Abstract
The sulfuric acid dew point in the flue gas from aluminium electrolysis cells increases with increasing concentrations of sulfur dioxide and water vapour. This represents a potential problem in a future scenario with reduced air draught for increased carbon dioxide capture and heat exchangers for cooling or heat collection. A pragmatic model was derived, based on kinetics for the sulfur dioxide to trioxide conversion and models for estimating the temperature and air penetration into the flame. The results indicate that oxidation of carbon monoxide as well as sulfur dioxide takes place mainly less than 10 cm above the crust opening. The dew point increases with decreasing current efficiency and with increasing area and diameter of the crust openings. In the base case, which was thought to be representative of a modern prebake cell, the dew point was 73 °C. The dew point may increase if sulfur trioxide is refluxed with the secondary alumina.