SO₃ Emissions and Removal by Ash in Coal-Fired Oxy-Fuel Combustion

Abstract

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.

Oxyfuel combustion for CO2 capture in power plants

“…This considerable increase is mainly a result of the increased SO 2 concentrations under oxyfuel fired conditions. In a studies by Fleig et al (2011b) and Spörl et al (2014d), no clear trends were observed on changes in the SO 2 /SO 3 conversion ratios between air and oxy-firing (see also Fig. 13).…”

“…Also shown is the SO2/SO3 conversion according to Marier and Dibbs (Marier and Dibbs, 1974) undertaken during air and oxyfuel combustion experiments from a number of different combustion test facilities (different locations and scales) using different fuels. It is obvious that higher sulphur contents in coal that lead to high SO 2 levels normally result in higher SO 3 concentrations (Fleig et al, 2011b;Spörl et al, 2014d).…”

High-Temperature Conversion of SO2 to SO3: Homogeneous Experiments and Catalytic Effect of Fly Ash from Air and Oxy-fuel Firing

“…Fly ash samples A, B, and C derived from three Australian sub-bituminous coals (A, B, and C) were obtained from the 20 kW th combustion rig located at the Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Stuttgart, Germany. 9 The rig was operated at 1350°C, capable of combustion investigations with 0.5−3 kg of pulverized fuel/h and having a constant product rate of 11.5 m 3 [standard temperature and pressure (STP)]/h to maintain comparable residence times for the different modes of firing, i.e., air, oxy-fuel with partial cleaning, and oxy-fuel full recycling without cleaning. The fly ash was obtained from the bag filter of the rig maintained at an inlet temperature of 225 ± 30°C and outlet temperature of 195 ± 15°C.…”

Predicting acid dew point with a semi-empirical model

“…However, Huijbrejts et al [3] found that the sulfuric acid vapor will condense first with the decreasing of flue gas temperature, because the boiling point of sulfuric acid is highest in flue gas under the same conditions. Actually, as mentioned, the acid dew point depends mainly on the content of sulfur trioxide and the partial pressure of water vapor in flue gas [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Thus, the multi-compound poly-phase system in ELECNRTL can be simplified to the binary sulfuric acid-water system.…”

“…When the temperature of outer wall of convection heating surface in the second pass of boilers is below the acid dew point, the acid vapors in flue gas will condense and form acidic droplets on the heating surfaces. Actually, the acid dew point depends on the content of sulfur trioxide and the partial pressure of water vapor in flue gas [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The condensed acidic droplets not only cause serious acidic corrosion to the metal heating surfaces [5][6], but also further react with the flaking rust and the fly ash in the second pass of boilers.…”