The invention provides a method of operating a thermal installation comprising a combustion chamber fed with a fuel contaminated with vanadium, with sulphur and possibly with sodium. In particular, the combustion chamber is also fed with boron and with magnesium, in quantities such that the sodium molar ratio s=Na2SO4/V2O5, the magnesium molar ratio m=MgO/V2O5 and the boron molar ratio b=B2O3/V2O5 satisfy the equation m≧3+2b−s, so that the combustion products comprise magnesium orthovanadate, mixed magnesium boron oxide and possibly sodium borate. The invention also provides for the use of such a method to inhibit corrosion of the thermal installation by vanadium oxide, possibly in the presence of sodium.