AbstractThis paper presents results on experiments carried out at a 20kW combustion rig simulating different extents of oxy-fuel recycle gas cleaning by impurities injection to the oxidant gas of the once-through combustion reactor. A comprehensive set of total (Hgtot), elemental (Hg0) and oxidized (Hg2+) mercury as well as SO3 concentrations was obtained before and after the combustion rig's baghouse filter for in total 14 air and oxy-fuel experiments with 3 Australian coals. Based on this data, an assessment in respect to Hg oxidation, SO2/SO3 conversion and Hg and SO3 capture on the test rig's filter was performed. The air and the oxy-fuel experiments with different extents of recycle gas cleaning, revealed differences in the Hg and SO3 formation and capture behavior: The Hg2+/Hgtot ratios in the flue gas are higher during oxy-fuel combustion compared to air-firing. This effect is even more pronounced at the filter outlet, after flue gas has passed through the filter ash. In some experiments, even a net oxidation of Hg0 entering the filter to Hg2+ was observed. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. However, the Hg capture was altered by the different oxy-fuel recycle configurations, leading to decreased Hg capture efficiencies on the filter for one of the coals. A coal-specific trend of increased SO2/SO3 conversion ratios with increased flue gas SO2 levels was observed that could be related to the ash composition of the three different coals. This and the higher SO2 concentrations in the flue gas lead to considerably higher SO3 levels in oxy-fuel combustion with SO2 recycling. During the experiments, also a considerable capture of SO3 in the baghouse filter was observed (up to 80% under air- and up to 66% under oxy-fired conditions). A reduction of the SO3 capture on the filter under oxy-fuel conditions may be related to the higher SO3 levels in this process.