Combustion enhancement through
electronically excited oxygen
K. Zähringer1, A. Bourig1,2,3, J.-P. Martin2, D. Thévenin1
LSS, Universität Magdeburg, Germany,
2 ICARE, CNRS, Orléans, France,
now at EDF, Centre des Renardières, France.
1
3
Outline
1. Introduction and context
2. Theoretical aspects
3. Experimental Set-up
4. Experimental results
4.1. Excited oxygen production (Post-discharge study)
4.2. Plasma-flame experiments (Co-flow partially premixed flame)
5. Conclusions and perspectives
Introduction (1/3)
Several plasma-assisted combustion technologies exist
One promising approach consists in plasma-enhanced activation of the
oxidizing substance
→ converting molecular oxygen to its electronically-excited singlet
delta O2(a1Δg) and singlet sigma O2(b1Σg+) states.
In contrast with non-excited reactants, singlet O2 molecules are more
chemically active and can affect reaction kinetics.
Using non-thermal electrical discharges to support combustion processes
should result in:
¾ Faster ignition (i.e. reduction of ignition delay or flame ignition),
¾ Better flame stabilization (lean and ultra-lean combustion,
hypersonic propulsion concepts),
¾ Extension of flammability limits.
These are key technical issues for combustion improvement.
Introduction (2/3)
The reason of these modifications:
intensification of chain reactions in the mixture owing to the presence of
electronically excited oxygen molecules in the flow.
E.g. for H2-combustion, the principal source of OH, O and H are the following
ramification reactions:
H2 + O → OH + H
(1)
H + O2 → OH + O
(2)
When considering O2 excitation, new pathways appear, in particular:
H + O2* → OH + O
H2 + O2* → OH + OH
O(1D) + H2 → OH + H
(3)
(4)
(5)
This leads to a significant increase of highly reactive atoms and radicals,
thus intensifying the reactions.
Introduction(3/3)
Additionally, these reactions, involving excited oxygen, have higher
preexponential factors and lower activation energies, e.g. :
A
n
E
H2 + OH → H2O + H
H2 + OH* → H2O + H
2.53E+8
1.0E+14
1.48
0
1700
276
H + O2 → OH + O
H + O2* → OH + O
8.65E+14
1.1E+14
-0.24 8200
0
3188
Chain branching
H2 + O → OH + H
H2 + O* → OH + H
5.1E+4
8.7E+13
2.67 3160
0
-14
Chain branching
Chain propagation
Reaction rate data has been collected from various litterature
sources, leading to a mechanism including 15 species and 142
elementary reactions (from Moscow State University)
• Species: O2, H2, H, O, H2O2, H2O, OH, HO2, O3, Ar, He, N2
• Excited species: O2(a1Δg), O(1D), OH (A2∑+)
Excited oxgen production
¾ Modelling shows important result :
For O2: Optimal singlet oxygen excitation: 10 Td (1 Td = 10-17V.cm2)
¾ Problem
This reduced electric field E/n value is much lower than those
achieved in self-sustained discharges! /
¾ Solution
A double discharge plasma chemical reactor has been developed
1. short, high voltage Dielectric-Barrier-Discharge (DBD) pulses
produce ionization
2. in-between pulses : comparatively low electric field supports the
electric current between ionizing pulses
⇒ This allows adjusting the electric field close to the optimum singlet
oxygen excitation
Pulse Dielectric-Barrier-Discharge(DBD)
and crossed-discharge (DC)
3
Stainless steel DC electrode
.5
R0
Flow
direction
R
1 .5
48
6 .5
50 mm
48 mm
60 mm
HVPG
• high electric field concentrations
38 mm
4 – 10 mm
64 mm
→ Rounded at the edges to prevent :
• “hot spot” formation in the plasma
Crossed discharge plasma reactor
DC electrodes
Pulsed
barrier electrodes
DBD electrodes
Optical access
CaF2 windows
DC electrodes
Flow direction
Flow direction
Combined Discharge Reactor
O2 + He : 10% - 90%
CDR - crossed discharge reactor
P = 120 Torr
HVPG – high voltage pulse generator
HV DCPS – high voltage direct current power supply
MFC – mass flow controller
Electrical characteristics of the discharge
High Voltage Pulse Generator
Chemical Physics Technology Inc.
Crossed discharge load is connected to HVPG
Over shoot
0
FWHM = 25 ns
-10000
1.2
Ionisation = pulse
DC current (A)
Voltage (Volts)
10000
DC power supply
Glassman Inc.
0.8
Sustain
0.4
-20000
-400
-200
0
200
Time (ns)
400
600
Typical single-pulse oscillogram
Pulse peak voltage -17 kV
10 % O2 – helium flow at P = 120 Torr
0
-4E-005 -2E-005
0
2E-005
4E-005
Time (s)
6E-005
8E-005
DC current temporal profile at 20 kHz
10 % O2 – helium flow at P = 120 Torr
1- High E/n during pulses Ä efficient ionization and dissociation of O2 by e-impact.
Short pulse duration and low duty cycle greatly improve plasma stability
2- DC voltage only moves electrons and does not ionize.
Optical diagnostics
Optical Emission Spectroscopy
Direct visualization by ICCD camera
+ optical interference filters
→ Excited species are visualized :
• O2* molecule :
CW = 762 nm/Δλ=20 nm + 775 nm SP
• OH* radical :
CW = 307 nm/Δλ=10 nm
• CH* radical :
CW = 431 nm/Δλ=10 nm
• C2* radical :
CW = 515 nm/Δλ=10 nm
- Species identification;
- Relative intensities;
- Plasma temperature measurements by comparisons between experimental
and simulated emission spectra.
⇒ For both plasma and flame
Singlet sigma oxygen production
The production of singlet sigma oxygen in the electrical discharge is
confirmed experimentally by spectroscopy :
reveals a well-defined peak of excited oxygen O2 (b1∑g+) at 762 nm
Intensity (a.u)
Experimental conditions:
P = 0.2 bar
• He + 6% O2 (+ NO = 500 ppm)
• Pulse repetition rate of 20 kHz.
• UDC = 1.3 kV
Optical
access
Hybrid burner
exit
Image of the discharge
O2(b1Σg+, v’=0) → O2(X3Σg-, v=0)
P - branch
R - branch
840
Zero
gap
820
800
780
760
762
764
Wavelength (nm)
766
Plasma burner (1/2)
Pictures of the flame: front view (left) and side view (right)
Barrier discharge
electrodes
CH4 + O2 + He
O2 + He
DC electrode
partially premixed flame,
stabilized at 0.2 bar and Φ = 1
Plasma burner (2/2)
Water inlet
Water outlet
Low pressure
combustion chamber
Cooling system
Thermocouple
Low pressure ignition
system
Pumping station
Premixed gases
CH4, O2, He, NO
Mass flow
controllers
DC sustainer voltage effects
He 0.65 m3/h + O2 0.04 m3/h flow + 500 ppm NO
f = 20 kHz
50
0.9
O2 (b1Σg+) (0-0)
40
0.8
R-branch
P-branch
IDC
30
O2 (b1Σg+) (0-0)
R-branch
P-branch
0.7
IDC
O2 (b1Σg+) (0-0) intensity (a.u)
O2 concentration and distance effects
20
0.6
10
0.5
0
0.4
0
10
20
30
40
% O2 in the mixture
50
• From 0.25 to 10% O2: decrease of the emission intensity of O2(b1Σg+) by a factor 10
• Reduction in the DC sustainer current by a factor 1.4
•Characteristic “bell” form for O2(b1Σg+) intensity distribution above the burner.
• Both O2(b1Σg+) and O (777 nm) species follow the same trend
0.8
25
0.7
20
O2 (b1Σg+) (0-0)
0.6
15
0.5
10
0.4
5
I_DC (A)
O 2 (b 1 Σ g + ) (0-0) intensity (a.u)
Pressure effects
R-branch
P-branch
IDC
0.3
0
0.2
0
200
400
600
Total pressure (Torr)
800
• Collisional de-excitation is an important removal channel for our excited species
• Pressure ↑ ⇒ density of neutral species ↑ and the mean free path ↓ and E/n ↓
• From 75 to 350 Torr : maximum DC current is reduced by a factor 2.5
Plasma-burner experiments
No (He/O2/NO)
flow out of the
secondary slot
With (He/O2/NO)
flow out of the
secondary slot
PLASMA
OFF
•
•
With (He/O2/NO)
flow out of the
secondary slot
Extra
flame
PLASMA
ON
Extension of the flame due to the presence of exited molecules
produced by the plasma discharge.
The pink signal is emission of excited oxygen species :
conical 2D distribution
⇒ In the absence of the pilot flame, no auto-ignition
when activating by the CDR !
Spontaneous emission images (1/2)
O2* (b1∑g+ - X3∑g-) emission
Experimental conditions:
• P= 0.2 bar and Φ = 1
OH* A2∑+ - X2Π emission
• Discharge slot :
He 0.65 + O2 0.04 m3/h
NO: 250 ppm
• DBD = 25 kHz
and UDC = 1.2 kV
CH* (A2Δ - X2Π) emission
PLASMA OFF
PLASMA ON
Spontaneous emission images (2/2)
OH*
O2* radical
radical
0.25 He + 0.02 O2 (m3/h)
Effect of secondary flow rate
0.4 He + 0.02 O2 (m3/h)
0.65 He + 0.04 O2 (m3/h)
Stronger effect on the flame:
Excited oxygen species reach more efficiently the reaction zone
O
OH*
radical
2* radical
UDC 1.2 kV
Effect of DC sustainer voltage
UDC 1.25 kV
UDC 1.4 kV
The quantity of O2* generated by the CDR is increased when increasing UDC
Plasma effects
Î The excited oxygen states have long enough lifetimes to be able to
produce direct chemical effects on the flame
Î A non-emitting zone between the post-discharge area and the
flame reaction zone exists
• This observation corroborates the fact that excited oxygen
concentration is locally reduced (increasing the flame reactivity)
Î Two major effects are visible from these plasma assisted
combustion experiments:
ƒ the oxidizer flow temperature increase is not significant !
ƒ the chemical effect of the reactive species created by the plasma
dominates the process.
Conclusions
Î Gas phase generation of singlet oxygen has been performed by a pulsed
dielectric barrier discharge excitation in conjunction with a DC discharge.
• Generation at low (100 Torr) up to atmospheric pressure !
• No significant increase of temperature ( T ~ 370 K).
Î Hybrid plasma burner : investigation of a partially premixed flame.
• Possibility to integrate the CDR in the burner !
• Possibility to transfer O2* to the flame reaction zone !
ÎComparison of spontaneous emission of flame front markers
taken without and with the discharge
Ø Flame structures are modified with the discharge :
particularly evident for OH*.
Ø Spatial extension of flame reaction zone with plasma ON.
Ø Radical species intensity increases with plasma ON
Î Optimal conditions for O2* production = higher flame effects
Perspectives
Laser-Induced Fluorescence and Raman-scattering could be
employed
Î This will allow to quantify the observed structural modifications
Experiments with hydrogen combustion
Development of a CH4 oxidation kinetic mechanism including
excited oxygen species allowing accurate experimental
validation
Non-self-sustained microwave discharge:
Replace DC excitation by a more efficient µ-wave discharge
excitation
Should allow O2 excitation at higher (at least up to atmospheric)
pressures
Acknowledgments
This project has been financed by :
Land Sachsen-Anhalt (Germany)
Région Centre (France)
French-German University
INTAS project
Many thanks also to:
INTAS project members (INTAS Ref. No: 03-51-4736)
Heat and Mass Transfer Institute, Minsk, Belarus
F. Pliavaka, S. Gorbatov, K. Pliavaka
Danke für lhre Aufmerksamkeit !