Milling and handling Cynara Cardunculus L. for use as solid biofuel: Experimental tests

The behaviour of Cynara Cardunculus L., for use as a solid biofuel, is evaluated during industrial grinding in a hammer mill and handling. Three distinct presentations are considered: whole Cynara, stems, or pellets. The evolution of performance figures of the milling process (specific energy requirement and drying effect) and the obtained product (particle size distribution and bulk density) are described as a function of the target particle size, from (5–0.5) mm, and the milling strategy, open or with external sieving and recirculation. Energy requirements, on dry ton basis, for (1 and 2) mm target sizes are about 20 kWh  t−1 for pellets and (50–60) kWh  t−1 for stems or whole Cynara. For target size 5 mm, the specific energy to grind pellets is as low as 6.12 kWh  t−1, and for stems or whole Cynara is below 15 kWh  t−1. The handling behaviour is also analysed, mainly the tendency to bridge in three different hoppers, before and after milling, showing that milling improves the handling behaviour in all cases. A major cause of problems was the presence of hairs from capitula in ground whole Cynara, so it is recommended to separate each fraction for the use as biofuel.