Combustion Kinetics Research Articles

Effects of alkali and alkaline earth metals on co-combustion of sewage sludge and coal slime: Combustion characteristics, interactions, and kinetics

The co-combustion of sewage sludge (SS) and coal slime (CS) is a preferred method for their resource utilization, however, alkali and alkaline earth metals (AAEMs) in SS may affect the co-combustion process. In this work, the co-combustion behavior of AAEMs-rich SS and CS was investigated in terms of combustion characteristics, interactions, and combustion kinetics using a thermogravimetric analyzer. Further, the role of AAEMs in co-combustion was evaluated by loading Ca, K, Na, and Mg individually after pickling. The results revealed that co-combustion compensated for the limitations of the individual combustion processes, with SS reducing ignition and burnout temperatures and CS improving the comprehensive combustion characterization. Principal component analysis (PCA) showed that the effect of CS on co-combustion was more significant compared to SS. Significant synergies were observed in the weight loss phase of fixed carbon in the blends with 40%, 50%, and 60% CS ratios, where the peak temperature of fixed carbon combustion was reduced by 9.8 °C, 12.6 °C, and 13.1 °C, respectively, compared to the theoretical values. The effects of AAEMs on combustion were as follows: all AAEMs promoted the precipitation of volatiles except Ca, which showed inhibition of light volatiles; AAEMs had a significant catalytic effect on fixed carbon combustion. The improvement effect of AAEMs on the comprehensive combustion characteristics during co-combustion was Na > K > Mg > Ca. The catalytic effect of Na on fixed carbon was strongest at a loading of 5%, leading to a decrease in the apparent activation energy of fixed carbon combustion by 22.2 kJ/mol and a change in reactor order from n = 1 to n = 1.2 during co-combustion. This work provides a better understanding of the role of AAEMs in SS-CS co-combustion.

Construction of Al@PTFE composites with excellent ignition and combustion properties through mechanical and thermal activation

Fluorine-based aluminum thermite have excellent ignition and combustion properties. Nonetheless, its combustion kinetics and energy release are hindered by slow mass diffusion and incomplete reactions. Therefore, this study successfully prepared an Al@PTFE composite with a core-shell structure, utilizing mechanical activation and sintering techniques, to enhance combustion properties and energy release rate. SEM, XRD, FTIR and XPS characterization results demonstrate the unique core-shell structure of the samples and the homogeneous distribution of elements on the surface. In comparison to the physical mixture, the Al@PTFE composite exhibits significantly higher values for Pmax (2.19 times), Pmax/Δt (11.30 times), and flame propagation velocity (4.5 times). Furthermore, the combustion duration required to reach the maximum flame height was reduced by 131.75 ms. The adsorption, decomposition and reaction processes of PTFE on the surface of aluminum particles were demonstrated based on RMD simulations. The promotion mechanism of PTFE on combustion in aluminum particles was elucidated.

Alternative fire performance screening method of cladding system using cone calorimeter

Small-scale cone calorimeter tests are commonly used to evaluate the flammability properties of building materials used in cladding systems. However, the combined effect of aluminium composite cladding panels (ACPs) and insulations used in cladding systems has rarely been investigated in the field of small-scale fire assessment. This could be due to a lack of device adaptation for testing multi-component materials, as well as difficulties in translating the results to real-world scenarios. In this study, an alternative fire performance screening method based on a cone calorimeter is developed to evaluate the flammability properties of the cladding system. Two types of assemblies were tested to highlight the interaction between panel and insulation in cladding systems: i) layer-by-layer and ii) side-by-side. The results showed that the more flammable insulation accelerated the global combustion kinetics in side-by-side assemblies. The presence of insulation to the back of the cladding panel in layer-by-layer assemblies caused a thermal thickening, which slowed the combustion of the front cladding panel. Finally, acceptance criteria are developed to assess the fire risk of cladding systems.

Effect of inorganic salts / lignin on the combustion performance of torrefied biochar

In this study, inorganic salts (KMnO4, Ba(NO3)2) and enzymolysis lignin were added into torrefied biochar to improve the drawback of high ignition temperature during combustion when biomass underwent torrefaction. Weight loss characteristics, gas products, combustion kinetics of torrefied biochar with different additives were investigated. The addition of KMnO4, Ba(NO3)2, enzymolysis lignin, and their blends can reduce the ignition point of torrefied biochar, and the addition of Ba(NO3)2 can also extent the burnout time. When KMnO4, Ba(NO3)2 and lignin were added at the same time, the first weight loss peak temperature was reduced, and the reaction rate was accelerated. The combustion kinetics of torrefied biochar with different inorganic salts/lignin can be described by a simple first-order reaction. The addition of different inorganic salts/lignin additives is beneficial to reduce the activation energy of the low temperature reaction stage, and the activation energy of torrefied biochar with lignin addition decreases the most.

Evaluating combustion kinetics and quantifying fuel-N conversion tendency of shoe manufacturing waste

Combustion is an effective and cost-efficient thermochemical conversion method for solid waste, showing promise for the resource utilization of shoe manufacturing waste (SMW). However, SMW is generally composed of different components, which can lead to unstable combustion and excessive pollutant emissions, especially NOx. To date, combustion characteristics, reaction mechanism and fuel nitrogen (fuel-N) conversion of different SMW components remain unclear. In this work, the combustion behavior of typical SMW components combustion was investigated using Thermogravimetric coupled with Fourier transform infrared spectrum (TG-FTIR). A simplified single-step reaction mechanism was proposed according to the temperature interval to estimate reaction mechanism of SMW. Additionally, the relationship between fuel-N conversion tendency and fuel properties was established. The results indicate that the values for the comprehensive combustion performance index (S) and flammability index (C) range from 1.65 to 0.44 and 3.98 to 1.37, respectively. This demonstrates the significant variability in combustion behavior among different SMW components. Cardboard, leather and sponge have higher values of S and C, suggesting a better ignition characteristic and a stable combustion process. During the combustion of SMW, nitrogen oxides (NO and N2O) are the main nitrogen-containing compounds in the flue gases, with NO being the major contributor, accounting for over 82.97 % of the nitrogen oxides. NO has a negative correlation with nitrogen content, but it is opposite for N2O, HCN and NH3. Furthermore, the conversion of NO, N2O and NH3 is proportional to logarithmic values of O/N, while its conversion to HCN is proportional to logarithmic values of VM/N. These findings facilitate the prediction of the fuel-N conversion of solid waste combustion. This work might shed light on combustion optimization and in-situ pollutant emission control in solid waste combustion.

Characterization and Combustion Kinetics of Binderless and Bindered Dry Cow Dung Bio-Pellets

The effect of molasses addition as a binder in the manufacturing of cow dung bio-pellets on their characteristics and combustion kinetics have been studied. The bio-pellets characterization included the physical and mechanical properties as well as the proximate analysis and calorific values. Thermogravimetric analysis (TGA) was carried out using a macro-TGA apparatus under a non-isothermal conditions and an oxidative atmosphere to study the thermal decomposition characteristics. Then, the first order Coast and Redfern method was used to determined the kinetic parameters of bio-pellets combustion. It was found that the ash content of bio-pellets were tended to decreased, while the volatile matter and fixed carbon were tended to increase with the addition of molasses. Nevertheless, the density, the axial compressive strength and the calorific values of bindered bio-pellets were decreased due to the higher amounts of water in the raw mixtures. Thermogravimetric analysis provided an information that the combustions of cow dung bio-pellets took place in three stages of decompositions The bindered bio-pellet began to decompose at lower temperatures than the binderless bio-pellet with a higher weight loss percentage. According to the comprehensive combustion characteristic index (S), the combustion performance of both binderless and bindered bio-pellets were similar. The addition of molasses as a binder tended to reduce the ignition temperature and activation energy for all stages of bio-pellets combustion.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Unveiling platinum's electronic and dispersion impacts for enhanced benzene combustion: From nanoparticles to nanoclusters and single atoms

Catalytic combustion using platinum (Pt) supported on metal oxides emerges as an eco-friendly approach for benzene removal. However, relationships between Pt dispersion, adsorptive properties, and reactivity across overall Pt size regimes remain unclear, as many previous studies have focused on only one or two Pt size ranges. This study systematically examines the oxidation state, electron density, and resulting adsorptive properties of Pt across a range of sizes, including isolated atoms, sub-nanometer clusters, and nanoparticles. The as-synthesized Pt/Fe2O3 catalysts with defined Pt dispersion and electronic states on faceted Fe2O3 supports. Benzene combustion kinetics are measured over Pt entities from single atoms to sub-nanometer clusters and 2 nm particles, elucidating mechanisms and structure–reactivity links. Findings reveal surprising non-monotonic activity trends that deviate from dispersion features, with an order of: Pt sub-nanometer clusters < Pt single atoms < Pt nanoparticles. Despite high dispersion, single Pt atoms and nanoclusters exhibit lower benzene combustion activity than 2 nm Pt particles on Fe2O3. This reduced activity stems from diminished adsorptive affinity of single atoms and nanoclusters for benzene and O2, tracing back to their oxidized Pt valence states. In contrast, 2 nm Pt particles show excellent benzene and oxygen adsorption capacities enabled by metallic Pt ensembles, thus maximizing reactivity. Additional insights indicate single Pt atoms supported on Fe2O3 avoid competitive benzene/O2 adsorption through a non-competitive mechanism, moderately enhancing performance versus nanoclusters with competitive adsorption. However, Pt valence state has a greater impact than adsorption mechanisms, as Pt nanoparticles are considerably more active than single atoms and nanoclusters. In situ analysis proposes plausible benzene decomposition routes over active Pt sites. The novel findings that fine-tuning Pt dispersion and electronic structure—along with kinetic mechanisms—lead to significant variations in benzene combustion reactivity will assist rational design of efficient Pt catalysts for benzene elimination.

Experimental and modeling study on ignition kinetics of ethyl methyl carbonate

Ethyl methyl carbonate (EMC) is an essential electrolyte of lithium-ion batteries (LIBs), and the spontaneous ignition of electrolyte is considered to be an important inducement for the thermal runaway (TR) combustion of LIBs. To better understand its ignition chemistry and combustion kinetics, the ignition delay times (IDTs) of ethyl methyl carbonate were measured by using a Rapid Compression Machine (RCM) over a wide range of temperatures (810–980 K), pressures (20 and 40 bar), equivalence ratios (φ = 0.5, 1.0, and 2.0), and a fuel concentration of 2 %, yielding results that are in substantial variance with those calculated with detailed kinetics in the literature over the entire range of conditions investigated. An improved chemical kinetic model was developed to simulate the ignition and combustion characteristics of EMC under intermediate-temperature region by employing the updated kinetic parameters. Results show that the overall performance of the present kinetics model is substantially improved with respected to the prediction of ignition delay times, intermediate species profiles, as well as laminar flame speeds across different temperature, pressure, and equivalence ratio conditions. Furthermore, sensitivity and pathway analyses show that the ignition chemistry of EMC is mainly controlled by retro-ene reaction and H abstraction reactions.

Numerical Evaluation of Sodium Droplet Swarm Combustion and Its Modeling Optimization

Because of its superior thermal-hydraulic qualities, liquid sodium has been applied to a variety of industries, including energy storage, solar energy, sodium-cooled fast reactors, and aerospace. However, fires brought on by sodium leaks at high pressure can have major thermodynamic repercussions and put employees and equipment in use at risk directly or indirectly. As a result, a realistic and accurate forecast of the combustion behavior of sodium droplet swarm can offer technical backing for the use of liquid sodium in engineering as well as a way of sodium fire prevention and control. Spray dynamics (droplet settling, droplet particle size distribution, etc.), combustion kinetics (premixed combustion, gas phase combustion, etc.), sodium aerosol diffusion, and other specialized phenomena all contribute to the complex process of sodium droplet swarm combustion. The NACOM code created by Brookhaven National Laboratory for sodium droplet swarm combustion is utilized in this paper as a framework. The code is first validated using the benchmark of the sodium droplet swarm combustion tests carried out by prestigious institutions. The validation results demonstrate that the code’s drag model, droplet combustion model, and heat transfer model are to blame for the significantly overestimated thermodynamic effects of sodium droplet swarm combustion. NACOM is subsequently developed twice for the authors’ previously developed vapor-liquid two-layer-structure drag model, chemical kinetic combustion model, and suitable heat transfer coefficient. It is then thoroughly assessed for the separate-effects tests and integral-effects test. The evaluation results demonstrate that the optimized drag model accelerates the settling of sodium droplets due to the consideration of the sodium-vapor drag reduction effect, reducing the thermodynamic effects of liquid sodium combustion; the optimized premixed combustion model can accurately predict the low-temperature sodium droplet swarm combustion conditions, resolving the issue of serious misvaluation of the original version of NACOM. The associated research findings can serve as valuable resources and tools for deeper comprehension of the combustion effects and mechanisms of sodium droplet swarm under various operating settings (such as leakage rate and oxygen concentration).

METHODOLOGY FOR STUDYING THE COMBUSTION OF SOLID AND LIQUID SUBSTANCES A CLOSED VOLUME

The development of new methods for testing substances and materials is a relevant area that allows to assess the fire hazard of substances, including modern materials, the composition of which is constantly improving and changing. There are and are created special conditions in the premises, under which combustion acquires a completely different character. An example of such conditions can be combustion in a closed volume, when there is no oxygen access and no energy exchange with the environment. We present a technique that allows us to study the combustion of solids and liquids in a closed volume and to control the parameters of the gas medium. The purpose of the technique is to study the kinetics of combustion of solids and liquids in a closed volume by controlling the parameters and investigating their influence on each other (temperature of the gas medium, reduction of light transmission of combustion products, total oxygen concentration, mass loss of samples). A description of the three stages and the order of their realization is given. The laboratory bench developed by the author is briefly described. The results of the research are supposed to be used to expand the understanding of the nature and kinetics of combustion of substances and materials in a closed volume, as well as to assess the fire hazard of substances whose combustion can occur in a closed volume.

Tracking the reaction networks of acetaldehyde oxide and glyoxal oxide Criegee intermediates in the ozone-assisted oxidation reaction of crotonaldehyde.

The reaction of unsaturated compounds with ozone (O3) is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs with different functionality, i.e. acetaldehyde oxide (CH3CHOO) and glyoxal oxide (CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde (CA), is investigated over a temperature range between 390 K and 840 K in an atmospheric pressure jet-stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5 with a mixture of 1% crotonaldehyde, 10% O2, at an fixed ozone concentration of 1000 ppm and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum-ultraviolet (VUV) radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of CA already at 390 K, which is below the temperature where the oxidation reaction of CA was observed in the absence of ozone. The observed CA + O3 product, C4H6O4, is found to be linked to a ketohydroperoxide (2-hydroperoxy-3-oxobutanal) resulting from the isomerization of the primary ozonide. Products corresponding to the CIs uni- and bi-molecular reactions were observed and identified. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs bimolecular reactions with species like aldehydes, i.e., formaldehyde, acetaldehyde, and crotonaldehyde and alkenes, i.e., ethene and propene. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH), and ethyl hydroperoxide (C2H5OOH). Detection of additional oxygenated species such as alcohols, ketene, and aldehydes are indicative of multiple active oxidation routes. This study provides important information about the initial step involved in the CIs assisted oligomerization reactions in complex reactive environments where CIs with different functionalities are reacting simultaneously. It provides new mechanistic insights into ozone-assisted oxidation reactions of unsaturated aldehydes, which is critical for the development of improved atmospheric and combustion kinetics models.

Theoretical and kinetic study of the H-atom abstraction reactions by Ḣ atom from alkyl cyclohexanes.

Reaction kinetics of hydrogen atom abstraction from six alkyl cyclohexanes, methyl cyclohexane (MCH), ethyl cyclohexane (ECH), n-propyl cyclohexane (nPCH), iso-propyl cyclohexane (iPCH), sec-butyl cyclohexane (sBCH) and iso-butyl cyclohexane (iBCH), by the Ḣ atom are systematically studied in this work. The M06-2X method combined with the 6-311++G(d,p) basis set is used to perform geometry optimization, frequency analysis and zero-point energy calculations for all species. The intrinsic reaction coordinate (IRC) calculations are performed to confirm the transition states connecting the reactants and products correctly. One-dimensional hindered rotors are used to treat the low frequency torsional models with potentials scanned at the M06-2X/6-31G level of theory. Electronic single-point energy calculations for all reactants, transition states, and products are performed at the QCISD(T)/CBS level of theory. High-pressure limiting rate constants of 39 reaction channels are obtained using conventional transition state theory with asymmetric Eckart tunneling corrections in the temperature range 298.15-2000 K. Reaction rate rules for H-atom abstraction by the Ḣ atom from alkyl cyclohexanes on primary, secondary and tertiary carbon sites on both the side chain and ring are provided. The obtained rate constants are given by the Arrhenius expression in the temperature range 500-2000 K, which can be used for the combustion kinetics model development for alkyl cyclohexanes.

EFFECTS OF FLUORINATED AND NON-FLUORINATED ADDITIVES ON BURNING RATES OF BORON-TEFLON BLENDS FOR SOLID ROCKET PROPULSION

Boron is considered a promising metal fuel for energetic propellants due to its high energy density, but in practice, boron experiences slow combustion kinetics. Fluorinated additives can improve boron combustion. The objective of this study was to determine how different fluorinated additives would affect the burn rate of a boron-based fuel in a high-pressure inert environment. Boron powder was sintered with polytetrafluoroethylene (PTFE) powder to create test pellets with variable fluorine:boron mass ratios. Three different purities of boron powders were tested, along with three different fluoropolymer additives and 14 other additives, which varied in molecular morphology and fluorination. The base boron-PTFE test pellets with no additives have a maximum burning rate at fluorine:boron mass ratio of &amp;#126; 3.5, which remains the same even with additives at 20 wt&amp;#37;. A commercial fluoropolymer, Aquivion, showed significant burn rate enhancement at 20 wt&amp;#37; but not at 5 wt&amp;#37;. At this lower loading level, materials that tend to enhance thermal conductivity (graphene, nanotubes) show some burn rate enhancement, even without fluorination. However, very significant burn rate enhancement is shown by a fluorinated carbon nanotube, likely due to the reduced bond dissociation energy of fluorine when attached to a nanotube. The data suggest this method of fluorination can have an important effect on enhancing boron combustion, and even small additives with readily available fluorine could significantly improve boron combustion rates for solid propellant applications.

Effect of non-ideal mixture on flame–spray interaction in counterflow n-heptane/ethanol flames

This work presents a numerical study of bi-component n-heptane/ethanol spray flames in a counterflow to improve the understanding of the effect of non-ideal mixtures on the flame–spray interaction. The simulations of this spray configuration allow for the detailed examination of the real fluid behavior and its influence on gas-phase combustion kinetics for a wide range of relevant parameters, i.e., droplet composition, size, and gas strain rate. The results reveal that due to the large real activity coefficient of n-heptane and the resulting selective evaporation of the heavier component, the burning characteristics of the spray with a high initial ethanol content do not differ if non-ideal mixture is considered. Conversely, the flame structure with a low ethanol content is substantially different, and displacement of the flame towards the spray side of the configuration is found. A multiple reaction zone, displaying both partially-premixed and diffusion flames, exists, which is overestimated spatially if the fluid is assumed ideal. In general, the ideal prediction of real fluid would misinterpret the composition evaporation order, prolonging the droplet lifetime and consequently, enhancing the flame–spray interaction locally, which results in a lower flame temperature. A parameter characterizing the multicomponent droplet penetration is introduced, which highlights the appreciable non-ideal effects that vary with droplet size and composition, and most importantly, further demonstrates the necessity of considering the real fluid properties in design of next-generation engines with blended fuels.

Dynamics of atomic oxygen production in an NH3/air flame assisted by a nanosecond pulsed plasma discharge

Atomic oxygen (O) is vital in plasma-assisted combustion, but its quantitative measurement is challenging and therefore rarely reported. This work performs femtosecond two-photon-absorption laser-induced fluorescence (fs-TALIF) imaging to quantify the dynamics of atomic oxygen production by a single nanosecond (ns) pulsed plasma discharge in an ammonia/air flame for the first time. First of all, the plasma kinetic enhancement is quantified by the plasma-produced O with a molar fraction of up to 2.8 × 10−3, which is at least one order higher than that in the flame without plasma. Then, the spatial distribution of O demonstrates significant differences between the unburnt and burnt zones, revealing the distinct mechanisms of O production in unburnt and burnt NH3/air mixtures. Specifically, a new O region is formed in the burnt zone of the lean flame, which is probably mainly caused by the pathway of NO dissociation into O by the ns plasma. Furthermore, the temporal dynamics of O in different zones are discussed in-depth. In the unburnt zone, from the continuous increment of O after hundreds of ns following the ns discharge initiation, it can be seen that the relaxation of the excited nitrogen plays an equally important role in O production as the direct electron impact. Subsequently, O atoms in the unburnt zone exhibit an exponential decay, which is attributed to the formation of O3 from O. Whereas, in the burnt zone, a bi-exponential decay of O is found for the first time. Such decay mainly results from the reaction between O and H2O, which importantly, is a reaction to form OH radicals. The transition between decays is due to the formation of the O region in the unburnt zone and the dominance of the combustion kinetics in the post-plasma stage.

Co-combustion characteristics and kinetics of sludge/coal blends in 21%–50% oxygen-enriched O2/CO2 atmospheres

This research investigated the thermogravimetric analysis (TGA) and combustion kinetics of sewage sludge and Australian sub-bituminous coal and their blends with different mixing ratios (sludge/coal = 100/0, 80/20, 50/50, 20/80, 0/100 by weight) under air atmosphere (O2/N2 = 21/79 by volume), oxy-fuel combustion (O2/CO2 = 21/79, 25/75, 35/65, and 50/50 by volume), and pyrolysis (pure N2 atmospheres). The gaseous products that evolved from the TGA experiments were also analyzed using a Fourier transform infrared (FTIR) spectrometer under pyrolysis conditions. Results showed that the mixed fuel with higher coal blending ratio had higher ignition temperature, lower burnout temperature and larger combustion characteristic index. Furthermore, TGA–FTIR experiments of the 20 % sludge + 80 % coal blended fuel under nitrogen atmospheres showed more gasification components in pure coal than in sludge. The combustion kinetics analytical results via the Flynn–Wall–Ozawa method indicated that the larger the conversion degree, the smaller the obtained activation energy value. Under the oxy-fuel combustion conditions, when the oxygen concentration increased from 21 % to 50 % for the 20 % sludge + 80 % coal blended fuel, the ignition and burnout temperatures decreased, and the combustion characteristic index increased nearly three times. Oxygen enrichment can clearly improve combustion characteristics and flammability.

A revised reaction kinetic mechanism for the oxidation of methyl formate

The combustion kinetics of methyl formate (MeFo), a promising oxygenated, highly knock-resistant e-fuel, is investigated in this work. The potential energy surfaces (PESs) of the low-temperature oxidation reactions of MeFo radicals were determined at the CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ level of theory and their rate coefficients were obtained with RRKM/master equation. The thermochemical properties of relevant intermediate species were calculated by using the G4 method. A chemical mechanism for the oxidation of MeFo was developed by adding fuel-specific reactions taken from a literature model into a well-validated base mechanism, and incorporating the newly calculated rate constants of elementary reactions and thermochemical properties of species. The fuel-specific mechanism was further revised in terms of reaction channels and rate coefficients for improved prediction accuracy. In particular, the O2 addition to hydroperoxy radicals (QOOH), the isomerization of peroxy hydroperoxy radicals (OOQOOH), the subsequent ketohydroperoxide (KET) decomposition, and the formally direct pathways of MeFo radicals (R) with O2 were added for the completeness of consumption pathways, in conjunction with the reaction channels of alkoxy radicals (RO) and hydroperoxides (ROOH). For good model prediction accuracy of ignition delay times, especially in the high-temperature range, the resulting mechanism was optimized by modifying the rate constants of several important H-atom abstraction reactions of MeFo within their uncertainty limits. The performance of the proposed mechanism is demonstrated through extensive validation against literature data obtained from different experimental configurations. Finally, the underlying reaction pathways of MeFo are explored by means of reaction flux analysis with the newly developed chemical mechanism.Novelty and significance statement: This work proposes a new reaction kinetic mechanism for the oxidation of methyl formate. Quantum chemistry calculation at high level of theory was performed to explore the low-temperature chemistry of MeFo. The reaction mechanism was improved by including missing reaction channels, incorporating theoretically calculated rate and thermochemical data, and modifying rate constants of sensitive reactions. The mechanism has been validated successfully against extensive data available in the literature covering a wide range of conditions and facilities. By using this new mechanism, reaction pathway analysis is performed to provide insights into the oxidation chemistry of MeFo.

Study of combustion characteristics of diesel-vegetable oil blends utilizing an industrial fuel burner

This study delves into the combustion characteristics of diesel-vegetable oil blends using an industrial fuel burner, shedding light on essential factors that impact the viability of these alternative fuels in industrial applications. Viscosity, a key concern in vegetable oil-based fuels, can be effectively mitigated by blending with diesel. This viscosity reduction enhances fuel atomization, optimizing combustion efficiency and mitigating nozzle blockages. The study also explores the density variation in these blends, indicating potential implications for combustion kinetics and injection dynamics. Furthermore, the research addresses the trade-off between energy content and viscosity reduction as the calorific value decreases with an increasing volume ratio of vegetable oil. Flame behavior, crucial for combustion system design, exhibits an inverse relationship with the volume ratio of vegetable oil, resulting in shorter and less intense flames at higher vegetable oil content. Through comprehensive experiments, the study demonstrates that increased vegetable oil content leads to reduced flame length and stability, primarily attributed to the elevated density and viscosity of vegetable oil. A comparative analysis highlights the similarity in combustion properties between a 40 % vegetable oil and a 60 % diesel blend, which exhibited a kinematic viscosity of approximately 1.58 cP emphasizing the potential of vegetable oil as a viable substitute for diesel in industrial fuel burners.

A novel machine learning based lumping approach for the reduction of large kinetic mechanisms for plasma-assisted combustion applications

The development of skeletal mechanisms has become essential for multi-dimensional simulations of plasma-assisted combustion (PAC). However, reduction tools developed for traditional combustion applications are not always applicable to PAC, due to the complex interplay between non-equilibrium plasma and combustion kinetics. Plasma direct relation graph with error propagation (P-DRGEP) is a recent plasma-specific reduction method developed in order to incorporate plasma energy branching in the reduction. In the first part of this work, the applicability of P-DRGEP to large kinetic mechanisms is investigated. A detailed isooctane/air plasma mechanism containing 2805 species and 18457 reactions is reduced to 415 species and 4716 reactions, keeping errors on ignition time within 3% for a wide range of initial conditions: from 750 K to 1200 K, 10 atm and equivalence ratios from 0.75 to 1.50. The second part focuses on isomer lumping, which is another reduction technique widely used in combustion. When applied to PAC, it is shown that the resulting lumped mechanism produces poor results. A novel plasma-specific isomer lumping strategy using machine learning is proposed instead. With the supervised algorithm of gradient boosting, predictive regression models are generated, which describe rate coefficients of lumped reactions adequately. These models are trained with simulation data. Leveraging this newly proposed lumping approach on the reduced mechanism, allows for an additional 28% reduction in the number of species and 19% reduction in the number of reactions. Two different versions are presented: in the first one the models are trained using one input feature (1D), while in the second one, two input features are selected (2D). The resulting lumped mechanism is shown to produce accurate predictions of PAC over the entire parameter space of interest, while significantly decreasing the computational time. Indicatively, with the 1D version the maximum error on ignition time in this range of conditions is 6%. The 2D approach produces even lower errors, which do not exceed 3%.Novelty and significance statementIn this work, a novel approach for isomer lumping, in plasma-assisted combustion mechanisms, is demonstrated. This plasma-specific approach, uses predictive machine learning regression models to describe the complex evolution of lumped reaction rate coefficients. Combining it with the plasma direct relation graph with error propagation, a powerful reduction framework is created, which is successfully demonstrated on a detailed isooctane/air plasma kinetic mechanism, via zero-dimensional ignition simulations. This framework constitutes a useful tool towards the creation of highly accurate skeletal mechanisms, which significantly reduce the computational costs of simulations.