Environmental Engineering Reference
In-Depth Information
taBle 7.1
Biomass densification characteristics
Biomass Feedstock
mass density (kg/m
3
dry basis)
energy density (GJ/m
3
)
Loose herbaceous biomass
50-95
0.8-1.5
Bales
120-200
a
2-3
Woodchips
200-250
4-5
Pellets
600-700
a
8-11
Torrefied pellets
750-850
15-18
Pyrolysis oil
1200-1300
15-16
Source:
Cundiff, JS. and Marsh, LS.,
Bioresour Technol,
56, 95-101, 1996; Venturi, P., Huisman, W., and Molenaar, J.,
J
Agric Eng Res.,
69, 209-215, 1998; Brown, RC.,
Biorenewable Resources: Engineering New Products from
Agriculture,
Blackwell Publishers, Boston, MA, 2003; Sokhansanj, S. and Turhollow, AF.,
Appl Eng Agric.,
20,
495-499, 2004; Bergman, PCA.,
Combined Torrefaction and Pelletisation—The TOP Process,
ECN Report, ECN-
C-05-073, ECN Biomass, Petten, The Netherlands, 2005; Shinners, KJ., et al.,
Biomass Bioenergy,
31, 211-221,
2007b; Uslu, A., Faaij, APC., and Bergman, PCA.,
Energy,
33, 1206-1223, 2008.
a
Significantly higher values are reported in the text using new equipment that is not widely available.
Herbaceous biomass has a low bulk density relative to wood and very much so relative to coal,
oil, and other energy carriers. Densification, including baling, palletizing, and briquetting has been
recognized as an effective way to significantly reduce transport and storage costs and associated
handling costs (Badger and Fransham 2006; Uslu et al. 2008; Petrolia 2008). Bulk density depends
on material composition, particle size, shape, distribution, and moisture content (Lam et al. 2007).
Table 7.1 gives the ranges of typical biomass mass and energy densities for fresh biomass and
several densification strategies.
7.6.1 p
EllEtizing
and
B
riquEtting
Pelletizing and briquetting are traditionally used for producing animal feed and compressing coals
and minerals. Biomass is first ground to the size of sawdust, then compressed and forced through a
metal die, where friction or supplemental heating seals the surface to keep the individual pellet or
briquette intact. The most popular shape of pellets and briquettes is cylindrical. The main difference
between pellets and briquettes is size, which affects material handling and downstream conversion
processes. The diameter of pellets typically ranges from 5 to 12 mm whereas briquettes have a
diameter of 50-100 mm or more. These technologies and equipment can be used for densifying
biomass materials with the end goal of bioenergy production.
In a drum pelletizer, many holes are drilled through the drum, and a roller inside squeezes mate-
rial through these holes. Another piece of metal outside of the drum may be mounted to shear off
the pellets at a desired length. Sometimes starch is added as a binder, but most biomass materials
can be pelletized with a small amount of electrical heat or steam applied while the material is forced
through the die; no binding material is needed. Appropriate moisture content of biomass materials
is between 8% to 15% dry basis (d.b.) depending on the type of biomass material. The bulk density
of pellets or briquettes normally ranges from 600 to 700 kg/m
3
, but it can reach as high as 1000
kg/m
3
(Mani et al. 2006). One cubic meter of wood pellets is equivalent to approximately 3 m
3
of
woodchips and 2 m
3
of solid wood.
Pellets can also be made with herbaceous biomass feedstocks, although few commercial
manufacturers currently exist. Moisture content of the feedstock and temperature control of the die
are critical because the lower lignin content in herbaceous material does not bind as well as sawdust
from wood. However, because herbaceous biomass is less dense than woodchips even when baled,
the densification advantages that pellets offer for these feedstocks warrant a close evaluation.
