Energy & Fuels 2007, 21, 2525-2530
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Kinetic Analysis of the Catalyst and Nonthermal Plasma Hybrid Reaction for Methane Steam Reforming Tomohiro Nozaki,* Hiroyuki Tsukijihara, Wataru Fukui, and Ken Okazaki Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 1528552, Japan ReceiVed March 7, 2007. ReVised Manuscript ReceiVed May 14, 2007
Bioresources, such as landfill gas and agricultural residues, attract considerable attention because of growing concerns of global energy and environmental protection. However, the efficient usage of bioresource poses major challenges and generally necessitates appropriate pre-reforming processes. We propose a low-temperature (300-500 °C) upgrading method using an atmospheric pressure nonthermal discharge generated in a reforming catalyst bed reactor for profitable recovery of poor bioresources. Excited species produced by high-energy electron impact, which proceed independent of the temperature, accelerate methane steam reforming at lower temperatures than normal catalyst reactions with minimum energy required. The resultant hydrogen-enriched biogas is then available for use in conventional energy utility systems, such as internal combustion engines. This paper introduces fundamental characteristics of nonthermal discharge and the catalyst hybrid reactor. Furthermore, a detailed mechanistic study of synergistic effects between nonthermal discharge and the reforming catalyst is presented on the basis of the Arrhenius plot method.
1. Introduction Biogas generally consists of 60% CH4 and 40% CO2 if methane fermentation of the organic matter is the main digestion process. Methane content decreases further below 40% if air leaks and oxidizes CH4 during fermentation, thereby creating much poorer biogas. Although a huge amount of poor biogas is obtainable from landfills, coal mines (coal mine methane, CMM), and agricultural residues, most are simply flared and wasted because the global warming potential (GWP) of biogas is 5-15 times as potent as carbon dioxide, depending upon CH4 contents. The poor ignition stability and low heating value of such biogas makes it difficult to use in a conventional energy utility system. The main purpose of this research is to promote the profitable recovery of CH4 from those bioresources using nonthermal plasma technology. We have developed a lowtemperature (300-500 °C) upgrade method of simulated biogas using nonthermal discharge generated in a reforming catalyst bed.1-3 The simulated biogas is partially converted into hydrogen via low-temperature steam reforming, and the resultant hydrogen-enriched biogas is fed into internal combustion engines. Ignition stability and combustibility of hydrogenenriched biogas is improved and efficiently drives internal combustion engines. Low-temperature steam reforming is also beneficial because part of the biogas, of which the heating value is already low (20-60% CH4), does not have to be flared to provide a high-temperature heat source. Low-temperature exhaust gas from a thermal engine (300-500 °C) is useful to activate the catalyst bed. In this paper, we first introduce the * To whom correspondence should be addressed: Telephone: +81-35734-2179. Fax: +81-3-5734-2893. E-mail:
[email protected]. (1) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 57-65. (2) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 67-74. (3) Nozaki, T.; Tsukijihara, H. Okazaki, K. Energy Fuels 2006, 20, 339345.
synergistic effect observed in the nonthermal discharge and catalyst hybrid reactor. Subsequently, a reaction enhancement mechanism by nonthermal discharge is discussed on the basis of the Arrhenius plot method. 2. Experimental Section Simulated biogas, which consists of CH4, N2, and H2O, was partially reformed in a barrier discharge and 12 wt % Ni/γ-Al2O3 catalyst bed reactor, which was described in detail in a precedent study.3 We used a well-established commercially available catalyst (ISOP; Su¨d-Chemie Catalysts Japan, Inc.)4 for the best understanding of the role of nonthermal discharge generated in the catalyst bed. A schematic diagram of the hybrid reactor is shown in Figure 1. Catalyst pellets (3 mm sphere) were packed in a sapphire tube with a volume of φi ) 20 × 50 mm. The catalyst reactor was also equipped with an internal high-voltage electrode (φr ) 3 mm) and an external grounded electrode. Bipolar pulsed voltage ((20 kV at 1-5 kpps) was applied between the electrodes to break down the simulated biogas. This type of atmospheric-pressure nonthermal plasma is known as dielectric barrier discharge (DBD).5 Gas breaks down at the pellet contacts where the electric field is concentrated, and a number of filamentary discharge channels propagate along the pellet surface. The discharge is terminated within 1-10 ns because the charge deposited on the pellet surface creates reverse fields and restricts further discharge development. Detailed characteristics of DBD are presented elsewhere.5-7 The total gas flow of simulated biogas was adjusted so that the gaseous hourly space velocity (GHSV) was 10 000-18 000 h-1. Moisture was removed after cold trapping, and H2 and N2 were analyzed using a gas chromatograph [GC-8A (TCD); Shimadzu Corp.] equipped with Molecular Sieve 13X (GL Sciences, Inc.). Methane, CO, and CO2 (4) Su¨d-Chemie Catalysts Japan, Inc. http://www.sud-chemie-jp.com/. (5) Gibalov, V. I.; Pietsch, G. J. J. Phys. D: Appl. Phys. 2000, 33, 26182636. (6) Nozaki, T.; Miyazaki, Y.; Unno, Y.; Okazaki, K. J. Phys. D: Appl. Phys. 2001, 34, 3383-3390. (7) Nozaki, T.; Unno, Y.; Okazaki, K. Plasma Sources Sci. Technol. 2002, 11, 431-438.
10.1021/ef070117+ CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007
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Figure 1. Plasma catalyst hybrid reactor.
were measured using GC-8A (FID) with PORAPAK-Q (GL Sciences, Inc.) after methanation.
3. Synergistic Effect of the Hybrid Reactor Figure 2a shows CH4 conversion obtained respectively in the (1) catalyst bed reactor (without plasma), (2) hybrid reactor, and (3) DBD reactor produced in R-Al2O3 pellets (without catalyst). The results are plotted with respect to the catalyst bed temperature measured using an infrared camera. The dotted line shown in Figure 2a represents the equilibrium CH4 conversion (S/C ) 1). The relationship between the discharge power and endothermic reaction enthalpy is also an important criterion because the bed temperature is determined as a result of the energy balance. The result for the hybrid operation is presented in Figure 2b. Detailed reaction characteristics of the hybrid reactor have been presented elsewhere.3 Briefly, methane was decomposed only slightly by DBD because the discharge power was minimized to the greatest degree, which is possible to avoid excess heating. In this case, H2O does not participate in the gas-phase reforming reaction. For that reason, the main product was C2H6;3,8 H2 selectivity was much lower than the thermal equilibrium. The catalyst reaction was more efficient than DBD. However, CH4 conversion does not reach thermal equilibrium. On the other hand, CH4 conversion in the hybrid reactor is clearly much greater than the simple sum of DBD and catalyst reactions at given bed temperatures. When DBD is applied, methane conversion increases by following points A400, B400, and C400, depending upon the discharge power. The subscript denotes the temperature of a ceramic radiant heater. The ceramic heater functioned as a temperature constant bath. It remains at a constant temperature independent of the catalyst bed temperature. The bed temperature was slightly lower than 400 °C because a small amount of heat was absorbed by CH4 steam reforming (see also Figure 3a for the bed temperature distribution) when the discharge power was 0 (at A400). Methane conversion increases steeply with an increasing discharge power, whereas the bed temperature slightly increases (A400 f B400). The average bed temperature was 420 °C at B400, and CH4 conversion reaches thermal equilibrium at this point; the bed temperature distribution at B400 is shown in Figure 3b. A further increase in the discharge power creates a localized temperature increase in the bed, as shown in Figure 3c. Once the chemical composition reaches (8) Hammer, T.; Kappes, T.; Schiene, W. Utilization of Greenhouse Gases; Liu, C.-J., et al., Eds.; ACS Symp. Ser. 2003, 852, 292-301.
thermal equilibrium at B400, a further increase in CH4 conversion beyond thermal equilibrium is not anticipated because the reverse reaction (CO2 + 2H2 f CH4 + 2H2O) is much more efficient than the forward reaction. In this equilibrium-limiting step, the synergistic effect is no longer anticipated and heat produced by the DBD becomes the main driving force of CH4 steam reforming.3 In fact, CH4 conversion increases along the equilibrium curve shown in Figure 2a (B400 f C400). It should be noted, at C400, CH4 conversion is plotted against a localized maximum temperature spot created downstream of the bed. When the initial bed temperature was 500 °C (at A500), the forward CH4 reaction is also accelerated with an input power until the chemical composition reaches thermal equilibrium (A500 f B500). Deposition of solid carbon on catalysts was negligibly small because the catalyst temperature was lower than 500 °C; the value of S/C ) 1 is sufficiently large to prevent carbon precipitation because methane conversion at 500 °C is approximately 25% and the equivalent S/C becomes 4. If the catalyst temperature is greater than 500 °C, carbon precipitation must be carefully monitored. If marked carbon precipitation occurs, the carbon balance becomes greater than 100% because accumulated solid carbon also contributes to the production of hydrogen via C + H2O f CO + H2; solid carbon precipitation was readily detectable during operations. We confirmed that all experiments were performed without carbon precipitation and that the carbon balance was obtained within 5% error. Excited species, produced by high-energy electron impact are inferred to accelerate the low-temperature catalysis of CH4. Simultaneously, heat generated by the DBD promotes catalytic conversion of CH4. Both play important roles in the hybrid reactor. The effects of radical production and heat generation must be assessed properly to elucidate the reaction enhancement mechanism in the hybrid reactor. For that purpose, the discharge power and resultant bed temperature distribution were carefully monitored; the overall CH4 steam reforming reaction was analyzed on the basis of the Arrhenius plot method. 4. Kinetic Analysis of the Forward CH4 Reaction 4.1. Overall Reaction Order of CH4 and H2O. The forward CH4 reaction rate based on overall steam reforming using powerlaw kinetics is expressed as eq 1.
r)
d[CH4] ) k[CH4]R[H2O]β dt
(1)
Kinetic Analysis of Plasma Methane Reforming
Figure 2. Synergistic effect between the reforming catalyst and DBD. GHSV, 11 500 h-1; S/C, 1; CH4/N2/H2O, 1100:733:1100 cm3/min; 2, DBD; O, catalyst; 0 and 9, hybrid; and dotted line, equilibrium conversion.
Therein, r is the forward CH4 reaction rate and k is the forward CH4 reaction rate constant for overall steam reforming. Numerous previous efforts suggest that the reaction order for overall CH4 steam reforming on a nickel catalyst is R ) 0.85-1.4 in CH4 and β ) -0.8-0 in H2O.9-12 The phenomenological understanding of those values is that the rate-determining step is the activation of CH4 on the metallic catalyst, whereas the overall forward reaction rate is less dependent upon the H2O concentration.13 The overall reaction order for H2O (β) takes a negative value if a nickel catalyst is oxidized with H2O. In the (9) Lee, A. L.; Zabransky, R. F. Ind. Eng. Chem. Res. 1990, 29, 766773. (10) Rostyup-Nielsen, J. R.; Bak Hansen, J.-H. J. Catal. 1993, 144, 3849. (11) Ahmed, K.; Foger, K. Catal. Today 2000, 63, 479-487. (12) Laosiripojana, N.; Assabumrungrat, S. Appl. Catal., A 2005, 290, 200-211. (13) Wei, J.; Iglesia, E. J. Catal. 2004, 224, 370-383.
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Figure 3. Temperature distribution of the catalyst bed. Conditions correspond to A400, B400, and C400, shown respectively in parts a and b of Figure 2.
hybrid reactor, both CH4 and H2O are excited primarily by electron impact before reaching the catalyst. Therefore, both R and β might take different values when DBD is superimposed on the catalyst bed. For this reason, we determined reaction orders in the given catalyst bed reactor with and without DBD. The overall rate constant was estimated using the method of the initial rate. First, take a logarithm of eq 1 and rewrite it in the following form:
ln(r) ) ln(k) + R ln[CH4] + β ln[H2O]
(2)
For simplicity, assuming that β and k are constant with respect to GHSV and S/C, eq 2 reduces to the following form under a constant H2O concentration:
ln(r) ∝ R ln[CH4]average + C
(3)
The reaction rate, r, and average CH4 concentration, [CH4]average, were determined experimentally using the following equations:
2528 Energy & Fuels, Vol. 21, No. 5, 2007
r)
Nozaki et al.
∆[CH4] [CH4]0 - [CH4] ) ∆t ∆t
∆t )
2πB2L 2V ) Qin + Qout Qin + Qout
[CH4]average )
[CH4]in + [CH4]out 2
(4) (5)
(6)
Here, B and L in eq 5 respectively represent the reactor radius (10 mm) and length (50 mm). The H2O concentration was maintained as a constant, and N2 was used for balance. Methane is more or less consumed after steam reforming. For that reason, the average reaction time and average CH4 concentration were defined for the estimation of the reaction order. Results are plotted in Figure 4a with and without DBD. The forward reaction rate increases monotonically with the CH4 partial pressure. In this case, the slope of each line corresponds to the overall reaction order for CH4. It corresponds to 0.45 in the catalyst reactor and is increased slightly by applying DBD to 0.54. The overall reaction rate for H2O was obtained similarly (Figure 4b), showing β ) 0.08 in the catalyst reaction and 0.20 in the hybrid reaction. The important fact is that R is much larger than β, and both of them increase in the presence of DBD. 4.2. Arrhenius Plot. Assume that the overall rate constant is expressed in the Arrhenius form.
(
k ) A exp -
E RT
)
(7)
Taking the logarithm of eq 7, we obtain
ln(k) ) ln(A) -
E RT
Figure 4. Effect of the (a) CH4 and (b) H2O concentration on the forward CH4 reaction rate. GHSV, 14 400 h-1; S/C, 2-4; and bed temperature, 500 °C.
(8)
In that equation, T refers to the catalyst bed temperature. To obtain the overall forward rate constant, k, eq 1 is integrated in the following form: [CH ] 1 d[CH4] ∫0tk dt ) k(∆t) ) - ∫[CH ] [CH ]R[H O] β 4
(9)
4 0
4
2
Then, we obtain
k)-
1 ∆t
[CH ] 1 d[CH4] ∫[CH ] [CH ]R[H O] β 4
(10)
4 0
4
2
Expansion of the integrand into the partial fraction is fairly complicated unless the reaction order is an integer. Therefore, eq 10 was numerically integrated. When the overall rate constant, k, is substituted into eq 8, an Arrhenius plot is obtained with respect to the inverse of the space-averaged catalyst bed temperature (Figure 5). The activation energy, E, and the preexponential factor, A, for the overall steam reforming reaction are listed in Table 1 along with respective reaction orders. During the experiment, the discharge power was controlled appropriately so that CH4 conversion did not reach thermal equilibrium. Otherwise, eq 1 must be modified so that it takes into account the reverse reaction. In addition, the bed temperature was monitored carefully using an infrared camera, clarifying that there is no localized temperature increase, such as that depicted in Figure 3c. In a normal catalyst reaction, the convex nature of the given Arrhenius plot implies that the overall steam reforming was in the reaction-limited regime when the bed temperature was lower
Figure 5. Arrhenius plot for the forward CH4 reaction rate constant. (O and b) GHSV ) 18 000 h-1, S/C ) 1; (0 and 9) GHSV ) 18 000 h-1, S/C ) 3; and (4 and 2) GHSV ) 10 800 h-1, S/C ) 1.
than 460 °C. It transits to the diffusion-limited regime when the bed temperature becomes greater than 460 °C. An analogous curve is obtained in the hybrid reactor where the ratedetermining regime is separated at 420 °C. Although the overall rate constant in the hybrid reaction is larger than that of the normal catalyst reaction, the overall activation energy is
Kinetic Analysis of Plasma Methane Reforming
Energy & Fuels, Vol. 21, No. 5, 2007 2529
Table 1. Reaction Order, Activation Energy, and Pre-exponential Factor of the Overall CH4 Steam Reforming Reactiona R
β
hybrid
0.54
0.20
catalyst
0.45
0.08
a
T (°C)
E (kJ mol-1)
420
53
460
48
A 3.90 × 107 (mol0.26 L-0.26 s-1) 6.15 × 103 (mol0.26 L-0.26 s-1) 8.01 × 105 (mol0.47 L-0.47 s-1) 8.20 × 102 (mol0.47 L-0.47 s-1)
GHSV ) 10 800-18 000 h-1, and S/C ) 1-3.
essentially unchanged independent of DBD. The overall activation energy in the reaction-limited regime is approximately 100 kJ/mol, which agrees well with the reported value, implying that CH4 dehydrogenation is the rate-determining step, independent of the DBD application.13 On the other hand, the preexponential factor is enhanced by a factor of 50 in the presence of DBD. The same trend is observed in the diffusion-limited regime, where only the pre-exponential factor was enhanced by a factor of 7, whereas the overall activation energy was not influenced by DBD. Molecular beam studies have elucidated the isothermal activation energy of 74 kJ/mol for CH4 dissociative chemisorption on a clean single-crystal nickel surface where the temperature of CH4 and nickel crystal are equally elevated. A noteworthy finding of molecular beam studies is that the nonequilibrium heating mechanism, such as an increase in the vibrational temperature of CH4, dramatically increases the probability of dissociative chemisorption of CH4 on the nickel surface.14,15 On the other hand, surface temperature dependence is rather moderate, showing that the activation energy of dissociative chemisorption with respect to the catalyst temperature remains unchanged even if the overall rate-limiting step (CH4 dissociative chemisorption) is promoted via vibrational excitation.16 Recent studies predicted that incident CH4 initially forms an energetic physisorbed intermediate with neighboring nickel atoms. The intermediate complex is then chemisorbed dissociatively if it carries sufficient internal energy. Otherwise, physisorbed CH4 is likely to desorb and bounce back to the gas phase. In this respect, the most abundant active species produced by high-energy electron impact is the vibrationally excited CH4, whose electron collision cross-section is the largest among electronic states.1,17 The production of vibrationally excited CH4 is thus expected to promote the overall forward reaction rate of CH4 without changing the overall activation energy. The translational (gas) temperature of CH4 is expected to be higher than the catalyst bed temperature in the hybrid reactor, which is also capable of promoting CH4 dissociative chemisorption.14,15 In this paper, the forward rate constant is analyzed on the basis of the catalyst bed temperature and respective contribution of the translational temperature; the vibrational temperature of CH4 was not taken into account. Detailed mechanistic studies of the nonequilibrium heating effect on reaction enhancement are being organized for future work. 4.3. Role of Excited H2O on CH4 Steam Reforming. Because CH4 dehydrogenation is promoted under the influence (14) Luntz, A. C. J. Chem. Phys. 1995, 102 (20), 8264-8269. (15) Holmblad, P. M.; Wambach, J.; Chorkendorff, I. J. Chem. Phys. 1995, 102 (20), 8255-8263. (16) Abbott, H. L.; Bukoski, A.; Kavulak, D. F.; Harrison, I. J. Chem. Phys. 2003, 119 (13), 6407-6410. (17) Davies, D. K.; Kline, L. E.; Bies, W. E. J. Appl. Phys. 1989, 65 (9), 3311-3323.
of nonthermal discharge, H2O chemisorption must be promoted simultaneously to remove the unreacted chemisorbed carbon intermediate from catalysts. Otherwise, chemisorbed carbon ultimately builds up carbon filaments, which block active sites and catalyst pores. As explained in section 3, carbon precipitation was not observed in this experiment. In this respect, excited H2O is expected to increase the concentration of chemisorbed oxygen-related intermediates, such as O and OH, and efficiently oxidize chemisorbed solid carbon, leaving many active sites available for successive CH4 dehydrogenation. In fact, the reaction order for H2O was more than doubled by applying DBD, which ultimately increases the pre-exponential factor of the overall forward rate constant, k. We speculate that excited H2O primarily oxidizes chemisorbed carbon to form CO. The CO readily establishes equilibrium with CO2 via a water-gas shift (WGS) reaction (CO + H2O f CO2 + H2). Therefore, CO coverage on the Ni catalyst is known to be low;13 the WGS reaction is so fast under the presence of the Ni-based catalyst that CO and CO2 selectivity was readily equilibrated independent of DBD.3 The removal of chemisorbed carbon is the vital reaction pathway together with the CH4 dehydrogenation reaction. Several important facts are noteworthy. First, no noticeable difference pertains among S/C ratios in the overall activation energy, E, as long as CH4 activation, such as dehydrogenation, is the rate-determining step. Second, excited H2O does not contribute to either CH4 dehydrogenation or the WGS reaction in the absence of the catalyst.3,8,18 Finally, although a nonthermal discharge is a promising technique for the preparation and modification of a catalyst,19 DBD did not seem to modify catalytic functions of the well-established catalyst.20 In fact, the overall reactivity of the given catalyst was fundamentally unchanged after a longterm operation in DBD. 5. Concluding Remarks A catalytic and nonthermal plasma (DBD) hybrid reactor was developed for profitable recovery of CH4 from poor bioresources. Production of excited species in the catalyst bed engenders a synergistic effect and clearly promotes CH4 steam reforming until CH4 conversion reaches thermal equilibrium at given bed temperatures. In the equilibrium-limiting step, however, the synergistic effect is no longer anticipated and the heat generated by DBD becomes the main driving force of CH4 steam reforming. A kinetic analysis of the forward CH4 steam reforming reaction in the hybrid reactor showed that the overall activation energy was not influenced by DBD, implying that CH4 dehydrogenation is the rate-determining step independent of DBD. On the other hand, the pre-exponential factor dramatically increased in the presence of DBD. We speculate that vibrationally excited CH4 produced by high-energy electron impact promotes dissociative chemisorption of CH4 on the nickel catalyst. Another possibility is that excited H2O elicits a decarbonizing reaction from the catalyst that also increases active sites on the catalyst. Both mechanisms contribute toward increasing the pre-exponential factor, but the overall activation energy remains unchanged as long as CH4 dehydrogenation is the rate-determining step. Acknowledgment. This project was supported by the New Energy and Industrial Technology Development Organization of (18) Sobacchi, M. G.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A.; Ahmed, S.; Krause, T. Int. J. Hydrogen Energy 2002, 27, 635-642. (19) Zou, J.-J.; Liu, C.-J.; Zhang, Y.-P. Langmuir 2006, 22, 2334-2339. (20) Nozaki, T.; Ohnishi, K.; Okazaki, K.; Kortshagen, U. Carbon 2007, 45, 364-374.
2530 Energy & Fuels, Vol. 21, No. 5, 2007 Japan (P03033) and Japanese Ministry of Education, Culture, Sports, Science, and Technology Grants-in-Aid for Scientific Research to Young Scientists (A) (18686018).
Nomenclature [CH4] ) Molar concentration of CH4 (mol/cm3) [H2O] ) Molar concentration of H2O (mol/cm3) GHSV ) Gaseous hourly space velocity, defined as 60Qin/V (h-1) S/C ) Steam/carbon ratio, defined as [H2O]0/[CH4]0 k ) Overall forward rate constant (cm3 mol-1 s-1) E ) Activation energy for the overall reaction (kJ/mol) R ) Gas constant (8.314 J mol-1 K-1) A ) Pre-exponential factor for the overall reaction (cf. Table 1)
Nozaki et al. T ) Temperature of the catalyst bed (K) V ) Reactor volume (cm3) Q ) Total gas flow rate including water vapor (cm3/min) P ) Discharge power (W) ∆H ) Reaction enthalpy of overall steam reforming (W) ∆t ) Reaction time (s) Subscripts 0 ) Initial value in ) Inlet of the reactor out ) Outlet of the reactor EF070117+
