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Ash deposits, fuel treatment and fuel specification are considered between the most important technical factors that require specific consideration for economical use of heavy fuel oils in gas turbines or combined cycles. Ash deposits built in the hot gas path of gas turbines can cause severe erosion and corrosion of blades and vanes, besides the fouling associated to the Mg-base additive used in order to inhibit vanadium, a significant contaminant in residual fuel oils. Erosion, corrosion and ash deposition constitute the central problem affecting turbine reliability and service life when using low quality fuel oils. The aim of this paper is to show how these detrimental processes are interrelated in gas tuibines using heavy high sulfur fuel oils.The deposition rate for an ash on the turbine is influenced by the duty cycle, the contaminants in the fuel (particularly Na, V and S), the type and dosage of Mg inhibitor and the gas path temperatures. The duty cycle significantly 267

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Heavy, brittle and very hard deposits built on the first row vanes have caused severe erosion of all the first stage blades of a gas turbine during operation with washed and treated heavy residual fuel oil. The high sulphur (3.5–4.0 wt.%) fuel oil consumed by the turbine is also high in vanadium (280–290 ppm) and asphaltene content. In the present work the results of an investigation on the physical and chemical characteristics of erosive ash deposits as a function of operation conditions and fuel oil characteristics are presented. The structure and chemistry of deposits were studied by chemical analysis, x-ray diffraction, microanalysis and scanning electron microscopy. It was confirmed that deposit friability is enhanced by its MgSO4 content and that its hardness depends on the amount of MgO present. It was also found a clear correlation between the gas inlet temperature and the size of the ash particles deposited, and on the degree of compactness and hardness of the deposit. The role of the unburned particles, unavoidable in the combustion of heavy fuel oils, is discussed in relation to their influence on the effectiveness of the magnesium inhibitor.