Conjugate heat transfer studies on steam reforming of ethanol in micro-channel systems

Abstract

The heat transfer aspects of Steam Reforming of Ethanol (SRE) to produce hydrogen in a micro-channel reactor heated by co-flowing flue gas, have been modeled. Conjugate heat transfer models coupled with convective diffusion of reactants and single step kinetics at the catalyst coated walls of the reformer, are considered. The effects of different geometrical configurations (i.e., square, wide and tall channels) and 2D/3D aspects have been addressed in detail. Irrespective of the channel geometry, reformer system attains isothermal condition at the catalyst–coated wall, at a short axial distance from the inlet. A thermal resistance model developed for conjugate heat transfer across adjacent channels shows that the convective resistances within the channels dominate over the conduction resistance across the separating walls. Hence, the thermal conductivity of wall material has negligible influence on the overall heat transfer coefficient of the system. For the endothermic steam reforming process, availability of heat is one of the most important factors governing hydrogen yield. In adiabatic micro-channel reforming process depending solely on preheating of the reactant mixture, the conversion rates are observed to be very low (less than 10%). When flue gas flowing in adjacent channels supplies the necessary heat, nearly 100% ethanol conversion is possible; however overall hydrogen yield further depends on reformer fluid temperature, due to water-gas shift and methanation reactions occurring in the reactant mixture. It is also observed that, at low reformer wall temperatures, both reaction kinetics and heat availability are important; at high wall temperatures, the system performance is primarily heat transfer controlled.