22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Transport phenomena in thermal plasmas
M.I. Boulos
Professor Émerit, University of Sherbrooke
Chief Technology Officer, (CTO), Tekna Plasma Systems Inc., Sherbrooke, Québec, Canada
Abstract: A review is presented of advances in our understanding of basic transport
phenomena under plasma conditions. These had a major impact on plasma torch and
process developments. Examples are given of results obtained using induction plasma
sources and in-flight powder treatment technology.
Keywords: transport phenomena, heat, mass and momentum transfer, induction plasmas,
plasma-particle interactions, inflight powder treatment
1. Introduction
Over the past four decades considerable progress was
made in our understanding of the fundamental concepts
behind thermal plasma sources, and their use for the
processing of materials. Special attention was given to
the potential of the technology for the manufacture of
unique high added value products on an industrial scale.
These varied widely depending on the plasma source
used, and the process needs:
DC plasma sources
o Plasma chemical synthesis;
o Plasma metallurgical applications, including plasma
smelting and refining;
o Multi-MW plasma wind tunnels for space reentry
simulation;
o Plasma cutting and welding;
o Plasma spray coating and deposition of near net shape
parts;
o Plasma
waste
treatment.
Waste–to-energy
conversion;
o Plasma treatment of powders and the synthesis of
nanopowders.
RF Inductively Coupled Plasma (ICP) sources
o Plasma wind tunnels for space reentry simulation;
o ICP spectrochemical analysis (ICP-AIS, or ICP-MS);
o Fiber optics preform cladding;
o Plasma spraying of coatings and deposition of near
net shaped parts;
o Plasma treatment of powders, purification,
densification and spheroidization;
o Plasma synthesis of nanopowders.
Developments in each of these areas has required
intensive research efforts and a basic understanding of the
transport phenomena under these conditions in order to
insure the reliability and scalability of the process to an
industrial scale production. In spite of their successful
technological performance, not all such developments
have survived important economic challenges offered by
alternate technologies. For example the use of dc plasma
torches in industrial scale metallurgical applications have
been limited to relatively specific cases of melting of high
purity alloys, ladel and tundish heating for the close
PCA
control of the temperature of the molten bath. The use of
plasma technology for waste treatment has also met
limited success due to economic constraints. A major
breakthrough in this area was reached recently through
the success of the concept of waste-to-energy conversion
where the plasma system ends up as being a net producer
of electrical energy to the grid rather, than an energy
consumer.
One of the most successful large scale industrial
applications of thermal plasmas has been in the cutting
and welding industry where a plasma cutting or plasma
welding equipment has become a standard piece of
hardware in any mechanical manufacturing environment.
Such units are available in the low to medium power
range (10 - 20 kW) with thousands of such units used
across the world. Plasma spraying is another application
of thermal plasma technology which has gained a rapid
acceptance especially in the aerospace industry for the
protective coating of critical components in a jet engine
such a turbine blades and combustion chamber liners.
Numerous other applications of plasma spraying has also
been developed and used on a regular basis in the
biomedical industry, in the mining and chemical industry
and more recently in the automotive industry. Most of
these units are developed around dc non-transferred arc
plasma torches operating at atmospheric pressure or soft
vacuum conditions, at power levels ranging between tens
of kW up to 100 or 200 kW. Plasma Transferred Arc
(PTA) units have also been developed and used on an
industrial scale for hard facing and wear resistance
coating of parts in the mining and petroleum industry.
Inductively Coupled Plasma sources, on the other hand,
have been successful in niche applications for the
synthesis of high purity, high added value, materials such
spherical, free flowing dense powders of a wide range of
materials for the powder metallurgical, Metal Injection
Molding (MIM) and added manufacturing or 3D printing.
These include high purity metals such as tungsten,
molybdenum and tantalum and titanium. The same
technology has also been used for the synthesis of
ultrafine powders and nanopowders of metals and alloys
such as nickel, copper and silicon, or ceramics such as
1
titanium and zinc oxides and specialized nano-glasses
mostly for the electronic industry and more recently for
potential use in battery developments.
As we take a close look at the basic phenomena at play
in each of these technological applications of thermal
plasmas, we recognize that a fundamental understanding
of the transport phenomena in the plasma source and
plasma reactor or processing unit, has been of critical
importance in order to achieve the transformation
efficiencies, production capacities and degree of
reliability required for an industrial scale acceptance of
the technology. In this paper, the critical role of
fundamental studies of transport phenomena is
demonstrated as applied to a few examples of induction
plasma source development, and their integration in
industrial scale process technology. The paper also aims
at identifying areas where more research and development
efforts are needed to continue our progress in this field.
2. Induction plasma source development
RF Inductively coupled plasma sources have gone
through a long history of development over the most of
the latter half of the twentieth century and the early part
of this century. A schematic representation of the
different stages of the development of these sources is
shown in Fig. 1.
Fig. 1. Evolution of RF induction plasm sources over the latter part of the 20th century and the beginning of the
21st century.
The first stages of the development of these sources,
which took place in the sixties and seventies was mostly
funded by the aerospace industry for the testing of
thermal shields under space re-entry conditions. This lead
to the development of the high power segmented metal
wall torches by Dresvin and his collaborators [1] in the
former Soviet union, and water-cooled, quartz tube
torches by Thorpe and his collaborators [2] at TAFA
corporation in the USA.
While early versions of
laboratory scale torches were also developed by Reeds [3]
at MIT and Roboux in France over the same period, it was
obvious that a fundamental understanding and control of
the flow and temperature fields in the discharge was an
essential condition for the further development of such a
source. This triggered a world wide effort for the
development of advanced mathematical models of such
devices operating under a wide range of conditions. This
was initiated by the work of Miller and Ayen [4],
2
followed by Boulos and his collaborators [5-25] at the
University of Sherbrooke, Yoshida et al. [26-27] at Tokyo
University, and more recently by the significant
development of 3D models of induction plasma systems
by Colombo and his collaborators [28-34] at the
University of Bologna. Such intensive modelling work
over essentially four decades helped considerably in
developing a fundamental understanding of the flow,
temperature and mixing patters in such sources and in the
development of a number of novel plasma torch designs
whether of the standard, ceramic wall type [14-17] or a
dc/rf or rf/rf hybrid plasma torches [26-27]. Typical
results of the flow and temperature fields in a standard
ceramic wall torch are given in Fig. 2. These show the
typical large volume discharge with a mean temperature
in the 8000 - 9000 K range and plasma jet velocities at the
torch exit in the range of 50 to 60 m/s. These models
were instrumental in determining the heat flux profile to
PCA
the plasma confinement tube and accordingly played a
key role in the design of commercially available torches
and the determination of their safe power ratings. Based
on these designs, standard RF induction plasma torches
are now available for operation at plate power levels up to
400 kW.
Fig. 2. Typical flow and temperature fields in an inductively coupled rf plasma torch (70 mm i.d., operated at
atmospheric pressure, at a plate power of 100 kW (65% coupling efficiency) using Ar/H 2 as the plasma gas with
12.6 vol.% of H 2 .
3. Plasma - particle interactions
One of the most widely used applications of induction
plasma torch is for the treatment of powders for the
purpose of their purification, densification and
spheoridization. Such powders have a wide range of
applications which benefits from their dense and free
flowing properties. The process is based on the axial
injection of the powders to be treated into the center of
the discharge using a water-cooled injection probe. As
the individual powder particles enter into contact with the
plasma, they are rapidly heated and melted forming
micron sized spherical droplets which on cooling at the
downstream end of the reactor chamber solidify to form a
dense spherical particle. The process is strongly affected
by plasma-particle interaction effects, which limits the
throughput of powder that can be injected into the
discharge. The effect is due to the local cooling of the
plasma by the particles which eventually leads to a
reduction of the fraction of the particles which could be
melted by the plasma.
Our understanding of the
fundamental laws governing the heat, mass and
momentum transfer under these conditions have helped
create rather elaborate models of the in-flight particle
melting process [6, 8, 10, 34] and optimize the powder
distribution in the discharge in order to achieve a
maximum through put of powder treated for a given
plasma power rating. Typical results obtained using the
same plasma conditions as those of Fig. 2. are given in
Fig. 3. These show the temperature fields in the discharge
when injecting a, 40 to 200 µm molybdenum powder, into
the center of the discharge at through puts of 15 and
25 kg/h. It is noted that with the increase of the powder
PCA
feed rate, there is a significant cooling of the central
region of the discharge and an overall reduction of the
temperature of the particles as shown on the right hand
side of Fig. 3. The effect results in an overall reduction of
the fraction of the powder that is fully melted which drops
from 100% of the powder for a powder feed rate of
15 kg/h to 67%wt. for a powder feed rate of 25 kg/h. The
model predictions are supported by experimental data
given in Fig. 4, which shows electron micrographs of the
molybdenum powder prior to plasma treatment and
following the plasma treatment at feed rates of 14 and
25 kg/h. On the lower part of the figure corresponding
powder properties are given. These show a drop of the
packing density of the plasma treated powder from 6.25 to
5.0 g/cm3, and an increase of the Hall flow index from
14 to 19 s/50g with the increase of the powder feed rate
from 14 to 25 kg/h as a result of loss in the percentage of
powder melted.
4. Acknowledgment
I would like to thank in particular the many graduate
students, post-doctoral research fellows, research
associates and colleagues who contributed to this research
program at the University of Sherbrooke. The financial
supported by the National Sciences and Engineering
Research Council of Canada and the Ministry of
Education of the Province of Quebec are gratefully
acknowledged. The reported mathematical modelling
work was carried out by Dr Siwen Xue, and the
experimental results were obtained by the research team,
on the pilot R&D facility at Tekna plasma Systems Inc.
whose contributions is gratefully acknowledged.
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Fig. 3. Plasma and in-flight molybdenum particle temperatures for powder throughputs of 15 and 25 kg/h.
Fig. 4. Electron micrographs of molybdenum powder,(a) prior to plasma treatment, and (b) after plasma treatment at a
through put of 15 kg/h and (c) 25kg/h.
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