The results of industrial utilisation of by-product from the semi-dry process flue gas desulphurisation in a fluidised bed boiler are presented. By products obtained by a semi-dry desulphurisation method was mixed in 50/50 proportion with calcium carbonate, traditionally used in the flue gas desulphurisation process in a CFB boiler. The mixture was then mechanically activated and blew into the boiler through a flue gas desulphurisation installation. Calcium sulphite was fully utilised and converted into sulphate. The increase of the boiler efficiency was observed as a result of incomplete and imperfect combustion loss decrease.