Destruction of Organic Halogen Liquid Waste in Thermal
Plasma
Bournonville Blandine, Thellier Christophe, Guenadou David, Meillot Erick
CEA Le Ripault
BP 16, 37 260 Monts, France
Abstract
A new patented process working with thermal plasma generated by inductive plasma
torch, for destruction of organic halogen liquid wastes is presented. Using Ar/H 2 plasma
as high temperature source, this safe system can destroy a wide variability of wastes,
even mixed together, by forming only non-toxic products such as atmospheric gases,
liquid water and halogen sodium salt. Liquid water is added to the wastes for obtaining
the correct stoichiometry for complete chemical reactions. The two fluids are introduced
directly into the hottest plasma zone. This process works with radiofrequency plasma
torch to dissociate completely molecules in atomic forms. Then, air addition permits
atom oxidation. After quick recombination, atoms form simple and no-toxic compounds
such as CO2, H2O and HCl. The formation of solid carbon is so avoided to prevent
migration of pollutants onto soot. Then, a wet treatment with soda traps HCl to leave
only clean gases for evacuation into the atmosphere.
The final aim is to develop a clean and safe process for treatment of radioactive organic
halogen compounds. The first step consists in the validation of the process using noradioactive organic halogen compounds such as chloroform. The process simulation has
been confirmed by experimental results and then extrapolation to other wastes, more
particularly in mixture, can be carried out.
Keywords: waste destruction, inductive thermal plasma, organic halogen liquid.
1. Introduction
Due to new developments of industrial applications, companies generate more and more
wastes: solids, gases, liquids have to be eliminated in earth environmental respects. In
the same time, environmental laws evolve and create severe stresses leading to more
and more important stocks of wastes without elimination way.
Destroying wastes can be really feasible by combustion if these wastes do not present
high temperature stability. With high stability wastes, high energy processes are
required to destruct them but also to control the exhaust products. That is why thermal
plasma processes have been first applied for vitrifying fly ashes from domestic waste
incinerators, for example, with the EUROPLASMA process. Another application
concerns also asbestos vitrifying by the same process. State of the art shows many
thermal plasma processes to destroy gases or to vitrify solid wastes. This is due to the
high energy power and the adaptability of this kind of processes. After solids, the
second application has been to destroy gases such as ozone depleting gases. For
example, the PLASCON process has been developed to remove CFC but also liquids
such as dichlorophenoxyacedic (2-4 D) or oils contaminated by PCB (Murphy, 1999).
Other plant is the one built by NIPPON STEEL CORP. in Japan to remove
chlorofluorocarbons (Takeuchi et al., 1995). Other research papers have been proposed
using different kinds of plasma (Snyder et al., 1997, Suzuki et al., 1997, Yargeau et al.,
1998).
In nuclear industry, the problem is more significant due to the adding of radioactivity.
The 1991’s French law, called BATAILLE law, stipulated that different ways of
elimination must be explored to stock or to diminish and to store the wastes issued from
nuclear activities. That is why, the French atomic energy agency, CEA, has high
interests for all new industrial treatments of diversified wastes.
To response to the directives, CEA has decided to study, in first step, chemical
destruction of no-radioactive halogen organic mixed wastes in a new process based on
thermal inductive plasma torch. This process is in progress to be patented.
Its aim is to destroy completely a wide range of halogen liquid wastes, pure or mixed,
and to exhaust only no-toxic gases with trapping the halogen molecules as mineral salts.
2. Description of the process
Short description of the pilot is presented here without details due to the in-progresspatent study. Inductive plasma torch, PL 50 gun from TEKNA Inc., working with
argon/hydrogen mixing, transforms the electric energy into mainly thermal energy at the
head of the reactor (figure 1). It is a double-flux gun, one in the central network of the
torch (only argon), the second along the wall as sheath gas (mixture of argon and
hydrogen). This kind of technology presents the interest to permit the use of a wide
range of gases and the introduction of compounds-to-destroy directly inside the torch, in
the highest temperature zone and that without any gun material degradation. The gases
are ionized at high temperature (more than 12 000 K) as thermal plasma and flow inside
the reacting chamber. This one presents a studied design to avoid cold areas, with a
refractory wall. Wastes and water adding are introduced in the plasma plume at the head
of the reaction chamber.
The high temperatures, allied with a long time life of the products in the hot zone, lead
to vaporize and to break up completely the molecules, even the most stabilized one, in
elementary atoms. The molecular flow rates of the two fluids (wastes and water) are in
correlation such as the carbon and halogen atom numbers from wastes and the oxygen
and hydrogen atom numbers can lead to the following complete reactions:
C  2  CO2
(1)
Cl  H  HCl
(2)
An excess of water can saturate the previous reactions. At the end of the first destruction
zone, an air introduction accelerates and achieves the oxidation of carbon and hydrogen.
After complete reactions, the off-gases are cleaned in a wet treatment unit with soda to
trap halogen atoms, and then are cooled by a heat exchanger. Pump unit has two goals:
firstly evacuating the off gases and secondly regulating the inside pressure in the
process.
Waste and water
injections
Electrical Power
Plasma gas
Inductive torch
Air introduction
Destruction
And
Recombination
Stage
Cooling water
Mass
spectrometer
Soda
Halogen
treatment unit
H2O
NaCl
HCl analysis
Heat
exchanger
Pump
Off gases
Ar , N2, O2, CO2, H2O
Figure 1: Simplified scheme of the process.
3. Operating conditions and analysis setup
3.1 Operating parameters
The main operating parameters are collected in Table 1. Argon and hydrogen are
introduced in the torch to generate the thermal plasma. Hydrogen increases the enthalpy
of the gases. The plate power is around 42 kW. Due to the rather bad thermal efficiency
of inductive gun, less than 50% of the electric power is transferred to the gases at the
torch exit. The products to destroy, here chloroform, are injected in the plume. For
chloroform destruction, the water flow rate ratio is depending on the equation (3). An
excess of water is introduced to saturate the basic reaction in the first steps.
CHCl 3  5H 2 O  CO 2 3HCl  3H 2 O  H 2
(3)
3.2 Analysis materials
Gases are analysed at the end of reaction zone, after air introduction, by mass
spectrometer (PFEIFFER VACCUM type MONISTAR GSD 300 C2). Several
thermocouples measure the gas temperature, before and after air injection for example,
to control the well-done oxidation reactions. Before exhaust into the atmosphere, HCl
concentration measurements are realized with an OLDHAM analyser (LASERGAS
2000).
Table 1: operating parameters of the process.
Parameters
Values and units
Argon central flow rate
30 slpm
Argon sheath flow rate
100 slpm
Hydrogen sheath flow rate
7 slpm
Plate power
42 kW
Additional air flow rate
180 slpm
Chloroform flow rate
13 g/mn
Additional water flow rate
11 g/mn
Soda initial concentration in treatment unit
7% mass
Pressure
0.9 105 Pa
4. Results
The results are presented versus experimental time. Several injections have been carried
out in the same test and so experiments can show the rather good stability of the process
because of test duration. Chloroform and water feedings are represented by darkest grey
zone while, before and after the injection, only water introduction is coloured in
brightest grey to clean the injection system and reactor.
4.1 Gas temperature
In the destruction and recombination stage, just before air adding and without liquid
injection, the gas temperature has risen up to around 1200 °C (first curve of Figure 2)
when the thermal equilibrium was steady. This temperature, measured quite far from the
plasma torch, is in good correlation with simulation results (Guenadou and Meillot,
2004). These results imply that the high upstream temperatures are efficient to destroy
completely the wastes.
The only water introduction (at the flow rate of 24 g/mn) into the plasma induces a
temperature drop of about 100°C (Figure 2), due to molecular decomposition, at the first
step of the temperature measurement. During the first product injection, the process has
stopped suddenly for external explanation (voltage drop). A few minutes later, the
plasma has been again generated and the injection has been performed. This leads to the
temperature fall in the middle of the experiment (around one hour and a half).
Introducing the waste/water mixture in the process has leaded to the severe temperature
drop: the temperature was less than 1000 °C during around three quarters after two
hours time. Although the mass flow rate of chloroform and water was similar than
single water injection, the temperature drop during the chloroform injection is more
important due to the endothermic reaction of the hydrochloric acid formation. After
three hours and a half experiment, the process has been stopped. The gas temperature
has decreased rapidly first and more slowly in a second time: this effect is due to the
energy stored in the refractory wall.
1400
temperature (°C)
after additional air
process halted
before additional air
1200
1000
800
process stopped
Water
injection
600
Chloroform
injection
400
200
0
0:00
1:00
2:00
time (h:mn)
3:00
4:00
5:00
Figure 2: Gas temperature during chloroform injection.
After air adding and still without liquid injection, the gas temperature has risen up to
nearly 400°C (second curve on Figure 3). The curve has the same trend than the
previous one.
4.2 Gas composition
1,E-08
H2
H2O
O2
1,E-09
Water
injection
ion current (A)
1,E-10
1,E-11
1,E-12
Chloroform
injection
1,E-13
1,E-14
0:00
0:30
1:00
1:30
2:00
2:30
time (h:mn)
Figure 3: H2, H2O and O2 composition signal during chloroform injection.
3:00
After molecular combinations, the mass spectrometer analyses show the presence of
only simple molecules: Ar, N2, O2, CO2, H2O, HCl and Cl2. No chloroform has been
detected. With these analyses, following the evolution of the gas composition versus
time is possible (Figures 3, 4 and 5). In Figure 3, only H2, H2O and O2 compositions are
first plotted, process working.
O2 appears in excess while H2 seems completely transformed in vapour. The water
evolution is particular. The quantity of steam has increased during the first injection of
water. Then, in the middle of the first chloroform injection, a notable decrease has
appeared. At the beginning of the second chloroform injection, a same evolution but
smoother has gone on. Then, the water quantity has increased regularly until the end of
the second water injection. Assumptions about water deposition by condensation on
cold walls and deposit saturation could be proposed.
1,E-08
N2,CO
NO
CO2
1,E-09
Water
injection
ion current (A)
1,E-10
1,E-11
1,E-12
Chloroform
injection
1,E-13
1,E-14
0:00
0:30
1:00
1:30
2:00
2:30
3:00
time (h:mn)
Figure 4: N2, NO, CO and CO2 composition signal during chloroform injection.
In Figure 4, results concerning carbon and nitrogen oxidation are presented. Firstly, if
CO and N2 are plotted together, this curve remains stable during test duration although
the CO2 line shows important increases when chloroform is injected. So, if the carbon
oxidation has not been completely achieved, that has stayed in low level. Secondly,
nitrogen oxides have left under low concentrations, process working, and so no nitrogen
oxide treatment unit is needful.
Figure 5 shows signals of chloride species and also argon for the main line. This one
does not evolve while chloroform injection or no. So, the evolution of the second line,
which indicates the presence of both argon and hydrochloric acid, proves only
appearing of hydrochloric acid when injection working. Unlike the CO 2 evolution which
has increased and decreased drastically when the chloroform injection started and
stopped, the hydrochloric acid evolution has been smoother. The delay before seeing the
hydrochloric acid is higher than for CO2 and the increase of HCl concentration, as its
decrease, has been progressive. At the end of the chloroform injection, a few Cl 2
molecules have been detected with the same smooth evolution.
The Cl2 has been formed certainly due to a local under-stoichiometry of hydrogen: the
high hydrogen diffusion could be responsible of this generation. The increases in H 2O
and HCl seem to follow the same slope. Their evolution seems to be linked by a
saturation phenomenon. At the exit of the process, the gas cools down on cold wall.
1,E-08
Ar
36Ar, HCl
Cl
Cl2
1,E-09
Water
injection
ion current (A)
1,E-10
1,E-11
1,E-12
Chloroform
injection
1,E-13
1,E-14
0:00
0:30
1:00
1:30
2:00
2:30
3:00
time (h:mn)
Figure 5: Ar, HCl, Cl and Cl2 composition signal during chloroform injection.
Consequently, there is an important condensation of steam on the reactor wall. When
chloroform is introduced into the plasma, a great HCl formation appears and the
hydrochloric acid has a great affinity with liquid water. Then, a part of HCl dissolves
into the condensed water. So, when the liquid is saturated by hydrochloric acid,
equilibrium is obtained and the HCl concentration in the gas increases enough to be
detected by the mass spectrometer. A great quantity of liquid highly concentrated in
HCl has been founded at the bottom of the reactor, before halogen trap unit. That’s
confirmed this hypothesis.
11
norm
HCl concentration (mg/Nm3)
10
9
8
Water
injection
7
6
5
Chloroform
injection
4
3
2
1
0
0:00
1:00
2:00
3:00
4:00
time (h:mn)
Figure 6: Hydrochloric acid ejection into atmosphere during chloroform injection.
5:00
After that, the gas treatment unit has trapped efficiently hydrochloric acid gas and has
limited its concentration in off gases (Figure 6). The HCl concentration is always lower
than 10 mg/sm3 (the French HCl off-gas concentration norm): when the treatment unit
parameters have been correctly fixed, equilibrium has started and the concentration was
less than 4 mg/sm3. This light concentration of gaseous hydrochloric acid during the
chloroform injection indicates also the well-operating treatment of the process.
5. Conclusion
Due to necessity of destroying high stability liquid halogen wastes, a new process has
been proposed and is in progress to patent. Based on thermal inductive plasma torch and
working with water adding in the hot destruction area, it is safe and robust.
The measured temperatures, before additional air injection, far from the plasma zone,
are high and in correlation with previous simulation results. They represent the first
evidence of the good destruction rate.
After additional air, the gas composition, caught by mass spectrometer analyses, is
exempt of initial products to destroy, here chloroform. Although no mass balance has
been performed, the process presents a very high destruction rate: only no toxic but
simple gas molecules have been rejected into the atmosphere after a gas treatment for
hydrochloric acid. In this case, chloride has been trapped as minerals salts.
New developments are carried on: after simple products such as chloroform,
destructions of mixtures of several halogen liquid wastes are in progress.
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