Method for combusting an organic waste concentrate containing alkali metal compounds under oxidative conditions

A method for combusting an organic waste concentrate containing alkali metal compounds, said combustion being carried out at a temperature of at least 850° C., and flue gases formed during the combustion are cooled by mixing a colder medium to the flue gases, wherein the combustion is carried out under oxidative conditions in the presence of excess air for recovering said alkali metal compounds as alkali metal carbonates, to be dissolved in water to form a recoverable aqueous solution, said flue gases being cooled below a sticking temperature range of the alkali metal carbonates simultaneously as water or a recirculated aqueous solution is poured on the walls of a cooling zone at least at the sticking temperature range to create a water film covering said walls.