Multistage milling and classification for improving both pellet quality and biogas production from hazelnut and olive pruning

The effects of a mechanical process were determined on the solid fuel quality and anaerobic biodegradability of hazelnut and olive pruning. Both the feedstocks did not meet the specification for industrial and residential pellets given in the European Standard EN ISO 17225-2, because of too high ash and nitrogen content. The coarser products from processing were notable for the high reduction in both the ash and nitrogen content. Therefore, as regards hazelnut, they met the requirement of the Standard for both the industrial and residential pellet, whereas those from olive processing met the requirements for the industrial pellet. The finest products showed a high concentration of nitrogenous matter and smaller C/N ratios, far better for anaerobic digestion. The best products from hazelnut and olive achieved methane yields of 118.1 and 176.5 Nm3 tVS-1, respectively, corresponding to 70.1 % and 93.5 % gains over the untreated substrates. The process was highly energy efficient, since consumption was low compared with the energy output from the residues of fuel upgrading, intended for anaerobic digestion. The investigated process could be successfully used to improve the fuel quality of pruning and to generate products suitable for anaerobic digestion and the production of advanced biofuels.