Temperature-staged methanation: An alternative method to purify hydrogen-rich fuel gas for PEFC

The preferential CO oxidation (PROX) is considered to be the only reliable technology for purifying hydrogen-rich fuel gases applied to the polymer electrolyte fuel cells (PEFCs), but the technology demands a close control of its oxidant supply. In this article we devised an oxidant-free technology called the temperature-staged methanation. Over a 0.5%Ru/Al2O3 catalyst we found that the complete selective CO methanation without CO2 methanation is possible for hydrogen-rich gases containing CO of up to 1.0 mol%, provided the reaction temperature is controlled to be lower than a critical value. This critical temperature decreased with decreasing the initial CO content in the gas. Consequently, with methanation high-selective CO removal from CO2-rich hydrogenous gases can be realized through dividing the required catalyst bed into more than one reaction zones that run at gradually lower temperatures. While at least one upstream zone at a higher reaction temperature performs to methanate most CO present in the treated gas under selectivity close to 1.0, the last zone at a lower temperature is conditioned to assure the desired outlet CO levels of lower than 100 ppm under selectivity above 0.5. The overall selectivity to methanate CO of the multi-staged reaction system depended on the number of the adopted reaction stages, which was shown to be possibly 0.95 in a three-stage system treating a simulated reformate consisting of (in volume) 69% H2, 30% CO2 and 1.0% CO and with water steam of 55% of the dry gas.