A method of operating an incinerator , said incinerator comprising - a chamber for incinerating solid fuel in the presence of oxygen-comprising gas, - a flue gas channel for passing flue gas emanating from the chamber to an exhaust opening, wherein said flue gas comprises ash, and - a device for separating ash from said flue gas into - flue gas having a reduced ash content, and - ash; wherein the method comprising the steps of - introducing oxygen-comprising gas and a solid fuel into the chamber to incinerate said solid fuel resulting in a stream of flue gas comprising ash, - capturing ash from the stream of flue gas comprising ash using the device , and - collecting ash deposits originating from the flue gas comprising ash from the incinerator resulting in collected ash; wherein the method comprises the step of introducing a powdery additive material comprising i) clay and ii) calcium carbonate using an injection port transverse to the flow of flue gas comprising ash into the flue gas comprising ash wherein the flue gas comprising ash has at the location where the additive material is introduced a temperature of at least 700°C and the additive material is introduced upstream of the device , wherein a powder particle of said powdery additive material comprises granules, each granule comprising a mixture of clay and calcium carbonate, at least 10% by weight relative to the calcium carbonate being calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10°C per minute has decomposed completely when a temperature of 875°C has been reached; and wherein the powdery additive material is introduced with a rate R of at least 0.1 times the mass of ash in the stream of flue gas comprising ash. - a chamber for incinerating solid fuel in the presence of oxygen-comprising gas, - a flue gas channel for passing flue gas emanating from the chamber to an exhaust opening, wherein said flue gas comprises ash, and - a device for separating ash from said flue gas into - flue gas having a reduced ash content, and - ash; - flue gas having a reduced ash content, and - ash; - introducing oxygen-comprising gas and a solid fuel into the chamber to incinerate said solid fuel resulting in a stream of flue gas comprising ash, - capturing ash from the stream of flue gas comprising ash using the device , and - collecting ash deposits originating from the flue gas comprising ash from the incinerator resulting in collected ash; wherein the method comprises the step of introducing a powdery additive material comprising i) clay and ii) calcium carbonate using an injection port transverse to the flow of flue gas comprising ash into the flue gas comprising ash wherein the flue gas comprising ash has at the location where the additive material is introduced a temperature of at least 700°C and the additive material is introduced upstream of the device , wherein a powder particle of said powdery additive material comprises granules, each granule comprising a mixture of clay and calcium carbonate, at least 10% by weight relative to the calcium carbonate being calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10°C per minute has decomposed completely when a temperature of 875°C has been reached; and wherein the powdery additive material is introduced with a rate R of at least 0.1 times the mass of ash in the stream of flue gas comprising ash. <EOS>