Characterization of pellets from industrial tomato residues

Large volumes of residual biomass (mainly peels and seeds) are generated by tomato industrial processing plants. After adequate drying and densification operations, those biomass wastes might be used for livestock food production, for lycopene extraction or even as fuel supply for thermal applications. The process described in the present paper is the manufacturing of 6mm diameter pellets from tomato waste samples obtained from industrial processing plants. As a previous step, the samples were dehydrated to five different values of moisture content (20, 25, 29, 34 and 38wt% w.b. approximately). Bulk and particle densities, hardness and durability of pellets were significantly influenced by the initial moisture content of the biomass raw materials, as well as by the subsequent reductions of such moisture content after pelletizing operations and storage periods. Increases of 21%, 41% and 67% in moisture content of the feeding biomass material – with respect to the lower value of the operating range, i.e. 20wt% w.b. – were observed to yield the following percentage increments in a series of physical properties of pellets, respectively: 14%, 25% and 50% in bulk density; 6%, 16% and 18% in particle density; 225%, 1737% and 3575% in hardness; 8%, 44% and 88% in durability. Nevertheless, higher increments in moisture content of feeding material led to significantly lower values of the aforementioned properties – not to mention technical operating problems due to roller skidding in the pelletizing device. Pellets obtained from raw biomass (peels and seeds) with 34wt% initial moisture content (9% final moisture content of pellets) were regarded as those with the most suitable configuration: durability 91.2%, hardness up to 88N and energy density approaching 8GJ/m³.