# Reducing Energy Cost of NOx Production in Air Plasmas

ArticleinThe Chemical Engineering Journal · January 2019with 261 Reads
Abstract
Current worldwide fixed nitrogen utilization efficiency (NUE) is low, leading to significant environmental problems. An important aspect of this problem involves NH3 released from bacterially degraded, N-containing organic waste such as animal manure. Acidifying organic waste with nitric acid traps NH3 as NH4NO3, thus increasing the nitrogen content of the resulting organic fertilizer and reducing the environmental effects of NH3 emission. Air plasmas have been proposed for this purpose through production of NOx from air which can be readily converted to nitric acid. However, a major challenge remains in reducing the energy cost of NOx production in air plasmas to make the technology economically competitive. Different types of discharges such as arcs, sparks, dielectric barrier discharges have been investigated for NOx production but they are challenging to compare due to differences in their structure, geometry and excitation modes etc. Different discharges also utilize potentially different chemical reaction pathways to produce NOx. As of yet, there are no general evaluation criteria to determine the qualities of an appropriate discharge configuration for energy-efficient NOx production. In this study, we report on four types of discharges; dielectric barrier, glow, spark and extending arc discharge to examine NOx production efficiency under different conditions. Based on our results and previously published results from the literature, we propose a dimensionless parameter to guide the design of appropriate plasma sources for reduced energy cost NOx production.
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• Article
This paper describes a recent advance on the conversion of molecular dinitrogen into ammonia or ammonia equivalent, silylamine, by using transition metal-dinitrogen complexes. Two effective systems for the catalytic transformation of molecular dinitrogen under ambient reaction conditions have been achieved by the use of molybdenum and iron complexes as catalysts. The former system employs a molybdenum-dinitrogen complex bearing two ferrocenyl diphosphines or an iron complex such as iron carbonyl or substituted ferrocene as a catalyst to afford up to 226 equiv of silylamine based on the catalyst. The latter system employs a dinitrogen-bridged dimolybdenum complex bearing two PNP-type pincer ligands as a catalyst to afford up to 23 equiv of ammonia based on the catalyst. Both systems provide a new aspect in the development of novel nitrogen fixation under mild reaction conditions.
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• Q Wang
• V Hessel
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• Nox production in lightning
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D. Burnette, A. Montello, I.V. Adamovich, W.R. Lempert, Nitric oxide kinetics in the afterglow of a diffuse plasma filament, Plasma Sour. Sci. Technol. 23 (4) (2014) 045007.
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• Article
A new atmospheric pressure non-equilibrium plasma source named the 'Propeller Arc' (PA) is developed using the concept of rotating electrodes. The PA device consists of a rotating cathode, driven by a motor, with one or more fixed anodes. Plasma is ignited at or near the narrowest gap as the rotating cathode passes by the anode and then it is extended up to a length of ∼66 mm or longer depending on the supplied power. This allows for efficient ignition, followed by a quick increase in plasma volume. The PA is similar to the widely used gliding arc (GA); however, unlike the GA, PA does not require imposed gas flow, and the PA discharge frequency can be easily controlled by the motor angular velocity. In this paper, the basic characteristics of PA are investigated using two different operation modes: pulse modulation and DC power. Discharge properties including electrical characteristics, time-resolved optical emission images, plasma electrical properties such as resistance and average electric field (discharge voltage divided by gap distance) and plasma power consumption are reported. Use of multiple anodes to increase the plasma volume is also demonstrated. As the PA has a compact design and is relatively easy to stabilize and control without the need for applied gas flow, it has potential to be adapted for many different applications such as nitrogen fixation, fuel and carbon dioxide conversion, waste, odor and hydrogen sulfide treatment, etc.
• Article
Improved utilization of organic waste for fertilizer has significant worldwide economic and ecological potential and the use of plasma can help unlock this potential. Organic waste that are used as fertilizer includes animal waste (manure and urine), human sewage, food waste and biogas digestate. Air plasma treatment of aqueous solutions of organic fertilizer (plasma activated organic fertilizer, or PAOF) has multiple advantages such as reduction or elimination of atmospheric emission of volatile organic carbon (VOC) compounds, CH$${_4}$$ and NH$${_3}$$. Although the emission of N$$_2\mathrm{O}$$ from the fertilized soil may be enhanced by PAOF, we surmise that N$$_2\mathrm{O}$$ emission at large is reduced because the losses of reactive nitrogen from the agro-ecosystem (which cause N$$_2\mathrm{O}$$ emissions elsewhere) are significantly reduced. In addition, PAOF will improve the commercial value of fertilizer that can be made from organic waste. This includes altering both the quantity and chemical form of N contained in the organic fertilizer, as well as odor reduction. PAOF appears to function using chemical reactivity similar to well-studied natural antimicrobial processes, resulting in significant antibacterial effects in treated waste. The commercial viability of PAOF depends on numerous factors, the most important of which are the energy efficiency and capital costs associated with the plasma process and associated processing equipment; the cost of electricity; and the nature and extent of government regulations regarding pollution from organic waste and all types of fertilizer. We estimate that if the total cost of plasma production of reactive nitrogen is below about €2/kg N–€3/kg N, the process will be economically viable in the absence of penalties or subsidies.
• Article
Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.
• Article
Developing an ammonia synthesis process from N2 and H2 is of interest in catalysis and hydrogen research community. Wool-like metal electrodes used to produce non-thermal plasma were determined to serve as efficient catalysts for ammonia synthesis under atmospheric pressure without heating. The catalytic activity of Pt, Pd, Ag, Cu, and Ni wools increased as the experiment was repeated, while that of Au, Fe, Mo, Ti, W, and Al was almost constant. The activity change was mainly due to migration of metals from the electrode to the inner wall of a silica reactor or increases in surface areas of metal catalysts. The order of the activity at each initial experiment was Au > Pt > Pd > Ag > Cu > Fe > Mo > Ni > W > Ti > Al. DFT calculations using Gaussian 09 and CASTEP were applied for energy changes in a reaction M3 + 1/2 N2 ---> M3N and in adsorption of a nitrogen atom on metal surface, in which M3 was a virtual minimum unit of the metal surface. The reactions were assumed to be an essential step in the ammonia production after plasma-activation of N2. The resulting values correlated with the respective initial catalytic activity, indicating that a more unstable M3N surface intermediate produced higher catalytic activity. Emission spectra in the plasma process using various electrodes were measured and showed that the efficiency of electrodes for plasma activation of nitrogen molecules was almost independent on the metals, while the reactivity of the activated species to form ammonia depended greatly on the metal used. The N2/H2 ratio dependence and formation/decomposition rate constants of ammonia were finally determined on Au and Cu, which were different from those for the conventional Haber-Bosch process. The decomposition of produced ammonia was suggested to proceed in a plasma-irradiated gas phase.
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Full-text available
Invited for this month's cover is the group of Prof. Dr. Annemie Bogaerts at the University of Antwerp and their collaborators at Eindhoven University of Technology in the laboratory of Prof. Dr. Volker Hessel. The cover image shows that gliding arc plasma leads to energy efficient nitrogen fixation by promoting the vibrational excitation of N2. The Full Paper itself is available at 10.1002/cssc.201700095.
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Full-text available
Treatment of irrigation water using plasma involves a series of electrochemical reactions that forms NOx? in water working as a fertilizer in plant cultivation. Here we report the results of an experimental study of the effects of different discharge conditions especially discharge frequency on air phase gliding arc discharge. The results show that, the low-frequency system consumed nearly three times more energy than the high-frequency system when generating the same NOx? production rate in water; gas flow rate has strong influence on the low-frequency system while slight influence on the high-frequency system; high-frequency discharge can generate much larger plasma area to promote electrochemical reactions resulting more NOx? produced in water. Frequency from 5 kHz to 80 kHz does not significantly influence on the nitrogen fixation of gliding arc.
• Article
Synthesis of reactive forms of nitrogen such as ammonia is important in modern agricultural productivity, but present agricultural technology uses reactive nitrogen inefficiently, leading to numerous and growing environmental problems. Animal, human, and food waste all contain significant quantities of organic nitrogen that are transformed into ammonia (NH3) by bacterial degradation of organic waste. If not captured, this volatile form of reactive nitrogen is lost to the environment, reducing N content and thus the agricultural value of organic waste. Furthermore, ammonia loss to the environment initiates a cascade of environmental problems. Nonequilibrium air plasma technology creates reactive nitrogen that can be readily converted to dilute aqueous nitric acid solutions. If mixed with decaying organic waste, NH3 loss is greatly reduced via the formation of involatile ammonium nitrate, a potent nitrogen fertilizer. Air plasma technology for fixed nitrogen manufacture is currently limited only by the availability of electricity and the energy efficiency of the process. The price of electricity via distributed renewable routes such as solar photovoltaic or wind turbines is rapidly decreasing. Increasingly, inexpensive wind and solar power sources, coupled with recent advances in air plasma energy efficiency, suggest that this technology could have a significant role in improving nitrogen use efficiency and reducing environmental and other threats associated with the current system.
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The direct synthesis of NOx from N2 and O2 by non-thermal plasma at an atmospheric pressure and low temperature is presented, which is considered to be an attractive option for replacement of the Haber-Bosch process. In this study, the direct synthesis of NOx was studied by packing different catalyst support materials in a dielectric barrier discharge (DBD) reactor. The support materials and their particle sizes both had a significant effect on the concentration of NOx. This is attributed to different surface areas, relative dielectric constants and particles shapes. The nitrogen could be fixed at substantially lowered temperatures by employing non-thermal plasma-catalytic DBD reactor, which can be used as an alternative technology for low temperature synthesis. The γ-Al2O3 with smallest particles size of 250-160 μm, gave the highest concentration of NOx and the lowest specific energy consumption of all the tested materials and particle sizes. The NOx concentration of 5700 ppm was reached at the highest residence time of 0.4 s and an N2/O2 feed ratio of 1 was found to be the most optimum for NOx production. In order to intensify the NOx production in plasma, a series of metal oxide catalysts supported on γ-Al2O3 were tested in a packed DBD reactor. A 5% WO3/γ-Al2O3 catalyst increased the NOx concentration further by about 10% compared to γ-Al2O3, while oxidation catalysts such as Co3O4 and PbO provided a minor (~5%) improvement. These data suggest that oxygen activation plays a minor role in plasma catalytic nitrogen fixation under the studied conditions with the main role ascribed to the generation of microdischarges on sharp edges of large-surface area plasma catalysts. However, when the loading of active metal oxides was increased to 10%, NO selectivity decreased, suggesting possibility of thermal oxidation of NO to NO2 through reaction with surface oxygen species.
• Article
Nitric oxide (NO) is a vasodilator and antihypertensive agent as well as a universal anti-microbial factor killing bacteria, fungi and parasites without killing human cells provided that an appropriate dose level and treatment time are applied. Exogenous NO is often employed in inhalation therapies for treating pulmonary hypertension in children and adults. NO generation from air in high voltage electrical discharges is being developed for medical uses because it is technologically simple, economical and portable. The related literature is reviewed here. The plasma can be a thermal plasma, where the temperature is of the order of 10,000 K, or it can be a non-thermal plasma, where the electron temperature is very high but the average gas temperature can vary over a wide range from close to room temperature to thousands of degrees above room temperature. The plasma temperature has significant effects on the chemical composition of the treated gas. These effects are explained based on the chemical reaction mechanism. Further, NO generated by electrical discharges is usually contaminated with nitrogen dioxide and sometimes with ozone and particulate matter. The techniques that have been successfully hybridized with the electrical discharge devices or that can potentially be hybridized for the purification of NO are also reviewed. Recent successful testing of electrical discharge-based NO generators for inhalation therapy on animal models in the US and routine use of them in Russia and east Europe for wound decontamination and fast heeling suggests that the technique has a great potential for applications in future.
• Article
Time-resolved, absolute NO and N atom number densities are measured by NO Laser Induced Fluorescence (LIF) and N Two-Photon Absorption LIF in a diffuse plasma filament, nanosecond pulse discharge in dry air, hydrogen-air, and ethylene-air mixtures at 40 Torr, over a wide range of equivalence ratios. The results are compared with kinetic modeling calculations incorporating pulsed discharge dynamics, kinetics of vibrationally and electronically excited states of nitrogen, plasma chemical reactions, and radial transport. The results show that in air afterglow, NO decay occurs primarily by the reaction with N atoms, NO + N → N2 + O. In the presence of hydrogen, this reaction is mitigated by reaction of N atoms with OH, N + OH → NO + H, resulting in significant reduction of N atom number density in the afterglow, additional NO production, and considerably higher NO number densities. In fuel-lean ethylene-air mixtures, a similar trend (i.e. N atom concentration reduction and NO number density increase) is observed, although [NO] increase on ms time scale is not as pronounced as in H2-air mixtures. In near-stoichiometric and fuel-lean ethylene-air mixtures, when N atom number density was below detection limit, NO concentration was measured to be lower than in air plasma. These results suggest that NO kinetics in hydrocarbon-air plasmas is more complex compared to air and hydrogen-air plasmas, additional NO reaction pathways may well be possible, and their analysis requires further kinetic modeling calculations.
• Article
Atmospheric pressure air plasma discharges generate potential antimicrobial agents, such as nitrogen oxides and ozone. Generation of nitrogen oxides was studied in a DC-driven self-pulsing (1–10 kHz) transient spark (TS) discharge. The precursors of NOx production and the TS characteristics were studied by nanosecond time-resolved optical diagnostics: a photomultiplier module and a spectrometer coupled with fast intensified camera. Thanks to the short (~10–100 ns) high current (>1 A) spark current pulses, highly reactive non-equilibrium plasma is generated. Ozone was not detectable in the TS, probably due to higher gas temperature after the short spark current pulses, but the NOx production rate of ~7 × 1016 molecules/J was achieved. The NO2/NO ratio decreased with increasing TS repetition frequency, which is related to the complex frequency-dependent discharge properties and thus changing NO2/NO generating mechanisms. Further optimization of NO2 and NO production to improve the biomedical and antimicrobial effects is possible by modifying the electric circuit generating the TS discharge.
• Article
Nitrogen fixed in the form of nitrogen oxides is essential to produce fertilizers and many other chemical products, which is vital to sustain life. The performance of a milli-scale gliding arc reactor operated under atmospheric pressure has been studied for nitrogen oxides synthesis. In this work, the electrical and process parameters of the gliding arc reactor, such as frequency, pulse width, amplitude and feed ratio were investigated respectively. The experiments were performed at 1 L/min in a gliding arc discharge regime. The highest concentration of NOx was found to be ~1 % at energy consumption of 10 kWh/kg of NOx. Increase in frequency, pulse width and amplitude resulted in an increased specific energy input and NOx concentration. The feed ratio (N2/O2) affected the amount of NO and NO2 produced, which gives possibility to independently obtain the desired ratio of NO/NO2 by tuning the electrical and process parameters.
• W Steffen
• K Richardson
• J Rockstrom
• S E Cornell
• I Fetzer
• E M Bennett
• R Biggs
• S R Carpenter
• W De Vries
• C A De Wit
• C Folke
• D Gerten
• J Heinke
• G M Mace
• V Ramanathan
• B Reyers
• S Sorlin
W. Steffen, K. Richardson, J. Rockstrom, S.E. Cornell, I. Fetzer, E.M. Bennett, R. Biggs, S.R. Carpenter, W. de Vries, C.A. de Wit, C. Folke, D. Gerten, J. Heinke, G.M. Mace, L.M. Persson, V. Ramanathan, B. Reyers, S. Sorlin, Planetary boundaries: guiding human development on a changing planet, Science 347 (6223) (2015) 1259855.
• Article
Atmospheric-pressure air plasmas, created by a variety of discharges, are promising sources of reactive species for the emerging field of plasma biotechnology because of their convenience and ability to operate at ambient conditions. One biological application of ambient-air plasma is microbial disinfection, and the ability of air plasmas to decontaminate both solid surfaces and liquid volumes has been thoroughly established in the literature. However, the mechanism of disinfection and which reactive species most strongly correlate with antimicrobial effects are still not well understood. We describe quantitative gas-phase measurements of plasma chemistry via infrared spectroscopy in confined volumes, focusing on air plasma generated via surface micro-discharge (SMD). Previously, it has been shown that gaseous chemistry is highly sensitive to operating conditions, and the measurements we describe here extend those findings. We quantify the gaseous concentrations of ozone (O3) and nitrogen oxides (NO and NO2, or NOx) throughout the established 'regimes' for SMD air plasma chemistry: the low-power, ozone-dominated mode; the high-power, nitrogen oxides-dominated mode; and the intermediate, unstable transition region. The results presented here are in good agreement with previously published experimental studies of aqueous chemistry and parameterized models of gaseous chemistry. The principal finding of the present study is the correlation of bacterial inactivation on dry surfaces with gaseous chemistry across these time and power regimes. Bacterial decontamination is most effective in 'NOx mode' and less effective in 'ozone mode', with the weakest antibacterial effects in the transition region. Our results underscore the dynamic nature of air plasma chemistry and the importance of careful chemical characterization of plasma devices intended for biological applications.
• Article
Chemical species that are generated by a plasma jet in a microhollow cathode discharge geometry when operated with air and a dc voltage were investigated. Nitric oxide (NO) is found as the dominant long-lived reaction product with concentrations of several hundreds of ppm. In comparison, the concentrations observed for nitric dioxide (NO2) and ozone (O3) are negligible. The concentrations of NO are increasing with increasing electric power but decreasing with increasing flow rates. Simultaneously, NO2 concentrations are increasing slightly. The results suggest that the observed far reaching biocidal effect of the plasma jet depends on the generation of nitric oxide.
• Article
This paper describes a usage of a low-current coaxial plasmatron for generation of nitrogen oxide molecules. Glow-type discharge in vortex air flow is sustained at an average current from 0.05 to 0.2 A that corresponds to an average discharge power from 65 to 160 W. The diameter of an exit nozzle of the plasmatron is of 0.5 cm, and the air flow is varied from 0.2 to 1.5 g/s. In such conditions, the discharge burns in nonsteady-state regime, when a sustainment of a plasma jet/plume and a plasma column in the plasmatron nozzle is accompanied by the spontaneous glow-to-spark transitions. Due to the special design of the anode nozzle, an efficient interaction of the air flow with the plasma plume and plasma column is provided. Typical contents of nitric monoxide in the output gas are of about several grams per cubic meter, and the cost for formation of one molecule is from 50 to 35 eV.
• Article
Ambient-gas plasma, or plasma created from air at ambient conditions, has been studied as a means of disinfecting surfaces. However, the parameters that determine the effectiveness of plasma disinfection are not well understood. We report results from surface disinfection experiments and determine the characteristics that are responsible for the antimicrobial effect against the model bacterium Escherichia coli. Our results suggest that neutral reactive species are more influential in surface disinfection than charged particles. In addition, non-biological surfaces are easier to disinfect than pig skin. Air plasma performed comparably to conventional antiseptics, indicating that ambient-gas plasma treatment could be a promising strategy for both clinical and community infection control.
• Article
We describe the photochemical and antimicrobial synergy between ambient-condition air plasma and ultraviolet photons at near-UV (UVA) wavelengths. Plasma treatments generate reactive oxygen and nitrogen species in aqueous solution. When UVA treatment followed plasma treatment of Escherichia coli, the antimicrobial effect exceeded the effect predicted from the two treatments alone. Exposing individual plasma-associated species to UVA photons indicated that the synergy is associated with nitrite and hydrogen peroxide in combination. An antioxidant present during UVA treatment prevented the synergistic effect, suggesting oxidant-mediated bacterial inactivation. Addition of nitrite to aqueous solution, followed by photolysis of nitrite by UVA photons, is hypothesized as the primary mechanism of synergy.
• Article
Ambient-condition air plasma produced by indirect dielectric barrier discharges can rapidly disinfect aqueous solutions contaminated with bacteria and other microorganisms. In this study, we measured key chemical species in plasma-treated aqueous solutions and the associated antimicrobial effect for varying discharge power densities, exposure times, and buffer components in the aqueous medium. The aqueous chemistry corresponded to air plasma chemistry, and we observed a transition in composition from ozone mode to nitrogen oxides mode as the discharge power density increased. The inactivation of E. coli correlates well with the aqueous-phase ozone concentration, suggesting that ozone is the dominant species for bacterial inactivation under these conditions. Published values of ozone-water antibacterial inactivation kinetics as a function of the product of ozone concentration and contact time are consistent with our results. In contrast to earlier studies of plasma-treated water disinfection, ozone-dependent bacterial inactivation does not require acidification of the aqueous medium and the bacterial inactivation rates are far higher. Furthermore, we show that the antimicrobial effect depends strongly on gas-liquid mixing following plasma treatment, apparently because of the low solubility of ozone and the slow rate of mass transfer from the gas phase to the liquid. Without thorough mixing of the ozone-containing gas and bacteria-laden water, the antimicrobial effect will not be observed. However, it should be recognized that the complexity of atmospheric pressure plasma devices, and their sensitivity to subtle differences in design and operation, can lead to different results with different mechanisms.
• Article
We report an experimental assessment of the contributions of the shockwave and the hot channel to the production of nitric oxide by simulated lightning. Lightning in the laboratory was simulated by a hot plasma generated with a pulsed Nd-YAG laser. The temporal evolution of electric breakdown in air at atmospheric pressure was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The shockwave front velocity was determined to be about 60 km s-1 at 20 ns and the temperature behind the shock front was estimated to be about 105 K. The production yield of nitric oxide by shock heating is estimated to be: P(NO) (3±2) × 1014 molecule J-1. In contrast it was calculated that the production yield of NO by the hot channel is as much as P(NO)=(1.5±0.5) × 1017 molecule J-1. To the extent our simulation is an accurate representation of natural lightning, the hot channel is the dominant region for nitrogen fixation.
• Article
Full-text available
The production of NO has been studied by means of arc discharges in the laboratory which simulate natural lightning in current waveform and amplitude (___ 30kA). Observations are compared to the results of a computational model that includes the dynamics of energy deposition and channel expansion, combined with the Zel'dovich equations to model the relevant chemical reactions. Results are expressed as NO produced per meter of arc length, and are measured as functions of dissipated energy and of peak current. It is found that at atmospheric pressure, the NO production per joule of dissipated energy is not constant. NO production per meter discharge length as a function of peak current appears to provide a more appropriate scaling factor for estimates of total global NO production. Production of NO2 was less than 10% of the production of NO, and by implication the production of 03 less still. From these data, and using published estimates of global lightning frequency, we derive an estimate of global NO production of 2.5 Tg(N)/yr for 30 flashes per second to 8.3 Tg(N)/yr for 100 flashes per second.
• Article
Human production of food and energy is the dominant continental process that breaks the triple bond in molecular nitrogen (N2) and creates reactive nitrogen (Nr) species. Circulation of anthropogenic Nr in Earth’s atmosphere, hydrosphere, and biosphere has a wide variety of consequences, which are magnified with time as Nr moves along its biogeochemical pathway. The same atom of Nr can cause multiple effects in the atmosphere, in terrestrial ecosystems, in freshwater and marine systems, and on human health. We call this sequence of effects the nitrogen cascade. As the cascade progresses, the origin of Nr becomes unimportant. Reactive nitrogen does not cascade at the same rate through all environmental systems; some systems have the ability to accumulate Nr, which leads to lag times in the continuation of the cascade. These lags slow the cascade and result in Nr accumulation in certain reservoirs, which in turn can enhance the effects of Nr on that environment. The only way to eliminate Nr accumulation and stop the cascade is to convert Nr back to nonreactive N2.
• Article
The mechanism of production of nitrogen oxides by electrical discharges has been examined. The velocity of shock fronts generated by laboratory scale discharges have been measured and are found to be too slow to raise the air temperature to the -3000 K necessary for nitrogen fixation by the Zel'dovich mechanism. The freeze-out mixing ratio of NO/in air has been measured directly for low-pressure discharges and is found to be of the order expected from the Zel'dovich mechanism for gas cooling over a timescale far longer than the duration of the shock front. Therefore it is concluded that NO/is formed in the gas in the slowly cooling hot channel region and not in the rapidly cooling shock front. Also, it is argued that NO x formation occurs by a freeze out mechanism due to a rapid drop in temperature, not density as has been suggested. NO 2 production is found to be significant, with the (NO2)/(NO) ratio being strongly dependent on the water content of the air. Discrepancies between previous experimental studies of the (NO2)/(NO) ratio and the quantity of NO/formed per unit energy (P) are discussed. P is also found to vary with spark gap and ambient pressure. It is thought that these effects may be due to a significant loss of heat from the spark gap to the electrodes. The inclusion of atmospheric levels of N20, CH4, and CO2 are found to have no measurable effect on the yields of NO or NO 2.
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Hexagonal (h-) and monoclinic (m-) WO3 nanoparticles with controlled composition (oxidized/yellow color or partially reduced/blue color) were prepared through annealing (NH4)(x)WO3-y. The formation, structure, composition, morphology, and optical properties of the samples were analyzed by powder X-ray diffraction, scanning and transmission electron microscopy combined with electron diffraction, and Raman, X-ray photoelectron, H-1 magic angle spinning nuclear magnetic resonance, diffuse reflectance ultraviolet visual, and photoluminescence spectroscopy. Their photocatalytic properties were tested by decomposing methyl orange in the aqueous phase and acetone in the gas phase. Oxidized m-WO3 (m-WO3 ox) was the most active photocatalyst both in the aqueous and in the gas phase, followed by the oxidized h-WO3 (h-WO3 ox) sample. Reduced h-WO3 (h-WO3 red) and m-WO3 (m-WO3 red) exhibited much lower activity. Thus, in contrast to TiO2, where crystalline structure (rutile or anatase) plays a key effect in photocatalysis, for WO3, it is the composition that is of greatest importance: the more oxidized the WO3 sample, the better a photocatalyst it is. The crystal structure of WO3 has only an indirect effect, in that it influences the composition of WO3 samples. While oxidized m-WO3 is completely oxidized, oxidized h-WO3 is always in a partially reduced state due to the presence of stabilizing positive ions in its hexagonal channels. Consequently, an oxidized monoclinic WO3 material will always provide better photocatalytic activity than an oxidized hexagonal one.
• Article
Nanosecond repetitively pulsed (NRP) spark discharges have been studied in atmospheric pressure air preheated to 1000 K. Measurements of spark initiation and stability, plasma dynamics, gas temperature and current–voltage characteristics of the spark regime are presented. Using 10 ns pulses applied repetitively at 30 kHz, we find that 2–400 pulses are required to initiate the spark, depending on the applied voltage. Furthermore, about 30–50 pulses are required for the spark discharge to reach steady state, following initiation. Based on space- and time-resolved optical emission spectroscopy, the spark discharge in steady state is found to ignite homogeneously in the discharge gap, without evidence of an initial streamer. Using measured emission from the N2 (C–B) 0–0 band, it is found that the gas temperature rises by several thousand Kelvin in the span of about 30 ns following the application of the high-voltage pulse. Current–voltage measurements show that up to 20–40 A of conduction current is generated, which corresponds to an electron number density of up to 1015 cm−3 towards the end of the high-voltage pulse. The discharge dynamics, gas temperature and electron number density are consistent with a streamer-less spark that develops homogeneously through avalanche ionization in volume. This occurs because the pre-ionization electron number density of about 1011 cm−3 produced by the high frequency train of pulses is above the critical density for streamer-less discharge development, which is shown to be about 108 cm−3.
• Article
Nitric oxide (NO) is increasingly being used in medical treatments of high blood pressure, acute respiratory distress syndrome and other illnesses related to the lungs. Currently a NO inhalation system consists of a gas cylinder of N2 mixed with a high concentration of NO. This arrangement is potentially risky due to the possibility of an accidental leak of NO from the cylinder. The presence of NO in the air leads to the formation of nitric dioxide (NO2), which is toxic to the lungs. Therefore, an on-site generator of NO would be highly desirable for medical doctors to use with patients with lung disease. To develop the NO inhalation system without a gas cylinder, which would include a high concentration of NO, NAMIHIRA et al have recently reported on the production of NO from room air using a pulsed arc discharge. In the present work, the temperature of the pulsed arc discharge plasma used to generate NO was measured to optimize the discharge condition. The results of the temperature measurements showed the temperature of the pulsed arc discharge plasma reached about 10,000 K immediately after discharge initiation and gradually decreased over tens of microseconds. In addition, it was found that NO was formed in a discharge plasma having temperatures higher than 9,000 K and a smaller input energy into the discharge plasma generates NO more efficiently than a larger one.
• Article
Full-text available
Nitric oxide (NO) is increasingly being used in medical applications. Currently, a gas cylinder of N2 mixed with a high concentration of NO is used in the NO inhalation system. However, this arrangement is potentially risky due to the possibility of accidental leak of NO from the cylinder. The presence of NO in air leads to the formation of nitric dioxide (NO2), which is toxic to the lungs. Therefore, an on-site generation of NO would be very desirable for patients with acute respiratory distress syndrome and other related illnesses. Previously, our group reported the production of NO using a pulsed arc discharge. In this work, the prototype of the on-site NO generator was developed and the performances of the NO generator were demonstrated for medical applications.
• Article
The concentration of nitric oxide in the effluent of an air plasma (2350–2600 K) was studied as a function of the rate of quenching. Rapid quenching leads to concentrations higher than those predicted from stagnant equilibrium conditions. This observation may be attributed to a lack of chemical equilibrium under the conditions investigated.
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The knowledge of the lightning-induced nitrogen oxides (LNOx) source is important for understanding and predicting the nitrogen oxides and ozone distributions in the troposphere and their trends, the oxidising capacity of the atmosphere, and the lifetime of trace gases destroyed by reactions with OH. This knowledge is further required for the assessment of other important NOx sources, in particular from aviation emissions, the stratosphere, and from surface sources, and for understanding the possible feedback between climate changes and lightning. This paper reviews more than 3 decades of research. The review includes laboratory studies as well as surface, airborne and satellite-based observations of lightning and of NOx and related species in the atmosphere. Relevant data available from measurements in regions with strong LNOx influence are identified, including recent observations at midlatitudes and over tropical continents where most lightning occurs. Various methods to model LNOx at cloud scales or globally are described. Previous estimates are re-evaluated using the global annual mean flash frequency of 445 s-1 reported from OTD satellite data. From the review, mainly of airborne measurements near thunderstorms and cloud-resolving models, we conclude that a “typical” thunderstorm flash produces 15 (2-40)1025 NO molecules per flash, equivalent to 250 mol NOx or 3.5 kg of N mass per flash with uncertainty factor from 0.13 to 2.7. Mainly as a result of global model studies for various LNOx parameterisations tested with related observations, the best estimate of the annual global LNOx nitrogen mass source and its uncertainty range is (53) Tg a-1 in this study. In spite of a smaller global flash rate, the best estimate is essentially the same as in some earlier reviews, implying larger flash-specific NOx emissions. The paper estimates the LNOx accuracy required for various applications and lays out strategies for improving estimates in the future. An accuracy of about 1 Tg a-1 or 20%, as necessary in particular for understanding tropical tropospheric chemistry, is still a challenging goal.
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An assessment of the global distribution of nitrogen oxides is required for an adequate description of tropospheric chemistry and in the evaluation of the global impact of increasing anthropogenic emissions of NOx. In the mathematical models utilized for this purpose, one needs to specify as inputs the natural as well as man-made sources of nitrogen oxides in the atmosphere. Lightning is one of the main natural sources of nitrogen oxides in the atmosphere, and it may be the dominant source of nitrogen oxides in the troposphere in equatorial and tropical South Pacific regions. Thus, an accurate quantification of nitrogen oxide production by thunderstorms is necessary for further development of the chemical models of the troposphere and in the evaluation of the effects of the man-made nitrogen emissions in the terrestrial atmosphere.
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Lightning is thought to represent an important source of tropospheric reactive nitrogen species NOx (NO + NO2), but estimates of global production of NOx by lightning vary considerably. We evaluate the production of NOx by lightning using a global chemical/transport model, satellite lightning observations, and airborne NOx measurements. Various model calculations are conducted to assess the global NOx production rate of lightning by comparing the model calculations with airborne measurements. The results show that the simulated NOx in the tropical middle and upper troposphere are very sensitive to the amount and altitude of the lightning NOx used in the model. A global lightning NOx production of 7 Tg N yr(-1) uniformly distributed in convective clouds or 3.5 Tg N yr(-1) distributed in the upper cloud regions produces good agreement between calculated and measured NOx concentrations in the tropics.
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