Oxygen enriched combustion is a promising retrofitting option for existing power plants to improve CO2 capture. In this study, the effect of oxygen enrichment of air as oxidant was investigated with a 30kWth fluidized bed combustor. Tests were conducted with two different Turkish lignites, one biomass and their blends. Biomass share was increased up to 20%. The oxygen concentration in the oxidant was kept between 21 and 30%.