Impact of boiler load and limestone addition on SO3 and corrosive cold-end deposits in a coal-fired CFB boiler

•Corrosion caused by hygroscopic fly ash deposits and SO3 were studied in a CFB boiler.•Changes in boiler operation had dramatic effects on cold-end deposits and corrosion risk.•At low load operation of the boiler, hygroscopic and corrosive NH4Cl formed.•With limestone addition, hygroscopic and corrosive CaCl2 was present in the deposits.•SO3 in the flue gases was low (<0.1 ppmv) in all cases.