Energy Valorization of Fruit Shells and Stones Deriving from the Food Industry

Shells and stones are the main residues deriving from nuts and fresh fruit processing, representing a cost for food industries due to their disposal. This study investigated the main physical and chemical characteristics of twenty fruit shells and stones deriving from Italian food companies, for their use for energy purposes in biomass boilers. For each sample the moisture content, heating value, ash content, bulk density, ash fusibility and analysis of micro and macro elements were determined. Low ash contents (<1.5%) and high higher heating values (19.5–23.6 MJ/kg) and bulk density (>270 kg/m3st) were found. The latter two are higher than forest wood chips. Low concentrations of S and Cl were detected, avoiding the risk of emissions corrosive agents emissions. Walnut shells and peach stones have N concentrations greater than 1.0%, causing possible risks of air pollutants release. High values of Na, K and P were found in the biomasses, increasing the ash fusibility. Shells have low melting points (<900 ℃) while the fruit stones have higher values (>1100 ℃). Overall, fruit shells and stones have suitable characteristics for being used for energy purposes. It would be advisable to use them in medium-large size power plants due to the presence of low melting ash and the possible risk of NOx emissions. The combine use of shells and stones with woody biomass, such as wood chips, could guarantee a reduction of the problem of ash fusibility and at the same time fill the lack of material that could occur during the year.