The equilibrium ash composition in a magnesium treated residual fuel-fired gas turbine is considered in detail. The stability of the condensed phases of interest—MgO, MgSO4, and the magnesium vanadates are shown to be determined by the temperature and the equilibrium SO3 pressure at the deposit. The ash present will depend on these parameters as well as the Mg : V ratio in the ash. A thermodynamic treatment of the ternary MgO-V2O5-SO3 system is presented and experimental measurements of the stability limits of Mg3V2O8 and Mg2V2O7 are presented. A simple graphical method is described for representing the condensed phases present in ash for any Mg : V ratio and for temperatures and equilibrium SO3 pressures of interest in gas turbines.

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