Recycling of the Flue Gas in Aluminium Electrolysis Cells

Recycling of the flue gas from aluminium reduction cells is a possible method for increasing the carbon dioxide concentration, thereby enabling carbon dioxide capture. The present report is a preliminary study concerning some of the consequences in the electrolysis cells. A hypothetic 400 kA cell was used as basis. The energy balance was estimated, and it turned out that the heat flow from the interior of the cell could be kept constant by decreasing the thickness of the anode cover material even with a very hot gas inside the superstructure. With gas recycling, there is no principal upper limit for the CO₂ concentration after removal of fluorides and sulphur dioxide. At very low air flow into the cell, oxygen will be the limiting factor for anode airburn, meaning that it can be avoided. Recycling gives a higher amount of collectible heat from the cells, mainly because of higher temperature in the gas going into the cell. It will be necessary to use a catalytic burner for oxidation of CO to CO₂, which represents considerable extra heat. Increased sulphuric acid dewpoint due to very high concentration of SO₂ as well as water vapour may represent a challenge. It is also necessary to assess the amount of hydrogen fluoride that re-evolves from the secondary alumina at high superstructure temperature.