Jintao Sun, Qi Chen, Xiaofang Yang and Bruce E. Koel

Published 4 December 2019 

The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30–100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4  +  O(1D)  →  CH3  +  OH, and O2(a1Δg)  +  H  →  O  +  OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v)  +  O  →  CH3  +  OH, CH4(v)  +  OH  →  CH3  +  H2O, O2(v)  +  H  →  O  +  OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13)  +  OH  →  H2O  +  CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.

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Development of earth-abundant electrocatalysts for hydrogen evolution and oxidation reactions in strong acids represents a great challenge for developing high efficiency, durable, and cost effective electrolyzers and fuel cells. We report herein that hafnium oxyhydroxide with incorporated nitrogen by treatment using an atmospheric nitrogen plasma demonstrates high catalytic activity and stability for both hydrogen evolution and oxidation reactions in strong acidic media using earth-abundant materials. The observed properties are especially important for unitized regenerative fuel cells using polymer electrolyte membranes. Our results indicate that nitrogen-modified hafnium oxyhydroxide could be a true alternative for platinum as an active and stable electrocatalyst, and furthermore that nitrogen plasma treatment may be useful in activating other non-conductive materials to form new active electrocatalysts.

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Qi Chen, Xiaofang Yang, Jintao Sun, Xiaojun Zhang, Xingian Mao, Yiguang Ju & Bruce E. Koel 

Published 10 October 2017

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH4 mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H2, C2 and C3 hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C2 and C3 formation dramatically decreased, and syngas (H2 and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH4 mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH3, CH2, CH, H, O and O (1D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.

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Aric Rousso, Yang Suo, Joseph Lefkowitz, Sun Wenting, Ju Yiguang

Published 5 October 2016

The present study seeks to explore the parametric effects of oxygen concentration, argon dilution and plasma discharge frequency on pyrolytic and oxidative reaction pathways of n-heptane assisted by non-equilibrium plasma discharge. Low temperature reaction pathways of n-heptane/O2/Ar mixtures with a nanosecond repetitively pulsed plasma discharge are experimentally investigated in both in situ time-dependent TDLAS and steady state gas sampling diagnostics. Fuel consumption is found to be more effective in higher argon than higher oxygen concentrations, indicating higher electron number densities with argon dilution is more effective than direct electron impact dissociation of oxygen. Steady state sampling results suggest a linear trend of n-heptane dissociation and product species formation with increasing plasma frequency, with different major product species for oxidation and pyrolysis. In the time-dependent measurements, the comparison between experiments and numerical modeling show that formation of a major intermediate species, formaldehyde, is significantly under-predicted while fuel and water production are over-predicted. This discrepancy suggests missing reactions in the current model, possibly involving excited alkyl peroxide radicals.

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Yiguang Ju, Wenting Sun

Published 10 February 2015

Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control.

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Yiguang Ju, Wenting Sun

Published 8 January 2015

Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control.

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Joseph K. Lefkowitz, Mruthunjaya Uddi, Bret C. Windom, Guofeng Lou, Yiguang Ju

Published 24 August 2014

In situ measurements by mid-IR laser absorption spectroscopy of C2H4/Ar dissociation and C2H4/O2/Ar oxidation activated by a nanosecond repetitively pulsed plasma have been conducted in a low temperature flow reactor (below 500 K) at a pressure of 60 Torr for both a continuously pulsed plasma discharge mode and a burst mode with 150 pulses. The measurements of the in situ diagnostics are validated and complemented by gas chromatography in the continuous discharge mode. A recently developed kinetic mechanism (HP-Mech-Plasma) for plasma activated C2H4 oxidation is assembled. The experiments of plasma activated dissociation show that the formation of acetylene by direct electron impact dissociation and dissociation by excited and ionized argon collision reactions is the major fuel consumption pathway. Plasma activated C2H4 oxidation experiments show that there exist three fuel consumption pathways, (1) a plasma activated low temperature fuel oxidation pathway via O2 addition reactions; (2) a direct fragmentation pathway via collisional dissociation by electrons, ions, and electronically excited molecules; and (3) a direct oxidation pathway by plasma generated radicals. It is found that the plasma activated low temperature oxidation pathway is dominant and leads to a large amount of formaldehyde formation with less acetylene and negligible large hydrocarbon molecules as compared to the dissociation experiment. The results also indicate that the latter two fuel consumption pathways are strongly dependent on O2 and Ar concentrations due to their effect on the production of atomic oxygen and excited Ar. Although the current model improves the overall prediction over USC-Mech II for plasma activated dissociation and oxidation, both models fail to predict quantitatively the H2O and CH4 formation. The present data provide good targets for future model development in plasma-assisted combustion.

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Wenting Sun, Sang HeeWon, Yiguang Ju

Published 17 February 2014

The effect of non-equilibrium plasma activated low temperature chemistry (PA-LTC) on the ignition and extinction of Dimethyl Ether (DME)/O2/He diffusion flames has been investigated experimentally in a counterflow burner with in situ nanosecond pulsed discharge at 72 Torr. A uniform discharge is generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of DME/O2/He are studied by employing OH and CH2O Planar Laser Induced Fluorescence (PLIF) techniques at constant strain rates and O2 mole fractions on the oxidizer side with varying the DME mole fractions. Contrary to the conventional understanding, strong low temperature reactivity during ignition process is observed for DME with non-equilibrium plasma activation even at 72 Torr and flow residence time of a few milliseconds. The OH PLIF shows strong OH signal at and after ignition, whereas extremely low OH signal before ignition. However, the CH2O PLIF experiments demonstrate that, with the increase of DME mole fraction on the fuel side, the CH2O PLIF signal intensity increases significantly before ignition and decreased rapidly after ignition. The low OH number density and high CH2O number density before DME ignition clearly demonstrates the existence of PA-LTC at low pressure. Moreover, at higher O2 mole fraction and discharge repetition frequency, the in situ discharge significantly modifies the characteristics of ignition and extinction, thus creating a new monotonically and fully stretched ignition S-curve without an extinction limit. Compared to our previous study of methane, the existence of strong low temperature reactivity in DME oxidation makes ignitions occur at much lower fuel mole fractions, thus accelerating the transition of ignition curve from conventional S-curve to the fully stretched S-curve. The transition from the conventional S-curve to the new stretched ignition curve at high plasma repetition rate indicates that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry even at low pressure. The chemical kinetic model for the plasma–flame interaction has been also developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling results reveal that the PA-LTC has a much shorter timescale comparing with that of thermally activated low temperature chemistry owing to the rapid radical production by plasma. The reaction pathways analysis shows that atomic O generated by the discharge is critical to controlling the population of radical pool.

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C.H.Skinnera, R. Sullenberger, B.E.Koel, M.A.Jaworski, H.W.Kugel

Published 16 January 2013

Lithium conditioned plasma facing surfaces have lowered recycling and enhanced plasma performance on many fusion devices. However, the nature of the plasma–lithium surface interaction has been obscured by the difficulty of in-tokamak surface analysis. We report laboratory studies of the chemical composition of lithium surfaces exposed to typical residual gases found in tokamaks. Solid lithium and a molybdenum alloy (TZM) coated with lithium have been examined using X-ray photoelectron spectroscopy, temperature programmed desorption, and Auger electron spectroscopy both in ultrahigh vacuum conditions and after exposure to trace gases. Lithium surfaces near room temperature were oxidized after exposure to 1–2 Langmuirs of oxygen or water vapor. The oxidation rate by carbon monoxide was four times less. Lithiated PFC surfaces in tokamaks will be oxidized in about 100 s depending on the tokamak vacuum conditions.

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Timothy Ombrello, Yiguang Ju and Alexander Fridman

Published 2 May 2012

Kinetic ignition enhancement of CH 4 –air and H 2 –air diffusion flames by a nonequilibrium plasma discharge of air was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species, as well as computations of rates of production and sensitivity analyses, were performed to understand the kinetic enhancement pathways for ignition by plasma discharge of air. It was found that plasma discharge of air led to significant kinetic ignition enhancement illustrated by large decreases in the ignition temperatures for a broad range of strain rates. Examination of the radical and NO x production in the plasma showed that the enhancement was caused primarily by the catalytic effect of NO x . The results of numerical simulations of the counterflow burner with preheated air and NO x addition showed the existence of different ignition regimes, which appeared due to the competition between radical production by NO x and other pathways, as well as heat release. There were two ignition regimes for small concentrations of NO x and three ignition regimes for large concentrations of NO x . Numerical simulations agreed well with the experimental measurements and suggested a new strategy for plasma-assisted ignition in supersonic flow, where a combination of thermal and nonthermal plasma would work more efficiently for ignition enhancement.

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Timothy Ombrello, Xiao Qin, Yiguang Ju, Alexander Gutsol, Alexander Fridman and Campbell Carter

Published 2 May 2012

A new piecewise nonequilibrium gliding arc plasma discharge integrated with a counterflow flame burner was developed and validated to study the effect of a plasma discharge on the combustion enhancement of methane-air diffusion flames. The results showed that the new system provided a well-defined flame geometry for the under- standing of the basic mechanism of the plasma-flame interaction. It was shown that with a plasma discharge of the airstream, up to a 220% increase in the extinction strain rate was possible at low-power inputs. The impacts of thermal and nonthermal mechanisms on the combustion enhancement was examined by direct comparison of measured temperature profiles via Rayleigh scattering thermometry and OH number density profiles via planar laser-induced fluorescence (calibrated with absorption) with detailed numerical simulations at elevated air temper- atures and radical addition. It was shown that the predicted extinction limits and temperature and OH distributions of the diffusion flames, with only an increase in air temperature, agreed well with the experimental results. These results suggested that the effect of a stabilized piecewise nonequilibrium gliding arc plasma discharge of air at low air temperatures on a diffusion flame was dominated by thermal effects.

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Xiaofang Yang, Yannick C. Kimmel, Jie Fu, Bruce E. Koel, and Jingguang G. Chen

Published 16 March 2012

Tungsten carbide (WC) has been considered a promising replacement for precious metal-based catalysts and electrocatalysts; however, synthesis of high-quality WC that is free of surface carbon remains a major challenge. Surface carbon adversely influences the catalytic activity of WC and hinders direct interaction between metal adlayer modifiers and the WC substrate. In this letter, we report the beneficial effects of pretreatments of WC foil by atomic oxygen generated in an oxygen plasma source. We found that the graphitic carbon at the WC surface could be removed controllably by the atomic oxygen without causing oxidation of WC, and this improved performances for electrocatalytic methanol oxidation and hydrogen evolution.

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Wenting Sun, Mruthunjaya Uddi, Sang Hee Won, Timothy Ombrello,  Campbell Carter, Yiguang Ju

Published 16 August 2011

The kinetic effects of low temperature non-equilibrium plasma assisted CH4 oxidation on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH4 by volume into the oxidizer stream of a counterflow system. The experiments showed that non-equilibrium plasma can dramatically accelerate the CH4 oxidation at low temperature. The rapid CH4 oxidation via plasma assisted combustion resulted in fast chemical heat release and extended the extinction limits significantly. Furthermore, experimental results showed that partial fuel mixing in the oxidizer stream led to a dramatic decrease of O concentration due to its rapid consumption by CH4 oxidation at low temperature. The products of plasma assisted CH4 oxidation were measured using the Two-photon Absorption Laser-Induced Fluorescence (TALIF) method (for atomic oxygen, O), Fourier Transform Infrared (FTIR) spectroscopy, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H2O and H2 concentrations and under-predicted CO2 concentration. A path flux analysis showed that O generated by the plasma was the critical species for extinction enhancement. In addition, the results showed that O was produced mainly by direct electron impact dissociation reactions and the collisional dissociation reactions of electronically excited molecules with O2. Moreover, these reactions involving electron impact and excited species collisional dissociation of CH4 contributed approximately a mole fraction of 0.1 of total radical production. The present experiments produced quantitative species and extinction data of low temperature plasma assisted combustion to constrain the uncertainties in plasma/flame kinetic models.

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Wenting Sun, Mruthunjaya Uddi, Sang Hee Won, Timothy Ombrello,  Campbell Carter, Yiguang Ju

Published 2 October 2010

A non-equilibrium plasma assisted combustion system was developed by integrating a counterflow burner with a nano-second pulser to study the effects of atomic oxygen production on the extinction limits of methane diffusion flames at low pressure conditions. The production of atomic oxygen from the repetitive nano-second plasma discharge was measured by using two-photon absorption laser-induced fluorescence (TALIF). The results showed that both the atomic oxygen concentration production and the oxidizer stream temperature increased with the increase of the pulse repetition frequency for a constant plasma voltage. The experimental results revealed that the plasma activated oxidizer significantly magnified the reactivity of diffusion flames and resulted in an increase of extinction strain rates through the coupling between thermal and kinetic effects. Numerical computations showed that atomic oxygen quenching strongly depends on the oxidizer stream temperature. The kinetic effect of atomic oxygen production by a non-equilibrium plasma discharge on the enhancement of flame extinction limits was demonstrated, for the first time, at high repetition frequencies with elevated oxidizer temperatures. The reaction paths for radical production and consumption were analyzed. It was concluded that in order to achieve significant kinetic enhancement from atomic oxygen production on flame stabilization, the plasma discharge temperature needs to be above the critical crossover temperature which defines the transition point from radical termination to chain-branching.

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Timothy Ombrello, Sang Hee Won, YiguangJu, Skip Williams

Published 10 March 2010

The isolated effect of O2(a1Δg) on the propagation of C2H4 lifted flames was studied at reduced pressures (3.61 kPa and 6.73 kPa). The O2(a1Δg) was produced in a microwave discharge plasma and was isolated from O and O3 by NO addition to the plasma afterglow in a flow residence time on the order of 1 s. The concentrations of O2(a1Δg) and O3 were measured quantitatively through absorption by sensitive off-axis integrated-cavity-output spectroscopy and one-pass line-of-sight absorption, respectively. Under these conditions, it was found that O2(a1Δg) enhanced the propagation speed of C2H4 lifted flames. Comparison with the results of enhancement by O3 found in part I of this investigation provided an estimation of 2–3% of flame speed enhancement for 5500 ppm of O2(a1Δg) addition from the plasma. Numerical simulation results using the current kinetic model of O2(a1Δg) over-predicts the flame propagation enhancement found in the experiments. However, the inclusion of collisional quenching rate estimations of O2(a1Δg) by C2H4 mitigated the over-prediction. The present isolated experimental results of the enhancement of a hydrocarbon fueled flame by O2(a1Δg), along with kinetic modeling results suggest that further studies of CnHm + O2(a1Δg) collisional and reactive quenching are required in order to correctly predict combustion enhancement by O2(a1Δg). The present experimental results will have a direct impact on the development of elementary reaction rates with O2(a1Δg) at flame conditions to establish detailed plasma–flame kinetic mechanisms.

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Published 14 August 2006

The effects of NO and NO2 produced by using a plasma jet (PJ) of a N2/O2 mixture on ignition of hydrogen, methane, and ethylene in a supersonic airflow were experimentally and numerically investigated. Numerical analysis of ignition delay time showed that the addition of a small amount of NO or NO2 drastically reduced ignition delay times of hydrogen and hydrocarbon fuels at a relatively low initial temperature. In particular, NO and NO2 were more effective than O radicals for ignition of a CH4/air mixture at 1200 K or lower. These ignition enhancement effects were examined by including the low temperature chemistry. Ignition tests by a N2/O2 PJ in a supersonic flow (M = 1.7) for using hydrogen, methane, and ethylene injected downstream of the PJ were conducted. The results showed that the ignitability of the N2/O2 PJ is affected by the composition of the feedstock and that pure O2 is not the optimum condition for downstream fuel injection. This result of ignition tests with downstream fuel injection demonstrated a significant difference in ignition characteristics of the PJ from the ignition tests with upstream fuel injection.

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