The sulfur oxide (SOx) concentrations during oxy-fuel combustion are generally higher compared to conventional air firing. The higher SOx concentrations, particularly sulfur trioxide (SO3) in combination with high concentration of water in the recycled flue gas, increase the sulfuric acid dew point temperature in oxy-fuel fired systems, thereby increasing allowable flue gas temperatures and reducing the thermal efficiency of a power plant. This paper presents results of experiments carried out at a 20 kW once-through combustion rig of the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart simulating different extents of oxy-fuel recycle gas cleaning by impurities injection to the oxidant gas of a once-through combustion reactor. Three Australian coals that have previously been tested under air and oxy-fuel conditions at the Aioi furnace of IHI in Japan were used in the experiments. The SOx emissions were measured, conversion ratios of sulfur dioxide (SO2) to SO3 were calculated, and results were compared with existing literature, finding good agreement. The experiments with different extents of recycle gas cleaning and therefore different SO2 levels in the system, revealed differences in the SO3 generation behavior: A coal-specific trend of increasing conversion ratios of SO2 to SO3 with increased flue gas SO2 levels was observed that could be related to the ash composition of the three different coals. The capture of SOx in a baghouse filter was also evaluated. Acid dew point temperatures (ADPs) for the flue gas were calculated for the various firing conditions. Acid dew point (ADP) temperatures increased by up to 50 C when changing from air to oxy-firing with recyding of H2O and SO2. Considerable differences in the ADPs were found for different extents of oxy-fuel recycle gas treatment and were evaluated in respect to power plant efficiency implications.