Page 163 - New Trends In Coal Conversion

126 New Trends in Coal Conversion

torrefactiondtypically 260 C or lower. The pressure is maintained at about 700 psi.
Both treatments result in the formation of a solid product as well as evolved gases, but
wet torrefaction also produces an aqueous solution consisting primarily of dissolved
sugars. Although both torrefaction strategies result in an improvement in the solid
biomass fuel for combustion/cofiring purposes, wet torrefaction seems the most favor-
able option based on hydrophobicity and energy density (Yan et al., 2009). However,
more extensive research has to be carried out with respect to the advantages and draw-
backs of each technique.
Pelletization. One of the main problems involving the use of biomass as a fuel is its
low density, which leads to high handling and transport costs. Pelletization is a process
to physically densify fine wood particles (e.g., sawdust) into compact, low-moisture,
and low-eroding capsules by applying pressure and heat, which improves the energy
density of the fuel. Advanced (“black”) pellets can also repel water, thus improving
logistics and storage options. The pelletization process consists of three main stages:
drying, grinding/milling, and compaction. The moisture content of the biomass is first
reduced to about 10% by weight with a drying equipment. Following this, the mechan-
ical size reduction can be carried out by using, for example, a hammer mill. A press
mill is then used to compact the dried and milled biomass particles into pellets of
the required size. The particles are bound together by moisture and natural binders
which are released from the biomass itself due to heat, although artificial binders

and stabilizers could also be added. The pellets are then cooled to about 5 C, which
causes further hardening (Madanayake et al., 2017). Both woody and herbaceous
biomass can be pelletized, and pellets could become the most suitable biomass-
derived feedstock for biomass cofiring operations.
Finally, one important aspect that has to be considered with respect to pretreatment
is the overall energy/efficiency benefit. Although the higher energy density of the pre-
treated fuel means lower storage and transportation costs, energy is also consumed in
the treatment processes (particularly in grinding, torrefaction, and compaction). How-
ever, pretreatment is necessary to overcome the issues inherent to fire biomass. The
high costs of pelletization can be justified by better operability of the fuel (handling,
transportation, storage, and feeding), resulting in improved boiler and combustion per-
formance (Dai et al., 2008). Processed biomass has higher energy density than raw
biomass (Fig. 5.2), which makes it suitable for long-distance transportation. While
biomass pretreatment and upgrade facilitate handling and improve combustion effi-
ciency, the energy density of biomass remains lower than that of coal. Therefore,
biomass use is economically viable if resources are readily available locally. Future
studies should be undertaken on the evaluation of the entire process from an economic
standpoint, as this would invariably affect the practical implementation of biomass pre-
treatment on an industrial scale. In addition, according to Madanayake et al. (2017),
there is a lack of standardization in existing research on biomass, and standard proto-
cols with regard to characterization and pretreatment for future research and practical
applications are needed.