼4.7 V vs Li/Li+ for LiCoO2), rather than electrochemical reaction, is the dominant decomposition process at the positive electrode surface in this system. This is ascribed to the well-known release of reactive oxygen at higher states-of-charge, indicating that reactions of the electrolyte at the positive electrode are intrinsically linked to surface reactivity of the active material. Soluble electrolyte decomposition products formed at both electrodes are characterized, and a detailed reaction scheme is constructed to rationalize the formation of the observed species. The insights on electrolyte decomposition through reactions with reactive oxygen species identified through this work have a direct impact on understanding and mitigating degradation in high-voltage/higher-energy-density LiCoO2-based cells, and more generally for cells containing nickel-containing cathode materials (e.g., LiNixMnyCozO2; NMCs), as they lose oxygen at lower operating voltages.

Analysis of the Water Addition Efficiency on Knock Suppression for Different Octane Ratings

<div class="section abstract"><div class="htmlview paragraph">Water injection can be applied to spark ignited gasoline engines to increase the Knock Limit Spark Advance and improve the thermal efficiency. The Knock Limit Spark Advance potential of 6 <i>°CA</i> to 11 <i>°CA</i> is shown by many research groups for EN228 gasoline fuel using experimental and simulation methods. The influence of water is multi-layered since it reduces the in-cylinder temperature by vaporization and higher heat capacity of the fresh gas, it changes the chemical equilibrium in the end gas and increases the ignition delay and decreases the laminar flame speed. The aim of this work is to extend the analysis of water addition to different octane ratings. The simulation method used for the analysis consists of a detailed reaction scheme for gasoline fuels, the Quasi-Dimensional Stochastic Reactor Model and the Detonation Diagram. The detailed reaction scheme is used to create the dual fuel laminar flame speed and combustion chemistry look-up tables. The Detonation Diagram is used as a novel approach in the Quasi-Dimensional Stochastic Reactor Model to evaluate the auto-ignition characteristic in the end gas and determine if it is a harmless deflagration or developing detonation. First, the Quasi-Dimensional Stochastic Reactor Model is trained for three engine operating points and a RON95 E10 fuel. Its performance is evaluated based on experimental results of a single cylinder research engine. Subsequently, different spark timings and water-fuel ratios are investigated for different Primary Reference Fuels. The results outline that water addition can effectively reduce the strength of auto-ignition in the end gas for different Primary Reference Fuels. Thereby, it can be stated that the reduction of the auto-ignition strength through water addition by 50 - 80 <i>%</i> water-fuel ratio for high octane number fuels corresponds to the spark timing delay of 6 <i>°CA</i> or an increase of research octane number by 10 points.</div></div>

Assessment of property estimation methods for the thermodynamics of carbon dioxide-based products

Carbon dioxide can be used as feedstock to produce chemicals. It represents a stimulating defiance to manufacture novel cost competitive materials with less environmental impact, besides to investigate new opportunities for catalysts and industrial chemistry. The contribution of carbon dioxide conversion goes beyond lowering global warming, by reducing fossil resource depletion or even yielding more benign production pathways. Albeit promising, the literature data regarding the quantity of energy needed to convert carbon dioxide into chemicals is limited and narrowed to the most studied processes and products. In order to understand and model the formation of species using carbon dioxide as raw material, some basic thermodynamic data are needed. The development of detailed reaction schemes in the field is also scarce. To enhance and further complete the database of the products obtained from carbon dioxide, this study investigates different procedures to estimate the basic thermodynamic properties of the reactants and products of these reactions. To date various methods have been developed and introduced to determine the gas-phase standard enthalpies of formation and Gibbs energy. Among them, group additivity and semi-empirical methods are widely employed due to their accuracy and effort time for implementation compared to more rigorous methods. Semi empirical quantum-chemistry methods were compared with group additivity methods. Available literature data were used to select the best method for property estimation of the whole set of species, whereby produced from carbon dioxide. The products from carbon dioxide were categorized in sixteen chemical classes, the reaction enthalpy for the direct route to manufacture the products were assessed and indicate a large difference among the classes. The results of this investigation show that semi empirical quantum-chemistry methods revealed to be more accurate for the studied species; additionally, the method demonstrates robustness in estimating the properties. Together, these results provide important insights into the thermodynamics of carbon dioxide related products.

Prediction of Premixed Flame Dynamics Using Large Eddy Simulation With Tabulated Chemistry and Eulerian Stochastic Fields

Abstract This paper demonstrates that a large Eddy simulation (LES) combustion model based on tabulated chemistry and Eulerian stochastic fields can successfully describe the flame dynamics of a premixed turbulent swirl flame. The combustion chemistry is tabulated from one-dimensional burner-stabilized flamelet computations in dependence on progress variable and enthalpy. The progress variable allows to efficiently include a detailed reaction scheme, while the dependence on enthalpy describes the effect of heat losses on the reaction rate. The turbulence-chemistry interaction is modeled by eight Eulerian stochastic fields. An LES of a premixed swirl burner with a broadband velocity excitation is performed to investigate the flame dynamics, i.e., the response of heat release rate to upstream velocity perturbations. In particular, the flame impulse response and the flame transfer function (FTF) are identified from LES time series data. Simulation results for a range of power ratings are in good agreement with the experimental data.

Unraveling the interaction of hydroxylamine and Fe(III) in Fe(II)/Persulfate system: A kinetic and simulating study

Hydroxylamine showed an outstanding performance on enhancing the oxidation of pollutants in Fe(II) involved advanced oxidation processes, while the detailed reaction schemes have not been fully revealed. Specific functions of hydroxylamine in the oxidation of benzoic acid with Fe(II)/persulfate (PDS) system were explored. With the addition of hydroxylamine, degradation kinetics of benzoic acid deviated from both two-stage kinetics and pseudo first order kinetics, but could be interpreted well with binomial regression analysis. Degradation rate constant (kobs) of benzoic acid was calculated and showed the same variation trend with [hydroxylamine][Fe(III)]2/([Fe(II)][H+])2, the value of which was changed during reaction processes. A detailed kinetic model for simulating the degradation profile of benzoic acid with hydroxylamine acceleration was proposed for the first time and indicated that interactions of hydroxylamine and Fe(III) were fast equilibrium reactions, which was a dominant factor influencing the oxidation kinetics of benzoic acid in Fe(II)/hydroxylamine/PDS system. Comparative study showed that when 1.4 mM of ascorbic acid was added into Fe(II)/PDS system, degradation kinetics of benzoic acid was similar to that enhanced by hydroxylamine. However, when 0.6 mM or 1.0 mM of ascorbic acid was added, oxidation kinetics still presented as the two-stage profile. Kinetic simulations indicated that Fe(II) was produced slower from Fe(III)-ascorbic acid complexes than that with hydroxylamine, which caused the difference in oxidation kinetics. This study could improve our understanding about the effect of hydroxylamine and other reductants in promoting pollutants elimination in Fe(II)/PDS system.

In situ FTIR analysis in determining possible chemical reactions for peroxide crosslinked LDPE in the presence of triallylcyanurate

Most of the current methods used in the investigation of crosslinking reactions between low density polyethylene (LDPE) and dicumylperoxide (DCP) alone or with DCP/triallylcyanurate (TAC) systems are normally inconclusive in determining its detailed reaction scheme. In this paper, an attempt to determine the reactions mechanism of this particular system was investigated using electron spin resonance (ESR), nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR). It was found that the ESR was unsuccessful in gaining insight into reaction mechanisms. Similarly, at the low degree of crosslinking necessary to allow samples to swell, the NMR technique was not suitable to illustrate structural changes after crosslinking. On the other hand, in situ FTIR was proven to be the most elucidating technique, positively indicating oxidation, full peroxide dissociation and indications that TAC reacted by forming both allyl and alkyl radicals. These radicals then appeared to take part in additional reaction incorporating into the network (improving the crosslinking efficiency) or possibly by self-polymerisation either as an independent species or more probably within the network.

Assessment of the validity of RANS knock prediction using the resonance theory

Following the resonance theory by Bradley and co-workers, engine knock is a consequence of an auto-ignition in the developing detonation regime. Their detonation diagram was developed using direct numerical simulations and was applied in the literature to engine knock assessment using large eddy simulations. In this work, it is analyzed if the detonation diagram can be applied for post-processing and evaluation of predicted auto-ignitions in Reynolds-averaged Navier–Stokes simulations even though the Reynolds-averaged Navier–Stokes approach cannot resolve the fine structures resolved in direct numerical simulations and large eddy simulations that lead to the prediction of a developing detonation. For this purpose, an engine operating point at the knock limit spark advance is simulated using Reynolds-averaged Navier–Stokes and large eddy simulations. The combustion is predicted using the G-equation and the well-stirred reactor model in the unburnt gases based on a detailed gasoline surrogate reaction scheme. All the predicted ignition kernels are evaluated using the resonance theory in a post-processing step. According to the different turbulence models, the predicted pressure rise rates and gradients differ. However, the predicted ignition kernel sizes and imposed gas velocities by the auto-ignition event are similar, which suggests that the auto-ignitions predicted by Reynolds-averaged Navier–Stokes simulations can be given a meaningful interpretation within the detonation diagram.