Development and validation of a CFD-based steam reformer model

Steam reforming of liquid biofuels (ethanol, bio-diesel etc.) represents a sustainable source of hydrogen for micro Combined Heat and Power (CHP) production as well as Auxiliary Power Units (APUs). In relation to the design of the steam reforming reactor several parameter are important including efficiency, start-up time, load following capabilities, and system weight and volume. In order to assist in the development phase of such systems, detailed computational models are useful and offer a potential reduction in the time-to-market as well as lower total development costs due to a reduced need for expensive prototypes. This paper presents an advanced Computational Fluid Dynamics based model of a steam reformer. The model was implemented in the commercial CFD code Fluent through the User Defined Functions interface. The model accounts for the flue gas flow as well as the reformate flow including a detailed mechanism for the reforming reactions. Heat exchange between the flue gas and reformate streams through the reformer reactor walls was also included as a conjugate heat transfer process. From a review of published models for the catalytic steam reforming of ethanol and preliminary predictions, it was found that ethanol decomposition is not a rate limiting step and can be disregarded without significant error. Model predictions were compared with experimental data in terms of detailed species concentration and temperature profiles inside the reforming reactor. The measurements were made in a commercial ethanol steam reformer. The illustrations below show the measurements locations and predicted and measured temperature profiles. From detailed comparison with the measurements it was concluded that a mechanism for catalytic steam reforming of methane gives a reasonably accurate representation of the ethanol steam reformer. Based on the model predictions, a detailed investigation of the processes controlling the hydrogen production rates is presented. It was found that efficient heat transfer from the flue gas to the endothermic steam reforming reactions is critical and represents a limiting factor with respect to the reactor dimensions.