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Energy & Fuels 2007, 21, 2066-2071
Biogas to Syngas by Microwave-Assisted Dry Reforming in the Presence of Char A. Domı´nguez, Y. Ferna´ndez, B. Fidalgo, J. J. Pis, and J. A. Mene´ndez* Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 OViedo, Spain ReceiVed February 26, 2007. ReVised Manuscript ReceiVed April 25, 2007
The aim of this paper is to study the reforming of CH4 with CO2 using as a catalyst a rich potassium char obtained from biomass pyrolysis. The reaction was carried out at 800 °C by means of two different methods of heating, microwave and electrical heating (MWH and EH, respectively). In addition, the individual reactions proposed for the dry reforming of methane, that is, (i) the decomposition of CH4 to form hydrogen and carbon and (ii) the dissociative adsorption of CO2 followed by reduction to give CO, were also studied with both heating methods at the same temperature. The results showed that MWH produces hot spots (microplasmas located inside the catalyst bed) that favor heterogeneous catalytic reactions. Thus, it was found that the conversion of CH4 and CO2 in the individual reactions was greater for MWH than for EH. An examination of the CH4 decomposition reaction proved the formation of coke deposits, which reduced catalytic activity and CH4 conversion. When a CH4/CO2 mixture was used, this problem was minimized since the CO2 partly removed the carbon deposits formed, thereby prolonging the activity of the catalyst. This gasification reaction, catalyzed by the high level of K contained in the char, provides an “in situ” route for catalyst regeneration. The results indicated that the presence of CO2 increased the conversion of CH4 to H2, the values being higher in MWH than in EH. Both heating methods produced an outlet gas composed mainly of syngas (CO + H2) and practically free of CO2 and CH4 (especially in the case of MWH). In addition, the study of the exhausted catalysts by scanning electron microscopy revealed the presence of significant amounts of carbon nanofibers on the char surface, but only in the case of MWH, these nanofibers being more abundant in the dry reforming reaction than in the single CH4 decomposition.
Introduction In a recent publication by our group,1 we investigated the conventional and microwave-induced pyrolysis of coffee hulls at different temperatures for the production of a hydrogen-rich fuel gas. These experiments gave rise to a large gas yield with a high syngas content, which was even higher in the case of microwave heating. To explain the results, it was postulated that self-gasification of the char with the CO2 produced during the pyrolysis and CH4 decomposition to H2 was taking place and that microwave heating would promote these reactions. These results are in agreement with other works where it is reported that heterogeneous reactions and heterogeneous catalytic reactions are favored under microwave heating.2-4 In order to confirm whether microwave heating promotes the CO2 gasification reaction (eq 1), even at relatively low temperatures, the char obtained from the pyrolysis experiments was subjected to reaction with CO2 at different temperatures using both methods of heating:5
C + CO2 T 2CO
∆H298 ) 173 kJ/mol
(1)
* Corresponding author. E-mail:
[email protected]. (1) Domı´nguez, A.; Mene´ndez, J. A.; Ferna´ndez, Y.; Pis, J. J.; Valente Nabais, J. M.; Carrott, P. J. M.; Ribeiro Carrott, M. M. L. J. Anal. Appl. Pyrolysis 2007, 79, 128-135. (2) Zhang, X.; Lee, C. S.-M.; Mingos, D. M. P.; Hayward, D. O. Catal. Lett. 2003, 88, 129-139. (3) Sekiguchi, H.; Mori, Y. Thin Solid Films 2003, 435, 44-48. (4) Cooney, D. O.; Xi, Z. Fuel Sci. Technol. Int. 1996, 14, 1111-1141. (5) Mene´ndez, J. A.; Domı´nguez, A.; Ferna´ndez, Y.; Pis, J. J. Energy Fuels 2007, 21, 373-378.
It was found that gasification of the char by CO2 was improved by microwave heating, and that the relatively large amounts of K present in the ashes of the char may also favor this reaction. This effect of K has also been mentioned by other authors.6 The hypothesis that CH4 decomposition in the presence of a char (eq 2) is also favored by microwave heating is investigated in the present paper.
CH4 T C + 2H2
∆H298 ) 75.6 kJ/mol
(2)
Moreover, the combination of these two reactions (eqs 1 and 2), which give rise to the dry reforming reaction of methane (eq 3), is also studied:
CH4 + CO2 T 2CO + 2H2
∆H298 ) 260.5 kJ/mol
(3)
The dry reforming of methane has received widespread interest in recent years due to its potential application in industry and energy storage7 and due to the environmental benefits it offers, since both methane and carbon dioxide contribute to the greenhouse effect. The major problem in the way of its industrial application8 is the formation of carbon, which leads to catalyst deactivation. Thus, the main objective of CO2 reforming studies has been to develop suitable catalysts and to optimize their lifetime stability. Numerous materials have been tested as potential catalysts for the reforming of CH4 with CO2. Catalysts (6) Juan-Juan, J.; Roma´n-Martı´nez, M. C.; Illa´n-Go´mez, M. J. Appl. Catal., A 2006, 301, 9-15. (7) Wang, S.; Lu, G. Q. Energy Fuels 1996, 10, 896-904. (8) Zhang, Z. L.; Verykios, X. E. Catal. Today 1994, 21, 589-595.
10.1021/ef070101j CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007
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Table 1. Main Chemical Characteristics of the Char Chemical Characteristics ultimate analysis (wt %)a
proximate analysis asha volatile mattera fixed carbonb
24.4 12.3 63.3
C (%) H (%) N (%) S (%) O (%) H/C (%)
69.3 0.7 1.6 0.3 3.7 0.12
Inorganic Composition of the Ashes (Expressed as wt % of Metal Oxidesa) K2O 40.3 a
CaO 29.6
MgO 11.3
SO3 7.0
P 2 O5 5.2
SiO2 4.7
Fe3O2 1.3
MnO 0.2
CuO 0.2
ZnO 0.2
SrO 0.2
Dry basis. b Calculated by difference.
based on noble metals7 are reported to be less sensitive to coking than transition metal catalysts (e.g., Ni, Fe, Co, Pd, etc.).9,10 Although some noble metals show a high activity and selectivity for carbon-free operation, their high cost and limited availability prevent them from being used commercially. Muradov et al.11-13 have proposed the use of carbon-based catalysts for hydrocarbon pyrolysis reactions since their availability, durability, and low cost offer several advantages over metal catalysts. In view of the fact that biogas is composed mainly of these two gases, the dry reforming of biogas could be considered as an important route in syngas production (H2 + CO). Thus, the main objective of this work is to investigate the possibility of converting biogas into syngas by the autoreforming of CH4 with CO2, both of which are present in the biogas. In order to carry this out, we used a combination of microwave heating and potassium-rich char, which acts as a catalyst and microwave absorber. In addition, the performance of microwave heating was tested for each single reaction (eqs 1 and 2), and the results were compared with those of conventional heating. Experimental Section Coffee hulls pressed into small pellets (approximately 3 mm in diameter × 2 cm in length) were pyrolyzed separately in an electrical furnace to 1000 °C under an inert atmosphere to produce char.5 The main chemical characteristics and the ash composition of the char are shown in Table 1. The proximate analysis (ash, volatile matter, and fixed carbon) and the elemental analysis (C, H, O, N, and S) were carried out in a LECO TGA-601 thermobalance and in a LECO CNHS-932 apparatus, respectively; the oxygen content was calculated by difference. The ash composition was determined by X-ray fluorescence using a BW 14-80 spectroscope from Phillips, equipped with a scandium-molybdenum dual tube as the source of X-rays. The reactions studied were performed in an electrical furnace and in a single-mode microwave oven in order to compare the different methods of heating. The experiments were conducted using a quartz reactor (45 cm in length × 2.2 cm i.d.). The gasification experiments were performed on 3.5 g of char (size 1-3 mm) using CO2 at a flow rate of 60 cm3 min-1, that is, a volumetric hourly space velocity (VHSV) of 0.166 L g-1 h-1. The methane decomposition reaction was performed on 13 g of char (size 0.5-3 mm) using CH4 at a flow rate of 36 cm3 min-1, that is, a VHSV of 0.166 L g-1 h-1. In order to simulate the composition of the biogas, and (9) Effendi, A.; Hellgardt, K.; Zhang, Z. G.; Yoshida, T. Catal. Commun. 2003, 4, 203-207. (10) Rostrup-Nielsen, J. R.; Bak Hansen, J.-H. J. Catal. 1993, 144, 3849. (11) Muradov, N.; Smith, F.; Huang, C.; T-Raisi, A. Catal. Today 2006, 116, 281-288. (12) Muradov, N.; Smith, F.; T-Raissi, A. Catal. Today 2005, 102103, 225-233. (13) Muradov, N. Catal. Commun. 2001, 2, 89-94.
to assess the possibility of directly converting biogas to syngas, a gaseous mixture of CH4 and CO2 (1:1 ratio) was used. Thus, the dry reforming reaction was performed on 13 g of char (size 0.5-3 mm) using CH4 and CO2 at flow rates of 36 cm3 min-1, in such a way that the final flow rate was 72 cm3 min-1, that is, a VHSV of 0.332 L g-1 h-1. All of the experiments were conducted at 800 °C for 30 min in the case of the gasification reaction, and for 120 min in the case of methane decomposition and the dry reforming of methane. Before introducing the reactant gases, the system was flushed with a flow rate of nitrogen of 60 cm3 min-1 for 20 min and then heated up to the reaction temperature under this inert atmosphere. At the end of the experiments, the mixture of the reactant gases was switched off and the sample was cooled down to room temperature under a nitrogen gas atmosphere. The product gas was collected in Tedlar sample bags at different reaction times. The gases were analyzed in a Varian CP-3800 gas chromatograph equipped with a thermal conductivity detector (TCD) and two columns connected in series. The first column was an 80/100 Hayesep Q (2 m × 1/8 in × 2 mm) and the second column was a 80/100 Molesieve 13X (1.5 m × 1/8 in × 2 mm). The second column was bypassed by a six-port valve for the analysis of CO2 and hydrocarbons (>C2). The initial oven temperature was 60 °C, which was held for 1.5 min. It was then programmed to rise from 60 to 90 °C at 30 °C/min with an isotherm held for 2 min. The temperature was then lowered from 90 to 60 °C at 50 °C/min and held for 2 min. The TCD was calibrated with a standard gas mixture at periodic intervals. In the gasification reaction, the gases leaving the reactor contained mainly CO2 and CO, and CH4 and H2 in the case of the decomposition of methane. The dry reforming reaction involves a combination of these reactions, so that all of these gases were found to be present in the effluent gas. The conversions of methane and carbon dioxide were calculated by means of the following equations: CH4 conversion, % ) 100[(H2)out/2]/[(CH4)out + (H2)out/2] CO2 conversion, % ) 100[(CO)out/2]/[(CO2)out + (CO)out/2] where (CH4)out, (H2)out, (CO2)out, and (CO)out are the methane, hydrogen, carbon dioxide, and carbon monoxide concentrations in the effluent gas (vol %), as determined by gas chromatography. Samples of the char before and after the CH4 decomposition and the reforming of methane with CO2 were examined using a scanning electron microscope (SEM), a DSM 942 from Zeiss equipped with an energy dispersive X-ray (EDX) detector (OXFORD LINK-ISIS).
Results and Discussion The char obtained from the pyrolysis of coffee hulls used in this work was found to perform remarkably well in the conversion of CO2 to CO (according to reaction 1).5 The reason for this is its high potassium content (see Table 1), which catalyzes the carbon gasification reaction; other metallic elements present in the ashes could also have certain catalytic activity in this reaction, but its content in the ashes is very low compared to that of K. The K present in the char surface was also observed by SEM-EDX. Thus, Figure 1 presents a general view of the char used as a catalyst in the experiments. Some clear areas are visible in the picture, extending over the whole surface but in a haphazard way. EDX analyses showed that these spots correspond to high K-content deposits. Furthermore, it was found that microwave heating (MWH) enhances the displacement of the equilibrium to CO production, with respect to conventional (electrical) heating (EH), the difference in performance increasing as the temperature at which the reaction takes place decreases.5 In Figure 2, it can be seen that, while at 800 °C the conversion of CO2 is practically 100% in the MWH, it falls to about 80% for EH. On the basis of these findings and
2068 Energy & Fuels, Vol. 21, No. 4, 2007
Figure 1. SEM image of the char.
Figure 2. Variation of the CO2 conversion at 800 °C with time, under conventional (EH) and microwave heating (MWH). VHSV ) 0.166 L g-1 h-1.
some results in the literature for other heterogeneous or heterogeneously catalyzed reactions,2,14 it can be inferred that microwave heating favors gas-solid reactions. Also gas-phase reactions catalyzed by a dielectric solid, that is, a solid that absorbs microwaves (e.g., a char or a carbon when used as a catalyst or catalyst support) were also promoted. Thus, during MWH, the char (catalyst) is at a much higher temperature than the surrounding gases, while in conventional heating, the temperature gradient is inverse. In addition, MWH gives rise to hot spots, which might be considered as “microplasmas”, located in the dielectric solid, where the temperature is much higher than the average temperature of the bed as measured by the optical pyrometer.5 It seems, therefore, that heterogeneous gas-solid reactions are favored by the higher temperature of the MWH. In accordance with the heating mechanisms outlined above, the decomposition of CH4 (reaction 2) catalyzed by the actives’ centers on the char surface might also be expected to be favored by MWH. Experiments performed without char, in the electrical furnace, showed that, at 800 °C, there is no conversion of CH4 at all. However, when CH4 is passed through a bed of char (see the Experimental Section), there is a considerable selective conversion to H2, no other compounds being produced. Figure 3 shows that this conversion is especially high at the beginning of the experiments. Moreover, it can be observed that the conversion is greater when MWH is used. This assessment, however, is only true for the initial stage of the process (approximately 40 min). Then, the methane conversion values (14) Zlotorzynski, A. Crit. ReV. Anal. Chem. 1995, 25, 43-76.
Domı´nguez et al.
Figure 3. Variation of the CH4 conversion at 800 °C with time, under conventional (EH) and microwave heating (MWH). VHSV ) 0.166 L g-1 h-1.
fall below those obtained using EH. In fact, at the beginning, the conversion was practically 100%, but it fell relatively quickly to values close to 20%. This can be due to blockage of the active centers on the catalyst surface by carbon deposits. Given that MWH is more efficient in converting CH4 to H2, the carbon deposits are also more abundant, and so the active centers are blocked more quickly. Therefore, the main problem in using this char as a catalyst for the CH4 decomposition reaction is its rapid loss of activity. Other researchers investigating the use of different carbonaceous-based catalysts (char,15 activated carbon,13,16 carbon black17) for the decomposition of CH4 (using only conventional heating) found a similar trend. One possible way to overcome the problem of carbon deposits is to combine reactions 1 and 2. Thus, it is postulated that, if a mixture of CO2 and CH4 is passed through the char bed and heated in a similar way to that described above, CH4 decomposition will give rise to H2 and carbon deposits (reaction 2). However, these deposits can be continuously removed by the CO2, giving rise to CO (reaction 1). To test this hypothesis, a 50/50 mixture of CH4/CO2 was treated as described in the Experimental Section. The results showed (Figure 4) that the conversion of CO2 remains very high during the first 2 h of treatment and is slightly higher in the case of MWH. In the case of EH, there is a synergistic effect that is also worth noting. Thus, CO2 conversion is higher for the CH4/CO2 mixture than when CO2 reacts alone. This increase may be due to the higher reactivity of a certain type of carbon atom (R-carbon7,8) present in the carbon deposits with respect to the carbon atoms contained in the char. Thus, reaction 4 would continue to be the main reaction, although reaction 5 would also occur.
C(char) + CO2 ) 2CO
(4)
C(CH4) + CO2 ) 2CO
(5)
Another possible mechanism that might explain the increase in CO2 conversion is based on the following reactions (reactions 6 and 7):
CO2 + H2 ) CO + H2O
(6)
(15) Bai, Z.; Chen, H.; Li, B.; Li, W. Int. J. Hydrogen Energy 2006, 31, 899-905. (16) Kim, M. H.; Lee, E. K.; Jun, J. H.; Kong, S. J.; Han, G. Y.; Lee, B. K. Int. J. Hydrogen Energy 2004, 29, 187-193. (17) Lee, E. K.; Lee, S. Y.; Han, G. Y.; Lee, B. K.; Lee, T.-J.; Jun, J. H. Carbon 2004, 42, 2641-2648.
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Figure 5. Variation of the gas composition with time in the reforming of methane with carbon dioxide at 800 °C, under microwave heating (white symbols) and conventional heating (black symbols). VHSV ) 0.332 L g-1 h-1.
Figure 4. Variation of the CH4 and CO2 conversions at 800 °C with time, under (a) microwave heating (MWH) and (b) conventional heating (EH). VHSV ) 0.332 L g-1 h-1.
However, the water produced in reaction 6 was not observed during the experiments; that is, no water condensation was detected, and the outlet gases did not contain any water vapor. The water was more probably consumed in the gasification of the more reactive carbon atoms giving H2 and CO:
C(CH4) + H2O ) H2 + CO
(7)
In any case, although this mechanism cannot be totally discarded, the combination of reactions 6 and 7 is equivalent to reaction 5, so the final result would be the same. The conversion of CH4 to H2 is kept at a much higher level than for mere CH4 decomposition during the 2 h of the experiment. In the case of EH, there was an initial drop in the conversion from 90% to 70% for a period of 10 min followed by a smooth decrease until it reached values of around 60% after 2 h. Consistent with the hypothesis that MWH is more efficient, MWH conversion ranges from 95 to 80% throughout the whole experiment. These results agree with the constant removal of carbon deposits from the active centers on the catalyst surface due to the action of CO2. The “cleaning” of the active centers seems to be more favored in MWH than in EH, indicating that MWH presents a certain degree of selectivity, with respect to EH, in the reaction of CO2 with the deposited carbons. Accordingly, reaction 5 would be more favored in MWH than in EH. Figure 5 shows the composition (vol %) of the gas produced in the CO2 reforming of CH4 using EH and MWH. As a consequence of the high conversions, an outlet gas with a high concentration of syngas (around 90% in EH and 95% in MWH) and practically free of CO2 (
