Energy Fuels 2010, 24, 2607–2613 Published on Web 03/25/2010
: DOI:10.1021/ef100022r
Ethanol and E85 Reforming Assisted by a Non-thermal Arc Discharge Guillaume Petitpas, Jose Gonzalez-Aguilar,† Adeline Darmon,‡ and Laurent Fulcheri* Center for Energy and Processes, MINES ParisTech, 06904 Sophia Antipolis, France †IMDEA Energı´a, c/Tulip an s/n, 28933 M ostoles, Madrid, Spain. ‡Renault SAS, DREAM-DTAA, 1 avenue du Golf, 78288 Guyancourt, France. Received January 6, 2010. Revised Manuscript Received March 8, 2010
Ethanol reforming could provide an interesting path for on-board hydrogen production from renewable resources for fuel cell powering. This paper presents a study on hydrogen production from both pure ethanol and E85 with a plasma reactor. Reforming of ethanol was systemically tested with a non-thermal arc discharge system based on a high voltage/low current power source. A short review on ethanol reforming is first presented as a reference point for this study. Effects of supplied power for plasma generation, air/fuel ratio, and addition of water were then experimentally investigated. Those results were at last compared with a 1D multistage model, exhibiting good correlation. In spite of the early stage of research, a fair conversion rate and fair H2 yields have been achieved (90% and 65%, respectively). The optimal conditions for ethanol reforming are found to be at an oxygen ratio higher than the reaction’s stoichiometry, with a slight addition of steam. Discussion is provided concerning the non-thermal plasma effect on the ethanol reforming.
Table 1. Reforming Reaction Enthalpies of Methane, Gasoline, and Ethanol19
Introduction A major drawback for the large-scale development of fuel cells for automotive applications is the commercial unavailability of hydrogen to the end users, at least at short- and midterms. This problem is caused by the absence of a real hydrogen distribution network and the lack of low cost, efficient, onboard hydrogen storage technologies. An alternative way to overcome these limitations consists in the onboard production of hydrogen from conventional liquid hydrocarbon fuels such as gasoline or diesel (available via existing gas stations) thanks to a process named reforming. Several studies on plasma assisted gasoline reforming have been carried out in our group.1-3 The growing importance of ethanol (and its various blends) as a commercially available fuel [more than 1500 fuel stations in Europe,4 about 2000 in the U.S.A.,5 and between 3 and 4 million ethanol-powered vehicles in Brazil] makes us consider the fuel as an alternative source of hydrogen. The use of renewable agrofuels;or biofuels (first, second, or third generation);in transportation is getting more and more attractive. Although there are still open discussions about the overall energy and carbon
balances concerning the use of agrofuels,6-8 it is generally agreed that the use of agrofuels can lead to energy and environmental benefits. Numerous studies have been focused on the optimization of catalytic ethanol steam reforming (see the review in ref 9). It appears that the catalyst choice plays a crucial role in hydrogen production: Rh and Ni are so far the best and the most commonly used catalysts for ethanol steam reforming, achieving 100% ethanol conversion and up to 89.1% hydrogen selectivity. Simulation of integrated systems with solid oxide and proton exchange membrane fuel cells has been performed, showing promising results.10-18 An important drawback of steam reforming is the fact that the reaction is strongly endothermic (see Table 1). Because external heat is required, reactor designs are typically limited both by heat transfer and by reaction kinetics. Techniques to increase heat transfer, such
*To whom correspondence should be addressed. Phone: þ33 (0)4 97 95 74 06. Fax: þ33 (0)4 93 95 75 35. E-mail: laurent.fulcheri@ mines-paristech.fr. (1) Rollier, J.-D.; Gonzalez-Aguilar, J.; Petitpas, G.; Darmon, A.; Fulcheri, L.; Metkemeijer, R. Energy Fuels 2008, 22 (1), 556. (2) Rollier, J.-D.; Petitpas, G.; Gonzalez-Aguilar, J.; Darmon, A.; Fulcheri, L.; Metkemeijer, R. Energy Fuels 2008, 22, 1888. (3) Gonzalez-Aguilar, J.; Petitpas, G.; Lebouvier, A.; Rollier, J.-D.; Darmon, A.; Fulcheri, L. Energy Fuels 2009, 23 (5), 4931. (4) Roulons Propre. http://www.roulonspropre-roulonsnature.com/ carte_europe/ [In French] (accessed Mar 2010). (5) Growth Energy Market Development. http://www.e85refueling. com/ (accessed Mar 2010). ~ ez, E. E.; Castillo, (6) Escobar, J. C.; Lora, E. S.; Venturini, O. J.; Yan E. F.; Almazan, O. Renewable Sustainable Energy Rev. 2009, 13, 1275. (7) Almeida D’Agosto, M.; Ribeiro, S. Renewable Sustainable Energy Rev. 2009, 13, 1137. (8) Fatih Demirbas, M.; Balat, M.; Balat, H. Energy Convers. Manage. 2009, 50, 1746.
(9) Ni, M.; Leung, D.; Leung, M. Int. J. Hydrogen Energy 2007, 32, 3238. (10) Arteaga, L. E.; Peralta, L. M.; Kafarov, V.; Casas, Y.; Gonzales, E. Chem. Eng. J. 2008, 36, 256. (11) Laosiripojana, N.; Assabumrungrat, S. J. Power Sources 2007, 163, 943. (12) Jamsak, W.; Assabumrungrat, S.; Douglas, P.; Laosiripojana, N.; Charojrochkul, S. Chem. Eng. J. 2006, 119, 11. (13) Hernandez, L.; Kafarov, V.; Marquardt, W.; Pantelides, C. Comput.-Aided Chem. Eng. 2006, 21, 1131. (14) Assabumrungrat, S.; Pavarajarn, V.; Charojrochkul, S.; Laosiripojana, N. Chem. Eng. Sci. 2004, 59, 6015. (15) Song, S.; Wang, Y.; Shen, P. Chin. J. Catal. 2007, 28, 752. (16) Francesconi, J. A.; Mussati, M. C.; Mato, R. O.; Aguirre, P. A. J. Power Sources 2007, 167, 151. (17) Giunta, P.; Mosquera, C.; Amadeo, N.; Laborde, M. J. Power Sources 2007, 164, 336. (18) Perna, A. Int. J. Hydrogen Energy 2007, 32, 1811. (19) Salge, J.; Deluga, G.; Schmidt, L. J. Catal. 2005, 235, 69.
r 2010 American Chemical Society
partial oxidation steam reforming
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methane
gasoline
ethanol
-35.7 kJ/mol 206 kJ/mol
-675 kJ/mol 1258 kJ/mol
14 kJ/mol 256 kJ/mol
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Table 2. Catalytic Partial Oxidation of Ethanol: Performances from the Literature Salge et al. Silva et al. Hsu et al. Mattos and Noronha Liguras et al. a
catalyst
Tfurnace (K)
eth conversion
Rh-Ce Ru/Y2O3 Pt/ZrO2 Pt/ZrO2 Pt/CeO2 Pt/CeO2 Ni/La2O3
1073 370 573 573 673 873
>95% 100% 51% 80% 70-80% 100% 100%
H2 selectivity
O/C
>80% 59% 97%
0.5 1
up to 97%
0.4 0.61a
ref 19 20 21 22 23 24 25
Here, steam has been added to reach higher H2 selectivity (H2O/ethanol = 3).
as the use of microchannel reactor systems or catalytic wall reactors, are being studied. The use of multiple feeds (fuel, steam) and expensive materials, together with catalyst deactivation due to carbon deposition, makes steam reforming unsuitable for transportation applications. Ethanol autothermal reforming and partial oxidation have been investigated more recently. Unlike most hydrocarbons, the ethanol partial oxidation is slightly endothermic (standard enthalpies for reforming of methane, gasoline, and ethanol are reported in Table 1), which implies an additional energy input. This is all the more necessary with partial oxidation combined with steam reforming. One can observe that the term “autothermal” is not appropriate here since thermal balance cannot be equal to zero when both partial oxidation and steam reforming are used, since the two chemical reactions are endothermic. Several studies have reported that ethanol conversion and hydrogen selectivity by autothermal reforming (or, let’s say, “combined” reforming) greatly vary with the nature of the catalyst, support, and oxygen/steam/ethanol molar ratios (see review in ref 9). A few studies on catalytic partial oxidation have been published and are summarized in Table 2. Total conversions are reached at temperatures higher than 673 K. O/C ratio stands for the molar ratio between oxygen atoms from the air and carbon atoms of the fuel (2 for the case of ethanol). No data on the overall energy efficiency of the reforming process have been found. However, that performance is of great importance when considering on-board application. Considerations like size and weight requirements, cost, catalyst poisoning (coke or soot), and limitations on time response have led to consideration of non-equilibrium plasma assisted reformers as a relevant alternative to the classical catalytic on-board application, as discussed in 26. Very few works on plasma assisted ethanol reforming have been published. Bromberg et al.27 studied ethanol partial oxidation with a second-generation plasmatron packed with a nickelbased catalyst on an alumina substrate. The authors stressed that homogeneous reformation of ethanol was difficult to achieve because of the presence of oxygen in the molecule, leading to a very mildly exothermic reaction. They found that the highest level of hydrogen production occurred at a higher O/C ratio than theoretically expected by the stoichiometry
balance of the global reaction (1.6 instead of 1). As a consequence, they claimed that the oxygen bounded in the fuel did not contribute to the exothermal character of the reaction, as the oxygen is already bound either to carbon or to hydrogen. The energy consumption varied between 8 and 10 MJ per kilogram of hydrogen for 270 W of electrical power input into plasma. The opacity measured during ethanol reforming was 0.1, which indicates that only very little soot was produced. The effect of plasma versus the catalytic counterpart has been studied.28 Two sets of values were presented, with and without plasma (ethanol flow rate: 0.58 g/s). It is interesting to note that, at high values of O/C, there was a relatively small impact on the presence of the plasma. Reformat composition at O/C > 2.1 is comparable with and without plasma. Tests here were still carried out with a plasmatron reformer that incorporated a nickel catalyst on a crushed alumina substrate. At last, tests without a catalyst gave 40% of the hydrogen production, while C2H4 was observed (4-5%). Aubry et al.29 studied ethanol steam reforming with a batch reactor, continuously fed with the mixture so that the bottom electrode was always submerged. A high electrode surface temperature ensured the mixture vaporization. The output H2 molar fraction laid between 60 and 72%. Steam to ethanol ratios were lower or equal to 0.7, while reaction stoichiometry occurs near unity. At last, dry and steam reforming of ethanol have been investigated with a dielectric barrier discharge (DBD) reactor in the work of Sarmiento et al.,30 achieving a conversion rate near 100%, with steam plus ethanol flow rates lower than 100 sccm. In the present study, reforming of ethanol and E85 has been investigated using a high voltage/low current plasma torch. Effects of electrical supplied power for plasma generation, the air/fuel ratio, and the addition of steam were systematically investigated, and consequent results on the conversion rate, selectivity, and efficiency were discussed. A kinetic analysis is proposed. Experimental Section The experimental arrangement used in this work was initially applied in gasoline reforming.2 It is briefly described here for clarity (see sketch in Figure 1). The reactor was composed of a non-thermal arc plasma torch, a post-discharge zone, and a cooling system. The plasma torch had a tip-cylinder configuration. The power supply uses a resonant converter technology31 and enables a continuous control of
(20) Silva, A.; Barandas, A.; Costa, L.; Borges, L.; Mattos, L.; Noronha, F. Catal. Today 2007, 129, 297. (21) Hsu, S.; Bi, J.; Wang, W; Yeh, C.; Wang, C. Int. J. Hydrogen Energy 2008, 33, 693. (22) Mattos, L.; Noronha, F. J. Power Sources 2005, 145, 10. (23) Mattos, L.; Noronha, F. J. Power Sources 2005, 152, 50. (24) Mattos, L.; Noronha, F. J. Catal. 2005, 233, 453. (25) Liguras, D. K.; Goundani, K.; Verykios, X. E. J. Power Sources 2004, 130, 30. (26) Petitpas, G.; Rollier, J.-D.; Darmon, A.; Gonzalez-Aguilar, J.; Metkemeijer, R.; Fulcheri, L. Int. J. Hydrogen Energy 2007, 32 (14), 2848. (27) Hadidi, K.; Bromberg, L.; Cohn, D.; Rabinovitch, A.; Alexeev, N.; Samokhin, A. Internal Report, PSFC-JA-03-28, 2003.
(28) Bromberg, L.; Cohn, D.; Rabinovitch, A.; Alexeev, A.; Samokhin, A.; Hadidi, K.; Palaia, J.; Margarit-Bel, N. Internal Report, PSFC-JA-063, 2006. (29) Aubry, O.; Met, C.; Khacef, A.; Cormier, J.-M. Chem. Eng. J. 2005, 106, 241. (30) Sarmiento, B.; Javier, J.; Viera, I.; Gonzalez-Elipe, A.; Cotrino, J.; Rico, V. J. Power Sources 2007, 169, 140. (31) Fulcheri, L.; Rollier, J.-D.; Gonzalez-Aguilar, J. Plasma Sources Sci. Technol. 2006, 6, 183.
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Petitpas et al. Table 3. Main Physical Properties of Gasoline, Ethanol (from ref 32), and E85 (computed) density (g/cm3) low heating value (kJ/mol) molecular weight (g/mol) empirical formula
gasoline
ethanol
E85
0.7 5061.6 114 C8H18
0.794 1234.9 46.07 C2H5OH
0.787 2995.8 50.03 C2.36H5.78O0.94
species “x”, while n stands for the number of C atoms in the empirical formula of the fuel (CnHmOp). Figure 1. Schematic of the experimental setup.
the arc current with a high accuracy in the range 200-660 mA and a maximum voltage of 15 kV. A gas feeding system supplied the gas mixture to the reactor. This device produced mixtures composed of fuel vapor, air, and steam in a controlled way. The air flow rate was controlled in the range 15-50 slpm using a mass flow controller, while fuel and water mass flow rates were controlled in their liquid phase via rotameters in the range 0.05-0.2 g/s 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. The reformed gas was dried by a chiller, then went through two analyzers set in series. The reformate gas composition was first analyzed using a nondispersive infrared (NDIR) sensor and a thermal conductivity detector (TCD) analysis bench (Rosemount, NGA 2000), which enables a continuous monitoring of H2, CO, CO2, and CH4 molar fractions (dry sample). The second analyzer was a gas phase chromatograph (Perichrom PR 2100). The measurement performed on the NDIR-TCD analysis bench is based on the assumption that outlet dry gas does not contain gases other than nitrogen and the above-mentioned. This assumption was verified by comparison with gas phase chromatograph analyses. The gas phase chromatograph contained two independent analysis channels. The first channel was used to measure hydrogen. It was equipped with a molecular sieve 5A column and a TCD. The carrier gas used was argon. The second channel measured CO, CO2, CH4, and C2 hydrocarbons. Its features consisted of a Poraplot U column and a molecular sieve 5A column, a methanizer, a TCD, and a flame ionization detector (FID) and used helium as a carrier gas. The methanizer option enables the FID to detect levels of CO and CO2. The fuels that were used for this study are ethanol and E85. The E85 experimental tests have been conducted with a blend of 15 vol % regular gasoline (commercially available unleaded 95) and 85 vol % ethanol. Table 3 reports the main characteristics of these three enumerated fuels. Isooctane has a similar structure to gasoline and is therefore used as its surrogate for calculations. The reactant mixture composition (air, steam, and fuel) 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 fuel. Equation 1 shows the balance for partial oxidation of ethanol (nitrogen is not represented), and eq 2 derives the O/C ratio from the molar ratio of air and ethanol. Accordingly, the stoichiometric balance for partial oxidation of ethanol corresponds to O/C = 0.5 (O/C = 1 for gasoline, whose surrogate is isooctane, O/C = 0.6 for E85). In the following equations, nx stands for the number of moles of
C2 H5 OH þ 0:5O2 f 3H2 þ 2CO
ð1Þ
O 2no2 0:21nair ¼ ¼ C 2nC2 H5 OH nC2 H5 OH
ð2Þ
Similarly H2O/C expresses the molar ratio of steam on carbon atoms from the fuel. The corresponding chemical balance is the steam reforming reaction (eq 3), for which the stoichiometry H2O/C ratio is 0.5 for ethanol, 1 for gasoline, and 0.6 for E85 (eq 4). C2 H5 OH þ H2 O f 4H2 þ 2CO
ð3Þ
H2 O nH2 O ¼ C 2nC2 H5 OH
ð4Þ
Plasma reformer performances were analyzed in terms of energy efficiency η and conversion rate χ, which are expressed as ðQ þ QCO ÞLHVH2 ð5Þ η ¼ H2 Qfuel LHVfuel þ W χ ¼
QCO þ QCO2 þ QCH4 nQfuel
ð6Þ
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 in the discharge. The input electric power is expressed in terms of the percentage of Qfuel 3 LHVfuel, and is noted as %LHV. Equation 5 assumes that the CO produced can potentially be totally transformed into CO2 by producing an equal molar amount of H2 via the water gas shift reaction with zero energy cost. The conversion rate (eq 6) accounts for the fuel transformation and is an indicator of the mass balance on carbon atoms. In the results presented in this paper, the inlet temperature and working pressure were set to 400 K and 0.1 MPa, respectively. For each measurement reported here, the system is considered to be at a thermal steady state, which is monitored by various thermocouples (type K). A waiting time of 5 to 15 min is generally observed in order to allow the system to reach this equilibrium. Results and Discussion Ethanol Reforming. Several ethanol reforming tests were performed in order to assess the effects of reaction conditions, such as fuel flow rate, input plasma power, and O/C (oxygen to carbon) and H2O/C (steam to carbon) ratios. The first two inputs are extensive parameters of the system. As a consequence, a set of preliminary tests (data not shown here) helped us determine the fuel flow rate and plasma power as 0.25 g/s and 1200 W (18%LHV), respectively. The space (32) Ballerini, D.; Alazard-Toux, N. IFP Publications, Editions Technip, 2006.
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Figure 2. Thermodynamics equilibrium (Gibbs free energy minimization) on the energy efficiency variations as a function of different reactants ratios, for the reforming of ethanol. Fuel flow rate and input electrical plasma power are held constant: Qfuel = 0.25 g/s, P = 1200 W, respectively.
Figure 3. Experimental results (Conversion rate (9) and energy efficiency (b) versus reactants ratios) of ethanol reforming. Fuel flow rate and input electrical plasma power are held constant: Qfuel = 0.25 g/s, P = 1200 W, respectively.
velocity ranged from 250 to 830 cm3/s (standard), depending on the H2O/C and O/C ratios. The fuel flow rate and plasma power are held constant during the subsequent tests. Before selecting optimal experimental conditions such as H2O/C and O/C ratios, preliminary calculations of thermodynamics equilibrium (see Figure 2) were performed using the EQUIL module from the CHEMKIN II package.33 This algorithm is based on Gibbs free energy minimization (see ref 1 for more details on this modeling approach). For a given flow rate and plasma power, pure steam reforming and pure partial oxidation modes give highest energy efficiencies at stoichiometry: about 61% at H2O/C = 0.5 and 82% at O/C = 0.5, respectively. Combined steam reforming/partial oxidation modes could achieve even better performances; thus another set of calculations has been performed around those ratios. Energy efficiencies as high as 85% can be reached (H2O/C = 0.5, O/C = 0.5). We observe that the system is not very sensitive to a steam ratio between 0 and 2 at constant O/C, then drops as more steam is added. At varying O/C (constant H2O/C = 0.5), the effect of oxygen is more noticeable as the conversion and the temperature increase together
with the injection of the oxidant. The production of H2O and CO2 is an increasing function of oxygen and limits synthesis gas concentration. As a result, the energy efficiency drops after O/C = 0.5. On the basis of the results from thermodynamics equilibrium calculations, the influence of initial composition was investigated via two sets of experimental tests: varying H2O/C at a constant O/C = 0.5 and vice versa. Figure 3 shows experimental results in terms of conversion rate and energy efficiency. Conversion rate and energy efficiency are respectively lower than 65% and 35%, while output H2 concentration varies between 22 and 30%, depending of the cases. The energy consumption lies between 53 and 60 MJ per kilogram of hydrogen, and the hydrogen yield is about 35-40%. With increasing H2O/C ratio, we observe an H2 concentration decrease from 30 to 22% while CO variation is less sensitive (concentration stays between 20 and 18%). Hydrogen yields and gas concentrations are of the same magnitude as the data reported by Bromberg et al. 27. Furthermore, no coke deposition has been observed after two hours of operation, contrary to deposition we usually observed with plasma assisted reforming of gasoline. Those tests are preliminary results that first showed that nonthermal plasma assisted reforming of ethanol could be achieved with the test bench, prior to any optimization step.
(33) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia National Laboratories Report, SAND 89-8009, 1989.
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Figure 4. Inside reactor picture of pure ethanol reforming (O/C = 1, Qethanol = 0.25 g/s, P = 1200 W). One can observe the tip of the nozzle on the left-hand side, while the flame-like phenomenon is the plasma extinction. The actual arc is confined inside the nozzle, as the nozzle’s length is too long for the arc to lengthen outside the nozzle.
Figure 6. Experimental dry output molar fraction of H2, CO, CO2, and CH4 for the partial oxidation of E85, at 20%LHV (mass flow of E85 set to 0.19 g/s), as a function of the O/C ratio.
should be produced. On the other hand, a higher flow rate induces less reaction time. The E85 flow rate was then held constant at 0.19 g/s. This value corresponds to a fuel heating value of 5.7 kW, similar to the gasoline heating value as used in plasma assisted gasoline reforming in our previous experiments. The O/C ratio was varied at constant energy input (20%LHV) from O/C = 0.55 to 1.30. Dry output molar flow rates of the main products (H2, CO, CO2, and CH4) as a function of the O/C ratio are reported in Figure 6 for the partial oxidation test. An extra O/C ratio (O/C = 2.05) is added in order to show the trend at a high O/C (“rich” mixture). At a low O/C, the fuel is scarcely oxidized. Not enough O2 molecules break the E85 molecule apart, resulting in a weak conversion (low rate of production of H2 and CO). As the amount of oxidizer in the mixture increases, more heat is brought in, and the main products’ composition shifts from CH4 to H2 and CO to CO2. The oxidation of the mixture is still low (hence the term “partial oxidation”), and the main products are partially oxidized molecules: H2 and CO. CO2 production is a direct function of the oxidation of the mixture, as long as the O/C ratio is kept below the combustion equivalence ratio (O/C = 2.83 for E85). The production of H2 reaches a peak around O/C =1, while the peak of CO occurs at higher O/C (about 1.16). At O/C = 1.16, the CO2 production rate undergoes an acceleration: most of the C atoms from the E85 molecules are converted into CO2 at O/C ratios greater than 1.7-2. Even if the H2O production could not be monitored here, the H2 production fall after O/C ∼ 1 very likely coincides with an increase of H2O (oxidized molecule) as the O/C ratio comes closer to combustion conditions. Figures 7 and 8 show the conversion rate, the energy efficiency, and the hydrogen yield as a function of the O/C ratio. The conversion rate is a growing function of the O/C ratio because the O/C increment promotes CO2 production after the syngas peak synthesis (see Figure 6). The highest synthesis gas (H2 þ CO) production is reached for an initial mixture close to O/C = 1.16, which corresponds to 47% energy efficiency and a 92% conversion rate. A H2 yield close to 70% was found, as seen in Figure 8, while the energy consumption varies between 30 and 50 MJ/kg of H2. No coke deposition was observed. One of the most interesting results here is the fact that the peak efficiency is reached at an O/C greater than the stoichiometric result (O/C = 1.16 versus O/C = 0.71), contrary to what thermodynamics calculations say (not shown here for E85, but behaviors very similar to the peaks observed on Figure 2).
Figure 5. Experimental results of the influence of the fuel flow rate on E85 partial oxidation, at various O/C ratios. The ratio of the plasma electrical power to the injected fuel heating value (%LHV) is maintained at approximately 20%.
However, Figure 4 shows that the nozzle undergoes high heating (red/orange skin): the arc is confined inside the nozzle. What the figure actually shows is the post plasma “recombining” zone. In a fully developed regime, the arc cathode spot should be located at the outer tip of the nozzle, as sketched on Figure 1. Subsequent modification of the setup for further E85 tests was then the shortening of the nozzle’s length from 75 mm to 55 mm. E85 Reforming: Experimental Results. The experimental effort focused then on E85 reforming, a fuel more likely usable on the market. Previous work on plasma-assisted reforming of gasoline (both theoretical1 and experimental2) was used for comparison. The multifuel capability of the plasma reformer, i.e., the possibility to efficiently produce hydrogen from different fuels, is a feature worth studying for future onboard application. Partial Oxidation of E85. Partial oxidation represents an interesting path for onboard E85 reforming applications, as it would not require multiple feeds (the only oxidant is the dioxygen from air) and would require less energy than steam reforming to maintain continuous plasma operation. On the other hand, the presence of N2 at high temperatures might result in the formation of nitrogen oxides (NOx). This aspect is beyond the scope of this study but should be addressed in the future. The influence of E85 flow rate on the energy efficiency has first been investigated with various O/C ratios at constant % LHV (ratio of plasma power to injected fuel heating value). One can observe in Figure 5 that energy efficiency decreases together with the fuel flow rate, for all the O/C ratios tested. This is the result of two opposite effects, the second one being predominant. On one hand, more energy is brought to the system as the fuel flow rate increases, so more synthesis gas 2611
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Figure 9. Experimental investigation of the influence of the H2O/C ratio on the reforming performances. Conversion rate, H2 yield, and energy efficiency (O/C = 1.16, 20%LHV).
Figure 7. Experimental conversion rate (9) and energy efficiency (b) as a function of the O/C ratios, for the partial oxidation of E85. Fuel flow rate is 0.19 g/s and %LHV is 20%.
Figure 10. Experimental investigation of the influence of the H2O/C ratio on the energy efficiency for various O/C ratios. Fuel flow rate is 0.19 g/s, and %LHV is 20%.
Figure 8. Experimental H2 yields versus O/C ratio. For the partial oxidation of E85, the fuel flow rate is 0.19 g/s and %LHV is 20%.
“Combined” Reforming of E85. In order to maximize the hydrogen production rate of E85, the addition of steam has been studied. First, combined reforming tests were thus conducted at constant O/C (1.16, as it is the best ratio in partial oxidation mode) and varying H2O/C. Figure 9 illustrates the evolution of reformer performances as a function of H2O/C. The highest hydrogen production was found to be around H2O/C = 0.2. The partial oxidation has a much quicker reaction rate than the steam reforming mode, as discussed in ref 1. As a consequence, working at such an O/C ratio might prevent any steam reaction from occurring, as the fuel is virtually converted by the O2 only. Steam reforming reactions, if any, should thus be given more “opportunities” to occur by decreasing the oxidizing power of air, i.e., decreasing the O/C ratio. Following tests on steam addition were thus realized at O/C = 0.59, 0.7, 0.82, 0.94, 1.05, and 1.16. As can be observed in Figure 10, the best conditions are at an O/C ratio slightly lower than for partial oxidation (1.05 instead of 1.16) and at a relatively small H2O/C ratio (about 0.2). With these initial ratios, energy efficiency, conversion rate, H2 yield, and energy consumption are 47%, 92%, 68%, and 40 MJ/kg H2, respectively. Steam Reforming of E85. The last experimental result concerns the pure steam reforming of E85. Although steam reforming is less advantageous in terms of system complexity and integration (need for an external heat source and water feed in), its higher hydrogen yield could compensate those drawbacks. The experimental investigation highlighted the strong endothermic behavior of the reaction, resulting in conditions difficult to handle with a plasma gas discharge. As a
Table 4. Outlet Dry Molar Fraction as Read from the Gas Chromatograph, in %, for the Steam Reforming of E85 composition H2 CO CO2 CH4 C2 O2 N2 total
55.54 17.8 5.9 5.58 10.5 0.9 3.37 99.65
consequence, only one data point was obtained (H2O/C = 0.81), with an input electrical power of about 20%LHV (E85 mass flow rate: 0.19 g/s). The outlet dry composition obtained via the gas chromatograph is reported in Table 4. Molecules of C2 (hydrocarbons with two carbon atoms) and traces of air (O2 and N2) are observed, enabling very good knowledge of the output composition (99.65% of the dry sample is characterized). Comparison with the Phenomenological Model. The partial oxidation of E85 was simulated by means of a 1D phenomenological model previously developed for octane reforming.3 The model assumes that a fraction of the reactants’ inlet flow passes through the arc discharge, and there is no mass transfer between the plasma zone and the remaining gas inside the nozzle. A perfectly stirred reactor (PSR) receiving an input heating power equal to the electric power describes the plasma region. Cold and hot streams are instantaneously mixed at the plasma torch exit. Mixture temperature is calculated from the global enthalpy balance from the PSR outflow and the remaining cold gas. Finally, a plug flow reactor (PFR) models the post discharge zone. The model 2612
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Petitpas et al.
hand, the non-thermal plasma does not break apart all the chemical bonds down to the single species. It action just focuses on one part of the species that enhances the reaction kinetics (electrons, charged radicals). A proof of principle of such an assumption would be some temperature measurements inside of the plasma zone. Even though some spectroscopic techniques are being implemented in the laboratory, the geometry and the high number of species made it difficult to achieve. One way to shift this ideal ratio to the stoichiometric side would be to increase the mixture temperature, by increasing the input plasma power for example. This would result in an increasing syngas yield on one hand, but in a dropping energy efficiency on the other hand. On the basis of that observation and our kinetic model investigation (see above), the following main steps are proposed for the ethanol decomposition under plasma reforming process: 1 ð7Þ CH3 CH2 OH f CH3 CH2 O þ H2 2
Figure 11. Partial oxidation of E85: model vs experiment, as a function of O/C ratios. Simulations have been calculated using the iso-octane mechanism. The fraction of reactant inlet that goes to the plasma is 6.25%. The residence time in the plasma zone varied between 1.42 and 1.72 10-4 s.
was implemented in FORTRAN code using the PSR and SENKIN modules of the CHEMKIN II33 package. The use of two kinetic oxidation mechanisms was investigated: the n-octane and the isooctane mechanisms both developed by Glaude et al.34 The two mechanisms include ethanol in their species. The n-octane mechanism is composed by 146 chemical species and 899 chemical reactions and the iso-octane mechanism by 160 species and 1540 chemical reactions. The octane rating for E85 is “105”, whereas n-octane is “-10”, and iso-octane is “100”. Consequently, the iso-octane mechanism showed the best correlation with experimental results, as depicted in Figure 11.
1 CH3 CH2 O f CH3 CHO þ H2 2
ð8Þ
1 CH3 CHO þ O f CH3 COO þ H2 2
ð9Þ
1 CH3 CH2 O f CH4 þ CO þ H2 2
ð10Þ
OH þ CH3 COO f CH4 þ CO þ O
ð11Þ
Discussion
Conclusion and Perspectives
As shown in this paper, the oxygen bound in the ethanol molecule seems to be a minor contributor to the plasma assisted reforming reaction: its presence does not decrease the necessary amount of oxygen atoms (coming from air) to convert the molecule. This result corroborates the observation from Bromberg et al.27 Similarly, thermodynamics modeling and catalytic studies (see Table 2) show that the ideal air ratio should be at O/C = 0.6, whereas our experimental results show a much higher ideal ratio. The ethanol molecule is already partially oxidized: its lower heating value (LHV) is 26.81 MJ/kg versus 47.80 MJ/kg for ethane, its “paraffin’s counterpart”. In other words, part of the ethanol molecule is nonreactive to the oxidizing mixture. Thermodynamics modeling assumes equilibrium based on the chemical species (C, H, O, N) as a function of the temperature (i.e., all chemicals bounds are broken). Consequently, it does not matter where the oxygen atom comes from (from the molecule or from the air). Catalytic and thermal partial oxidations occur at high temperatures, at which the decomposition of the molecules is important, similarly to what is happening in the thermodynamics modeling. On the other
The reforming of pure ethanol and E85 has been investigated using a non-thermal high voltage-low current arc plasma torch. First, results show that it is possible to produce hydrogen from three fuels (gasoline;previous paper1-3, pure ethanol, and E85) with minor modifications of the setup (nozzle length) and at a fuel heating value of 5.7 kW. This characteristic is an important advantage of the nonthermal plasma assisted technology, showing its multifuel capability. Conversion rates obtained with partial oxidation of E85 are somewhat close to catalytic results (see Table 2): about 67% hydrogen yield and 90% conversion rate (O/C∼1.16). It is worth noting that, with plasma assisted reforming, the oxygen bound in the ethanol from the fuel seems to not contribute to the exothermal character of the reaction, which is not the case in catalytic application. This shift helps understand the mechanisms of the nonthermal plasma assisted reforming. The addition of steam proved to give slightly better performances at H2O/C ∼ 0.2. Pure steam reforming was difficult to maintain and is therefore not relevant for our automotive application. Acknowledgment. The authors gratefully acknowledge the financial support of Renault SAS and ADEME (French Agency for Energy and Environment Management).
(34) Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc, F.; C^ ome, G. M.; Scacchi, G. Int. J. Chem. Kinetics 1998, 30, 949.
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