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Hydrogen Enrichment of Low-Calorific Fuels Using Barrier Discharge Enhanced Ni/γ-Al2O3 Bed Reactor: Thermal and Nonthermal Effect of Nonequilibrium Plasma Tomohiro Nozaki,* Tsukijihara Hiroyuki, and Ken Okazaki 2-12-1 O-okayama, Meguro, Tokyo 1528552, Japan ReceiVed May 12, 2005. ReVised Manuscript ReceiVed September 25, 2005
We have developed a dielectric barrier discharge (DBD) and catalyst hybrid reactor for reforming lowcalorific fuels such as biogas at low temperature (300-500 °C). This technique allows the use of low-temperature thermal energy wasted from various industries, which ultimately provides a variety of energy utility options. The idea behind the project is that radicals produced by DBD can be decomposed at much lower temperatures than in a normal reforming condition. However, the situation becomes even more complicated because DBD enhances chemical reactions in different ways: (1) Excited species, radicals, and ions decompose on the catalyst at a lower temperature than the stable molecule. (2) Byproducts such as acetylene and ethane decompose at lower temperatures than methane. (3) Heat that is generated by DBD also enhances regular catalytic reforming. A mechanistic study of steam reforming in a plasma hybrid reactor was performed to distinguish their respective contributions to the synergistic effect. Results are discussed on the basis of the catalyst bed temperature, which was measured accurately with an infrared camera with and without DBD. Thermal and nonthermal effects of DBD on the catalytic reforming of methane are discussed extensively.
1. Introduction Renewable energy from bioresources such as organic wastes, landfills, and agricultural residues are attracting considerable attention through growing concerns of global energy and the environment because bioresources are carbon-neutral and potentially minimize fossil fuel consumption and CO2 emissions. However, the efficient usage of bioresources poses major challenges. First of all, the heating value of bioresources is generally 20-80% of natural gas, and many materials are flameresistant. Diverse composition and both the moisture and energy contents of bioresources further complicate the situation. Those “poor” resources are typically upgraded primarily for desired purposes. However, reforming reactions generally require a hightemperature heat source (ca. 800 °C), in which a large part of the initial fuel must be burned out, losing enthalpy of the initial fuel. Although conventional catalytic reforming presents a promising option, further improvements of existing processes are demanded along with an increasing demand for bioresource use. More recently, atmospheric pressure nonthermal plasma systems such as Plasmatron1 and Glidarc2-4 are used in economically competitive plasma fuel converters. Electrical energy consumption of those plasma reactors is typically less than 10% of the heating value of the initial feed when those plasma reactors are combined with partial oxidation. The driving force of the reforming reaction is mostly heat released by partial oxidation, whereas the plasma preliminarily enhances the combustion process of low-calorific fuels. The heating value * To whom correspondence should be addressed: 2-12-1 O-okayama, Meguro, Tokyo 1528552, Japan. Telephone: +81-3-5734-2179. Fax: +813-5734-2893. E-mail:
[email protected]. (1) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Heywood, J. Int. J. Hydrogen Energy, 2001, 26, 1115-1121. (2) Crernichowski, A. Pure Appl. Chem. 1994, 66, 1301-1310. (3) http://www.glidarc-tech.com/. (4) Mutaf-Yardimci, A. V.; Saveliev, A. A.; Fridman, L. A.; Kennedy, J. Appl. Phys. 2000, 87, 1632-1641.
of upgraded fuel might be low if air is used as an oxidant that introduces a large amount of nitrogen into the resultant fuel. From these perspectives, the authors have proposed combustion-free, low-temperature (300-500 °C) steam reforming of biogas, which generally contains 20-80 vol % of methane, using the dielectric barrier discharge (DBD) and catalyst hybrid reactor.5,6 The idea behind the project is that radicals produced by nonthermal plasma are decomposable at much lower temperatures than those of normal reforming conditions. For that reason, low-temperature wasted thermal energy can be used in this process: reforming reactions are completely free from combustion of initial biogas. That fact does not necessarily mean that incoming biogas is fully reformed because partially hydrogen-enriched biogas remarkably improves combustibility and enables internal combustion engines to run at high efficiency. On the contrary, polymer electrolyte fuel cells, for instance, might not be economically compatible because their exhaust temperature is much too low to drive a plasma hybrid reactor. Plasma hybrid reactors must be combined with appropriate energy utility systems so that a small amount of electrical energy input improves the entire energy system. To date, the authors have demonstrated that production of nonequilibrium plasma in a catalyst bed reactor enhances the hydrogen production rate remarkably during steam reforming of methane.5 Excited species produced by nonequilibrium plasma are necessary to derive a synergistic effect between the catalyst and nonequilibrium plasma. However, the situation becomes even more complicated in hybrid reactors because nonequilibrium plasma enhances chemical reactions differently: (1) Excited species, radicals, and ions are decomposed on the catalyst at a lower temperature than on the ground-state (5) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 57-65. (6) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 67-74.
10.1021/ef050141s CCC: $33.50 © 2006 American Chemical Society Published on Web 10/29/2005
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Figure 1. Barrier discharge and catalyst hybrid reactor.
molecule. The lifetime of a radical species strongly affects the lowering of the reaction temperature. (2) Byproducts such as acetylene and ethane decompose at lower temperatures than methane. Lifetimes of these products are sufficiently long; therefore, they reach the catalyst surface to decompose it. (3) Heat generated by DBD also enhances the regular thermal reaction. A mechanistic study of steam reforming in the plasma hybrid reactor was performed to distinguish each contribution to the synergistic effect. Results are discussed for the temperature of the catalyst bed, which was measured accurately with an infrared camera with and without DBD. Thermal and nonthermal effects of DBD on the catalytic reforming of methane are discussed extensively. 2. Experimental Setup and Procedures Figure 1 shows the detailed configuration of the hybrid reactor. To avoid liquid condensation, the rod-tube (φr 3 × φi 20 mm, stainless steel) reactor was located in a constant-temperature bath, of which the ambient temperature was 150 °C. Catalyst pellets of 12 wt %/γ-Al2O3 (3 mm spheres) were packed in the volume of φi 20 × 50 mm. The catalyst bed was sandwiched by layers of R-Al2O3 (2 mm spheres), which has no catalytic activity. Bipolar pulsed voltage ((20 kV at 1-5 kpps) was applied between the center and external electrode. We used simulated biogas, which comprises 60% methane and 40% nitrogen (CH4/N2 ) 6:4). The initial steam and methane ratio was set to 1 (S/C ) H2O/CH4 ) 1). The value of S/C ) 1 is sufficiently large to prevent carbon precipitation because our purpose is the partial conversion of methane to enrich hydrogen: Initial feed composition:
CH4/N2/H2O ) 6:4:6 steam/carbon ratio (S/C) ) 1
Steam reforming: CH4 + 2H2O ) CO2 + 4H2 assume that low-temperature reforming yields no CO Gas composition after 20% of methane converted (dry base): CH4/N2/CO2/H2 ) 4.8:4:1.2:4.8
Hydrogen in the output gas is enriched by 32 vol % if 20% of the initial methane is converted. The total gas flow (including CH4, H2O, and N2) was selected so that the gas hourly space velocity (GHSV) was 3600 or 11 500 h-1, as calculated using eq 1. The discharge power was selected carefully so that the external electrode temperature did not increase too much (see section 2.1). Some reaction gas was sampled after cold trapping. Both H2 and N2 were analyzed using a gas chromatograph (Shimadzu GC-8A) equipped with a TCD detector. Methane, CO, and CO2 were measured using GC-8A (FID) after methanation. Methane conversion and product selectivity are defined as shown below. GHSV ) 60
Qtotal (cc/min) V (cc)
percent conversion of CH4 )
percent selectivity to H2 )
(h-1)
CH4|inlet - CH4|outlet × 100 (2) CH4|inlet
0.25H2|outlet × 100 CH4|inlet - CH4|outlet
percent selectivity to COx )
(1)
(3)
COx|outlet × 100 (4) CH4|inlet - CH4|outlet
A ceramic radiant heater surrounded the hybrid reactor. The ceramic heater temperature is controlled as a constant; it indirectly maintains the 300-600 °C external electrode temperature. A thermocouple, which is embedded in the external electrode, monitors the temperature variation. This configuration is necessary to analyze the energy balance. If a resistive heater was wrapped directly around the external electrode and the electrode temperature was controlled using a thermostat, the electric heater would supply thermal energy to the reactor when the electrode temperature falls as a result of steam formation. In such a situation, it is nearly impossible to distinguish the power supply from DBD and the external electric heater to the hybrid reactor.
Methane Reforming by Discharge
Energy & Fuels, Vol. 20, No. 1, 2006 341 Table 1. Methane Conversion in Various Hybrid Reactorsa
reference
a
temperature (°C)
(5) TW
400 580 511
(7) (8)
400 200-300
(9)
UN
b
feed gas (sccm)
volume (cc)
CH4
N2
H2O
2.7 2.7 15 47 47 4.5 4.7 4.7
7 8 1100 333 333 25 15 15
0.7 0.8 733 UN UN 5 UN UN
16.5 16 1100 667 667
CO2
Qtotal
power (W)
GHSV (h-1)
SEICH4 (kJ/mol)
conversion CH4
energy cost (kJ/molH2)
efficiency (%)
75 15 15
24 25 2933 1000 1000 105 30 30
3.3 4.4 47.8 50 250 10.5 130 80
528 542 11467 1274 1274 1393 382 382
634 739 58 202 1009 564 11648 7168
0.10 0.64 0.28 0.15 0.33 0.02 0.60 0.40
3168 328 53 448 1246 14 112 19 413 17 067
3 15 80 15 7 1 1 1
a
TW, this work; UN, unknown; a, steam reforming; b, dry reforming; (5), Ni/SiO2; TW, Ni/γ-Al2O3; (7), Ni/ceramic; (8), Ni, Ni-Ca, Rh/ceramic form; (9), Ni/γ-Al2O3.
2.1. Energy Balance and Energy Efficiency. Although atmospheric pressure nonequilibrium plasma is recognized as weakly ionized low-temperature plasma from the fact that the electron temperature is much higher than the gas temperature, electrons are likely to lose their energy through vibrational, rotational, and even momentum transfer collisions at atmospheric pressure. Excited species produced by energetic electrons are the main driving force of chemical conversion processes. However, heat generated by nonequilibrium plasma also accelerates thermal reactions in a hybrid reactor. The plasma reactor temperature, especially the catalyst bed temperature, markedly affects the reforming reaction and is determined by various factors such as input power, external heating condition (isothermal or constant heat flux), reaction enthalpy (mostly endothermic), and so on. Table 1 presents a comparison of steam and dry reformation of methane in DBD and the catalyst hybrid reactor in various studies. Specific energy input (SEI) and energy efficiency are calculated using the following equations: P (W) ) f ∆Q (W) )
QCH4
∫
τ
0
i(t)V(t) dt
(5)
(mol/s) × (percent conversion of CH4) ×
60 × 22.4
∆Hreform (kJ/mol) (6) SEI (kJ/molCH4) ) 60 × 22.4 × precent energy efficiency )
P (W) QCH4 (cc/min)
∆Q (W) × 100 P (W)
(7)
(8)
Here, P represents discharge power (W), where f is the operating frequency (s-1). Also, ∆Q represents the enthalpy absorbed by steam reforming (W), and QCH4 is the flow rate of methane (cm3/ min). SEI was also calculable based on the total gas flow, but such a result underestimates the energy consumption of the plasma reactor. The authors recommend eq 7 or its equivalent for appropriate estimation of the energy consumption of a plasma hybrid reactor. Energy efficiency achieved by those studies was 15% at most: 85% of discharge energy is presumed to transfer to thermal energy. However, we probably could not measure the appropriate temperature increase associated with heat generation simply because it is not easy to determine an accurate energy balance in a small reactor system. Our previous study achieved input power of 3.3 W, where the energy efficiency was 15%.5 Heat produced by DBD corresponds to 2.8 W. However, even 1 W of heat loss decreases the reaction temperature remarkably. The DBD serves as an isothermal radical injection device in a small reactor, where the chemical composition seems not to follow the thermodynamic equilibrium at a given temperature. An identical situation is presented in ref 8 Hammer and co-workers examined a large hybrid reactor and (7) Hammer, T.; Kappes, T.; Schiene, W. Utilization of Greenhouse Gases. ACS Symp. Series 2003, 852, 292-293.
discussed energy balance analysis.10 They pointed out that excess energy input increases the reactor temperature remarkably. It is noteworthy that excess energy input produces C2 and C3 hydrocarbons.5,7 Considering those facts, our previous setup was scaled up to 1100 cm3/min of methane flow (0.73 kW in HHV basis), which enables a sufficiently accurate analysis of the energy balance.
3. Results and Discussion 3.1. Measurement of Catalyst Bed Temperature. We used an infrared camera to measure the catalyst bed temperature. An infrared camera was focused to a 10 × 50 mm square window of the external electrode (see Figure 1). The reactor is made of sapphire to allow for temperature measurement using an infrared camera. The infrared camera was calibrated in advance. The advantage of using an infrared camera is that the temperature distribution in the bed medium is obtained in situ and that results are not disturbed by electrical noise nor by visible light emission from DBD. Figure 2a shows an infrared image of the catalyst bed without DBD. Parts b-d of Figure 2 correspond to the bed temperature with DBD. A detailed temperature profile along the center of the catalyst bed is also shown in Figure 3. Captions of respective images explain the discharge power, endothermic enthalpy of steam reforming, and temperature of the external electrode measured using a thermocouple. In the case of Figure 2a, the temperature of the external electrode was initially 400 °C, but it decreases to 385 °C because of steam reforming, whereas the bed temperature is kept uniform and equal to the electrode temperature. The external-electrode temperature does not recover to the initial temperature (400 °C) because the ceramic heater does not boost heat flux to the reactor, as explained previously. In the case of Figure 2b, the discharge power was added until the electrode temperature reverted to its initial value. The energy efficiency (∆Q/P) in this case is 65%. Although it was not detected, 35% of the input power is presumed to have been converted to thermal energy. The missing heat flux corresponds to the heat that escapes through the bed media and free convection around the external electrode. This operating condition is defined as the quasi-isothermal regime: the electrode temperature recovered to the initial setting value and did not induce an additional temperature increase of the catalyst bed. At that instant, chemical composition almost reaches thermodynamic equilibrium. Temperatures of the bed and external electrode clearly increase, as shown in the cases of parts c and (8) Kraus, M.; Eliasson, B.; Kogelschatz, U.; Wokaun, A. Phys. Chem. Chem. Phys. 2001, 3, 294-300. (9) Song, H. K.; Choi, J.-W.; Yue, S. H.; Lee, H.; Na, B.-K. Catal. Today 2004, 89, 27-33. (10) Kappes, T.; Schiene, W.; Hammer, T. Proceedings of the 8th International Symposium on High Pressure and Low Temperature Plasma Chemistry; July 21-25; Pu¨haja¨rve, Estonia, 2002; pp 196-200.
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Figure 2. Temperature distribution of catalyst bed. P, discharge power; ∆Q, endothermic enthalpy; GHSV, 11 500 h-1; CH4/N2/H2O, 1100:733: 1100 cm3 min-1.
Figure 4. Methane conversion in the hybrid reactor at different GHSV. Figure 3. Temperature distribution along the center of the catalyst bed.
d of Figure 2, when input energy is further increased. The energy efficiency decreased respectively to 47 and 41%. The bed temperature is not uniform and increases to a level that is much higher than the electrode temperature downstream. 3.2. Effect of the Gas Hourly Space Velocity (GHSV). Figure 4 shows methane conversion at different GHSVs. The operation regime is quasi-isothermal so that bed temperature maintains an initial setting. Methane conversion reaches equilibrium without DBD when GHSV is small (3600 h-1): thermal energy sufficiently promotes steam reforming if the reaction time is sufficiently long. However, it is well below the equilibrium curve when GHSV is 11 500 h-1: barrier discharge has a strong effect on methane conversion when the reaction time is not long enough. Therefore, the following experiments
were conducted at GHSV ) 11 500 h-1. Note that the precedent studies identified in Table 1 have worked on GHSV between 100 and 1500 h-1, where thermal energy might promote methane conversion until it reaches equilibrium without DBD. 4. Synergistic Effect 4.1. Contribution of Byproduct to the Synergistic Effect. Figure 5 compares methane conversion and product selectivity obtained in three different conditions: (1) Ni/γ-Al2O3 (without DBD), (2) hybrid (DBD and Ni/γ-Al2O3), and (3) DBD with R-Al2O3 (without the catalyst). The results are plotted with respect to the bed temperature. They show that methane is decomposed only slightly by DBD because the discharge power was minimized to avoid excess heating. The main product was C2H6. Therefore, selectivity of CO2, CO, and H2 is much lower than equilibrium. The carbon balance was obtained within 5%
Methane Reforming by Discharge
Energy & Fuels, Vol. 20, No. 1, 2006 343
Figure 5. Enhancement of methane conversion in the hybrid reactor. SV, 11 500 h-1; CH4/N2/H2O, 1100:733:1100 cm3 min-1. (b) Ni/γ-Al2O3, (2) DBD, and (9) hybrid.
error throughout the experiment. The Ni/γ-Al2O3 catalyst is more effective than DBD, but methane conversion does not reach equilibrium because the space velocity is high, as shown in Figure 4 (GHSV ) 11 500 h-1). On the other hand, the combined result maintained equilibrium through the tested temperature range. The existence of DBD demonstrably promotes methane conversion at a given bed temperature. Barrier discharge produces not only H2 and CO2 but also C2H6 and C2H2. Those C2 products further contribute to low-temperature catalysis because they decompose at lower temperatures than methane does. However, the yield of C2 products is negligibly small. For that reason, the contribution of those products to the synergistic effect can be reasonably neglected. It is noteworthy that the conversion of water vapor reached 75% at 600 °C of the bed temperature and that we observed carbon precipitation, but catalyst activity remained unchanged. 4.2. Contribution of Radical Injection to the Synergistic Effect. In this experiment, we used a plasma preprocessing
Figure 6. Plasma preprocessing reactor.
reactor to distinguish the contributions of both the radical and heat produced by DBD. Figure 6 depicts a schematic diagram of the plasma preprocessing reactor. Radicals produced in front of the catalyst bed are likely to be deactivated before flowing into the catalyst zone. On the other hand, the feed gas is heated
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Figure 9. Reaction mechanism in the net-zero production regime. Figure 7. Contribution of radical injection to the synergistic effect. SV, 11 500 h-1; CH4/N2/H2O, 1100:733:1100 cm3 min-1.
by DBD; it then flows into the catalyst bed. The plasma zone must be placed immediately in front of the catalyst bed to minimize heat loss through the reactor wall. The result obtained in the preprocessing reactor is shown in Figure 7, which presents a comparison of (1) Ni/γ-Al2O3 (without plasma), (2) a hybrid reactor, and (3) a plasma preprocessing reactor. Methane conversion in a preprocessing reactor reached equilibrium when the bed temperature was less than 400 °C. The addition of thermal energy sufficiently promotes steam reforming of methane in the low-temperature regime. A contribution of the radical injection might also be possible, but it cannot contribute to the synergistic effect that is attributable to the thermodynamic limitation (see section 5.1). On the other hand, methane conversion in the preprocessing reactor approaches that of the result obtained in the normal catalyst bed reactor as the reaction temperature increases. The difference between the hybrid and preprocessing reactor corresponds to the contribution of the radical injection by DBD, which remarkably enhances methane conversion in the high-temperature regime. 5. Limitation of Synergistic Effect 5.1. Backward Reaction. Methane conversion is enhanced by DBD, and chemical composition achieves a thermodynamic equilibrium. It also does not exceed equilibrium and almost perfectly follows the equilibrium curve. Moreover, methane conversion is apparently governed by thermodynamic equilibrium in the presence of a catalyst because of the backward reaction. f
CH4 + 2H2O T CO2 + 4H2 b
(9)
The CO2 and H2 mixture was fed through the catalyst bed with
and without DBD to ensure that the backward reaction occurs. The temperature of the external electrode was initially 400 °C. Discharge power and GHSV were 12 W and 1080 h-1, respectively. Conversion of CO2 and product selectivity are shown in Figure 8 along with the equilibrium curve at given conditions. The result shows that CO2 and H2 react together to form CH4. Even if methane conversion is beyond the equilibrium curve because of the nonequilibrium effect of the barrier discharge, the backward reaction limits that synergistic effect. We propose the reaction mechanism described in Figure 9. In the presence of DBD, the methane conversion increases from 0 to 1; it then would move beyond the equilibrium curve at a given temperature (1 f 2 at Tinitial). However, the excess amount of CO2 and H2 present in the reactor drives the backward reaction and goes back to CH4 and H2O (2 f 1). This process is a net-zero production of hydrogen. The energy absorbed by reforming during the 1 f 2 process, which could be either electrical energy or heat, is re-released as thermal energy through the backward reaction. This thermal energy simply increases the bed temperature from 1 f 3. The increase in methane conversion ∆CH4 shown in Figure 9 is induced by the net temperature increase of the bed. Here again, DBD might enhance methane conversion from 3 f 4; it reverts to the equilibrium through the backward reaction (4 f 3). Even if DBD produces excited species, it simply works as a heat source once chemical composition reaches the thermodynamic equilibrium. Synergistic effects are no longer anticipated. This operation mode is defined as the net-zero production regime. 5.2. Thermal and Nonthermal Effects of Barrier Discharge on the Catalytic Reaction. Figure 10 shows endothermic enthalpy as plotted with respect to the input power. External electrode temperatures were set to 300, 400, 500, and 600 °C. The maximum input power increases with an increasing reaction temperature because methane conversion, ∆Q, also increases with temperature. Energy efficiency is between 65 and 85%:
Figure 8. Backward reaction (CO2 + 4H2 f CH4 + 2H2O). CO2/H2/N2, 1:2:0.7; GHSV, 1080 h-1; P, 12 W.
Methane Reforming by Discharge
Figure 10. Reaction enthalpy (endothermic) (∆Q) versus input power (P). (A) Quasi-isothermal regime. (B) Net-zero production regime.
input power is optimized according to the quasi-isothermal regime at a given temperature, which is expressed as curve A. The energy efficiency is remarkably higher than that attained in previous studies (see Table 1). Energy efficiency decreases markedly when the input power is increased steadily beyond the quasi-isothermal regime. Although both radical and thermal energy provided by DBD enhance steam reformation, the energy efficiency decreases to 45-50% (curve B). The operation regime transits from quasi-isothermal to net-zero production when excess energy is supplied because the chemical composition has already reached thermodynamic equilibrium in the quasiisothermal regime. The hybrid reactor temperature increases greatly, which thereby increases various energy losses to the surroundings. Consequently, the energy efficiency decreases in the net-zero production regime. Table 1 presents a comparison of GHSVs. The GHSVs are between 100 and 1500 h-1. According to our findings, each hybrid reactor operated in the net-zero production regime. At lower GHSVs, chemical composition can reach equilibrium without DBD (see Figure 4). Once thermodynamic equilibrium is established, the DBD serves as a heat source even if it supplies radical species: electrical energy fed to chemical species is released as thermal energy through the backward reaction. Much experimental evidence indicates that the excited species produced by nonequilibrium plasma plays a unique role in chemical conversion processes.11-13 Nevertheless, the energy efficiency of the hybrid reactor has been limited to 15% at most, probably because hybrid reactors were operated in the net-zero production regime. Input power and GHSV must be selected carefully so that hybrid reactors operate in the quasi-isothermal regime. 6. Conclusion We investigated combustion-free, low-temperature steam reforming of simulated biogas, which is a mixture of methane (11) Savinov, S. Y.; Lee, H.; Song, H. K.; Na, B.-K.; Plasma Chem. Plasma Process. 2003, 23, 159-173. (12) Chen, X.; Marquez, M.; Rozak, J.; Marun, C.; Luo, J.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 1998, 178, 372-377. (13) Kraus, M.; Egli, W.; Haffner, K.; Eliasson, B.; Kogelschatz, U.; Wokaun, A. Phys. Chem. Chem. Phys. 2002, 4, 668-675.
Energy & Fuels, Vol. 20, No. 1, 2006 345
and nitrogen at a ratio of CH4/N2 ) 6:4, using the barrier discharge and a catalyst hybrid reactor. The reaction mechanism, particularly synergistic effects of the hybrid reactor, was discussed on the basis of the catalyst bed temperature, which was measured accurately with an infrared camera with and without DBD. Thermal and nonthermal effects of DBD on catalytic reforming of methane are thereby clearly distinguishable. 1. Synergistic effects caused by radical injection are anticipated in the quasi-isothermal operation regime. 1-1. In the quasi-isothermal regime, input power is optimized so that the bed temperature does not increase from the initial value after plasma operation. In this regime, the bed temperature is almost uniform; it is equal to the electrode temperature. 1-2. Optimum discharge power is determined in the quasiisothermal regime, where chemical composition already reached thermodynamic equilibrium. Energy efficiency reaches 6585%. About 15-35% of the input power was not detected, which corresponds to the heat loss to the surroundings. 1-3. The contribution of radical injection to the synergistic effect is observed when the bed temperature is higher than 400 °C. Radical injection remarkably enhances methane conversion at a given condition. 1-4. Contributions of byproducts such as C2H2 and C2H6 are negligibly small over the tested temperature range. 2. In the net-zero production regime, DBD works as a heat source and a synergistic effect attributable to radical injection is no longer anticipated. 2-1. Methane conversion and product selectivity are governed by thermodynamic equilibrium in the presence of the catalyst. Once chemical composition reaches thermodynamic equilibrium, DBD behaves as a thermal energy source. Synergistic effects attributable to radical injection are no longer anticipated. 2-2. Methane conversion reaches equilibrium without DBD when GHSV is small (GHSV , 10 000 h-1). This condition is inevitably clarified into the net-zero production regime. 2-3. Excess energy input markedly increases the bed and external electrode temperatures. This situation is also considered as the net-zero production regime. An increase in the methane conversion is engendered by the net temperature increase of the bed medium. In this regime, the energy efficiency decreases are attributable to various heat losses. 2-4. A surface corona was observed around the edges of the external electrode. Such unfavorable discharge must be removed for further improvements of the present system. Acknowledgment. This project was supported by the New Energy and Industrial Technology Development Organization of Japan, Strategic Development of Technology for Efficient Energy Utilization (P03033) and Grants-in-Aid for Scientific Research for Young Scientists from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (16760151). T. N. would like to thank Dr. Shigeru Kado of the University of Tsukuba for very helpful discussions related to catalyst preparation. EF050141S
