Molecular Beam Mass Spectrometry Research Articles

Effect of HTC and Water-Leaching of Low-Grade Biomasses on the Release Behavior of Inorganic Constituents in a Calcium Looping Gasification Process at 650 °C.

The release of alkali metals (K, Na) and nonmetals (S, Cl) during a calcium looping (CaL) gasification process of waste derived-hydrochars, water-leached samples, and CaO-biomass blends was investigated. Special attention was paid to biomasses that are not particularly promising for gasification requirements but have a large occurrence in Europe, including Grape Bagasse, Organic Fraction of Municipal Solid Waste (OFMSW), Green Waste, and Out-of-use woods from construction debris and discarded furniture. The release experiments were performed at 650 °C in a flow channel reactor to investigate the behavior of inorganic trace substances. Hot-gas analysis was performed by Molecular Beam Mass Spectrometry (MBMS). Thermodynamic equilibrium calculations via FactSage indicate H2S, carbonyl sulfide (COS), KCl, NaCl, and HCl as the main inorganic impurities. Thus, the focus of the experiments was placed on these species. It was found that the concentrations of trace elements released during gasification at 650 °C, such as H2S, SO2, KCl, and NaCl, are hardly affected by intense water-leaching. In contrast, carbonaceous materials from hydrothermal carbonization exhibit a higher concentration of trace potassium substances (K, KCl, and K2Cl+). When biomass samples are combined with CaO, the total amount of inorganic trace compounds (K, Na, and S compounds) in the resulting syngas could be decreased.

Experimental Study of the Reaction of OH Radicals with Carbonyl Sulfide between 365 and 960 K: Kinetics and Products

Reaction OH + OCS ® products (1) has been studied in a discharge–flow reactor combined with modulated molecular beam mass spectrometry. The reaction rate constant has been determined under pseudo-first-order conditions through monitoring OH decays in a high excess of OCS: k1 = (2.35 ± 0.25) × 10−12 exp(−(2144 ± 56)/T) cm3 molecule−1 s−1 at T = 365–960 K (the uncertainties represent precision at the 2σ level, the total 2σ relative uncertainty including statistical and systematic errors on the rate constant being 20% at all temperatures). The rate constant of reaction (1) was found to be similar at a total helium pressure of 1, 2, and 8 torr at around 500 K. The SH radical was identified as the primary product of the reaction, and its yield was determined to be about 100% at T = 500 and 750 K. The kinetic and mechanistic data from the present study are compared to previous experimental and theoretical work.

NO formation by N2/O2 plasma catalysis: The impact of surface reactions, gas-phase reactions, and mass transport

Pathways and timescales relevant to facilitate plasma-assisted N2-O2 reactions are assessed by measuring the consumption of plasma-derived N and the formation of NO in the gas phase and over Ag catalytic surfaces. These measurements are enabled by a setup that enables N2 activation in an atmospheric pressure RF plasma jet, enables O2 addition in the plasma afterglow, facilitates reactions over an Ag wire catalyst, and allows species density quantification by molecular beam mass spectrometry. Gas-phase reactions consume N but do not form NO with high selectivity. The presence of the non-porous Ag wire catalyst increases the rate of N conversion to NO, though mass transfer processes, not surface reactions, dictate the rate of N consumption. When O2 concentrations and the ratio of the surface area of the catalyst to the void volume of the reactor are high (3–5 mol% O2, 10900 m−1), N conversion to NO reaches 100 % selectivity. When both N2 and O2 are fed through the plasma jet, gas-phase NO production increases 10×, although plasma and gas-phase processes do not exclusively produce NO. Above a threshold NO density, N cannot diffuse to the catalyst surface faster than it is consumed in the gas phase by reactions with NO. Thus, the use of heterogeneous catalysts to enhance plasma-driven NxOy formation and control NxOy product selectivity is limited to cases where diffusive transport of N from the gas phase to the catalyst surface is faster than consumption of N from gas-phase reactions with NO.

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.

Combining mass spectrometry, i2PEPICO, and FTIR spectroscopy: Comprehensive speciation in DMM/NO oxidation

Oxymethylene ethers (OMEs) are potential alternative fuels that can be produced in a sustainable way based on CO2 and electricity. Additionally, they exhibit low tendencies to emit NOx or soot during practical combustion. Dimethoxy methane (DMM, CH3OCH2OCH3) has received increasing attention as the smallest OME with a O-CH2-O moiety. In the face of a possible application in exhaust gas recirculation (EGR) scenarios, understanding the influence of NOx on DMM oxidation is of importance. In this work, eleven mixtures of DMM/NO/O2/Ar were investigated in an atmospheric laminar flow reactor employing an extensive set of diagnostics. Molecular-beam mass spectrometry (MBMS), Fourier-transform infrared (FTIR) spectroscopy, NOx chemiluminescence detection, and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy were combined to obtain a reliable speciation. All in all, species concentrations from different instruments showed a good agreement and one technique could counterbalance the physical limitations of another technique in many cases. In this manner, accurate mole fraction profiles of intermediates like e.g. CH4, C2H4, NO2, and methyl formate (C2H4O2) were gained. Based on i2PEPICO results, nitromethane (CH3NO2) and trans-HONO could additionally be identified as crucial intermediates in the NO-assisted oxidation of DMM. The present data set therefore provides an excellent basis to enhance future model development.

Availability and reactivity of N2(v) for NH3 synthesis by plasma catalysis

Production of vibrationally excited N2 (N2(v)) in atmospheric pressure nonthermal plasma and loss of N2(v) by gas-phase reactions and reactions on catalytic surfaces are analyzed to examine the role of N2(v) in NH3 formation by plasma catalysis. Vibrational state-to-state kinetic models complemented with molecular beam mass spectrometry (MBMS) measurements demonstrate that N2(v> 0) is produced with densities 100× greater than the density of N radicals by a radiofrequency atmospheric pressure plasma jet. The experimentally measured loss of N2(v) corresponds with a state-to-state kinetic model that describes loss of N2(v) by surface-mediated vibrational relaxation without consideration of reactions that convert N2(v) to NH3 over the catalyst surface. Rate constants for vibrational relaxation of N2(v) on catalyst surfaces exceed upper bounds on proposed rate constants for NH3 formation reactions from N2(v) over Fe when v < 9, Ni when v < 18, and Ag when v < 39, which indicates that only higher vibrational levels can possibly contribute to catalytic NH3 formation faster than they undergo vibrational relaxation on the surface. Densities of N2(v> 8), vibrational levels that can possibly react over Fe to form NH3 faster than they undergo vibrational relaxation, are less than or similar to N densities at the inlet of the catalyst bed and measured NH3 formation for the investigated conditions in this work, while densities of N2(v> 17) and N2(v> 38) are orders of magnitude below the N density at the inlet of the catalyst bed and the measured NH3 formation. The loss of N2(v) by vibrational relaxation on the surface limits the ability of N2(v) to contribute to catalytic NH3 formation and explains why N2(v) does not produce NH3 in quantities that are comparable to NH3 formation from N even though N2(v > 0) is more abundantly produced by the plasma.

Development of a pulse modulated sub-radio frequency power supply for atmospheric pressure plasma devices.

The effect of pulse-modulated sub-RF range (100 kHz-1MHz) excitation on atmospheric pressure argon plasma jet characteristics is studied. For this, a suitable power supply is developed, offering a sub-µs rise time with control of different parameters, such as voltage amplitude, pulse modulation frequency in the range of 1-30kHz, and an oscillation frequency of ∼520kHz, which can affect the plasma behavior. Plasma characteristics, such as reactive species generation, ionic composition, plasma plume length, and gas temperature, are evaluated qualitatively and quantitatively by employing diagnostics such as optical emission spectroscopy, molecular beam mass spectrometry, and optical imaging. Experimental observations indicate that the gas temperature of the plasma jet and plume length increase with the applied voltage for all pulse modulation frequencies, with a maximum value of ∼(325 ± 2K) and a maximum length of ∼(23 ± 3mm), respectively, at 30kHz and 9 kVpp. The emission intensities of OH• and O• lines show an incremental behavior with the applied voltage across all pulse modulation frequencies. The relative yield of different positive (OH+, O+, etc.) and negative (OH-, O-, etc.) ions also increases with the applied voltage for all pulse modulation frequencies with maximum values of ∼(7.6%, 9.9%) and (3.9%, 9.4%), respectively; these are relatively close to RF excited ionic concentrations reported previously. Attaining a high plasma length and species yield signify the features of both kHz and RF atmospheric plasmas. This study offers significant insights and flexibility into exploring the impact of different RF frequency regimes on plasma characteristics.

Temperature-Dependent Kinetic Study of the Reactions of Hydrogen Atoms with H2S and C2H4S

A discharge-flow reactor combined with modulated molecular beam mass spectrometry technique was employed to determine the rate constants of H-atom reactions with hydrogen sulfide and thiirane. The rate constants for both reactions were determined at a total pressure of 2 Torr from 220 to 950 K under pseudo-first-order conditions by monitoring either consumption of H atoms in excess of H2S (C4H4S) or the molecular species in excess of atomic hydrogen. For H + H2S reaction, a suggested previously strong curvature of the Arrhenius plot was confirmed: kl = 8.7 × 10−13 × (T/298)2.87 × exp(−125/T) cm3 molecule−1 s−1 with a conservative uncertainty of 15% at all temperatures. Non-Arrhenius behavior was also observed for the reaction of H-atom with C2H4S, with the experimental rate constant data being best fitted to a sum of two exponential functions: k2 = 1.85 × 10−10 exp(−1410/T) + 4.17 × 10−12 exp(−242/T) cm3 molecule−1 s−1 with an independent of temperature uncertainty of 15%.

Inhibition of Laminar Premixed Flames of Dimethyl Ether by a Phosphorus-Containing Compound

ABSTRACT The present paper reports experimental and modeling results on the laminar burning velocity and structure of stoichiometric and fuel-rich premixed laminar flames of DME with and without TMP additives. The spatial variations of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames with various equivalence ratios are measured by the flame-sampling molecular beam mass spectrometry. The novel measurement data for the DME flames are used to validate a kinetic mechanism available in the literature for flame inhibition by organophosphorus compounds. The TMP inhibition effectiveness of DME flames has been determined and revealed to be lower than that of CH4/air flame. The speeds of CH4/air and DME/air flames were found to be sensitive to the same reactions involving phosphorus compounds, while the sensitivity of the methane flame speed to inhibition reactions is higher than that of DME/air flame. The observed differences were explained by the higher concentration of the chain carriers H, OH, and O in the DME flame. Based on the results obtained, we conclude that the phosphorus-containing inhibitors are promising candidates for reducing the flammability and ensuring the fire safety of DME mixtures with air.

A VUV photoionization study on the gas-phase synthesis of the first five-membered carbon ring - C7H6 isomers

Utilizing synchrotron photoionization and molecular-beam mass spectrometry methodologies, this investigation deciphered the gas-phase reaction of propargyl with vinylacetylene by employing a high-temperature micro reactor within a time scale of tens to hundreds of microseconds, thereby precluding interference from secondary reactions. One of the typical C5-membered cyclic structures, fulvenallene, along with its isomers 1-ethynyl-1,3-cyclopentadiene and 5-ethynyl-1,3-cyclopentadiene, were synthesized. The latter two isomers were detected and identified for the first time within the confines of our experiments. The detailed formation mechanisms of these C7H6 isomers were meticulously examined with the incorporation of quantum computations on the potential energy surfaces and rate constants.

Flame Structure at Elevated Pressure Values and Reduced Reaction Mechanisms for the Combustion of CH4/H2 Mixtures

Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.

Experimental and kinetic modeling study of the autoignition and oxidation of ammonia/ethane mixtures in a rapid compression machine and a jet-stirred reactor

Ignition delay times and speciation data of ammonia/ethane (NH3/C2H6) fuel blends were measured in a rapid compression machine (RCM) and in a jet-stirred reactor coupled with molecular beam mass spectrometry (JSR-MBMS), respectively. Specifically, the RCM experiments were conducted at elevated pressures of 20 and 40 bar, intermediate temperatures between 890 and 1110 K, three equivalence ratios of 0.5, 1.0 and 2.0 and for three C2H6 mole fractions from 1% to 10%. Besides, the JSR measurements were conducted at atmospheric pressure, intermediate temperatures between 700 and 1180 K, three equivalence ratios of 0.5, 1.0 and 2.0 and three C2H6 mole fractions from 10% to 50%. The experimental results demonstrate that adding C2H6 into NH3 leads to a more reactive mixture. A kinetic model from our recent work on ammonia/ethanol fuel blends (M. Li et al., Proc. Combust. Inst., 2022) has been updated and validated with the new data obtained in this work. The updated model (PTB-NH3/C2 mech) can reproduce the effects of ethane fraction and equivalence ratio on IDT/speciation and show an overall satisfactory agreement between experimental results and predictions. Based on the kinetic analyses, for all conditions investigated in this study, the generation of OH radicals from HO2 and H radicals dominates the characteristic of the auto-ignition process in the RCM and the NH3 consumption process in the JSR, respectively. Apart from that, the addition of ethane provides additional OH from a relatively low temperature (e.g., around 850 K in a JSR condition) and triggers the interactions between hydrocarbon and ammonia systems, e.g., the reactions of C2H6/C2H4/CH3+NH2, which play an important role in the combustion of ammonia.

Resonance enhanced multiphoton ionization detection of aromatics formation in fuel-rich flames

Polycyclic aromatic hydrocarbons (PAHs) are formed in fuel-rich and sooting hydrocarbon flames. These compounds and smaller aromatic flame constituents, such as benzene, toluene and naphthalene can be detected by flame-sampling molecular beam mass spectrometry (MBMS). They are readily ionized by photoionization with UV light, either in resonant or non-resonant ionization processes. The multiphoton ionization of benzene, toluene, and styrene with MBMS detection was investigated as a function of laser power and wavelength and the quantitative relation between ion signal and mole fractions in the flames was established. These measurements were prerequisites for the measurement of quantitative mole fraction profiles of these species in flames of various fuels. In addition, the measurements provide relative mole fraction profiles of various other aromatic flame species. The results are compared to measurements of aromatics in butadiene, propyne, and allene flames by single-photon ionization with vacuum ultraviolet radiation from a synchrotron source and other literature data.

Revisit flame chemistry of propene at elevated pressures: Insight into pressure effects on chemical structure and laminar flame propagation

This work reports an experimental and kinetic modeling investigation on flame chemistry of propene at elevated pressures. The chemical structures of premixed propene/O2/Ar flames at 2–5 atm and the equivalence ratio of 1.5 including reactants, major products and a series of intermediates were measured using the flame sampling molecular beam mass spectrometry. The maximum mole fractions of H, CH3 and C2H6 decrease with increasing pressure, while that of CH4 follows the opposite trend. Laminar burning velocities (LBVs) of propene/air mixtures at equivalence ratios of 0.7–1.5 and initial pressures up to 10 atm were measured using the spherically propagating flame method. Remarkable pressure effects can be observed, especially under the lean and rich conditions. A kinetic model for propene combustion was developed based on our recent model for ethylene combustion and recently reported chemically termolecular reactions and HCO prompt dissociation reactions. Key reactions in the propene flames were revealed using the modeling analysis, especially for those playing significant roles in the pressure effects. Two chain termination reactions including the self-combination of CH3 and the recombination between CH3 and H are found to be crucial for the observed pressure effects in maximum mole fractions of H, CH3, CH4 and C2H6. The HCO prompt dissociation can significantly promote the laminar flame propagation of propene/air mixtures due to the substantial formation of CH2O through CH2+O2 and CH3+O pathways, while the chemically termolecular reactions exhibit weak inhibition effects at equivalence ratios smaller than 1.2. The reason for the stronger pressure effects of LBVs under the lean and rich conditions is that the excess O2 and abundantly produced CH3 enhances the roles of H+O2(+M)=HO2(+M) and CH4(+M)=CH3+H(+M) in chain termination, respectively.

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.

Absolute densities of O and O_3 in the effluent of an atmospheric pressure plasma jet (kINPen) operated with He and Ar determined by molecular beam mass spectrometry

The absolute densities of O and O_3 in the effluent of a kINPen, an atmospheric pressure plasma jet, are determined by molecular beam mass spectrometry. A nitrogen gas curtain surrounding the effluent provides a stable environment and prevents interactions with the ambient air. Both He and Ar were used as feed gas with admixtures of O_2 up to 1%. Measurements are performed at discharge voltages from 1600 to 3000 {textrm{V}}_{textrm{pp}} and at distances from 5.5 to 10.5 mm from the nozzle of the gas shielding device to the orifice of the mass spectrometer. The measured O atom densities are in the order of 1 times 10^{14}~{textrm{cm}}^{-3} to 2 times 10^{15}~{textrm{cm}}^{-3}, whereas the O_3 molecule densities are in the order of 1 times 10^{14}~{textrm{cm}}^{-3} to 5 times 10^{14}~{textrm{cm}}^{-3}, respectively, when operated in argon the plasma generates higher densities of reactive species. Additionally, a novel molecular beam chopper design with linear movement has been successfully tested in the mass spectromerty setup as an alternative to a rotating chopper.Graphic

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.

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH4) and ethylene (C2H4) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron-ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 °C (300-500 °C) and 300-800 °C (300-600 °C) for the plasma and nonplasma conditions. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 °C, where no oxidation was observed under nonplasma conditions. For the oxidation of methane at higher temperatures, three different oxidation stages were observed: (I) a period of complete oxidation, (II) a period of incomplete CO oxidation, and (III) a period of carbon buildup. For the C2H4 experiments, and unlike the CH4 experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH2O, CH3OH, C2H4O, and C2H6O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF (reactive force field) simulations were performed for the oxidation of CH4 and C2H4 in a nonplasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH2O and C2H6O were not observed in experiments under nonplasma conditions. A reaction pathway analysis for CH4 and C2H4 reacting with NiO was created based on the observed species from the MBMS spectra along with ReaxFF simulations.

Generation of a low-temperature plasma in a nozzle to initiate ion-cluster reactions in jets of mixtures of methane with a buffer gas

We have considered the option of generating cluster ions directly at the nozzle by initiating an electric discharge. Here, a low-temperature plasma in the flow is formed as a result of the outflow of a partially ionized gas. Conditions for the formation of clusters in the presence of excited particles are provided downstream. To implement this generation option, a scheme for creating an effective discharge in the diffuser part of a supersonic nozzle was proposed and implemented. The diagnostic system of the LEMPUS-2 gas-dynamic facility was adapted to register processes in a supersonic flow of ionized gas with clusters. Molecular nitrogen, which is weakly condensing under experimental conditions, was used as the primary reference gas. The methods of molecular beam mass spectrometry were used to study gas mixtures consisting of 20% methane in an inert carrier gas. Highly condensable argon and practically non-condensable helium were used as carrier gases. It was found that when using the ionized particle selection system for detection, only methane monomers and dimers are registered in mixtures with argon, whereas in mixtures with helium larger clusters are registered. A version of the explanation of the detected dependencies was presented.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.