PEM FUEL CELL HYDROGEN PRODUCTION BY METHANOL-STEAM REFORMING: THE EFFECT OF CATALYST DEACTIVATION

摘要

Methanol-steam reforming, as a source of hydrogen for PEM fuel cell systems has been extensively studied with Cu/ZnO/Al_2O_3 being a commonly employed catalyst. The thermodynamics, mechanisms and kinetics of this process have been well-studied [1,2] and a number of potential commercial applications have been identified (e.g. automobiles). Many of these applications require the reformer to be as small as possible requiring that the catalyst be run at the highest possible temperatures in order to achieve the desired rate of hydrogen production. However, thermal deactivation of the catalyst is usually significant at these temperatures. A complicated dynamic is set-up between the activity of the catalyst and the (non-uniform) temperature field within the catalyst bed. Initially the catalyst activity is uniform but, with time, will develop axial and radial gradients. Over significant lengths of time, the performance of the reformer will change leading to declines in hydrogen yield that will adversely effect, for example, system efficiency. In particular, there are concerns about the level of carbon monoxide in the reformate as this will impact on the downstream processing to control its concentration to harmless levels. In this presentation, we report on a preliminary but fruitful simulation of the catalyst activity―bed temperature dynamic and its impact on reformer performance. The key assumption of this work is that the reformer is at quasi-steady state: temperature and concentration profiles are at steady state and determined by the instantaneous catalyst activity. Any transients in temperature or concentration are fast and simply ensure that the temperature or concentration profile is continually in 'equilibrium' with the catalyst activity profile.