PRODUCTION OF SYNTHESIS GAS THROUGH PLASMA-ASSISTED REFORMING OF GREENHOUSE GASES

Greenhouse Gas Control TechnologiesP. Riemer

, B. Eliasson and A. Wokaun, editors© 1999 Elsevier Science Ltd All rights reserved 385

PRODUCTION OF SYNTHESIS GAS

THROUGH PLASMA-ASSISTED REFORMING OF GREENHOUSE GASES

U. Kogclschatz, L. M. Zhou , B. Xue and B. Eliasson

ABB Corporate Research Ltd, 5405 Baden, Switzerland

*On leave from Xi,an Jiaotong University, Xi,an, P.R. China

ABSTRACT

Low temperature conversion of the two major greenhouse gases C02 and CH4 to synthesis gas (a mixture of H2 and CO)is investigated theoretically and experimentally in a high power dielectric-barrier discharge (DBD). Utilising thisnonequilibrium discharge technique high conversion rates can be achieved in this special gas mixture. A pronouncedsynergetic effect caused by free radical reactions was observed using these two gases simultaneously. This way C02separated from flue gases could be combined with methane to produce syngas which then can be processed to yieldliquid fuels like e.g. methanol or dimethyl ether. Parameters studied are CH4/CO2 mixing ratio (0 - 100% of C02),electric power (100 - 800 W), flow rate (0.1 - 4 Nl/min), gas pressure (0.35 - 2 bar) and reactor wall temperature (80°C -

250°C). This technique of plasma reforming of methane with carbon dioxide can produce syngas with different H2/COratios depending mainly on the C1VC02 mixing ratio. The amount of syngas produced rises almost linearly withincreasing discharge power. Up to 66 moles of syngas with a H2/CO ratio of 3.7 were obtained from 100 moles of feedgas in a single pass through the DBD reactor of 31 cm active length. The minimum required specific energy was40 eV/molecule for the production of syngas (II2 plus CO) and the highest energy efficiency (electric energy convertedto chemical energy in the syngas) reached so far was about 7 %.

INTRODUCTION

Continued excessive anthropogenic emissions of the greenhouse gases carbon dioxide and methane to the atmospheremay lead to an increase of the average global temperature. Therefore disposal or reuse of these gases has become amajor issue. The hydrogenation of C02 to methanol has been investigated with copper-based catalysts at elevatedpressure and temperature or, in combination with electrical discharges, at less demanding operating conditions. At about220°C and moderate pressures of less than 30 bar, a methanol yield of 5 - 10% per single pass was obtained in acatalytic packed-bed reactor [1,2]. This methanol yield is already close to the equilibrium value. A methanol selectivityof about 40% was reached. When a catalyst was inserted into a dielectric-barrier discharge (DBD) gap the simultaneouspresence of the discharge shifted the temperature region of maximum catalyst activity from 220°C to 100°C [3, 4]. Inthis temperature range, the theoretical equilibrium yield of methanol is 34% at 8 bar, roughly ten times higher than thatat 220°C

. So, in principle, there is more room to increase methanol yield at this low temperature. Obviously,hydrogenation of C02 will only be attractive if the required hydrogen is produced in a cheap way and without causingadditional C02 emissions [5]. The partial oxidation of methane to methanol with oxygen or air was investigatedexperimentally and theoretically in DBDs [6, 7]. The highest methanol yield of 3% and the highest methanol selectivityof about 30% were achieved in CHi/02 mixtures. In CHVair mixtures, up to 2% methanol yield was obtained. Thepartial oxidation of methane is always accompanied by the undesired by-products COx and H20, which weretheoretically shown to be the main substances limiting methanol yield. Based on these experimental results our effortsare now directed to plasma reforming of C02 and CH4 to synthesis gas, an important intermediate for chemicalsynthesis. Syngas can then be turned into methanol and other valuable oxygenated components in a second process step.

Today steam reforming of methane (CH4 + H20 -» CO + 3H2, 229.7 kJ/mol) is the major industrial process forsyngas production. The disadvantages are high cost, about 60 - 70% of the total investment cost of the methanol plant,and a resulting H2/CO molar ratio > 3 which is too high for methanol synthesis and for many other synthesis processes.Carbon dioxide reforming of methane, or dry reforming of methane to syngas is considered an attractive reaction witheconomic and environmental benefits [8]. This highly endothermic reaction (C02 + CH4 -> 2CO + 2 H2, 247 kJ/mol)can be utilised to transfer and store energy (e.g. solar energy or electricity) in the form of CO and H2. Although theconversion of C02 and CH4 to syngas has potential applications in industry it has not yet been practicallycommercialised. A major problem is the deactivation of the catalyst due to carbon deposition.

Recently, the utilisation of plasma techniques for syngas or hydrogen production has attracted considerableattention. With short high voltage pulses DBD processing of C02/CH4 mixtures led to the formation of syngas [9]. In aspecial GlidArc discharge [10] syngas production from C02/CH4 mixtures was reported with high efficiencies at

Present address: Chemical Engineering Department, The University of Oklahoma, Norman. OK 73019, U. S. A.

386

atmospheric pressure. Also plasma reforming of methane to hydrogen in thermal plasmas [ 111 has been investigatedrecently.

In this study, we used a high power dielectric-barrier discharge to producc synthesis gas with pre-selectedarbitrary H2/CO ratios from the two major greenhouse gases C02 and CII4. The discharge is used to establish non-equilibrium plasma conditions in which the major fraction of the electrical energy is transferred to electrons, typically inthe energy range of 1 - 10 eV. In dielectric-barrier discharges at about atmospheric pressure the discharge consists of alarge number of short-lived microdischarges. The plasma conditions in these microdischarges resemble those oftransient high pressure glow discharges and can be optimised for specific plasmachemical reactions [12]. This has, forexample, been demonstrated in industrial ozone production in which large-scale DBD configurations are used [13, 14].The electrons in the microdischarges are heated to high temperatures while the actual gas temperature remains nearambient. Through electron-impact ionisation, dissociation and excitation of the source gas active radicals, ionic andexcited atomic and molecular species are generated which in turn initiate the plasmachemical reactions. As a result,considerable amounts of H2 and CO are produced from CII4 and C02 even at ambient temperature and low pressure.

EXPERIMENTAL SET-UP

The experimental set-up was already described in detail [4, 7]. The discharge is maintained in an annular discharge gapof 1 mm radial width and 310 mm length, formed by an outer steel cylinder and an inserted cylindrical quartz tube. Theouter steel cylinder serves as the ground electrode and an alternating sinusoidal high voltage of up to 20 kVpp amplitudeand about 30 kHz frequency is applied to the centre electrode which is connected to a metal brush pressing a metal foilagainst the inner surface of the quartz tube.

uartz Tube

Discharge Gap

GroundElectrode

Insulation

Temperature ControlBP

High Voltage Electrode

Fig. 1: Diagram of dielectric-barrier discharge reactor showing the high-voltage feedthrough to the centre electrode onthe left side and a cross section on the right side.

The power supply (Arcotec corona generator CG 20) can feed between 50 W and 1000 W into the discharge reactor.CH4 and C02 are introduced into the reactor from high pressure bottles via mass flow controllers, admitting a total gasflow of 0.1 to 4.0 Nl/min. A back pressure valve and a vacuum pump at the exit are used to adjust the gas pressure in thereactor between 0.1 bar and 10 bar. The pre-selected temperature of the steel ground electrode can be maintained by a

closed loop of re-circulating oil from a thermostat between room temperature and 400°C

. A MTI (Microsensor

Technology Inc., M200H) dual-module micro gas chromatograph with a TCD (thermal conductivity detector) is used toanalyse most of the gaseous products. A second gas chromatograph (HP 5890A) is connected to detect Hj by a TCD and

to monitor high hydrocarbon products on-line by a FID (flame ionisation detector). As a reference gas a controlled flowof nitrogen is added to the product stream at the exit of the reactor to detect changes of the volume flow due to chemical

reactions in the discharge. This way the mass balances for H, C and O could be established.

387

EXPERIMENTAL RESULTS

Experiments were performed with varying mixing ratio CH4/C02 in the feed at a pressure of 1 bar. A specific electricenergy (electric discharge power in [kW] divided by the flow rate measured in [Nm3/h]) of 16.7 kWh/m3 was used. Thesyngas production from 100 moles feed gas is shown together with the obtained H2/CO ratio as a function of C02content in feed (Fig 2). The syngas amount reaches a pronounced maximum in gas mixtures containing between 70%and 80% C02. The molar ratio of H2/CO strongly depends on the feed gas composition, varying from about 3.5 toalmost zero.

The required specific energy for the production of syngas was calculated to determine the used electric energyper syngas molecule (H2 plus CO). The lowest energy requirement of slightly less than 40 eV/molecule of syngas wasobtained close to the maximum at a C02 content in the range of 70 - 80%. In the product gas, the concentrations of allhigher hydrocarbon products decrease with increasing C02 content in feed. In contrast, water production increases withrising C02. CO formation increases with the C02 content in the feed up to 80% and then drops sharply. 02 formation ishardly detected in mixtures containing up to 80% C02. At these operating conditions higher.hydrocarbons (C3 - C6)amount totally to less than 1% in the product gas.

20 40 60

CO, content In teed (°/<J

80 10010 15 20 25 30 35

Specific Input energy (kWh/Nm>)

40 45

Fig. 2: Syngas production from 100 moles feed versus C02 Fig. 3: Syngas production from 100 moles feed andcontent (Power: 500 W; Flow rate: 0.5 Nl/min; obtained H2/CO ratio versus specific input energyWall temperature: 80°C; Pressure: 1 bar) (Flow rate: 0.2 Nl/min; Wall temperature: 80°C;

Pressure: 1 bar; CHVC02 = 30/70)

In a mixture of CHVC02 = 30/70, a discharge power from 0.1 to 0.5 kW was applied, resulting in the specific inputenergies in the range of 8.3 - 41.7 kWh/Nm

, at a very low total gas flow of 0.2 Nl/min. In general, increasing thespecific input energy leads to an increase in the conversion of CH4 and C02 and the production of syngas. The syngasamount increases almost linearly with the specific input energy (Fig. 3). The molar ratio of H2/CO around 0.5 is hardlydependent on the specific input energy in this parameter range. Other gas mixing ratios yielded other H2/CO ratios,again independent of applied power or flow rate. This way any desired II2/CO ratio between about 3.5 and practicallyzero can be pre-selected. Very high conversion rates were obtained for high specific input energies. At 87 kWh/Nm3

, 52

moles of H2 and 14 moles of CO were obtained from 100 moles feed CH4/C02 = 80/20. In this case CH4 conversion

reached 64% and C02 conversion reached 54%. Under ihese extreme conditions the H2/CO molar ratio eventuallyincreases slightly with specific input energy and carbon deposition and wax formation are observed.

The gas pressure is normally regarded as an important operating parameter for thermal chemical reactions andelectrical discharges. The influence of operating pressure on syngas production is shown in Fig. 4. Low pressure clearlyfavours the conversion of CH4 and C02 to syngas. The H2/CO ratio is practically not affected by changes of theoperating pressure. The variation of operating pressure also leads to a change in the gas residence time. Therefore, this

effect was examined in a separate experiment by varying of total flow rate at a fixed pressure. The results demonstrated

that the variation of the residence time had no obvious influence on syngas production. It therefore appears that theenhancement of the conversion of CH4 and C02 to syngas at lower gas pressures is due to more favourable dischargeconditions.

The temperature is the most important parameter in thermal reactions. The equilibrium calculations demonstratethat normal chemical reactions between CII4 and C02 cannot be expected at temperatures lower than 250°C (Fig. 5). In

endothermal reactions normally high temperatures are required to add enthalpy. In our DBD reactor, the conversion ofCH4 and C02 to syngas is a non-equilibrium plasma process. Radical reactions in the discharge play the major role in thesyngas production. Therefore, in this case syngas production proceeds already at low temperatures in the range of 80°C.Again, the H2/CO molar ratio hardly depends on the temperature. This is true only in C02-rich feed gas mixtures.

388

Experiments conducted in CH4-rich feed were hardly reproducible at high specific input energies and high operatingtemperatures. Wax and carbon production strongly interfered with discharge stability.

80

1.0 1.5

Pressure (bar)

60-

40-

20-

/Equilibrium Calculation:

/ H,»CO

H,*CO .- /

Simulation J_ _

HVCO

Hyco

2.0

. 1.5

1.0

0.5

I

0.0

200 400 600

Temperature ("C)

800 1000

Fig. 4: Syngas production from 100 moles feed gas versusoperating pressure(Power: 200 W; Flow rate: 0.2 Nl/min;Wall temperature: 80°C; CH4/C02 = 20/80)

Fig. 5: Comparison of theoretical and experimental data(Power: 500 W; flow rate: 0.5 Nl/min; Pressure: 1bar; CH CO;, = 20/80)

NUMERICAL SIMULATION

The equilibrium calculations are based on the principle of Gibbs free energy minimisation. Five molecules C02, CH4,H2, CO and H20 are involved in the reactions in which C02 and CH4 arc chosen as the basic components and the rest asproducts. Equilibrium curves of H2+CO and H2/CO are also plotted in Fig. 5. These computations demonstrate thatsyngas production by ordinary chemical CH4/C02 reactions starts at temperatures above 250°C and that up to 80 molesof syngas can be obtained from 100 moles of feed gas if the temperature is above 700°C

.

A simplified microdischarge model including 60 species and 353 reactions has been used to simulate the processof syngas production in the discharge. The microdischarges in DBDs are sources of electrons with mean energies highenough to dissociate C02 and CH4 molecules.

e + CH4 -> CH3 + H + e

e + C02 -» CO + O + e

(1)

(2)

In a next step the generated CO molecules and H2 molecules formed by recombination of H atoms can also bedissociated by electron impact.

e + CO

e + H,

C + O + e

2 H + e

(3)

(4)

By integrating the differential equations for the reactions (1) - (4) we get the number density of atoms generated in eachmicrodischarge:

[CHj] = e [CH4| (5)

[H] = e [CH4I + a [H2] (6)

[CO] = 3 [C02] - Y [CO] (7)

[C] = Y [CO] (8)

[O] = P [C02] + y [CO] (9)

389

The coefficients e, (3, y and a, corresponding to reactions (1) to (3), and to reaction (4), respectively, are functions of theelectron density generated by one microdischarge and the field dependent rates of the dissociation reactions. In thissimulation the microdischarge are simulated as a series of bursts of dissociation products [15, 16]. Fair agreementbetween experiment and simulation (Fig. 5) was achieved by using following values in the mixture of C02/CHi = 80/20:a = 4.1 x 10

"4, P = 1.3 x 10*", y = 1-4 x 10"< and e = 0.9 x 10"1. Both experiment and simulation show only a small

temperature influence in the range between 50°C and 250°C. This is typical for free radical reactions. The mostimportant radicals for the formation of H2 and CO are H and CHO, respectively. The major reactions leading to H2 andCO formation and destruction in this kinetic scheme and their reaction rates are discussed in more detail in a

forthcoming publication [17].

DISCUSSION

In our DBD reactor, we can produce synthesis gas with an arbitrary, desired H2/CO ratio simply adjusting the mixingratio of the two greenhouse gases C02 and CH4. Surprisingly enough, for a fixed CH4/C02 feed mixture the H2/COmolar ratio is practically independent of gas pressure and temperature in the reactor and the specific input energy up to afairly high level. This advantage over catalytic reactions makes the DBD technique very convenient for producingsyngas with a desired H2/CO molar ratio.

In catalytic syngas production, operating temperatures must be higher than a certain limiting temperature toprevent carbon formation [8]. Plasma assisted conversion of CH4 and C02, on the contrary, takes place at lowtemperatures and without equilibrium limitation (Fig. 5). No carbon formation was observed in C02-rich feed and no 02was detected in feed mixtures containing less than 80% C02. These results suggest that the presence of O atoms canavoid carbon deposition. Since the deposited carbon is an active species, it might react with chemically active oxygenatoms to form CO again (C + O -> CO). Strong carbon formation was observed in CH4-rich feed at high specific inputenergy and high temperatures. We believe that this is mainly due to CO decomposition and total decomposition of CH4.In CH4-rich feed gas mixtures the number of oxygen atoms generated may not be sufficient to fix all deposited carbon.

The pronounced maximum in syngas production at a C02 content of 70 to 80% in the feed found in themeasurements (Fig. 2 ) is a consequence of synergetic free radical conversion reactions in the C02/CH4 gas mixture. Hatoms resulting initially from CH4 dissociation react with C02 molecules and O atoms originally stemming from C02dissociation react with CII4 molecules to produce this pronounced synergetic effect in the gas mixture. Previousexperiments investigating the decomposition products of pure C,H4 and pure C02 in our DBD reactor showedconsiderably smaller conversion rates [18].

The strong pressure effect exhibited in Fig. 4 is probably at least partly due to changes of the dischargeconditions. For a given discharge gap the reduced elcctric field E/n required for electrical breakdown rises withdecreasing gas density n. The electron energy, on the other hand, is a monotonously rising function of E/n [12], So wecan expect more energetic electrons at lower operating pressure which apparently is beneficial for the dissociationreactions.

The formation of syngas from a CHVCO2 mixture requires external energy. The highest amounts of H2 and COper single pass was obtained at the highest specific input energy used. The best result of required specific energy forsyngas production is about 40 eV/molecule of syngas (H2 plus CO) at low specific input energy. As far as H2 productiononly is concerned, the present required specific energy level in the DBD reactor at this stage is 7 - 10 times higher thanthat of hydrogen produced by the electrolysis of water. Further improvements with this system to lower the requiredspecific energy may lead to an economic process for the preparation of synthesis gas.

CONCLUSIONS

Conversion of the greenhouse gases CH4 and C02 to syngas was investigated in a high power dielectric-barrierdischarge reactor at low temperature and pressure. Our experiments show that surprisingly high conversion rates of C02

and CHi can be obtained in a single pass through our small DBD reactor of 31 cm active length. Kinetic simulations

with a rather extended reaction scheme show that mainly free radical reactions are of importance. Due to these radical

reactions there is a strong synergetic effect when C02 and CILt are simultaneously present in the discharge plasma. A

simplified discharge model combined with the extensive chemical code including 60 species and 353 reactions has beenused to simulate syngas production in the discharge. Fair agreement is achieved between experiment and simulation.

The

results demonstrate that DBDs can be used for producing syngas of varying, pre-selected H2/CO ratio. The desiredH2/CO molar ratio can lie between about 3.5 and close to zero. It is primarily determined by the CH4/C02 ratio in thefeed gas mixture. Specific input energy, gas pressure and temperature hardly influence syngas composition in theabsence of carbon formation. The amount of syngas produced, however, depends strongly on the electric input energy.For a given discharge power the maximum amount of syngas with low H2/CO molar ratio is produced from a mixturecontaining 70% - 80% C02. The minimum required specific energy was 40 eV/molecule for the production of syngas(H2 plus CO) and the highest energy efficiency (electric energy converted to chemical energy in the syngas) reached sofar was about 7%. Extremely high amounts of 52 moles H2 and 14 moles CO were obtained starting from 100 molesfeed gas in a single pass through our DBD reactor in a 80/20 mixture of CH4/C02 at the highest used specific input

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ABSTRACT



INTRODUCTION






386



EXPERIMENTAL SET-UP


uartz Tube

Discharge Gap

GroundElectrode

Insulation










387




20 40 60


80 10010 15 20 25 30 35


40 45





, 52








388


80

1.0 1.5

Pressure (bar)

60-

40-

20-


/ H,»CO

H,*CO .- /

Simulation J_ _

HVCO

Hyco

2.0

. 1.5

1.0

0.5

I

0.0

200 400 600

Temperature ("C)

800 1000





.


e + CH4 -> CH3 + H + e

e + C02 -» CO + O + e

(1)

(2)


e + CO

e + H,

C + O + e

2 H + e

(3)

(4)


[CHj] = e [CH4| (5)

[H] = e [CH4I + a [H2] (6)

[CO] = 3 [C02] - Y [CO] (7)

[C] = Y [CO] (8)

[O] = P [C02] + y [CO] (9)

389





DISCUSSION






CONCLUSIONS






The









ABSTRACT



INTRODUCTION






386



EXPERIMENTAL SET-UP


uartz Tube

Discharge Gap

GroundElectrode

Insulation










387




20 40 60


80 10010 15 20 25 30 35


40 45





, 52








388


80

1.0 1.5

Pressure (bar)

60-

40-

20-


/ H,»CO

H,*CO .- /

Simulation J_ _

HVCO

Hyco

2.0

. 1.5

1.0

0.5

I

0.0

200 400 600

Temperature ("C)

800 1000





.


e + CH4 -> CH3 + H + e

e + C02 -» CO + O + e

(1)

(2)


e + CO

e + H,

C + O + e

2 H + e

(3)

(4)


[CHj] = e [CH4| (5)

[H] = e [CH4I + a [H2] (6)

[CO] = 3 [C02] - Y [CO] (7)

[C] = Y [CO] (8)

[O] = P [C02] + y [CO] (9)

389





DISCUSSION






CONCLUSIONS






The









ABSTRACT



INTRODUCTION






386



EXPERIMENTAL SET-UP


uartz Tube

Discharge Gap

GroundElectrode

Insulation










387




20 40 60


80 10010 15 20 25 30 35


40 45





, 52








388


80

1.0 1.5

Pressure (bar)

60-

40-

20-


/ H,»CO

H,*CO .- /

Simulation J_ _

HVCO

Hyco

2.0

. 1.5

1.0

0.5

I

0.0

200 400 600

Temperature ("C)

800 1000





.


e + CH4 -> CH3 + H + e

e + C02 -» CO + O + e

(1)

(2)


e + CO

e + H,

C + O + e

2 H + e

(3)

(4)


[CHj] = e [CH4| (5)

[H] = e [CH4I + a [H2] (6)

[CO] = 3 [C02] - Y [CO] (7)

[C] = Y [CO] (8)

[O] = P [C02] + y [CO] (9)

389





DISCUSSION






CONCLUSIONS






The









ABSTRACT



INTRODUCTION






386



EXPERIMENTAL SET-UP


uartz Tube

Discharge Gap

GroundElectrode

Insulation










387




20 40 60


80 10010 15 20 25 30 35


40 45





, 52








388


80

1.0 1.5

Pressure (bar)

60-

40-

20-


/ H,»CO

H,*CO .- /

Simulation J_ _

HVCO

Hyco

2.0

. 1.5

1.0

0.5

I

0.0

200 400 600

Temperature ("C)

800 1000





.


e + CH4 -> CH3 + H + e

e + C02 -» CO + O + e

(1)

(2)


e + CO

e + H,

C + O + e

2 H + e

(3)

(4)


[CHj] = e [CH4| (5)

[H] = e [CH4I + a [H2] (6)

[CO] = 3 [C02] - Y [CO] (7)

[C] = Y [CO] (8)

[O] = P [C02] + y [CO] (9)

389





DISCUSSION






CONCLUSIONS






The


390

energy of 87 kWh/Nm3

. In this case CH4 conversion reached 64% and C02 conversion was 54%. According to ourexperiments CCVrich mixtures prevent carbon and wax formation. Wax formation and carbon decomposition isobserved only in CH4-rich feeding mixtures especially at high operating temperatures and high discharge powers. Lowoperating pressures favour syngas production.

This process of plasma reforming of methane with carbon dioxide as an oxygen source offers several advantagesover the classical steam reforming process like lower operating pressure and temperature and adjustable H2/CO ratio. Sofar the specific energy required for the conversion is too high to be of economic interest. It is expected that the use ofpulsed discharges will provide the required free radicals at lower energy input.

ACKNOWLEDGEMENTS

Thanks are due to Eric Killer for help with the experiments and to Walter Egli for help with the computations. One ofthe authors (LMZ) would like to thank ABB Corporate Research Ltd for providing the opportunity to stay as a visitingscientist for 10 months during 1997.

REFERENCES

[1] Bill A, Eliasson B, Killer E, 11th World Hydrogen Energy Conf., Stuttgart, Germany, 1996, II, 1989-1995

[2] Bill A, Wokaun A., Eliasson B, Killer E, Kogelschatz U, Energy Convers. Mgmt., 1997, 38, S415-S422

[3] Eliasson B, Kogelschatz U, Xue B, Zhou L M, 13th Int. Symp. Plasma Chem., Beijing, China, 1997, IV, 1784-1789

[4] Eliasson B, Kogelschatz U, Xue B, Zhou L M, Ind. Eng. Chem. Res., 1998, 37, 3350-3357

[5] Eliasson B, CO2 Chemistry: An Option for CO2 Emission Control? in Carbon Dioxide Chemistry: EnvironmentalIssues, Paul J, Pradier C M, Eds., The Royal Society of Chemistry, Cambridge, UK, 1994, 5-15

[6] Kogelschatz U, Eliasson B, Phys. Blatter, 1996, 52, 360-362

[7] Zhou L M, Xue B, Kogelschatz U, Eliasson B, Plasma Chem. Plasma Proc., 1998, 18, 375-393

[8] Wang S; Lu G Q, Millar G, J. Energy & Fuels, 1996, 10, 896-904

[9] Hibert C, Gaurand I, Motret O, Nikravech M, Pouvesle J M, 13th Int. Symp. Plasma Chem., Beijing, China, 1997,II, 704-707

[10] Lesueur H, Czernichowski A, Chapelle J, Int. J. Hydrogen Energy, 1994, 19, 139-144

[11] Bromberg L, Cohn D R, Rabinovich A, O,Brien C, Hochgreb S, Energy & Fuels, 1998 12, 11-18

[12] Eliasson B, Kogelschatz U, IEEE Trans. Plasma Sci., 1991, 19, 309-322

[13] Kogelschatz U, Eliasson B, Ozone Generation and Applications in Handbook of Electrostatic Processes, Chang J S,Kelly A J, Crowley J M, Eds., Marcel Dekker, New York , 1995, 581-605

[14] Kogelschatz U, Eliasson B, Egli W, J. Phys. IV, France, 1997, 7, C4-47-C4-66

[15) Eliasson B, Simon F G, Egli W, Hydrogenation of C02 in a Silent Discharge in Non-Thermal Plasma Techniquesfor Pollution Control, Penetrante B M, Schultheis S E, Eds., Springer, Berlin, 1993, NATO ASI Series G, 34 B,321-337

[16] Eliasson B, Egli W, Kogelschatz U, Pure & Appl. Chem., 1994, 66, 1275-1286

[17] Zhou L M, Xue B, Kogelschatz U, Eliasson B, Energy & Fuels, 1998 (in print)

[18] Eliasson B, Kogelschatz U, Killer E, Bill A, 11th World Hydrogen Energy Conf., Stuttgart, Germany, 1996, III,2449-2459

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PRODUCTION OF SYNTHESIS GAS THROUGH PLASMA-ASSISTED REFORMING OF GREENHOUSE GASES