NOVELTY - A combustion process carried out in an isothermal and flameless combustor involves feeding a fuel, a comburent, and component B) comprising sulfur or sulfur containing compounds, in an amount to have a specific molar ratio of the amount of sulfur present in component B) and the amount of sulfur contained in the fuel and the amount of alkaline and/or alkaline-earth metals contained in the fuel and the amount of the alkaline and/or alkaline-earth metals contained in component B).    USE - For reducing the basic ashes contained in the fumes from combustors and prevents corrosion of the walls of the thermal recovery plants.    ADVANTAGE - By operating in the combustion under the above mentioned conditions, the fumes coming out from the combustor are aggressive basic ash-free. Also the walls of the combustor and of the thermal recovery plants remain unaffected. They are neither attacked by the basic ashes nor by the combination of the basic ashes with other components present in the fuels, as for example vanadium. The basic ashes are transformed into inert compounds which neither attack the refractories of the combustor walls, nor the metallic materials, in particular steels and metallic alloys which, form the walls of the plants downhill of the combustor. It is also possible to use in the plants downhill of the combustor, for example the thermal recovery plants, metal alloys e.g. AISI 304H steel in operating conditions where as in the prior art alloys having a high nickel content, as Inconel and Hastelloy, were used. This is a remarkable advantage since it allows to save cost. The combustor used is isothermal and flameless, since it is operated at greater than or equal to 1500 (preferably higher than 1700-2100) K, and at a pressure higher than 101.3 (preferably higher than 200, more preferably higher than 600 to 2026) kPa. The method also enables use of less expensive fuels or biomasses, derived from sugars, animal meals, carbon, industrial scraps from neutralization reactions, high-boiling refinery fractions, bitumens and oil shales, processing scraps of tar sands, peats, exhausted solvents, pitches, in general industrial process scraps and wastes, including the residual fractions from urban scraps, optionally comprising CDR (fuel from scraps).    DETAILED DESCRIPTION - A combustion process carried out in an isothermal and flameless combustor involves feeding a fuel, a comburent, and component B) comprising sulfur or sulfur containing compounds, in an amount to have a molar ratio B'/A1 greater than or equal to 0.5, where B'is the sum by moles between the amount of sulfur present in component B) and the amount of sulfur contained in the fuel; and A1 is the sum by moles between the amount of alkaline and/or alkaline-earth metals contained in the fuel and the amount of the alkaline and/or alkaline-earth metals contained in component B).