Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

The low reactivity of ammonia (NH₃) is the main barrier to applying neat NH₃ as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH₃-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H₂), methane (CH₄), and methanol (CH₃OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O₃) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH₃ consumption can be triggered at lower temperatures than in the neat NH₃ case. CH₃OH has the most prominent effect on enhancing the reactivity, followed by H₂ and CH₄. Furthermore, two-stage NH₃ consumption was observed in NH₃/CH₃OH blends, whereas no such phenomenon was found by adding H₂ or CH₄. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH₃ oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH₂O + NH₂ ⇄ HCO + NH₃ is responsible for the underestimation of CH₂O in NH₃/CH₄ fuel blends. The discrepancies observed in the modeling of NH₃ fuel blends are mainly due to the deviations in the neat NH₃ case. The total rate coefficient and the branching ratio of NH₂ + HO₂ are still controversial. The high branching fraction of the chain-propagating channel NH₂ + HO₂ ⇄ H₂NO + OH improves the model performance under low-pressure JSR conditions for neat NH₃ but overestimates the reactivity for NH₃ fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH₃OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH₃ consumption at temperatures below 450 K but unexpectedly inhibit the NH₃ consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH₃-related species and O₃ is effective for improving the model performance, but their rate coefficients have to be refined.

Parametrical investigation for the optimization of spherical jet-stirred reactors design using large eddy simulations

Due to the importance of gas-phase chemical reaction kinetics in low-emission combustion, stirred tank reactors have been used for decades as an experimental tool to study high- and low-temperature oxidation. A Jet-Stirred Reactor (JSR) setup is valuable to determine the evolution of species mole fractions. For the accuracy of the experimental results, it is important that a JSR is designed such that the concentration field is as homogeneous as possible in order to avoid disturbance of the chemical kinetics. In this work, numerical simulations were performed to investigate the mixing in a JSR chamber. The turbulent structures inside the JSR and the nozzles are captured using Large Eddy Simulations. We conducted numerically a parametric study to evaluate the effects of thermodynamic conditions and geometrical parameters on the mixing characteristics. More specifically, the diameter of the spherical chamber is modified together with the diameter of the nozzles through which fresh gases are fed. The characterization of the gas flow inside a typical spherical JSR layout and results derived by the normalized standard deviation of a tracer mass fraction show that a reduction of the JSR diameter at high pressures improves the homogeneity. Further, we propose a new optimized configuration consisting of six nozzles pointing to the center of the reactor which provides a more uniform composition compared to the standard JSR design.

Molecular-Weight Growth in Ozone-Initiated Low-Temperature Oxidation of Methyl Crotonate

We report experiments of ozone-initiated low-temperature oxidation of methyl crotonate (MC, CH₃-CH═CH-C(O)OCH₃) from 420 to 660 K in a near-atmospheric-pressure jet-stirred reactor using photoionization molecular-beam mass spectrometry as a sampling technique. In this temperature regime, no typical low-temperature combustion (LTC) reactions have been observed for MC when oxygen (O₂) is used as the oxidizer. Upon ozone addition, significant oxidation of methyl crotonate is found. On the basis of experimentally observed energy-dependent mass peaks in combination with temperature-dependent mole fraction profiles and photoionization efficiency curves, we provide new insights into the methyl crotonate ozonolysis reaction network. The observed MC + O₃ products, C₅H₈O₅, are found to be related to the keto-hydroperoxides resulting from the isomerization of the primary ozonide. Evidence is also provided that molecular growth mainly results from cycloaddition reactions of the Criegee intermediate into aldehydes and alkenes as well as addition reactions of the Criegee intermediates to the double bond of methyl crotonate and sequential decomposition into ketones. Furthermore, species that contribute in large amounts to the low-temperature oxidation of methyl crotonate, like H₂O₂, CH₃OOH, CH₃OH, and HC(O)OH, are identified, and their mole fractions are reported. Additionally, preliminary modeling is performed which qualitatively captures the observed NTC behavior and reveals future research opportunities.

Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran

Keto-hydroperoxides (KHPs) are reactive, partially oxidized intermediates that play a central role in chain-branching reactions during the gas-phase low-temperature oxidation of hydrocarbons and oxygenated species. Although multiple isomeric forms of the KHP intermediate are possible in complex oxidation environments when multiple reactant radicals exist that contain nonequivalent O₂ addition sites, isomer-resolved data of KHPs have not been reported. In this work, we provide partially isomer-resolved detection and quantification of the KHPs that form during the low-temperature oxidation of tetrahydrofuran (THF, cycl.-O-CH₂CH₂CH₂CH₂-). We describe how these short-lived KHPs were detected, identified, and quantified using integrated experimental and theoretical approaches. The experimental approaches were based on direct molecular-beam sampling from a jet-stirred reactor operated at near-atmospheric pressure and at temperatures between 500 and 700 K, followed by mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation, and the identification of fragmentation patterns. The interpretation of the experiments was guided by theoretical calculations of ionization thresholds, fragment appearance energies, and photoionization cross sections. On the basis of the experimentally observed and theoretically calculated ionization and fragment appearance energies, KHP isomers could be distinguished as originating from H-abstraction reactions from either the α-C adjacent to the O atom or the β-C atoms. Temperature-dependent concentration profiles of the partially resolved isomeric KHP intermediates were determined in the range of 500-700 K, and the results indicate that the observed KHP isomers are formed overwhelmingly (∼99%) from the α-C THF radical. Comparisons of the partially isomer-resolved quantification of the KHPs to up-to-date kinetic modeling results reveal new opportunities for the development of a next-generation THF oxidation mechanism.

Influences of the molecular fuel structure on combustion reactions towards soot precursors in selected alkane and alkene flames

In this study, we experimentally investigate the high-temperature oxidation kinetics of n-pentane, 1-pentene and 2-methyl-2-butene (2M2B) in a combustion environment using flame-sampling molecular beam mass spectrometry. The selected C5 fuels are prototypes for linear and branched, saturated and unsaturated fuel components, featuring different C-C and C-H bond structures. It is shown that the formation tendency of species, such as polycyclic aromatic hydrocarbons (PAHs), yielded through mass growth reactions increases drastically in the sequence n-pentane < 1-pentene < 2M2B. This comparative study enables valuable insights into fuel-dependent reaction sequences of the gas-phase combustion mechanism that provide explanations for the observed difference in the PAH formation tendency. First, we investigate the fuel-structure-dependent formation of small hydrocarbon species that are yielded as intermediate species during the fuel decomposition, because these species are at the origin of the subsequent mass growth reaction pathways. Second, we review typical PAH formation reactions inspecting repetitive growth sequences in dependence of the molecular fuel structure. Third, we discuss how differences in the intermediate species pool influence the formation reactions of key aromatic ring species that are important for the PAH growth process underlying soot formation. As a main result it was found that for the fuels featuring a C[double bond, length as m-dash]C double bond, the chemistry of their allylic fuel radicals and their decomposition products strongly influences the combination reactions to the initially formed aromatic ring species and as a consequence, the PAH formation tendency.

Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds

Decades of research on the autooxidation of organic compounds have provided fundamental and practical insights into these processes; however, the structure of many key autooxidation intermediates and the reactions leading to their formation still remain unclear. This work provides additional experimental evidence that highly oxygenated intermediates with one or more hydroperoxy groups are prevalent in the autooxidation of various oxygenated (e.g., alcohol, aldehyde, keto compounds, ether, and ester) and nonoxygenated (e.g., normal alkane, branched alkane, and cycloalkane) organic compounds. These findings improve our understanding of autooxidation reaction mechanisms that are routinely used to predict fuel ignition and oxidative stability of liquid hydrocarbons, while also providing insights relevant to the formation mechanisms of tropospheric aerosol building blocks. The direct observation of highly oxygenated intermediates for the autooxidation of alkanes at 500-600 K builds upon prior observations made in atmospheric conditions for the autooxidation of terpenes and other unsaturated hydrocarbons; it shows that highly oxygenated intermediates are stable at conditions above room temperature. These results further reveal that highly oxygenated intermediates are not only accessible by chemical activation but also by thermal activation. Theoretical calculations on H-atom migration reactions are presented to rationalize the relationship between the organic compound's molecular structure (n-alkane, branched alkane, and cycloalkane) and its propensity to produce highly oxygenated intermediates via extensive autooxidation of hydroperoxyalkylperoxy radicals. Finally, detailed chemical kinetic simulations demonstrate the influence of these additional reaction pathways on the ignition of practical fuels.

Photoelectron-photoion coincidence spectroscopy for multiplexed detection of intermediate species in a flame

Complex reactive processes in the gas phase often proceed via numerous reaction steps and intermediate species that must be identified and quantified to develop an understanding of the reaction pathways and establish suitable reaction mechanisms. Here, photoelectron-photoion coincidence (PEPICO) spectroscopy has been applied to analyse combustion intermediates present in a premixed fuel-rich (ϕ = 1.7) ethene-oxygen flame diluted with 25% argon, burning at a reduced pressure of 40 mbar. For the first time, multiplexing fixed-photon-energy PEPICO measurements were demonstrated in a chemically complex reactive system such as a flame in comparison with the scanning "threshold" TPEPICO approach used in recent pioneering combustion investigations. The technique presented here is capable of detecting and identifying multiple species by their cations' vibronic fingerprints, including radicals and pairs or triplets of isomers, from a single time-efficient measurement at a selected fixed photon energy. Vibrational structures for these species have been obtained in very good agreement with scanning-mode threshold photoelectron spectra taken under the same conditions. From such spectra, the temperature in the ionisation volume was determined. Exemplary analysis of species profiles and mole fraction ratios for isomers shows favourable agreement with results obtained by more common electron ionisation and photoionisation mass spectrometry experiments. It is expected that the multiplexing fixed-photon-energy PEPICO technique can contribute effectively to the analysis of chemical reactivity and kinetics in and beyond combustion.

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + ĊH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OĊHOOH intermediate, which predominantly leads to the HPMF.

Formation of Oxygenated and Hydrocarbon Intermediates in Premixed Combustion of 2-Methylfuran

This paper focuses on the combustion chemistry of 2-methylfuran (2-MF), a potential biofuel, and it is built on the previous work of Tran et al. [Combust. Flame 161 (2014) 766]. In their work, they had combined detailed flame chemistry modeling with flame speciation data based on flame-sampling molecular beam mass spectrometry (MBMS) with electron ionization and gas chromatography with MS detection. In this work, we significantly extend those previous studies by in-situ isomer-resolving species identification and quantification. Specifically, we have determined the detailed chemical structure of a premixed laminar 2-MF flame using flame-sampling high-resolution MBMS with synchrotron-generated vacuum-ultraviolet radiation. Mole fraction profiles of 60 intermediate, reactant, and product species were measured in order to assess the pollutant potential of this possible next-generation biofuel. Special emphasis is paid towards the fuel's ability to form aromatic and oxygenated intermediates during incomplete combustion processes, with the latter species representing a variety of different classes including alcohols, ethers, enols, ketones, aldehydes, acids, and ketenes. Whenever possible the experimental data are compared to the results of model calculations using the 2-MF combustion chemistry model of Tran et al., but it should be noted that many newly detected species are not included in the calculations. The experimental data presented in this work provides guidance towards to development of a next-generation 2-MF combustion chemistry model.