Chemical Kinetic Calculations Research Articles

Calibrating the Livengood–Wu integral knock model for differently sized methanol engines

Experimental test campaigns have begun to demonstrate the potential of methanol as an alternative fuel for heavy-duty spark-ignited engines. However, there is no consensus yet on the scope of this solution in terms of maximum power and engine size. A zero-dimensional combustion model is therefore being developed outside the scope of this work. Its main objective will be to predict key performance parameters such as power and efficiency as function of engine size. Due to the high loads typically encountered in heavy-duty engines, knock will be the main constraint to maximize the engine's potential. This work therefore aims to find an accurate knock model that can be implemented in the modelling framework. The Livengood–Wu knock integral model is being considered as a good candidate, as it is computationally inexpensive and thus allows for a large number of engine configurations to be modelled within a reasonable time. Due to a lack of autoignition delay times of methanol at conditions relevant to heavy-duty engines, a large database was created using chemical kinetics calculations. A neural network model was trained with the tabulated data for fast data retrieval. To validate whether the knock integral approach is robust enough to be applied to a wide range of engine sizes, a calibration constant was added to match the knock predictions to experimental data. Its value was calculated for three different engines, a light and heavy-duty SI engine and a large-bore dual-fuel engine. They highlight a remarkable difference in calibration constant across the different engines investigated.

Experimental study and kinetic analysis of NO emission characteristics in ammonia/coal co-firing process with different ammonia injection methods

Ammonia/coal co-firing in coal-fired power plants is an effective way to achieve CO2 reduction. In this paper, the ammonia/coal co-firing study was carried out in a 45 kW wall-temperature-controlled staged combustion downstream furnace at 1100 °C. Bituminous coal was used as the experimental object to analyze the effects of ammonia injected at the main combustion zone, reduction zone, with the burnout air, and at the burnout zone on NO release characteristics at different ammonia co-firing ratios. To ascertain the significant reaction of NO production in the ammonia/coal co-firing, chemical reaction kinetics calculations were performed. When co-firing ratios of 10∼30%, the NO emission concentration is lower by the ammonia injection position of the main combustion zone which is 275–346 mg/m3. When co-firing ratios reach to 40% and 50%, NO releases are lower by the ammonia injection position of burnout zone which is 511–574 mg/m3. In addition to exhibiting a greater capacity to decrease NO in environments with low oxygen concentrations and high NO concentrations, ammonia can also, under specific circumstances, have a significant capacity to decrease NO in environments with high oxygen concentrations. Based on the experimental results and chemical kinetic calculations, the NO release pattern and mechanism during ammonia-coal co-combustion with different ammonia injection methods under air staged combustion were investigated, which provides a technically feasible solution to reduce CO2 and NOx emissions from coal-fired power plants.

Combustion and emission characteristics of a gasoline/ammonia fueled SI engine and chemical kinetic analysis of NOx emissions

Mitigating climate change involves the greater adoption of carbon–neutral and renewable energy sources within the transportation sector. Ammonia (NH3), as a carbon-free and sustainable fuel, has garnered growing interest in recent years. The present study aims to investigate the impact of NH3 blending on combustion and emission characteristics of a stoichiometric spark-ignition gasoline engine, with a particular emphasis on nitrogen-based emissions. The experimental investigation was complemented by chemical kinetic calculations. The results showed that NH3 blending could effectively suppress engine knock, optimize combustion phase and improve thermal efficiency. For pure gasoline, advancing the spark timing resulted in increased NOx emissions. However, when NH3 was blended, NOx emissions decreased with advancing spark timing, indicating a negative correlation with pressure. The NH3 emission was attributed to the ‘crevice mechanism’ as well as the absorption/desorption in the lubricant oil film on the cylinder wall. Chemical kinetic analysis revealed that the NOx emission from NH3 blended combustion is closely related to reactive radicals such as OH, H and O. The reduction in NOx emissions under high-pressure conditions was primarily attributed to the consumption of these reactive radicals via three-body reactions. Interestingly, NOx emissions initially increased with increasing NH3 blending ratio but eventually followed a decreasing trend. This can be attributed to the lower combustion temperature, lower concentration of reactive radicals, and enhanced de-NOx reactions.

Theoretical Insights into the Gas-Phase Oxidation of 3-Methyl-2-butene-1-thiol by the OH Radical: Thermochemical and Kinetic Analysis.

3-Methyl-2-butene-1-thiol ((CH3)2C═CH-CH2-SH; MBT) is a recently identified volatile organosulfur compound emitted from Cannabis sativa and is purported to contribute to its skunky odor. To understand its environmental fate, hydroxyl radical (•OH)-mediated oxidation of MBT was conducted using high-level quantum chemical and theoretical kinetic calculations. Three stable conformers were identified for the title molecule. Abstraction and addition pathways are possible for the MBT + OH radical reaction, and thus, potential energy surfaces involving H-abstraction and •OH addition were computed at the CCSD(T)/aug-cc-pV(T+d)Z//M06-2X/aug-cc-pV(T+d)Z level of theory. The barrier height for the addition of the OH radical to a C atom of the alkene moiety, leading to the formation of a C-centered MBT-OH radical, was computed to be -4.1 kcal mol-1 below the energy of the starting MBT + OH radical-separated reactants. This reaction was found to be dominant compared to other site-specific H-abstraction and addition paths. The kinetics of all the site-specific abstraction and addition reactions associated with the most stable MBT + OH radical reaction were assessed using the MESMER kinetic code between 200 and 320 K. Further, we considered the contributions from two other conformers of MBT to the overall reaction of MBT + OH radical. The estimated global rate coefficient for the oxidation of MBT with respect to its reactions with the OH radical was found to be 6.1 × 10-11 cm3 molecule-1 s-1 at 298 K and 1 atm pressure. The thermodynamic parameters and atmospheric implications of the MBT + OH reaction are discussed.

Study on the characteristics parameters of magnesium dust explosion suppression by various inert gases

Alongside the extensive use of magnesium, dust explosion accidents have become increasingly frequent. Conducting research on suppressing magnesium dust explosions is of great significance for the healthy development of the magnesium industry. In this paper, the effects of four inert gases (CO2, N2, Ar, and He) on the explosion characteristics parameters of magnesium dust clouds with different concentrations were investigated using a closed explosion device. It was observed that at low volume fractions of inert gases in the explosion system, there were no significant differences in their effects on the explosion characteristics parameters and the combustion time of magnesium particles. In situations where there was a high level of oxygen, N2 was more effective than Ar in suppressing explosions. However, when the concentration of magnesium dust cloud was higher and the oxygen content was lower within the hybrids, N2 reacted with magnesium dust that was unable to react with oxygen. As a result, the explosion suppression effect gradually shifted from N2 being better than Ar to Ar being better than N2. Moreover, the differences in the effects of four inert gases on particle combustion time became increasingly prominent. However, in all operating conditions, CO2 demonstrated the most effective suppression of magnesium dust explosions, while He exhibited the least effective suppression effect. Furthermore, by employing a chemical reaction kinetics calculation model, it has been discovered that the content of inert gases significantly affected the combustion time and reaction mode of particle combustion, thereby causing variations in the propagation behavior of magnesium dust cloud flames.

An integrated 0D/1D/3D numerical framework to predict performance, emissions, knock and heat transfer in ICEs fueled with NH3–H2 mixtures: The conversion of a marine Diesel engine as case study

In the maritime transportation, e-fuels represent a valid alternative to fossil energy sources, in order to accomplish the European Union goals in terms of climate neutrality. Among the e-fuels, the ammonia-hydrogen mixtures can play a leading role, as the combination of the two allows to exploit the advantages of each one, simultaneously compensating their gaps.The main goal of the present publication is the proposal of a robust numerical framework based on 0D, 1D and 3D tools for CFD analyses of internal combustion engines fueled with ammonia-hydrogen mixtures.The 1D engine model provides boundary conditions for the multi-dimensional investigations and estimates the overall engine performance. 3D in-cylinder detailed analyses are proficiently used to predict combustion efficiency (via the well-established G-equation model supported by laminar flame speed correlations for both ammonia and hydrogen) and emissions (with a detailed chemistry based approach). Heat transfer and knock tendency are evaluated as well, by in-house developed models. As for the 0D/1D chemical kinetics calculations, firstly they support 3D analyses (for example via the generation of ignition delay time tables). Moreover, they allow insights on aspects such as NOx formation, to individuate mixture qualities able to strongly reduce the emissions.The present paper offers also indications for designers on the effectiveness of NH3–H2 as valid substitutes of traditional fuels in marine applications. In fact, the proposed framework is preliminary tested to convert an existing marine Diesel engine to ammonia-hydrogen mixtures. Different proportions (among the two fuels) are investigated, included pure ammonia and pure hydrogen and the NH3–H2 80mol%-20mol% mixture is selected as the best compromise. Thanks to both the anti-knock quality of the ammonia and the (limited) addition of hydrogen as combustion enhancer, the engine supplied with NH3–H2 mixtures is able to operate free of knock and keep the same performance as the original one. Simultaneously (neglecting the presence of lubricant oil in the combustion chamber) unburned hydrocarbons, carbon monoxide and carbon dioxide are eliminated. Still in comparison with the Diesel version, the heat transfer is similar, the NOx formation is much greater and the engine range (here evaluated in terms of working hours) noticeably reduces.

Experimental Study on Flammability Limits Behavior of Methane, Ethane, and Propane with Dilution of Nitrogen.

The dilution inerting process of multi-component flammable gaseous mixtures is an important emergency disposal technology that has been widely applied in the explosion-proof field of flammability gases (vapors). In this study, we examined the flammability limits (LFLs and UFLs) of mono and binary alkane mixtures of methane, ethane, and propane when nitrogen is used for dilution inerting. The HY12474B explosion limit test device was used to determine the flammability limits, and the obtained data were compared with the literature data and Chatelier's law. Additionally, the sensitivity coefficient of the chemical reaction chain for LFLs and UFLs of the binary alkane mixtures was analyzed. The minimum inerting concentration (MIC) of methane was found to be sequentially higher than that of ethane and propane when using nitrogen for dilution inerting, and the MIC of the binary alkane mixtures follows the rule of methane/ethane > methane/propane > ethane/propane. Chemical kinetics calculation revealed that the maximum positive sensitivity coefficient of methane/ethane, methane/propane, and ethane/propane are both R5 H + O2 ↔ O + OH, and the reaction with the maximum negative sensitivity coefficients are both R34 H + O2(+M) ↔ HO2(+M) and R43 CH3 + H(+M) ↔ CH4(+M), respectively. The limiting oxygen concentration (LOC) for both mono alkane and binary alkane mixtures ranged between 10 and 13%. The region of the triangular flammability diagram for methane and ethane was greater than the regions for methane/ethane and methane/propane. In contrast, propane had a smaller region compared to other mono alkane or binary alkane mixtures.

Accelerating Chemical Kinetics Calculations With Physics Informed Neural Networks

Abstract Detailed chemical kinetics calculations can be very computationally expensive, and so various approaches have been used to speed up combustion calculations. Deep neural networks (DNNs) are one promising approach that has seen significant development recently. Standard DNNs, however, do not necessarily follow physical constraints such as conservation of mass. Physics Informed Neural Networks (PINNs) are a class of neural networks that have physical laws embedded within the training process to create networks that follow those physical laws. A new PINN-based DNN approach to chemical kinetics modeling has been developed to make sure mass fraction predictions adhere to the conservation of atomic species. The approach also utilizes a mixture-of-experts (MOE) architecture where the data is distributed on multiple subnetworks followed by a softmax selective layer. The MOE architecture allows the different subnetworks to specialize in different thermochemical regimes, such as early stage ignition reactions or post-flame equilibrium chemistry, then the softmax layer smoothly transitions between the subnetwork predictions. This modeling approach was applied to the prediction of methane-air combustion using the GRI-Mech 3.0 as the reference mechanism. The training database was composed of data from 0D ignition delay simulations under initial conditions of 0.2–50 bar pressure, 500–2000 K temperature, an equivalence ratio between 0 and 2, and an N2-dilution percentage of up to 50%. A wide variety of network sizes and architectures of between 3 and 20 subnetworks and 6,600 to 77,000 neurons were tested. The resulting networks were able to predict 0D combustion simulations with similar accuracy and atomic mass conservation as standard kinetics solvers while having a 10-50× speedup in online evaluation time using CPUs, and on average over 200× when using a GPU.

The atmospheric oxidation mechanism of acetophenone initiated by the hydroxyl radicals

Acetophenone (C8H8O, AP) is a common indoor air component but with no information on its atmospheric oxidation. The atmospheric oxidation mechanism of AP in gas phase, initiated by its reaction with the OH radical, is investigated here using quantum chemistry and chemical kinetics calculations. The reaction starts mainly by OH additions to the aromatic ring, and the predicted rate coefficient suggests a lifetime of ∼8 days in the atmosphere. The additions form adducts AP-i-OH (Ri) (i = 1, 2, 3, 4, the position on the aromatic ring). The adducts Ri would react with O2 or undergo unimolecular decompositions. The adduct R1 reacts with O2 rather slowly with an effective rate coefficients of <10−20 cm3 molecule−1 s−1, and it would alternatively decompose to phenol and acetyl radical. The dominate fate of R2, R3, and R4 in their reaction with O2 proceeds via direct hydrogen-atom abstraction, forming hydroxyacetophenones, and only R3 would have a fraction of bicyclic radical formation as in oxidation of alkyl benzenes. The main products from oxidation of AP include phenol and three isomers of hydroxyacetophenones, suggesting that oxidation of AP by OH would reduce its unpleasant effect to human beings.

Growth and Evolution of Secondary Volcanic Atmospheres: 2. The Importance of Kinetics

AbstractVolcanism is a major and long‐term source of volatile elements such as C and H to Earth's atmosphere, likely has been to Venus's atmosphere, and may be for exoplanets. Models simulating volcanic growth of atmospheres often make one of two assumptions: either that atmospheric speciation is set by the high‐temperature equilibrium of volcanism or that volcanic gases thermochemically reequilibrate to the new, lower, temperature of the surface environment. In the latter case, it has been suggested that volcanic atmospheres may create biosignature false positives. Here, we test the assumptions underlying such inferences by performing chemical kinetic calculations to estimate the relaxation timescale of volcanically derived atmospheres to thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry and reaction catalysis. We demonstrate that for planets with volcanic atmospheres, thermochemical equilibrium over geological timescales can only be assumed if the atmospheric temperature is above ∼700 K. Slow chemical kinetics at lower temperatures inhibit the relaxation of redox‐sensitive species to low‐temperature thermochemical equilibrium, precluding the production of two independent biosignatures through thermochemistry alone: 1. ammonia and 2. the cooccurrence of CO2 and CH4 in an atmosphere in the absence of CO. This supports the use of both biosignatures for detecting life. Quenched at the high temperature of their degassing, volcanic gases also have speciations characteristic of those produced from a more oxidized mantle, if interpreted as being at thermochemical equilibrium. This therefore complicates linking atmospheres to the interiors of rocky exoplanets, even when their atmospheres are purely volcanic in origin.

Theoretical Study of the Atmospheric Chemistry of Methane Sulfonamide Initiated by OH Radicals and the CH3S(O)2N•H + 3O2 Reaction

In the present work, we have revisited the reaction mechanism of the atmospheric oxidation of methane sulfonamide (CH3S(═O)2NH2; MSAM) initiated by hydroxyl (OH) radicals in the gas phase. The present reaction has been studied for the first time using quantum calculations combined with chemical kinetic modeling. The abstraction of an H-atom from the -NH2 group of MSAM by OH radical to form the products CH3S(═O)2N•H + H2O was found to be a major path with a barrier height of ∼2.3 kcal mol-1 relative to the energy of the separated MSAM + •OH starting reactants. This study is the first to identify the reaction of MSAM with •OH as exclusively generating N-centered MSAM radicals. The chemical kinetic calculations for various paths associated with the MSAM + •OH reaction were performed under pre-equilibrium approximation conditions using canonical variational transition state theory, employing the small curvature tunneling method in the temperature range of 200-400 K. A recent experimental study reported that OH radical-mediated degradation of MSAM proceeds via the formation of the C-centered MSAM radical (•CH2S(═O)2NH2) product. However, the energetics and rate coefficient calculations in the present work suggest that the formation of the N-centered MSAM radical is a major path compared to that which proceeds via the C-centered MSAM radical. The overall rate coefficient for the MSAM + •OH reaction was calculated in the 200-400 K temperature range. The overall rate coefficient for the MSAM + •OH reaction was estimated to be k = 1.2 × 10-13 cm3 molecule-1 s-1 at 298 K. This rate coefficient at 298 K agrees well with the reported experimental value (1.4 × 10-13 cm3 molecule-1 s-1) at the same temperature. We also provide branching ratios for each path associated with the MSAM + •OH reaction. In addition, the atmospheric implications for the title reactions are discussed. The oxidation mechanism of the MSAM + •OH reaction suggests that the formed CH3S(═O)2N•H further reacts with atmospheric oxygen (3O2) to form the corresponding RO2 radical adduct. The downstream products of the CH3S(═O)2N•H + 3O2 reaction in the present work indicate that sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2), nitric acid (HNO3), nitrous oxide (N2O), and formic acid [HC(O)OH] are formed as final products.

Influence of H2 blending on NOx production in natural gas combustion: Mechanism comparison and reaction routes

Hydrogen blending in natural gas is an effective way of carbon reduction. The effect of hydrogen blending on the reaction pathway of NOx emission from natural gas combustion in industrial furnaces is studied by chemical kinetic calculations. To ensure the reliability of kinetic calculation, the NOx prediction accuracy of the mechanisms GRI and PG2018 were compared by validation with corresponding experiments. It is found that the PG2018 mechanism is more accurate for NOx prediction than GRI 2.11 and GRI 3.0, especially for N2O-intermediate and NNH pathways. The effect of hydrogen blending on the chemical pathways of NOx production was obtained based on PSR reactor and burner stabilized laminar flame with a well-controlled temperature profile. The results show that in the PSR reactor, N2O-intermediate and thermal NOx are predominating pathways for NOx formation. NOx emissions of unit fuel heating value first rise with the increase of blending ratio and then reduces rapidly while the blending ratio is greater than 50%, which is mainly due to the suppression of thermal NOx and the increase of N2O-intermediate NOx. It can be found from calculation results of burner stabilized flame that the prompt NOx decreases dramatically in the laminar flame and the NOx production via NNH increases slightly as hydrogen blending ratio increases. With increasing hydrogen blending ratio, thermal NOx is suppressed, and the N2O-intermediate pathway is suppressed also at the flame sheet but enhanced in the flue gas due to the increment of the H2O content. NOx emission decreases with increasing hydrogen blending ratio in laminar flame at a given combustion temperature.

Experimental study on effects of air-staged strategy and NH3 co-firing ratios on NO formation characteristics in ammonia/coal co-firing process

Ammonia/coal co-firing in coal-fired power plants is an effective and promising route to achieve CO2 reduction. This paper studies ammonia/coal co-firing in a 45 kW wall-temperature controlled staged combustion furnace employing overall air-staged combustion technology to meet the goal of low carbon and low NOx emissions. The effect of different burnout air addition positions and combustion environmental temperatures on the NO emissions, as well as the CO and H2S release characteristics of ammonia/coal co-firing was studied for ammonia co-firing ratios ranging from 0 % to 100 % (calorific value), with bituminous coal as the experimental object. To offer a qualitative overview of the experimental results, chemical reaction kinetic calculations were performed. The position of the burnout air addition has significant effects on the ammonia/coal co-firing products. The further the burnout air addition position is from the burner outlet, the lower the NO emission. When the distance is too great, however, the improvement in NO reduction efficiency is limited, which might have a negative impact on combustion efficiency. At 1300 °C, the NO emission concentration is substantially higher due to the deterioration of SNCR (selective non-catalytic reduction) capabilities of NH3 at high temperatures. High temperatures also reduce coal char production, which results in less heterogeneous reduction reactions on char surface. Based on experimental results and chemical kinetic calculations, the release patterns and mechanisms of combustion products during ammonia/coal co-firing under various air-staged strategies were explored in this work, presenting a promising technically achievable solution for reducing CO2 and NOx emissions from the coal-fired power plants.

Experimental-Based Laminar Flame Speed Approximation Formulas of Efficiency-Optimized Biofuels for SI-Engine Modeling

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The transition towards sustainable mobility encourages research into biofuels for use in internal combustion engines. For these alternative energy carriers, high-fidelity experimental data of flame speeds influenced by pressure, temperature, and air-fuel equivalence ratio under engine-relevant conditions are required to support the development of robust combustion models for spark-ignition engines. E.g., physicochemical-based approximation formulas adjusted to the fuel provide similar accuracy as high fidelity chemical kinetic model calculations at a fraction of the computational cost and can be easily adopted in engine simulation codes. In the present study, a workflow to enable predictive combustion engine modeling is applied first for a gasoline reference fuel and two biofuel blends recently proposed by Dahmen and Marquardt [Energy Fuels, 2017]. They identified one promising high-octane rating biofuel blend, expected to be optimized for SI combustion engines, and one promising low carbon high energy density blend with an optimized production pathway. The first blend consists of ethanol, 2-butanone, cyclopentane, and cyclopentanone, and the second blend consists of 1-butanol, ethanol, and cyclopentane. In the present study, the reference fuel RON95 E10 and both biofuel blends were experimentally examined for their flame speed in RWTH-ITV’s closed combustion chamber at 423 K and 2.5 bar, with equivalence ratios (&lt;i&gt;Φ&lt;/i&gt;) ranging from 0.8 to 1.3. Then, pressure (1 atm and 5 bar) and temperature variations (398 K and 450 K) were conducted for the blends at &lt;i&gt;Φ&lt;/i&gt; = 1.1. Due to its good agreement with the experimental results, a detailed kinetic mechanism was selected and used for comprehensive flame speed calculations at engine conditions. The approximation formula was parametrized in the next step, showing good agreement with the detailed calculations. Finally, the flame speed model is adopted for engine simulations, and the 0-2% burn duration of gasoline is used as a benchmark against engine data, showing the improved predictability of the newly derived approximation compared to a standard correlation. The biofuels’ burn durations indicate slight improvements due to higher flame speeds.&lt;/div&gt;&lt;/div&gt;

Experimental study and kinetic analysis of the impact of ammonia co-firing ratio on products formation characteristics in ammonia/coal co-firing process

Ammonia is a carbon-free, hydrogen-rich fuel, which is considered as an alternative energy source with high utilization potential. Ammonia/coal co-firing in air-staged coal-fired boilers has attracted the attention of a number of academics in order to meet the aim of low carbon and NOx emissions. In this paper, ammonia/coal co-firing experiments were conducted under air-staged condition in a one-dimensional combustion-temperature controlled down-fired furnace with multi-path air inlets at 1100℃. The influence of ammonia co-firing ratio on NO and CO2 emissions, as well as CO and H2S release characteristics, was studied using Shenhua bituminous coal in air-staged and non-staged combustion conditions. Chemical reaction kinetic calculations were conducted to provide a qualitative analysis of the experimental results. Ammonia plays a complex role in the ammonia/coal co-firing process. An effective CO2 reduction was achieved with ammonia co-firing, and it was found that air-staged combustion is also effective for NO emission control in the ammonia/coal co-firing process. Considering the economy issue and NO emission control, 20–30 % ammonia co-firing ratio is recommended. However, ammonia/coal co-firing can greatly promote the coal gasification reaction, which leads to a higher concentration of CO in the reduction zone in air-staged combustion, compared with coal combustion. Meanwhile, ammonia co-firing has a significant inhibitory effect on the release of H2S from coal and will promote the subsequent decomposition process. This study explains the NO release pattern in ammonia/coal co-firing processes based on experimental results and chemical kinetics calculations, which provides a promising technical advancement direction for the coal-fired power plants to achieve a signification reduction of both CO2 and NO emissions.

Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

Hydrogen and its derivatives, including ammonia, are gaining increasing attention as carbon-neutral fuel alternatives. An intermediate step in the transition to hydrogen and ammonia is the blending of these fuels with hydrocarbons, introducing the challenge of soot formation. The impact of ammonia on soot formation has recently been the focus of several studies, but a complete understanding of its chemical effects is lacking. Hydrogen, by comparison, has received significant attention from the soot community. However, controversy remains with regards to hydrogen’s chemical impact, and the dependence of this impact on fuel and flame configuration. This work investigates the effect of both hydrogen and ammonia on soot formation in laminar coflow flames of both ethylene and methane. Hydrogen or ammonia are introduced either by addition or substitution, with parallel studies of helium and argon, in order to isolate their chemical effects. Time- and spectrally-resolved laser-induced emissions from UV and IR excitation are used to quantify differences in soot and soot precursor formation. Additionally, chemical kinetics calculations and analyses are used to elucidate the effects of ammonia introduction to ethylene flames. Ammonia is found to chemically inhibit soot when mixed with either ethylene or methane, with increasing effects on larger precursors. Calculations suggest that this suppression is due to carbon consumption in the formation of HCN and CN. Hydrogen is found to chemically enhance soot formation in both ethylene and methane flames.

Quantitative calculation method to compare the reactive sites of different methyl esters

Methyl ester is the main component of biodiesel, which plays a crucial role in constructing chemical kinetic models for the combustion. The carbon chain length and the degree of unsaturation have a significant influence on the molecular properties and reactivity of methyl esters. In this study, to explore the difference in chain length and reactive sites of methyl esters, the quantum chemical approach is used to examine the chemical reaction sites of methyl butyrate, methyl crotonate, methyl heptanoate, and methyl enanthate. Based on density functional theory, the structure of the four methyl ester molecules is optimized under the M06-2X method, and the global reactivity (including molecular electrostatic surface potential, frontier molecular orbital) and local reactivity (including Fukui function, charge, and natural bond orbital) are analyzed. At the same time, the visualization results are also obtained. The results show that the reactive sites of saturated methyl ester occur in the structure of the ester group, and the reactive sites of methyl crotonate occur on the ester group and the double bond. Further, the reactivity of methyl crotonate is stronger than that of the three saturated methyl esters. As the carbon chain grows, the reactivity of saturated methyl esters gradually increases. Overall, the results provide a theoretical basis for revealing the different structural properties of methyl esters and subsequent chemical kinetic calculations.

The influence of the environment in chemical reactivity: the HCOOH formation from the H2O + CO reaction.

The reaction between carbon monoxide and water was studied occurring in an aerosol medium rich in methanol. This environment is plausible for the primitive and prebiotic Earth atmosphere. The chemical environment is expressed in terms of dielectric constant (ε) and the chemical system was modeled employing the polarizable continuum model (PCM). The main results were acquired from calculations employing the M06-2X density functional for the electronic structure calculations and the canonical variational theory with small curvature tunneling for the chemical kinetic calculations. The rise of ε affects both the thermochemistry and the kinetics of the reaction, increasing the barrier height and decreasing the rate constant for the reaction occurring at room temperature. For example, the rate constant at 300 K is 5-10× 10- 53 cm3 ⋅molecule- 1 ⋅s- 1 for low dielectric constant (ε < 3) and around 2-4× 10- 53 cm3 ⋅molecule- 1 ⋅s- 1 for ε between 7 and 40. Our results indicate that the ε variation allows a fine tuning to the rate of the reaction.

A comprehensive study on low-temperature oxidation chemistry of cyclohexane. I. Conformational analysis and theoretical study of first and second oxygen addition

To understand the low-temperature oxidation chemistry of cyclohexane, conformational analysis and theoretical study of the first and second oxygen addition are performed using quantum chemical calculations and kinetic calculations. Pressure- and temperature-dependent rate constants and branching ratios for major reaction channels are determined with RRKM/master-equation simulations over 298–2000 K and 0.01–1000 bar. The theoretical results indicate that the rapid inversion-topomerization processes facilitate fast equilibrium between axial and equatorial conformers. This can greatly counterbalance the influence of initial positions of side-chain groups in ROO, QOOH, cis-OOγQOOH and trans-OOγQOOH conformers. Conformational effects are found to be influential on the chain branching reaction sequences in second oxygen addition. The carbon ring prevents the conventional intramolecular H-transfer of cis-OOγQOOH conformers to yield ketohydroperoxides, as well as the inversion-topomerization from cis-OOγQOOH conformer to trans-OOγQOOH conformers. cis-OOγQOOH conformers mainly undergo alternative isomerization channel (cis-OOγQOOH→γP(OOH)2→alkenylhydroperoxides+OH→oxy radical+OH+OH), while trans-OOγQOOH conformers have both conventional isomerization channel (trans-OOγQOOH→ketohydroperoxides+OH→oxy radical+OH+OH) and alternative isomerization channel (trans-OOγQOOH→γP(OOH)2→alkenylhydroperoxides+OH→oxy radical+OH+OH). Kinetic calculation results also support the application of the thumb rule widely used in acyclic alkane oxidation that the rate constant of QOOH+O2 is roughly half of that of R+O2 in cyclic alkane oxidation, while it is indicated that estimating the rate constants of OOQOOH reactions from similar reactions of ROO may cause significant uncertainties.

Dependence of Biofuel Ignition Chemistry on OH-Initiated Branching Fractions

Abstraction of hydrogen by ȮH is the dominant initiation step in low-temperature oxidation of biofuels. Theoretical chemical kinetics calculations for such reactions provide a direct means of quantifying rates of abstraction, which are critical to modeling biofuel combustion. However, in several cases and despite agreement on total rate coefficients, branching fractions (i.e. the distribution of initial radicals) can vary depending on the level of theory, which leads to variations in ignition delay time predictions. To examine the connection between branching fractions and ignition delay time predictions, simulations were conducted for 1-butanol, cyclopentanone, and methyl propanoate at 10 atm and from 500–1000 K. For each case, the simulations utilized recent combustion mechanisms to produce an initial set of ignition delay time trends. H-abstraction rates were then replaced using rates from the literature to examine the effects of ȮH-initiated branching fractions on ignition chemistry. Branching fractions were found to significantly influence ignition chemistry, specifically in the case of 1-butanol, even when total rate coefficients were relatively consistent. From comparison of site-specific rates in the literature, branching fractions for initiation of 1-butanol and methyl propanoate are not consistent, which resulted in ignition delay times differing by factors of up to 6.3 and 1.2 respectively. Conversely, in the case of cyclopentanone, for which both the total and the site-specific rate coefficients agree, ignition delay times were unaffected. From the observed dependence of ignition delay times on ȮH-initiated branching fractions, an intermediate step in the development of combustion mechanisms is necessary to validate site-specific rate coefficients and ensure accurate model predictions. Speciation measurements are one example that can provide a critical link to radical-specific, fundamental chemical pathways and determine accurate branching fractions.