a comparative study of non-thermal plasma assisted reforming technologies.pdf

International Journal of Hydrogen Energy 32 (2007) 2848–2867www.elsevier.com/locate/ijhydene

Review

A comparative study of non-thermal plasma assisted reforming technologies

G. Petitpasa,!, J.-D. Rolliera, A. Darmonb, J. Gonzalez-Aguilara, R. Metkemeijera, L. Fulcheria

aCenter for Energy and Processes, Ecole des Mines de Paris, Rue Claude Daunesse, 06690 Sophia Antipolis, FrancebTechnocentre Renault, DREAM/DTAA - Service 64240, 1 Avenue du Golf, 78288 Guyancourt Cedex, France

Received 30 May 2006; received in revised form 15 November 2006; accepted 17 March 2007Available online 23 May 2007

Abstract

On board hydrogen production out of hydrocarbons (reforming) for fuel cells feed is subject to problems when used with traditional catalysts.High device weight, a relatively long transient time and poisoning problems make the integration onboard a vehicle complex. In response to thesechallenges, hydrocarbons reforming processes assisted by non-thermal plasmas for hydrogen production have been implemented over recentyears. This paper aims to provide an overview of the setting up, feasibility and efficiency of the existing technologies here investigated. Thisstate-of-the-art technology review explains the key characteristics of plasma reforming through various original approaches. The performancesof some of the systems are then compared against each other and discussed.! 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen production; Non-thermal plasma; Reforming

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28492. Plasma assisted reforming technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2850

2.1. Thermal and non-thermal plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28502.2. Non-thermal phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28502.3. Non-thermal plasma assisted technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2851

2.3.1. PSFC/MIT (MA, USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28512.3.2. Department of Mechanical Engineering of the University of Illinois at Chicago (IL, USA)–DPI (PA, USA) . . . . . . . . . . . . . . . . . . . . . . . . . .28512.3.3. GREMI (Orléans, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28522.3.4. ECP (Sarl) GlidArc Technologies (La Ferte Saint Aubin, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28542.3.5. Siemens AG (Erlangen, Germany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28542.3.6. Center for Energy and Process (Sophia Antipolis, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28552.3.7. Waseda University (Tokyo, Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28562.3.8. Tokyo Institute of Technology (Tokyo, Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28572.3.9. Kurchatov Institute Russian Research Center (Moscow, Russia) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28582.3.10. Laboratoire de Physique des Gaz et des Plasma (Orsay, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2858

3. Discussion on performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28583.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28583.2. Comparison of non-thermal plasma performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2859

3.2.1. Influence of the presence of an additional catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28613.2.2. Nitrogen oxides (NOx) production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28613.2.3. Transient performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2861

! Corresponding author. Tel.: +33 493957445; fax: +33 43957535.E-mail address: [email protected] (G. Petitpas).

0360-3199/$ - see front matter ! 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2007.03.026

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G. Petitpas et al. / International Journal of Hydrogen Energy 32 (2007) 2848–2867 2849

3.2.4. Summary of non-thermal plasma reforming assisted test benches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28623.2.5. Performances of on-board catalytic reforming applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2862

4. Numerical modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28634.1. PSFC/MIT (MA, USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28634.2. Universidade da Madeira (Funchal, Portugal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28644.3. Center for Energy and Processes (CEP) (Sophia Antipolis, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28644.4. Department of Mechanical Engineering of the University of Illinois at Chicago (IL, USA)–DPI (PA, USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28644.5. GREMI (Orléans, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28644.6. Korea Institute of Science & Technology (Seoul, Korea) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28654.7. University of Cassino (Cassino, Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28654.8. Comments on modeling works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2865

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2865Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2865References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2865

1. Introduction

Recent years have seen a real enthusiasm for fuel cell de-velopment as a good alternative to fuel (diesel, gasoline) basedcombustion engines in terms of efficiency and environmentalimpact. The technology based on hydrogen however has sig-nificant drawbacks due to its storage properties. Despite a highmass heating value: 120 kJ/g compared to gasoline (42.8 kJ/g),methanol (20 kJ/g) and methane (50 kJ/g), hydrogen is char-acterized by a very low volumetric heating value as a resultof its low density (2.016 g/mol): 11 kJ/l at atmospheric pres-sure against 16 000 kJ/l for methanol. Its low density and highflammability risk make it very difficult to store on-board ve-hicles, where available space is limited. Possible storage so-lutions such as cryogenic system (hydrogen cooled down to20.3 K, where it is in liquid phase) allows to increase the fueldensity up to 70.8 kg/m3, which is still low: the equivalent vol-ume of hydrogen (stored in these conditions) required to reachthe same heating value of 1 l of gasoline is about 3.78 l (gaso-line density in atmospheric conditions is 750 kg/m3). Further-more, loss rates of 1–2% per day have to be countered and10–25% of the fuel boils off during refueling [1]. Others so-lutions like high pressure storage (700 bars—equivalent vol-ume of 1 l of gasoline is equal to 6.81 l), that presents a safetyhazard, and metal hydrides (like magnesium, where hydrogencapacity could reach 7.6% mass [2]—equivalent volume of 1 lof gasoline lies around 4 l) are under study to develop relevantoptions.

In response to the storage problematics of hydrogen, onepossible way to ensure the feed in of hydrogen on the vehiclewould be to store it in liquid fuels (methanol, gasoline, . . .)and then to produce hydrogen out of these fuels: this chemicaltransformation is called reforming. The main advantage to suchan approach is the exploitation of the existing distribution net-work (gas stations), which could play a key role before theemergence of new technologies.

The hydrocarbon reforming is an oxidation reaction in whichoxygen, water or carbon dioxide play the role of the oxidant.Thus, three main reforming reactions are possible:

• Partial oxidation (POx):

CnHm + n

2O2 " nCO + m

2H2; (1)

• steam reforming:

CnHm + nH2O " nCO +!m

2+ n

"H2; (2)

• dry CO2 reforming:

CnHm + nCO2 " 2nCO + m

2H2. (3)

The partial oxidation is strongly exothermic whereas thesteam reforming and the dry CO2 reforming are endother-mic. The enthalpies corresponding to the different reactions aregiven in Table 1 for the methane and isooctane. Oxygen, waterand carbon dioxide can be mixed together to achieve an auto-thermal reaction (global enthalpy equal to zero). Steam reform-ing potentially leads to best hydrogen yields. The reformingoperation produces a mixture of hydrogen and carbon monox-ide known as synthesis gas (syngas).

By using the water gas shift (WGS) reaction:

H2O + CO " H2 + CO2, !H = #41 kJ/mol, (4)

carbon monoxide can be converted into hydrogen in a secondreactor. A third reactor makes possible to lower CO concen-tration (PrOX—preferential oxidation) that would poison low-temperature fuel cells. Most of the time, these two reactionsare supposed to be complete.

Note that a fourth chemical reaction, strongly endothermic,exists:

Thermal decomposition

CnHm " nCs + m

2H2. (5)

The difficulty of handling the solid carbon makes the appli-cation not suitable for on-board hydrogen generation. An ex-ception though is the thermal decomposition of methanol [1],because it produces only syngas:

Thermal decomposition of methanol

CH3OH " 2H2 + CO. (6)

Industrial scale reforming of hydrocarbons ranging from nat-ural gas to heavy oils has been widely used for many years andall thanks to traditional catalytic processes, which can hardlybe scaled down for on board systems due to specific problems

2850 G. Petitpas et al. / International Journal of Hydrogen Energy 32 (2007) 2848–2867

Table 1Standard enthalpies (298 K, 1 atm), in kJ/mol, of methane [8] and isooctane

Methane [8] Isooctane

Partial oxidation #36.1 #675.8Steam reforming 205.7 1258.8Dry CO2 reforming 246.9 1596.3

such as: sulfur or soot catalyst poisoning, compactness, dy-namic behavior, weight, costs. . . . Some applications of onboard catalyst reforming are under development, focused ontransient performances and compactness [3,4]. They targetmainly auto thermal or steam reforming.

Among the innovative possible reforming options, plasmasystems could provide original responses to these drawbacksin terms of reactivity, compactness and efficiency. Differentpaths have been investigated for the two last decades usingvarious plasma technologies such as plasmatron [5], glidingarc [6–12], dielectric barrier discharge (DBD) [13–15], corona[16], microwave [17–19], pulsed discharge [20] and reformedhydrocarbons, such as methane [6,7,20–22], diesel [23] andbio fuels [5,24–26]. The purpose in this article is to give anoverview of the literature dedicated to this subject in terms ofexisting technologies, performance capabilities and modelingapproaches.

2. Plasma assisted reforming technologies

2.1. Thermal and non-thermal plasma

Plasma is an ionized gas that can be generated by a numberof methods, like combustion, flames, electrically heated fur-naces, electric discharges (corona, spark, glow, arc, microwavedischarge, plasma jets and radio frequency plasma), and shocks(electrically, magnetically and chemically driven) [27]. There-fore, plasma media exhibits a high energetic state of matter,characterized by a high electrical conductivity.

The reforming process aiming hydrogen production is theconversion of an hydrocarbon into a more valuable product(higher heating value). The challenge is to perform the oper-ation with the best energetic efficiency. Because of their abil-ity to release high energetic densities, plasmas have then beenstudied with interest for reforming applications.

Depending on their energy level, temperature and electronicdensity, plasma state is usually classified as a high tempera-ture (or thermal) plasma and a cold (or non-thermal or non-equilibrium) plasma. For example, neon lights are non-thermalplasmas and the sun is a thermal one. Historically, first plasmaassisted reformers were thermal ones, e.g. direct current (DC)plasma torch. Chemical reactions were thus enhanced becauseof the presence of very reactive species (ions, electrons) in avery hot medium but the energy consumption was high. It hasbeen shown, by Cohn et al. [28], that comparable H2 yieldscould be attained with both kinds of plasma, but at significantlylower energy consumption in the case of non-thermal plasma(see Fig. 1): “new plasmatron” and “old plasmatron” refer to

Fig. 1. Comparisons of energy costs for non-thermal and thermalplasmas—reforming of diesel (from [28]).

a non-thermal and thermal plasma assisted reformers, respec-tively. The thermal application is therefore not necessarily veryeffective in that most of the energy is used in heating particles.

2.2. Non-thermal phenomena

Non-thermal plasmas [27] have been applied for fuel gastreatment and have been considered very promising for or-ganic synthesis because of its non-equilibrium properties, lowpower requirement and its capacity to induce physical andchemical reactions within gases at relatively low temperatures.The electrons in non-thermal plasma can reach temperaturesof 10 000–100 000 K (1–10 eV) while the gas temperature canremain as low as room temperature. It is the high electrontemperature that determines the unusual chemistry of non-thermal plasmas. Based upon mechanisms of which plasma isgenerated, pressure applied and the electrode geometry, non-thermal plasmas comprise very different types including glowdischarge, corona discharge, silent discharge, DBD, microwavedischarge and radio frequency discharge. The glow dischargeis a low-pressure discharge (less than 10 mbar) usually oper-ating between flat electrodes. Electrons in the glow dischargeare highly energetic. The excited neutral atoms and moleculesgenerate a typical glow (e.g., fluorescent tubes). Due to its low-pressure characteristics, the glow discharge is not very suitablefor chemical synthesis. The corona discharge is an inhomo-geneous discharge and can be initiated at atmospheric pres-sure using inhomogeneous electrode geometries, like a pointedwire electrode with a plate one. It is the small radius of cur-vature at the top of the wire electrode that results in a highelectric field required for ionizing the neutral molecules. Thesilent discharge or DBD combines the large volume excitationof the glow discharge with the high-pressure characteristicsof the corona discharge. A dielectric layer covers at least oneelectrode in the silent discharge. The entire electrode area willbe effective for discharge reactions. Once the silent dischargeis initiated at any location within the gap between electrodes,the charge accumulates on the dielectric to form an opposite


electric field and interrupts the current flow in a few nanosec-onds to generate microdischarges. The duration of the currentpulse relates to the pressure, properties of gases and the di-electric material applied. The RF discharge operates at highfrequencies (several MHz) and very low pressure to achievethe non-equilibrium conditions. This discharge is also not suit-able for chemical synthesis. The microwave discharge operatesat very high frequencies, e.g. 2.45 GHz in the range of mi-crowaves, with which only light electrons can follow the os-cillations of the electric field. Therefore, this discharge is farfrom local thermodynamic equilibrium and can be operated ina wide pressure range. More recently, gliding arc discharge, acombination of high power equilibrium arc discharge and bet-ter selectivity of non-thermal plasmas, has also been reportedto be used for reforming application.

Since a non-thermal plasma is a mixture of electrons, highlyexcited atoms and molecules, ions, radicals, photons, . . . , thechemistry is very complex. One should not expect a very selec-tive product from this plasma chemistry. The rate of all thesedischarge reactions depends on the electron energy, electrondensity, gas temperature, gas pressure, properties of gases andso on. However, the present understanding of plasma chem-istry is limited. It is still difficult to predict final products the-oretically. Most of present achievements are mostly based onexperiences.

2.3. Non-thermal plasma assisted technologies

The following is a brief description of existing reformingsystems as a function of the laboratories where they have beendeveloped. The evolution between the different models elabo-rated herein highlights the optimization steps in order to achievethe best performances; that will further be discussed.

2.3.1. PSFC/MIT (MA, USA)The Plasma Science and Fusion Center (PSFC) group from

Massachusetts Institute of Technology (MIT) is one of themost advanced in plasma assisted reforming. Although Cohn,Bromberg, Rabinovich and co-workers started developing athermal plasma assisted reformer, they re-oriented researchestowards non-thermal arc systems for energetic efficiency rea-sons. Potential applications involve hydrogen production forfuel cell feed-in, NOx absorber regeneration, gas enrichmentor spark ignition engine [5].

Two non-thermal reactors, called “plasmatron”, have beendeveloped:

• Low current plasmatron fuel converter, named GEN 2, inwhich fuels and oxidants are injected near the electrode gapand the cathode can be a spark plug.

• Wide area electrode, low current plasmatron fuel reformer,named GEN 3, in which the species can be injected at differ-ent inputs and electrodes are concentric with an axial gap.

GEN 2 and GEN 3 are called such as to be opposed toGEN 1 [29–31], that is a thermal plasma reformer.

Various fuels were tested: gasoline, methane [5], diesel fuel[32,33], propane [34] and bio fuels such as ethanol [24], corn,

Fig. 2. Scheme of the GEN 2 plasmatron reformer (MIT, USA) [5].

soy bean, and canola oil [32]. Oxidants used were dry air andwater–air mixtures. The after treatment with catalysts has beeninvestigated [32] and has shown the best results. Catalysts (typ-ically Pt or Pd-based) were used on a ceramic or an honeycombframe [23,28]. Experiments were carried out at atmosphericpressure. Fig. 2 shows the sketch of the plasmatron GEN 2reformer. This 2 l, 3 kg reactor is supplied with a 15–120 mAcurrent, corresponding to power ranges in the order 50–300 W.Reactant species are pre-heated before entering the system.

In order to achieve a liquid fuel injection and to minimizethe electrode erosion, a plasmatron GEN 3 has been developed,with concentric electrodes. Air can be injected at three differentlocations (axial, near the wall or between the two electrodeswith a swirl motion) whereas fuel is injected through an axialnozzle. Dimensions of the compact set up are 12 cm high, 5 cmouter diameter. The power supply works at constant current andproduces voltages in the order 500–2000 V during the dischargemaintaining phase. Contrary to plasmatron GEN 2, air and fuelare injected at ambient temperatures. A swirl flow rotates andpushes the discharge towards the axis of the reactor.

PSFC’s work has led to a technology commercialized byArvin Meritor. The application purpose is to enrich the fuelwith syngas in order to improve the combustion for traditionalinternal engines (lower consumption, reduced emissions of par-ticles and NOx) [5].

2.3.2. Department of Mechanical Engineering of theUniversity of Illinois at Chicago (IL, USA)–DPI (PA, USA)

Researches on plasma applications have been transferredfrom the Department of Mechanical Engineering of the Uni-versity of Illinois at Chicago to the laboratory of the DrexelPlasma Institute (DPI) in the frame of Fridman et al.’s works[6,7,16,35]. This group has investigated the field of plasma


assisted technologies, including reforming for the productionof syngas. Studies of the application of corona discharges[16,35] and gliding arc [6,7] for reforming purposes have beenperformed. Second plasma reactors allow achieving a higherpower.

2.3.2.1. Corona discharge. A pulsed corona discharge tech-nology was used to investigate the dry reforming of methaneand the auto thermal reforming of isooctane. Here the deviceincluded catalytic post-processing.

The corona reactor consists of a wire-into-cylinder coaxialelectrode system, where the inner electrode is a 0.5 mm diame-ter Inconel wire and the outer electrode is a 1.2 m long, 22.2 mminner diameter cylindrical stainless steel tube. The central 0.9 mlong section of the reactor is placed inside a high-temperaturefurnace that helps in the pre-heating process of the species. Thepower of the plasma source varies from 1 to 20 W.

The catalyst section employed consists of a 50 cm long,1.3 cm diameter stainless steel reactor that can be placed up-stream or downstream of the plasma zone. Experimental resultsshowed that the efficiency of the plasma assisted reforming sys-tem is much better when coupled with the catalyst, especiallywhen it is before the corona discharge zone.

Considering the low range of plasma power achieved, non-thermal arcs plasma were studied trough two different reactors,which were able to work at higher power and to give largerplasma volume.

2.3.2.2. Gliding arc discharge reactor. This reactor helped theexperimental investigation of methane conversion [6]. It con-sists of a 2.5 l reactor walled with quartz (8 cm inner diame-ter, 50 cm long). Two 7 mm thick, 2 cm wide and 9 cm longdiverging electrodes were used; hold by a stainless steel platelocated at the top of the setup. Reactant species were preheatedat temperatures in the range 573–873 K before entering the testchamber.

Experimental runs conditions were atmospheric pressure,specific energy input in the range 3–12.10 MJ/kg (ratio of theinput electric power on the inlet gas flow rate), and partial ox-idation was considered both for the air and 50% oxygen en-riched air.

2.3.2.3. Gliding arc in Tornado (GAT) reactor. The GAT tech-nology is named after the natural tornado, of which it is simi-lar to [7]. Indeed, the phenomenon of reverse vortex flow is itsmain principle. Fig. 3 shows the scheme of the GAT principle.Two flows enter the cylindrical volume. At first the flow is axi-ally injected (3) while a second enters the volume tangentially(2). The reaction products exit the cylinder in (4), the sameside as the tangential gas entry. Solid arrows (6) represent thethree-dimensional sketch of the rotating flow and dotted arrows(5) show the streamline in the axial plane. The plasma zone isthen well stabilized by the near wall tangential flow. Dimen-sions of the tube (in quartz for convenient reasons) are 40 mminner diameter and 50 mm length. Note that the plasma tube isfollowed by a post reactor of bigger volume, where most of thereforming reaction takes place.

Fig. 3. GAT reactor principle (University of Illinois, USA) [7].

Two high voltage electrodes geometries have been developed,to show different ways one can ignite the discharge (shownon Fig. 4). The first configuration consists of a movable ringelectrode (8) that moves away from the top. Initially, the cathodering is 3 mm away from the ground electrode (7), or anode disc,and that is where the arc ignites. Next, the cathode is moveddown and thus the arc elongates, up to a point where it is in non-equilibrium conditions. The second configuration has a spiralcathode (6) placed coaxially with the tube. The arc dischargeignites at the top of the spiral electrode and then glides along ituntil it reaches a smaller diameter ring (10) and thus stabilizes.

The GAT reactor was applied to investigate the partial oxida-tion of methane, by using the movable circular electrode tech-nology. Methane was axially injected while air was injectedtangentially. GAT reactor volume is 0.2 l and is coupled to aheat exchanger that permits the preheating of the reacting com-ponents. The high voltage DC power supply delivers poweraround 200 W.

2.3.3. GREMI (Orléans, France)The Groupe de Recherches sur l’Energétique des Milieux

Ionisés (GREMI), composed of Chapelle, Cormier and co-workers, has been studying plasma assisted reforming for manyyears and has played an important role in its development,working together with Czernichowski [36,37] and Fridman etal. [37] on the first investigations on gliding arc development.Different types of reactor have been studied in the laboratory.

2.3.3.1. Three-discharge glidarc reactor (RotArc). Re-searchers from GREMI have used a reactor called “RotArc”[9,38] to study the steam reforming of methane with oxygen.


The system (see Fig. 5) has an inner cone-shaped electrodeand an external metallic tube as a second electrode (diameter:40 mm, thickness: 1 mm; length: 200 mm). Turning gliding arcdischarges are then generated because of the radial injection ofthe gas mixture. Three different anode–cathode couples allowthe creation of three discharges.

2.3.3.2. Magnetic blow out glidarc reactor. This second gen-eration reactor [39,40] was designed for the study of the steamreforming of methane giving the discharge a rotating effect,and thus the ability to sweep a larger volume. The effect of anoxygen addition was then studied.

Fig. 4. Reverse vortex reactor design for GAT stabilization: (a) movable ringelectrode and (b) spiral configurations [7].

Fig. 5. Scheme of the RotArc reactor (GREMI, France) [9].

Fig. 6 shows the scheme of the reactor: a rotating magneticfield is applied to the ionized particles thanks to a magnet.Three anodes are set around one axial cathode (tungsten rod-6 mm diameter). Reactive species enter the reactor at a temper-ature of 500 K, along the cathode through a 16 mm diameterceramic tube. One rotating arc discharge is then created andwinds around the cathode. Furthermore, the transition betweennon-thermal and thermal regime is controlled by the power ofthe magnetic field, without the use of any external limitations(resistance . . .). The three cascading discharges are then pow-ered up by a three channel electrical power supply.

2.3.3.3. Electrical discharge. The steam reforming of ethanolhas been studied with an electrical discharge, in a batch

Fig. 6. Scheme of magnetic blow-out glidarc reactor (GREMI, France) [39](acknowledgment to International Plasma Chemistry Society).


reactor working at low temperatures and at atmospheric pres-sure. [25]. The water/ethanol mixture is heated by graphiteelectrodes (10 mm gap), whose dimensions are 150 mm longand 25 mm diameter. Reforming was studied with ethanol/watermole ratio in the range 0–0.72. A 50 Hz high voltage trans-former delivers sinusoidal current at constant value 155 mA.

2.3.3.4. Sliding discharge. The hydrogen enrichment of amethane–air mixture by a sliding discharge plasma has beeninvestigated [10], to operate according to the principle ofgliding arc.

A quartz tube (length: 300 mm, inner diameter: 22 mm) con-tains two knife shaped electrodes between which a dischargedevelops (electrode gap: 1 mm).

2.3.4. ECP (Sarl) GlidArc Technologies (La Ferte SaintAubin, France)

This firm, founded in 1997, has been developing gliding arc(“glidarc”) prototypes and pilot model systems for several years[8,26,41–46]. Its founder, Czernichowski, is considered to bethe inventor of the gliding arc principle. He worked at GREMI(see previous section) before creating its own company.

Experimental investigations concerned combinations of re-forming processes (dry or/and steam, partial oxidation) forvarious fuels: methane [8,36], propane [41,42] cyclohexane,heptane, toluene, gasoline (SP95), diesel oil [43], naturalgas [8,43–45] JP8 [46], bio oils (rapeseed oil [26], soybeanoil [46]); and were mostly performed with a reactor calledGlidarc I.

Two versions of Glidarc I reactor have been constructedbased on the gliding arc technology (see Fig. 7). The firstversion was employed to the reforming of natural gas. Thissix-phase reactor works at atmospheric pressure and has sixstainless steel, 0.8 mm thick, 14 cm long and 25 mm wide knifeshaped electrodes, symmetrically disposed around the flow axis.Inner diameter and capacity of the reactor are, respectively,80 mm and 1.5 l. Power is supplied from 0.6 to 1.1 kW.

The second version is very similar to the previous and waselaborated in order to work at higher gas preheated tempera-tures, reactor wall temperatures, pressure and total gas flow.The electrodes in this reactor are 2 mm thick, 8 cm long and25 mm wide. Total volume of the void reactor (plasma and post-plasma zones) is about 1.56 l. The electrical power supplied tothe plasma during the different runs was up to 980 W.

2.3.5. Siemens AG (Erlangen, Germany)This firm has successively developed three different plasma

assisted reforming reactors for methane conversion, thanks tothe competences of Kappes, Hammer and co-workers: DBD[13,47] low energy electron beams [48] and then later arc dis-charge (for gas enrichment) [49]. This evolution aimed higherhydrogen yields and energetic density.

2.3.5.1. DBD reactor. The first of these tests concerned thesteam reforming of methane [13], investigated thanks to twoDBD reactors, different in size and thermal insulation (the

Fig. 7. Scheme of Glidarc I reactor (ECP, France) [8].

Fig. 8. Scheme of DBD reactor (Siemens AG, Germany) [13].

second one operating at larger volume and flow rates, and inadiabatic conditions). Main characteristics of the two reactorsare thus, respectively: inner alumina diameter (20 mm, 46 mm),number of discs (38, 30), electrode gap (2 mm, 4 mm), waterflow rate (5.5 $ 10#6 m3/s, 3.3 $ 10#5 m3/s), methane flowrate (1.1 $ 10#5 m3/s, 1.6 $ 10#5 m3/s). Reactive species arepreheated up to a temperature of 250 %C.

Fig. 8 shows the sketch of the DBD reactor that is an alu-mina ceramic (Al2O3) tube playing the role of the groundedelectrode (cathode). The inner high voltage axial anode con-sists of a metal rod equipped with metal parallel disks with


serrated edges. The reactor is thermally controlled at mediumtemperature (200–600 %C).

Power supply delivers pulsed voltage of 15–20 kV/"s, withan energy per pulse set at 10 mJ and a frequency range of5–20 kHz. Thus, the discharge was induced at powers in therange 5–200 W.

Combination of DBD with catalysts has also been tested:good yields and efficiency at high temperature (400–600 %C)were observed, the activation energy being provided by theplasma.

2.3.5.2. Low energy electron beams [48]. In this setup, elec-trons are generated in the vacuum and then accelerated witha strong electric field before entering the gas mixture throughan extraction window. The latest is able to withstand the pres-sure difference between the vacuum and the reaction volumeat atmospheric pressure, and sufficiently permeable for energyelectrons, in order to minimize energy loss. For high energyelectrons (E > 100 keV) metallic membrane, in titanium for ex-ample, are used. For lower energy ones (which is the case here),ceramic materials like SiN are convenient. For this setup, a sil-icon plate containing a SiN membrane with a size of 1 $ 1 mmand thickness of about 300 nm is employed.

The use of low energy electron beams help generation ofradicals, ions and secondary electrons. Furthermore, electronsemissivity is set out of the chamber and is therefore independentof pressure, temperature and composition conditions.

A direct voltage (10–14 kV) source supplies current up to33 "A, limiting the power delivered to 462 mW. The plasmazone created in this method is a spherical one with a low ener-getic density (about 1 W/cm3). The electron beam is at a BaOheated cathode and is further focused thanks to an anode and amagnetic system, similar to a TV or oscilloscope system. Twotest chambers are used: cell A (transverse flow) for dry reform-ing and partial oxidation (CH4: CO2 =1: 1, CH4: O2 =2: 1) andcell B (radial/axial flow) for steam reforming (CH4: H2O=1: 2).

Methane conversion and hydrogen production were betterachieved in the case of dry and steam reforming. The author[48] claims that the required energy for steam reforming canbe divided by 4 with electron beams in comparison with DBD.However, hydrogen production is limited by the input power.

2.3.5.3. Discharge plasma. In parallel to fuel cells feed-in andbecause of the difficulty to produce a clean exhaust H2 rich gas,possibilities of reforming systems applications to gas enrich-ment has been studied. This latter aims H2 production amongothers species in order to have a better combustion process.Indeed, H2 provides higher heating value and higher flame ve-locity [49]. A reforming unit (arc discharge based) was inte-grated co-axially inside a compact burner set-up. Part of therich methane mixture is converted into mainly H2, CO and H2Othen injected into a leaner mixture where it burns. The aim isto have a flame with excess air compare to usual burner andsteadier thanks to the enrichment in hydrogen.

The plasma unit in this approach is similar to a tube of10 mm outer diameter and 1 mm thick. Its diverging outlet hasa 2.2 mm initial diameter. A 1.5 mm diameter electrode goes

through the reforming unit up to the grounded diverging outlet.The electrode gap is 0.35 mm. A baffle plate slows down theplasma and mixes the syngas with the lean mixture of the burner.The burner is 240 mm long and 22 mm inner diameter. A DCpower supply provides the discharge in the range 45–75 W.Total flow rates were about 3.3–6.6 $ 10#5 m3/s. Thus, theair/ratio was increased from 1.67 (usual burner) to 2.01 withthe reforming unit.

2.3.6. Center for Energy and Process (Sophia Antipolis,France)

The CEP has been studying plasma applications to hydrocar-bons conversion for about 10 years, including synthesis of car-bon nanostructures and hydrogen production, and supervisedby Fulcheri.

In the field of reforming studies, two successive plasma as-sisted reforming reactors based on gliding arc technology havebeen developed.

The first one [11] was designed to work in auto-thermal orsteam reforming conditions for pressures up to 3 bars and preheating temperatures up to 773 K. The fuel studied in the ex-periments was a non-sulfur synthetic gasoline called Califor-nia gasoline-Syntroleum (average formula: C7H15,2). Two setsof electrode creating two gliding arcs are placed in series in a6.3 l stainless-steel cylinder. Scheme of the reactor is shown onFig. 9. The knife-shaped electrodes are 45 mm length, 20 mmheight and 2 mm width.

A power supply has been specially developed, composedof two high voltage systems, for each set of electrodes. Bothcan work in AC (50 Hz) or DC configurations, at maximumpotential difference equal to 5000 and 10 000 V, respectively.

During the different runs, a maximum power of 1000 W wasattained. O/C ratios varied from 0.3 to 1.2 and H2O/C ratiosfrom 1 to 5. Prior to their introduction, fuel and water werepremixed whereas the air was introduced separately.

A second reactor was designed in order to achieve a largerreactive volume [12]. The study of the auto-thermal reformingof isooctane, at atmospheric pressure, was carried out. Thiswork is supported by the automobile manufacturer Renault.Isooctane iC8H18 is chosen as a substitute of gasoline, althoughoctane has slightly higher H/C ratio (effective gasoline formulais C6.9H12.5) and hence may give higher hydrogen yield.

The plasma reformer is composed by a compact non-thermalarc plasma torch and a post discharge reactor. Plasma torchgeometry is very similar to those encountered in classicalhigh current DC plasma devices. An electric arc is establishedbetween a central-peak shaped electrode and a 8 mm innerdiameter annular electrode. The two concentric electrodesare separated by a high voltage insulating ceramic material.A low current–high voltage arc discharge generated betweenthe electrodes is blown down by a high velocity gas mixture(preheated separately at temperatures being adjusted freelybetween ambient temperature and 800 K) injected radially atthe vicinity of the central electrode. As a consequence, thegas flow, and thus the arc, rotates. This vorticity stabilizes thenon-thermal arc, additionally to the wall effect.


Fig. 9. Scheme of Glidarc reactor (CEP, France) [10].

The plasma reactor geometry has been designed to further ahigh plasma homogeneity and an efficient reagents mixing. Thereactor volume is about 350 cm3. Radical species are createdin the plasma zone and then the main chemical process occursin the post plasma reactor.

Gasoline flow rates up to 0.3 g/s can be achieved while O/Cand H2O/C ratios can be adjusted continuously.

The test chamber is thermally insulated. Products arequenched at the exit of the test chamber by a water spray forthe gas analysis. Thermocouples are placed along the reactor.Sketch of the set up is represented Fig. 10.

The power supply operates as a current source and is basedon a resonant converter technology. A 15 kV maximum voltagecan be obtained while maximum current is 660 mA. Contraryto high voltage transformers commonly used for such applica-tions, this power supply allows the continuous control of thearc current with a high accuracy in the range 200–600 mA. De-pending on the conditions (gas flow rate, current), the regimeof the discharge can vary from streamer over gliding arc tocontinuous discharge [50].

2.3.7. Waseda University (Tokyo, Japan)The Department of Applied Chemistry has developed a non-

equilibrium pulsed discharge reactor and a diaphragm reactorin the frame of its study on hydrogen production at atmosphericpressure and low temperature (393 K), through the works ofSekine, Urasaki, Kado, et al. Attention was as well focused onthe introduction of reactive species in liquid phase (water andethanol for example). The electrical power is typically under100 W and the flow rates under the mol/min.

2.3.7.1. Non-equilibrium pulsed discharge. This first reactorhas been designed for the steam reforming of hydrocarbons andalcohols (methane, propane, hexane, cyclohexane, methanoland ethanol) and the dry reforming and partial oxidation ofmethane [20,51,52].

The pulsed discharge reactor is made of a 4 mm inner diam-eter quartz tube with two stainless steel rods of 2 mm diameterinserted from each end, used as electrodes. Reactive species ofthe reaction are preheated (393 K) prior to their introduction sothat they are in gas phase.


5Thermocouples

1 3

CondensedProducts

Gaseous Products

Reactive SpeciesEntrance

Plasma Torch

6

Post Plasma Zone QuenchingChamber

Thermal Insulation

Stainless Steel

b c

d

a

ArcDischarge

Axial AnodePlasma Zone Post Plasma Reactor

Grounded cylinderElectrodeCeramic Insulator

ReactiveSpecies

ThermalInsulation

2 4

Fig. 10. Scheme of plasma torch reactor (CEP, France) [12].

Fig. 11. Scheme of diaphragm reactor developed at Waseda University [53](acknowledgment to International Plasma Chemistry Society).

For a second version, authors have chosen to use a fibercarbon electrode in order to “pump” by capillarity an ethanol-water mixture in its liquid phase.

2.3.7.2. Diaphragm reactor. This second reactor (Fig. 11)aimed the steam reforming of ethanol in liquid phase [52].

It is made of two Pyrex tubes (inner diameter: 100 mm, thick-ness: 5 mm, length: 150 mm) separated by an insulated Teflonmembrane having single pinhole (diameter: between 0.25 and2 mm). The role of the pinhole is to centralize the charge in or-der to generate the discharge in the liquid phase. The electrodegap is about 6 mm.

The effect of the shape of the electrodes (needle type orflat-plate type) on the formation rate of the products has beeninvestigated.

2.3.8. Tokyo Institute of Technology (Tokyo, Japan)Two departments of the Tokyo Institute of Technology have

been studying plasma assisted reforming.

2.3.8.1. Department of Mechanical and Control Engineering.The Department of Mechanical and Control Engineering(Nozaki, Okazaki, et al.) has investigated the possibilities ofhydrogen production out of methane thanks to two reactors:a microscale non-equilibrium one used for partial oxidation[54], and a DBD reactor coupled to a catalyst for hydrogenenrichment of gas, designed for steam reforming [14].


The microplasma reactor, whose principle is similar to DBD,consists of a Pyrex thin glass tube (inner diameter: 60 mm)which has twisted along its length a 100 long metallic tube. Itis maintained in heat grounded reservoir to achieve a constanttemperature. Between the metallic wire and the reservoir isapplied a high voltage sine wave (2 kV at 75 kHz).

In a second reactor, a DBD/catalyst hybrid reactor is set up.The system is located in a constant temperature bath (150 %C)to avoid liquid condensation. Ni/!-Al2O3 pellets of 3 mm di-ameter are packed into the 2 mm inner diameter and 50 mmlong DBD reactor. Bipolar pulsed voltage (±20 kV) is appliedbetween center and external electrode.

2.3.8.2. Department of Chemical Engineering. The Depart-ment of Chemical Engineering (Sekiguchi et al.) has conductedexperimental investigation on the steam reforming of n-hexane,using a microwave discharge reactor at atmospheric pressure[17].

Reactive species are preheated prior to their introduction ina 12 mm inner diameter and 500 mm long quartz tube, whichitself gets inserted in a wave-guide where the electromagneticfield is concentrated. Microwave discharges are delivered at2.45 GHz at a maximum power of 2.8 kW.

2.3.9. Kurchatov Institute Russian Research Center (Moscow,Russia)

A non-equilibrium plasma of pulse microwave discharge wasused to investigate the methane and kerosene partial oxidation[18,19,55], tanks to Rusanov et al.’s investigation.

Experimental investigation of methane conversion was con-ducted in two types of microwave discharge: a pulsed periodicregime (streamer pseudo corona discharge, 3 cm wavelength,pulse power up to 3 kW, 1 "s pulse duration, and repetition fre-quency of 1 kHz) and a continuous regime (coaxial torch dis-charge, 2.45 GHz frequency and power in the range 1–5 kW).Air/methane mixture could be heated up to 500–900 %C beforeentering the test chamber.

Two pulse microwave discharge reactors were developed(coaxial and resonant type) for the partial oxidation of kerosene.Resonant set up is formed by a break in the central part of theco-axial inner electrode. The frequency used is 2.45 GHz andthe power range from 500 to 3000 W. Air flow rate was var-ied within 1–9.5 $ 10#3 m3/s. The vaporizer-heater heats thefuel/air mixture to a temperature of 250–300 %C.

2.3.10. Laboratoire de Physique des Gaz et des Plasma(Orsay, France)

In the framework of a partnership with the automobile man-ufacturer Peugeot has been investigated the reforming of isooc-tane, with a DBD reactor [15] (Pasquiers and co-workers). Theexperimental set up consists of a quartz tube (14 mm inner di-ameter, 2 mm thick) coated with a 14.5 cm long copper layer(external electrode). The axial stainless steel electrode is of5 mm diameter width. The reactor volume is about 19.5 cm3.

The power supply in this approach can deliver voltages inthe range and up to 40 kV and function either pulsed or in

alternative currents (up to 120 Hz). Experiments were carriedout at atmospheric pressure and isooctane is introduced at am-bient temperature (thanks to a capillary).

3. Discussion on performances

3.1. Definitions

The amounts of products are given in different ways: mol,mol percentage, yield or selectivity. The definition of the twolast magnitude amounts is more ambiguous and has been there-fore taken in all calculations as followed:

Yield (H2)

= Amount of H atoms in the formed H2

Total amount of H atoms injected, (7)

Selectivity (H2)

= Amount of H atoms in the formed H2

Amount of H atoms in the formed product, (8)

Yield (H2)

= Amount of C atoms in the formed COTotal amount of C atoms injected

, (9)

Selectivity (CO)

= Amount of C atoms in the formed COAmount of C atoms in the formed product

. (10)

The link between the two is the conversion rate ":

Yield = " $ Selectivity. (11)

In order to make comparisons on the same basis, efficienciesand conversion rates have been calculated as defined below:

Efficiency: Hydrocarbon reacts with oxygen or water to pro-duce hydrogen, whose heating value is higher than any otherhydrocarbon. Therefore, the efficiency of a reforming system isthe lower heating value (LHV) of hydrogen produced dividedby the input energy, that is the summation of the electrical en-ergy of the plasma and the LHV of the hydrocarbon injected:

# = (H2 + CO)produced $ LHV(H2)

Input plasma energy + fuel injected $ LHV(fuel). (12)

We consider that the entire CO produced is then convertedinto H2 by Water Gas Shift reaction (Eq. (4)). Therefore, wetake into account the CO produced.

The compositions of the reformed products are usually givenin dry percentage. The syngas flow rates were thus calculatedassuming that the balance was realized by N2 for the caseof partial oxidation with air, the latest not being transformedinto NOx .

Notice that this study is focused on the efficiency of theplasma reactor and not in the overall system. Therefore, thiscalculation does not take into consideration the energy spent inthe preheating of the reactant species.

Conversion rate: In order to produce hydrogen, hydrocarbonmolecules have to be “cracked”, to break the C–C and C–Hlinks. The performance of this operation is evaluated by usingthe conversion rate which the ratio of atomic carbon contained


in reforming products to the atomic carbon contained in theinjected hydrocarbon:

" = [CO + CO2 + CH4 + · · · ]produced

n $ [CnHm]injected. (13)

Specific energy requirement: This value is the input electricalpower used by the plasma that is required for producing one molof H2. Still considering the CO produced, the specific energyrequirement is

$ = Input plasma power[H2 + CO]produced

. (14)

3.2. Comparison of non-thermal plasma performances

The efficiency, the specific energy requirement and the con-version rate, as defined in equations above, appear to be goodindicators for quantifying reforming systems in the field of syn-gas production. These three parameters have been calculatedfor every non-thermal plasma processes found in the literature(see Section 2) in order to be able to compare together theirdifferent performances. As a result, some values reported herecould be different from those claimed by the authors in theirarticles or proceedings.

Figs. 12–14 illustrate the efficiency, the specific energy re-quirement and the conversion rate, respectively. The figurescontain information on the hydrocarbon feedstock, the non-thermal plasma device as well as the institution involved in thedevelopment of the plasma technology. Notice that most of theresults published concern arc discharge based technologies.

Fig. 12. Efficiency of non-thermal plasma processes described (see text for description).

Fig. 12 includes 121 entries collected from literature. Effi-ciency distribution is widely spread from 0.49% to 79%. Thehighest values correspond mainly to arc discharge. The GATreactor achieves the top value with 79%.

The specific energy requirement data (see Fig. 13), which arerepresented on a logarithmic based scale, have a range goingfrom 3.8 up to 6907 kJ/mol. The specific energy requirementis the expression of how well the heat released by the plasmais used into the reforming reaction. A very high value for atest bench can be explained by two factors: either the non-thermal plasma technology is not appropriate for reformingapplications; either thermal losses are too large because of thetest bench design. In order to optimize this thermal effect, somereactors have been designed so that a heat exchanger transfersthe heat released in the reactive chamber to the reactant speciesprior to their introduction (for instance, the GAT reactor [7]):this heat lowers then the total energy needed for the reformingprocess and increases the overall performances.

Fig. 14 contains 113 conversion rate entries, which have beenworked out from data found in publications. Notice that someresults are greater than 100%, which might be due to someapproximations done for the gas analysis.

Conversion rate reports to how much hydrocarbons has gonethrough the plasma region, how much C–H links have been“broken”. Therefore, the homogeneous nature of the plasmazone can be evaluated. The gliding arc has been first developedfor an arc igniting and increasing in the plane of two electrodes.This two dimensional set up leads to weak conversion ratebecause a small part of the gas mixture would go through thearc. As a consequence, further improvements concerned three


Fig. 13. Specific energy requirement of non-thermal plasma processes described (see text for description).

Fig. 14. Conversion rate of non-thermal plasma processes described (see text for description).


Fig. 15. Effect of additional catalyst on conversion rate and efficiency. Isooctane reforming with a corona discharge. Plasma power: 2 W. White bar: plasmaonly/solid bar: plasma + catalyst (from [16]).

dimensional construction (plasmatron GEN 2 and 3, GAT, Gl-idarc I, plasma torch) and showed then good conversion capa-bilities. This has to be taken into consideration together withthe fact that arc discharge is a high energetic density mean. Atlast, in arc discharge system can appear a flame phenomenonthat is also involved in the conversion process.

3.2.1. Influence of the presence of an additional catalystA catalyst placed before or after the plasma process can

considerably increase the performances.For instance, the reforming of isooctane performed with the

same energy input and reactive flow rates increases from 1.91%to 47.51% in efficiency when added a pre-processing catalyst[16]—see Fig. 15. Specific energy requirements and conver-sion rate follow a similar improvement (from 977 to 15 kJ/moland from 17.51% to 75%, respectively). In this case, the cat-alytic section is a stainless steel reactor placed inside a tubularfurnace. The tube is packed with catalytic media (about 3 cm3)supported by two layers of quartz wool. The volume of catalystuse to produce 1 kW of H2 is then about 272 cm3, whereas thisvolume is 37 cm3 for pure catalytic systems [4].

Observing the significant increases of the overall per-formances, one can say that the most of the reforming isperformed thanks to the catalyst. Efficiency of the stand alonecatalyst is 39%. Thus, coupling the corona reactor to a cata-lyst increases the efficiency in the sense that plasma allows toreach higher temperatures more rapidly. The application of thecoupling of a plasma reactor with a catalyst would be the useof the plasma during transient regimes, when quick response isneeded, whereas the catalyst would be more likely to producehydrogen efficiently in permanent use.

In the field of catalyst addition, PSFC team has tried vari-ous systems such as honeycomb, ceramic or metallic catalyst

[23,28] on different fuels (diesel, methane, ethanol, soybeanoil). Again, performances of the reforming process were in-creased (around an extra 15% for both efficiency and conver-sion rate results).

The combination of a plasma reactor with a catalyst has notbeen studied yet from the view point of improvements for coldstart up and transient regimes.

3.2.2. Nitrogen oxides (NOx) productionPartial oxidation of fuel can cause NOx production when the

oxygen is brought by the air. This has to be taken into account ifthe reformer is the source of hydrogen for a fuel cell, especiallyfor car applications. Indeed, NOx are toxic substances and theirproduction is regulated.

Measuring NOx production is complicated because of thepresence of hydrogen. Anyway very few studies focusing thissubject have been published so far [5,49]. However, plasmareactors usually work in rich regimes and are consequentlylikely to produce a non-negligible amount of NOx .

Fig. 16 shows a continuous decrease of NOx production atrising air numbers, i.e. at rising air flows, due to dilution effect.Furthermore, the increase air flow induces a decreasing heatrelease: a lower flame temperature lower NOx production. Sincethe most important NOx production is a thermal one (i.e. occursat high temperatures), one can thus interpret the relative positionof the input power curves: the greater the injected power, thegreater the temperature and the greater the NOx production.

3.2.3. Transient performancesOne of the advantages of plasma assisted reforming is its

good reactivity, especially for on board applications. The in-tegration of the reformer on a vehicle needs indeed good per-formances in terms of transient time, cold startup . . . similarly


Fig. 16. NOx production during plasma reforming (from [49]) (acknowledg-ment to International Plasma Chemistry Society).

Fig. 17. Time response of H2, O2 and CH4 molar concentration for methanepartial oxidation (O/C = 2.01; Electrical power, 160 W) (from [5]).

effect to a traditional engine. The investigation of transient be-havior has not been widely performed yet (attention being fo-cused on steady state). Efforts should be made in the future onthis very important point.

The measurements of products concentrations in transientregimes can be achieved thanks to a mass spectrometer. Fig. 17shows such an analysis. H2 concentration reaches permanentregime in about 1 s [5]. It has also been proven that for sufficientO/C ratio, the reforming reaction can be self sustainable: if thepower is shut down for a few seconds after the ignition, thehydrogen production keeps going.

In our knowledge, the only work concerning transient per-formances has been performed by the PSFC group (Bromberg,Cohn, Rabinovich and co-workers [5]).

3.2.4. Summary of non-thermal plasma reforming assistedtest benches

Performances of most promising technologies in the fieldof non-thermal plasma assisted reforming are presented inTable 2. The “ratio to thermodynamic efficiency” is the effi-ciency of the reformer divided by the thermodynamic compu-tation of the efficiency. “kW H2” accounts for the power of H2produced by the reformer (considering that CO is completelyconverted into H2). This parameter should be compared withthe power needs of the fuel cell vehicle, which is approximately

160 kW. This estimation is obtained by assuming that the on-board fuel cell provides around 80 kW of electrical power withan mean efficiency of 50%. Finally, the power density is thepower of H2 delivered divided by the reactor volume. Thisvalue accounts for the compactness of the reactor. The reactorvolume does not take into account the surroundings of thereacting zone itself such as valves, thermal insulation. . . . Forcomparison purposes, the power density of a car with internalcombustion engine is about 53 kW/l (100 horse power car,cylinder chamber volume equal to 1.4 l) and the one of a fuelcell is 1 kW/l.

Even if all of the technologies presented in Table 2 are basedon arc discharge, ranges of power of fuel injected, power of hy-drogen and power density are widely spread. Best productionof H2 is achieved by MIT team (between 5.15 and 11.4 kW),with efficiencies close to thermodynamic calculations. Further-more, power density of the plasmatron GEN 3 is very good,due to its small reactor volume (235 cm3). Even if results fromFridman et al. exhibit efficiency very close to thermodynamic(93% and 97%), the weak values of fuel input power do notallow to reach power density greater than 0.476 kW/l. Experi-ments conducted at GREMI and CEP show power densities inthe order of 10 kW/l.

3.2.5. Performances of on-board catalytic reformingapplications

In order to give the reader external standards, performancesof some catalytic reformers for on-board applications are givenbelow.

Goebel et al., from General Motors Corporation, have devel-oped a fuel processor (auto-thermal reformer+water gas shift+PrOx) incorporating two burners in order to achieve rapid start-up [3]. The idea is to burn part of the fuel (here, derived fromgasoline, average composition C7.494H14.53) near combustionconditions to reach quickly high temperature levels: start timeof 20 s to 30% power and 140 s to full power was observed. Thereformer itself is a 80 mm diameter monolithic catalyst (approx-imately 250 mm long), and whit a weight of about 5.3 kg. Thewhole apparatus {ATR + WGSreactor + PrOxreactor + burners}is 34.1 kg.

Houseman and Cerini, from Jet Propulsion Lab (USA),worked on the partial oxidation of a federal test gasoline,called Indolene 30, with a nickel based catalyst [4]. Inlet fuelflow is 1.12 g/s and typical composition of outlet gas is (involume): 21.6% H2, 23.6% CO, 1% CH4, 1.2% CO2, 1.2%H2O and 51.2% N2. Dimensions of the tubular catalyst bedreactor is 25.4 cm long, 10.2 cm wide. No data on transientperformances is given.

Aidi Qui et al. [56] have developed a stand-alone integratedfuel processor, incorporating three reaction zones, i.e., an au-tothermal reformer, a high temperature WGS reactor and a lowtemperature WGS reactor, and thermally coupled with embed-ded heat exchangers, using commercial gasoline and its surro-gate n-octane as a hydrogen generator for fuel cell application.The stages are based on different catalysts, and detailed studyof the temperature evolution is provided. Outlet composition is


Table 2Performance of some non-thermal plasma assisted reforming processes

Team Reactor Fuel Fuelinjected(kW)

Reformingprocess

Plasmapower (W)

Efficiency(%)

Ratio tothermody-namic effi-ciency (%)

Conversionrate (%)

kW H2 Powerdensity(kWH2/l)

MIT [5] GEN 2 Diesel 10.78 Pox 270 46.67 80.9 5.15 2.6Methane 11 ATR+catalyst 210 63.59 84 103.98 7.14 3.57

GEN 3 Ethanol 26 Pox+catalyst 200 43.5 84 11.4 57Methane 22.5 Pox 375 36.70 57 73.38 8.4 42

Fridman Gliding arc Methane 1.52 Pox 50 75.81 97.8 87 1.19 0.476et al. [7,16] Isooctane 0.019 ATR+catalyst 4 48.78 70 75 0.011 0.004

GAT Methane Pox 200 74 93

GREMI Sliding Methane 2.68 ATR 830 24.59 40 22.53 0.86 7.8[10,22] discharge Methane 4.63 Pox 75 27.53 42 58.55 1.3 11.8

CEP [11,12] Glidarc Gasoline 7 SR 1000 29 65.8 2.24 0.86Plasma torch Isooctane 5.72 ATR 1000 41.78 72 90.55 2.8 8

ECP [41,43] Glidarc I Diesel 12.01 Pox 280 54.28 100.14 6.67 4.44Gasoline 95 6.23 Pox 350 49.06 95.78 3.23 2.15

Table 3Performances of catalytic reforming systems

Team Fuel Powerinjected(kW)

Reformingprocess

Efficiency(%)

Ratio tothermody-namic effi-ciency (%)

kW H2 Powerdensity(kWH2/l)

Startuptime

Timeto fullpower

Houseman and Cerini [4] Indolene 30 42.63 Pox, nickel based catalyst 78.5 96 33.4 16.12

Goebel et al. [3] C7.494H14.53 86.15 ATR, monolith catalyst 78 67.2 53.4 20 s 140 s

Aidu Qui et al. [56] Octane 1.44 ATR, house catalyst 68–70 1 0.4Gasoline 1.56 62–65 1 0.3 5 min 50 min

around 35% H2 (below theoretical value) and 1% CO. Catalystdeactivation is observed at high temperature.

Performances of the three catalytic reforming systems aresummarized in Table 3. These results should be compared tothose of plasma technologies given in Table 2. In general terms,plasma reformers have lower performances than catalytic. Thiscan be explained by the early stage of research on non-thermalplasma reforming. More complementary studies will must to beperformed for achieving performances comparable to catalyticreforming technologies. In addition, notice that other parame-ters such deactivation of the catalyst due to sulfur-containinggasoline feeds or coke deposition is a real issue [57] for whichplasma reforming provide an original solution. For instance,plasma reactor such as plasmatron GEN3 (MIT [5]) experiencesno soot deposition.

4. Numerical modeling

Numerical simulation is essential for the understanding andthus the further developments of plasma assisted reforming.Theoretical modeling of the reforming process deals with

thermodynamic and chemical kinetic as well as fluid flowcalculations. Discharge simulation is also investigated.

4.1. PSFC/MIT (MA, USA)

The methane partial oxidation was first analyzed with a ther-modynamic model [58].

Theoretical studies of the plasmatron were then performedin two different ways [5]: computational fluid dynamics (CFD)by using the commercial CFD code Fluent [59] and chemicalkinetic by employing the commercial code CHEMKIN, version3.7 then 4.0 [60].

First addresses to understand the average composition in theplasmatron, very inhomogeneous in time and spatially, whereasthe second focuses on the mixing effects at the molecular level.

The fluid model helped the understanding of the mixing up-stream from the reactive volume, through different configu-rations (axial or radial injection of fuel, effect of air . . .) ofmethane, propane or gasoline reforming with air.

Chemical kinetic studies were based on a two stage per-fectly stirred reactor (PSR) and a partially stirred reactor (PaSR)


models. This focused on studies of transient times. Kineticmechanism used for methane conversion was GRI 3.0 [61],having 53 species and 350 equations.

4.2. Universidade da Madeira (Funchal, Portugal)

Simulations of stationary discharges were first performedusing local thermodynamic equilibrium densities of the neutralcomponents [62,63].

Chemical kinetic processes including plasma effects havethen been studied. Plasma discharge was considered by usingtwo approaches [63]. First assumes that the gas flow separatesin two parts, one passing through the discharge region and theother not heated by the discharge. Both gas streams are mixedat the entrance of the reactor chamber. The second considersthe discharge and the reactor chamber as a whole system. Thedischarge is homogenously distributed inside the reactor andtreated as a power source. Taking into account the fact thatthe difference between the two approaches is not large, theestimation of parameters has been conducted using the secondand simplest one. Three codes from CHEMKIN II package [64](EQUIL, SENKIN and PSR) have been successively used forthe thermodynamic and kinetic analysis.

Methane reforming has been modeled with a PSR model[65] by applying two reaction mechanisms: GRI-MECH 3.0[61] and the Leeds methane oxidation mechanism version 1.5[66], observing the influence of the model selection. Methanereforming was studied by SENKIN code with the second reac-tion mechanism, without addition of power source (plasma), inorder to point out the long time needed for the reaction to beachieved.

n-Octane reforming study [65] was performed using the EN-SIC reaction mechanism [67], involving about 1700 reactionsbetween 145 species. The influence of several parameters (O/C,H2O/C, initial temperature, discharge power . . .) on H2 flowrates, composition and efficiency of the reformer have been in-vestigated. Good comparison with experimental data has beenachieved (see Fig. 18).

4.3. Center for Energy and Processes (CEP) (SophiaAntipolis, France)

n-Octane and isooctane reforming simulations were per-formed in parallel with experimental investigation. Thermody-namic analysis was conducted using T&Twinner [68], takinginto account 47 species, including solid carbon.

Chemical kinetic modeling was performed through three suc-cessive models based on CHEMKIN II [64] codes and the EN-SIC [67] reaction mechanisms for n-octane and isooctane. Thefirst model a PSR used for studying the reforming of n-octane.By using this approach, the influence of the main processingparameters such as gas mixture, flow rate and composition (O/Cand H2O/C ratios), inlet temperature, pressure, plasma powerand reactor volume, can easily be described. Second model wasa plug flow reactor used to understand the chemical compo-sition evolution of the isooctane reforming inside the reactor.

Fig. 18. Comparison of the simulation versus experimental data [65]: molarfractions of products (in %) of octane reforming as a function of O/C ratio.Lines—simulation; points—experiment (from [31]).

The third model was developed in order to achieve a close-to-experiment simulation, highlighting the catalytic effects of theplasma. The model was based on Benilov and Naidis’s work(see previous section). Reactive species flow separates in twoflows: one part goes through the plasma discharge, modeled bya PSR, while the second part is unchanged. The second stepof the model is the mixing of the two parts based on a per-fect mixing with no chemical reactions, based on calculationsof the two enthalpies and temperatures (from each part). Thelast step of the model is the time evolution in an adiabatic plugflow reactor. Difference in respect to first Benilov and Naidis’sinvestigation is the achievement of a more precise model, tak-ing into consideration the post discharge zone (with a plug flowreactor), where most of the reforming process occurs. A goodagreement with the experimental data was obtained.

4.4. Department of Mechanical Engineering of the Universityof Illinois at Chicago (IL, USA)–DPI (PA, USA)

Works of arc discharge understanding and modeling werefirst performed [69–71]. In the field of reforming units, a numer-ical code was then specially developed, including three stagesrelated to the electric discharge, the air–hydrocarbon mixingand the post discharge zone [72]. Chemical kinetic simulationof methane conversion was carried out by using CHEMKINand the reaction mechanism GRI 2.11 [61], which includes 65species and 200 reactions. The reverse-vortex reactor under-standing was investigated [73,74] with the fluid flow and heattransfer FLUENT programming package [59].

4.5. GREMI (Orléans, France)

The numerical study of the steam reforming of methanewith the rotating discharge reactor experimental set up hasbeen performed [75]. Thermodynamic calculations weredone by means of Chemical Workbench code, version 2.5


(Kinetic Technologies, Russia), using the thermodynamicallyequilibrium reactor model (TER).

4.6. Korea Institute of Science & Technology (Seoul, Korea)

Methane conversion (without adding any oxidizer) in a glid-ing arc reactor has been achieved using a chemical kinetic re-action mechanism [76] composed of 28 first order reactions,obtained and chosen from literature, numerically coded andsolved using Matlab program [77]. The effect of frequency onconversion and product yields was also investigated.

4.7. University of Cassino (Cassino, Italy)

ASPEN-PLUS software [78] was applied for thermodynamicanalysis of methane, propane, heptane, toluene and gasolinereforming [79]. The reformer was assumed to be at thermo-dynamic equilibrium and simulations were run using a Gibbsreactor.

4.8. Comments on modeling works

One can discuss the fact of using thermodynamic modelsto phenomena that are by nature in non-thermodynamic equi-librium. However, the relative simplicity of thermodynamic incomparison to kinetics calculations facilitate the modeling ofthe major parameter of the process. Thermodynamic calcula-tions are well justified in the region where the reaction time ismuch more lower that the residence time [65]. This is particu-larly true in the plasma zone, especially in the case of arc dis-charges where temperatures are typically greater than 2000 K.

Chemical kinetic modeling seems more realistic, since ittakes into account the properties of mixing and residence time.All of kinetic studies are based on chemical mechanisms de-veloped in combustion. The role of charged chemical specieshas not been studied. The relevance of a combustion kineticmechanism have been proven by Benilov and Naidis [65] (seeFig. 29) in low-current arcs discharges. However, the valid-ity to other plasma discharge remains open, more particularlyin order to rise the question whether the reforming process isdue to thermal effects or to the presence of charged species orother types.

CFD codes have been developed to help the understandingof the hydrodynamic phenomena inside the reactor, e.g. mixingof the reactive species before entering the plasma zone [5] orthe velocity field helping the establishment of the arc discharge[73]. Considering the presence of high temperature and velocitygradients inside plasma reactors and thus their influence onkinetics, an even more accurate approach will be to couple thehydrodynamic together with chemical kinetic modeling.

5. Conclusion

In the field of assisted production of hydrogen, non-thermalplasma reforming exhibits interesting results in terms of effi-ciency, conversion rate and H2 yields.

So far, most of the existing reforming reactors are still be-ing developed and further advanced in research laboratories.Most of the studies performed have concerned the developmentof the non-thermal plasma assisted reforming, through the in-vestigation of various fuels and plasma types. The review oftheir set up gives characteristic keys of non-thermal plasmaassisted reforming for H2 production such as technology andoperating conditions. In addition, comparisons of their outputs(efficiency, conversion rate and specific energy requirement)have been worked out. Test benches developed so far exhibitgood results, the best ones being rather close to thermodynamiccalculations. The evaluation of the existing non-thermal tech-nologies shows that arc discharge based ones meets the bestperformances because of their relative simplicity of set up, theirhigh energetic densities and their ability do create a large reac-tive volume.

In the field of modeling, different investigations haveachieved good understandings of the phenomenon thanks tochemical kinetics modeling, using standard oxidation mech-anisms. The next step would be the accounting of fluid flowand chemical kinetics interaction.

Non-thermal reforming for onboard applications is relevant:good H2 yields, compactness, reactive system, non-deactivationbecause of coke deposition, sulfur presence or high temper-ature. However, the technology is not mature yet and moreworks have to be made in order to compare it with existingonboard catalytic systems. The lack of information concerningmainly performances during transient regimes (cold start-up,acceleration, shutdown, . . .) and NOx production (when air isused—ATR or Pox) makes the evaluation of the technology foronboard applications difficult.

Acknowledgment

This work was supported by RENAULT SAS.

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