(419c) Fate of Sulfur Trioxide Under Oxy-Coal Combustion Conditions

In coal combustion, most sulfur present in the fuel is oxidized to sulfur dioxide, with only a small portion being converted to sulfur trioxide. However, in oxy-fuel combustion, due to the increased oxygen concentration, it is possible that more of the fuel-bound sulfur can be converted to sulfur trioxide. Many studies to date have focused on the formation of NOx under oxy-coal conditions; however, the formation of sulfur trioxide under oxy-coal conditions is not well understood. Since sulfur trioxide can form sulfuric acid with water vapor in flue gas and subsequently lead to corrosion, it is important to understand the formation of sulfur trioxide during oxy-coal combustion.

The purpose of this study is to investigate the effect of various operating parameters in combustion on the formation of sulfur trioxide. Modified ASTM method D3226-73T (also called the Controlled Condensation Method) was used to measure sulfur trioxide in bench-scale and pilot-scale experiments. Experiments were performed at various temperatures to understand the relationship between the combustion temperature and the amount of sulfur trioxide formed. The effects of oxygen and carbon dioxide concentration on the amount of sulfur trioxide were investigated for both air and oxy-fired combustions by changing the concentration of oxygen in the oxidizer. Also, this study investigated the influence of limestone (CaCO₃) on the amount of sulfur trioxide emitted during both air-fuel and oxy-fuel combustion. Since the formation of sulfur trioxide is directly related to the concentration of sulfur dioxide, a decreased amount of sulfur dioxide due to limestone capture should, in principle, affect the concentration of sulfur trioxide. The experimental results presented will be explained in the context of the relevant sulfur chemistry, and compared with the outcomes predicted by thermochemical and kinetic calculations.