(424d) Development of a Catalytic Micro-Combustor for a Microchannel Methanol Steam Reformer

Methanol is often considered the best liquid fuel to generate hydrogen due to its high energy density, low cost and easy transportation and reforming. The use of metallic micro-reactors for hydrogen production is gaining an increasing importance regarding portable and low-cost hydrogen sources for numerous applications, particularly for feeding fuel cells. Methanol steam reforming is an endothermic reaction. Therefore, a heat source such as e.g. combustors should be incorporated into the system to maintain reaction temperature and to supply the heat required for the reforming reaction and reactants vaporization. The design, construction and operation of plate-heat-exchanger reactors for application as reformers have been investigated intensively within the last decade [1]. It has been demonstrated that an internal heating by integrated combustion is more efficient than using hot gas from external burners. Methanol combustion can be used because of simplicity.

In the present work, an integrated catalytic microchannel reactor with a micro-combustor coated with Pd/Al2O3 catalyst and a methanol steam micro-reformer coated with Pd/ZnO catalyst was designed and fabricated. The performance of the micro-reformer with the integrated combustor was evaluated under various conditions.

Catalysts were deposited on the microchannel block by washcoating. Several preliminary tests of slurry formulation for washcoating the structural substrates were carried out and the coating characteristics (specific load, homogeneity and adherence) were analyzed to establish the best recipe.

The catalytic tests were carried out at atmospheric pressure in a Computerized Microactivity Reference Catalytic Reactor from PID Eng&Tech. 6 thermocouples were used to monitor temperature: 4 in the same horizontal plane, 2 of them at the microchannels entry and the other 2 at the microchannels outlet, and 2 additional thermocouples at the upper and bottom sides of the block. The shells used to connect the microchannels block also included 4 heating cartridges for the catalyst reduction step. This auxiliary electrical heating system was also used to compensate the heat losses of the microchannel reactor.

The micro-combustor was studied alone using Fecralloy micro-monoliths with different
combustion catalyst load. In all
the cases total combustion of methanol was achieved, but as methanol/air flow increased, temperature increased as well, and the bulk of
combustion occurred in the first 10-15 mm of
monolith.

Based on the results of the combustion monolith, micro-reformer coupled to the combustor was investigated. First, the performance of the micro-reformer without combustion was investigated heating the micro-channel block with the electric cartridges. The methanol reforming conversion was 95 % at 350 ºC and stable selectivity was obtained, producing a very
low CO content (<1 %) in the reformate stream.
Measured temperatures were very similar (DT≤5 ºC)
in all points. The power required to
maintain this situation included the
power required to reform the methanol and
the heat lost. In subsequent experiments the power of the cartridges was fixed at decreasing values from initial value to zero and the temperature (350 °C) was controlled through the methanol combustion with a cascade flow- temperature controller. As the electrical power was reduced, the combustion power increased (increased methanol/air flow) producing an increase in the temperature at the combustion channels entry. This increase was due to the fact that the combustion is completed near the entry of the microchannels. Nevertheless, the excellent heat conduction characteristics of the microchannels block homogenized the temperatures.

Acknowledgements

Financial support by the Spanish Ministry of Science and Innovation and Ministry of Economy and Competitiveness (ENE2009-14522-C05 and ENE2012-37431-C03 grants,) and UPV/EHU (GIU11/13) are gratefully acknowledged. I. Uriz and I. Velasco gratefully acknowledge the fellowships granted by the Innovation Department of the Navarre Government and the Spanish Ministry of Science and Innovation (program FPI, BES-2010-030021 and BES-2010-034034).

References

1 G. Kolb, “Fuel Processing for Fuel Cells”, Wiley-VCH Verlag GmbH & Co. KGaA Weinheim, Germany (2008).