New Method To Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. II. Molybdenum Salts

Abstract

The use of fuels other than natural gas in gas turbine generators still is fraught with blade corrosion problems that result from the formation of sodium sulfate or potassium sulfate, in the presence of chlorine. The present work illustrates that the addition of molybdenum salts to synthetic fuels (synfuels) modifies this deposition process and benign protective coatings of alkali polymolybdates are produced instead. This study is a follow-up to research published earlier in this journal that showed closely similar behavior with tungsten salt additives. In the case of molybdenum, a new preferential ranking of product formation on the surface is established that is closely related to the thermodynamic stabilities, namely, Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl and K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl. This chemistry exists under fuel-lean conditions and is otherwise not sensitive to the fuel, combustion conditions, surface temperature, or material. The required additive trace levels of molybdenum salt are on the order of twice that of the gaseous alkali, on an atomic basis. Consequently, when used as a final pre-gas turbine polishing technique and following other fuel precleaning methods, the approach offers a very practical and inexpensive solution.