*2.5. Olive Pomace Biomass Slag (OPBS)*

During biomass combustion, two types of waste are generated: bottom ash or slag and fly ash. Biomass slag comprises the coarser fraction of ash produced on the grate in the primary combustion chamber. The residual ash forms molten aggregates that are not transported out of the burner grate and/or furnace, thus forming slag [42]. The presence of alkali metals in biomass decreases the melting point of ash, allowing for faster slag formation [43]. The biomass slag is often mixed with mineral impurities contained in biomass fuel, such as sand, stones, and mud, or with bedding material in fluidized bed combustion plants. These impurities can be mineral, especially in fixed-bed combustion plants, and give rise to slag formation (due to a lowering of the melting point) and the presence of sintered ash particles in the bottom ash. In this study, biomass slag residue was sampled from a biomass-fired power plant for the combustion of olive pomace (Figure 1). The raw biomass slag residue was air dried at room temperature, crashed manually, and then sieved at 2 mm mesh size. The pH of the OPBS was measured following the 9045D method using a Thermo Orion pH meter (Waltham, MA USA) equipped with a low-sodiumerror electrode. For trace-element determination in the OPBS, the sample was digested by HNO3-HCL 1:3 (*v*/*v*) using a DigiPREP blocks digestion and heating system. The leached trace elements were analyzed by direct injection in (ICP/OES). The moisture content or humidity percentage in the OPBS was calculated by measuring the loss in weight after drying the sample at 105 ◦C for 24 h. The unburned carbon fraction in the collected Biomass Slag was quantified by the loss on ignition (LOI) method.
