Engineering Properties of Biomass - Engineering and Science of Biomass Feedstock Production and Provision - page 23

Table 2.2 Dimensions of circular hole sieves
Screen No. Hole diameter (mm) Screen thickness (mm) Open area (%)

1 48 26.0 35.98

2 32 20.0 33.00

3 16 9.53 35.65

4 8 6.35 32.45

5 4 3.00 32.77

Pan

Because of the mentioned problem associated with long and thin particles, the
application of a dynamic online image analysis can improve its effectiveness.
This new classification method can sort particles based on more than one dimen-
sion, but it has the problem of particles overlapping. So, the most reliable method of
characterizing the size of particles is still direct measurement of size by hand, using
a digital caliper.
ASABE S424 specifies the use of a stack of thick plates with square holes to
analyze the size of cut forages in the field [ 22 ]. The thickness of the plates is pro-
portional to the dimension of the hole. Larger holes have a thicker dimension and
this makes sure the long particles do not have enough space to pass through the
holes during sieving. A biomass sieving system was developed at the University of
British Columbia (UBC), Vancouver, based on the ASABE 424 standard. The siev-
ing system consists of a stack of five round sieves plus pan. Each sieve has a height
of 85 mm and a diameter of 305 mm except the sieve with the smallest hole that has
a height of 4 mm and rests on a pan with the height of 45 mm. The dimensions of
each sieve are summarized in Table 2.2 .
The sieve shaker (Retsch Model AS 400, Newton, PA) applies a horizontal
circular motion. The speed ranges from 50 to 300 rpm and can be electronically
controlled. The actual value of the number of revolutions is digitally displayed.

2.2.2.2 Digital Imaging Technique

Digital imaging technique provides an accurate measurement of particle size by
processing the particle's projected area in the image and counting the digital pixels
with a preset of scanning resolution [ 20 , 23 - 26 ]. The images can be taken either by
a scanner or by a scanning electron microscope (Fig. 2.2 ). For the digital scanning
method [ 26 ], the sample of ground fir softwood particle's images were taken by a
CanoScan 4,400 F high-resolution scanner (Canon, Lake Success, NY). The resolu-
tion of the image was determined by the number of pixels per inch (DPI). The par-
ticles were scattered on a transparent plastic sheet before images were taken by the
scanner. Prior to imaging, the individual particles were deliberately separated so that
they did not touch or overlap with each other, which would affect the particle size
analysis results. This manual separation of the particles was performed with the aid
of a magnifying glass. The resulting images were analyzed with MATLAB software