Spouting Characteristics of SPF Wood Pellets
J. V. H. Haddou, N. Ellis, X. Bi and N. Epstein*
Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, BC,
Canada, V6T 1Z3
The spouting characteristics with ambient air of cylindrical wood pellets, 6.5 mm in diameter and 11.1 ± 4.1 mm long, were investigated using a
transparent 152.4-mm diameter semi-circular cone-based cylindrical column. Despite the previously unexplored shape of the pellets in the spouted
bed mode, the measured minimum spouting velocities at different bed heights, the maximum spoutable bed height and the mean spout diameter
above the conical base were all well represented by equations from the literature developed mainly for more rounded particles. Pellet attrition
was found to be severe, so that the possible torrefaction of the pellets by the spouting technique without sacrifice of their integrity appears to be
unlikely.
Keywords: spouting, wood pellets, minimum spouting velocities, maximum spoutable bed height
INTRODUCTION
S
pruce-pine-fir (SPF) wood pellets are an extruded biomass
product of British Columbia’s forest industry. Consideration
is being given to upgrading this product by torrefaction, that
is, mild pyrolysis in the low temperature range of 200–300
◦
C
under atmospheric pressure conditions in the absence of oxy-
gen. As a result of the devolatilisation caused by torrefaction
of woody biomass, van der Stalt et al. (2011) report that 70%
of the original mass is retained, containing 90% of the original
energy content. In the case of SPF pellets, the higher heating value
increases more modestly from 18 to 21–22MJ/kg (rather than to
90 × 18/70 = 23.1 MJ/kg). This energy densification allows, eco-
nomically, larger transportation distances of the pellets. Torrefied
pellets are also favoured by co-firing with coal in thermal power
plants, because of their higher heating values than regular pellets.
Torrefaction, like conventional high temperature pyrolysis, can
be carried out by a variety of methods for contacting discrete solids
with a gas, usually nitrogen in this case. Fluidisation is a com-
mon choice, but because the size of the SPF pellets are in the
range above which smooth (non-slugging) fluidisation normally
occurs, it was decided instead to try out spouting, which was orig-
inally developed precisely for gas–solids contacting of such large
particles.
Additionally, the non-spherical, imperfect-cylindrical pellets
were of a shape that had not been tested before in the spouting
mode. It was therefore of interest to measure their spouting char-
acteristics and to learn whether or not existing equations for more
spherical particles could be applied to these pellets. The present
study was thus limited to examining spouted bed hydrodynamics
using air under ambient conditions.
PARTICLE CHARACTERISTICS
The pellets (Figure 1) had a uniform cylinder diameter (d) of
6.5 mm, but the length (L) varied from 3.4 to 22.4 mm, averaging
at 11.1 mm with a standard deviation of 4.1 mm, in a sample size of
150 pellets. The L
i
of an individual pellet was taken as the mean
of the longest and shortest sides of the cylinder, the difference
having been caused by partial diagonal cleavage (or splintering)
of the pellet at one or both jagged ends. The average equi-volume
sphere diameter d
v
of the particles was then determined as:
d
v
=
150
i=1
V
i
150
6
1/3
= 8.898 mm (1)
where V
i
= volume of a pellet of length L
i
= d
2
L
i
/4, and the aver-
age equi-surface sphere diameter as:
d
s
=
150
i=1
S
i
150
1
1/2
= 9.663 mm (2)
∗
Author to whom correspondence may be addressed.
E-mail address: helsa@chbe.ubc.ca
Can. J. Chem. Eng. 9999:1–6, 2012
©
2012 Canadian Society for Chemical Engineering
DOI 10.1002/cjce.21706
Published online in Wiley Online Library
(wileyonlinelibrary.com).
| VOLUME 9999, 2012 | | THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING | 1 |