The Fate of Sulfur during Oxy-Fuel Combustion

Licentiate thesis, 2011

Oxy-fuel combustion, where air is replaced by O2 and recycled flue-gas to enrich combustion products, is one of the main CO2 capture technologies suitable for large-scale coal-fired power plants. The changed combustion conditions in oxy-fuel combustion influences the sulfur chemistry, which has recently been the subject of increased attention due to the importance of sulfur species in corrosion and flue-gas cleaning issues. In this thesis, the sulfur chemistry during oxy-fuel combustion was examined by both experimental and modeling studies. Since the formation of SO3 plays a critical role in corrosion processes, the SO3 formation was studied separately. Experiments were conducted in the Chalmers 100 kWth oxy-fuel test unit using pulverized coal as fuel. Oxy-fuel and air-fired conditions were compared in the experimental analysis aimed at quantifying the sulfur sinks. In each test case the quantity of sulfur in the fuel, flue-gas, ashes, and condensed water was determined. The composition of the fuel and ashes in the experiments was analyzed; the distribution of alkaline earth and alkali metals was of special concern because of their ability to retain sulfur in the ash. In a separate study, the gas-phase chemistry was investigated by means of a detailed kinetic gas-phase model. This part of the work mainly focused on the impact of the combustion atmospheres and parameters critical for SO3 formation. The overall modeling results were compared to data obtained from experiments in the Chalmers oxy-fuel test unit. The experimental results show that the conversion of fuel-S to SO2 is significantly lower in oxy-fuel combustion than in air-fired conditions and, consequently, the SO2 emission, in mg/MJfuel, is reduced. However, the SO2 concentration was more than three times higher during oxy-fuel combustion. The higher SO2 concentration in oxy-fuel combustion is likely to increase sulfate formation, resulting in higher sulfur self-retention by ash. The increased SO2 concentration promotes the formation of SO3. This was confirmed by the model, which revealed SO3 outlet concentrations that were several times higher under oxy-fuel conditions compared to air-fired conditions. The gas-phase formation of SO3 is strongly dependent on the O2 concentration and only significant in zones where O2 is available and the temperature above 550˚C. The residence time of the flue-gas in the critical temperature window is another important factor in the formation of SO3. Flue-gas recycling conditions and the location of flue-gas desulfurization equipment are therefore both important design criteria with respect to the formation and resulting concentration of SO3.