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Register a new userCH$_4$ reforming with CO$_2$ in a nanosecond pulsed discharge. The importance of the pulse sequence
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Added by
Luca Matteo Martini
Publication date
2021
fields
Physics
and research's language is
English
Authors
Cesare Montesano
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The plasma dry reforming reaction of methane with carbon dioxide is investigated in a nanosecond repetitively pulsed discharge, a type of plasma that offers one of the highest non-equilibrium and performance characteristics. The experiments purpose was to examine the effect of varying the sequence of high-voltage pulses for the same plasma energy. We find that when successive pulses are closer than 500 $rm mu$s, a memory-dominated regime gradually develops, which influences subsequent breakdown events. As a result, both a higher reactant conversion and more efficient use of the energy injected into the discharge plasma are achieved.
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In this work, we present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in single-pulse mode (no memory effects). Various discharge parameters, including voltage, current, gas density, rotational/vibrational/gas temperature, and electron number density, were measured. Several different measurement techniques were used, including microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, fast photography, and electrical parameter measurements. Spark and corona discharge regimes were studied with discharge pulse duration of 90 ns and electrode gap sizes ranging from 2 to 10 mm. The spark regime was observed for gaps < 6 mm using discharge pulse energies of 0.6-1 mJ per mm of the gap length. Higher electron number densities, total electron number per gap length, discharge currents, and gas temperatures were observed for smaller electrode gaps and larger pulse energies, reaching maximal values of about 7.5x10^15 cm-3, 3.5x10^11 electrons per mm, 22 A, and 4,000 K (at 10 us after the discharge), respectively, for a 2 mm gap and 1 mJ/mm discharge pulse energy. Initial breakdown was followed by a secondary breakdown occurring about 30-70 ns later and was associated with ignition of a cathode spot and transition of the discharge to cathodic arc. A majority of the discharge pulse energy was deposited into the gas before the secondary breakdown (85-89%). The electron number density after the ns discharge pulse decayed with a characteristic time scale of 150 ns governed by dissociative recombination and electron attachment to oxygen mechanisms. For the corona regime, substantially lower pulse energies (~0.1 mJ/mm), peak conduction current (1-2 A), and electron numbers (3-5x10^10 electrons per mm), and gas temperatures (360 K) were observed.
Nanoparticles grown in a plasma are used to visualize the process of film deposition in a pulsed radio-frequency (rf) atmospheric pressure glow discharge. Modulating the plasma makes it possible to successfully prepare porous TiO2 films. We study the trapping of the particles in the sheath during the plasma-on phase and compare it with numerical simulations. During the plasma-off phase, the particles are driven to the substrate by the electric field generated by residual ions, leading to the formation of porous TiO2 film. Using video microscopy, the collective dynamics of particles in the whole process is revealed at the most fundamental kinetic level.
We report results from grand-canonical Monte Carlo simulations of methane and carbon dioxide adsorption in structure sI gas hydrates. Simulations of pure component systems show that all methane sites are equivalent, while carbon dioxide distinguishes between two types of sites, large or small. The adsorbed mixture can be regarded as ideal, as long as only large sites are occupied. A strong preference is demonstrated for methane, when the smaller sites become filled. The molar heat of adsorption of methane decreases with composition, while the molar heat of adsorption for carbon dioxide passes an extremum, essentially in accordance with the observation on the site sizes. The Helmholtz energies of the hydrate with CO$_2$-CH$_4$ gas mixture for temperatures between 278 and 328 K and pressures between 10$^4$ and 10$^9$ Pa indicate that certain mixtures are more stable than others. The results indicate that a thermodynamic path exists for conversion of a pure methane hydrate into a pure carbon dioxide hydrate without destroying the hydrate structure.
We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as $n_text{e}=7times10^{19}$ cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.
The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution, even though these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. Here, we present a coupled numerical framework that links an evolutionary, vertically-resolved model of the planetary silicate mantle with a radiative-convective model of the atmosphere. Using this method we investigate the early evolution of idealized Earth-sized rocky planets with end-member, clear-sky atmospheres dominated by either H$_2$, H$_2$O, CO$_2$, CH$_4$, CO, O$_2$, or N$_2$. We find central metrics of early planetary evolution, such as energy gradient, sequence of mantle solidification, surface pressure, or vertical stratification of the atmosphere, to be intimately controlled by the dominant volatile and outgassing history of the planet. Thermal sequences fall into three general classes with increasing cooling timescale: CO, N$_2$, and O$_2$ with minimal effect, H$_2$O, CO$_2$, and CH$_4$ with intermediate influence, and H$_2$ with several orders of magnitude increase in solidification time and atmosphere vertical stratification. Our numerical experiments exemplify the capabilities of the presented modeling framework and link the interior and atmospheric evolution of rocky exoplanets with multi-wavelength astronomical observations.
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