Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Physical/mechanical tests - durability and raw density of pellets and briquettes

The physical fuel quality of densified biofuels like pellets and briquettes is primarily characterised by durability (i.e. fuel resistance towards shocks and tensions) and particle density (i.e. ratio between mass and volume of a sample that is appropriate to estimate durability). Thus, durability is important with regard to handling, transportation and end-conversion processes.

For the estimation of the briquettes durability the most repeatable and reproducible method is to tumble the briquettes for 105 rotations corresponding to 5min treatment. Nevertheless, the briquettes durability testing leads to highly variable results. But the variability of the method is influenced by the fuel itself and is smaller for briquettes of high durability. Furthermore, if all the tested briquettes are considered, it seems illusive to reach a higher accuracy than 10 %.

Also for determination of pellets durability, using the tumbling device (ASAE standard) shows better results compared to the pneumatic tester (ÖNORM standard). Though, there is no accurate relation between the results of pellets durability obtained by both devices. It clearly appears that the level of durability influences the variability of results: i.e. the lower the pellet durability, the higher the variability. Taking into account the number of replication needed, an accuracy level of 1 % could be reached in practice with the tumbling device.

Particle density
The different results revealed that the hydrostatic and buoyancy methods based on liquid displacement give lower repeatability and reproducibility than the stereometric methods and thus performed better. For all tested methods, it clearly appears that the fuels type influences the variability of the results.

For each analysed parameter the stereometric methods led to higher variability, higher bias between participated laboratories, higher values of repeatability and reproducibility and needed more replications to reach a given exactness.

The liquid displacement methods showed similar standard deviation. The values of repeatability and reproducibility were similar. This is particularly true for briquettes. Methods with addition of wetting agent give higher results than those obtained with paraffin coating methods. In the case of pellets, the buoyancy method using non-coated samples and wetting agent mixed with water gives the lowest values in repeatability and reproducibility.

However, the choice of the liquid displacement method used may be based on the material available at the laboratory. Thus, e.g. hydrostatic with paraffin coating needs to have a balance that can weight in a range of 10 to 15kg with an accuracy of 0.1g. Indeed applying buoyancy without paraffin coating decreases the time of sample preparation but the water has often to be changed because briquettes start to disintegrate very fast.

Generally, it is concluded that for given fuel types, the number of replications needed to reach a given accuracy level is by far smaller than for the others. It is suggested to further determine on which other parameters the number of replications (or the expected accuracy) may be based.

Comparing particle density and durability by using the most accurate methods for both parameters, no relation has been found between those two parameters. Thus, the particle density cannot be used to estimate the durability.

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