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Energy & Fuels 2008, 22, 556–560
Experimental Study on Gasoline Reforming Assisted by Nonthermal Arc Discharge Jean-Damien Rollier,† José Gonzalez-Aguilar,*,† Guillaume Petitpas,† Adeline Darmon,‡ Laurent Fulcheri,† and Rudolf Metkemeijer† Center for Energy and Processes, Ecole des Mines de Paris, Rue Claude Daunesse BP 207, 06904 Sophia Antipolis Cedex, France, and Technocentre Renault, DREAM/DTAA - SerVice 68240, 1 aVenue du golf, 78288 Guyancourt Cedex, France ReceiVed September 10, 2007. ReVised Manuscript ReceiVed October 31, 2007
On-board hydrogen production out of hydrocarbons reforming for fuel cells feed-in is subject to problems when using traditional catalytic reformers. High device weight, a relatively long transient time, and catalyst poisoning make their integration in a vehicle complex. In response to these challenges, reforming processes based on cold plasma have been implemented over recent years. This paper presents a nonthermal arc discharge system based on a high voltage, low current power source (about 2 kV and 0.5 A), designed to convert gasoline into hydrogen rich gas under autothermal or partial oxidation conditions for car applications.
Introduction A major drawback for the large-scale development of fuel cells for automotive applications is the commercial unavailability of hydrogen by the end users at least at short- and midterm. This problem is caused by the absence of a real hydrogen distribution network and the lack of low cost, efficient, on-board hydrogen storage technologies. An alternative way to overcome these limitations consists of the on-board production of hydrogen from conventional liquid hydrocarbon fuels such as gasoline, diesel, or ethanol (available via widespread existing gas stations) thanks to a process named reforming. Fuel reforming, particularly methane, is a well-developed technique that has been widely employed for many years at the industrial scale for hydrogen production. Some studies have been devoted to on-board catalytic gasoline reforming since the 1970s. Houseman and Cerini1 studied the partial oxidation of Indolene 30, a U.S. federal test gasoline whose atomic hydrogen-to-carbon ratio close to 1.92, with a nickel-based catalyst. Efficiency was found to be equal to 78.5%, which should be compared to the thermodynamic limit of 98%, with an equivalent input fuel power equal to 42.63 kWth. No information was given on transient performances. Goebel et al.2 developed a fuel processor comprising an autothermal reforming, a water gas shift, and a partial oxidation stage, designed to reform a fuel whose average formula was C7.494H14.53. The device achieved a 78% efficiency with an equivalent input fuel power of 86.16 kWth. The authors claimed a 20 s start-up time and 140 s to full power. Qui et al.3 presented a stand-alone integrated fuel processor for the octane and gasoline reforming. The efficiency ranged from 62 to 70% with a quite low equivalent input fuel power (1.44 and 1.56 * To whom correspondence should be addressed. Phone: +33 4 97 15 70 52. Fax: +33 4 93 95 75 35. E-mail:
[email protected]. † Ecole des Mines de Paris. ‡ Technocentre Renault. (1) Houseman, J.; Cerini, D. J. SAE paper 740600, 1974. (2) Goebel, S. G.; Miller, D. P.; Pettit, W. H.; Cartwright, M. D. Int. J. Hydrogen Energy 2005, 30, 953. (3) Qi, A.; Wang, S.; Fu, G.; Wu, D. J. Power Sources 2006, 2 (22), 1254.
kWth). The start-up and full-time power times were 5 and 50 min, respectively. Recently, Renault and Nuvera Fuel Cells presented a compact on-board fuel processor for use on a medium-size car.4,5 The system was able to operate up to 200 kWth and hydrogen efficiencies above 77% at a working pressure of 3 bar. The start-up time was less than 4 min. Considerations like size and weight requirements, catalyst poisoning (for instance, due to sulfur presence), and limitation on time response have led to consider nonequilibrium plasma assisted reformers as a real alternative to the classical on-board application.6 In the field of gasoline reforming, Sobacchi et al.7 coupled a negative pulsed corona discharge and a catalytic reactor to enhance the partial oxidation reforming of isooctane, which is a common surrogate fuel of gasoline, at atmospheric pressure. The authors pointed out the synergy of the coupled system, which produced higher levels of hydrogen than each one of the individual treatments (49% energy efficiency). However, the input fuel power was equal to 0.019 kWth, which was extremely lower than that required for on-board applications. Czernikowski et al.8 applied a gliding arc reactor, named Glidarc I, for reforming commercial gasoline 95. Partial oxidation (Pox) reforming conditions gave performances similar to previously mentioned work with a higher injected fuel power (6.23 kWth). However, the authors did not provide any information on arc discharge stability. Paulmier and Fulcheri9 developed a gliding arc reactor for steam and autothermal reforming of California Gasoline-Syntroleum (whose average (4) Mitchell, W.; Bowers, B. J.; Garnier, C.; Boudjemaa, F. J. Power Sources 2006, 154, 489. (5) Bowers, B. J.; Zhao, J. L.; Ruffo, M.; Khan, R.; Dattatraya, D.; Dushman, N.; Beziat, J.-C.; Boudjemaa, F. Int. J. Hydrogen Energy 2007, 32, 953. (6) Petitpas, G.; Rollier, J.-D.; Darmon, A.; Gonzalez-Aguilar, J.; Metkemeijer, J.; Fulcheri, L. Int. J. Hydrogen Energy 2007, 32, 2848. (7) Sobacchi, M. G.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A.; Ahmed, S.; Krause, T Int. J. Hydrogen Energy 2002, 27, 635. (8) Czernichowski, M.; Czernichowski, P.; Czernichowski, A. Proceedings of France Deutschland fuel cell conference on “material, engineering, systems, applications”, Oct 7–10, 2002. (9) Paulmier, T.; Fulcheri, L. Chem. Eng. J. 2005, 106, 59.
10.1021/ef700540v CCC: $40.75 2008 American Chemical Society Published on Web 12/06/2007
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Figure 1. Diagram of the experimental setup. Table 1. Test Series Characteristic Results (Qfuel ) 0.13 g/s, P ) 0.1 MPa, O/C ) 0.8, H2O/C ) 0.43) dry reformate composition (molar fraction in %) geometry
H2
CO
CO2
CH4
1 (D ) 15 mm) 2 (D ) 8 mm)
13.8 16.6
11.4 16.5
7.2 4.8
3.6 1.2
plasma temperature power (W) (K) 500 700
1120 1410
chemical formula is C7H15.2). The authors indicated a low performance of the device even though the fuel inputs were 7 kWth. This paper aims to present a new plasma reformer, whose performance is close to on-board reforming requirements. The device is based on a low current, high voltage nonthermal arc discharge, and it has been developed at the Center for Energy and Processes (CEP) of Ecole des Mines de Paris in collaboration with Renault. Experimental Section A schematic view of the experimental setup is depicted in Figure 1. The reactor, which is shown in Figure 2, was composed of a nonthermal arc plasma torch, a postdischarge zone, and a cooling system. The plasma torch had a tip-cylinder configuration. It operated in nontransferred mode with inversed polarity. The anode was a 2 mm diameter electrode located on the axis, while the cathode was a stainless steel cylinder with an axial internal passage. In the experiments, various cathodes with different inner channel geometries were tested. The postdischarge zone consisted of a cylindrical chamber of 25 mm inner diameter and 710 mm long aligned with the plasma torch axis. The chamber wall was composed of a 25 mm thickness alumina pipe, in order to attain a high thermal insulation. The axial temperature profile was measured with four K-type thermocouples. The first thermocouple was located at 15 cm from the plasma torch exit. The wall temperature in the middle of the postdischarge zone was also recorded with a fifth K-type thermocouple. A pressure gauge was used for monitoring the reactor pressure. The power supply, which was specially developed for the application, was based on a resonant converter technology.10 Contrary to classical high voltage transformers currently used for similar applications, this power supply allowed a continuous control of the arc current to be provided with a high accuracy in the range 200–660 mA and a maximum voltage of 15 kV. Discharge voltage (10) Fulcheri, L.; Rollier, J.-D.; Gonzalez-Aguilar, J. Plasma Sources Sci. Technol. 2006, 6, 183.
and current measurements were performed using a 1:1000 probe (Elditest, GE3830) and a Hall effect current probe (PR 30 LEM). The electrical signals were analyzed by a two-channel digital oscilloscope (HP 54615 B). A gas feeding system supplied the gas mixture to the reactor. This device produced mixtures composed of gasoline vapor, air, and steam in a controlled way. Air flow rate was controlled in the range 15–50 slpm using a mass flow controller (Brooks, 5851S), while gasoline and water mass flow rates were controlled in their liquid phase via rotameters in the range 0.05–0.2 and 0–0.1 g/s, respectively. The reactants were injected tangentially in a mixing chamber located right in front of the anode tip. The inlet temperature could be freely adjusted between ambient temperature and 500 K. Commercially available gasoline (unleaded 95) was used. Reformate gas composition was analyzed using an NDIR-TCD analysis bench (Rosemount, NGA 2000), which allows continuous monitoring of H2, CO, CO2, and CH4 molar fractions (dry sample). The principle of the measurement is based on the assumption that outlet dry gas does not contain other gases than nitrogen and the above-mentioned ones. This assumption was verified by gas analysis with gas phase chromatography. Thermocouples, the oscilloscope, and the gas analyzer were connected to a data acquisition system, HP34970A, which was linked to a personal computer for recording and analyzing data.
Results and Discussion 1. Nozzle Geometry. A first series of experiments was conducted with a tubular cathode of 15 mm inner diameter and a length of 100 mm. As seen in Table 1, fair H2 and CO yields were obtained, demonstrating the high potential of the process under development. However, these experiments were characterized by high power supply instabilities and important erosion on the electrodes, especially the cathode. Such a result was attributed to the hydrodynamics inside the cathode. The gas velocity was too weak inside the nozzle for blowing out the arc, and this remained near to the near anode region. As a result, the chain reaction mechanisms occurring in the partial oxidation of the gasoline were confined in a small volume inside the plasma torch. This situation engendered a series of explosions (and the corresponding shock waves) inside the nozzle that eventually disturbed the arc discharge and damaged the electrodes. In order to avoid these shortcomings, a new cathode having another internal geometry was elaborated. It consisted of a 100 mm long channel with an 8 mm inner diameter and ending with a converging-diverging nozzle. Reducing the internal diameter, the mean gas velocity inside the nozzle was increased and then
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the drag force on the arc. In addition, the converging-diverging nozzle allowed providing a favorable region for the arc root attachment increasing the velocity locally at the nozzle neck. These modifications improved the plasma reformer operating function, preventing electrode erosion, reaching stable discharges, and increasing the reformer performance, as shown in Table 1. 2. Arc Regimes. Figure 3 shows the dry molar fraction of the output reformate gas as a function of time for autothermal reforming conditions (O/C ) 0.8 and H2O/C ) 0.43) and a 700 W net input electric power. As seen in the figure, mean dry volume fractions were almost constant with fluctuations lower than 1%. The carbon monoxide, hydrogen, carbon dioxide, and methane mean molar concentrations were 16.5, 16.6, 4.8, and 1.2%, respectively. Experiments have shown that the reformate composition was very sensitive to the electric discharge regime. Best performances were achieved in the continuous regime. A previous study showed that three different discharge regimes could be obtained by varying the mean arc current and the gas flow rate.10 Figure 4 illustrates the three domains, named the streamer, gliding arc, and continuous regimes, for the second geometry working with dry air. Given a gas flow rate, the streamer regime is produced at low current, the continuous regime at high current, and the gliding arc regime at intermediate current. The boundaries between these different regimes are found to increase toward higher currents when increasing the gas flow rate. The streamer regime is characterized by high frequency discharges (typically higher than 2 kHz) confined close to the anode. The gliding arc regime, which is associated with a lower frequency (500 Hz), corresponds to a repetitive process in which an arc starts between the shortest interelectrode distance and spreads by gliding along the cathode until it extinguishes at a certain length. In the continuous regime, the electric arc has a quasi-constant length and the anodic arc spot remains on the central electrode while the cathodic spot moves freely on the nozzle tip. The gasoline conversion efficiency decreased when the electric discharge regime was changing from continuous to gliding arc and from gliding to streamer. This decreases to the electrical power reduction associated with the regime alteration. 3. Influence of O/C and H2O/C Ratios. The reactant mixture composition (air, steam, and gasoline) is represented in terms of O/C and H2O/C ratios. H2O/C ) 0 and O/C ) 1 define a complete partial oxidation reaction, while H2O/C ) 1 and O/C ) 0 indicate a steam reforming reaction. In order to get higher H2 yields, one would preferentially increase the H2O/C ratio. However, the endothermic nature of steam reforming reaction makes it necessary to balance with partial oxidation, which is exothermic, thus giving autothermal conditions. In general terms, the reforming is more sensitive to the O/C ratio rather than to the H2O/C ratio. Therefore, it was decided to focus first on the influence of the O/C ratio and perform a more precise H2O/C analysis afterward. Figure 5 presents the dry molar fraction of the reformate composition as a function of the O/C ratio. These results have been obtained for H2O/C ) 0.43 and an electrical power corresponding to 20% of the lower heating value of the inlet gasoline mass flow rate. Error bars are mainly due to fluctuations in the reformate composition. As can be seen in this figure, both hydrogen and carbon monoxide molar fractions decrease from 23 to 17% and from 20 to 17%, respectively, when the O/C ratio increases from 0.91 to 1.26. Carbon dioxide increases slightly from 3 to 5%, and the methane molar fraction volume
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Figure 2. Scheme of the plasma reactor: (I) plasma torch; (II) postdischarge chamber.
remains almost constant around 2% in the same O/C range. Increasing the O/C ratio promotes the combustion reactions (production of water and carbon dioxide) and a rise in temperature. The first thermocouple indicates an increment from 1300 to 1540 K. As a result, the syngas content of reformate gas is oxidized and their molar fraction decreases. The plasma reformer performances have been analyzed in terms of the conversion rate and the energy efficiency. The energy efficiency, η, is expressed as η)
(QCO + QH2)LHVH2 QgasolineLHVC8H18 + W
(1)
where Qi and LHVi are the molar flow rate and the lower heating value of chemical species i, respectively, and W is the net electrical power applied for maintaining the discharge. In this formula, the lower heating value of gasoline is taken to be that of isooctane (44 × 106 J/kg). Equation 1 assumes that the produced CO can potentially be totally transformed into H2 by the water gas shift (WGS) reaction with zero energy cost. In on-board hydrogen production units, WGS transformation is usually carried out in a second stage in series with the reformer and followed by a final CO reduction in a preferential oxidation reactor (PrOx). So far, there has been intensive research on onboard WGS, mainly on high performance catalysts,11 and some experimental units have been reported.4 The gasoline conversion rate, τ, is defined by τ)
QCO + QCO2 + QCH4 8Qgasoline
(2)
Here, the number 8 stands for the number of atomic carbons contained in the gasoline molecule, which is represented by isooctane. The energy efficiency gives information on synthesis gas production in terms of available energy, while the conversion rate evaluates the efficiency of gasoline cracking. Figure 6 illustrates the gasoline energy efficiency and conversion rate corresponding to compositions of Figure 5. Increasing the O/C ratio causes an opposite effect on both parameters. As the hydrocarbon oxidation dominates, a high gasoline conversion is achieved with a low efficiency, since CO2 content increases. Note that the conversion rates increase from 75 to nearly 90% in the studied range, while the energy efficiency remains between 40 and 50%. The highest energy efficiency equal to 48% was obtained for an O/C ratio around 1, which should be compared with 67% obtained by thermodynamics calculations. As a consequence, this value was retained for the following tests with steam. The reformate composition is plotted against the H2O/C ratio in Figure 7. These results have been obtained with O/C ) 1 and an electrical power corresponding to 12% of the inlet gasoline lower heating value. Moving from low H2O/C ratio, a smooth increase of syngas production occurs when H2O/C varies from 0 (partial oxidation only) to 0.2. Then, when increasing (11) Pasel, J.; Cremer, P.; Wegner, B.; Peters, R.; Stolten; D., J. Power Sources 2004, 126, 112.
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Figure 3. Evolution in time evolution of the output dry reformate molar fraction for O/C ) 0.8 and H2O/C ) 0.43 and a 700 W net input electric power. Figure 6. Efficiency and conversion rate variations versus O/C ratio with H2O/C ) 0.43 and a net electrical power corresponding to 20% of the inlet gasoline low heating value: (O) experimental conversion rate; (b) experimental energy efficiency; ()) thermodynamic equilibrium energy efficiency.
Figure 4. Different discharge regimes versus mean current and dry air mass flow rate with second anode geometry (from ref 10).
H2O/C ) 0.2. In the following range, both the energy efficiency and the conversion rate decrease when the H2O/C ratio increases. This decrease is explained by the reformate composition, whose content in H2 and CO goes down. In the interval, the conversion rate remains higher than 70%. Thermodynamic equilibrium energy efficiency is almost constant in this interval and about 67.8%. Parameters study on initial composition (O/C and H2O/C ratios) shows that the best performances are obtained when O/C ) 1 and H2O/C ) 0.2, under which the plasma reformer works under autothermal conditions. As the electric power released by the plasma together with the partial oxidation (exothermic reaction) balances the heat losses and the energy needed to achieve the steam reforming reaction (endothermic), one can expect a better performance after improving the reactor insulation, which will make it possible to operate at higher H2O/C ratios. Conclusion and Perspectives
Figure 5. Output reformate molar fraction versus O/C ratio with H2O/C ) 0.43 and an electrical power of 20% of inlet gasoline low heating value: (O) H2; (b) CO; (4) CO2; (2) CH4.
the H2O/C ratio, a slight drop of hydrogen and carbon monoxide together with a smooth rise of CO2 and CH4 have been observed. This is explained by the reaction temperature that decreases when the H2O/C ratio increases. It should be pointed out that, at O/C ) 1, gasoline is preferentially oxidized by oxygen from the air. The addition of steam will cause mainly a reduction of the temperature due to endothermic steam reforming. This diminution, which is about 150 K, reduces the rate of conversion due to the high activation energy of the forward water gas shift reaction. Figure 8 presents the energy efficiency and the gasoline conversion rate as a function of the H2O/C ratio. According to observations from Figure 7, maximum efficiency is reached at
The reforming of unleaded commercial gasoline has been investigated using a nonthermal high voltage, low current arc plasma torch. The results presented in this paper show that the plasma reformer is able to generate synthesis gas under stationary conditions. In addition, good gasoline conversion rates and energy efficiencies have been achieved in the explored range of the O/C ratio with respect to other plasma reformers. In particular, the highest performances were τ ) 80% and η ) 42% at O/C ) 1 and H2O/C ) 0.2. These rates have been attained before any optimization of the plasma device. It is expected that the improvements on the plasma reformer, such as thermal insulation and hydrodynamic optimization, will increase the synthesis gas production. In the present device, the important heat losses are balanced with the exothermic reactions (partial oxidation reforming), which are not favorable for endothermic steam reforming. Improving the thermal insulation will allows one to take advantage of the steam reforming contribution increasing the hydrogen production. With respect to thermal plasma technologies, the nonthermal technology makes it possible to greatly reduce the specific electrical energy consumption and the electrode wear, which are of paramount importance when it comes to industrial development. Moreover, the size allows its application for onboard applications. Compared with catalytic reformers, plasma
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Figure 7. Output reformate composition versus H2O/C ratio with O/C ) 1 and a net electrical power corresponding to 12% of the inlet gasoline low heating value: (O) H2; (b) CO; (4) CO2; (2) CH4.
devices have so far lower performances, which is explained by the early stage of research on nonthermal plasma reforming and the need of an external energy input (electricity). However, nonthermal plasma technologies have the potential to be ahead of catalytic reformers in terms of time response, catalyst poisoning, and multifuel requirements. Only further work for improving and optimizing the plasma technology for on-board
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Figure 8. Efficiency and conversion rate variations versus H2O/C ratio with O/C ) 1 and a net electrical power corresponding to 12% of the inlet gasoline low heating value: (O) conversion rate; (b) energy efficiency.
specific applications will elucidate the nonthermal plasma technology alternative. Acknowledgment. The authors gratefully acknowledge the financial support of Renault SAS. EF700540V
