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An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming
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 x x x ( 2 0 1 4 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming Nadia O. Richards*, Paul A. Erickson Mechanical and Aeronautical Engineering, University of California Davis, One Shields Ave, Davis, CA 95616, USA
article info
abstract
Article history:
This research project explored a methanol reforming system using a stratified catalyst bed.
Received 20 December 2013
Commercially available catalysts were used within a single reactor and the system was run
Received in revised form
autothermally (fuel, steam and oxidizer). The investigation explored the fuel conversion
9 March 2014
and reactor temperature profile at various O2/CH3OH ratios. The experiments showed that
Accepted 18 March 2014
the stratified system had fairly high conversions at low O2/CH3OH ratios. Additionally, it
Available online xxx
showed high selectivity towards hydrogen, and low selectivity for carbon monoxide. The experimental results gathered show a promising use of stratified catalyst beds for small-
Keywords:
scale reforming systems.
Autothermal reformer
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Oxygen-to-carbon ratio Methanol Stratified Dual-bed catalyst
Introduction The production of hydrogen for fuel cells continues to be a vital area of research as the world moves into a new era of alternative fueled vehicles. Industrially hydrogen has been produced by natural gas steam reforming on a large-scale basis. However, for transportation purposes, smaller-scale hydrogen production methods continue to present challenges, and so have emerged as an exciting area of research. The most common method of reforming is the steam reforming process (SR) which occurs over an activated catalytic bed. SR consists of a highly endothermic fuel breakdown accompanied by the exothermic water gas shift (WGS) reaction. The WGS acts to remove the CO produced from the fuel
breakdown step, thus increasing the hydrogen produced. The SR Eq. (1) and WGS Eq. (2) reactions are shown below: CH3 OH þ H2 O/3H2 þ CO2 ; DHð298 KÞ ¼ 49 kJ=mol
(1)
CO þ H2 O4H2 þ CO2 ; DHð298 KÞ ¼ 41 kJ=mol
(2)
SR experiences heat transfer limitations due to the endothermic reaction process. External heat application is typically used to provide the energy needed to activate the catalyst. The catalyst along the reactor walls get heated through conduction. However within the packed bed the random ordering and pellet point-to-point contact cause low conduction rates, therefore convection becomes the dominant mode of heat transfer.
* Corresponding author. Tel.: þ1 530 754 5352. E-mail address: [email protected] (N.O. Richards). http://dx.doi.org/10.1016/j.ijhydene.2014.03.131 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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Fig. 1 e Catalysts used in reactor.
Consequently, large temperature gradients are observed near the walls for a SR packed bed reactor which can affect the catalyst performance [1]. In an effort to reduce that particular limitation in SR, researchers have explored different techniques. Active [2] and passive [3] methods have been shown to improve the heating of the center of the reactor where the endothermic reaction is occurring. Once the catalyst is active the fuel can be quickly broken apart and reassembled to the desired products based on the catalyst selectivity. Another reforming process uses no steam but rather oxygen, and is called partial oxidation (POX). The POX of methanol is shown in Eq. (3) and is highly exothermic. CH3 OH þ 0:5 O2 /2H2 þ CO2 ; DHð298 KÞ ¼ 192 kJ=mol
(3)
When POX is carried out over a catalytic bed it has been shown [4,5] that hydrogen is produced by an indirect mechanism where a portion of the fuel is combusted to produce CO2 and H2O, then the remaining fuel undergoes SR to produce H2. This is known as the two-step mechanism. However, in POX the combustion reactions produce a localized elevated temperature which can possibly sinter the catalyst and reduce performance. Additionally, using air as an oxidizer lowers the concentration of hydrogen produced thereby lowering the Nernst potential in a fuel cell. Combining SR and POX results in a process where steam, fuel and an oxidizer are reacted over a catalyst to produce H2. This is called autothermal reforming (ATR), and this method is used in this research project. ATR is a thermoeneutral reaction which allows for direct thermal contact for SR and POX over a single catalytic bed. The ideal overall ATR reaction for methanol is shown in Eq. (4) below. CH3 OH þ 0:23O2 þ 0:54H2 O/2:54H2 þ CO2 ; DHð298 KÞ ¼ 0 kJ=mol
(4)
ATR is a great candidate for use in small-scale hydrogen production given that it is compact, light-weight and has the ability to reform multiple fuels. Typical ATR reactions use a noble metal catalyst supported on a monolithic structure. The monolith does not easily allow for access to the inner active catalyst sites, and therefore ATR suffers from mass transfer limitations which hinder its performance. For these limitations the rate of the oxidation step is dependent on the rate of the reactants diffusion to and from the active catalyst sites [6]. This limitation is a critical factor to determine catalyst light-off.
Operating below the light-off temperature places the reactions in a kinetically limited region, since the combustion process is unable to sustain the reaction; whereas operating above the light-off temperature allows the reaction to be sustained with proper selection of O2/CH3OH ratios. ATR has also been shown to closely follow an equilibrium reaction [7] and therefore does not achieve high selectivity for hydrogen compared to a SR reaction. Possible ways to overcome that limitation have been investigated by using alternative catalyst compositions [8e10]. From the literature regarding stratified catalytic beds most authors [11e14] have focused on POX of methane using a dual bed process to promote the two-step mechanism. They have shown a benefit in using different catalysts to promote the separated oxidation and reforming steps. Some work explores operational methods to minimize the hot zone of the combustion section by using two reactors in series [11] or by splitting the oxygen feed to the system [12]. While proving beneficial for enhancing conversion these methods introduce a more complex system of operation in having to use multiple reactors, or requiring careful balance of the split feed to maintain high conversions. Other researchers have focused on developing and testing alternate catalysts made from different materials to lower the system costs [13e15]. It is useful to explore and develop new materials, it is also important to seek out new methods of using existing resources. The contribution of the stratified reformation method as presented here is to potentially stretch the performance of available catalysts in a single bed. This project seeks to utilize commercially available catalysts in a stratified configuration for methanol ATR. The system consists of a platinum group metal, typically used in ATR, placed upstream of a copper-based catalyst used for lower temperature SR, within a single reactor. The catalyst configuration seeks to support the desired reactions at each location inside the reactor. A key part of this research relies on using fuel, oxidizer, and steam as the reactants. The main focus is to explore the influence of the catalysts on reducing the limitations that are experienced in ATR/POX systems as well as SR systems. With a stratified bed, improved heat transfer to the cold regions of the SR section is expected. In turn this is expected to maintain an active SR catalyst. We seek to lower the amount of oxidizer required so that there will be less dilution for hydrogen in the product stream. Additionally, going to lower operating conditions can potentially allow for cheaper
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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 x x x ( 2 0 1 4 ) 1 e7
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systems to be built that do not require special high temperature materials.
Experimental facility The stratified bed used consists of commercially available catalysts from Su¨d Chemie and are shown in Fig. 1. The upstream catalyst was a monolithic ceria wafer that is wash-coated with platinum group metals (PGM) of a proprietary composition. Immediately downstream following the monolith, pelletized FCRM-2 catalyst was filled into the reactor bed. The composition is shown in Table 1. The stratified catalyst bed was loaded into the reactor as depicted in Fig. 2. The length of the monolith wafer was 0.635 cm (0.25 in.) and 143.2 g of pellets were used which had a packed length of 8.763 cm (3.45 in.). This is accordance to previous work that showed ATR on the PGM wafer performed similarly at varying lengths [16,17]. Therefore the shortest length of 0.635 cm (0.25 in.) was selected for operation. The reactor was constructed from a 4.09 cm diameter (1.5 in., Sch. 40) stainless steel pipe, threaded at both ends. Five nozzle band heaters (5.08 cm (2 in.) ID, 3.81 (1.5 in.) width) each with a rating of 120 V, 300 W were used to externally heat the system. The reactor outer surface was wrapped with a conductive aluminum tape to improve the heat transfer from the heaters. Along the length of the reactor twelve 0.32 cm (0.125 in.) MNPT to 0.32 cm (0.125 in.) pipe fittings were welded and used as thermocouple ports to measure the inner wall and centerline temperatures. The temperatures were measured using 0.159 cm (0.0625 in.) diameter stainless steel, sheathed, ungrounded K-type thermocouples. Additional temperature readings of the reactor outer wall and at the monolith exit/ pellet bed inlet were measured. The reactor was fully insulated using a high temperature cerawool blanket followed by a mineral wool clamshell for added rigidity. A schematic of the ATR research facility is shown in Fig. 3. The reforming process starts with pumping a water-methanol premix through a series of vaporizers. The vaporizers allow for a phase change of the premix from liquid to gas. At the same time clean compressed air is supplied using a mass flow controller and then routed to an air heater. The reactants meet at the inlet of the super heater to ensure sufficient mixing, while maintaining a steady temperature (w250 C) prior to entering the reactor. The products from the reaction are sent to a condensing unit, where any unreacted fuel and water are collected. The amount of methanol in the condensate was determined by empirical calibration which relied on the condensate density in a similar fashion to other research [7]. A sample of the dry gases are sent to a four-gas NOVA
Table 1 e Chemical composition of FCRM2 pellets. Chemical name
wt%
Zinc oxide Copper oxide Aluminum oxide Graphite
60e75 25e40 10e1 5e1
Fig. 2 e Reactor schematic.
Analytical Systems Inc. analyzer. The gas analyzer measures the concentrations of hydrogen, methane, carbon dioxide and carbon monoxide from the dry gas product stream. Hydrogen is detected by a temperature controlled thermal conductivity (TC) cell, while the remaining gases are detected by a single, microprocessor based Non-Dispersive Infra-Red (NDIR) detector.
Approach For all tests, the length of the monolith wafer and the amount of pellets were held constant, while the oxygen to methanol molar ratio (O2/CH3OH) was varied. The total volume of the stratified bed was 0.122 L. The experiments were all carried out using a steam: carbon molar ratio (S/C) of 1.5 and premix flow rate of 8.5 ml/min. The fuel energy input was calculated as 1.39 kW based on the product of the methanol molar flow rate and the LHV of methanol. Inlet temperatures and reactor wall temperatures were set to 250 C for all tests. The flow parameter of liquid hourly space velocity of methanol (LHSV-M) was used in the investigation. It characterizes the flow rate of liquid methanol per catalyst volume as shown in Eq. (5). For the premix flow of 8.5 ml/min the LHSV-M is 2.59 h1. LHSV M ¼
V_ CH3 OH ðlÞ Vcatalyst
(5)
Methanol conversion was calculated from the difference of the amount of fuel consumed in the reactor in relation to the fuel input to the reactor as shown in Eq. (6) where values are given on a molar basis. XCH3 OH ð%Þ ¼
CH3 OHIN CH3 OHOUT 100 CH3 OHIN
(6)
The gas selectivity for CO and H2 were each calculated from the ratio of the desired product to the amount of fuel
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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Fig. 3 e Schematic of ATR system. consumed shown in Eq. (7). In order to normalize the ratio to 100% the selectivity was multiplied by a stoichiometric factor. In the case of hydrogen the SF was 1/2.8 at 0.1 O2/CH3OH. Sð%Þ ¼
Desired Product Produced SF Reactant Consumed
(7)
Results and discussion Each test point was randomized and ran in triplicate. The average values were used in the analysis. All plots for the stratified system show the uncertainty at 2 standard deviations. The representative uncertainty at 0.1 O2/CH3OH was 1.81% in conversion, 0.59% in H2 selectivity and 2 C inlet temperature. The experimental data were collected at steady state to observe the non-transient nature of the stratified system.
temperature and S/C gave methanol conversions near 35%. Results from the stratified bed show that the pelletized section was catalyzing the reaction and was more active than the monolith at these conditions, as expected. Fig. 5 shows the methane concentration in the dry product gas. Under SR test conditions there was a large amount of methane present, which was not expected based on previous research of SR. Conversion was also low (34%) at this point as shown in Fig. 4. From these results operating the stratified bed under SR conditions is not beneficial when trying to improve fuel conversion or output gas purity. Some amount of oxidizer is required to gain a benefit to using a stratified system. The CH4
Stratified performance The stratified system was tested at various O2/CH3OH ratios ranging from 0 to 0.16. Fig. 4 shows the stratified bed performance compared to previous results for two single composition beds made of SR [2] and ATR [7] catalyst. Both single component beds were operated at 250 C inlet temperature, as was the stratified system. Having no air flow (or O2/CH3OH of zero and running in SR mode) the plot shows that the conversion for the stratified catalyst was about 34%. This result was similar compared to the SR catalyst bed. Other comparable copper-based SR systems [18,19] that had similar operating conditions of
Fig. 4 e Conversion vs. O2/CH3OH ratios.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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 x x x ( 2 0 1 4 ) 1 e7
Fig. 5 e CH4 concentrations for various O2/CH3OH ratios. concentration was drastically reduced in low (0.05) O2/CH3OH ratios. This is correlated to conversion increases. The true benefit of the stratified bed comes when operating as an ATR reaction under low O2/CH3OH ratios. The stratified bed showed that as the air flow increased, the conversion also started to increase. This is expected since there is increasing oxygen for POX mechanism to combust the fuel. However, the aim was to lower the O2/CH3OH required for ATR operation. The testing conditions at 0.1 and 0.16 O2/CH3OH were selected for their significant drop-off in conversion in the ATR catalyst. From our testing at these O2/CH3OH values, the stratified bed had greater conversions than the ATR catalyst. This can be attributed to the increased heat release from the monolith being used to maintain the activity of the pellets. This increased activity essentially allowed the plot to be shifted to the left to have higher conversion at lower O2/CH3OH ratios. Additionally, our system tested the use of a having a shorter ATR section compared to the SR section. The stratified reformation method demonstrated the ability to use a low amount of ATR catalyst to obtain improved fuel conversion and higher output gas purity than ATR alone, as discussed below. This supports the findings from other researchers using the twostep method [13,15,20] for methane POX in having smaller amounts of the oxidation catalyst than reforming catalyst. It is expected that increasing the packed bed length will allow an increased conversion for low O2/CH3OH ratios. This highlights a benefit of using a stratified system in being able to use lower amounts of air to reduce product dilution.
5
Fig. 6 e Hydrogen selectivity vs. O2/CH3OH for ATR and Stratified Catalysts.
SR section downstream seems to enhance the selectivity of the products, while allowing high conversion as shown above. Specifically in Fig. 6 at an O2/CH3OH of 0.1 and 0.16 the stratified system shows an increase for hydrogen selectivity as compared to the ATR catalyst alone by about 15%. This is also a result of having the pellets remain highly active from being heated by the upstream reaction. It supports the two-step mechanism of having the oxidation step being used to support the endothermic reforming reaction but allows for close coupling in a single bed. Similarly, Fig. 7 shows that the carbon monoxide was drastically reduced from 70% to under 10% for an O2/CH3OH of 0.1. Once again the pelletized section seems to benefit from the convected heat to remain active for the SR reaction. Other authors [13,14] demonstrate these improvements for the dual bed system, however they were obtained at higher O2/C ratios which heavily influence the performance. This work was focused on exploring lower O2/CH3OH ratios over the stratified bed to limit product dilution by nitrogen. Our findings showed that over the ATR catalyst the O2/CH3OH value corresponding to light-off occurred at 0.2. The effect on
Gas selectivity comparison of stratified catalyst to ATR catalyst Looking at the selectivity of hydrogen (Fig. 6) and carbon monoxide (Fig. 7), the stratified system shows that there is an improvement at lower O2/CH3OH values than were previously obtained from the conventional ATR system [7]. This improvement means that with this method one can increase the hydrogen selectivity while simultaneously lowering the carbon monoxide selectivity. This phenomenon has also been shown in single bed copper-based catalyst by other researchers [21,22]. For the stratified configuration placing the
Fig. 7 e Carbon monoxide selectivity vs. O2/CH3OH for ATR and stratified catalysts.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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hydrogen and carbon monoxide selectivity is highly visible in the plots (Figs. 6 and 7) at that value. For the stratified plots in both figures there was no drastic change in selectivity as would be indicative of catalyst light off. Therefore, all stratified tests were carried out below light-off and were still able to successfully maintain high selectivity for hydrogen and low selectivity for carbon monoxide.
bed temperatures influencing the WGS reaction causing the reverse to reaction to be favored. The 0.16 O2/CH3OH tests show there is a temperature increase across the monolith unlike before, followed by a decrease throughout the entire pellet section. This may be attributed to approaching possible light-off conditions on the pellets. Given their proximity, the temperature conditions at the inlet to the pellet section causes the back end of the monolith to see a temperature rise.
Reactor temperatures The axial centerline temperatures for each tested O2/CH3OH shown in Fig. 8 assist in the visualization of the stratified bed activity. The vertical dashed lines indicate the location of the monolith and pellet sections. In previous studies reactor temperatures have been shown at the inlet and/or exit bed temperatures [13,14]. Centerline temperatures shown here add insight to where reactions are taking place in the stratified reformer. The stratified reformer centerline temperature profile shows the temperature spikes at the inlet followed by a temperature decrease. This indicates a two-step mechanism occurring within the single bed where generally speaking the exothermic oxidation reactions occur upstream and endothermic reforming reactions occur downstream. For the case of 0 O2/CH3OH, the sharp decrease across the monolith wafer and continued decrease halfway through the pellets clearly shows the strong endothermic nature of the SR reaction. The latter half of the pellet bed has a slight increase in temperature indicating the exothermic WGS reaction may be occurring and helping to increase the temperature. As air is introduced to the reaction it causes a detectable difference in the temperature at the inlet of the monolith, and even further upstream to the incoming gases. The exothermic reactions are being supported by the monolith and heat is being transferred to the pellet section as expected. With the increased heat to the pellets they become more active and show similar bed temperature profiles as the SR case. Interestingly, at 0.10 O2/ CH3OH there is a much smaller temperature drop across the monolith. In the pelletized section the familiar decrease is present, but at the tail-end there is a slight decrease rather than an increase as before. This is possibly due to the higher
Fig. 8 e Centerline temperatures vs. reactor length for various O2/CH3OH ratios.
Conclusion A stratified catalyst bed consisting of a PGM monolithic wafer followed by copper-based pellets was tested with methanol fuel at a single flow rate. The system showed fairly high conversions at lower O2/CH3OH ratios than were previously used for ATR reactions. The monolithic catalyst upstream was beneficial in supplying heat to the downstream pellets. The highly active pellets resulted in the system having high hydrogen selectivity and low carbon monoxide selectivity. The findings from this study show a promising outcome in stratified ATR reforming of methanol and potentially for other longer chain fuels.
references
[1] Davieau DD. An analysis of space velocity and aspect ratio parameters in steam-reforming hydrogen production reactors. In: Mechanical and aeronautical engineering. Davis: University of California; 2004. [2] Erickson PA. Enhancing the steam-reforming process with acoustics: an investigation for fuel cell vehicle applications. United States e Florida: University of Florida; 2002. p. 161. [3] Liao C. Hydrogen production enhancement and the effect of passive mixing using flow disturbers in a steam-reforming reactor. Davis: United States e California: University of California; 2008. p. 181. [4] Mallens EPJ, Hoebink JHBJ, Marin GB. The reaction mechanism of the partial oxidation of methane to synthesis gas: a transient kinetic study over rhodium and a comparison with platinum. J Catal 1997;167(1):43e56. [5] Pengpanich S, Meeyoo V, Rirksomboon T. Methane partial oxidation over Ni/CeO2eZrO2 mixed oxide solid solution catalysts. Catal Today 2004;93e95(0):95e105. [6] Pfefferle L, Pfefferle W. Catalysis in combustion. Catal Rev Sci Eng 1987;29(2e3):219e67. [7] Dorr J. Methanol autothermal reformation: oxygen-to-carbon ratio and reaction progression. In: Mechanical and aeronautical engineering. Davis: University of California; 2004. [8] Lindstro¨m B, Agrell J, Pettersson LJ. Combined methanol reforming for hydrogen generation over monolithic catalysts. Chem Eng J 2003;93(1):91e101. [9] Yoon H, Erickson P, Kim H. Lowering the O2/CH3OH ratio in autothermal reforming of methanol by using a reduced copper-based catalyst. Int J Hydrogen Energy 2008;33(22):6619e26. [10] Ma L, Trimm DL. Alternative catalyst bed configurations for the autothermic conversion of methane to hydrogen. Appl Catal A Gen 1996;138. [11] Zhu J, Mujeebur Rahuman MSM, van Ommen JG, Lefferts L. Dual catalyst bed concept for catalytic partial oxidation of methane to synthesis gas. Appl Catal A Gen 2004;259(1):95e100.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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[12] Xu Jian, Wei Weisheng, Tian Aizhen, Fan Yu, Bao Xiaojun, Yu Changchun. Temperature profile in a two-stage fixed bed reactor for catalytic partial oxidation of methane to syngas. Catal Today 2010;149(1e2):191e5. [13] Tong GCM, Flynn J, Leclerc CA. A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas. Catal Lett 2005;102(3e4):131e7. [14] Nguyen BNT, Leclerc CA. Metal oxides as combustion catalysts for a stratified, dual bed partial oxidation catalyst. J Power Sources 2007;163(2):623e9. [15] Mark W, Smith DS, Berry David A, Haynes Daniel J, Floyd Donald L, Spivey James J, et al. Effect of the catalyst bed configuration on the partial oxidation of liquid hydrocarbons. Energy Fuels 2013;27. [16] Kim Hyung-Man, Choi Kap-Seung, Yoon Hyung Chul, Dorr J Lars, Erickson Paul A. An investigation of reaction progression through the catalyst bed in methanol autothermal reformation. J Mech Sci Technol 2008;22(2):367e73. [17] Yoon H, Larsdorr J, Erickson P. Fuel-lean and fuel-rich startup and shut-down processes in an autothermal reformer. Int J Hydrogen Energy 2008;33(12):2942e9.
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[18] Huang T-J, Chen H-M. Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2x catalysts. Int J Hydrogen Energy 2010;35(12):6218e26. [19] Yong-Feng L, Xin-Fa D, Wei-Ming L. Effects of ZrO2-promoter on catalytic performance of CuZnAlO catalysts for production of hydrogen by steam reforming of methanol. Int J Hydrogen Energy 2004;29(15):1617e21. [20] Bell Christa J, Leclerc Corey A. Effect of Relative bed lengths on a platinum/nickel stratified dual bed catalyst for the catalytic partial oxidation of methane at millisecond contact times. American Chemical Society; 2007. [21] Agrell J, Boutonnet M, Fierro JLG. Production of hydrogen from methanol over binary Cu/ZnO catalysts: part II. Catalytic activity and reaction pathways. Appl Catal A Gen 2003;253(1):213e23. [22] Velu S, Suzuki K, Okazaki M, Kapoor MP, Osaki T, Ohashi F. Oxidative steam reforming of methanol over CuZnAl(Zr)oxide catalysts for the selective production of hydrogen for fuel cells: catalyst characterization and performance evaluation. J Catal 2000;194(2):373e84.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming Nadia O. Richards*, Paul A. Erickson Mechanical and Aeronautical Engineering, University of California Davis, One Shields Ave, Davis, CA 95616, USA
article info
abstract
Article history:
This research project explored a methanol reforming system using a stratified catalyst bed.
Received 20 December 2013
Commercially available catalysts were used within a single reactor and the system was run
Received in revised form
autothermally (fuel, steam and oxidizer). The investigation explored the fuel conversion
9 March 2014
and reactor temperature profile at various O2/CH3OH ratios. The experiments showed that
Accepted 18 March 2014
the stratified system had fairly high conversions at low O2/CH3OH ratios. Additionally, it
Available online xxx
showed high selectivity towards hydrogen, and low selectivity for carbon monoxide. The experimental results gathered show a promising use of stratified catalyst beds for small-
Keywords:
scale reforming systems.
Autothermal reformer
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Oxygen-to-carbon ratio Methanol Stratified Dual-bed catalyst
Introduction The production of hydrogen for fuel cells continues to be a vital area of research as the world moves into a new era of alternative fueled vehicles. Industrially hydrogen has been produced by natural gas steam reforming on a large-scale basis. However, for transportation purposes, smaller-scale hydrogen production methods continue to present challenges, and so have emerged as an exciting area of research. The most common method of reforming is the steam reforming process (SR) which occurs over an activated catalytic bed. SR consists of a highly endothermic fuel breakdown accompanied by the exothermic water gas shift (WGS) reaction. The WGS acts to remove the CO produced from the fuel
breakdown step, thus increasing the hydrogen produced. The SR Eq. (1) and WGS Eq. (2) reactions are shown below: CH3 OH þ H2 O/3H2 þ CO2 ; DHð298 KÞ ¼ 49 kJ=mol
(1)
CO þ H2 O4H2 þ CO2 ; DHð298 KÞ ¼ 41 kJ=mol
(2)
SR experiences heat transfer limitations due to the endothermic reaction process. External heat application is typically used to provide the energy needed to activate the catalyst. The catalyst along the reactor walls get heated through conduction. However within the packed bed the random ordering and pellet point-to-point contact cause low conduction rates, therefore convection becomes the dominant mode of heat transfer.
* Corresponding author. Tel.: þ1 530 754 5352. E-mail address: [email protected] (N.O. Richards). http://dx.doi.org/10.1016/j.ijhydene.2014.03.131 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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Fig. 1 e Catalysts used in reactor.
Consequently, large temperature gradients are observed near the walls for a SR packed bed reactor which can affect the catalyst performance [1]. In an effort to reduce that particular limitation in SR, researchers have explored different techniques. Active [2] and passive [3] methods have been shown to improve the heating of the center of the reactor where the endothermic reaction is occurring. Once the catalyst is active the fuel can be quickly broken apart and reassembled to the desired products based on the catalyst selectivity. Another reforming process uses no steam but rather oxygen, and is called partial oxidation (POX). The POX of methanol is shown in Eq. (3) and is highly exothermic. CH3 OH þ 0:5 O2 /2H2 þ CO2 ; DHð298 KÞ ¼ 192 kJ=mol
(3)
When POX is carried out over a catalytic bed it has been shown [4,5] that hydrogen is produced by an indirect mechanism where a portion of the fuel is combusted to produce CO2 and H2O, then the remaining fuel undergoes SR to produce H2. This is known as the two-step mechanism. However, in POX the combustion reactions produce a localized elevated temperature which can possibly sinter the catalyst and reduce performance. Additionally, using air as an oxidizer lowers the concentration of hydrogen produced thereby lowering the Nernst potential in a fuel cell. Combining SR and POX results in a process where steam, fuel and an oxidizer are reacted over a catalyst to produce H2. This is called autothermal reforming (ATR), and this method is used in this research project. ATR is a thermoeneutral reaction which allows for direct thermal contact for SR and POX over a single catalytic bed. The ideal overall ATR reaction for methanol is shown in Eq. (4) below. CH3 OH þ 0:23O2 þ 0:54H2 O/2:54H2 þ CO2 ; DHð298 KÞ ¼ 0 kJ=mol
(4)
ATR is a great candidate for use in small-scale hydrogen production given that it is compact, light-weight and has the ability to reform multiple fuels. Typical ATR reactions use a noble metal catalyst supported on a monolithic structure. The monolith does not easily allow for access to the inner active catalyst sites, and therefore ATR suffers from mass transfer limitations which hinder its performance. For these limitations the rate of the oxidation step is dependent on the rate of the reactants diffusion to and from the active catalyst sites [6]. This limitation is a critical factor to determine catalyst light-off.
Operating below the light-off temperature places the reactions in a kinetically limited region, since the combustion process is unable to sustain the reaction; whereas operating above the light-off temperature allows the reaction to be sustained with proper selection of O2/CH3OH ratios. ATR has also been shown to closely follow an equilibrium reaction [7] and therefore does not achieve high selectivity for hydrogen compared to a SR reaction. Possible ways to overcome that limitation have been investigated by using alternative catalyst compositions [8e10]. From the literature regarding stratified catalytic beds most authors [11e14] have focused on POX of methane using a dual bed process to promote the two-step mechanism. They have shown a benefit in using different catalysts to promote the separated oxidation and reforming steps. Some work explores operational methods to minimize the hot zone of the combustion section by using two reactors in series [11] or by splitting the oxygen feed to the system [12]. While proving beneficial for enhancing conversion these methods introduce a more complex system of operation in having to use multiple reactors, or requiring careful balance of the split feed to maintain high conversions. Other researchers have focused on developing and testing alternate catalysts made from different materials to lower the system costs [13e15]. It is useful to explore and develop new materials, it is also important to seek out new methods of using existing resources. The contribution of the stratified reformation method as presented here is to potentially stretch the performance of available catalysts in a single bed. This project seeks to utilize commercially available catalysts in a stratified configuration for methanol ATR. The system consists of a platinum group metal, typically used in ATR, placed upstream of a copper-based catalyst used for lower temperature SR, within a single reactor. The catalyst configuration seeks to support the desired reactions at each location inside the reactor. A key part of this research relies on using fuel, oxidizer, and steam as the reactants. The main focus is to explore the influence of the catalysts on reducing the limitations that are experienced in ATR/POX systems as well as SR systems. With a stratified bed, improved heat transfer to the cold regions of the SR section is expected. In turn this is expected to maintain an active SR catalyst. We seek to lower the amount of oxidizer required so that there will be less dilution for hydrogen in the product stream. Additionally, going to lower operating conditions can potentially allow for cheaper
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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 x x x ( 2 0 1 4 ) 1 e7
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systems to be built that do not require special high temperature materials.
Experimental facility The stratified bed used consists of commercially available catalysts from Su¨d Chemie and are shown in Fig. 1. The upstream catalyst was a monolithic ceria wafer that is wash-coated with platinum group metals (PGM) of a proprietary composition. Immediately downstream following the monolith, pelletized FCRM-2 catalyst was filled into the reactor bed. The composition is shown in Table 1. The stratified catalyst bed was loaded into the reactor as depicted in Fig. 2. The length of the monolith wafer was 0.635 cm (0.25 in.) and 143.2 g of pellets were used which had a packed length of 8.763 cm (3.45 in.). This is accordance to previous work that showed ATR on the PGM wafer performed similarly at varying lengths [16,17]. Therefore the shortest length of 0.635 cm (0.25 in.) was selected for operation. The reactor was constructed from a 4.09 cm diameter (1.5 in., Sch. 40) stainless steel pipe, threaded at both ends. Five nozzle band heaters (5.08 cm (2 in.) ID, 3.81 (1.5 in.) width) each with a rating of 120 V, 300 W were used to externally heat the system. The reactor outer surface was wrapped with a conductive aluminum tape to improve the heat transfer from the heaters. Along the length of the reactor twelve 0.32 cm (0.125 in.) MNPT to 0.32 cm (0.125 in.) pipe fittings were welded and used as thermocouple ports to measure the inner wall and centerline temperatures. The temperatures were measured using 0.159 cm (0.0625 in.) diameter stainless steel, sheathed, ungrounded K-type thermocouples. Additional temperature readings of the reactor outer wall and at the monolith exit/ pellet bed inlet were measured. The reactor was fully insulated using a high temperature cerawool blanket followed by a mineral wool clamshell for added rigidity. A schematic of the ATR research facility is shown in Fig. 3. The reforming process starts with pumping a water-methanol premix through a series of vaporizers. The vaporizers allow for a phase change of the premix from liquid to gas. At the same time clean compressed air is supplied using a mass flow controller and then routed to an air heater. The reactants meet at the inlet of the super heater to ensure sufficient mixing, while maintaining a steady temperature (w250 C) prior to entering the reactor. The products from the reaction are sent to a condensing unit, where any unreacted fuel and water are collected. The amount of methanol in the condensate was determined by empirical calibration which relied on the condensate density in a similar fashion to other research [7]. A sample of the dry gases are sent to a four-gas NOVA
Table 1 e Chemical composition of FCRM2 pellets. Chemical name
wt%
Zinc oxide Copper oxide Aluminum oxide Graphite
60e75 25e40 10e1 5e1
Fig. 2 e Reactor schematic.
Analytical Systems Inc. analyzer. The gas analyzer measures the concentrations of hydrogen, methane, carbon dioxide and carbon monoxide from the dry gas product stream. Hydrogen is detected by a temperature controlled thermal conductivity (TC) cell, while the remaining gases are detected by a single, microprocessor based Non-Dispersive Infra-Red (NDIR) detector.
Approach For all tests, the length of the monolith wafer and the amount of pellets were held constant, while the oxygen to methanol molar ratio (O2/CH3OH) was varied. The total volume of the stratified bed was 0.122 L. The experiments were all carried out using a steam: carbon molar ratio (S/C) of 1.5 and premix flow rate of 8.5 ml/min. The fuel energy input was calculated as 1.39 kW based on the product of the methanol molar flow rate and the LHV of methanol. Inlet temperatures and reactor wall temperatures were set to 250 C for all tests. The flow parameter of liquid hourly space velocity of methanol (LHSV-M) was used in the investigation. It characterizes the flow rate of liquid methanol per catalyst volume as shown in Eq. (5). For the premix flow of 8.5 ml/min the LHSV-M is 2.59 h1. LHSV M ¼
V_ CH3 OH ðlÞ Vcatalyst
(5)
Methanol conversion was calculated from the difference of the amount of fuel consumed in the reactor in relation to the fuel input to the reactor as shown in Eq. (6) where values are given on a molar basis. XCH3 OH ð%Þ ¼
CH3 OHIN CH3 OHOUT 100 CH3 OHIN
(6)
The gas selectivity for CO and H2 were each calculated from the ratio of the desired product to the amount of fuel
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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Fig. 3 e Schematic of ATR system. consumed shown in Eq. (7). In order to normalize the ratio to 100% the selectivity was multiplied by a stoichiometric factor. In the case of hydrogen the SF was 1/2.8 at 0.1 O2/CH3OH. Sð%Þ ¼
Desired Product Produced SF Reactant Consumed
(7)
Results and discussion Each test point was randomized and ran in triplicate. The average values were used in the analysis. All plots for the stratified system show the uncertainty at 2 standard deviations. The representative uncertainty at 0.1 O2/CH3OH was 1.81% in conversion, 0.59% in H2 selectivity and 2 C inlet temperature. The experimental data were collected at steady state to observe the non-transient nature of the stratified system.
temperature and S/C gave methanol conversions near 35%. Results from the stratified bed show that the pelletized section was catalyzing the reaction and was more active than the monolith at these conditions, as expected. Fig. 5 shows the methane concentration in the dry product gas. Under SR test conditions there was a large amount of methane present, which was not expected based on previous research of SR. Conversion was also low (34%) at this point as shown in Fig. 4. From these results operating the stratified bed under SR conditions is not beneficial when trying to improve fuel conversion or output gas purity. Some amount of oxidizer is required to gain a benefit to using a stratified system. The CH4
Stratified performance The stratified system was tested at various O2/CH3OH ratios ranging from 0 to 0.16. Fig. 4 shows the stratified bed performance compared to previous results for two single composition beds made of SR [2] and ATR [7] catalyst. Both single component beds were operated at 250 C inlet temperature, as was the stratified system. Having no air flow (or O2/CH3OH of zero and running in SR mode) the plot shows that the conversion for the stratified catalyst was about 34%. This result was similar compared to the SR catalyst bed. Other comparable copper-based SR systems [18,19] that had similar operating conditions of
Fig. 4 e Conversion vs. O2/CH3OH ratios.
Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131
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Fig. 5 e CH4 concentrations for various O2/CH3OH ratios. concentration was drastically reduced in low (0.05) O2/CH3OH ratios. This is correlated to conversion increases. The true benefit of the stratified bed comes when operating as an ATR reaction under low O2/CH3OH ratios. The stratified bed showed that as the air flow increased, the conversion also started to increase. This is expected since there is increasing oxygen for POX mechanism to combust the fuel. However, the aim was to lower the O2/CH3OH required for ATR operation. The testing conditions at 0.1 and 0.16 O2/CH3OH were selected for their significant drop-off in conversion in the ATR catalyst. From our testing at these O2/CH3OH values, the stratified bed had greater conversions than the ATR catalyst. This can be attributed to the increased heat release from the monolith being used to maintain the activity of the pellets. This increased activity essentially allowed the plot to be shifted to the left to have higher conversion at lower O2/CH3OH ratios. Additionally, our system tested the use of a having a shorter ATR section compared to the SR section. The stratified reformation method demonstrated the ability to use a low amount of ATR catalyst to obtain improved fuel conversion and higher output gas purity than ATR alone, as discussed below. This supports the findings from other researchers using the twostep method [13,15,20] for methane POX in having smaller amounts of the oxidation catalyst than reforming catalyst. It is expected that increasing the packed bed length will allow an increased conversion for low O2/CH3OH ratios. This highlights a benefit of using a stratified system in being able to use lower amounts of air to reduce product dilution.
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Fig. 6 e Hydrogen selectivity vs. O2/CH3OH for ATR and Stratified Catalysts.
SR section downstream seems to enhance the selectivity of the products, while allowing high conversion as shown above. Specifically in Fig. 6 at an O2/CH3OH of 0.1 and 0.16 the stratified system shows an increase for hydrogen selectivity as compared to the ATR catalyst alone by about 15%. This is also a result of having the pellets remain highly active from being heated by the upstream reaction. It supports the two-step mechanism of having the oxidation step being used to support the endothermic reforming reaction but allows for close coupling in a single bed. Similarly, Fig. 7 shows that the carbon monoxide was drastically reduced from 70% to under 10% for an O2/CH3OH of 0.1. Once again the pelletized section seems to benefit from the convected heat to remain active for the SR reaction. Other authors [13,14] demonstrate these improvements for the dual bed system, however they were obtained at higher O2/C ratios which heavily influence the performance. This work was focused on exploring lower O2/CH3OH ratios over the stratified bed to limit product dilution by nitrogen. Our findings showed that over the ATR catalyst the O2/CH3OH value corresponding to light-off occurred at 0.2. The effect on
Gas selectivity comparison of stratified catalyst to ATR catalyst Looking at the selectivity of hydrogen (Fig. 6) and carbon monoxide (Fig. 7), the stratified system shows that there is an improvement at lower O2/CH3OH values than were previously obtained from the conventional ATR system [7]. This improvement means that with this method one can increase the hydrogen selectivity while simultaneously lowering the carbon monoxide selectivity. This phenomenon has also been shown in single bed copper-based catalyst by other researchers [21,22]. For the stratified configuration placing the
Fig. 7 e Carbon monoxide selectivity vs. O2/CH3OH for ATR and stratified catalysts.
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hydrogen and carbon monoxide selectivity is highly visible in the plots (Figs. 6 and 7) at that value. For the stratified plots in both figures there was no drastic change in selectivity as would be indicative of catalyst light off. Therefore, all stratified tests were carried out below light-off and were still able to successfully maintain high selectivity for hydrogen and low selectivity for carbon monoxide.
bed temperatures influencing the WGS reaction causing the reverse to reaction to be favored. The 0.16 O2/CH3OH tests show there is a temperature increase across the monolith unlike before, followed by a decrease throughout the entire pellet section. This may be attributed to approaching possible light-off conditions on the pellets. Given their proximity, the temperature conditions at the inlet to the pellet section causes the back end of the monolith to see a temperature rise.
Reactor temperatures The axial centerline temperatures for each tested O2/CH3OH shown in Fig. 8 assist in the visualization of the stratified bed activity. The vertical dashed lines indicate the location of the monolith and pellet sections. In previous studies reactor temperatures have been shown at the inlet and/or exit bed temperatures [13,14]. Centerline temperatures shown here add insight to where reactions are taking place in the stratified reformer. The stratified reformer centerline temperature profile shows the temperature spikes at the inlet followed by a temperature decrease. This indicates a two-step mechanism occurring within the single bed where generally speaking the exothermic oxidation reactions occur upstream and endothermic reforming reactions occur downstream. For the case of 0 O2/CH3OH, the sharp decrease across the monolith wafer and continued decrease halfway through the pellets clearly shows the strong endothermic nature of the SR reaction. The latter half of the pellet bed has a slight increase in temperature indicating the exothermic WGS reaction may be occurring and helping to increase the temperature. As air is introduced to the reaction it causes a detectable difference in the temperature at the inlet of the monolith, and even further upstream to the incoming gases. The exothermic reactions are being supported by the monolith and heat is being transferred to the pellet section as expected. With the increased heat to the pellets they become more active and show similar bed temperature profiles as the SR case. Interestingly, at 0.10 O2/ CH3OH there is a much smaller temperature drop across the monolith. In the pelletized section the familiar decrease is present, but at the tail-end there is a slight decrease rather than an increase as before. This is possibly due to the higher
Fig. 8 e Centerline temperatures vs. reactor length for various O2/CH3OH ratios.
Conclusion A stratified catalyst bed consisting of a PGM monolithic wafer followed by copper-based pellets was tested with methanol fuel at a single flow rate. The system showed fairly high conversions at lower O2/CH3OH ratios than were previously used for ATR reactions. The monolithic catalyst upstream was beneficial in supplying heat to the downstream pellets. The highly active pellets resulted in the system having high hydrogen selectivity and low carbon monoxide selectivity. The findings from this study show a promising outcome in stratified ATR reforming of methanol and potentially for other longer chain fuels.
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Please cite this article in press as: Richards NO, Erickson PA, An investigation of a stratified catalyst bed for small-scale hydrogen production from methanol autothermal reforming, International Journal of Hydrogen Energy (2014), http:// dx.doi.org/10.1016/j.ijhydene.2014.03.131