Chemical-looping steam methane reforming for hydrogen production in a fluidized bed reactor

Abstract

Chemical-looping steam methane reforming (CL-SMR) system is the one of the alternative methods for hydrogen production and it is a novel concept to produce the high concentrated hydrogen and separate the carbon dioxide from methane oxidation inherently without any post treatment. It consists of fuel reactor (FR) for methane oxidation and steam reactor (SR) for steam reduction, where oxygen carrier (metal oxides) circulates between the two reactors. With this concept, conventional SMR process for hydrogen production is divided into methane oxidation and steam reduction steps with oxygen carriers. For development of CL-SMR system, four types of redox cycle such as $Fe_3O_4/Fe$, $WO_3/W$, $VO_2/VO$, and $SnO_2/SnO$ was examined through the literature and thermodynamic analysis. Based on the chemical and physical properties (oxygen ratio, reaction rate) iron oxides redox cycles of the selected oxygen carriers was selected for this system. The effect of reaction temperature on the redox reactivity of iron oxides was determined in thermo-gravimetric analyzer (TGA). The reaction mechanisms are proposed with variation of phase changes and the kinetic data (order of reaction, reaction rate constant, and activation energy) were obtained. In particular, oxidation reaction is well represented by the reaction control model at the initial oxidation state $(Fe\rarrFeO)$ while the ash-layer diffusion was dominant at the later part $(FeO\rarrFe_3O_4)$. This is reason why the thickness of dense metal oxide layer increases with time during the oxidation reaction and then penetration and diffusion of oxygen molecules through the dense metal oxide layer is rather difficult. Based on the average conversion rate from the kinetic data, the CL-SMR system for hydrogen production was designed, which consists of two bubbling fluidized bed reactors for methane oxidation (Fuel Reactor, FR) and steam reduction (Steam Reactor, SR), L-valve as a non-mechanical valve to control the solid ci...