Conceptual Design of a Dual Fluidized Bed System for an Intensified Steam Methane Reforming Concept Coupled with Ca-Ni Looping

摘要

The severe environmental issues along with the foreseen depletion of fossil fuels have accentuated the need of finding alternative fuels and energy production technologies that reduce the dependence on fossil energy and moderate CO2 emissions. Hydrogen is regarded as an important primary industrial gas as well as a promising and sustainable energy carrier due to its combustion having high energy efficiency and zero carbon emissions. The dominant large-scale production path of hydrogen is steam reforming of natural gas which is considered an energy intensive process and requires multiple steps and complexity to achieve the generation of high-purity hydrogen. Furthermore, due to thermodynamic limitations the process is usually coupled with water-gas shift reactors downstream the reformer to recover additional hydrogen and thus not avoiding the co-production of CO2. To address the aforementioned issues, a novel technology which provides a path for intensified conversion of natural gas directly to high purity hydrogen in a single step has been developed; the so-called sorption enhanced chemical looping steam methane reforming (SE-CL-SMR) [1]. This concept combines the reformer and WGS reactors in a single unit by introducing a solid sorbent material such as CaO that can in-situ remove the produced CO2. The exothermic carbonation reaction of CaO with the produced CO2 enables the performance of both the reforming and water-gas shift reactions in a single vessel, overcoming the thermodynamic limitations of the overall reaction, while the heat generated by the strongly exothermic carbonation is consumed in-situ for the endothermic reforming reaction. When the solid sorbent material reaches saturation, it needs to be regenerated in a separate reactor for it to be re-utilized. The necessary heat for the regeneration is addressed with the introduction of a second chemical loop, where an oxygen transfer material (OTM) such as NiO is also circulating between the two reactors by undergoing continuing redox reactions. During the regeneration stage, a sweep gas such as pure oxygen can be used for the strongly exothermic oxidation of the OTM thus moderating the thermal demands of the regeneration. The oxidized OTM returns to the reformer wherein is reduced by methane and the reformate gases. In addition to the oxygen transfer properties as an OTM, NiO in the reduced form presents excellent catalytic properties for steam methane reforming and water gas shift reactions. A schematic representation of the combined process is presented in Fig. 1.