Chemistry of Ash-Deposits on Gas Turbines Hot Parts: Reactivity of Nickel, Zinc and Iron Oxides in (Na, V, S) Molten Salts

Abstract

When ash-forming oils or contaminated distillate oils are used as fuels in land-based, marine or aero gas turbines, the hot gas path components, mainly the partition vanes and the blades of the expansion turbine are subjected to the deposition of slags that are corrosive at high temperature due to their low liquidus temperature. This hot corrosion process - if not properly inhibited - entails a dramatic life reduction of the hot gas path parts. MgO is a traditional, efficient inhibitor. Recently, it has been found that NiO also suppresses the corrosiveness of the (Na,S,V) melts by trapping vanadium in a refractory vanadate (Ni3V2O8); this compound is friable and does not tend to accumulate on turbine blades. The use of inhibitors entails losses in both machine performance and availability. Moreover, other metals can interfere with the inhibition process. In particular, zinc and iron are often inadvertently introduced in gas turbines fuels during their transportation or storage and they can significantly interact with nickel. This paper distinguishes the interactions between NiO on one hand and both ZnO and Fe2O3 on the other hand in the general complex chemistry of ash. The thermochemical study of (Na,S,V) melts in presence of Ni confirms that nickel is a good "trapper" of vanadium oxide at high temperature. However, they also show that nickel can react with iron to form the very stable ferrite NiFe2O4 and a low melting point vanadate phase. On the contrary, the presence of zinc affects to a lesser extent the reactivity of NiO versus V2O5 despite the formation of Ni1-xZnxO solid solutions.