Mechanistic insights into slag formation and mineral evolution during olive kernel residue combustion

Biomass combustion delivers renewable green energy through its inherently carbon-neutral cycle. The objective of this study is to elucidate the mechanism behind slag formation and mineral evolution during the industrial-scale combustion of olive kernel residues (OKR) with the dual aim of improving slag-mitigation strategies and identifying valorization routes for biomass-derived by-products. OKR feedstock was fully characterized for its chemical and thermal properties and slag deposits collected from a 5-MW boiler were investigated using SEM, XRF, and XRD/Rietveld methods to assess their micro-morphology, elemental makeup, and phase assemblage. Results showed that slag formation is driven by a two-step volatilization-condensation mechanism dominated by potassium and sulfates rather than sodium and chlorine. Compared with the 823 K OKR ash, the slag exhibited ∼50% reduction in Na, K, Ca and Mg, confirming extensive volatilization and redeposition. OKR slag consisted predominantly of an amorphous glassy matrix accompanied by crystalline phases, i.e., quartz, cristobalite, tridymite, wollastonite, sanidine, diopside, sylvite, arcanite and aphthitalite. The high aluminosilicate content combined with alkali vapors promoted the formation of low-melting eutectics and extensive vitrification. Slag micro-textures ranged from rock-like to compact reflecting the progressive accumulation and sintering of molten ash. The strong chemical and mineralogical similarity between boiler slag and bottom ash indicated shared utilization potential; these aluminosilicate-rich, nutrient-bearing materials can be useful in the construction industry and as alkaline soil amendments for pH restoration and nutrient elements’ release. Overall, the mechanistic insights obtained provide a basis for targeted slag-mitigation measures and support the broader circular-economy deployment of OKR biomass.