1. Introduction
Biomass is an important renewable energy resource that could
potentially replace traditional fossil energy. It can be directly
combusted to produce heat or be transformed to electrical energy
and other forms of energy [1]. But biomass materials have low bulk
densities and have low volumetric heating values. Densification of
biomass is an effective method to decrease the volume of loose
biomass and the cost of storage, transport and utilization [2e4].
Lignocellulosic biomass such as agricultural and forest residues,
e.g. crop straw and sawdust, respectively, have been reported to
have been densified and used as fuel [3]. Compaction of sawdust
and wheat straw was tested by pressure and temperature applications
[5]. In the briquetting of olive cake, a byproduct in the
extraction of olive oil, it was reported that moisture content and
pressure had significant effects on briquette durability and stability
[6]. Variables of densification (pressure, moisture, particle size, etc.)
are very important in the forming process of materials as reported
in previous research works. It is well known that the increase of
compression pressure increases the briquette densities, but should
be selected at an optimum value to produce high quality briquettes
[7e9]. Several typical levels of pressure and moisture content were
preset to compress the briquettes; for instance, good unit density
and durability could be produced with low to medium moisture
contents of 0.09e0.12 (wet basis (w.b.)) and high compression
pressure of 12.5 MPa [10]. Low moisture corn stover (0.08e0.10
(w.b.)) resulted in denser, more stable and more durable briquettes
than high moisture stover (0.15 (w.b.)) that had a lower density.
This is because high moisture content results in the expansion of
the briquette volume after extrusion due to moisture evaporation
at high temperature [9,10]. Smaller particle sizes can produce
higher density briquettes, but more energy is expended to grind the
raw materials [11]. Tumuluru et al. [10] reported that large hammer
mill screen sizes resulted in higher durability briquettes due to
better interlocking of the variable sizes of particles.
In most of the previous studies, the plunger-die system was used
to investigate the compaction characteristics of biomass on a