(487b) Effect Of Oxygen Addition To The Feed Of A 5 Kwel Microstructured WATER-GAS Shift Reactors

The Water-gas shift reaction is a mandatory reaction to reduce the CO content of reformate produced by decentralized fuel processors dedicated for all kind of PEM fuel cell applications. When adiabatic water-gas shift reactors such as fixed beds or monoliths are applied for this task, this is usually performed in two independent reactor stages with intermediate water-cooling. Plate heat-exchanger technology bears the advantage of combining these two stages to a single reactor (1). When microchannels are applied, the improved heat and mass transfer allows a size reduction of 50% compared to conventional technology (2). The plate heat-exchanger reactor has to be operated with a declining temperature profile, which allows high reaction rate at the reactor inlet and shift of the thermodynamic equilibrium of the reaction towards the outlet (3). Medium temperature water-gas shift catalysts such as platinum/ceria formulations are required to allow the operation at temperatures between 400°C and 250°C, which show high activity and especially high utilization in case microchannels are applied (4). The declining temperature profile is achieved by a coolant gas guided counter-currently through dedicated cooling passages of the reactor. The feasibility of this principle has been demonstrated for microreactors of the kW scale by the group of the authors of the current paper (5, 6). However, the adjustment of the temperature profile is not easily achieved by only cooling the reactor from the product side. The coolant flow rate and temperature mostly determines the reactor exit temperature, while the temperature in the inlet region is dependent on the reformate inlet temperature and the degree of CO conversion in the reactor. The addition of oxygen to noble metal water-gas shift catalysts has been investigated recently as an option to improve catalyst activity at the penalty of certain hydrogen losses owing to unselective combustion (7).

The addition of air to a water-gas shift reactor with an electric power equivalent of 5 kW_el was evaluated in the work described by the current paper. It allows the precise adjustment of the reactor inlet temperature and, depending on the catalyst technology applied,  increases the CO conversion considerably. On top of that, the air addition is beneficial during transient operation of the reactor and during start-up, the latter because heat can be generated inside the reactor to heat the reactor internally.

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(6)      M. O'Connell, G. Kolb, K.P. Schelhaas, J. Schuerer, D. Tiemann, A. Ziogas, V. Hessel, Int. J. Hydrogen Energy 35 (2010) 2317-2327.

(7)      J. Kugai, J.T. Miller, N. Guo, C. Song, J. Catal. 277 (2011) 46-53.