(617a) An Investigation of a Stratified Catalyst Bed for Small-Scale Hydrogen Production From Methanol Autothermal Reforming

Introduction

Autothermal reforming (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. Unfortunately, as with a steam reforming (SR) system, ATR requires downstream cleanup stages to purify the product stream for use with fuel cell systems.

Research Objectives and Contribution

The overall purpose of this research project is to explore a methanol reforming system using a stratified catalyst bed. This means that by using different catalyst types the reaction will progress according to the catalyst selectivity. The system will include two spatially separated catalysts within a single reactor and will be run autothermally (fuel, steam and oxidizer). The types of catalysts to be used are a standard monolithic noble-metal combustion catalyst, typically used in ATR; along with a commercially available pelletized copper-based water gas shift catalyst used in SR. The ATR catalyst will be upstream and the SR placed immediately behind it. Including a SR section behind the ATR catalyst will allow for a reduction in the limitations experienced by each system. 

ATR systems experience mass transfer limitations since the rate of the oxidation step is dependent on the rate of the reactants diffusion to and from the active catalyst sites [1]. 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  ratios. ATR closely follows an equilibrium reaction [2] and therefore does not achieve high selectivity for hydrogen compared to a SR reaction. Possible ways to overcome this limitation have been investigated by using alternative catalyst compositions [3-5] however the combined effects of geometry and configuration have yet to be explored.

Within the SR catalyst bed, heat transfer limitations are experienced 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 contacts cause low conduction rates, therefore convection becomes the dominant mode of heat transfer. Consequently, large temperature gradients are observed near the walls for a SR packed bed reactor which can affect the catalyst performance [6]. In an effort to reduce that particular limitation in SR researchers have explored different techniques. Active [7] and passive [8] 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.

The motivation for the stratified system is that by strategically placing the catalysts in the reactor the reaction will progress according to the catalyst selectivity thereby reducing unwanted intermediates. Essentially this should allow the system to maintain high conversion while ensuring the output gas has a higher concentration of hydrogen than a comparable traditional ATR system. It is also expected to be better equipped to handle transients and have shorter start-up times than a SR system. Additionally, it will be beneficial to small-scale systems since the amount of catalyst can be reduced while maintaining high fuel conversion and hydrogen selectivity. Therefore it should be possible to go to a smaller system in which the fuel breakdown can occur over a small section of noble metal catalyst to be further reformed by a pellet catalyst bed. Overall the findings will contribute to the knowledge of small-scale reforming systems using commercially available catalysts.

References

[1] Pfefferle, L., and Pfefferle, W., 1987, "Catalysis in combustion," Catalysis Reviews Science and Engineering, 29(2-3), pp. 219-267.

[2] Dorr, J., 2004, "Methanol Autothermal Reformation: Oxygen-to-Carbon Ratio and Reaction Progression," M.S., University of California, Davis.

[3] Lindström, B., Agrell, J., and Pettersson, L. J., 2003, "Combined methanol reforming for hydrogen generation over monolithic catalysts," Chemical Engineering Journal, 93(1), pp. 91-101.

[4] Yoon, H., Erickson, P., and Kim, H., 2008, "Lowering the O2/CH3OH ratio in autothermal reforming of methanol by using a reduced copper-based catalyst," International Journal of Hydrogen Energy, 33(22), pp. 6619-6626.

[5] L. Ma, D. L. T., 1996, "Alternative catalyst bed configurations for the autothermic conversion of methane to hydrogen," Applied Catalysis A: General.

[6] Davieau, D. D., 2004, "An Analysis of Space Velocity and Aspect Ratio Parameters in Steam-Reforming Hydrogen Production Reactors," M.S., University of California, Davis.

[7] Erickson, P. A., 2002, "Enhancing the steam-reforming process with acoustics: An investigation for fuel cell vehicle applications," Ph.D. 3065931, University of Florida, United States -- Florida.

[8] Liao, C., 2008, "Hydrogen production enhancement and the effect of passive mixing using flow disturbers in a steam-reforming reactor," Ph.D. 3329636, University of California, Davis, United States -- California.