PURPOSE: To prevent the corrosion of a heat exchanger tube at high temp. without using chemicals by washing and separating ash recovered from a boiler after combustion and adding the resulting Na2SO4-rich soln. to fuel so as to recycle the soln.

CONSTITUTION: Ash produced in the body 1 of a boiler by combustion is recovered with an electrostatic dust precipitator 4, sent to an ash washer 9, washed and separated into an Na2SO4-rich soln. and V2O5-rich sludge. The sludge is discharged from a discharge line 12. The soln. is sent to an air line 15 for combustion and blown into the fire furnace 16 of the boiler. Fuel is also fed into the furnace 16 from a line 17. The concn. of Na2SO4 in ash produced by combustion is increased and the mol.% of V2O5 is reduced, so the corrosion of the surface of a heat exchanger tube can be effectively prevented.