Nanosecond Pulsed Discharge Plasma Research Articles

Plasma-assisted NH3/air flame: Simultaneous LIF measurements of O and OH

In the emerging field of plasma-assisted ammonia (NH3) combustion, the evolution of key intermediate species has rarely been reported. This work establishes a simultaneous measurement system of laser-induced fluorescence (LIF) for hydroxyl (OH) and quantitative two-photon-absorption LIF for atomic oxygen (O), to explore the OH and O dynamics in an NH3/air flame affected by a nanosecond (ns) pulsed plasma discharge. Firstly, with the plasma on, the molar fraction of O is quantified to reach 8.7 × 10–3 in the burnt zone, about two orders of magnitude higher than that without plasma. In addition, the OH LIF signal intensity is four times higher, indicating a significant kinetic enhancement. Then, the spatial characteristics of OH and O are discussed and compared, showing remarkable discrepancy. The discrepancy between them indicates that O production is dominated by plasma kinetics, however, the OH production, primarily stemming from reactions between O and NH3/H2O, still depends on parameters associated with combustion kinetics. We further study the temporal dynamics of O and OH. It is concluded that O and OH peaks at 1.75 μs are mainly attributed to the pathway of quenching of the excited species. After that, O and OH start to decay but show significant differences between unburnt and burnt zones, which are characterized by a single-exponential decay and a bi-exponential decay, respectively. In the unburnt zone, the OH decay is much slower than the O decay due to the diverse pathways for OH production. In the burnt zone, the bi-exponential decay of O and OH can essentially be regarded as a process in which the NH3/air reactive system reaches chemical equilibrium. At this stage, the impacts of the excited species from the plasma gradually diminish and combustion kinetics dominates alone.

Nanosecond pulsed discharge plasma modified porous polymer adsorbent materials for efficient removal of low‐concentration bisphenol A in liquid

AbstractThe efficient removal of low‐concentration endocrine disruptors is crucial for the protection of the aquatic environment. In this study, porous polymer adsorbent materials were modified by nanosecond pulsed discharge plasma to achieve efficient adsorption of low‐concentration bisphenol A (BPA). The removal efficiency of BPA reached 99% after 10 min of plasma modification at a pulse peak voltage of 28 kV, which increased by 25.8% compared to the raw materials. This enhancement was attributed to the increase of active sites and oxygen‐containing functional groups. The adsorption behaviors of the porous polymer materials were primarily dominated by monolayer chemisorption. Subsequently, comparative experiments further verified the high‐efficiency adsorption performance of porous polymer materials after plasma treatment.

Generation of meter-scale nanosecond pulsed DBD and the intelligent evaluation based on multi-dimensional feature parameter extraction

This study introduces a novel meter-scale dielectric barrier discharge (m-DBD) reactor designed to generate large-scale, low-temperature nanosecond pulsed discharge plasma. By employing a modularized gas path, this reactor enables a comprehensive analysis of discharge patterns and uniformity using multi-dimensional discharge parameters. Simulation results reveal optimal gas distribution with ten gas holes in the variable plate and a 40 mm slit depth in the main reactor. Besides, a diagnosis method based on electro-acoustic-spectrum-image (E-A-S-I) parameters is developed to evaluate nanosecond pulsed m-DBD discharge states. It is found that the discharge states are closely related to the consistency of segmental discharge currents, the fluctuation of acoustic signals and the distribution of active particles. Machine learning methods are established to realize the diagnosis of m-DBD discharge pattern and uniformity by E-A-S-I parameters, where the optimized BPNN has a best recognition accuracy of 97.5%. Furthermore, leveraging nanosecond pulse power in Ar/m-DBD enables stable 1120 × 70 mm2 discharge, uniformly enhancing hydrophobicity of large-scale materials from a 67° to 122° water contact angle with maximal fluctuations below 7%. The modularized m-DBD reactor and its intelligent analysis based on multi-dimensional parameter provide a crucial foundation for advancing large-scale nanosecond pulsed plasma and their industrial applications.

Streamer propagation characteristics of nanosecond pulsed discharge plasma on fluidized particles surface: experimental investigation and numerical simulation

Plasma fluidized-bed contributes to strengthening the interaction between active species in plasma and fluidized powder particles, resulting in higher active species utilization efficiency and superior powders processing/modification performance. However, the plasma streamer dynamics on the fluidized powder particles are still unclear due to the intricacy of plasma fluidized-bed. In this work, the time-resolved evolution behavior of plasma streamers on fluidized powder particles surfaces has been explored in plasma fluidized-bed system based on a simplified pin-cylinder configuration. The results reveal that the entire streamer propagation process includes volume discharge and surface discharge. The maximum electron density generated by surface discharge is one order of magnitude higher than that produced by volume discharge, indicating that surface discharge plays a dominant role in powder particles modification. The presence of fluidized particle will cause streamer branching, and the main streamer splits into two independent sub-streamers for propagation in a ‘parabola-like’ shape. Compared with large-size fluidized particles (1000 µm), streamer wraps a larger area on micron-size fluidized particles (200 µm), with a 78% increase in the coverage area, which is favorable to the modification of powder particles. Furthermore, the evolution of active species on fluidized particle surface is analyzed. The active species (N, O, O2 −) are mainly distributed around the north pole, and N2 + is mainly distributed between 25° and 50° of the particles. With the decrease of fluidized particle size, the polarization effect between particles is significantly enhanced, and the maxima of the number densities of active species increase. These findings help to get a better understanding of the interaction between plasma and fluidized particles in fluidized systems.

Application of emission spectroscopy in plasma-assisted NH3/air combustion using nanosecond pulsed discharge

This work investigates the effects of atmospheric-pressure nanosecond (ns) pulsed plasma discharges on NH3/air flames using optical emission spectroscopy (OES). Firstly, spatially-resolved OES in the 200–800 nm range of NH3/air flames with and without discharges are presented to identify the plasma-induced excited species (such as N2* and H*). Then, time-resolved OES with ns timescale resolution are achieved to illustrate the discharge dynamics using a statistical imaging strategy, which is specifically designed to overcome the timing jitter of the discharge. From the time-resolved spectra of excited N2, the first and second emission events in NH3/air flames with a time interval of 80 ns are revealed, and each exhibits a profile with two exponential decays. Furthermore, spatial- and time-resolved rotational and vibrational temperatures are derived from the spectral fitting of N2(C→B) to quantify the thermal effects of ns plasmas, thus revealing the ultrafast and slow gas heating processes. In addition, the electron number density along the flame is obtained from the intense Hα line, which locally reaches up to 1 × 1017 cm−3. Besides, the spatial-resolved Hα line intensity suggests that the flame chemistry does not directly affect, but the discharge dominates the production of the atomic hydrogen. It provides insight into the kinetic effects of ns pulsed discharge on NH3 combustion enhancement.

(Invited) Plasma-Assisted Ammonia Combustion and Flare Gas Reforming to Enhance Reactivity and Control Emission

Increasing emphasis on the de-carbonization of energy production has led researchers to consider carbon-free, renewable, and green fuels such as hydrogen (H2) and ammonia (NH3). While H2 suffers from the major challenges of production difficulties, transportation, and storage, NH3 is plagued with challenges of low flame speeds causing unstable flames, high autoignition temperatures resulting in longer ignition delays, narrow flammability limits, and higher levels of NOx emission. Among the different solutions to overcome these challenges of NH3 combustion, non-equilibrium plasma-based igniters are significant owing to the promotion of localized volumetric ignition kernel development by both thermal and chemical assistance. Computational investigation of plasma-assisted combustion of ammonia-air mixtures in constant volume and constant pressure reactors are conducted, to determine the impact of operating conditions on ignition delays and NOx emissions. A mechanism has been assembled in this work using well-validated plasma reactions of NH3 with O2 and N2, alongside plasma kinetics of air from the literature. Subsequently, the newly developed mechanism was used to investigate the plasma-assisted oxidation of NH3. In particular, the impact of the reduced electric field (E/N), equivalence ratio, pressure, pulse frequency, and energy density on the ignition delays and NOx emission were investigated. A Global Pathway-based Analysis algorithm for plasma-assisted systems (PGPA) is used to analyze the nanosecond pulsed nonequilibrium plasma-assisted combustion of NH3/air mixtures. Firstly, a faster ignition and lower production of NOx are observed in the case of plasma discharges compared to thermal energy deposition, owing to the enhanced production of OH radicals and the early reforming of NH3 to produce N2 and H2 with plasma, respectively. At lower reduced electric fields (E/N), PGPA analyses elucidated the significance of gas heating due to vibrational-translational cycles of NH3 and N2 on the increased reactivity of NH3/air mixtures as compared to ignition at a higher E/N. The fuel-lean mixture is observed to exhibit higher production of NOx than stoichiometric and fuel-rich mixtures, resulting from plasma chemistry involving oxygen radical and electronic excited states of N2. Higher rates of collisional quenching at higher pressures during the inter-pulse gaps are found to result in a lesser amount of electronically excited states of N2 and O2, resulting in lower production of air-bound NOx during the pulses.Complementing combustion enhancements, the study also considers the role of plasma-assisted systems in gas reforming, thereby imparting specific desired characteristics lacking in the original mixture. For instance, plasma-assisted reforming can be utilized to control emissions by reforming specific emission precursors or by improving the gas reactivity to promote clean combustion. Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring/burning of the associated gas removes greenhouse gases like methane (CH4) and other hydrocarbons. Our study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. A well-studied conventional combustion chemistry for small alkanes is coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed nonequilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of H2 and C2H4 in the reformed gas mixture, owing to the electron-impact dissociations of alkanes and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3. The reformed mixture exhibits significantly high reactivity as exhibited by their increased flame speeds and shorter ignition delays. The reformed mixture is also observed to promote increased CH4 destruction levels and complete flaring, thereby reducing the emissions of CH4 and other hydrocarbons.

Simultaneous hydrodynamic cavitation and nanosecond pulse discharge plasma enhanced by oxygen injection

A novel Hydrodynamic Cavitation-Assisted Oxygen Plasma (HCAOP) process, which employs a venturi tube and oxygen injection, has been developed for enhancing the production and utilization of hydroxyl radicals (·OH) in the degradation of organic pollutants. This study has systematically investigated the fluid characteristics and discharge properties of the gas–liquid two-phase body in the venturi tube. The hydraulic cavitation two-phase body discharge is initiated by the bridging of the cavitation cloud between the electrodes. The discharge mode transitions from diffuse to spark to corona as the oxygen flow rate increases. The spark discharge has the highest current and discharge energy. Excessive oxygen results in the change of the flow from bubbly to annular and a subsequent decrease in discharge energy. The effects of cavitation intensity, oxygen flow rate, and power polarity on discharge characteristics and ·OH production were evaluated using terephthalic acid as a fluorescent probe. It was found that injecting 3 standard liter per minute (SLPM) of oxygen increased the ·OH yield by 6 times with only 1.2 times increase in power, whereas<0.5 SLPM of oxygen did not improve the ·OH yield due to lower breakdown voltage. Negative polarity voltage increased the breakdown voltage and ·OH yield due to asymmetric density and pressure distribution in the throat tube. This polarity effect was explained by numerical simulation. Using indigo carmine (E132) as a model pollutant, the HCAOP process degraded 20 mg/L of dye in 5 L water within 2 min following a first-order reaction. The lowest electric energy per order (EEO) was 0.26 (kWh/m3/order). The HCAOP process is a highly efficient flow-type advanced oxidation process with potential industrial applications.

Methane coupling in nanosecond pulsed plasmas: Correlation between temperature and pressure and effects on product selectivity

We present a zero-dimensional kinetic model to characterise specifically the gas-phase dynamics of methane conversion in a nanosecond pulsed discharge (NPD) plasma reactor. The model includes a systematic approach to capture the nanoscale power discharges and the rapid ensuing changes in electric field, gas and electron temperature, as well as species densities. The effects of gas temperature and reactor pressure on gas conversion and product selectivity are extensively investigated and validated against experimental work. We discuss the important reaction pathways and provide an analysis of the dynamics of the heating and cooling mechanisms. H radicals are found to be the most populous plasma species and they participate in hydrogenation and dehydrogenation reactions, which are the dominant recombination reactions leading to C2H4 and C2H2 as main products (depending on the pressure).

A parametric study on control authority and vorticity transport in a compressor airfoil with plasma actuation at low Reynolds number

In order to obtain the general criterion of control authority for suppressing flow separation in the compressor airfoil at low Reynolds number by nanosecond pulsed dielectric barrier discharge plasma actuation with different actuation parameters, a parametric study on the actuation power and actuation frequency is performed using a large eddy simulation. A non-dimensional actuation power is proposed to reveal the relationship between the actuation power and the characteristic power of the baseline flow field, in order to provide a reference for obtaining a balance between energy consumption and flow control. With different actuation powers, the mechanism behind the evolution process of vortex structures induced by the plasma actuation is revealed through a decomposition of the vorticity according to the vorticity transport equation and the analysis of the non-dimensional circulation according to the Q criterion. It is found that the evolution shows a relatively consistent tendency and can be divided into three stages corresponding to different disturbance processes induced by Kelvin–Helmholtz instabilities. Finally, a criterion for the effective actuation frequency based on the continuous generation of induced vortex structures is established considering different actuation powers and simplified to specific parameters within the flow field.

Mechanism of runaway electron generation in nanosecond pulsed plate-plate discharge at atmospheric-pressure air

Classical discharge theory (Townsend theory and streamer theory) has limitations in explaining nanosecond pulsed gas discharge. In recent years, the research on nanosecond pulsed gas discharge theory based on the high-energy runaway electrons has attracted extensive attention. But so far, there have been few studies of the generation mechanism of runaway electrons in atmospheric-pressure-air nanosecond pulsed plate-to-plate discharge, which seriously hinders the application and development of nanosecond pulse discharge plasma. In this paper, a one-dimensional implicit particle-in-cell/Monte Carlo collision (PIC/MCC) model is developed to investigate the mechanism of runaway electron generation and breakdown in a 1 mm-long atmospheric-pressure-air gap between the plate electrode and plate electrode driven by a negative nanosecond pulse voltage with an amplitude of 20 kV. The results show that under the influence of space charge dynamic behavior, the electric field enhancement region appears between the plate electrode and plate electrode, so that electrons can satisfy the electron runaway criteria and behaves in the runaway mode. In addition, it is also observed that the pre-ionization effect of the runaway electrons in front of the discharge channel can cause the secondary electron avalanches. As the secondary electrons avalanche and the discharge channel continues to converge, the discharge is guided and accelerated, eventually leading to the breakdown of the air gap. This study further reveals the mechanism of nanosecond pulsed plate-plate discharge, expands the basic theory of nanosecond pulsed gas discharge, and opens up new opportunities for the application and development of nanosecond pulsed discharge plasma.

High-Efficiency Adsorption of SARS-CoV-2 Spike 1 Protein by Plasma-Modified Porous Polymers

Under the background of the COVID-19 pandemic, this study reports an affordable and easily prepared porous material modified by nanosecond-pulsed discharge plasma, which can adsorb SARS-CoV-2 S1 protein efficiently. Both Western blotting and an enzyme-linked immunosorbent assay were used to detect the adsorption efficiency of SARS-CoV-2 S1 protein. The physical and chemical properties of the modified porous polymer were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. We found that the new type of porous polymer material presented an excellent performance on SARS-CoV-2 S1 protein adsorption, whose adsorption efficiency reached 99.99% in 1 min. Both the physical and chemical characterizations showed that the material has many fresh pores on the material surface and that the surface is implanted with polar functional groups (C−O, C=O, O−C=O and −NH), which gives the material a high chemisorption performance along with an enhanced physical adsorption performance. Notably, the material can be prepared at prices ranging in the tens of dollars per kilogram, which shows that it could have great applications for respiratory virus protection in global epidemic states.

Nanosecond pulsed plasma discharge for remediation of simulated wastewater containing thiazine and azo dyes as model pollutants

Plasmas generated over and under the surface of water have immense scope for wastewater remediation due to in situ generation of redox-rich species. Remediation of simulated wastewater using underwater plasma discharge is reported in this paper. During the plasma discharge, an increase in energy coupling from source to discharge channel was noted with an increase in the applied pulsed DC voltage from 5 to 9kV. Higher energy coupling resulted in elevation of gas temperature and electron temperature. Concentrations of hydroxyl radicals and hydrogen peroxide, estimated spectrochemically in effluent water, also increased with the applied voltage. On the other hand, ozone concentration showed a decreasing trend with increasing applied voltage and the peak ozone density was obtained at the lower end of the applied voltage, i.e., at 5kV. With higher energy dissipation, a pronounced discharge and, hence, greater emission intensities of ultraviolet radiation, atomic oxygen, and atomic hydrogen were observed in discharge spectra. The oxidative-reductive stress produced by underwater discharge resulted in a degradation efficiency of 75.5% for methylene blue solution (thiazine dye) corresponding to energy yields of 3.55g/kWh. Similarly, the degradation efficiency of 81.4% for methyl orange solution (azo dye) with an energy yield of 4.73g/kWh was achieved with this discharge. From the considerations of collision theory, activation energies of 67.17kJ/mol for methylene blue and 75.32kJ/mol for methyl orange degradation were estimated for the plasma discharge. Fourier-transform infrared spectroscopy was utilized to further reveal the possible effects of plasma therapy on different bonds of the dyes. Additionally, gas chromatography mass spectrometry analyses were used to suggest the possible degradation pathways of the two dyes. Instant dye degradation in flowing samples was compared with contemporary dye degradation results listed in literature to show the effectiveness of the used technique.

Investigation of the interaction between NS-DBD plasma-induced vortexes and separated flow over a swept wing

The effect of nanosecond pulsed dielectric barrier discharge (NS-DBD) plasma flow separation control is closely related to the actuation frequency, because it involves the interaction between plasma-induced vortexes and separated flow. In order to study the mechanism of NS-DBD plasma flow separation control over a swept wing, especially the influence of the actuation frequency, at first, experimental studies of the actuation frequencies at 100 Hz are conducted to validate the numerical simulation method. Then, numerical studies of different actuation frequencies which are 50 Hz, 100 Hz, 160 Hz, 200 Hz, 500 Hz, and 1000 Hz, respectively are conducted. The interaction between the plasma-induced vortexes and the separated flow is analyzed. Results show that there is a range of the actuation frequency which includes the frequency (160 Hz) calculated by the average aerodynamic chord length to make the control effect good, but when the actuation frequencies are too low (50 Hz) or too high (1000 Hz), the control effect will get worse. The former is because plasma-induced vortexes disappear in a period within an actuation cycle; the latter is because plasma-induced vortexes cannot develop completely, resulting in a weak vortex intensity.

A novel, green, low-cost regeneration method for surface enhanced raman scattering (SERS) solid substrates based on nanosecond pulsed cold plasma technology

Surface enhanced raman scattering (SERS) technique has been widely implemented for the detection/quantification of numerous compounds. The development of reusable SERS substrates through regeneration is a constant concern of scientists in the field, related to the sustainability of the method. Cold atmospheric plasma (CAP) is an advantageous green method well-known for its effectiveness towards the successful degradation of organic molecules and materials’ activation/modification. In the present study, we explored for the first time nanosecond pulsed dielectric barrier discharge (NSP-DBD) plasma as a rapid, energy efficient method for SERS solid substrates regeneration, implemented either directly in the gas or in the liquid phase as well as through immersion of the substrates in plasma activated water (PAW). We investigated the critical cold plasma factors (e.g. feeding gas, plasma treatment and retention time) in order to propose the most cost-effective alternative and shed light on the underlying regeneration mechanisms. The different SERS analysis case scenarios (analyte's class , concentration andcross check) were considered, in order to simulate real SERS measurements conditions/requirements. In practical terms, such an approach will contribute to a significant reduction of the detection costs, revealing the NSP-DBD process as a flexible, fast, green, effective and low-cost solution towards the SERS substrates regeneration.

Kinetic investigation of plasma catalytic synthesis of ammonia: insights into the role of excited states and plasma-enhanced surface chemistry

The present work investigates the kinetics of catalytic ammonia synthesis in a N2/H2 mixture activated by a nanosecond pulsed discharge plasma experimentally and numerically. X-ray diffraction, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy are combined to characterize the morphology and surface electronic properties of the catalyst. Special attention is placed on the role of excited species in promoting the formation of important intermediates and the plasma-enhanced surface chemistry. A detailed kinetic mechanism consisting of atoms, radicals, excited species, molecules, ions, and surface species is developed and studied by incorporating a set of the electron impact reactions, reactions involving excited species, ionic reactions, direct and dissociative adsorption reactions, and surface reactions. A zero-dimensional model incorporating the plasma kinetics solver is used to calculate the temporal evolution of species densities in a N2/H2 plasma catalysis system. The results show that the coupling of Fe/γ–Al2O3 catalyst with plasma is much more effective in ammonia synthesis than the Fe/γ–Al2O3 catalyst alone and plasma alone. The numerical model has a good agreement with experiments in ammonia formation. The path flux analysis shows the significant roles of excited species N(2D), H2(v1), N2(v) in stimulating the formation of precursors NH, NH2, and adsorbed N(s) through the pathways N(2D) + H2 → NH + H, H2(v1) + NH → NH2 + H and N2(v) + 2Fe(s) → N(s) + N(s), respectively. Furthermore, the results show that the adsorption reaction N + Fe(s) → N(s) and Eley–Ridel interactions N(s) + H → NH(s), N + H(s) → NH(s), NH + H(s) → NH2(s) and NH2 + H(s) → NH3(s) can kinetically enhance the formation of ammonia, which further highlights the plasma-enhanced surface chemistry. This work provides new insights into the roles of excited species and plasma-enhanced surface chemistry in the plasma catalytic ammonia synthesis.

A high‐efficiency adsorbent resin treated by nanosecond pulsed discharge for the removal of trace heavy metal ions in water

AbstractIn this paper, a high‐efficiency adsorbent resin is prepared by nanosecond pulsed discharge plasma to remove Cu(II), Pb(II), and Cd(II). The effects of discharge parameters, pH, and adsorbent dose on adsorption capacity of resins are evaluated. Correspondingly, the pore structure and chemical groups are analyzed using scanning electron microscopy, N2 adsorption–desorption, and X‐ray photoelectron spectroscopy. The equilibrium adsorption capacity of modified resins for Cu(II), Pb(II), and Cd(II) can reach up to 6.84, 10.87, and 9.22 mg/g, respectively, which is increased by 62%, 49%, and 48% compared with that of the raw. The improvement of adsorption capacity is attributed to the enhancement of specific surface area and mesopores number, as well as the generation of plentiful amino and O‐containing functional groups.

Micro-mesoporous polystyrene resins prepared by nanosecond pulsed discharge plasma for the efficient adsorption of PAHs

Adsorption method has been widely used in purification, trace pollutant control, biomedicine, and other fields, and preparing an efficient adsorbent with low cost is very important for the applications. In this work, we employed nanosecond pulsed discharge plasma as a pore-forming agent to produce micro-mesoporous in polystyrene macroporous resins and reported the adsorption behavior of naphthalene on resins. By plasma induced pore-forming and functional groups-grafting, the equilibrium adsorption amount of micro-mesoporous resins for naphthalene is 324.4 % higher than that of the raw polystyrene macroporous resins. The characterization results show that a new pore size ranging from 0.5 to 2 nm and a vastly increased content of oxygen group of CO appears on the surface of the resin after plasma treatment. Additionally, the novel micro-mesoporous resins had good reusability without apparent capacity loss after five adsorption–desorption cycles.

Experimental investigation of non-equilibrium plasma-assisted ammonia flames using NH2* chemiluminescence and OH planar laser-induced fluorescence

This study investigates the effect of nano-second pulsed non-equilibrium plasma on ammonia combustion in a model gas turbine combustor using NH2* chemiluminescence and OH planar laser-induced fluorescence (PLIF). Nano-second pulsed plasma discharge is generated at the exit of the premixed ammonia/air mixture injection tube. Broadband chemiluminescence observation shows that plasma can improve the stabilization of ammonia/air flame and extend the attached flame regime to a lower equivalence ratio. To understand the coupling effect between plasma kinetics and flame dynamics, NH2* chemiluminescence and OH PLIF of ammonia/air flames are measured at different conditions. With the increase of discharge voltage or discharge power, both NH2* chemiluminescence intensity and OH PLIF signal intensity increase, and this observation may explain the improved stabilization of ammonia flames. At very lean condition (equivalence ratio between 0.48 and 0.57, no visible flame existed), both NH2* chemiluminescence and OH PLIF signal are observed in the proximity of electrode region with plasma activation. If air is replaced by pure nitrogen (N2), NH2* chemiluminescence is measurable but extremely weak. As the oxygen (O2) concentration in the oxidizer stream gradually increases, NH2* chemiluminescence intensity increases linearly with O2 concentration under plasma activation. This finding indicates that the NH2* production in plasma-assisted ammonia oxidation process is possibly related to the production of OH or oxygen-related species. The direct electron impact on NH2* production might be secondary.

CH4 reforming with CO2 using a nanosecond pulsed dielectric barrier discharge plasma

Non-thermal plasma reforming of CH4 with CO2 into value-added fuels and chemicals is very promising but challenging due to the low energy efficiency of the reforming process. One of the most important factors influencing energy efficiency is plasma power supply, and nanosecond pulsed power sources are gaining increasing interest in enhancing the energy efficiency of plasma chemical processes. Here, we investigate plasma-assisted CH4 reforming with CO2 using a nanosecond pulsed dielectric barrier discharge (DBD) plasma. Both electrical characteristics and reaction performance were investigated under different operating parameters, including applied voltage, total flow rate, CH4/CO2 molar ratio, and discharge length. Increasing applied voltage (from 20 to 28 kV) and discharge length (from 6 to 14 cm) enhanced the total gas conversion by 53.2% and 41.2%, respectively, but decreased the energy efficiency by 30.6% and 14.3%, respectively. The highest energy efficiency (12.4%) was obtained at an applied voltage of 20 kV, a total flow rate of 50 ml/min, a CH4/CO2 molar ratio of 1:1 and a discharge length of 10 cm, while the highest total gas conversion of 30.3% was achieved at a higher applied voltage of 26 kV and a lower total flow rate of 25 ml/min. An artificial neural network (ANN) model was developed to predict the plasma reforming process, and to investigate the relative contributions of key operating parameters to the reaction performance of this process. The results of the ANN model indicate that the CH4/CO2 molar ratio and total flow rate affect the gas conversion most significantly, while the CH4/CO2 molar ratio plays a dominant role in determining the product selectivity and energy efficiency.

Dry reforming of methane in a nanosecond repetitively pulsed discharge: chemical kinetics modeling

Nanosecond pulsed discharge plasma shows a high degree of non-equilibrium, and exhibits relatively high conversions in the dry reforming of methane. To further improve the application, a good insight of the underlying mechanisms is desired. We developed a chemical kinetics model to explore the underlying plasma chemistry in nanosecond pulsed discharge. We compared the calculated conversions and product selectivities with experimental results, and found reasonable agreement in a wide range of specific energy input. Hence, the chemical kinetics model is able to provide insight in the underlying plasma chemistry. The modeling results predict that the most important dissociation reaction of CO2 and CH4 is electron impact dissociation. C2H2 is the most abundant hydrocarbon product, and it is mainly formed upon reaction of two CH2 radicals. Furthermore, the vibrational excitation levels of CO2 contribute for 85% to the total dissociation of CO2.