Alexander A. Fridman

Gliding arc gas discharge

Abstract The sliding arc discharge starts at the shortest distance between the electrodes, then moves with the gas flow at a velocity of about 10xa0m/s and the length l of the arc column increases together with the voltage. When the length of the gliding arc exceeds its critical value l crit , heat losses from the plasma column begin to exceed the energy supplied by the source, and it is not possible to sustain the plasma in a state of thermodynamic equilibrium. As a result, a fast transition into a non-equilibrium phase occurs. The discharge plasma cools rapidly to a gas temperature of about T 0 =1000xa0K and the plasma conductivity is maintained by a high value of the electron temperature T e =1xa0eV (about 11u2008000xa0K). After this fast transition, the gliding arc continues its evolution, but under non-equilibrium conditions ( T e ≫ T 0 ). The specific heat losses W crit in this regime are much smaller than in the equilibrium regime (numerically about three times less). The discharge length increases up to a new critical value of l ≅3 l crit . The main part of the gliding arc power (up to 75–80%) can be dissipated in the non-equilibrium zone. After the decay of the non-equilibrium discharge, the evolution repeats from the initial break-down. This permits the stimulation of chemical reactions in regimes quite different from conventional combustion and environmental situations. It provides an alternative approach to addressing energy conservation and environmental control. In the first part of this paper, the gas discharge physics are described. The second part reviews the chemical reaction in the gliding arc plasma and some possible applications.

Soot and NO formation in methane–oxygen enriched diffusion flames

NO and soot formation were investigated both numerically and experimentally in oxygen-enriched counterflow diffusion flames. Two sets of experiments were conducted. In the first set, the soot volume fraction was measured as a function of oxygen content in the oxidizer jet at constant strain rate (20 s−1). In the second set of experiments, the soot volume fraction was measured as a function of strain rate variation from 10 to 60 s−1 and at constant oxygen content on the oxidizer side. A soot model was developed based on a detailed C6 gas phase chemistry. The soot and molecular radiation were taken into account. Numerical results were verified against experimental data. The soot volume fraction was predicted with the maximum discrepancy less than 30% for all cases considered. It was found that oxygen variation significantly modified the diffusion flame structure and the flame temperature, resulting in a substantial increase of soot. The temperature increase promotes aromatics production in the fuel pyrolysis zone and changes the relative contributions of the thermal and Fenimore mechanisms into NO formation. As the strain rate increases, the residence time of incipient soot particles in the high temperature zone is reduced and the total amount of soot decreases. High concentration of soot in the flame leads to enhancement of radiant heat exchange: the reduction of temperature due to radiation was found to be between 10 and 50 K. This caused a reduction of peak NO concentrations by 20%–25%. The increase of oxygen content in the oxidizer stream resulted in a reduction of the distance between the plane of the maximum temperature and the stagnation plane.

Experimental assessment of a combined plasma/catalytic system for hydrogen production via partial oxidation of hydrocarbon fuels

A combined reformation system, which includes both auto-thermal catalytic and non-equilibrium plasma units, is studied experimentally. The system is assessed for the practical application of hydrogen production via reforming of liquid gasoline-like fuels. The catalyst has been previously used for reforming of different types of hydrocarbons, demonstrating good performances in terms of hydrogen production for temperatures as high as 800°C. In this work, a non-equilibrium plasma source is coupled to the catalytic unit. A pulsed corona reactor is used as a non-equilibrium plasma source at atmospheric pressure. The performances of combined reformation system are characterized experimentally in terms of hydrogen yield and electric power consumption. Hydrogen conversion and byproduct composition are determined and quantified with respect to power consumption, reactor temperature, input reactant composition, and configuration of the experimental setup.

Thermal and nonthermal regimes of gliding arc discharge in air flow

Gliding discharges comprising both equilibrium and nonequilibrium plasma conditions offer high energy efficiency and selectivity for chemical processes. Prevailing parameters satisfying nonequilibrium plasma conditions at relatively high power levels should be well understood and characterized. In the present work, gliding discharges formed between diverging electrodes in air flow were studied experimentally over a wide range of gas velocities and power levels. Depending on the system parameters the following discharge regimes were observed: low power nonequilibrium discharge; thermal quasiequilibrium discharge; and gliding discharge with equilibrium to nonequilibrium transition. The effect of system parameters on discharge characteristics is analyzed. The equilibrium to nonequilibrium transition was experimentally observed as a change of voltage increase rate with discharge length growth. The local electric field, defined as dV/dl, increased up to three times, indicating the change of plasma conditions. ...

Chemical structures of methane-air filtration combustion waves for fuel-lean and fuel-rich conditions

Chemical structures of filtration combustion waves in an inert porous media were analyzed comparatively for lean and rich methane-air mixtures. Temperature, velocity, and chemical products of the combustion waves were studied experimentally in the range of equivalence ratios from 0.2 to 2.5. Downstream (superadiabatic) wave propagation was observed for ultralean ( ≤0.45) and ultrarich ( ≥1.7) mixtures. Upstream (underadiabatic) propagation corresponds to the range of equivalence ratios from 0.45 to 1.7. It was found that with the equal heat content, rich mixtures have essentially higher combustion temperatures than corresponding lean mixtures. The products of partial methane oxidation (H 2 , CO, and C 2 hydrocarbons) are dominant for ultrarich superadiabatic combustion where up to 60% of methane is converted to CO and H 2 . A numerical model, based on a two-temperature approximation and detailed combustion chemistry, is developed to analyze species profiles and the combustion mechanism of the filtration waves. The model predictions for combustion temperatures and chemical products are in good agreement with experimental data. Kinetic simulation revealed the complex chemical structure of the ultrarich superadiabatic waves. It is shown that this wave is composed of an exothermic “partial oxidation” reaction zone followed by an endothermic “steam reforming” zone.

Syngas production using superadiabatic combustion of ultra-rich methane-air mixtures

Two common methods for the production of synthesis gas (syngas) are: (1) methane partial oxidation and (2) methane steam re-forming. This paper discusses the experimental results obtained from the partial oxidation of “ultrarich,” (=4), methane-air mixtures in a new type of chemical reactor based on filtration combustion. Experimental results show that the reciprocal flow burner (RFB), due to its high heat recuperation efficiency (approximately 90%), can support self-sustained combustion of ultrarich methane-air mixtures up to an equivalence ratio of 8, well beyond the conventional flammability associated with a methane-air flame in free space. For the range of equivalence ratios (2 Parametric studies demonstrate that the maximum temperature in the combustion zone, which varies from 1100 to 1400°C, is a function of the equivalence ratio, filtration velocity, reactor pressure, and porous body diameter. Kinetic simulations reveal that methane partial oxidation occurs in a two-stage process: (1) ignition, a fast process that accounts for approximately 60% of the total hydrogen conversion relative to thermodynamic equilibrium and (2) steam reformation, a slow process where the remaining conversion of hydrogen occurs when water reacts with unburned methane.

Formation of carbon nanotubes in counter-flow, oxy-methane diffusion flames without catalysts

In oxygen enriched methane diffusion flames, carbon nanotubes were discovered to be formed in the region on the fuel-rich side of the flame front at an oxygen enrichment of 50%. No catalyst was employed. An opposed flow diffusion flames with varying strain rate and oxygen content in the oxidizer stream was used. Substantial quantities of nanotube material are produced at atmospheric pressure in this continuous (non-batch) process. Thermophoretic sampling of the flame and collecting the carbon material deposited near the exhaust was done. Both confirm the growth of carbon nanotubes and other carbon clusters.

Hydrogen production in ultra-rich filtration combustion of methane and hydrogen sulfide

Abstract Filtration combustion waves formed in an inert porous media were analyzed comparatively for methane (CH4)/air and diluted hydrogen sulfide (20% H 2 S +80% N 2 )/ air mixtures. Temperature, velocity, and chemical products of the combustion waves were studied experimentally in the range of equivalence ratios (ϕ) from 1 to 2.5 for methane and from 1 to 5.5 for hydrogen sulfide, at a filtration velocity of 12 cm / s . The practical goal of the study in the rich and ultra-rich region was to explore the extent of conversion of these reactants into commercially viable products such as hydrogen (H2), syngas (H2+CO), and sulfur (S2). For both methane and hydrogen sulfide combustion, upstream (underadiabatic) propagation corresponds to the range of equivalence ratios from stoichiometry to 1.7, and downstream (superadiabatic) wave propagation was observed for ultrarich (ϕ⩾1.7) mixtures. The products of methane partial oxidation, dominant for ultra-rich waves, were: H2, CO, and C2 hydrocarbons where up to 60% of the methane was converted to CO and H2. Similar observation for H2S partial oxidation products reveals a maximum conversion rate of 20% to H2 and 50% for S2.

Effect of “overshooting” in the transitional regimes of the low-current gliding arc discharge

A linear stability analysis of the low-current gliding arc discharge in the transitional regime is performed. It is shown that the gliding arc remains stable during the evolution and gradually transforms into a more nonequilibrium one. The low-current arc discharge can propagate with the effect of “overshooting” at which the gliding arc extinguishes long after its maximum power has been achieved. Analytical and numerical solutions explain the general behavior of the low-current gliding arc and are in a good agreement with our experiment.

Employing plasma as catalyst in hydrogen production

Abstract A novel approach in hydrogen production via reforming of hydrocarbons that will use catalytic properties of non-equilibrium plasma gas discharge is presented. CH4/CO2 mixtures are heated to a temperature of 900°C, so that mixture will have approximately 80% of the energy required for thermal reforming. Preheated mixtures are then processed in a Pulsed Corona Discharge type plasma source and the catalytic effect of a non-equilibrium plasma is observed for promotion of hydrogen generation.