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Unusually large fouling deposits on the heat transfer tubes of a power plant using a residual product from olive extraction, which led to boiler shutdowns, have been attributed to an unusually high sodium content in the feed biomass.To confirm this and to evaluate the mechanisms of deposit formation, different techniques were used to analyze feed biomass and mature deposits sampled at different locations of the convection section: inductively coupled plasma, fusion test, energy dispersive X-ray fluorescence spectrometry, and scanning electron microscopy. All the deposits sampled were divided into two layers: a thin inner layer mainly composed of potassium and chlorine and a more porous outer layer with higher content in less volatile elements such as Ca, Si, Al, and Fe, both layers showing high sodium content. Differences in composition and morphology were also found depending on the temperature of the region where the deposits were formed and on the sampling position, frontside or backside, with respect to gas stream. Frontside deposits showed higher porosities and were mainly composed of granular and spherical structures, whereas backside ones showed higher compaction and are mainly composed of crystalline structures formed on the deposit surface. Structures were categorized based on their shape, and some relationships between shape and composition were proposed, which supported hypotheses about their formation mechanisms. Structures defined as amorphous molten were sticky, therefore increasing the capturing efficiency and tenacity of the deposit and leading to large deposits difficult to remove. The high sodium content of these structures was associated with the high sodium content of the feed biomass. Limiting this element is recommended to prevent uncontrolled deposit formation in the boiler.

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