Experimental and Modeling Studies of Sulfur-Based Reactions in Oxy-Fuel Combustion

Doctoral thesis, 2012

Oxy-fuel combustion is a technology for CO2 capture that is suitable for large-scale, coal-fired power plants. In this system, the combustion air is replaced by a mixture of O2 and recycled flue gases, so as to enrich for CO2 in the flue gas. These changes in combustion conditions have impacts on the degree of corrosion and flue-gas cleaning issues. In this thesis, the chemistry of sulfur-containing compounds during oxy-fuel combustion is examined through experimental and modeling studies. Since the concentration of formed SO3 is important for high- and low-temperature corrosion, the formation of SO3 is studied in detail in the present work. The formation of SO3 was characterized experimentally under different post-flame conditions in a quartz reactor. The gas-phase chemistry was analyzed using a detailed kinetic gas-phase model. The influences of the operating conditions of the Chalmers 100-kWth oxy-fuel test unit on the formation of SO3 were investigated using C3H8 as the fuel and SO2 injection into the feed gas. Different SO3 measurement techniques were applied and compared. A sulfur mass balance was established in the Chalmers 100-kWth oxy-fuel test unit during oxy-fuel and air-fired combustion with lignite as the fuel. In each test case, the quantities of sulfur in the fuel, flue gas, ash, and condensed water were determined. Although the SO2 emissions (mg/MJfuel) decreased, the SO2 concentrations were several-fold higher in oxy-fuel combustion than in air-fired combustion conditions. Simultaneously, the level of sulfur self-retention by the ash was increased during oxy-coal combustion, possible due to the increased concentration of SO2 during oxy-fuel combustion. The increased concentration of SO2 during oxy-fuel combustion generally results in an increase in SO3 concentration, which leads to an increased acid dew-point temperature, especially if a wet flue-gas recycle (FGR) is applied. The experiments and modeling show that SO3 formation is favored to a greater degree in a CO2 atmosphere than in an N2 atmosphere. However, during the conversion of CO, the rate of SO3 formation may be significantly reduced in a CO2 atmosphere, as compared to formation in an N2 atmosphere. The SO3 formation increased with residence time, as well with the temperature in the furnace. Therefore, the formation of SO3 is enhanced for a decrease in the FGR ratio in oxy-fuel combustion. That means that the FGR ratio and the location of flue-gas desulfurization are important design criteria with respect to SO3 formation in an oxy-fuel power plant.