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Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
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Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor Qi Wang, Huiliang Shi, Binhang Yan, Yong Jin, Yi Cheng* Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
Considering the inevitable high energy input to implement the CO2 reforming of methane
Received 25 January 2011
under high-temperature operation using conventional catalysis method, the low temper-
Received in revised form
ature conversion of CO2 and methane in the coaxial dielectric barrier discharge (DBD)
12 April 2011
plasma reactor was investigated in this work. Steam was introduced to enhance the CO2
Accepted 13 April 2011
reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The
Available online 17 May 2011
experimental results showed that a certain percent of steam could promote the conversion
Keywords:
with the dry reforming of methane. With the increase of steam input, the steam reforming
Carbon dioxide reforming
occurred predominantly. As a result, the hydrogen volume percentage in the product gases
Steam reforming
increased. In this way, the products with different H2/CO ratio could be achieved by
Hydrogen production
changing the mole ratio of CH4/CO2/H2O at the reactor inlet. In particular, when the mole
Plasma
ratio of H2O/CH4 increased to almost 3 corresponding to the pure steam reforming process,
Dielectric barrier discharge (DBD)
the conversion of CH4 reached almost 0.95 and the selectivity to H2 was almost 0.99 at 773K.
of both CH4 and CO2. Meanwhile, the carbon deposition was evidently reduced compared
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The requirements to clean energy have been increased significantly all over the world in these years. Hydrogen, as one of the cleanest energies or energy carriers, has potential capability to resolve the environment problems resulted from the usage of large amount of fossil fuels [1e5]. Meanwhile, hydrogen is one of the most important feedstock in the petroleum industry, especially for the hydrogenation processes. Nowadays, the steam reforming of hydrocarbons is a mature process for typical large-scale hydrogen production in industry [6e8]. However, the steam reforming is highly endothermic and is normally carried out at temperatures around 973e1073K catalyzed by noble catalysts. On the contrary, the reforming reaction can be implemented at low temperature in non-thermal plasma, where the high-energy
electrons (e.g., temperature > 500K) play the role of catalyst to stimulate the chemical reactions at low system temperature. In the literature, the processes of steam methane reforming (SMR) and other hydrocarbon reforming [9] were reported to take place effectively in microwave plasma reactors [10e12], gliding arc reactors [13,14], radio-frequency plasma reactors [15] and DBD reactors [16]. The hybrid catalysis of plasma and catalyst has also been investigated [12,17e19]. In our previous work, the CO2 reforming of methane was investigated in a DBD reactor [20,21], where the catalyst could be in-situ reduced at 673K and reach certain activity at such low temperature. However, considering the potential risk of carbon deposition during the CO2 reforming, the steam was introduced into this system to reduce the carbon deposition in this work. The Eqs. (1) and (2) show the reactions of methane reforming.
* Corresponding author. Tel./fax: þ86 10 62794468. E-mail address: [email protected] (Y. Cheng). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.084
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
Fig. 1 e Schematic diagram of the experimental setup: 1, 2. gas cylinders of CH4 and CO2; 3, 4. mass flow controller; 5. gas mixer; 6. power source; 7. oscillograph; 8. temperature controller; 9. GC; 10. steam generator; 11. temperature controller and heating band; 12. condenser; 13. DBD reactor; 14. furnace.
CH4 þ H2 O ¼ CO þ 3H2 ; DH ¼ þ206 kJ=mol
(1)
CH4 þ CO2 ¼ 2CO þ 2H2 ; DH ¼ þ247 kJ=mol
(2)
Although the investigations about the methane reforming by both the steam and CO2 with low temperature plasma were reported [22,23], the steam enhanced carbon dioxide reforming by the catalysis of low temperature plasma and Ni/g-Al2O3 was firstly studied in this paper, where the reforming could take place at 623K, having good activity at 773K.
Fig. 2 e Steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the same space velocity and different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 40% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 25 mL/min.
2.
out of the furnace because of the decreased temperature. If the reactor is horizontally placed, the water may flow back to the central part and accordingly influence the discharge seriously. Therefore, the reactor is placed with a certain angle to the horizontal. The calculation of the discharge power, the charge and the gas analysis has been all described in our previous work [21]. The conversion and the selectivity are calculated as shown in Eqs. (3)e(6).
Experimental section
A schematic diagram of the experimental setup is shown in Fig. 1, illustrating the details about the reactor and main facilities used in the present work. The mixture of CH4 and CO2 is fed into the steam generator together with the water. Then CH4 and CO2 take the steam out of the steam generator and enter into the DBD reactor together. In some cases, the un-reacted steam can condense to water at the downstream
Table 1 e Flow rate of reactants and the temperature of the catalyst bed in this work. Case
Temperature (K)
CH4 (mL/min)
CO2 (mL/min)
H2O (mL/min)
Space velocity (mL/(gCat h))
Case Case Case Case Case Case Case
623e773 623e773 623e773 623e773 623e773 773 773
25 25 25 25 25 25 25
25 12.5 12.5 0 0 0 0
12.5 25 12.5 25 37.5 50 75
1667.0 1667.0 1333.6 1333.6 1667.0 2000.6 2667.2
1 2 3 4 5 6 7
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
CCH4 ¼
CCO2
moles of CH4 converted moles of CH4 introduced
moles of CO2 converted ¼ moles of CO2 introduced
SCO ¼
moles of CO produced moles of CH4 converted þ moles of CO2 converted
SH2 ¼
moles of H2 produced 2 moles of CH4 converted þ moles of H2 O converted
(3)
3.
(4)
3.1. Steam enhanced carbon dioxide reforming of methane
(5)
(6) A commercial catalyst Ni/g-Al2O3 with the NiO concentration greater than 10% is crushed to 150e224 mm grains. The catalyst is reduced at 973K for 3 h with 10% hydrogen in argon. The operating conditions are listed in Table 1, where the applied input power is 85 W under each condition. The synergetic effect of the catalysis by both plasma and catalyst is defined as follows: the ratio of the reactants conversion when using plasma and catalyst simultaneously over the sum of the conversions when using plasma and using catalyst individually.
Fig. 3 e Steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the different space velocity and different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 25% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 12.5 mL/min.
8303
Results and discussion
The reforming reactions with different mole percentage of steam in the reactants were investigated with a residence time of about 1.4 s. The mole percentage of steam in the reactants was 20% and 40%, respectively, when the flow rates of CH4, CO2 and H2O were 25 mL/min, 25 mL/min and 12.5 mL/ min or 25 mL/min, 12.5 mL/min and 25 mL/min (see cases 1 and 2 listed in Table 1). Under these operating conditions, the space velocity was 1667 mL/(gCat h). In order to identify the synergetic effect of catalyst and plasma, experiments were firstly performed with the plasma discharge and the Ni/gAl2O3 catalyst individually. Then the comparison was made between the hybrid catalysis of plasma plus catalyst and the individual catalysis by plasma or catalyst. The impact of the increasing temperature at discharge area on the reaction performance with only the plasma discharge had been discussed in our previous paper [21]. The conversions of CH4 and CO2 were really low at room temperature when only the plasma catalyzed the reaction(s). The catalyst showed the initial activity from about 723K when only the catalyst worked for catalysis. Fig. 2 shows the experimental results of steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the same space velocity and power input. For the joint effect of catalyst and plasma, the conversions of CH4 and CO2 were all significantly improved to 0.62 and 0.41, respectively, with 20% steam at 773K. However, the space velocity, 1667 mL/(gCat h), was more than two times of 800 ml/ (gCat h) in the former experiments with only CO2 and CH4 [21]. Under these two experimental conditions, the same
Fig. 4 e The dependence of mole ratio of H2/CO in the product gases on the temperature under different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 25% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 12.5 mL/min, RH2 O [ 40% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 25 mL/min.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
Fig. 5 e TGeDTG analysis of the catalyst for steam enhanced CO2 reforming of methane in the DBD plasma reactor.
Fig. 7 e Comparison of hybrid steam and CO2 reforming of methane and the individual steam reforming of methane in the DBD plasma reactor at space velocity of 1333 mL/(gCat h).
conversions have been observed (Fig. 6 in [21]). This indicated that the steam addition significantly improved the process efficiency. As such, the activity of the catalyst was higher with the steam addition than that with only CH4 and CO2 at 623K. Besides, the selectivities to CO and H2 were all above 0.98 at the temperature range between 623K and 773K. Hence the CO2 reforming was evidently enhanced by adding proper percentage of steam. The conversion of CH4 with 40% steam in the reactants was higher than that with 20% steam versus the increased temperature but lower at 675K, while the conversion of CO2 was even lower with 40% steam content. The difference could be attributed to the generated CO2 produced from steam reforming and water gas shift (WGS) reactions, which inhibited the further conversion of reactant CO2. In other words, when the steam content was 40%, the steam reforming took place more predominantly, leading to a lower conversion of CO2, while the conversion of CH4 was still at the same level as that of steam content at 20%. By decreasing the flow rate of CO2 from 25 mL/min to 12.5 mL/min and keeping CH4 and H2O at 25 mL/min and
12.5 mL/min, respectively, the steam content was increased while the space velocity was decreased, see the case 3 listed in Table 1. From the conversions of CH4 and CO2 shown in Fig. 3, the synergetic effect of catalyst and plasma was lower with high steam content and low space velocity. Especially for the conversion of CO2, lower CO2 content and higher steam content resulted in a decreased conversion rate of CO2. Also, the product gases with different H2/CO mole ratio were obtained corresponding to the different inlet gases, as shown in Fig. 4. Compared to the catalyst status after synthetic catalysis with the dry reforming of methane [21], the carbon deposition with steam enhanced CO2 and CH4 reforming was different. The quantity of carbon deposition was less with steam enhanced CO2 and CH4 reforming than that with only CO2 and CH4 reforming (see the TGeDTG curve in Fig. 5). In the DBD packed bed, the bed was easily blocked because of the serious carbon deposition in only 1 h with carbon dioxide reforming of
Fig. 6 e The XRD patterns of spent catalysts after CO2 reforming of methane in the DBD plasma reactor (a) without steam, (b) with steam.
Fig. 8 e Comparison of hybrid steam and CO2 reforming of methane and the individual steam reforming of methane in the DBD plasma reactor at space velocity of 1667 mL/(gCat h).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
methane. But with steam added carbon dioxide reforming of methane, the operating time was significantly increased that the bed was not blocked at all after one day continuously operation. This could be explained by the steam reaction with the formed carbon on the catalyst surface to reduce the carbon deposition. In order to further prove the difference of the carbon deposition, the XRD patterns of catalysts were given in Fig. 6, i.e., the carbon peak at around 26 is much weaker with spent catalysts after CO2 reforming of methane with steam. The NiAl2O4 peak was weak after adding the steam to the reactant, which suggested that the NiAl2O4 was reduced to Ni and Al2O3 in the synergetic catalysis with steam, CO2 and CH4.
Fig. 9 e Steam reforming of methane in DBD plasma reactor under different mole ratio of H2O/CH4 at 773K.
3.2.
8305
Plasma enhanced steam reforming of methane
After the experiments of the steam enhanced CO2 reforming of methane, the steam reforming catalyzed by plasma and catalyst was also investigated in this work. The CO2 in the reactant gas was replaced by H2O, and the space velocity was kept at 1333 mL/(gCat h). The mole ratio of CH4/H2O was 1:1, see the case 4 listed in Table 1. It was observed that the conversion of CH4 was largely increased and the synergetic effect decreased versus the increase of temperature, as shown in Fig. 7. Then we increased the flow rate of CO2 to 25 mL/min, and kept CH4 and H2O at 25 mL/min and 12.5 mL/min, where the space velocity was 1667 mL/(gCat h). Substitute CO2 with steam, then the flow rate of H2O was 37.5 mL/min and the CH4 was 25 mL/min, see the case 5 in Table 1. As a result, the conversion of CH4 increased because of the high steam content. Also the synergetic effect at 623K and 673K reached 1.88 and 2.13, respectively, as shown in Fig. 8. By increasing the mole ratio of H2O/CH4 (see case 6 and case 7 in Table 1), the conversion of CH4 reached almost 0.9 at 773K, as shown in Fig. 9(a). And the conversion of CH4 would level off after the mole ratio of H2O/CH4 reached 3. The synergetic effect increased first and then decreased with the increase of the mole ratio of H2O/CH4, which meant that there was an optimum mole ratio of H2O/CH4 for the most significant synergetic effect. The production of hydrogen was the main reason to investigate the steam reforming of methane with the plasma intensification. The test results showed that the maximum selectivity to the hydrogen could reach almost 99% under different mole ratio of H2O/CH4 while the maximum selectivity to the CO was a little bit lower than that of H2. As shown in Fig. 9 (c), when increasing the mole ratio of H2O/CH4 in the reactant gas, the selectivity of CO decreased versus the increased mole ratio of H2O/CH4 due to more CO2 production with the increase of steam. Compared to the products catalyzed only by plasma, the selectivities to CO and H2 were significantly improved under the synergetic catalysis of plasma and catalyst.
Fig. 10 e The selectivities to CO and H2 for the steam reforming of methane in DBD plasma reactor under different temperature and different mole ratio of H2O/CH4.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
Fig. 10 shows the selectivity to CO and/or H2 by the synergetic catalysis of catalyst and plasma versus the temperature (see case 4 and case 5 listed in Table 1). With the increase of temperature, the selectivity to H2 reached almost 99% at 773K and the selectivity to CO was improved to almost 75% because of the improvement of catalyst activity. Under this circumstance, the conversion of CH4 reached almost 90% (at 773K).
4.
Conclusions
Considering the inevitable high-temperature operation to implement the steam/CO2 reforming of methane using conventional catalysis method, this work investigated the low temperature conversion of steam, CO2 and methane to hydrogen and CO by the catalysis of plasma and catalyst. Firstly, the steam enhanced CO2 reforming of methane was studied in a DBD plasma reactor with catalyst. By adding a certain amount of steam to the reactants of CH4 and CO2, the conversions of CH4 and CO2 can be both improved and the carbon deposition was also reduced. Different mole ratio of H2/CO in the product gases was achieved by changing the percents of H2O, CO2 and CH4 in the reactants. With the increase of the steam content, the process efficiency was evidently improved, while the steam reforming of methane became more and more predominant than CO2 reforming of methane. Secondly, the pure steam reforming of methane was tested in the DBD plasma reactor. The conversion of CH4 can reach 90% catalyzed by plasma and catalyst at 773K, while the selectivity to H2 can reach almost 99%. There was an optimal mole ratio of H2O/CH4 to obtain the best synergetic effect of plasma and catalyst. The evidence of the reaction performance (i.e., the high conversion of CH4 and the high selectivity to H2) in the plasma reactor would attract much attention in academics for a clear understanding on the mechanisms behind the synergetic effect of plasma and catalyst as well as the steam enhanced reaction performance, which would be also the key to make the process practical to industrial application.
Acknowledgments Financial supports from National Natural Science Foundation of China (No. 20990223) and the Program for New Century Excellent Talents in University are acknowledged.
references
[1] Holladay JD, Hu J, King DL, Wang Y. An overview of hydrogen production technologies. Catal Today 2009;139:244e60.
[2] Barreto L, Makihira A, Riahi K. The hydrogen economy in the 21st century: a sustainable development scenario. Int J Hydrogen Energy 2003;28:267e84. [3] Dunn S. Hydrogen futures: toward a sustainable energy system. Int J Hydrogen Energy 2002;27:235e64. [4] Bockris JO’M. The origin of ideas on a hydrogen economy and its solution to the decay of the environment. Int J Hydrogen Energy 2002;27:731e40. [5] Fan M, Abdullah AZ, Bhatia S. Hydrogen production from carbon dioxide reforming of methane over Ni-Co/MgO-ZrO2 catalyst: process optimization. Int J Hydrogen Energy 2011;36: 4875e86. [6] Song C. Fuel processing for low-temperature and hightemperature fuel cells: challenges, and opportunities for sustainable development in the 21st century. Catal Today 2002;77(1e2):17e49. [7] Van Hook JP. Methane-steam reforming. Catal Rev 1981;21(1): 1e51. [8] Rostrup-Nielsen JR. Catalytic steam reforming. Berlin, GE: Springer-Verlag; 1984. [9] Paulmier T, Fulcheri L. Use of non-thermal plasma for hydrocarbon reforming. Chem Eng J 2005;106:59e71. [10] Wang Y, Tsai C, Chang W, Kuo Y. Methane steam reforming for producing hydrogen in an atmospheric-pressure microwave plasma reactor. Int J Hydrogen Energy 2010;35:135e40. [11] Jasinski M, Dors M, Mizeraczyk J. Production of hydrogen via methane reforming using atmospheric pressure microwave plasma. J Power Sour 2008;181:41e5. [12] Sekiguchi H, Mori Y. Steam plasma reforming using microwave discharge. Thin Sol Films 2003;435:44e8. [13] Rusu I, Cormier J. On a possible mechanism of the methane steam reforming in a gliding arc reactor. Chem Eng J 2003;91: 23e31. [14] Yang Y, Lee B, Chun Y. Characteristics of methane reforming using gliding arc reactor. Energy 2009;34:172e7. [15] Patino P, Perez Y, Caetano M. Coupling and reforming of methane by means of low pressure radio-frequency plasmas. Fuel 2005;84:2008e14. [16] Zhang X, Wang B, Liu Y, Xu G. Conversion of methane by steam reforming using dielectric-barrier discharge. Chin J Chem Eng 2009;17(4):625e9. [17] Pietruszka B, Heintze M. Methane conversion at low temperature: the combined application of catalysis and nonequilibrium plasma. Catal Today 2004;90:151e8. [18] Hammer T, Kappes T, Baldauf M. Plasma catalytic hybrid processes: gas discharge initiation and plasma activation of catalytic processes. Catal Today 2004;89:5e14. [19] Hong R, Lai M, Huang H, Chang Y. Reforming performance of a plasma-catalyst hybrid converter using low carbon fuels. Energ Conver Manage 2009;50:2632e7. [20] Wang Q, Yan B, Jin Y, Cheng Y. Investigation of dry reforming of methane in a dielectric barrier discharge reactor. Plasma Chem Plasma Process 2009;29(3):217e28. [21] Wang Q, Yan B, Jin Y, Cheng Y. Dry reforming of methane in a dielectric barrier discharge reactor with Ni/Al2O3 catalyst: the interaction of catalyst and plasma. Energ Fuel 2009;23: 4196e201. [22] Supat K, Chavadej S, Lobban LL, Mallinson RG. Carbon dioxide reforming with methane in low temperature plasmas. Div Fuel Chem Prepr 2002;47(1):269e72. [23] Futamura S, Kabashima H. Synthesis gas production from CO2 and H2O with nonthermal plasma. Stud Surf Sci Catal 2004;153:119e24.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor Qi Wang, Huiliang Shi, Binhang Yan, Yong Jin, Yi Cheng* Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
Considering the inevitable high energy input to implement the CO2 reforming of methane
Received 25 January 2011
under high-temperature operation using conventional catalysis method, the low temper-
Received in revised form
ature conversion of CO2 and methane in the coaxial dielectric barrier discharge (DBD)
12 April 2011
plasma reactor was investigated in this work. Steam was introduced to enhance the CO2
Accepted 13 April 2011
reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The
Available online 17 May 2011
experimental results showed that a certain percent of steam could promote the conversion
Keywords:
with the dry reforming of methane. With the increase of steam input, the steam reforming
Carbon dioxide reforming
occurred predominantly. As a result, the hydrogen volume percentage in the product gases
Steam reforming
increased. In this way, the products with different H2/CO ratio could be achieved by
Hydrogen production
changing the mole ratio of CH4/CO2/H2O at the reactor inlet. In particular, when the mole
Plasma
ratio of H2O/CH4 increased to almost 3 corresponding to the pure steam reforming process,
Dielectric barrier discharge (DBD)
the conversion of CH4 reached almost 0.95 and the selectivity to H2 was almost 0.99 at 773K.
of both CH4 and CO2. Meanwhile, the carbon deposition was evidently reduced compared
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The requirements to clean energy have been increased significantly all over the world in these years. Hydrogen, as one of the cleanest energies or energy carriers, has potential capability to resolve the environment problems resulted from the usage of large amount of fossil fuels [1e5]. Meanwhile, hydrogen is one of the most important feedstock in the petroleum industry, especially for the hydrogenation processes. Nowadays, the steam reforming of hydrocarbons is a mature process for typical large-scale hydrogen production in industry [6e8]. However, the steam reforming is highly endothermic and is normally carried out at temperatures around 973e1073K catalyzed by noble catalysts. On the contrary, the reforming reaction can be implemented at low temperature in non-thermal plasma, where the high-energy
electrons (e.g., temperature > 500K) play the role of catalyst to stimulate the chemical reactions at low system temperature. In the literature, the processes of steam methane reforming (SMR) and other hydrocarbon reforming [9] were reported to take place effectively in microwave plasma reactors [10e12], gliding arc reactors [13,14], radio-frequency plasma reactors [15] and DBD reactors [16]. The hybrid catalysis of plasma and catalyst has also been investigated [12,17e19]. In our previous work, the CO2 reforming of methane was investigated in a DBD reactor [20,21], where the catalyst could be in-situ reduced at 673K and reach certain activity at such low temperature. However, considering the potential risk of carbon deposition during the CO2 reforming, the steam was introduced into this system to reduce the carbon deposition in this work. The Eqs. (1) and (2) show the reactions of methane reforming.
* Corresponding author. Tel./fax: þ86 10 62794468. E-mail address: [email protected] (Y. Cheng). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.084
8302
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
Fig. 1 e Schematic diagram of the experimental setup: 1, 2. gas cylinders of CH4 and CO2; 3, 4. mass flow controller; 5. gas mixer; 6. power source; 7. oscillograph; 8. temperature controller; 9. GC; 10. steam generator; 11. temperature controller and heating band; 12. condenser; 13. DBD reactor; 14. furnace.
CH4 þ H2 O ¼ CO þ 3H2 ; DH ¼ þ206 kJ=mol
(1)
CH4 þ CO2 ¼ 2CO þ 2H2 ; DH ¼ þ247 kJ=mol
(2)
Although the investigations about the methane reforming by both the steam and CO2 with low temperature plasma were reported [22,23], the steam enhanced carbon dioxide reforming by the catalysis of low temperature plasma and Ni/g-Al2O3 was firstly studied in this paper, where the reforming could take place at 623K, having good activity at 773K.
Fig. 2 e Steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the same space velocity and different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 40% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 25 mL/min.
2.
out of the furnace because of the decreased temperature. If the reactor is horizontally placed, the water may flow back to the central part and accordingly influence the discharge seriously. Therefore, the reactor is placed with a certain angle to the horizontal. The calculation of the discharge power, the charge and the gas analysis has been all described in our previous work [21]. The conversion and the selectivity are calculated as shown in Eqs. (3)e(6).
Experimental section
A schematic diagram of the experimental setup is shown in Fig. 1, illustrating the details about the reactor and main facilities used in the present work. The mixture of CH4 and CO2 is fed into the steam generator together with the water. Then CH4 and CO2 take the steam out of the steam generator and enter into the DBD reactor together. In some cases, the un-reacted steam can condense to water at the downstream
Table 1 e Flow rate of reactants and the temperature of the catalyst bed in this work. Case
Temperature (K)
CH4 (mL/min)
CO2 (mL/min)
H2O (mL/min)
Space velocity (mL/(gCat h))
Case Case Case Case Case Case Case
623e773 623e773 623e773 623e773 623e773 773 773
25 25 25 25 25 25 25
25 12.5 12.5 0 0 0 0
12.5 25 12.5 25 37.5 50 75
1667.0 1667.0 1333.6 1333.6 1667.0 2000.6 2667.2
1 2 3 4 5 6 7
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 3 0 1 e8 3 0 6
CCH4 ¼
CCO2
moles of CH4 converted moles of CH4 introduced
moles of CO2 converted ¼ moles of CO2 introduced
SCO ¼
moles of CO produced moles of CH4 converted þ moles of CO2 converted
SH2 ¼
moles of H2 produced 2 moles of CH4 converted þ moles of H2 O converted
(3)
3.
(4)
3.1. Steam enhanced carbon dioxide reforming of methane
(5)
(6) A commercial catalyst Ni/g-Al2O3 with the NiO concentration greater than 10% is crushed to 150e224 mm grains. The catalyst is reduced at 973K for 3 h with 10% hydrogen in argon. The operating conditions are listed in Table 1, where the applied input power is 85 W under each condition. The synergetic effect of the catalysis by both plasma and catalyst is defined as follows: the ratio of the reactants conversion when using plasma and catalyst simultaneously over the sum of the conversions when using plasma and using catalyst individually.
Fig. 3 e Steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the different space velocity and different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 25% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 12.5 mL/min.
8303
Results and discussion
The reforming reactions with different mole percentage of steam in the reactants were investigated with a residence time of about 1.4 s. The mole percentage of steam in the reactants was 20% and 40%, respectively, when the flow rates of CH4, CO2 and H2O were 25 mL/min, 25 mL/min and 12.5 mL/ min or 25 mL/min, 12.5 mL/min and 25 mL/min (see cases 1 and 2 listed in Table 1). Under these operating conditions, the space velocity was 1667 mL/(gCat h). In order to identify the synergetic effect of catalyst and plasma, experiments were firstly performed with the plasma discharge and the Ni/gAl2O3 catalyst individually. Then the comparison was made between the hybrid catalysis of plasma plus catalyst and the individual catalysis by plasma or catalyst. The impact of the increasing temperature at discharge area on the reaction performance with only the plasma discharge had been discussed in our previous paper [21]. The conversions of CH4 and CO2 were really low at room temperature when only the plasma catalyzed the reaction(s). The catalyst showed the initial activity from about 723K when only the catalyst worked for catalysis. Fig. 2 shows the experimental results of steam enhanced CO2 reforming of methane with the synergetic effect of catalyst and plasma under the same space velocity and power input. For the joint effect of catalyst and plasma, the conversions of CH4 and CO2 were all significantly improved to 0.62 and 0.41, respectively, with 20% steam at 773K. However, the space velocity, 1667 mL/(gCat h), was more than two times of 800 ml/ (gCat h) in the former experiments with only CO2 and CH4 [21]. Under these two experimental conditions, the same
Fig. 4 e The dependence of mole ratio of H2/CO in the product gases on the temperature under different steam content, RH2 O [ 20% with CH4 25 mL/min, CO2 25 mL/min and H2O 12.5 mL/min, RH2 O [ 25% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 12.5 mL/min, RH2 O [ 40% with CH4 25 mL/min, CO2 12.5 mL/min and H2O 25 mL/min.
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Fig. 5 e TGeDTG analysis of the catalyst for steam enhanced CO2 reforming of methane in the DBD plasma reactor.
Fig. 7 e Comparison of hybrid steam and CO2 reforming of methane and the individual steam reforming of methane in the DBD plasma reactor at space velocity of 1333 mL/(gCat h).
conversions have been observed (Fig. 6 in [21]). This indicated that the steam addition significantly improved the process efficiency. As such, the activity of the catalyst was higher with the steam addition than that with only CH4 and CO2 at 623K. Besides, the selectivities to CO and H2 were all above 0.98 at the temperature range between 623K and 773K. Hence the CO2 reforming was evidently enhanced by adding proper percentage of steam. The conversion of CH4 with 40% steam in the reactants was higher than that with 20% steam versus the increased temperature but lower at 675K, while the conversion of CO2 was even lower with 40% steam content. The difference could be attributed to the generated CO2 produced from steam reforming and water gas shift (WGS) reactions, which inhibited the further conversion of reactant CO2. In other words, when the steam content was 40%, the steam reforming took place more predominantly, leading to a lower conversion of CO2, while the conversion of CH4 was still at the same level as that of steam content at 20%. By decreasing the flow rate of CO2 from 25 mL/min to 12.5 mL/min and keeping CH4 and H2O at 25 mL/min and
12.5 mL/min, respectively, the steam content was increased while the space velocity was decreased, see the case 3 listed in Table 1. From the conversions of CH4 and CO2 shown in Fig. 3, the synergetic effect of catalyst and plasma was lower with high steam content and low space velocity. Especially for the conversion of CO2, lower CO2 content and higher steam content resulted in a decreased conversion rate of CO2. Also, the product gases with different H2/CO mole ratio were obtained corresponding to the different inlet gases, as shown in Fig. 4. Compared to the catalyst status after synthetic catalysis with the dry reforming of methane [21], the carbon deposition with steam enhanced CO2 and CH4 reforming was different. The quantity of carbon deposition was less with steam enhanced CO2 and CH4 reforming than that with only CO2 and CH4 reforming (see the TGeDTG curve in Fig. 5). In the DBD packed bed, the bed was easily blocked because of the serious carbon deposition in only 1 h with carbon dioxide reforming of
Fig. 6 e The XRD patterns of spent catalysts after CO2 reforming of methane in the DBD plasma reactor (a) without steam, (b) with steam.
Fig. 8 e Comparison of hybrid steam and CO2 reforming of methane and the individual steam reforming of methane in the DBD plasma reactor at space velocity of 1667 mL/(gCat h).
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methane. But with steam added carbon dioxide reforming of methane, the operating time was significantly increased that the bed was not blocked at all after one day continuously operation. This could be explained by the steam reaction with the formed carbon on the catalyst surface to reduce the carbon deposition. In order to further prove the difference of the carbon deposition, the XRD patterns of catalysts were given in Fig. 6, i.e., the carbon peak at around 26 is much weaker with spent catalysts after CO2 reforming of methane with steam. The NiAl2O4 peak was weak after adding the steam to the reactant, which suggested that the NiAl2O4 was reduced to Ni and Al2O3 in the synergetic catalysis with steam, CO2 and CH4.
Fig. 9 e Steam reforming of methane in DBD plasma reactor under different mole ratio of H2O/CH4 at 773K.
3.2.
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Plasma enhanced steam reforming of methane
After the experiments of the steam enhanced CO2 reforming of methane, the steam reforming catalyzed by plasma and catalyst was also investigated in this work. The CO2 in the reactant gas was replaced by H2O, and the space velocity was kept at 1333 mL/(gCat h). The mole ratio of CH4/H2O was 1:1, see the case 4 listed in Table 1. It was observed that the conversion of CH4 was largely increased and the synergetic effect decreased versus the increase of temperature, as shown in Fig. 7. Then we increased the flow rate of CO2 to 25 mL/min, and kept CH4 and H2O at 25 mL/min and 12.5 mL/min, where the space velocity was 1667 mL/(gCat h). Substitute CO2 with steam, then the flow rate of H2O was 37.5 mL/min and the CH4 was 25 mL/min, see the case 5 in Table 1. As a result, the conversion of CH4 increased because of the high steam content. Also the synergetic effect at 623K and 673K reached 1.88 and 2.13, respectively, as shown in Fig. 8. By increasing the mole ratio of H2O/CH4 (see case 6 and case 7 in Table 1), the conversion of CH4 reached almost 0.9 at 773K, as shown in Fig. 9(a). And the conversion of CH4 would level off after the mole ratio of H2O/CH4 reached 3. The synergetic effect increased first and then decreased with the increase of the mole ratio of H2O/CH4, which meant that there was an optimum mole ratio of H2O/CH4 for the most significant synergetic effect. The production of hydrogen was the main reason to investigate the steam reforming of methane with the plasma intensification. The test results showed that the maximum selectivity to the hydrogen could reach almost 99% under different mole ratio of H2O/CH4 while the maximum selectivity to the CO was a little bit lower than that of H2. As shown in Fig. 9 (c), when increasing the mole ratio of H2O/CH4 in the reactant gas, the selectivity of CO decreased versus the increased mole ratio of H2O/CH4 due to more CO2 production with the increase of steam. Compared to the products catalyzed only by plasma, the selectivities to CO and H2 were significantly improved under the synergetic catalysis of plasma and catalyst.
Fig. 10 e The selectivities to CO and H2 for the steam reforming of methane in DBD plasma reactor under different temperature and different mole ratio of H2O/CH4.
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Fig. 10 shows the selectivity to CO and/or H2 by the synergetic catalysis of catalyst and plasma versus the temperature (see case 4 and case 5 listed in Table 1). With the increase of temperature, the selectivity to H2 reached almost 99% at 773K and the selectivity to CO was improved to almost 75% because of the improvement of catalyst activity. Under this circumstance, the conversion of CH4 reached almost 90% (at 773K).
4.
Conclusions
Considering the inevitable high-temperature operation to implement the steam/CO2 reforming of methane using conventional catalysis method, this work investigated the low temperature conversion of steam, CO2 and methane to hydrogen and CO by the catalysis of plasma and catalyst. Firstly, the steam enhanced CO2 reforming of methane was studied in a DBD plasma reactor with catalyst. By adding a certain amount of steam to the reactants of CH4 and CO2, the conversions of CH4 and CO2 can be both improved and the carbon deposition was also reduced. Different mole ratio of H2/CO in the product gases was achieved by changing the percents of H2O, CO2 and CH4 in the reactants. With the increase of the steam content, the process efficiency was evidently improved, while the steam reforming of methane became more and more predominant than CO2 reforming of methane. Secondly, the pure steam reforming of methane was tested in the DBD plasma reactor. The conversion of CH4 can reach 90% catalyzed by plasma and catalyst at 773K, while the selectivity to H2 can reach almost 99%. There was an optimal mole ratio of H2O/CH4 to obtain the best synergetic effect of plasma and catalyst. The evidence of the reaction performance (i.e., the high conversion of CH4 and the high selectivity to H2) in the plasma reactor would attract much attention in academics for a clear understanding on the mechanisms behind the synergetic effect of plasma and catalyst as well as the steam enhanced reaction performance, which would be also the key to make the process practical to industrial application.
Acknowledgments Financial supports from National Natural Science Foundation of China (No. 20990223) and the Program for New Century Excellent Talents in University are acknowledged.
references
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