Steel converter slag as an oxygen carrier in a 12 MWth CFB boiler - Ash interaction and material evolution

Steel converter slag or LD slag is a byproduct of the basic oxygen steel production process, where raw iron from the blast furnace is converted to steel. LD slag contains mainly calcium and iron compounds and smaller amounts of magnesium, silicon, manganese and vanadium. The iron content, about 17 wt.% of the slag, makes this material a potential oxygen carrier for combustion processes such as Oxygen Carrier Aided Combustion (OCAC) or Chemical Looping Combustion (CLC). This study will present an investigation of the use of LD slag as an oxygen carrier in OCAC at semi-industrial scale. The Chalmers 12MW(th) biomass circulating fluidized bed boiler was operated using LD slag as bed material under OCAC conditions. During the operation, bed samples from the boiler were extracted and analyzed with ICP-SFMS, SEM-EDS, XRD and different mechanical tests to analyze chemical and physical changes of the bed material as a function of time. The samples were also investigated in a laboratory fluidized bed reactor to determine the change in reactivity towards common volatile fuel components, i.e. CO, H-2, CH4 and C6H6. It was found that LD slag can be utilized as an oxygen carrier in a combustion process for biofuel. However, the reactivity towards syngas, CH4 and C6H6 is reduced as a function of time in the boiler, which is believed to be caused by accumulation of, and interaction with, alkali from the biofuel ash. Sulfur addition may decrease the adverse effects of alkali on combustion efficiency, but not eliminate them completely.