RESEARCH NOTE pubs.acs.org/IECR
Hydrogen Generation by Pulsed Gliding Arc Discharge Plasma with Sprays of Alcohol Solutions Radu Burlica,† Kai-Yuan Shih,‡ Bogdan Hnatiuc,† and Bruce R. Locke‡,* † ‡
Technical University “Gh.Asachi” Iasi, Iasi, Romania Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Florida State University, Tallahassee, Florida, United States ABSTRACT: The formation rates and energy yields of H2 formed from methanol, propanol, and ethanol solutions exposed to a nonthermal pulsed plasma-gliding arc reactor equipped with a spray nozzle were determined. The H2 energy yield formation was the highest with Ar carrier and pure methanol giving 176 ( 25% kW h at the highest liquid flow rate. This value is higher than other reported plasma technologies and within error of the energy yields for more conventional thermal reforming. Liquid droplets can enhance energy yield; however, the low chemical yields of H2 suggest the need for reactor operation with a recycle stream.
’ INTRODUCTION A wide variety of methods (e.g., partial oxidation, thermal, catalytic, and plasma reforming, and pyrolysis) and feed stocks (e.g., oil, natural gas, coal, liquid fuels such as gasoline, diesel, and liquid alcohols, biomass, and water) have been investigated for the production of H2.1,2 Of these methods, plasma processes may provide significant advantages in terms of fast reactions and energy efficiency particularly for use in mobile applications.3 Plasma processes including corona, microwave, dielectric barrier, gliding arc, and plasmatron differ in their electrode types and configurations, power supply properties, and ultimately their plasma properties (e.g., electron energy and density).4 While thermal plasma is well suited for incineration, pyrolysis, and large scale generation of syngas from biomass and other solid materials,5,6 nonthermal plasma may be suited for smaller scale energy efficient generation of hydrogen especially from liquid fuels.3 The reforming of alcohols, including methanol and ethanol, to produce H2 has been investigated in a wide range of plasma processes including dielectric barrier discharge,7 surface wave discharge,8 microwave plasma,9 AC discharge,1012 glow discharge plasma electrolysis,13 ferroelectric packed bed silent discharge,14,15 and other plasma systems.1618 Alcohols, particularly methanol, can provide significant advantages as a liquid fuel for hydrogen generation, including a high hydrogen to carbon ratio, low boiling point, readily biodegradable, low temperature for conversion to H2, no sulfur content, and high miscibility with water.1 Although one of the major advantages of using alcohols, either pure or in aqueous solutions, is the convenience, as well as the high density, of the liquid solutions, most of these plasma processes use sufficiently high temperatures to vaporize the liquid in the plasma or to vaporize the liquid prior to feeding them into the plasma. Generally this cost must be included in an analysis of the total energy cost for hydrogen production; indeed in thermal/catalytic steam reforming, the energy to vaporize water and methanol to the high operating temperatures (e.g., 400 °C) is a major fraction of the total.19 In previous work, we have shown that H2 and H2O2 can be efficiently generated from water droplets flowing through a low r 2011 American Chemical Society
power (0.30.4 W) pulsed plasma gliding arc reactor.20,21 The energy yield for H2 generation in a carrier gas of argon was very close to a kinetically controlled limit that in turn was about 45% of the thermodynamic limit22 for the direct conversion of H2O into H2 and H2O2. The nonthermal plasma reactor was of sufficient power to initiate water dissociation reactions, but the energy was not large enough to vaporize the water droplets. Thus, it was conjectured that the condensed water droplets improved the energy yields of both H2 and H2O2 by reducing the effects of radical reactions that limit product yield. Since the thermodynamic limits of the energy yields for H2 generation from some organic fuels such methanol23 are higher than those of direct conversion from water the goal of the work reported in this paper is to determine the formation rates and energy yields of H2 formation from various alcohol solutions when spraying the liquid droplets into the plasma. All solutions are sprayed into a low power gliding arc reactor with an argon carrier gas and with different solution flow rates and concentrations of alcohol in aqueous solutions.
’ EXPERIMENTAL SECTION The gliding arc reactor, power supply, and injection nozzle are identical to those used in previous work.20,21 In the experimental setup, Figure 1, the electrodes are connected to a pulse power supply (25 kV load free), 1.5W (input power) and 250 Hz frequency), which is made from an automobile ignition coil (VW-AG Germany), driven by a pulse generator (BK Precision 4010A). In the present work a high pressure pulse injection pump (Eldex 5kpsi/140 mL/min) was used to deliver the liquid in pulses of 0.2 mL and a variation of the solution flow rate was made by adjusting the pump frequency. The temperature of the liquid at the outlet of the reactor was 12 °C higher than the Received: September 17, 2010 Accepted: June 22, 2011 Revised: June 16, 2011 Published: June 29, 2011 9466
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Figure 1. Experimental setup.
temperature of the liquid injected into the reactor. The argon carrier gas flow rate, Q g, was 2 L/min and the water flow rate, Q w, varied for different experimental situations between 2 and 20 mL/min. The power of the discharge, measured at the electrodes, is between 300 and 450 mW. The liquid sprayed into the reactor forms a fine aerosol mist and at the exit of the reactor, the aerosol particles coalesced into larger liquid droplets. The entire gas stream, with aerosols, flowed through a cold trap (with ice) where, in addition to the droplets, condensable vapor were collected. The gas stream exiting the cold trap was collected in a gas sampling vessel. Liquid solutions of methanol were made by adding specific amounts of methanol by mass to a given mass of water. The methanol in the liquid phase was measured using a SR International Gas Chromatograph. H2 was measured using a PerkinElmer Autosystem XL gas chromatograph (GC) with thermal conductivity detector and a Restek ShinCarbon ST 100/120 column and Ar carrier. The voltage and current waveforms of the discharge were measured with a Tektronix DPO 3014 oscilloscope. The power was calculated for an average of five measurements of the voltage and the current. The sampling rate of the oscilloscope was 104 points per acquisition window. The voltage of the discharge was measured with a high-voltage probe, Tektronix 1/1000, connected to the electrodes. The current was measured on a shunt (R) to the ground of 10 Ω in the secondary of the ignition coil. Typical waveforms for the current and voltage are shown in Figure 2 and they do not differ significantly from those for water droplets used in previous work. Three repeated measurements of gaseous H2 were taken after 5 min of reactor operation. It took 2 min to analyze each sample, and samples were obtained for 4, 6, 10, 15, and 20 mL/min solution flow rates. The average of the three repeated values of the hydrogen concentration in the gas phase was reported. The hydrogen peroxide concentration in water was measured after 10 min for the same values of the solution flow rates, and no detectable hydrogen peroxide was found in methanol solutions after plasma treatment. The methanol conversion (for the water solutions only) is defined by Δ½CH3 OH ¼
½CH3 OH0 ½CH3 OHf 100 ½% ½CH3 OH0
ð1Þ
where [CH3OH]0 is the feed concentration of methanol and [CH3OH]f is the methanol concentration in water measured after the treatment by the plasma.
Figure 2. Current and voltage waveforms for the 10% methanol in water solution with an argon carrier gas flow of 2 L/min and solution flow of 2 mL/min.
The hydrogen production energy yield is given by EER ðH2 Þ ¼
R½H2 M H2 3:6 ½g=ðkW hÞ P
ð2Þ
where R[H2] is the hydrogen production rate (μmol/sec), MH2 is hydrogen molecular mass [g/mol] and P is the power [W]. Error bars of 25% based upon variation of the input power were determined for the energy yields and 20% on the measured H2 concentrations. The factor R (H2/CH3OH) is determined from mCH3 OH ¼ ½CH3 OH ð%Þ Q w ðmL=minÞ Fmeth ðg=mLÞ ðmoles=secÞ 100 Mmeth 60 mH2 ðmoles=secÞ ð3Þ R ¼ mCH3 OH ðmoles=secÞ where Q w is the solution flow rate, [CH3OH] is the concentration of CH3OH in solution in V/V%, Fmeth is the methanol mass density, and Mmeth is the molecular mass of methanol at 32 g. With a utilization of a thermal energy balance and a conservative assumption that all energy from the plasma is transferred as heat to the flowing liquid, the increase in temperature of the liquid droplets cannot exceed 3 to 4 °C.
’ RESULTS AND DISCUSSIONS Hydrogen from Methanol. Figure 3 shows the hydrogen production rate and corresponding energy yield as functions of liquid flow rate and liquid composition, and Figure 4 shows the methanol conversion. Clearly the methanol conversion decreases with increasing feed flow rate, but tends to gradually level off at high flow. Since, the liquid droplets contain a large amount of methanol in the liquid phase that is less subject to interaction with the surrounding gas phase plasma (compared to methanol transferred into the vapor phase) and the energy of the plasma is deliberately low so as to not lead to complete vaporization of the liquid droplets, the chemical yield in the presence of the liquid droplets is expected to be relatively small; methanol must transfer from the liquid phase into the gas phase to react in the plasma. The production rate and corresponding energy yields gradually increase with increasing flow rate, unlike in the case of pure water where an abrupt increase occurred at the lowest liquid feed flow rates.20 The maximum for hydrogen energy yield (176 g/kWh) 9467
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Figure 3. H2 production rate (μmol/sec) and corresponding energy yields for different feed concentrations of CH3OH (% mass) and liquid flow rates.
Figure 4. Methanol conversion as a function of liquid flow rate for various feed compositions (% mass).
was found for an average power of 0.45 W for pure methanol and 15 mL/min solution flow rate. The significant increase in production rates with liquid feed flow rate indicates a strong effect of flow on the chemical reactions occurring in the production of hydrogen; perhaps the corresponding lower residence times allows for more rapid quenching and suppression of radical reactions that might destroy the product.20 While the cost of the argon carrier gas is an important consideration, we can note that for maximum efficiency the argon gas can be recirculated since it is not consumed in the process. The recirculation would require a gas separation device, and membranes for such purposes have been prepared.24 Figure 5 shows that the stoichiometric ratio of hydrogen production to methanol consumed decreases with increasing methanol content indicating that other reaction products are formed in increasing amounts as the amount of methanol increases. Further work on byproduct analysis is necessary and is currently underway. Figure 6 shows the energy yield, at the highest liquid flow rate, is approximately linearly dependent upon the amount of methanol in the liquid droplets; this linear increase with methanol concentration might be indicative of mass transfer limitations for methanol vaporization from the liquid droplet into the gas phase. In addition, since pure methanol has the highest energy and
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Figure 5. Molar ratio of hydrogen produced to methanol consumed as a function of the feed methanol concentration (% mass) for the highest liquid flow rate of 15 mL/min.
Figure 6. H2 energy yield as a function of CH3OH (% mass) in feed for 15 mL/min solution flow rate.
chemical yields with the lowest conversion, it is clear that water suppresses the formation of hydrogen relative to pure methanol. Kabashima et al.14 found higher yields of hydrogen from methanol vapor in Ar than with water vapor in Ar (Table 1), and the Ar carrier gas gave the best results compared to nitrogen, air, and oxygen. They attributed these results to the lack of O2 formation in the absence of water which thereby eliminated oxidation reactions that tend to destroy or prevent the formation of H2. Generally high yields, up to 90% (% mole), of H2 were found in the ferroelectric packed bed reactor, but with correspondingly lower energy yields in comparison to the droplet aerosol spray reactor in our work. Arabi et al.12 compared hydrogen formation in a range of methanol/water, ethanol/ water, and ammonia/water vapor mixtures. The best energy yield was found for a mixture of 76% (% vol) methanol in water (Table 1), but experiments with pure methanol were not reported. Comparison of energy yields for various technologies, including electrolysis, photocatalysis, and corona discharge, is shown in Table 1. Results for pure water reported previously are included for reference.20 The maximum possible energy yields, based upon the standard enthalpies of reaction, are significantly higher for methanol than pure water. Significantly, glow discharge 9468
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Table 1. Hydrogen Energy Yields for Various Plasma Reactors method thermodynamic limit
H2O/MeOH
H2 (g/(kW h))
100:0
(ΔH) 22
2H2O f H2 + H2O2
23.4
H2O f H2 + 1/2O2
28.7
kinetic limits absolute quenching
ref
22 100:0
5.2
ideal quenching
10
superideal quenching
13
electrolysis photocatalysis
100:0 100:0
20 0.01
15,25 26
microwave plasma
100:0
10
22
AC gliding arc spray
100:0
1.3
27
pulsed gliding arc spray
100:0
13
20
pulsed corona
100:0
2
28
ferroelectric packed bed
100:0
0.12
14
thermodynamic limits CH3OH f 2H2 + CO
0:100
290
(ΔH298) 29 +49.51 kJ/mol
CH3OH(liq) + H2O(liq) f CO2 + 3H2
1:1 (moles/mole)
238
+ 90.7 kJ/mol
pulsed corona
16:84 (mass/mass)
32.7
23
AC corona w/Ar
0:100
4.1
30
ferroelectric packed bed
0:100
0.5
14
dielectric barrier
0:100
0.30
31
AC corona
0:100
1.2
30
arc discharge (15 kV at 50 Hz) glow discharge electrolysis
76:24 (vol/vol) 0:100
18.5 69
12 13
glide arc spray
0:100
176 ( 35
this work (highest)
thermal/steam reforming
various
128
19
electrolysis in pure methanol has a relatively high energy yield, while the highest energy yield for a plasma process is the current result with the pure methanol sprays at the highest liquid flow rate. In glow discharge electrolysis the plasma is closely associated with the condensed liquid phase, and this may enhance energy yield in a way similar to that of the present work. Further, glow discharge electrolysis in contact with the condensed liquid phase might have a lower energy yield than the liquid spray due to the enhancement in mass transfer and gasliquid contact area in the aerosol reactor. In addition, it is interesting to note that the maximum energy yield for the liquid spray is comparable to that for thermal steam reforming.19 As in the case of pure water droplets,20,21 the presence of the condensed liquid phase (droplets) enhances the reaction and improves energy efficiency. It is conjectured that two factors contribute to this improvement. The first is the increased supply of reactants through the liquid droplets that provide high local concentrations of, in this case, methanol. The second is related to the effects of the liquid droplets on quenching side reactions that lead to the destruction of the products. While we do not have direct evidence to prove these hypotheses, the higher energy yields with the liquid spray even at the lowest methanol concentration in water at the lowest flow rate (24 g/kWh) compared to the other plasma processes listed in Table 1 provide partial support. While the water in the liquid droplets may suppress the energy yield relative to pure methanol droplets, the water/ methanol droplets enhance the energy yield relative to the pure
Figure 7. Comparison of hydrogen production rates for pure alcohols and various flow rates.
vapor results reported in the literature. We note that thermal quenching also stabilizes reactions products in thermal gasification and reforming. Hydrogen production rates for 40% aqueous solutions of methanol, ethanol, and propanol are shown in Figure 7 for 4, 10, and 20 mL/min solution flow rate. The highest molecular hydrogen production rate was obtained for methanol (up to 6 μmol/sec), followed by ethanol (5 μmol/sec), and propanol (about 4.2 μmol/sec). 9469
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Industrial & Engineering Chemistry Research Arabi et al. did not see a large difference in energy yield between 19% (mass) solutions of methanol and ethanol for plasma production of hydrogen from vaporized mixtures of the alcohols.12 And while the thermodynamics of steam reforming of alcohols also shows very similar energy costs for hydrogen production from methanol and ethanol (56 and 60 kJ/mol, respectively19), the radical plasma chemistry may be significantly different.
’ CONCLUSIONS The hydrogen energy yield increases with the flow rate and the amount of methanol in the liquid solution sprayed into the plasma zone with a maximum in energy yield of 176 ( 35 g/(kW h). The low chemical yields result from the large amount of methanol inside the liquid droplets that are not subject to plasma reactions and imply the need for operation in a recycle mode. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: (850) 410-6165. Fax: (850) 410-6150. E-mail: locke@eng. fsu.edu. Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310-6046.
’ ACKNOWLEDGMENT This work was financially supported by the Florida State University GAP award (to B.R.L.), the National Science Foundation (CBET-0932481), and Romanian Grant PCE-Idei 331 (to R.B.). We also thank Mr. Richard Crisler for assistance with GC measurements. ’ REFERENCES (1) Palo, D. R.; Dagle, R. A.; Holladay, J. D. Methanol steam reforming for hydrogen production. Chem. Rev. 2007, 107 (10), 3992–4021. (2) Holladay, J. D.; Hu, J.; King, D. L.; Wang, Y. An overview of hydrogen production technologies. Catal. Today 2009, 139 (4), 244–260. (3) Petitpas, G.; Rollier, J. D.; Darmon, A.; Gonzalez-Aguilar, J.; Metkemeijer, R.; Fulcheri, L. A comparative study of nonthermal plasma- assisted reforming technologies. Int. J. Hydrogen Energy 2007, 32 (14), 2848–2867. (4) Fridman, A.; Kennedy, L. A. Plasma Physics and Engineering; Taylor and Francis: New York, 2004. (5) Hrabovsky, M.; Konrad, M.; Kopecky, V.; Hlina, M.; Kavka, T.; Chumak, O.; van Oost, G.; Beeckman, E.; Defoore, B. Pyrolysis of wood in arc plasma for syngas production. High Temp. Mater. Process 2006, 10 (4), 557–570. (6) Hrabovsky, M.; Konrad, M.; Kopecky, V.; Hlina, M.; Kavka, T.; van Oost, G.; Beeckman, E.; Defoort, B. Gasification of biomass in water/gas-stabilized plasma for syngas production. Czech. J. Phys. 2006, 56, B1199–B1206. (7) Sarmiento, B.; Brey, J. J.; Viera, I. G.; Gonzalez-Elipe, A. R.; Cotrino, J.; Rico, V. J. Hydrogen production by reforming of hydrocarbons and alcohols in a dielectric barrier discharge. J. Power Sources 2007, 169 (1), 140–143. (8) Jimenez, M.; Yubero, C.; Calzada, M. D., Study on the reforming of alcohols in a surface wave discharge (SWD) at atmospheric pressure. J. Phys. D-Appl. Phys. 2008, 41, (17). (9) Deminsky, M.; Jivotov, V.; Potapkin, B.; Rusanov, V. Plasmaassisted production of hydrogen from hydrocarbons. Pure Appl. Chem. 2002, 74 (3), 413–418. (10) Sun, D. P.; Yang, X. H.; Liu, Y. Y.; Chen, Y. Q. Study on decomposition products of methanol in AC discharge by spectroscopy. Spectrosc. Spectral Anal. 2008, 28 (9), 1983–1986.
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