Engineering Properties of Biomass - Engineering and Science of Biomass Feedstock Production and Provision - page 22

2.2.2 Particle Size

Woody biomass and herbaceous crops are of irregular shapes. Some are of a needle
form with high an aspect ratio (length divided by diameter) and the finely ground
particles have a round shape with an aspect ratio close to one. The range of the par-
ticle size of woody biomass is huge, from spanning wood logs to ground powders
after milling. For herbaceous crops, the particles include leaves and stalks which are
fluffy in nature [ 16 ]. There are three dimensions for these particles, including par-
ticle length, width, and thickness. However, some of the traditional particle size
measurement techniques, e.g., sieving, are limited to the measurement of one single
dimension of the fibrous particles (e.g., particle width) only. In addition, a long
piece may actually pass through a sieve because it is oriented perpendicular to the
sieve and therefore passing through due to small width/thickness. This makes the
exact dimension measurement difficult and challenging. Therefore, there is a strong
need to develop accurate characterization techniques for biomass particle size and
shape for designing the handling, storage, and processing units including chemical
reactors for treatment. Sieve analysis and digital imaging techniques are two major
characterization methods for particle size analysis.

2.2.2.1 Sieve Analysis

A particle size analysis of biomass ground particles using sieves with square holes
opening was studied [ 19 ]. Prior to sieving analysis, samples were conditioned to
consistent low moisture content (e.g., 10 % moisture content [w.b.]) at a drying tem-
perature of 50 °C. This conditioning ensures that the particles do not stick to each
other by capillary force of moisture during sieving process. Particle size distribu-
tions were determined according to the ASABE Standard S319.4 JUL97 [ 20 ], using
a Ro-Tap sieve shaker (Tyler Industrial Products, OH, USA). A sample of approxi-
mately 20 g wheat straw, switchgrass, and corn stover grinds was placed on top of a
stack of sieves, arranged from the smallest to the largest mesh number. Sieves used
for 1.6-mm (1/16″) samples were 18, 25, 35, 45, 60, 80, 100, 120, 170, and 230,
corresponding to nominal sieve openings of 1.00, 0.707, 0.500, 0.354, 0.250, 0.177,
0.149, 0.125, 0.088, and 0.063 mm [ 19 ]. Sieving time was 5 min for each sample.
The mass retained on each sieve was weighed to obtain the particle size distribution
of the biomass. The geometric mean diameter ( d gw ) of the sample and geometric
standard deviation of particle diameter ( S gw ) were calculated accordingly.
To verify the characteristics of particles retained on each sieve, a representative
sample of particles from each sieve was selected. The length and the maximum
diameter of the particles belonging to each sample were measured with a caliper. It
is known that the sieve opening is not a representative of particle length, but it is a
representative of the particle maximum diameter. However, this relationship is
weakened as particles size increases.
Different methods of size classification of wood chips were discussed by
Hartmann et al. [ 21 ]. He mentioned that screens are common in wood classification.