MECHANISTIC STUDIES OF CARBON DEPOSITION IN METHANE REDUCTION WITH IRON-BASED COMPOSITE PARTICLES

Abstract

Chemical Looping Combustion (CLC) process has been widely considered to be clean energy conversion technology which utilizes an oxygen carrier to oxidize the fuel to H2O and separated CO2 efficiently. Therefore, the performance of the oxygen carrier (normally iron oxides) is critical to CLC. Although it has been found that the performances for reduction stage among these supported iron oxides are distinct from one to another, indicating complex iron oxide-support interaction, the understanding of these interactions including carbon deposition in methane reduction process remain limited, which could hamper the development of more efficient methane conversion system in CLC. Therefore, it is essential that a careful mechanistic study of methane reduction process be undertaken. In this study, efforts have been focused on understanding the mechanistic aspects of methane reduction process, among which carbon deposition and other key factors (such as supporting materials) to this process were carefully investigated. Iron oxides with three supporting materials, MgAl2O4, MgO and Al2O3, have been synthesized. Their combined reactivity, effect of supporting materials on the reaction, and corresponding reaction mechanisms have been studied using Thermogravimetric Analyzer (TGA), Brunauer-Emmett-Teller (BET), and Scanning Electron Microscope (SEM). It has been found that all three supported iron oxides, MgAl2O4, MgO and Al2O3, have the largest surface area and pore volume at point 4 (on TGA figure), where significant methane decomposition took place. The huge surface area and pore volume are believed to be largely attributed to the deposited carbon on solid samples. Correlations between the surface area (along with total pore volume) and the reaction pattern have been established by thoroughly analyzing results obtained from BET. During this stage of reduction, the MgO-supported iron oxides displayed the greatest reaction rate among all three synthesized particles. The reasons for the different behaviors may be attributed to the crystal structure change in the particles and different reaction pathways involved during redox reactions. Reactive sites in iron oxides are critical to the redox reactions, and they are closely related to the morphology of porous surface for the particles. Therefore, SEM was used to directly observe the morphological change of surfaces of unreacted and reacted particles among the reaction cycles.