19 research outputs found
Internal particle size distribution of biofuel pellets
Several methods for disintegration of biofuel pellets were tested and compared for their ability to break
up the pellets into the original particles of the raw material. Analyses performed on softwood pellets and
straw pellets concluded that wet disintegration in water at ambient temperature is insufficient for a
determination of the internal particle size distribution of wood- and straw pellets. When the wet disintegration
was performed with water heated to the boiling point and coupled with mechanical disintegration
in terms of stirring a more complete disintegration of the pellets was obtained. Based on the results
obtained in the initial study a round robin was set up including six European laboratories where the
selected method was tested. In the round robin test the method combining heated water and stirring
of the slurry was tested on solid biofuel pellets produced of comminute straw, deciduous wood and coniferous
wood respectively. With the method a satisfactory disintegration was obtained of all three types of
pellets.
Further wet disintegration of coniferous pellets was compared to a dry disintegration using a hammer
mill. The dry disintegration of the coniferous pellets resulted in smaller particle sizes compared to the
wet disintegration using heated water and stirring of the slurry indicating a further disintegration of
the original particles in the hammer mill process.
Overall the wet disintegration combined with mechanical impact was found to be the most suitable
method for disintegration of solid biofuel pellets. Combined with sieving analysis the method gives realistic
image of the internal particle size distribution of solid biofuel pellets
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.BioNor
Methods for size classification of wood chips
Methods for size classification of wood chips were analysed in an international round robin using 13 conventional wood chip samples
and two specially prepared standard samples, one from wood chips and one from hog fuel. The true size distribution of these two samples
(according to length, width and height) had been determined stereometrically (reference method) using a digital calliper gauge and by
weighing each of the about 7000 wood particles per sample. Five different horizontal and three rotary screening devices were tested using
five different screen hole diameters (3.15, 8, 16, 45, 63 mm, round holes). These systems are compared to a commercially available
continuously measuring image analysis equipment.
The results show that among the devices of a measuring principle—horizontal and rotary screening—the results are quite comparable,
while there is a severe incompatibility when distributions are determined by different measuring principles. Highest conformity with the
reference values is given for measurements with an image analysis system, whereas for all machines with horizontal screens the median
value of the size distribution only reached between one-third to half of the reference median value for the particle length distribution.
These deviations can be attributed to a higher particle misplacement, which is particularly found in the larger fractions. Such differences
decrease when the particle’s shape is more roundish (i.e. sphericity closer to one). The median values of length distributions from
screenings with a rotary classifier are between the measurements from an image analysis and horizontal screening devices
Particle density determination of pellets and briquettes
Several methods and procedures for the determination of particle density of pellets and briquettes were tested and evaluated. Round
robin trials were organized involving five European laboratories, which measured the particle densities of 15 pellet and five briquette
types. The test included stereometric methods, methods based on liquid displacement (hydrostatic and buoyancy) applying different
procedures and one method based on solid displacement.
From the results for both pellets and briquettes, it became clear that the application of a method based on either liquid or solid
displacement (only tested on pellet samples) leads to an improved reproducibility compared to a stereometric method. For both, pellets
and briquettes, the variability of measurements strongly depends on the fuel type itself.
For briquettes, the three methods tested based on liquid displacement lead to similar results. A coating of the samples with paraffin did
not improve the repeatability and the reproducibility.
Determinations with pellets proved to be most reliable when the buoyancy method was applied using a wetting agent to reduce surface
tensions without sample coating. This method gave the best values for repeatability and reproducibility, thus less replications are
required to reach a given accuracy level. For wood pellets, the method based on solid displacement gave better values of repeatability,
however, this instrument was tested at only one laboratory
Von Rittinger theory adapted to wood chip and pellet milling, in a laboratory scale hammermill
The study draws upon the milling theories developed for the ore processing industry
(Von Rittinger, Kick and Bond theories) in order to define a method for characterising wood
chip and pellet energy consumption during milling.
Energy consumption during wood milling depends on three main factors: the material
moisture content, the particle size difference between the feed and the milled product, and
the material itself. The latter may be characterised by a single parameter based on an
adaptation of Von Rittinger’s constant.
A relation characterising wood pellet energy consumption as a function of the particle
size distribution of the pellet ingredients and the milled pellets is proposed. This is characteristic
of each type of pellet for each moisture content value considered