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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: xin.tu@liv.ac.uk

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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