Table Of ContentNon-Thermal
Plasma Techniques
for Pollution Control
Part B: Electron Beam
and Electrical Discharge Processing
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Series G: Ecological Sciences, Vol. 34, Part B
Non-Thermal
Plasma Techniques
for Pollution Control
Part B: Electron Beam
and Electrical Discharge Processing
Edited by
Bernie M. Penetrante
Lawrence Livermore National Laboratory
Livermore, CA 94550, USA
Shirley E. Schultheis
San Ramon, CA 94583, USA
Springer-Verlag
Berlin Heidelberg New York London Paris Tokyo
Hong Kong Barcelona Budapest
Published in cooperation with NATO Scientific Affairs Division
Proceedings of the NATO Advanced Research Workshop on Non-Thermal
Plasma Techniques for Pollution Control, held at Cambridge, England,
September 21-25, 1992
ISBN-13: 978-3-642-78478-1 e-ISBN-13: 978-3-642-78476-7
DOl: 10.1007/978-3-642-78476-7
Library of Congress Cataloging-in-Publication Data. Non-thermal plasma techniques for pollution
control I edited by Bernie M. Penetrante, Shirley E. Schultheis. p. cm. - (Nato ASI series. Series G,
Ecological sciences; vol. 34) "Proceedings of the NATO Advanced Research Workshop on Non
Thermal Plasma Techniques for Pollution Control, held at Cambridge, England, September 21-25,
1992" - T.p. verso. Includes bibliographical references and index. Contents: pt. A. Overview,
fundamentals, and supporting technologies -pt. B. Electron beam and electrical discharge process
ing.
1. Air-Purification-Congresses. 2. Plasma devices-Industrial applications-Congresses. I. Penetrante,
Bernie M., 1960-.11. Schultheis, Shirley E., 1957-. III. NATO Advance Research Workshop on Non
Thermal Plasma Techniques for Pollution Control (1992: Cambridge, England) IV. Series: NATO ASI
series. Series G, Ecological sciences; no. 34. TD889.N66 1993 628.5'32-dc20 93-21307
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Softcover reprint of the hardcover I st edition 1993
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PREFACE
Acid rain, global warming, ozone depletion, and smog are preeminent envi
ronmental problems facing the world today. Non-thermal plasma techniques offer
an innovative approach to the solution of some of these problems. This field has
grown dramatically in recent years. Many new types of air pollutants have been
tackled; new kinds of plasma reactors have been developed, and existing kinds have
been improved greatly. It is in the interest of many nations to assure that all practical
and economically feasible steps are taken to apply and further develop these
techniques to help solve environmental pollution problems. To accomplish this, it is
necessary to get the best solutions from industry, academia, and government
laboratories, and to pool this information for the benefit of all. The NATO Advanced
Research Workshop on Non-Thermal Plasma Techniques for Pollution Con
trol, held in Cambridge University on 21 -25 September 1992, was conceived as a
step towards that goal. These two volumes contain the proceedings of that workshop.
Non-thermal plasmas are plasmas in which the electron mean energies are
considerably higher than those ofthe components ofthe ambient gas. The majority
of the electrical energy goes into the production of energetic electrons, rather than
into gas heating. The energy in the plasma is thus directed preferentially to the
electron-impact dissociation and ionization of the background gas to produce
radicals that, in turn, decompose the toxic molecules. This is in contrast to the use
of plasma furnaces or torches and several chemical techniques in which the whole
gas is heated in order to break up the undesired molecules. For many applications,
particularly in the removal of very dilute concentrations of air pollutants, the non
thermal plasma approach would be most appropriate because of its energy selectiv
ity and its capability for the simultaneous removal of various pollutants. For others,
including many mixed waste streams, the best approach might be to use a thermal
plasma to incinerate the complete waste, including its container, and to use a non
thermal plasma reactor to clean the off-gases.
There are many types of non-thermal plasma devices that have been devel
oped for environmental applications. The potential ofthese devices for the destruc
tion of pollutants or toxic molecules has already been demonstrated in many
contexts, such as nitrogen oxides (NOx) and sulfur dioxide (S02) in flue gases, heavy
metals and volatile organic compounds (VOCs) in industrial effluents, and chemical
agents such as nerve gases. The articles in these two volumes contain a comprehen
sive account of the latest developments in non-thermal plasma devices and their
application to the disposal of a wide variety of gaseous pollutants.
The cost of implementation underlies almost all of the decision-making in
selecting the technology for pollution control. In many cases, the only existing
economic analyses for new technologies derive from small-scale, laboratory-level
applications. Full-scale research in industrial pilot plants should result in a better
knowledge of actual costs. The articles in these proceedings discuss both laboratory
studies and industrial implementation of the different non-thermal plasma tech-
VI
nologies available. From the laboratory tests we learn about the feasibility of the
technique and understand the fundamental aspects of the process. From the
industrial implementation we learn not only what the scalability of the process is,
but also what the typical investment and operating costs are at full-scale industrial
facilities.
The proceedings are presented in two volumes. The subjects are subdivided
as follows: in Part A: Overview, Fundamentals and Supporting Technologies; in
Part B: Electron Beam and Electrical Discharge Processing. In the following I will
present a synopsis of the volume sections. This is meant to allow the reader to choose
those topics of principal interest and to identify the connection between the various
contributions. In many instances, each type of plasma reactor can be applied to many
different kinds of pollutants. Similarly, each pollutant can be processed by different
types of reactors. The chapters in Part B are categorized firstly in terms of reactor
type and secondly in terms of pollutant kind. Some authors have made studies with
more than one type of plasma reactor. Their results are not presented in separate
papers for each type of reactor or pollutant; however, the following synopsis will be
categorized in terms of reactor type. The papers are referenced again whenever
befitting. .
OVERVIEW OF TECHNIQUES AND APPLICATIONS
Non-thermal plasmas can be created in essentially two different ways: by
electron-beam irradiation, and by electrical discharges. Electrical discharge tech
niques can be implemented in many ways, depending on the electrode configuration
and electrical power supply. Each of these techniques can be applied to the disposal
of a wide variety of gaseous pollutants, and in some cases has the capability for the
simultaneous removal of coexisting pollutants. In some applications, the advantages
of non-thermal plasma techniques over conventional pollution control alternatives
are only recently being recognized. Section 1 of the first volume deals with an
Overview of Techniques and Applications. The paper by J.-S. Chang on "Energetic
Electron Induced Plasma Processes for Reduction of Acid and Greenhouse Gases in
Combustion Flue Gas" provides an overview of the myriad of plasma reactors and a
comprehensive review of all recent work related to the application ofthese reactors
to the control of NO x' SOx, COx, toxic gases, VOCs, hazardous emissions, and ozone
synthesis.
Thermal plasma systems are being considered for primary destruction of
hazardous wastes consisting of radioactive materials mixed with hazardous chemi
cal substances in various forms. The next two papers address the possible applica
tion of non-thermal plasma systems as secondary treatment units for the destruction
of mixed wastes. The paper by W. H. McCulla and D. M. French discUsses "The Need
for Waste Treatment Options Within the US Department of Energy" and addresses
the potential advantages of non-thermal plasma systems for the destruction of very
low concentrations ofVOCs and for secondary treatment of mixed wastes. Conven
tional thermal treatment of mixed wastes has the disadvantages of creating NOx
VII
problems. Innovative thermal plasma technologies such as that discussed in the
paper by M. P. Schlienger on "plasma Arc Processing of Solid and Liquid Wastes" can
be applied to difficult waste streams, but also produce NOx levels that are of concern
to regulatory agencies. The development of an economical exhaust gas treatment
system, such as those based on non-thermal plasmas, may be beneficial to the overall
implementation of these thermal technologies for the treatment of mixed wastes.
The use of non-thermal plasma processing for detoxifYing chemical warfare
gases has been investigated in several laboratories. The paper by D. E. Tevault
discusses the "Application of Plasma Technology for Proposed Air Purification
Systems." It is pointed out that even though some non-thermal plasma systems have
been shown to separate chemical warfare agents from air streams, they have not yet
been demonstrated to produce breathable air alone; thus, they cannot presently be
considered as a candidate air purification technology for military applications. The
paper identifies several problem areas for further work required to mature plasma
reactors for potential military application.
FUNDAMENTALS
Section 2 of the first volume reviews the fundamental aspects relevant to non
thermal plasma processing. There is a great need to assemble the relevant funda
mental data in order to guide researchers involved in the development of pollution
control technologies. There is a need to determine, and develop theoretical models
for, the underlying physical processes involved in the energy deposition and electri
cal breakdown of gases in order to minimize the power consumption of the pollution
control devices. A thorough knowledge of the relevant chemical reactions and the
development of diagnostic techniques are essential to fully elucidate the mecha
nisms of pollutant removal and to ensure that the processing does not itself create
hazardous by-products.
Chemical Kinetics
The paper by H. Matzing on "Chemical Kinetics Model of S021N0x Removal
by Electron Beam" discusses how chemical kinetics calculations have contributed
significantly to the understanding of the electron-beam flue gas treatment chemistry
and the major factors that determine the removal efficiency. The mechanisms for
NOx and S02 removal from coal-fired flue gas in the presence of ammonia are
discussed. The paper discusses the basis for suggesting the use of multiple-zone
irradiation for improving the NOx removal efficiency, and points some unresolved
discrepancies with experiments that would require further investigatiops.
Recent enthusiasm on the potential of using non-thermal plasmas for exhaust
treatment in mobile combustion engines has led to speculations that these tech
niques could reduce NOx with reasonable power consumption, and without the use
of catalysts or additives. Iftrue, the technique would be an important breakthrough
VIII
in post-combustion NOx control technology. The paper by B. M. Penetrante on
"Plasma Chemistry and Power Consumption in Non-Thermal DeNOx" tries to
answer some important basic questions regardirig the capabilities and limitations of
non-thermal deNOx. A comparison of discharge processing with electron beam
processing is presented in terms of power consumption, energy dissipation mecha
nisms and process by-products.
The importance of ion-molecular chain oxidation mechanisms in reducing the
energy cost for S02 removal is discussed by Potapkin et al. in their paper on "The
Effect of Clusters and Heterogeneous Reactions on Non-Equilibrium Plasma Flue
Gas Cleaning." Possible experimental evidence of these chain oxidation mechanisms
is presented in the second volume by A T. Kunavin et al. in the paper "Intensification
ofE-Beam Processing ofS02 Removal from Flue Gas" and by E. 1. Baranchicov et al.
in the paper "Investigation ofS02 Oxidation in Humid Air Stream by High Current
Density Pulsed Electron Beam."
The paper by J. T. Herron describes the design of "Chemical Kinetics Data for
Modeling Non-Thermal Plasmas" and discusses the proper representation of rate
data with respect to their functional dependence on temperature and pressure.
Examples of evaluated data relevant to flue gas cleanup are presented.
The analysis of the chemical kinetics is often complicated by the presence of
unknown heterogeneous reactions. For applications such as air purification from
nerve agents, it is imperative to ensure that the processing does not create other
hazardous by-products. There are many active species and in order to clarify the
chemistry it is sometimes necessary to look at individual reactions. The paper by R.
L. Sharpless and T. G. Slanger reports on experiments to study the "Interaction of
Organophosphonates with O(sp), N(4S), 02(a1L\g), and Os." The authors conclude
that surface-mediated heterogeneous reactions are important and probably domi
nant in this particular application.
The paper by Y. Sakai and H. Tagashira presents measurements on the
"Decomposition ofN02 by Glow Discharge Plasma." The reactor was operated at low
pressure (1 -5 Torr) and was meant mainly to study the decomposition kinetics of
N02·
The paper by J. D. Skalny et al. discusses experiments on the "Negative
Corona Induced Decomposition ofCC12F2." The main goal of the experiments is to
study the decomposition kinetics ofCFC-12; the reactor could only accomodate low
power input and small gas flow rates. The decomposition efficiency ofCFC-12, as well
as various chlorinated hydrocarbons like CC14 and TeE, are presented as a function
of the energy density input and initial pollutant concentration.
Discharge Physics
Many different types of high-pressure discharges may be used as reactors for
pollution control. The electrical and chemical properties of the plasma species
IX
produced in each reactor depends upon the mechanisms of discharge development
and ionization growth. These discharge mechanisms are discussed by E. Marode et
al. in the paper "High Pressure Discharge as a Trigger for Pollution Control." Regions
of different electron energies and chemical activities are identified and various
regimes are analyzed.
Many electrical discharge devices achieve non-thermal conditions through
the production of micro discharges called streamers. Streamers are plasma filaments
produced by highly localized space-charge waves. These space-charge waves en
hance the applied field in front of the wave and propagate because of electron
avalanching in this high field. Streamers yield good power efficiency because, within
the short lifetime of the streamer, the ions do not experience significant movement
and therefore do not contribute to the power consumption. The short lifetime of the
streamer is accomplished with the use of very-short high-voltage pulses (as in pulsed
corona discharges) and/or with the use of dielectric coatings on the electrodes (as in
dielectric barrier discharges). The term "streamer discharge" is generic in the sense
that it can refer to a discharge created in a wire-cylinder corona reactor, or a
discharge created in a parallel-plate dielectric-barrier reactor. The next six papers
deal with the production, diagnostics and modeling of streamer discharges.
The energy consumption of plasma reactors depends on both the chemical
reaction efficiency of the plasma and the energy conversion efficiency from the main
power source to the plasma. For the pulsed corona technique it is important to
determine how the voltage pulse parameters affect the discharge properties. The
paper by M. Rea and K. Yan discusses experiments on the "Energization of Pulse
Corona Induced Chemical Processes" to determine the conditions that lead to
maximum streamer number density and maximum energy injection. The current
voltage characteristics ofthe pulsed corona produced under different applied voltage
waveforms are presented. The conditions for optimal streamer formation and the
criteria for designing a voltage pulse generator are discussed.
An "Electrical and Optical Study of Pulsed Positive Corona" is presented in
the paper by Y. L. M. Creyghton et al. Diagnostic measurements to determine the
effect of voltage pulse parameters on the streamer inception voltage and time lag,
and on the energy input into the gas, are presented. The streamer discharge
structure is visualized using Schlieren and CCD pictures.
Emission spectroscopy can yield information on gas composition, population
of energetic electrons and processes in the discharge. In the paper by T. H. Teich on
"Emission Spectroscopy of Corona Discharges" procedures are outlined for spectral
and time-resolved single photon counting. The use of this method with repetitive
discharges makes it possible for the time history of emission to be recorded with
extreme sensitivity and sub-nanosecond resolution. Emission examples for air,
water vapor, nitrous oxide and flue gas containing nitric oxide are given.
Several distinct phases in the evolution of the structure of streamers are
presented by P. A. Vitello et al. in the paper on "Multi-Dimensional Modeling of the
Dynamic Morphology of Streamer Coronas." The paper demonstrates the transition
x
between these phases, showing how the self-consistent radial and axial structure
varies with time. The paper considers how the dynamical characteristics of stream
ers are determined by their structure.
The evolution of streamers in a dielectric barrier discharge is discussed by G.
J. Pietsch et al. in the paper on the "Modeling of Dielectric Barrier Discharges." The
production of radicals during the different phases of streamer evolution is discussed.
The significance of several discharge regions on radical production is also addressed.
In order to make the simulation of streamer evolution computationally
tractable, assumptions are usually made regarding the dependence of the electron
transport and rate coefficients on the local and instantaneous value of the electric
field. The difference in the simulation results because of differences in these
assumptions is discussed by J.-M. Guo and C.-H. Wu in their paper on the
"Comparisons of Multi-Dimensional Fluid Models for Streamers."
In order to determine the electrical energy dissipated in the various plasma
chemical processes, it is necessary to have a complete and self-consistent set of
electron-molecule cross sections. The derivation of these cross sections is discussed
by M. Yousfi et al. in the paper "Electron Data Base Needed for Discharge Modeling
in Flue Gas Treatment." The paper also presents an alternative finite element
method for simulating the discharge dynamics.
The electrical breakdown voltage is an important quantity in determining the
performance of many pollution control devices. For example, the efficiency of
electrostatic precipitators increases markedly with the voltage that can be applied
between the wires and collector plates, provided that electrical breakdown does not
occur. This is also true for corona reactors. An analysis ofthe effect of basic electron
molecule processes on the breakdown voltage is presented by J. J. Lowke in the paper
"The Role of Electron Attachment to Gas Molecules in Electrostatic Precipitators."
SUPPORTING TECHNOLOGIES
Electron Beam Sources
Conventional accelerators use thermionic cathodes that are subject to poison
ing and therefore requires high vacuum near the cathode region. Plasma cathodes,
which can operate at higher pressures, can be substituted for the beam source, but
have not been used in most applications because the emittance is usually not as good.
For pollution control applications, the use of plasma cathodes may significantly
reduce the capital cost for implementing electron beam processing in both small
scale problems and those requiring high-current beam densities.
The paper by S. J. Scott describes"A Long Life, High Repetition Rate Electron
Beam Source" based on a corona plasma cathode. The operating principles and
typical characteristics - peak voltage, peak current, emitting area, pulse duration,
repetition rate and lifetime - of this device are described. The paper then discusses
the scaling of this device.
