Page 161 - New Trends In Coal Conversion

124 New Trends in Coal Conversion

industry, waste wood, and energy crops. Using wastes for cofiring would minimize the
environmental problem associated with its disposal. Woody biomass is considered to
be the most convenient option for cofiring activities due to its naturally low content in
ash, sulfur, and nitrogen, but other biomass feedstocks, such as forest residues, mill
residues, or agricultural products such as straw, switchgrass, corn stover, rice hulls,
and olive pits, could be used in cofiring with coal.
Important barriers associated with cofiring biomass and coal can be the unstable
supply of biomass and the availability of quality biomass fuels, which limit the per-
centage of biomass that can be fired. The costs of biomass acquisition and transporta-
tion determine to a large extent the economic feasibility of cofiring. The acquisition
costs depend on possible competition with other biomass energy uses (e.g., biofuels)
or nonenergy applications. A stable and cheap flow of biomass is needed to sustain a
biomass cofiring project. The price of biomass is strongly dependent on the following:
(1) the feedstock’s origin, type, and composition; (2) the cost of handling, preparing,
and transporting the feedstock; and (3) the plant’s geographic location. The transpor-
tation cost over long distances is influenced strongly by the energy density or the heat-
ing value of the biomass feedstock (Agbor et al., 2014).

5.5.2 Biomass pretreatment
Drying, size reduction, storage, transport, feeding, and handling of biomass fuels pre-
sent problems in achieving stable conditions. Biomass produces a nonfriable, fibrous
material during milling, grinding, or any other process used to reduce the particle size.
It is generally unfeasible to reduce biomass to the same size or shape as coal. The shape
and size of biomass particles result in very low packing densities and could create chal-
lenges when pneumatically or otherwise transporting biomass fuels. Thus, the pretreat-
ment of the biomass involves changing it into a form which can be integrated into the
fuel chain of the generation plant without much change to the plant itself. Cofeeding of
blended fuels, such as coal and biomass, presents more problems than separate
feeding. However, if the proportion of biomass in the coal is small (<10% on an en-
ergy basis), the effect of biomass addition has been found to be almost insignificant
(Sami et al., 2001).
Various measures can be applied to avoid or reduce problems in biomass or blends
feeding, such as biomass densification as pellets and briquettes, or torrefaction. These
techniques can be effectively used to reduce their high moisture content and increase
the heat value per volume of biomass, which will improve their transportation and stor-
age and reduce the transportation costs (Agbor et al., 2014). Verma et al. (2017)
reviewed the different technologies used for biomass drying to reduce its inherent
moisture content. On the other hand, a recent publication by Madanayake et al.
(2017) has reviewed the state of the art of biomass pretreatment techniques as precur-
sors to combustion or cofiring. These authors aimed to place a particular emphasis on
torrefaction and leaching, which are potentially two of the most important strategies to
improve the fuel properties of biomass.
Leaching. One of the main concerns with using biomass is its high alkali content,
which causes a number of ash-related problems. The alkali (K) and alkaline earth (Ca)