Static Bifurcation Characteristics of an Autothermal Circulating Fluidized Bed Hydrogen Generator for Fuel Cells

Professor Milorad P. Dudukovic is the Chairman of the Chemical Engineering Department at Washington University in St. Louis, MO, and a leading figure in multiphase reactors. He has authored several highly cited papers on chemical reaction engineering involving kinetic-transport interactions in multiphase systems. He has a commitment to the development of reaction engineering theory and practice for solving a variety of practical and fundamental problems. He is the director of the Chemical Reaction Engineering Laboratory (CREL) at Washington University, which is supported by over a dozen major chemical and petroleum companies. Some of his recent research accomplishments include the development of computerassisted radioactive particle tracking for studies of multiphase flows for improving computational fluid dynamics (CFD) models for multiphase systems and for selecting environmentally friendly reactors for the conversion of natural and synthesis gas to fuels and chemicals and novel coupling of exothermic and endothermic reactions in a reverse flow reactor to achieve potential improved efficiencies and energy savings. My students, Dr. Pradeep Prasad and Zhongxiang Chen, and I are very proud to contribute this paper to this special issue of Ind. Eng. Chem. Res. on the occasion of the 60th birthday of this outstanding academician, Professor Milorad P. Dudukovic, and we all wish him a long and healthy continuation of his productive and innovative career. -Said Elnashaie Reforming reactions are endothermic and reversible. The present paper proposes a novel concept of an autothermic reactor-regenerator type of circulating fluidized bed. Carbon is optimally allowed to form on the catalyst in the riser reactor section through the use of relatively low steam-to-methane ratios. Coke formation occurs through the methane cracking and Boudouard coking reactions. The deactivated catalyst is regenerated in the regenerator by the burning of carbon. From the methane cracking and coke burning reactions, there can be a net energy production of 318.5 kJ per mole of CH4 cracked and carbon burned [CH4 -C + 2H2 ((delta)H2r = 75 kJ/mol), C + O2 CO2 ((delta)H3r = -393.5 kJ/mol)]. This concept of carbon formation and burning is very well suited to the proposed novel autothermic circulating fluidized bed (CFB) configuration. By using a carefully controlled low steam-to-methane ratio, both the carbon formation and steam reforming reactions can be made to occur simultaneously, ensuring relatively high hydrogen productivity under autothermal conditions. However, this mode of operation exhibits static bifurcation behavior (multiplicity). This paper presents an introductory investigation of the hydrogen productivity of this autothermic process in relation to its static bifurcation characteristics. The paper also investigates an alternate configuration in which the off-gases from the reactor are combusted along with the carbon in the regenerator. The effects of hydrogen-permeable membranes and in situ CO2 sequestration on the performance of this autothermic CFB reformer are discussed, and it is shown that, despite the considerable improvements, certain complexities arise when in situ CO2 sequestration is used in this CFB configuration.