Plasma-catalysis for greenhouse gas conversion into fuels ... Tu-2012.10.17.pdf · Plasma-catalysis...
Transcript of Plasma-catalysis for greenhouse gas conversion into fuels ... Tu-2012.10.17.pdf · Plasma-catalysis...
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Plasma-catalysis for greenhouse gas conversion
into fuels and carbon nanomaterials
Xin Tu
Department of Electrical Engineering and Electronics
University of Liverpool, UK
E-mail: [email protected]
Thermal Plasma Jet Coaxial DBD Gliding Arc Packed Bed DBD
Third Vacuum Symposium UK - Vacuum and Plasma for Industry
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Outline
I. Introduction
II. Plasma-catalytic dry reforming of methane for H2 production
III. Plasma-reduction of a NiO/Al2O3 catalyst in pure CH4
IV. Dry reforming of CH4 using gliding arc discharge
V. Summary and outlook
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Renewable energy – Hydrogen production
Steam reforming CH4 + H2O → CO + 3H2
Partial oxidation CH4 + 0.5O2 → CO + 2H2
Dry reforming CH4 + CO2 → 2CO + 2H2
Plasma-catalysis
Plasma-catalyst interactions, synergistic effect, improved selectivity
Catalyst activation/reduction
Thermal reduction (high temperature, long time)
Plasma reduction (in H2/Ar, H2/N2, low temperature, short time)
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Plasma-catalysis: advantages
Combine a non-thermal, atmospheric pressure, plasma with a catalyst
• To enhance the conversion of the environmental pollutants
• To improve the energy efficiency of the processing
• To vary the selectivity of the processing to minimise unwanted by-products (e.g. organic intermediates or NOx when processing in air, methane reforming)
• To improve the stability of the catalyst (reduce poisoning and coking, reduce operating temperature to enhance thermal stability)
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Plasma-catalysis configurations
One Stage Plasma –
Catalysis
Catalyst in Discharge
Two Stage Plasma –
Catalysis
Catalyst downstream
of Discharge
6/36 Adapted from Chen, Lee, Chen, Chang, Yu and Li, E.S.&T, 43 (2009) 2216.
Plasma-catalyst interactions
Enhance
Energy
Efficiency
Improve
Selectivity
Enhance
Catalytic
Activity
Improve
Catalyst
Durability
Increase electric field
strength
Influence of
catalyst on plasma
Influence of
plasma on catalyst
Adsorption of reactants
Alter catalyst surface
Formation of
radicals and
excited species
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DBD reactor
Both electrodes: Stainless steel mesh
Electrode gap: 1.5/3/4.5 mm
Discharge length: 55 mm
Power supply
Frequency: 30-40 kHz
Voltage (pk-pk): up to 30 kV
Gas
Catalyst reduction in plasma:
20 % H2/Ar (100 ml/min) DBD
Reforming: CH4/CO2 (50 ml/min, 1:1)
Catalyst pellets
Ni/Al2O3 (home made/JM)
Packing methods
3 different packing methods
Experimental setup
Gap (packed with catalyst)
Gas Inlet
Catalyst
50 mm
HV
Gas outlet
Outer Electrode
Inner Electrode
Quartz tubes
Oscilloscope
GC
Catalyst
55 mm
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Front view
Cata
lyst
C
ata
lyst
Section
Different packing methods
A: fully packing
10-18 g catalyst,
pellets (packed
bed effect)
Vc>>Vg
B: partial packing
1 g catalyst, pellets,
held by quartz
wool, Vc<<Vg
C: partial packing
1 g catalyst, flake,
no quartz wool,
Vc<<Vg
Gas Inlet
Gas Outlet
Catalyst
Catalyst pellet +Plasma
Catalyst
Plasma
Void fraction
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Plasma dry reforming reaction without catalyst
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Plasma-catalytic dry reforming reaction: packing method C (partial packing)
the conversion rate of CH4 and CO2 significantly increases when the Ni
catalyst (cal. 300 degree) is packed into the gap (double conversion)
the yield of H2 and C2H2/C2H4 is also doubled.
high calcination temperature leads to improve the selectivity towards C2H6
and C3H8
Tu et al, Appl. Catal. B: Environ. 2012, 125, 439-448
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800 oC 500 oC 300 oC
XRD patterns of the Ni catalyst before reaction
Ni-Al2O3 interactions are affected by
the calcination temperature.
formation of spinel NiAl2O4 at high
calcination temperature (strong Ni-
support interactions), is unfavorable to
the reduction of catalyst (low catalyst
activity).
strong Ni-Al2O3 interactions can
suppress carbon deposition on the
catalyst surface.
Tu et al, Appl. Catal. B: Environ. 2012, 125, 439-448
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Optical emission spectroscopy of the discharges
Calculation of rotational temperature of CH
Plasma gas temperature: 230-270 oC (discharge power 50 W)
Species: CO, CO2+, CO2, N2, N2
+, CH, C2, H, OH, e
Tu et al, Appl. Catal. B: Environ. 2012, 125, 439-448
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Synergistic effect of
plasma-catalysis
Thermodynamic equilibrium conversion rates of CH4 and CO2 as a function
of temperature with CH4/CO2 molar ratio of 1 at 1atm (without plasma)
At 300 oC, very low conversion rate
The synergistic effect of the combination of plasma with catalysis at constant
discharge power and low temperatures (without extra heating) for the reforming
reaction depends on the balance between the change in discharge behavior
induced by the catalyst and the plasma generated activity of the catalyst.
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Energy efficiency for conversion
Energy efficiency decreases with the discharge power and increases with feed
flow rate
The synergistic effect of plasma-catalysis enhances the energy efficiency of
the plasma dry reforming reaction
Tu et al, Appl. Catal. B: Environ. 2012, 125, 439-448
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NiO → Ni, increased conductivity → increased power → increased temperature
The decrease in carbon balance corresponds to the loss of carbon from the gas stream
as a result of solid carbon deposition.
CH4 → C + 2 H2
Power/temperature/H2/Carbon balance during the plasma reduction process
Gallon et al, Appl. Catal. B: Environ. 2011, 106, 616
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Cotton-wool
like structure
20-80 nm
Mean: 55 nm
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Gliding arc discharge
Arc ignition Arc development Arc extinction
Gliding arc plasma reactor
Electrodes: Al (semi-ellipsoidal )
Electrode gap: 3.2 mm
Power supply
AC 220/10 kV, 50 Hz
Applied voltage: 7 to 10 kV
Working gas
CH4/CO2 2.5 – 7.5 L/min
molar ratio: 3:7, 1:1, 7:3 High speed camera (5000 fps)
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Optical emission spectroscopy
Electron density (calculated from Stark broadening of Al I)
(2.2 – 3.2) × 1023 m-3 along the jet axis
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Effect of different operating parameters
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Carbon nanomaterials
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Using plasma-catalysis, we have shown
• enhanced conversion and yield for end-product
formation
• improved energy consumption
• reduced operating temperature
• plasma preparation/modification of the catalyst
• preliminary understanding of the mechanism of
plasma-activated catalysis
Conclusions
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Where do we go from here?
• In situ identification of active species on surface (e.g. by FTIR-ATR) with simultaneous gas-phase profiles
• Adsorption /desorption versus catalysis
• Development of catalysts specifically for plasma systems
• Synthesis of liquid fuels such as methanol from CO2 using plasma-catalysis
• Plasma preparation and treatment of catalysts
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Acknowledgement
• Prof. J. C. Whitehead
•Dr. H. J. Gallon
• Dr. M.V. Twigg (Johnson Matthey Plc. UK)
• Support of this work by SUPERGEN XIV - Delivery of Sustainable
Hydrogen, the Joule Centre and the UK EPSRC Engineering Instrument
Pool is greatly appreciated. The Energy Programme is an RCUK cross-
council initiative led by EPSRC and contributed by ESRC, NERC, BBSRC
and STFC.
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