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Energy & Fuels 2006, 20, 1674-1679
Steam Reforming of Dimethyl Ether by AC Corona Discharge Plasma with Various Waveforms Ji-Jun Zou,† Chang-Jun Liu,*,† and Yue-ping Zhang‡ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering, and Department of Chemistry, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed March 5, 2006. ReVised Manuscript ReceiVed May 31, 2006
Dimethyl ether (DME) is an ideal resource for hydrogen energy. In the present work, steam reforming of DME using AC corona discharge plasma has been investigated. The effects of H2O/DME feed ratio and input frequency and waveforms have been discussed. Steam reforming can inhibit the formation of undesired carbon deposits, improve the rate of H2 production, and decrease the energy consumption, compared to the decomposition of DME (to H2 and CO). The formation of carbon deposits is completely avoided when the H2O/DME ratio increases to 1.35, with which the H2 production rate is 1.8 times higher and the energy efficiency is 2.0 times higher than those of the decomposition reaction. AC plasma was found to be more efficient than DC plasma, and the optimum frequency is 2 kHz. The order of desired waveforms is sinusoid > sinusoid triangular > ramp > square. Steam plasma can eliminate the already formed carbon deposits, and the elimination rate is in linear proportion with the conversion rate of H2O. The plasma reaction pathway is analyzed according to the radical mechanism.
Introduction Hydrogen energy has attracted great attention worldwide. Hydrogen is normally produced from the reforming of hydrocarbons and methanol via catalytic conversion thermally.1-4 The reforming of natural gas and gasoline is usually operated at elevated temperatures (600∼800 °C), which requires an intense energy input. Methanol was considered as a more favorable option because it can be converted at relatively lower temperatures (250∼300 °C). Recently, dimethyl ether (DME) has been recognized as an ideal alternative fuel for power and domestic uses. DME is relatively inert, noncorrosive, and noncarcinogenic. Its physical properties are similar to those of liquefied petroleum gas (LPG). It can be easily liquefied, stored, and transported using the well-established facilities providing LPG. DME is also an ideal hydrogen carrier with a high H/C ratio and intense energy density. Galvita et al. reported the steam reformation of DME to hydrogen for the first time.5 Since then, studies on the steam reforming and partial oxidation of DME have quickly increased in number.6-11 The steam reforming of * To whom correspondence should be addressed. Fax: +86 22 27890078. E-mail:
[email protected] or
[email protected]. † Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering. ‡ Department of Chemistry. (1) Murata, K.; Wang, L.; Saito, M.; Inaba, M.; Takahara, I.; Mimura, N. Energy Fuels 2004, 18, 122-126. (2) Posada, A.; Manousiouthakis, V. Ind. Eng. Chem. Res. 2005, 44, 9113-9119. (3) Conte, M.; Iacibazzi, A.; Ronchetti, M.; Vellone, R. J. Power Sources 2001, 100, 171-187. (4) Yong, S. T.; Hidajat, K.; Kawi, S. J. Power Sources 2004, 131, 9195. (5) Galvita, V. V.; Semin, G. L.; Belyaev, V. D.; Yurieva, T. M.; Sobyanin, V. A. Appl. Catal. A 2001, 216, 85-90. (6) Takeishi, K.; Suzuki, H. Appl. Catal. A 2004, 260, 111-117. (7) Matsumoto, T.; Nishiguchi, T.; Kanai, H.; Utani, K.; Matsumura, Y.; Imamura, S. Appl. Catal. A 2004, 276, 267-273. (8) Tanaka, Y.; Kikuchi, R.; Takeguchi, T.; Eguchi, K. Appl. Catal. B 2005, 57, 211-222.
DME is normally carried out at 200∼400 °C. Multicomponent catalysts with metals such as Cu, Zn, Pd, or Pt, supported on acidic oxide(s), are often used for this reaction.6-9 Partial oxidation is normally conducted at 400∼600 °C with the presence of supported Ni catalysts.10,11 In this work, we attempt to develop a novel hydrogen production method from the steam reforming of DME via corona discharge with no use of any heterogeneous catalysts. Corona discharge is one of the conventional cold plasma phenomena. Cold plasma is characterized by highly energetic species (electrons, ions, excited atoms, and molecules) and low gas temperatures (as low as room temperature). The energy of the energetic species can be effectively transferred to other molecules through inelastic collisions. Chemical reactions that are traditionally conducted at elevated temperatures with catalysts can be easily conducted by cold plasma at ambient conditions. For example, stable molecules such as CH4 and CO2 can be easily converted to syngas, higher hydrocarbons, and oxygenates using cold plasmas.12-16 Recently, cold plasmas have been utilized to produce hydrogen from water, methane, and methanol.17-20 Hydrogen is produced quickly this way without the induction period that exists in conventional catalytic (9) Mathew, T.; Yamada, Y.; Ueda, A.; Shioyama, H.; Kobayashi, T. Appl. Catal. A 2005, 286, 11-22. (10) Wang, S.; Ishihara, T.; Takita, Y. Appl. Catal. A 2002, 228, 167176. (11) Zhang, Q.; Li, X.; Fujimoto, K.; Asami, K. Appl. Catal. A 2005, 288, 169-174. (12) Li, M.; Xu, G.; Tian, Y.; Chen, L.; Fu, H. J. Phys. Chem. A 2004, 108, 1687-1693. (13) Yao, S.; Nakayama, A.; Suzuki, E. AIChE J. 2001, 47, 419-426. (14) Yao, S.; Suzuki, E.; Nakayama, A. Energy Fuels 2001, 15, 13001303. (15) Zou, J.-J.; Zhang, Y.-P.; Liu, C.-J.; Li, Y.; Eliasson, B. Plasma Chem. Plasma Process. 2003, 23, 69-82. (16) Li, Y.; Liu, C.-J.; Eliasson B.; Wang, Y. Energy Fuels 2002, 16, 864-870. (17) Chen, X.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 2001, 201, 198-205.
10.1021/ef060098o CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006
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Figure 1. Schematic experimental apparatus for steam reforming of DME.
processes. In this regard, we have previously utilized corona discharge plasma to decompose methanol into hydrogen with high energy efficiency.21 This process is easily manipulated with a quick response, demonstrating great potential for practical uses. We further investigated the decomposition of DME using corona discharge, which also shows an excellent performance for hydrogen production.22 However, large amounts of carbon filaments are produced during the DME decomposition via corona discharge, which has a negative effect on the operation. To inhibit the carbon formation and to further increase energy efficiency, we attempt to use the corona discharge plasma to produce hydrogen through the steam reforming of DME. The effects of the reaction conditions including the H2O/DME feed ratio, input frequency, and waveforms were investigated. The elimination of carbon using steam is also studied to solve the problem existing in other plasma reactions. Finally, the reaction mechanism is analyzed according to a radical mechanism. Experimental Section The schematic experimental apparatus is shown in Figure 1. The reactor was a quartz tube with an inner diameter of 6 mm. A pinplate electrode configuration was used with a pin electrode and a plate electrode inserted in the tube. The gap between the electrodes was 6 mm. The pin electrode was connected to a high-voltage source, and the plate electrode was grounded. The signal, with an adjustable waveform and frequency generated by a signal generator (HP 33120A), was magnified by a high-voltage amplifier (Trek, 20/20B) and transferred to the pin electrode. When the voltage was high enough, a bright corona discharge was formed between the two electrodes. The discharge voltage and current were measured with a high-voltage probe (Tektronix P6015) and a pulse current transformer (Pearson Electronics 411) and recorded with a digital oscilloscope (Tektronix 2440), respectively. The input power was measured with a digital multimeter (Keithley 2000). The feed gas, DME/H2O/Ar, was supplied for the steam reforming of DME. The flow rates of DME and argon were fixed at 22.9 Ncm3/min (1.02 mmol/min) and 50.6 Ncm3/min (2.25 mmol/ min), respectively. Water was introduced through bubbling, and the feed content of the steam was controlled by adjusting the temperature of the water bath. The reactor was heated to 100 °C using an oven to avoid the condensation of steam. The reactant gas was converted in the plasma zone. The effluent gas, at a temperature below 120 °C, was cooled by a mixture of ice and water and then measured with an on-line mass spectrometer (MS; AVI-GmbH, Omnistar). The MS was previously calibrated with standard gas. The component was determined according to the m/z ratio of ionized species, and the concentration was calculated with the intensity of corresponding ions. The gas product includes (18) Kabashima, H.; Einaga, H.; Futamura, S. IEEE Trans. Ind. Appl. 2003, 39, 340-345. (19) Tanabe, S.; Matsuguma, H.; Okitsu, K.; Matsumoto, H. Chem. Lett. 2000, 1116-1117. (20) Sekine, Y.; Urasaki, K.; Asai, S.; Matsukata, M.; Kikuchi, E.; Kado, S. Chem. Commun. 2005, 78-79. (21) Li, H.; Zou, J.; Zhang, Y.; Liu, C. Chem. Lett. 2004, 33, 744-745. (22) Zou, J.-J.; Zhang, Y.-P.; Liu, C.-J. J. Power Sources In press.
Figure 2. Effect of H2O/DME ratio on the selectivities of products (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; input waveform: 2.0 kHz sinusoid).
H2, CO, CO2, and small amount of CH4. The amount of CH4 was neglected in the present study. Carbon deposits were measured by a gravimetric analysis. The total carbon balance was 97∼102%. The conversions were defined as DME: xDME % ) (moles of DME consumed/moles of DME introduced) × 100% H2O: xH2O % ) (moles of H2O consumed/moles of H2O introduced) × 100% The selectivities were defined as H2: SH2 % ) [moles of H2 produced × 2/(moles of DME consumed × 6)] × 100% CO: SCO % ) [moles of CO produced/ (moles of DME consumed × 2)] × 100% CO2: SCO2 % ) [moles of CO2 produced/ (moles of DME consumed × 2)] × 100% The selectivity of carbon was calculated on the basis of carbon balance: C: SC % ) 100 - SCO - SCO2 The energy efficiency was defined as the mass of H2 produced with 1 kWh consumed: energy efficiency (g H2/kWh) ) [rate of H2 production (g/min) × 60]/power consumed (kW) The specific energy density (SED) was calculated as SED (kWh/Nm3) ) input power (kW)/gas flow rate (Nm3/h) Because the input power was fixed and the gas flow rate did not change much in the present work, the SED was in the narrow range of 1.3∼1.7 kWh/Nm3 for AC plasma.
Results and Discussion Effect of H2O/DME Feed Ratio. Figure 2 shows the effect of the H2O/DME ratio on the selectivities of the product. For the DME/Ar feed (H2O/DME ratio ) 0), the selectivities of CO and C are both 50% and the selectivity of H2 is 100%. The H2/CO/C ratio is nearly 3:1:1, indicating that DME is decomposed in the plasma zone:
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CH3OCH3 f 3H2 + CO + C
Zou et al.
(1)
In this reaction, carbon filaments are formed. These carbon filaments cover the inner wall of the reactor tube and the surface of the electrodes in the present design of the reactor, which induces a problem for the stability of plasma reactions. Particularly, carbon filaments tend to accumulate between the electrodes. Once the electrodes are conjoined with each other by conductive carbon filaments, the plasma reaction is ceased because of a short circuit, and the carbon deposit has to be cleaned to restart the reaction. The presence of steam can significantly suppress the formation of carbon deposits. As shown in Figure 2, the selectivity of carbon decreases quickly with an increase of the H2O/DME ratio, while that of CO increases correspondingly. This tendency indicates that steam reforming inhibits carbon deposition and transfers it into carbon monoxide:
CH3OCH3 + H2O f 4H2 + 2CO
(2)
The selectivity of H2 also increases because extra hydrogen is produced from H2O and is as high as 143.4% with a H2O/ DME ratio of 4.23. The selectivity of carbon becomes zero when the H2O/DME ratio increases to 1.35. And the inner wall of the reactor and the surface of the electrodes are clean without any carbon deposits for a long period of reaction. This indicates that carbon deposition is completely inhibited. As a result, the selectivity of CO increases to nearly 100%. The further increasing H2O/ DME ratio reduces the selectivity of CO. On the contrary, CO2 is formed, and its selectivity increases with an increasing H2O/ DME ratio, indicating that part of CO is oxidized into CO2 when the feed content of H2O is sufficiently high:
CH3OCH3 + 3H2O f 6H2 + 2CO2
Figure 4. Effect of H2O/DME ratio on the rate of H2 production and energy efficiency. (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; input waveform: 2.0 kHz sinusoid).
Figure 5. Effects of frequency and waveform on the rate of H2 production. (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; H2O: 30.9 Ncm3/min).
(3)
It can be seen that this reaction would be more favorable than reaction 2 because more hydrogen can be produced. In catalytic processes, DME and H2O are stoichiometrically converted according to reaction 3.5-9 However, it is difficult for this to occur in plasma. Even if the H2O/DME ratio rises up to 4.23, the selectivity of CO2 is still much lower than that of CO. This shows that the reaction pathway of plasma is different from that of catalysis. Figure 3 shows the effect of the H2O/DME ratio on the conversion and reaction rate of the reactants. The addition of steam significantly enhances the conversion of DME. The conversion of DME increases with an increase of the H2O/DME Figure 6. Effects of frequency and waveform on the energy efficiency. (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; H2O: 30.9 Ncm3/min).
Figure 3. Effect of H2O/DME ratio on the conversion and conversion rate of feed. (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; input waveform: 2.0 kHz sinusoid).
ratio and reaches the highest value with a ratio of 1.35. Compared with DME decomposition, the conversion of DME is 1.34 times higher. The conversion of H2O is 100% when the concentration is relatively low and gradually decreases when its concentration is further increased. The conversion rate of H2O increases along with the H2O/DME ratio. However, the conversion rate of H2O is only slightly higher than that of DME, much less than 3:1, so the reaction is not in stoichiometric equilibrium as described in reaction 3. Figure 4 exhibits the effect of the H2O/DME ratio on the H2 production rate and energy efficiency. The highest reaction rate appears when the H2O/DME ratio is 1.35, in agreement with the highest conversion rates of DME and H2O. The energy
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Figure 7. Discharge current waveforms of different input waveforms with a time scale (horizontal direction) of 0.1 ms/grid. (Ar: 50.6 Ncm3/min; DME: 22.9 Ncm3/min; H2O: 30.9 Ncm3/min; input frequency: 2 kHz).
efficiency increases quickly with an increase of the H2O/DME ratio, demonstrating that steam reforming can greatly improve the energy efficiency. Effects of Frequency and Waveform. Corona discharge plasma can be ignited by sufficiently high voltage with various waveforms and frequencies, which determine the discharge parameters and thus affect the reaction rate and energy efficiency. Four types of waveformsssinusoid, sinusoid triangular, square, and rampswith the frequency ranging from 0 to 15 kHz were investigated in the present work to optimize the operation conditions. Figure 5 shows that the rate of H2 production is very low when DC voltage (frequency ) 0) is applied. AC voltage induces an obviously higher reaction rate. The rate increases quickly with the increase of frequency and reaches the highest value at an input frequency of 2 kHz. After that, it does not change much with the frequency. Figure 6 shows that the energy efficiency of AC plasma is significantly higher than that of DC plasma. In the ranges studied, the frequency range from 0.5 to 2 kHz is more favorable. Frequency can adjust the lifetime of reacting species. Thus various pathways can be turned on or off. Additionally, the energy distribution among ions and electrons is different at different frequencies.23 Therefore, the chemical kinetics is greatly dependent on the frequency, but more work is needed toward the detailed effects. As to the effect of waveforms, the rate of H2 production decreases in the following order: sinusoid > sinusoid triangular > ramp > square (Figure 5). Figure 7 shows the waveforms of (23) Brock, S. L.; Shimojo, T.; Marquez, M.; Marun, C.; Suib, S. L.; Matsumoto, H.; Hayashi, Y. J. Catal. 1999, 184, 123-133.
discharge current. Although the input waveform is different, a similar discharge current waveform is obtained. The current (peak-peak value) decreases as sinusoid ) sinusoid triangular (40.8 mA) > ramp (28.8 mA) > square (20.4 mA). A higher current means more electrons in the discharge that favors chemical reactions. So, a higher current should be accommodated with a higher reaction rate. At the same time, the current of the sinusoid waveform contains many pulses, while that of the square waveform shows few pulses. A pulse in the current is the character of a pulsed streamer that is most efficient for chemical reactions.13,14 Therefore, the sinusoid waveform is most desirable among the four waveforms studied. Also, a waveform may have an effect on the plasma-chemical kinetics such as frequency does. Because the power consumed for these waveforms is similar, the energy efficiency follows the same order: sinusoid > sinusoid triangular > ramp > square (Figure 6). Elimination of Carbon Deposit. As described above, the decomposition of DME produces large amounts of carbon filaments. In fact, carbon deposition is a serious problem for many plasma reactions where carbon-containing compounds are converted.12-16,22,24 Carbon filaments often contaminate and even block the reaction system, and carbon on the surface of the electrodes and the wall of the reactor will deteriorate the discharge, resulting in a decreased reaction rate. Therefore, it is necessary to inhibit carbon deposition to achieve a long-period and stable operation. The steam reforming of DME can effectively inhibit carbon deposition, suggesting that it may be used to solve this problem. To test this, about 0.2 g of carbon filaments were formed using DME/Ar as the feedstocks, and (24) Zou, J.; Liu, C. Plasma Sci. Technol. 2004, 6, 2585-2588.
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further dissociate into smaller radicals or molecules:
CH3O f CH2O (+ H) ...f CO (+ 3H)
(8)
CH3OCH2 f CH3OCH (+ H) f CH3OC (+ 2H) (9) CH3OC f CH3 + CO
(10)
CH3 f CH2 (+ H) ...f C (+ 3H)
(11)
OH f O + H
(12)
Because a large amount of active radicals is present in plasma, radical-induced reactions easily occur. In particular, the abstractions of hydrogen atoms by abundant H species are the dominant reactions:
CH3OCH3 + H f CH3OCH2 + H2
(13)
CH3O + H f CH2O (+ H2) ...f CO (+ 3H2)
(14)
CH3OCH2 + H f CH3OCH (+ H2) f CH3OC (+ 2H2) (15) CH3 + H f CH2 (+ H2) ...f C (+ 3H2)
Figure 8. Elimination of carbon with H2O. (a) Conversion and rate of conversion of H2O. (b) Elimination rate as a function of the rate of H2O conversion. (Ar: 50.6 N cm3/min; input waveform: 2.0 kHz sinusoid).
then H2O/Ar was input for an eliminating reaction. Figure 8 shows the result of the eliminating reaction. Although the conversion of H2O decreases, the conversion rate increases when its concentration increases. The rate of carbon elimination is in proportion to the conversion rate of H2O, and the rates of H2 and CO production show the same tendencies. This confirms that steam can be used to eliminate the carbon deposits formed in other plasma reactions. The elimination reaction can be described as follows:
H2O + C f H2 + CO
(4)
Plasma Reaction Pathway. As described above, the plasma reaction pathway is different from the catalysis reaction. For the catalytic reaction, reactant molecules have to be adsorbed and then activated on the catalyst surface. As to the plasma reaction, gaseous reactants are directly activated by highly energetic species including electrons, atoms, and radicals. The reaction pathway can be explained according to a radical mechanism. Plasma reactions are induced by the dissociation of reactant molecules through inelastic collisions with energetic electrons:
(16)
The presence of water enhances the conversion of DME because H species derived from the dissociation of water (eq 7) can activate DME molecules (eq 13). Moreover, O species offered by steam (eqs 7 and 12) can inhibit the formation of carbon deposits:
OH + C f H + CO
(17)
O + C f CO
(18)
If the amounts of O or OH species are enough, CO2 will be produced. However, the conversion rate of steam is not high enough to provide the required oxidative species. So, only a small amount of CO2 is produced. It is worthy to note that we have used another cold plasma, namely, dielectric-barrier discharge (DBD), to convert DME. Large amounts of liquid dimethoxy-containing hydrocarbons were formed with a selectivity of up to 39.0%.25,26 This result is dramatically different from the corona discharge in the present work where no hydrocarbons are produced. This indicates that the reaction pathway is different for different discharges. Dissociation reactions are the dominant processes for corona discharge plasma. As to DBD plasma, however, most of the originally produced radicals (eqs 5 and 6) combine with each other, instead of further dissociating, to form larger hydrocarbons. Our previous work has shown that the discharge characters and chemical properties are significantly different for corona discharge and DBD plasmas, which may be the main reason for the different resulting reaction pathways.24 Conclusions
CH3OCH3 f CH3O + CH3
(5)
CH3OCH3 f CH3OCH2 + H
(6)
H2O f H + OH
(7)
Once radicals are produced, they will be activated again via collisions with energetic electrons. These activated radicals will
The steam reforming of DME to hydrogen using corona discharge plasma has been studied. The use of steam effectively inhibits the formation of undesired carbon deposits that occur in the decomposition of DME. Moreover, the rate of hydrogen (25) Jiang, T.; Liu, C.-J.; Rao M.-F.; Yao, C.-D.; Fan, G.-L. Fuels Process. Technol. 2001, 73, 143-152. (26) Wang, Y.; Liu, C. Energy Fuels 2005, 19, 877-881.
Steam Reforming of Dimethyl Ether
production and energy efficiency are greatly improved. The optimal H2O/DME ratio is 1.35, where the rate of H2 production is 1.83 times higher and the energy efficiency is 2.0 times higher than those of DME decomposition. AC plasma is more efficient than DC plasma, and the optimal frequency is 2 kHz. The sinusoid waveform is the best to ignite the plasma, leading to the highest rate of H2 production and energy efficiency. This work demonstrates that the steam reforming of DME using corona discharges is a simple and effective technology to produce hydrogen. In addition, steam reforming may be a good
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alternative to solve the carbon deposition problem existing in many plasma reactions. Acknowledgment. Part of the instruments and equipment was donated by ABB Switzerland Ltd., which is greatly appreciated. Partial support from the National Natural Science Foundation of China (under Contract 20376060) and the Key Fundamental Research Project of Ministry of Science and Technology of China (2005CB221406) are appreciated. EF060098O
