Disclosed is a process for cleaning a boiler, wherein, while fumes are emitted in a combustion chamber of the boiler and circulate up to exchangers of the boiler, an aqueous solution of dissolved magnesium chloride and/or sulfate and/or dissolved calcium chloride is injected into the combustion chamber in the form of droplets which, by vaporization of the water of the aqueous solution, then thermal decomposition, are transformed in the combustion chamber into magnesium and/or calcium oxide particles reacting in the combustion chamber by mixing with molten salts and/or molten oxides, present in the fumes, to crystallize these molten salts and/or to vitrify these molten oxides before these molten salts and/or these molten oxides come into contact with the exchangers. Also disclosed is a device for implementing this process and a boiler equipped with this device.