Mercury and SO3 emissions in oxy-fuel combustion

This paper presents results on experiments carried out at a 20 kW 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 (Hg^tot), elemental (Hg⁰) and oxidized (Hg) 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 Hg/Hg^tot 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 Hg 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.