PURPOSE: To prevent the high-temperature corrosion, low-temperature corrosion, etc., of matrix in a boiler, to promote combustion of fuel and to save energy consumption, by adding a composition containing Cr (compound) as an active component to a fuel or throwing the composition into burning atmosphere.

CONSTITUTION: A composition containing Cr or arbitrary Cr compound [e.g. Cr2O3, Cr(OH)3, CrF2, CrSO4, Na2Cr2O7, (CH3COO)3Cr.H2O, chromium naphthenate, etc.] is added to a fuel or thrown into a burning atmosphere by injecting with a pump or pneumatically introducing into a furnace under pressure. The necessary amount of the additive is 10W1,000ppm (preferably 50W200ppm) in terms of Cr for preventing high-temperature corrosion, 20W500ppm (preferably 30W300ppm) for preventing low-temperature corrosion and 200W1,000ppm (preferably 500W1,000ppm) for preventing the generation of unburnt fuel or dust.