Alkali Control for Gas Turbines by a Combined Hot Gas Clean-Up Approach

Abstract

Gas turbines require very clean fuels if they are to avoid high temperature corrosion due to alkali sulfates and chlorides. Indications are that the alkali (sodium and potassium) salt levels need to be well below parts per million by volume in the hot gases and probably no more than 50 parts per billion by volume (0.01 mg/m3) for maintenance free operation. Moreover, it is the alkali that is the dominant factor and the actual concentrations of sulfur or chlorine are of minor relevance. Recently, numerous studies have shown that by using inexpensive absorbent type clay materials such as kaolin, bentonite, emathlite, or even bauxite and boehmite that these can reduce the alkali content very significantly (≥ 90%) in the hot gases. Coupled now to a final polishing method to remove the remainder, a new viable two step approach appears technically and economically feasible. This polishing method involves adding traces of either ammonium paramolybdate or ammonium metatungstate, both of which are water soluble, as a dilute aerosol into the hot gases. The ratio of molybdenum, or tungsten, to alkali needs to be in a ratio of about two to one on an atomic basis. This modifies the deposition process and alkali polymolybdate or polytungstate bronzes are preferentially produced. These are low melting point molecules that are benign and create with any remaining alkali a protective coating on the blades. They are thermodynamically more stable and rank above the sulfates and chlorides for formation. Also, being added in quantities of only parts per million or less there are no repercussions on NOX emissions or performance and remain cost effective at these levels. This coupling of the two complimentary approaches now appears feasible for final developmental testing to permit the use of alternate fuels without a need for cool-down cleaning cycles.