Page 173 - New Trends In Coal Conversion

136 New Trends in Coal Conversion

from the use of agricultural residues or demolition waste, which would otherwise
constitute a disposal challenge.
The trend in cofiring is to increase the biomass/coal ratio and to utilize a wider range
of biomass fuels. Thus, the development of efficient technologies to cofire new types
of biomass such as energy crops, waste wood, and agricultural residues is needed. The
net electric efficiency of a cofired coal/biomass power plant ranges from 36% to 44%,
depending on plant technology, size, quality, and share of biomass. Although a 20%
cofiring (as energy content) is feasible and more than 50% is technically achievable,
the usual biomass share is below 5% and rarely exceeds 10% on a continuous basis.
However, high biomass shares involve technical issues, such as securing sufficient
biomass, as well as potential combustion problems, such as slagging, fouling (which
reduces heat transfer), and corrosion. A more expensive alternative (in terms of invest-
ment costs) as the introduction of more advanced cofiring modes, such as parallel cofir-
ing or indirect cofiring, in which not only fuel preparation and feeding lines but also
conversion units for biomass and coal are independent, could be an interesting option
in some cases (Tumuluru et al., 2012).
The main difficulties and drawbacks associated to the biomass cofiring technology
are primarily caused by the differences in properties between coal and biomass. There-
fore, pretreatment techniques seem to be a potential and promising way to favor the
development of cofiring. Chemical composition issues of the biomass can be reduced
by methods such as washing and leaching, which can save costs involved in mainte-
nance of cofiring systems and minimize ash-related issues such as slagging and
fouling. The local availability of large quantities of cheap biomass makes biomass
cofiring more economically attractive. However, if local sources are insufficient,
high-energy density and pretreated biomass (e.g., wood pellets) can be used. In these
cases, long-distance transportation and logistics play an important role in the economic
viability. Both handling and combustion characteristics of biomass can be substan-
tially improved through torrefaction and pelletization, which increase the energy den-
sity of biomass, reduce transportation costs, and improve storage performance. The
pelletization of torrefied biomass is a promising option to increase the bulk density
and the net calorific value of biomass to higher levels than the typical pelletization.
Although the biomass pretreatment process has been developed technically, proper
collection and transportation is still required for its efficient utilization, as well as
large-scale application of the pretreatment process has to be developed. Seasonal/
annual fluctuation in biomass supply, due to its biological nature and environmental
diversity, is another important characteristic. Taken together, these issues indicate
that securing good quality feedstock at affordable prices over a plant’s lifetime is
crucial for biomass power projects.
Current operating experience and available solutions indicate that most technical
concerns do not materialize or can be easily solved when cofiring woody biomass at
relatively low thermal shares. As the biomass thermal share increases and more prob-
lematic fuels are utilized, further research and demonstration activities will be needed
to evaluate potential impacts of cofiring (Karampinis et al., 2014).