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Energy & Fuels 2008, 22, 1888–1893
Thermodynamics and Kinetics Analysis of Gasoline Reforming Assisted by Arc Discharge Jean-Damien Rollier, Guillaume Petitpas, José Gonzalez-Aguilar, 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 ReceiVed NoVember 6, 2007. ReVised Manuscript ReceiVed February 27, 2008
Onboard hydrogen production out of hydrocarbons for fuel cells is subject to problems when using traditional catalytic reformers. High device weight, a relatively long transient time, and catalyst poisoning all serve to make their integration in a vehicle complex. In response to these challenges, reforming processes based on cold plasma have been recently implemented . This paper presents a theoretical analysis of hydrogen production out of gasoline assisted by arc discharge. A wide range of O/C and H2O/C ratios (including partial oxidation and pure steam reforming) have been investigated, together with different forms of injected power and reactor conditions. Both thermodynamic equilibrium and kinetic calculations (e.g., perfectly stirred reactor, plug flow reactor) were performed. Thermodynamic equilibrium calculations provide theoretical upper limits of the process via input parameters, while the kinetic computations provide a more realistic estimation of both output composition and process efficiency.
Introduction A major drawback for the large-scale development of fuel cells for automotive applications is the lack of available hydrogen, at least in the short and midterm. This problem is caused by the absence of a real hydrogen distribution network and the lack of low cost, reliable, efficient, onboard hydrogen storage technologies. An alternative pathway to overcome these limitations consists in producing hydrogen on board the vehicle, starting with conventional liquid hydrocarbon fuels such as gasoline, diesel, or ethanol (available in conventional gas stations) via the so-called reforming process. It has been demonstrated in previous studies that hydrogen could be efficiently produced in compact plasma-assisted reformers by conversion of a variety of hydrocarbons.1–9 At present, study of arc discharge assisted reforming is at an early stage and even the fundamental reaction mechanisms are not clearly understood.9 In addition, arc discharge reforming is a multiparameter problem (reagents composition, reactor volume * To whom correspondence should be addressed. Telephone: +33 (0)4 97 15 74 06. Fax: +33 (0)493 95 75 35. E-mail:
[email protected]. † Technocentre Renault, DREAM/DTAA-Service 64240, 1 avenue du golf, 78288 Guyancourt Cedex, France. (1) Bromberg, L.; Cohn, D. R.; Rabinovich, A. Int. J. Hydrogen Energy 1997, 22 (1), 83. (2) Petitpas, G.; Rollier, J.-D.; Darmon, A.; Gonzalez-Aguilar, J.; Metkemeijer, R.; Fulcheri, L. Int. J. Hydrogen Energy 2007, 32 (14), 2848. (3) Bromberg, L.; Cohn D. R.; Rabinovich, A.; Alexeev, N.; Samokhin, N.; Hadidi, K.; Palaia, J.; Margarit-Bel, N. PSFC/JA-06-03, 2006. (4) Iskenderova, K.; Porshnev, P.; Gutsol, A.; Saveliev, A.; Fridman, A.; Kennedy, L.; Rufael, T. Proceedings of the 15th International Symposium on Plasma Chemistry (Orleans), 2001. (5) Kalra, C. S.; Gutsol, A. F.; Fridman, A. A. IEEE Trans. Plasma Sci. 2005, 33 (1). (6) Czernikowski, A. Oil Gas Sci. Technol.-ReV. IFP 2001, 2 (56), 181. (7) Cormier, J.-M.; Rusu, I. J. Phys D: Appl. Phys. 2001, 34, 2798. (8) El Ahmar, E.; Met, C.; Aubry, O.; Khacef, A.; Cormier, J. M. Chem. Eng. J. 2006, 116, 13. (9) Lee, D. H.; Kim, K.-T.; Cha, M. S.; Song, Y. H. Proc. Combust. Inst. 2007, 31, 3343.
and pressure, ratio of input energy to fuel heating value) for which a complete sensitivity analysis has not yet been performed. Increasing complexity levels in the chemistry modeling have already been proposed in previous studies. Lutz et al.10,11 assumed that the reaction went to completion, using a global reaction balance to provide maximum limits in the form of algebraic expressions involving O/C and H2O/C ratios (these parameters describing the initial composition will be defined later in this paper). A more realistic estimation of the output composition and thermal efficiency of the process could be provided by thermodynamic calculations based on entropy maximization (Bromberg et al.,12 Rusu and Cormier13) or Gibbs free energy minimization algorithms (Lutz et al.,10,11 Minutillo14,15). More recently, there have been papers published where the primary focus is kinetic modeling. Benilov and Naidis16 postulated that the role of the plasma lies mainly in reaction ignition, in a manner analogous to spark ignition engines. The authors considered the possibility that the major reforming reactions could be simulated with a perfectly stirred reactor (PSR) model (case of methane and n-octane). Bromberg et al.3 used a more sophisticated reactor composed of a PSR and a partially stirred reactor (PaSR) in series. Kalra et al.17 developed (10) Lutz, A. E.; Bradshaw, R. W.; Keller, J. O.; Witmer, D. E. Int. J. Hydrogen Energy 2003, 28, 159. (11) Lutz, A. E.; Bradshaw, R. W.; Bromberg, L.; Rabinovich, A. Int. J. Hydrogen Energy 2004, 29, 809. (12) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Alexeev, N. Int. J. Hydrogen Energy 1999, 24 (12), 1131. (13) Rusu, I.; Cormier, J. M. Chem. Eng. J. 2003, 91, 23. (14) Minutillo, M. Int. J. Hydrogen Energy 2005, 30, 1483. (15) Galloni, E.; Minutillo, M. Int. J. Hydrogen Energy 2007, 32 (13), 2532. (16) Benilov, M. S.; Naidis, G. V. Int. J. Hydrogen Energy 2006, 31, 769. (17) Kalra, S.; Cho, Y. I.; Gutsol, A.; Fridman, A.; Rufael, T. S.; Deshpande, V. A. Electronic Proceedings of the 2004 Technical Meeting, The Combustion Institute, University of Texas at Austin, TX, 21–23 March 2004.
10.1021/ef700665f CCC: $40.75 2008 American Chemical Society Published on Web 04/18/2008
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a numerical code to model three main process stages (discharge, mixing, postdischarge). Both authors simulated plasma-assisted reforming of methane. The present paper presents three different approaches of increasing complexity. The first model is based on thermodynamic equilibrium calculations and is used for examining the upper limits of thermal efficiency. The second and third models are based on kinetic calculations. These models allow a systematic study of the main significant parameters that influence: reactant composition, reactor volume and pressure, injected power, and inlet temperature, highlighting the most important parameters and key steps of arc discharge assisted reforming of gasoline. Note that the analyses presented in this paper do not include ionized species since, up to now, most of the reaction mechanisms derived from combustion models do not include ionized species. However, this limitation makes it possible to extrapolate toward other systems heated up by an external source as well. In a recent paper,18 we presented experimental results obtained with a low current-high voltage tip cylinder plasma torch specially developed in our laboratory for this study. Global Reactions Balances The hydrocarbon reforming is an oxidation reaction in which oxygen or water plays the role of the oxidant. Thus, two main reforming reactions are possible: partial oxidation steam reforming
n m CnHm + O2 f nCO + H2 2 2
CnHm + nH2O f nCO +
(1)
( m2 + n)H
2
(2)
The partial oxidation is strongly exothermic whereas the steam reforming is endothermic. Oxygen and steam can be mixed together to achieve a reforming reaction, e.g., to aim an autothermal reaction (where global enthalpy equals zero).The reforming operation produces a mixture of hydrogen and carbon monoxide known as synthesis gas (syngas). For low-temperature fuel cell applications, this carbon monoxide needs to be transformed into carbon dioxide since the former is an electrocatalytical poison. The hydrogen/carbon monoxide mixture is cooled down to approximately 450 °C by injecting water, and this mixture is led to a catalytic reactor where the slightly exothermic water gas shift (WGS) reaction takes place: H2O + CO f H2 + CO2
(3)
The WGS reaction refers to the chemical reaction between carbon monoxide and water producing hydrogen and carbon dioxide. This reaction is exothermic and as a consequence, thermodynamically limited at high temperatures. It is an important step in the hydrogen production and is commonly carried out by a combination of shift reactors working at different temperatures. A first WGS reactor works at high temperature (350–450 °C) on iron and chromium catalysts to promote the kinetic aspect of the reaction. A second WGS reactor works at lower temperatures (180–250 °C) on Cu and Zn catalysts to achieve thermodynamic equilibrium.19–21 From (18) Rollier, J.-D.; Petitpas, G.; Gonzalez-Aguilar, J.; Darmon, A.; Fulcheri, L.; Metkemeijer, R. Energy Fuels 2008, 22, 556. (19) Luengnaruemitchai, A.; Osuwan, A.; Gulari, E. Catal. Commun. 2003, 4, 215. (20) Utaka, T.; Sekizawa, K.; Eguchi, K. Appl. Catal. A: Gen. 2000, 194, 21. (21) Tanaka, Y.; Utaka, T.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Appl. Catal. A: Gen. 2003, 242, 287.
Table 1. Theoretical Models Characteristics model
aims
(A) thermodynamic
reaction upper limits
(B) PSR (C) PFR
residence time influence kinetics stages
input parameters
output parameters
O/C, H2O/C reactor pressure inlet temperature total mass flow rate input electric power (A) + reactor volume (A) + heating time
chemical species molar fraction outlet temperature
(A) (A) time dependent
the reaction it can be seen that 1 mol of CO gives 1 mol of H2, and in this paper the CO is considered as fully transformable into H2 without any energy outflow because of the exothermic character of the reaction. The reactant mixture composition (air, steam, and isooctane) is represented in terms of O/C and H2O/C ratios. The O/C ratio stands for the molar ratio between oxygen atoms from the air and carbon atoms of the isooctane; H2O/C expresses the molar ratio of steam on carbon atoms from isooctane. As a consequence, H2O/C ) 0 and O/C ) 1 define a partial oxidation reaction, while H2O/C ) 1 and O/C ) 0 indicate a steam reforming reaction. Modeling Approaches. Thermodynamic analysis consists in the calculation of equilibrium composition and temperature and includes the input (electric) power. This calculation is performed in the following way: Given the elemental composition defined by isooctane mass flow rate, O/C and H2O/C ratios, and the reactor pressure, the equilibrium composition is evaluated as a function of the temperature. In a second step, final equilibrium composition and temperature are calculated as a function of injected power from the following enthalpy balance: final mixture enthalpy is equal to the initial enthalpy obtained from the initial temperature Tin plus the injected power coming from the plasma. Contrary to thermodynamic modeling where residence time is assumed to be infinite, kinetic modeling considers time dependence and enables the inclusion of hypotheses that may yield results closer to experimental ones.16 Two kinetic models have been developed. In the first one, the electric discharge is accounted for as a homogeneous energy source inside the whole reactor. The plasma reformer is considered to be a perfectly stirred reactor (PSR) whereby the calculation is based on the assumption of a perfect mixing of species inside the reactor chamber. The second kinetic model includes the injection of the input power in a heterogeneous way. Energy is injected only during a period that is of shorter duration than the residence time. With this approach, the plasma reformer is considered as a plug flow reactor (PFR). Table 1 summarizes the three models and their main characteristics and input and output parameters. Thermodynamic calculations have been performed using T&Twinner software,22 which is based on global minimization of the Gibbs free energy algorithm, using the following set of chemical species: C(s), C(g), CH, CHN, CHO, CHO2, CH2, CH2O, CH2O2, CH3, CH3O, CH4, CH4O, CN, CNO, CN2, CO, CO2, C2H, C2HN, C2H2, C2H4, C2O, C8H18, H, HN, OH, HO2, H2, H2N, H2N2, H2O, H2O2, H3NO, H4N2, N, NO, NO2, NO3, N2, N2O, N2O3, N3, O, O2, O3. (22) Pateyron, B.; Delluc, G.; Calve, N. Méc. Ind. 2005, 6 (6), 651. (free download from http://www.unilim.fr/spcts)
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Figure 1. Thermal equilibrium composition of the reformate gas (molar fraction) as a function of the temperature for partial oxidation reforming (O/C ) 1, H2O/C ) 0).
Figure 2. Thermal equilibrium composition of the reformate gas (molar fraction) as a function of the temperature for steam reforming (O/C ) 0, H2O/C ) 1).
Kinetic computations have been carried out using PSR and PFR (SENKIN) codes from CHEMKIN II.23 The mechanism that was utilized is the oxidation mechanism of isooctane developed by ENSIC.24 It is composed of 353 chemical species and 1481 chemical reactions which, as has been previously discussed, include neither ionized species nor NOx production. In all models, isooctane C8H18 is used as a model molecule for gasoline. The plasma reformer performance has been analyzed in terms of energy efficiency η, which is expressed as η)
(QCO + QH2)·LHVH2 QgasolineLHVC8H18 + W
(4)
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 values of isooctane and hydrogen are 5016 kJ/mol (44 kJ/g) and 240 kJ/mol (120 kJ/g), respectively. The input electric power is expressed in the following results as a percentage of Qisooctane · LHVisooctane. (For clarity, in the following we use “% LHV” instead of “% Qisooctane · LHVisooctane”.) Equation 4 assumes that the produced CO can potentially be totally transformed into CO2 by producing an equal molar amount of H2 via the water gas shift reaction (eq 3) with zero energy cost. In the calculations presented in this paper, inlet temperature, gasoline mass flow rate, and working pressure were set at 400 K, 0.1 g/s, and 0.1 MPa, respectively, unless specified otherwise. Results Model A. Thermodynamics. For any given composition, gasoline is not completely transformed into synthesis gas unless a minimum reactor temperature is attained. Conversion temperature has been arbitrarily defined as the temperature at which synthesis gas production reaches 98% of its maximum value. Figures 1 and 2 show the reformate composition versus temperature for the cases of partial oxidation and steam reforming respectively. Note that isooctane is not present at 400 K since other hydrocarbons, mainly methane, are more stable from a thermodynamic point of view. As can be seen in the diagrams, the conversion temperature for partial oxidation and steam reforming is about 1200 K. (23) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia National Laboratories Report SAND 89-8009, 1989. (24) Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Lecrec, F.; Côme, G. M.; Scacchi, G. Int. J. Chem. Kinet. 1998, 30, 949.
Figure 3. Conversion temperature versus O/C and H2O/C ratios.
The role of reactant composition has been studied by varying H2O/C and O/C ratios in the ranges {0, 2.5} and {0, 2}, respectively. Figure 3 illustrates the conversion temperature in the studied ranges. The conversion temperature is almost constant at approximately 1200 K when H2O/C and O/C ratios are such that H2O/C + O/C < 1. This quasi-constant temperature region constitutes a triangle whose summits correspond to the three elemental reforming reactions: cracking (H2O/C ) O/C ) 0), partial oxidation (H2O/C ) 0, O/C ) 1), and steam reforming (H2O/C ) 1, O/C ) 0). Increasing the oxygen content causes the production of CO2 and steam to the detriment of the synthesis gas and the diminution of the final temperature. The conversion temperature decreases abruptly at H2O/C + O/C > 1. Figure 4 shows the energy efficiency (calculated with eq 4) as a function of O/C and H2O/C ratios. The white dashed line indicates the level at which maximum energy efficiency is obtained. At O/C ratios higher than 1, the energy efficiency no longer depends on the H2O/C ratio. It shrinks down to zero at O/C ) 3.125 (not shown in Figure 4), which corresponds to complete combustion of the isooctane. This behavior can be explained by the major role of O2 in the reforming process. Without steam (H2O/C ) 0), energy efficiency reaches its maximum at O/C ) 1, which indicates a pure partial oxidation. Steam addition (H2O/C > 0) leads to higher levels of energy efficiency, since steam reforming reaction provides more hydrogen per mole of hydrocarbon than partial oxidation. However, two conditions are needed to promote steam reforming: the presence of hydrocarbon molecules to react with water (O/C < 1) and a temperature high enough to transform these remaining hydro-
Modeling Gasoline Arc Discharge Reforming
Figure 4. Thermodynamic energy efficiency versus O/C and H2O/C ratios for an input power of 10% LHV.
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Figure 6. Highest energy efficiencies in the range of O/C between 0 and 2 and H2O/C between 0 and 2.5 as a function of the input power.
Figure 5. Thermodynamic energy efficiency versus O/C ratio at various input powers (H2O/C ) 1).
carbons into synthesis gas. Given an input power, the O/C ratio must go beyond a threshold value in order to achieve a final temperature higher than the conversion temperature as well as to convert the remaining hydrocarbon via steam reforming. The highest level of conversion will be achieved under such conditions. Input power supports the exothermic reactions in order to increase temperature thus improving the gasoline conversion. Figure 5, which shows the energy efficiency versus O/C ratio, illustrates the role of the input power. The optimal O/C ratio value decreases with increasing injected power. All curves tend to zero at O/C ) 3.125 independently of the input power. For each electric power, the energy efficiency curve is composed of an increasing arch followed by a decreasing one forming a bell. In the first part, inlet steam is at sufficient levels to react with gasoline that has not been consumed by partial oxidation. However, the temperature, which is lower than the conversion temperature, does not allow for the total steam reforming of remaining hydrocarbons. In the decreasing arch, energy efficiency is a decreasing function of the O/C ratio. At O/C < 1, part of the hydrocarbons which could have been converted into syngas by steam reforming preferentially reacts with oxygen. Thus, the energy efficiency decreases since partial oxidation provides less hydrogen per mole of hydrocarbon than steam reforming. At O/C > 1, decreasing efficiency is due to oxidation of H2 and CO into H2O and CO2. The maximum energy efficiency corresponds to the case where the remaining hydrocarbon molecules, not consumed by partial oxidation, are completely converted by steam reforming. The highest energy efficiency corresponds to pure steam reforming (O/C ) 0) and, in the particular case of H2O/C ) 1, an injected power of 38% LHV.
Figure 7. O/C and H2O/C ratios leading to the highest energy efficiencies as a function of the input power.
Figure 6 shows the energy efficiency maxima as a function of the injected electric power for O/C and H2O/C ratios in the range comprised between 0 and 2 as well as 0 and 2.5, respectively. Figure 7 illustrates the O/C and H2O/C ratios that lead to those values. Note: each couple of O/C and H2O/C corresponds to a maximum of energy efficiency. The high dispersion for H2O/C can be explained by a very flat slope of the optimal curve in the H2O/C direction (poor sensitivity of this parameter at the vicinity of the optimum). The dispersion of the data is directly linked with the optimization algorithm sensitivity with the calculation grid. Increasing the injected power makes it possible to decrease the O/C ratio and the gasoline fraction consumed by partial oxidation (exothermic) decreases, thus promoting the steam reforming reaction (endothermic). Above 38% LHV, the hydrocarbon is totally converted by steam reforming and the additional power is only used for heating the products, thus reducing the energy efficiency. From a thermodynamic point of view, the optimal energy efficiency is reached when the injected power is high enough for the mixture to attain a temperature equal to conversion temperature. Among all the three considered parameters (O/C, H2O/C, injected power) and with an isooctane mass flow rate of 0.1 g · s-1 at atmospheric pressure, the highest energy efficiency corresponds to 86%. This is reached with the followings operating conditions: O/C ) 0, H2O/C ) 1.5, and injected power corresponding to 38% LHV. Model B. Perfectly Stirred Reactor (PSR). The difference in results from those obtained through thermodynamic calculations can be attributed to residence time and final temperature. At constant fuel flow rate, residence time is directly related to the reactor volume and pressure. Both parameters are essential
1892 Energy & Fuels, Vol. 22, No. 3, 2008
Figure 8. Partial oxidation energy efficiency versus O/C ratio (no steam) at 0.2 MPa and an input power of 20% LHV.
Figure 9. Autothermal energy efficiency (H2O/C ) 1) versus O/C ratio at 0.2 MPa and an input power of 20% LHV.
Figure 10. Thermodynamic and kinetic energy efficiencies versus pressure under autothermal reforming conditions (O/C ) 0.5, H2O/C ) 0.5) in a 3 L reactor and an input power of 20% LHV.
for a reforming system in onboard applications in which the space required must be as small as possible. Figures 8 and 9 illustrate the reactor volume influence. In general, kinetic calculations show the same trends as those of the thermodynamic calculations. However, the energy efficiencies are usually lower due to the residence time and the temperature effects. As expected, kinetic and thermodynamic energy efficiencies get close when either the reactor volume or the O/C ratio increases. In the first case, it is primarily due to the increase of residence time. In the second case, this can be explained by the acceleration of the reaction promoted by the higher temperature. In addition, there is a correspondence between higher H2O/C ratios and lower energy efficiency. This is due to the low steam reforming reaction rate. At low O/C ratios, the H2O/C ratio influence is more significant since it decreases the temperature and thus the kinetic rates. Thermodynamically, the energy efficiency decreases with the pressure, as seen in Figure 10. Following Le Chatelier’s Principle, the gas mixture tends to shift its equilibrium position to counteract the effects of pressure. Therefore, pressure increase
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Figure 11. Energy efficiency versus input power under autothermal reforming conditions (H2O/C ) 2).
is balanced by minimizing the number of moles. Thus, the reactants side is promoted with respect to the products side and the energy efficiency decreases. Kinetically, pressure increase enhances kinetics for a given volume, since it permits longer residence times. Deviation between thermodynamics and kinetics becomes more pronounced when conditions tend to steam reforming. Consequently, an efficient way to increase efficiency without enlarging the reactor size (for compactness purposes) could consist in pressurizing the reactor. Energy efficiency increases markedly along with the inlet temperature. An increase of the initial temperature from 400 to 950 K implies a 20% energy efficiency rise. Even though it might prove interesting to operate at high inlet temperatures, the energy cost of preheating has not been taken into account here. Figure 11 shows the influence of the input power on the energy efficiency. The reactor temperature is a function of the injected electrical power on the one hand and the chemical reactions on the other. Higher efficiencies are reached at low O/C ratios and high levels of injected power, so it looks more promising to work with such conditions. Furthermore, the lower the O/C ratio, the lower the amount of air required as well as temperature. This explains the lower NOx production. However, once the overall system is taken into consideration, this may not necessarily be the optimal solution, especially considering the cost of onboard electricity production. This first approach allowed for distinguishing between the different parameters and to model their influence on the output efficiency. The steam reforming reaction, which is the most interesting from a thermodynamic point of view, suffers however from very slow kinetics. A way to increase the reaction rate toward final state is to increase temperature and/or the residence time inside the reactor. The reactor temperature is a function of the injected electrical power and the chemical reactions (i.e., initial composition; O/C and H2O/C ratios). The residence time is a function of pressure, total flow rate, and the reactor volume. Optimal efficiency is thus the result of a highly complex relation between those various parameters. Model C. Plug Flow Reactor (PFR) with Localized Injected Energy. With this model, special attention is paid to the composition variation inside the reactor and the phenomena involved. Figure 12 represents the evolution of temperature and gas mixture composition as a function of the time. In this calculation, chemical species have been heated for 5 s with a total input power of 200 W, and adiabatic conditions were obained afterward. The mixture evolution analysis leads to identify four main stages.
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undergo oxidization are ethylene (C2H4) and methanol (CH3OH). Radicals are partly (H, CH3, OH) or totally (HO2) consumed. Finally, the last stage concerns the steam reforming from 3.4 s onward. Its beginning is indicated by the absence of molecular oxygen. The heaviest hydrocarbons are the first to react with steam. The input power and the inlet temperature control the duration of the first step. It takes 3.4 s to reach 900 K with 200 W, while only 0.6 s are necessary with 1000 W. The steam reforming stage is usually the longest due to its low kinetic rate. Conclusion
Figure 12. Evolution of temperature and chemical species molar fraction as function of the time, corresponding to an input power of 200 W during 5 s (O/C ) 1, H2O/C ) 2, P ) 0.2 MPa): (top) main products; (center) C8H18, O2, H2O, CH4, C2H2, C2H4, CH3OH; (bottom) HO2, H, OH, CH3, CHO, CH2OH.
The first one, the duration of which is around 3.2 s, corresponds to the reactants heating and the isooctane cracking. Below 900 K, input power heats up the reactive species without producing any significant chemical reactions. Radicals such as HO2, OH, H, CH3, and light hydrocarbons (CH4, C2H2, C2H4) are formed in low concentrations. The second stage consists of radical concentration increase. In Figure 12, it is shown to occur between 3.2 and 3.4 s. At 900 K, only oxygen and methanol start being consumed while light hydrocarbons are continuously being produced. Radical production speeds up until the threshold when C8H18 is suddenly consumed by oxygen. Isooctane partial oxidation constitutes the third stage. Around 3.4 s, chain reactions corresponding to C8H18 partial oxidation take place. A sharp temperature spike from 1000 to 1485 K in 20 ms characterizes this stage. H2, CO, CO2, CH4, and C2H2 are produced, whereas hydrocarbons are transformed from the heavier to the lighter until there is total O2 consumption. In the case presented in Figure 12, this occurs before CH4 and C2H2 have been totally converted. The last chemical species that
Arc discharge assisted reforming of gasoline is a complex phenomenon. Listed below are the various findings and observations made from the study conducted. 1. Thermodynamically, a 86% maximum energy efficiency can be obtained at atmospheric pressure in the case of pure steam reforming with an injected power level equal to 38% LHV. 2. Even though the thermodynamic analysis shows that steam reforming leads to higher H2 yields, kinetics shows that the reaction requires either large residence times and/or high injected power levels. 3. The reactor temperature and gasoline conversion increase together with the O/C ratio and the injected power. However, this does not necessarily imply higher energy efficiency, because excess energy leads to exiguous heating of the mixture. 4. The higher the inlet temperature (between 400 and 1000 K), the higher the energy efficiency. 5. The lower the H2O/C ratio and the greater the reactor volume and pressure (higher residence time), the smaller the deviation between kinetic and thermodynamic equilibrium. 6. Assuming optimal conditions, partial oxidation (H2O/C ) 0) and steam reforming (O/C ) 0) may have energy efficiency levels between 60% and 70%, which are close to equilibrium. 7. Reforming reaction is composed of four stages, which are cracking, radical production, partial oxidation, and then steam reforming. 8. This multiparameter study shows that optimal performances are functions of, in decreasing degree of significance and from a kinetics point of view, O/C ratio, injected power, H2O/C ratio, reactor volume, entrance temperature, and pressure. 9. Introduction of ionized species may have a strong influence on the results. This very challenging task, yet to be performed, should provide more insight on the understanding of the arc discharge interaction with chemical phenomena. The nonthermal aspects of the process would also be clarified. As a general conclusion, we can say that model A would be more appropriately used to draw the upper limit of the process and evaluate its potential application, while the aim of models B and C is to provide information relevant for the designing of tools (reactor size, residence time, mean temperature). Better accuracy could be reached with more complex models, coupled with fluid dynamics equations and accounting for ionized species. These represent the next challenging steps for future research. The next task will be the comparison of modeling works with experimental results, in order to discuss the pertinence of the models. Acknowledgment. The authors gratefully acknowledge the financial support of Renault SAS. EF700665F
