Comparative study of durability test methods for pellets and briquettes

Different methods for the determination of the mechanical durability (DU) of pellets and briquettes were compared by international
round robin tests including different laboratories. The DUs of five briquette and 26 pellet types were determined. For briquettes,
different rotation numbers of a prototype tumbler and a calculated DU index are compared. For pellets testing, the study compares two
standard methods, a tumbling device according to ASAE S 269.4, the Lignotester according to O¨ NORM M 7135 and a second tumbling
method with a prototype tumbler. For the tested methods, the repeatability, the reproducibility and the required minimum number of
replications to achieve given accuracy levels were calculated. Additionally, this study evaluates the relation between DU and particle
density.
The results show for both pellets and briquettes, that the measured DU values and their variability are influenced by the applied
method. Moreover, the variability of the results depend on the biofuel itself. For briquettes of DU above 90%, five replications lead to an
accuracy of 2%, while 39 replications are needed to achieve an accuracy of 10%, when briquettes of DU below 90% are tested. For
pellets, the tumbling device described by the ASAE standard allows to reach acceptable accuracy levels (1%) with a limited number of
replications. Finally, for the tested pellets and briquettes no relation between DU and particle density was found.