Geopolymer Composite Oxygen Carriers for Chemical Looping Processes

Chemical Looping Combustion (CLC) allows the inherent separation of CO2, through the heterogeneous oxidation and reduction of an oxygen carrier (OC) which transfers oxygen for the combustion from air to the fuel [1]. In this work, geopolymer (GP) composites were proposed as a novel class of OCs for CLC. GPs are quasi-amorphous and nanostructured ceramic materials obtained by alkaline activation of alumino-silicate precursors through an easy, low-cost and green process at low to moderate temperature (25-100°C). They possess optimal features for this application as a diffused meso-porosity and an high surface area, combined to substantially high mechanical strength, abrasion resistance and stability to temperature up to 1000°C [2]. Within the composite the GP phase served as support matrix, while Fe2O3 and Mn2O3 were selected as nontoxic and largely available active phases. A relevant advantage of GPs lies in their production process. In fact, while OCs are generally produced through conventional methods based on the impregnation of supports with metal salts, the GP composites were produced through a one-step synthesis, thereby drastically optimizing the time and the cost of the overall process. Together with the distinct Fe and Mn-based composites, the two oxides where combined in a mixed OC in order to evaluate the synergy between them. Laboratory tests of the novel OCs were carried out in a CLC plant for the combustion of a CO rich syngas at 800, 850 and 900°C. The materials showed a suitable stability to the reaction conditions, with no cracking or agglomeration after repeated CLC cycles. The tests pointed out the better performance of the Mn-based oxygen carrier at 900°C, which achieved efficiency in CO conversion up to 98%, and exhibited the ability to release O2 in inert conditions. Moreover, the GP matrix resulted to actively influence the OC behavior through the formation of Mn-Si phases, which could improve the oxygen release capacity of the material. The Mn-Fe sample reached performance very close to the Mn-based one, probably related to the formation of an intermediate Mn-ferrite phase. The release of O2 under certain conditions of temperature and pressure opens the possibility of utilization in processes different from CLC (e.g. O2 separation from air).