Combustion Mechanism Research Articles

Mechanistic insights into ethylene catalytic combustion and CO2 formation in vinyl acetate synthesis

Vinyl acetate is a crucial monomer for the production of polyvinyl alcohol and polyvinyl acetate, with extensive applications in the photovoltaic and pharmaceutical industries. CO2 is the primary by-product in the synthesis of vinyl acetate, significantly burdening the separation process and impacting the ecological environment. Reducing CO2 generation at the source is an effective solution. In the vinyl acetate synthesis process, CO2 is primarily produced through the catalytic combustion of ethylene. However, the reaction mechanism of ethylene catalytic combustion on Pd-based catalysts in vinyl acetate synthesis remains incomplete, with existing mechanisms presenting certain limitations. Therefore, further investigation into the reaction mechanism of ethylene catalytic combustion leading to CO2 formation on Pd-based catalysts is necessary.This study employs density functional theory to calculate the reaction mechanism of ethylene catalytic combustion on the Pd(100) surface, obtaining elementary reaction kinetics data. Based on these data, the kinetic Monte Carlo method was used to investigate the reaction process of ethylene catalytic combustion, revealing the predominant pathway for CO2 formation. This research provides targeted strategies for catalyst modification. It aims to improve the understanding of the reaction mechanism of ethylene catalytic combustion in vinyl acetate synthesis, reduce the consumption of ethylene in by-product formation, enhance vinyl acetate yield and ethylene utilization, and ultimately decrease CO2 production, thereby lowering carbon emissions.

Mechanistic understanding of oxygen spillover enables efficient propane combustion over Pt/AlSOx catalyst

Understanding of the mechanism of saturated alkanes catalytic combustion is crucial for the petrochemical industrial exhaust elimination. Noble metals are commonly proposed to be the active sites for C–H bond activation of saturated alkanes, whereas the critical role of support is largely underestimated. Herein, we reveals a novel pathway for propane activation and combustion on acidic support of Pt catalyst, driven by oxygen spillover, which differs significantly from previously reported reaction mechanisms. Specifically, the Pt/AlSOx-I catalyst was prepared by pyrolyzing the Al2(SO4)3 sol–gel to obtain the acidic micro-mesoporous AlSOx support, followed by loading the Pt active sites using wetness impregnation. In comparison to Pt catalysts supported by Al2O3 nanoparticles (10 nm) or commercially available common γ-Al2O3, the Pt/AlSOx-I catalyst demonstrates superior catalytic activity for propane combustion. It is found that gaseous oxygen species activated on Pt active sites spillovers to acidic support of Pt/AlSOx-I and then drives propane to activation and further oxidation on acidic support, thereby remarkably decreasing the apparent activation energies (Ea) from 112.4 to 72.0 kJ·mol−1. Kinetic results suggest that the negative effect of oxygen competitive adsorption is weakened because propane can be activated on the acidic AlSOx support. Additionally, high proportion of metallic Pt species benefits to gaseous oxygen activation as observed on Pt/AlSOx-R, leading to the further increased catalytic activity with highest specific mass reaction rate of 170.2 μmol gPt−1 s−1 and TOFPt of 0.289 s−1 at 220 °C. Our primary results provide novel insights into revealing the reaction mechanism of saturated alkanes combustion on dual active sites.

Unveiling the electron-induced ionization cross sections and fragmentation mechanisms of 3,4-dihydro-2H-pyran.

The interactions of electrons with molecular systems under various conditions are essential to interdisciplinary research fields extending over the fundamental and applied sciences. In particular, investigating electron-induced ionization and dissociation of molecules may shed light on the radiation damage to living cells, the physicochemical processes in interstellar environments, and reaction mechanisms occurring in combustion or plasma. We have, therefore, studied electron-induced ionization and dissociation of the gas phase 3,4-dihydro-2H-pyran (DHP), a cyclic ether appearing to be a viable moiety for developing efficient clinical pharmacokinetics and revealing the mechanisms of biofuel combustion. The mass spectra in the m/z = 10-90 mass range were measured at several different energies of the ionizing electron beam using mass spectrometry. The mass spectra of DHP at the same energies were simulated using on-the-fly semi-classical molecular dynamics (MD) within the framework of the QCxMS formalism. The MD settings were suitably adjusted until a good agreement with the experimental mass spectra intensities was achieved, thus enabling a reliable assignment of cations and unraveling the plausible fragmentation channels. Based on the measurement of the absolute total ionization cross section of DHP (18.1 ± 0.9) × 10-16cm2 at 100eV energy, the absolute total and partial ionization cross sections of DHP were determined in the 5-140eV electron energy. Moreover, a machine learning algorithm that was trained with measured cross sections from 25 different molecules was used to predict the total ionization cross section for DHP. Comparison of the machine learning simulation with the measured data showed acceptable agreement, similar to that achieved in past predictions of the algorithm.

Numerical investigation of the effects of adding different gases on the performance and emissions of an ammonia-hydrogen dual-fuel engine

As an alternative to fossil fuels, there is growing interest in using ammonia in combustion systems, particularly in internal combustion engines. As a competent competitor to hydrogen, it has various advantages. However, adding an ignition promoter such as hydrogen is still necessary to maintain combustion stability. Since the engine using ammonia also suffers from high NOx emissions, in this study, the effects of adding different gases on the performance and emissions of an ammonia-hydrogen dual-fuel engine were numerically investigated. The base engine was a diesel engine whose parameters were accustomed to running with ammonia-hydrogen as fuel. Hydrogen was injected via the port in the intake stage at 180 crank angle degrees (CAD) and mixed with the cylinder charge. Ammonia was directly injected into the cylinder at 350–370 CAD. Different gasses, including argon, nitrogen, carbon dioxide, and oxygen, were injected into the cylinder at various crank angles before, during, and after ammonia injection (330–350 CAD, 350–370 CAD, and 370–390 CAD). A MATLAB code was prepared to solve the governing equations, and the combustion mechanism was implemented in Cantera. The results showed that adding CO2 before or concurrent with the ammonia injection timing had undesirable impacts on the peak in-cylinder pressure and NO and NO2 emissions. The O2 addition had a negative on the emissions. Adding N2 and Ar concurrently with the ammonia injection (350–370 CAD) could diminish NO and NO2 emissions without drastically affecting the peak in-cylinder pressure and temperature.

Optimization of kinetic mechanism for hydrogen combustion based on machine learning

Optimization of kinetic mechanism for hydrogen combustion based on machine learning

Thermodynamic cycle performance modeling and numerical simulation of higher Mach scramjet with inlet pre-injection

This paper focuses on the thermodynamic cycle performance modeling and numerical investigation of a high Mach number scramjet with inlet pre-injection of hydrogen. The scramjet model used in this study was the Hyshot-Ⅱ engine model. Flight experiments using this model at high Mach number have been conducted previously, which provides a unique testbed for validating the computational prediction of supersonic combustion. A thermodynamic cycle performance model with inlet pre-injection was developed for operating characteristics studies of the scramjet engine. A set of three-dimensional Reynolds average Navier-Stockes (RANS) simulations of the reactive flow was performed with a 9-specie and 19-step kinetic mechanism for hydrogen combustion. The turbulence is modeled by k-ω shear stress transport (SST) turbulence model. The thermodynamic cycle performance analysis results indicate that inlet pre-injection can enhance the hydrogen mixing efficiency, and a small amount of advanced heat release can improve the overall thermal cycle efficiency. Detailed flow analysis from the three-dimensional simulation results indicates flow can be ignited in the first shock wave induced boundary layer separation (SIBLS) region. Inlet pre-injection enables the propagation of the flame toward the core flow compared to the throat injection. The inlet fuel pre-injection proved to be a promising injection method due to the higher combustion efficiency, elevated from 0.75 to 0.85, and higher specific impulse potential, increased from 1570 s to 1810 s. As the ER of inlet pre-injection increases within the examined range, the performance curve exhibits an optimal point. As the geometric compression ratio increases, the entire combustion process advances, leading to a significant increase in the performance potential of the engine. The comparative analysis of the thermodynamic cycle performance modeling and computational fluid dynamics (CFD) modeling indicates the model shows a good prediction on the performance of the scramjet engine with the inlet pre-injection, with the same trend of performance curves and an error of less than 10 % compared to the numerical simulation.

Development of an ammonia/diesel combustion mechanism for high ammonia energy ratio: Validation of the mechanisms in kinetic simulation and RCCI optical engine simulation

Ammonia blending in diesel is an efficient combustion strategy that overcomes the combustion resistance of ammonia while maintaining high engine adaptability. A smaller and accurate chemical kinetic mechanism is crucial for exploring the application of ammonia/diesel in engine. This study, based on the stepwise validation and optimization mechanisms, developed a novel ammonia/diesel mechanism comprising 130 species and 772 reactions. The mechanism was validated through kinetic verification for ammonia, n-dodecane, diesel and ammonia/diesel fuel. For profound understanding of the low-temperature autoignition chemistry of ammonia/diesel, applying this mechanism construct precise CFD models in RCCI optical engine. The results indicate that this mechanism can accurately capture ignition under high AER, wide working conditions, and low-temperature conditions. In the kinetic analysis, the C–N interaction reaction, particularly C2H4+NH2=C2H3+NH3, plays a crucial role in predicting the IDT of ammonia/diesel. In addition, NH2 undergoes deoxygenation reaction with HO2, converting inactive radicals HO2 into OH active radicals, enhances the reactivity of ammonia/diesel under low-temperature conditions. Applying the mechanism to CFD models, the model accurately predicts the pressure and heat release rate in RCCI optical engine, capturing the phenomenon of the high-temperature flame rapidly spreading towards the low-temperature regions in the cylinder. The research on this mechanism can construct accurate CFD model for optimizing efficient and clean combustion simulations of ammonia/diesel.

Evolution of macromolecular structure during coal oxidation via FTIR, XRD and Raman

The analysis of the macromolecular structure and morphology in coal during oxidation is the basis to explore the mechanism of spontaneous combustion. To explore the evolutionary rules of coal macromolecular structure during oxidation, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Raman Spectroscopy (Raman) were employed to analyze the coal samples with different oxidation degrees. The results revealed that the oxidation action led to the decrease of the aliphatic structures and aromatic hydroxyl groups in coal, while promoting the formation of oxygen-containing functional groups and aromatic structures. It also led to a relative increase of free hydroxyl groups linked to hydrogen bonds. The aromatic layer spacing (d002) decreased with increasing oxidation degree, while the microcrystal stacking height (Lc), the aromatic layer diameter (La), the average number of crystal stacking layers (n) generally increased. It indicated that small aromatic ring molecules in coal could undergo continuous polymerization during oxidation to form a single aromatic layer structure. The variation of Raman spectrum parameters exhibited a consistent decreasing trend in WD/WG, ID/IG, AD/AG, and A(GR+SL)/AG value, indicating an increase in the vibration of sp2 hybridization carbon atoms within the lattice structure of coal. Conversely, PG-D, AS/AD and A(GR+VL+VR)/AD value increased overall, suggesting that small aromatic rings decreased in content during oxidation while polymerizing into larger aromatic rings. The coal structure underwent a brief stage of disordered evolution during oxidation, followed by removal of impurity structures and condensation of aromatic structures due to increasing oxidation temperatures, ultimately leading to a highly ordered crystalline state. The oxidation process significantly influenced the development of coal's aromatic structure, particularly in less metamorphic coal. The research findings provide a theoretical basis for analyzing the underlying mechanism behind spontaneous combustion induced by coal oxidation.

A numerical study comparing the performance of a self-aspirating domestic LPG porous burner and that of a conventional LPG burner

The study involves in the design of a domestic two-layer self-aspirating porous burner that operates at a thermal load of 1 kW using liquefied petroleum gas (LPG). The porous burner is intended to function within India’s standard domestic regulator fuel inlet pressure of 3000 Pa. For the same thermal load, this work also provides a numerical comparison between the domestic conventional burner and the designed porous burner. For both porous and conventional burners, a full-scale 3D model is developed to calculate the flow, combustion, heat transfer to the cooking vessel, thermal efficiency, and emissions. The combustion process of both burners is numerically computed using a detailed chemical kinetic mechanism of LPG combustion, the San Diego Mechanism (SDM) with 57 species and 268 reactions are used. The porous burner is simulated using a non-thermal equilibrium condition to better calculate the heat recirculation within the porous domain. The self-aspirated porous burner has an equivalence ratio ϕ of 0.75 at 1 kW and an efficiency of 84.2%; conventional burner at the same load had an efficiency of 68%. 10 and 6 parts per million (ppm), respectively, are the measured CO and NOx emissions from the domestic porous burner and 660 and 80 ppm for domestic conventional burner, respectively.

Theoretical study of the anharmonic effect on the main reactions in the combustion mechanism of methanol

The harmonic and anharmonic rate constants, kinetic and thermodynamic parameters of the important reactions of the combustion mechanism of CH3OH are calculated from 300 K to 4000 K. Furthermore, the effect of pressure and tunneling effect are considered in this paper. The calculations indicate that the bimolecular reaction exhibits a particularly obvious anharmonic effect than the unimolecular reaction. The calculated anharmonic rate constants are more in line with other theoretical or experimental results. In summary, these calculations complement the basic database of methanol, thereby improving the accuracy of the combustion reaction mechanism.

Methane Combustion Mechanisms on Ni10/CeO2 Studied by DFT, Microkinetic Modeling, and Kinetic Monte Carlo Simulation

Methane Combustion Mechanisms on Ni₁₀/CeO₂ Studied by DFT, Microkinetic Modeling, and Kinetic Monte Carlo Simulation

Novel NSTEX System Based on Ti/CuO/NC Nanothermite Doped with NTO

Modern energetic materials (EMs), e.g., nanothermite and NSTEX (Nanostructured Thermites and Explosive) compositions are attracting increasing research interest. In this work, we present the results of our investigation on the properties of a novel Ti/CuO nanothermite system doped with cellulose nitrate (NC) and 5-Nitro-1,2-dihydro-3H-1,2,4-triazin-3-one (NTO). In terms of safety parameters, the friction (40–>360 N), impact (40–>50 J) and laser irradiation sensitivity were determined for tested systems, which indicated tunable properties. The combustion velocity (up to 735 m/s), pressure parameters for combustion in closed vessel, thrust parameters and open-air combustion behaviour were measured. Moreover, in order to deeply study the impact of NTO on the combustion mechanism, SEM, DSC/TG and XRD analyses were conducted. The obtained results indicate that the Ti/CuO/NC/NTO system is extremely promising for future applications.

Impact of mixing low-reactivity gases on the mechanism of hydrogen spontaneous combustion: A ReaxFF MD study

The phenomenon of spontaneous combustion during the release of high-pressure H2 is critical to the safe storage and transportation. Besides physical methods to suppress self-ignition, the approach of mixing low-reactivity gases has gained increasing attention. Previous work has mainly focused on the flow, physical heating effects, and boundary conditions of self-ignition in mixed gases, while the intrinsic chemical reaction mechanisms of spontaneous combustion remain underexplored. To clear this issue, this work uses the ReaxFF MD simulation method to study the spontaneous combustion process of H2/CO and H2/CH4 mixtures with various concentrations. The results indicate that: (1) in pure H2, H2/CO, and H2/CH4 systems, the primary initial reaction mechanism is H2+O2HO2+H. Additionally, in some mixed systems, CO + O2CO2+O and CH4+O2CH3+HO2 may also serve as initial reaction mechanisms. (2) Under fuel-lean combustion conditions, the RIDTs of the systems decrease, supporting the viewpoint that self-ignition typically occurs first in fuel-lean regions. The addition of CO and CH4 increases the RIDTs, which helps inhibit the development of self-ignition. (3) In H2-rich environments, CO combusts to CO2 through the formation of HOCO, while CH4 undergoes dehydrogenation and oxidation processes through intermediates such as CH3, CH3O, and HCHO, leading to the formation of CO and CO2. The combustion of H2 is also influenced by these processes. (4) The addition of CO and CH4 increases the activation energy of H2 combustion, raising the reaction energy barrier that must be overcome for spontaneous combustion. This effectively helps to suppress the occurrence and development of self-ignition during the release of high-pressure gases.

Study of ignition and combustion of aluminum/ethanol nanofluid based on reactive molecular dynamics simulation

Aluminum/ethanol nanofluid fuel offers high energy density, high combustion efficiency, and low pollutant emissions, making it highly promising for aerospace applications. In recent years, the fundamental combustion characteristics for aluminum/ethanol nanofluid fuel have been extensively studied. However, current experimental techniques are still difficult to reveal the micro-mechanisms ignition and combustion of aluminum/ethanol nanofluid fuel. Hence, the ignition and combustion mechanisms of aluminum/ethanol nanofluid fuel were investigated from a microscopic point of view through reactive molecular dynamics simulation. The simulation results show that the mechanisms of enhanced ethanol combustion by aluminum nanoparticles mainly consists of micro-explosion at high temperature and small particle size, chain reaction at low temperature and large particle size, melt-dispersion in the mild oxidation state and diffusive oxidation in the moderate and heavy oxidation states. In addition, the initial stage of the combustion of aluminum nanoparticles with core-shell structure in ethanol is mainly a non-homogeneous surface reaction. This work reveals the combustion characteristics and mechanisms of aluminum/ethanol nanofluid fuel from an atomic perspective, which is expected to provide insights for the exploration and application of ethanol-based nanofluid fuel in the future.

Reaction rate of HO2+ 5-methyl-2-furanyl methyl/2-furanyl methyl and its effect on combustion of 2, 5-dimethylfuran/2-methylfuran

A valuable alternative to traditional fuels, bio-oxygenated fuels provide benefits. Oxygenated fuels 2,5-dimethylfuran (DMF) and 2-methylfuran (2MF) perform well. This study used quantum chemical calculations and combustion simulations to evaluate the combustion characteristics of low and medium temperature combustion mechanisms. Conclusions: In the combustion reaction of DMF and 2MF, the H-abstraction reaction occurs mainly at the methyl site. The reaction between macromolecular radicals and HO2 at low and middle temperatures plays a critical role in fuel auto-ignition. Thus, R∙+HO2 ↔ ROOH→R-CHO+H2O and R∙+HO2 → vdw complex → R+3O2 were added to the mechanism. Modified mechanism improves temperature prediction accuracy. The HOMO-LUMO gap is narrowed by the methyl group, which is closely related to the reaction sites and the molecular structure. After H-abstraction, the methyl site is the most likely to react, followed by the C3 and C5 sites.

Low-Temperature Methane Combustion Using Ozone over Coβ Catalyst.

Catalytic methane (CH4) combustion is a promising approach to reducing the release of unburned methane in exhaust gas. Here, we report Co-exchanged β zeolite (Coβ) as an efficient catalyst for CH4 combustion using O3. A series of ion-exchanged β zeolites (Co, Ni, Mn, Fe, and Pd) are subjected to the catalytic test, and Coβ exhibits a superior performance in a low-temperature region (<100 °C). The results of X-ray absorption spectroscopy (XAS) and catalytic tests for Coβ with different Co loadings indicate the isolated Co species is the plausible active site. The reaction mechanism of CH4 combustion over the isolated Co2+ cation is theoretically investigated by the single-component artificial force-induced reaction (SC-AFIR) method to thoroughly search for possible reaction routes. The resulting path toward CO2 formation shows an activation energy of 73 kJ/mol for the rate-determining step and an exothermicity of 1025 kJ/mol, which supports the experimental results. During a long-term catalytic test for 160 h without external heating, the CH4 conversion gradually decreases from 80 to 40%, but the conversion fully recovers after dehydration at 500 °C (0.5 h). The copresence of H2O and CO exhibits a negative impact on the catalytic activity, while NO and SO2 do not markedly change the catalytic activity.

Reaction Mechanism of Pyrolysis and Combustion of Methyl Oleate: A ReaxFF-MD Analysis

Reaction Mechanism of Pyrolysis and Combustion of Methyl Oleate: A ReaxFF-MD Analysis

Combustion characteristics and mechanism of hydrothermally aged asphalt in underwater tunnels

Asphalt could suffer hydrothermal ageing in underwater tunnels, which varies its combustion characteristics significantly. The hydrothermally aged asphalt (HA) was comparatively studied with base asphalt (BA) and thermal-oxidatively aged asphalt (TA). Major findings are achieved: (1) The C＝O and S＝O bonds are more abundant in TA/HA than BA. HA contains more prominent –OH and H2O but less substantial aliphatics. And it has lower SO than TA by about 20 %, attributing to the oxygen isolation of aqueous immersion during hydrothermal ageing. (2) Remarkably bimodal heat release rate (HRR) peaks are found for HA, while only unimodal HRR peak is observed for TA/BA. Importantly, the primarily main HRR peak of HA occurs at around 110 s, which exhibits to be earlier than those of TA/BA by over 120 s. Meanwhile, the fire growth rate index (FGRA) of HA is higher than those of TA/BA by more than 15 %, implying that hydrothermal ageing dramatically increases the asphalt's fire risk. (3) Herein, the combustion mechanism of bimodal HRR peaks found for HA is analyzed, attributing to thin-layer boilover effect. This could be supported by plentiful pores observed on the combusted HA via scanning electron microscopy technique.

Investigation of sodium–manganese oxides with various crystal phases for the efficient catalytic removal of diesel soot particles

Sodium–manganese oxides with various crystal phases (Na2Mn5O10, Na2Mn3O7, and Na4Mn9O18) were synthesized using a facile sol-gel method, and applied in the catalytic purification of soot particles from diesel engines. In this study, as-prepared samples with a single Na2Mn3O7 phase exhibited higher soot oxidation activity, with the lowest T10, T50, and T90 values of 279 ℃, 320 ℃, and 349 ℃, respectively. Numerous characterization techniques reveal that the Na2Mn3O7 catalysts have superior reducibility, NO oxidation, and storage capacity compared with the other crystal phases. In the meantime, the Na2Mn3O7 catalyst surface possesses a greater abundance of oxygen vacancies because of the emergence of Mn3+, which serve as crucial active sites during the dynamic cycling process of gaseous oxygen adsorption and activation. Moreover, the E-R reaction mechanism for soot combustion over the steps of (100) crystal plane of Na2Mn3O7 catalysts is proposed based on density functional theory (DFT) calculations and multiple characterization results. This study presents a facile and cost-efficient method for designing and preparing superior performance sodium–manganese composite oxides for soot combustion.

Enhancement effect of Mo electrodes on the ignition characteristics of HAN-based gel propellant fuel and its combustion mechanisms

Hydroxylammonium nitrate (HAN) serves as a monopropellant fuel, renowned for its low toxicity and high-performance in space propulsion systems. HAN-based gel propellant may offer additional advantages, including long-term stability and excellent throttling capabilities. To better understand the ignition performance and combustion characteristics of this gel propellant, we prepared HAN-based gel propellant, and monitored its decomposition products using thermogravimetry (TG), Fourier Transform Infrared Spectroscopy (FTIR) and mass spectrometry (MS). Electrode materials of Mo, Cu, Ti and SUS 304 were fabricated to investigated their impact on ignition performance. It was found that the Mo electrode exhibited the shortest ignition delay time (ti) compared to Cu, Ti, and SUS 304. Meanwhile, the role of surface roughness of Mo electrode on the ignition delaytime (ti) and the ignition energy (Qi) were also studied. Our results revealed that, as the voltage increased from 100 V to 200 V, the fine-surfaced Mo electrode demonstrated a lower sensitivity to changes in ti compared to the rough-surfaced electrode. Additionally, the Qi of the rough-surfaced electrode decreased from 43.5 J to 16.7 J, while the Qi of the fine-surfaced electrode decreased from 52.5 J to 17.6 J. Combining a thermocouple, the combustion wave structure of the gel propellant was divided into pre-heating zone, condensed phase, electro chemical reaction zone, microexplosion-gases mixing zone and flame zone. Finally, the combustion model of HAN-based gel propellant was inferred to explain its combustion mechanism.