-
I. INTRODUCTION
Nanosecond repetitively pulsed (NRP) discharges are
being increasingly used in various applications, in particular
for plasma-assisted combustion [1-3], nanomaterials synthesis [4],
and aerodynamic flow control [5]. The key advantage of NRP
discharges is their high-energy efficiency, and thus low power
consumption, to produce highly reactive atmospheric pressure air
plasmas. We seek to explore this property for water vapor
dissociation. In this objective, water dissociation final products,
i.e. H2 and O2 are measured and their production energy efficiency
should be estimated and compared to other processes. A general
overview of hydrogen production technologies [6] shows that besides
commercial technologies such as partial oxidation and steam
reforming processes, thermal and non-thermal plasma techniques
could be considered as possible reforming technologies. Focusing on
non-thermal plasma processes, different types of discharges were
used to produce hydrogen from various liquid or gaseous fuels
(mainly methane, natural gas and biogas), such as glow discharge,
corona discharge, dielectric barrier discharge, microwave discharge
and radio frequency discharge [7, 8]. On the other hand, water
dissociation using non-thermal plasma technique was previously
investigated, mainly in Russia, as reported by Fridman [9], using
low and moderate pressure microwave discharges at high specific
energy (1-4 kJ/L), and to a lower extent, low pressure glow
discharges. The energy efficiency was found to decrease with gas
pressure (from 0.05 to 0.4 atm). The initiation reactions claimed
by authors for water vapor dissociation are dissociative
attachment
e- + H2O → H- + OH
and dissociation of vibrationally excited molecules
e- + H2O → e- + H2O*
H2O*+ H2O → H + OH + H2O
These mechanisms correspond to major paths of energy transfer
for relatively low reduced electric field (E/N) conditions (lower
than 80 Td).
From a simulation using inelastic electron-molecule collision
cross-sections, the channels of energy transfer from electrons to
water molecules can be established [9]. This calculation was
performed using Bolsig+ [10] with the cross-section database of
Morgan [11] for the case of pure water vapor at 400K and
atmospheric pressure. The results are presented in Fig. 1 for a
reduced electric field (E/N) ranging from 1 to 1000 Td. As can be
seen, increasing the E/N over 80 Td leads to a shift of the
conversion of electrons energy from vibrational excitation to
dissociation
e- + H2O → e- + H + OH
and ionization.
Experimental Study of Nanosecond Repetitively Pulsed Discharges
in Water Vapor
F. P. Sainct1, 2, D. A. Lacoste1, 2, M. J. Kirkpatrick3, E.
Odic3, and C.O. Laux1, 2 1CNRS UPR 288, Laboratoire E.M2.C.,
France
2 Ecole Centrale Paris, France 3SUPELEC – E3S Department of
Power and Energy Systems, France
Abstract—Experimental study of a Nanosecond Repetitively Pulsed
(NRP) discharge in atmospheric pressure water vapor at 450K is
reported. The discharge is produced between two pin electrodes by
application of voltage pulses (0-15 kV amplitude), 10 ns in
duration, with a repetition frequency up to 30 kHz. Electrical
measurements were done to determine the energy deposited in the
discharge. The energy per pulse ranges from 1 to 10 mJ. In order to
determine the efficiency of water vapor dissociation, the
concentration of the final reaction products (H2 and O2) and their
respective flow rates were measured.
Keywords—NRP discharge, atmospheric pressure, water vapor
dissociation
Corresponding author: Florent P. Sainct e-mail address:
[email protected] Presented at the 8th International Symposium
on Non-Thermal/Thermal Plasma Pollution Control Technology &
Sustainable Energy, in June 2012
Fig. 1. Energy deposited by electrons in pure water vapor at 1
atm and 400K, using Bolsig+ [7] with Morgan’s database [8].
125Sainct et al.
-
The objective of this work is thus to evaluate the energy
efficiency of water dissociation using the NRP discharge in a
100-1000 Td reduced field range in pure water vapor at atmospheric
pressure.
II. EXPERIMENTAL SETUP A. Gas Flow Setup
The experimental setup (Fig. 2) consisted of a water
vapor generator without carrier gas, a reactor where the plasma
discharge was generated, and a cold-trap that re-condensed the
water vapor. The exhaust gases went through a mass flow meter and
reached the gas chromatograph (GC) injection valve, thus allowing
online gas analysis.
The vapor generator (Bronkhorst) was composed of 3 modules: a
regulation system, a heater and a closed loop temperature control.
The input of liquid water was regulated at a flowrate of 200 g/h in
this experiment. A heater vaporized the water to produce a flow of
pure water, without carrier gas. The water tank was pressurized
with helium at 4 bar.
The flow of pure water vapor was transferred to the reactor
through a heated line. The line temperature was set at the vapor
temperature, i.e. 450K in this experiment. The reactor was made of
an electrically grounded steel tube (4 cm inner diameter) heated to
450K. The total volume of the reactor was 120 cm3. The discharge
was generated in this tube, between two pin electrodes (insulated
from the steel tube) placed perpendicular to the gas flow. The gas
flow velocity at 200 g/h was 0.1 m/s in the discharge zone. Two
quartz windows mounted on the reactor walls allowed optical access
to the discharge zone.
The reactor outlet line was connected to a cold trap at 273K
that condensed the water vapor. A mass flow meter (full scale 100
mL/min) connected at the cold trap outlet measured the total flow
rate of all the gases that do not condense.
The chemical species concentrations were then measured using an
Agilent 6890A gas chromatograph with a Restek 100/120 Shincarbon ST
2 m × 1 mm micro-packed column and a thermal conductivity detector.
Argon was used as carrier gas. B. Plasma Production Setup The NRP
discharge setup and the electrical measurement system are sketched
in Fig. 3. NRP discharges were generated between two tungsten pin
electrodes (2 mm diameter), in a pin-to-pin configuration,
separated by an adjustable gap distance ranging from 1 to 10 mm. A
pin was grounded while the other one was stressed by high voltage
pulses of 10 ns duration, 1-30 kHz pulse repetition frequencies
(PRF) and 0-15 kV amplitude. High voltage pulses were obtained
using a solid state pulse generator (FID Technologies FPG 10-30MS).
The NRP discharges were produced in pure water vapor at atmospheric
pressure.
The voltage applied across the electrodes gap was measured with
a 1:1000, 100 MHz bandwidth high-voltage probe (LeCroy PPE20kV).
The current was measured with a Pearson coil current monitor (Model
6585) connected to a low-voltage attenuator (Barth Model 2-20).
Both signals were recorded simultaneously with a 1 GHz LeCroy
Wavepro7100 oscilloscope. In Fig. 4 are presented typical voltage
and current waveforms. The energy deposited in the discharge was
deduced from the instantaneous product of the two waveforms. The
delay between the two signals was compensated to take into account
the time response of
Fig. 2. Schematic diagram of the gas flow setup.
Fig. 3. Schematic diagram of the electrical setup.
Fig. 4. Voltage and current typical waveforms.
126 International Journal of Plasma Environmental Science &
Technology, Vol.6, No.2, SEPTEMBER 2012
-
the probes and the cables length. After the voltage pulse,
voltages oscillations occur because of non-matched reflections. No
discharges were observed during these oscillations.
III. METHODOLOGY The first step was to identify the various
accessible
regimes for a NRP discharge in pure water vapor. With a fixed
set of parameters, the focus was placed
on the gas characterization. Prior to the experiments, the gas
line connecting the reactor outlet to the GC injection port
(through the cold trap and the mass flow meter) was first purged of
air using argon (carrier gas for the GC). It is important to
mention that during operation, the total amount of water vapor was
re-condensed in the cold trap. The flow rate measured at the cold
trap outlet was then solely due to gas production (analyzed using
GC) initiated by water dissociation (this flowrate is small
compared to the reactor inlet water vapor flow).
IV. RESULTS A. Discharge Regimes
The regimes of operation of the NRP discharge in water vapor
were studied as a function of the pulse repetition frequency,
inter-electrode distance, gas temperature and applied voltage. The
results of the regimes available as a function of the distance and
the applied voltage are presented in Fig. 5. These results were
obtained at a PRF of 30 kHz and with water vapor at a temperature
of 450K and flow of 0.15 m/s. From 2 mm to 8 mm inter-electrode gap
distance, the corona onset voltage remains approximately constant
(∼6-7 kV). For small inter-electrode distances, the corona regime
is unstable and transient transitions to spark regime are observed.
For larger discharge gaps, the spark regime occurs for an electric
field of about 20 kV/cm. A narrow region appears for gaps larger
than 5 mm with another regime. It starts with two coronas, tending
to fill the whole gap. Additional measurements are needed to
confirm that this discharge regime is similar to the glow regime as
defined by Pai [12]. However, the regime here chosen was the spark
regime [13], which is more stable than the glow regime in these
experiments, and leads to a greater energy deposition into the gas.
The discharge gap was fixed at 5 mm. A preliminary study of the
discharge was done by Optical Emission Spectroscopy (OES) in the
UV-Visible range. Fig. 6 shows the emission intensity from 250 to
850 nm in the center of the discharge. The signal is integrated
over 5 ms, which correspond to 50 pulses at 10 kHz pulse repetition
frequency. Strong emission from atomic hydrogen is measured: the Hα
intensity is 60000 and Hβ is 10000. Atomic oxygen and OH also have
strong emission, which suggests that a time and/or space resolved
study would be interesting in the near future.
B. Gas Chromatography and Flow analysis
Without discharge, the gases detected were N2, O2 and He. These
gases are water dissolved gases. Even if partially degassed prior
to experiments, liquid water still contained air traces. Helium
also partially dissolved into the liquid water in the pressurized
tank (4 bar of He). This total gas flow remained constant at ∼3
mL/min, which corresponds to the flowrate created by the condensed
water in the cold-trap, for a 200 g/h liquid water flowrate (3.3
mL/min).
When the discharge was turned on, both the gas composition and
gas flowrate evolved. The gas composition reached a new equilibrium
slowly, due to the large volume to fill/purge (reactor volume and
condenser volume: 2 L) and low flowrate. In order to determine the
concentration of O2 actually produced by water dissociation, the O2
concentration resulting from dissolved air (previously measured
with the discharge off) was subtracted from the total measured
concentration. The dissolved air flow was also subtracted from
total measured flowrate. However, it is interesting to note that no
emission of N2 (C-B) at 337 nm is visible
Fig. 5. Regimes available (30 kHz voltage pulses repetition
frequency, T = 450K, Qwater vapor = 300 g/h).
Fig. 6. Optical Emission Spectroscopy in the spark regime (20
W).
127Sainct et al.
-
under these conditions (Fig. 6). It is also important to note
that the gas composition was taken into account in order to correct
the flow outputs (Table I) for each gaseous species. Table I shows
that only minor corrections were required, as all the major gases
measured have a correction factor close to 1.
Results are presented in Fig. 7, where it can be verified that
the 2/3 H2 and 1/3 O2 gas composition is in agreement with the
theoretical ratio and confirms pure dissociation of water. It
should also be mentioned that using a colorimetric method, H2O2 was
not detected in the condensed water, probably because of the
relatively high temperature of the condensed water (~ 60°C). In
these conditions, assuming that the hydrogen flow (3.1 mL/min)
corresponds to the water flow dissociated, it can be estimated that
0.04% of the inlet water vapor was dissociated. This result could
be expected since the discharge (5 mm length × approx. 1 mm
diameter) represents only 0.4% of the reactor cross-section. The
fraction of dissociated water is not a relevant performance
criterion in these conditions and so the hydrogen formation energy
cost will be used for further analysis. C. Energy cost
considerations
After stabilization of the water dissociation products
formation, the energy deposition was progressively raised to 47,
64, and 81 W (i.e. from 110 to 440 J/L). For
each operating condition, a steady state was reached,
corresponding to an increase of the total flowrate (for which GC
measurements were performed), and the hydrogen production was then
measured as a function of the discharge input power. Fig. 8 shows
the dependence of the H2 production with input power. From these
results, the energy efficiency could be estimated (Fig. 9). A
maximal value of 0.85 g-H2/kWh was obtained.
The usual values of energy efficiency for high purity hydrogen
using electrolysis are in the range 12-20 g-H2/kWh; 20 g-H2/kWh
corresponds to 80% of the maximum thermodynamic efficiency. The
observed energy efficiency is then low compared to these reference
values. Furthermore, the slope of the curve in Fig. 8 indicates
that there is more energy loss in other processes than H2 formation
when the energy deposited increases.
Assuming a gas temperature between 450 and 2000K in the
discharge column, and assuming E = V/d (d: inter-electrode gap
distance), which was shown to be valid for the spark regime [13],
the calculated E/N values range from 200 to 850 Td in these
conditions. Thus the spark discharge regime does not appear to be
the most suitable regime for effective water dissociation.
TABLE I CONVERSION FACTOR FOR BRONKHORST FLOW METER [14]
Gas Conversion factor Air 1.00 He 1.41 H2 1.01 O2 0.98
Fig. 7. Hydrogen and oxygen flow rates measured at the condenser
outlet. Gas compositions measured by GC-TCD. Total flow rate
measured by mass flow meter (specific flows corrected according
to Table I). 20 W NRP discharge (spark regime).
Fig. 8. Hydrogen production as a function of input power.
Fig. 9. Hydrogen production efficiency as a function of input
power.
128 International Journal of Plasma Environmental Science &
Technology, Vol.6, No.2, SEPTEMBER 2012
-
V. CONCLUSION
These experiments of NRP discharges in pure water vapor at
atmospheric pressure show that hydrogen and oxygen are produced
with a molar ratio corresponding to water dissociation. Among the
various regimes available, we have chosen to focus on the spark
regime (high E/N). By measuring the power consumption and the
hydrogen production using this discharge, we have shown that the
energy efficiency of spark discharges is at least one order of
magnitude lower than that of industrial processes (specifically
electrolysis) and also lower than low pressure microwave discharges
[9]. Rather than decreasing the water vapor pressure, the next step
will be to decrease the reduced electric field. Corona and glow
discharge regimes in water vapor at atmospheric pressure will be
investigated.
REFERENCES [1] G. Pilla, D. Galley, D. A. Lacoste, F. Lacas, D.
Veynante, and C.
O. Laux, "Stabilization of a Turbulent Premixed Flame Using a
Nanosecond Repetitively Pulsed Plasma," IEEE Transactions on Plasma
Science, vol. 34, pp. 2471-2477, 2006.
[2] G. Pilla, "Etude Experimentale de la Stablisation de Flammes
Propane-Air de Premelange par Decharges Nanosecondes
Impulsionnelles Repetitives" (in French), Ecole Centrale Paris,
2008.
[3] S. V. Pancheshnyi, D. A. Lacoste, A. Bourdon, and C. O.
Laux, "Ignition of Propane/Air Mixtures by a Repetitively Pulsed
Nanosecond Discharge," IEEE Transactions on Plasma Science, vol.
34, pp. 2478-2487, Dec. 2006.
[4] D. Z. Pai, "Nanomaterials synthesis at atmospheric pressure
using nanosecond discharges," Journal of Physics D:Applied Physics,
vol. 44, 174024, 2011.
[5] D. F. Opaits, A. V. Likhanskii, G. Neretti, S. Zaidi, M. N.
Shneider, R. B. Miles, and S. O. MacHeret, "Experimental
investigation of dielectric barrier discharge plasma actuators
driven by repetitive high-voltage nanosecond pulses with dc or low
frequency sinusoidal bias," Journal of Applied Physics, vol. 104,
043304, 2008.
[6] J. D. Holladay, J. Hu, D.L. King, and Y. Wang, "An overview
of hydrogen production technologies," Catalysis Today, vol. 139,
pp. 244–260, 2009.
[7] G. Petitpas, J. -D. Rollier, A. Darmon, J. Gonzalez-Aguilar,
R. Metkemeijer, and L. Fulcheri, "Acomparative study of non-thermal
plasma assisted reforming technologies," International Journal of
Hydrogen Energy, vol. 32, pp. 2848-2867, 2007.
[8] J. Luche, O. Aubry, A. Khacef, and J. -M. Cormier, "Syngas
production from methane oxidation using a non-thermal plasma:
Experiments and kinetic modeling," Chemical Engineering Journal,
vol. 149, 35–41, 2009.
[9] A. Fridman, Plasma Chemistery (2008) Cambridge Univ. Press.
[10] G. J. M. Hagelaar and L. C. Pitchford, "Solving the
Boltzmann
equation to obtain electron transport coefficients and rate
coefficients for fluid models," Plasma Sources Science and
Technology, vol. 14, pp. 722-733, 2005.
[11] W. L. Morgan, "Kinema Research & Software" [Online].
Available: http://www.kinema.com/. [Accessed: 01-Dec-2010].
[12] D. Z. Pai, G. D. Stancu, D. A. Lacoste, and C. O. Laux,
"Nanosecond repetitively pulsed discharges in air at atmospheric
pressure---the glow regime," Plasma Sources Science and Technology,
vol. 18, 45030, 2009.
[13] D. Z. Pai, D. A. Lacoste, and C. O. Laux, "Nanosecond
repetitively pulsed discharges in air at atmospheric pressure---the
spark regime," Plasma Sources Science and Technology, vol. 19,
65015, 2010.
[14] "General instructions digital Mass Flow / Pressure
instruments laboratory style / IN-FLOW," in Instruction manual,
2011.
129Sainct et al.