The Generation Mechanism Of SO₃ In Oxy-fuel Combustion

As the increase of greenhouse effect, CO2 capture become more and more important. Oxy-fuel combustion is a promising technology which is implemented in the energy industury for CO2 capture. During oxy-fuel combustion process, the factors such as flue gas circulation lead to increase of SOx and H2O. When the boiler is working, SO3 may cause high temperature corrosion, low temperature corrosion, SCR catalysts and air preheater jam. In addition, these hazards will become more prominent in oxy-fuel combustion. Thus predicting SO3 concentration by calculation mode in oxy-fuel combustion and studying the generation mechanism of SO3 is importance.In order to study SO3 generation in oxy-fuel combustion, chemical thermal equilibrium calculation is proposed to study the distribution of SO3 respectively from the simplest SO2-O2 system to oxy-fuel combustion system. Results indicate that SO3 thermodynamic equilibrium concentration increases when O2 and SO2 concentration increase, while the other gas components have no effects on SO3 concentration. For oxy-fuel combustion, the concentration of SO2 increases dramatically when flue gas circulation is adopted with the decline of flue gas amount and the increase of O2 concentration, as the result that SO3 concentration and formation rate increase.Chemical reaction kinetics model is adopted to research conversion mechanism of sulfur gas in oxy-fuel combustion. Results show that in oxy-fuel combustion H2S oxidizes to SH at first, and then SH generates important intermediate HSO, S and SO, the final product is SO2. Approximately 2.6% of SO2 is further oxidized to SO3 in cooling process. SO2 main production pathway is SO+O2= SO2+O in the early stage, the main production pathway become SO+OH= SO2+H in later stage. In oxy-fuel combustion, SO3 main generated path is SO2+O (+M)= SO3 (+M) at high temperature, the main production path become HOSO2+02= S03+H02 and SO2+NO2= S03+NO at medium temperature. In oxy-fuel combustion, SO3 concentration improves deeply because of the flue gas circulation and the improvement of O2 concentration when the SO2 concentration is basically stable. Results are valuable for understanding the sulfurous gas migration rules and main way of sulfurous gas generation.Chemical reaction kinetics model and chemical thermal equilibrium calculation are adopted to research the influence of flue gas circulation modes and flue gas composition on SO3 generation in oxy-fuel combustion. Results show that SO3 concentration is 69.6×10-6 in wet cycle which is the highest in oxy-fuel combustion, it is 6.8 times of conventional combustion and SO3 concentration is the lowest in dry cycle which is 2.0 times of conventional combustion. The increase of O2 and SO2 concentrations in flue gas promote SO3 generation at high and medium temperature. The increase of H2O concentration promotes free radical OH increases, and then promotes the SO3 generation at medium temperature; NO is weak for SO3 generation in high concentration of H2O, while NO has weak inhibitory effect for SO3 generation at low water content. Acid dew point is 154.8℃ in wet cycle which is the highest in oxy-fuel combustion, it is 35.5℃ higher than conventional combustion, and 25.2℃ higher than dry flue gas circulating. Therefore, SO3 concentration depends on flue gas circulation mode in oxy-fuel combustion.