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The reaction of SO 2 with fly ash in the presence of O 2 and H 2 O involves a series of reactions that lead to the formation of SO 3 and eventually H 2 SO 4 . Homogeneous experiments were conducted to evaluate the effects of the procedural variables, i.e., temperature, gas concentrations, and residence time, on the post-combustion conversion of SO 2 to SO 3 . The results were compared to existing global kinetics and found to be dependent upon SO 2 , O 2 , residence time, and temperature and independent of H 2 O content. For a residence time of 1 s, temperatures of about 900°C are needed to have an observable conversion of SO 2 to SO 3 . Literature suggested that the conversion of SO 2 to SO 3 is dependent upon the iron oxide content of the fly ash. Experiments using three different fly ash samples from Australian sub-bituminous coals were used to investigate the catalytic effects of fly ash on SO 2 conversion to SO 3 at a temperature range of 400−1000°C. It was observed that fly ash acts as a catalyst in the formation of SO 3 , with the largest conversion occurring at 700°C. A homogeneous reaction at 700°C, without fly ash present, converted 0.10% of the available SO 2 to SO 3 . When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO 3 /SO 2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.

This paper presents results of experiments performed at a 20 kW once-through combustion rig of the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart. A methodology to investigate oxy-fuel process configurations was used in which impurities were injected to the oxidant gas of the once-through reactor to simulate different extents of oxy-fuel recycle gas treatment. Three Australian coals, which had previously been tested in the Aioi furnace of IHI in Japan, were used in the experiments. A comprehensive set of total (Hg tot ), elemental (Hg 0 ), and oxidized (Hg 2+ ) mercury concentrations was measured for various air and oxy-fuel combustion conditions. These data enable an evaluation of process parameters that influence the Hg emissions of an oxy-fuel combustion process. A theoretical mass balance between Hg fed to the process (fuel and Hg 0 injection) and Hg measured before the filter matched well, indicating that no mercury was captured by fly ash at high temperatures. The capture of Hg 0 and oxidized Hg 2+ by ash in a baghouse filter has been determined for all experiments. Measured Hg concentrations show an increase when switching from air to oxy-fuel operation for all investigated coals and oxy-fuel settings, even when no additional Hg 0 is injected to the oxidant gas. Moreover, the Hg 2+ /Hg tot ratios in the flue gas are higher during oxy-fuel combustion. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. Reduction rates in a range between 18 and 51% for air and between 11 and 29% for oxy-fuel combustion were observed.

The sulfur oxide (SO x ) concentrations during oxy-fuel combustion are generally higher compared to conventional air firing. The higher SO x concentrations, particularly sulfur trioxide (SO 3 ) 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 SO x emissions were measured, conversion ratios of sulfur dioxide (SO 2 ) to SO 3 were calculated, and results were compared with existing literature, finding good agreement. The experiments with different extents of recycle gas cleaning and therefore different SO 2 levels in the system, revealed differences in the SO 3 generation behavior: A coal-specific trend of increasing conversion ratios of SO 2 to SO 3 with increased flue gas SO 2 levels was observed that could be related to the ash composition of the three different coals. The capture of SO x 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 recycling of H 2 O and SO 2 . 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.