Catalytic microchannel reactor development for the removal of carbon monoxide from hydrogen-rich gas streams
Abstract
Currently, hydrogen (H₂) fuel cells are among the fastest emerging clean power generation technologies worldwide. The emergence of H₂ energy and fuel cells for power generation originates from the renewable energy sector, and the storage of renewable energy in the form of a chemical energy carrier to provide power grid balancing. At the moment, H₂ production is still dominated by fossil fuel processing, leading to carbon-based impurities such as carbon monoxide (CO) in the H₂ streams––which is known to deactivate fuel cell anode catalysts. Selective methanation and preferential oxidation of CO are two common methods of catalytic CO removal from H₂-rich gas streams. In this work, three of the most popular catalysts used for CO abatement (Ni-Pt/Al₂O₃, Au/Al₂O₃, and Ru-Cs/Al₂O₃) were tested for their ability to remove ca. 1.4 vol.% CO from a synthetic H₂-rich gas stream. Stainless steel microchannel reactors, containing the three respective washcoated catalysts, were used during the experimental work. Due to micro-scale dimensions of the channels, limit gas diffusional effects were expected, with the channels providing close contact between the bulk gas phase and the catalytic layer. Experiments were conducted isothermally at reaction temperatures of 80–400°C (depending on the CO abatement reaction applied), and space velocities of 32.6–130.4 NL.gcat⁻¹.h⁻¹. The Ru-Cs/Al₂O₃ catalyst was found to be the most suitable catalyst. CO concentrations lower than 100ppm were obtained via CO preferential oxidation at reaction temperatures of 120–180°C, with a peak CO conversions of more than 99.7% at 120–140°C and space velocities of 65.2–97.8 NL.gcat⁻¹.h⁻¹. This corresponds to CO levels as low as 42 ppm in the product gas. The conversion of H₂ did not exceed 6.5%. Additionally, the inability to characterise the dynamic region and the transport phenomena within the microchannels led to a theoretical study of the preferential CO oxidation process. A full three-dimensional model (using COMSOL Multiphysics® V4.4 software) showed that CO oxidation was the dominant reaction in a temperature range of 80–160°C. At temperatures above 160°C, the effects of the RWGS reaction was more pronounced, leading to a noticeable decrease in the CO conversion. Kinetic approximations were used to validate the reactor model to the full set of experimental data, and the model fit was noticed to yield accurate approximations of CO conversion in the simulated microchannel reactor over the range of reaction parameters. It has been demonstrated that microchannel reactor technology is suitable for CO removal from H₂-rich gas streams by preferential oxidation, at relatively low reaction temperatures (below 200°C) and high gas throughput compared to the reactor’s physical size. H₂ originating from fossil or bio-based processes can be successfully treated to near complete CO purification standards, before fuel cell technology is applied for power generation purposes.
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