Tidal Current Energy

The Future of Clean Coal
3.2 . Combustion technologies
Combustion is the prevailing mode of fossil energy utilization, and coal is the
principal fossil fuel of electric power generation [44] . There are three fundamental
ways of generating electricity from coal: pulverized coal (PC) combustion,
fluidized bed combustion (FBC) and integrated gasification combustion cycle
(IGCC).
Pulverized coal (PC) combustion boilers have been used worldwide (in
developed and developing nations [7] ) in relatively large-scale boilers with the
attendant technologies almost approaching the maximum thermal efficiency
under the present operational conditions [45] . For hard coal, supercritical PC
combustion presently operates at efficiencies of 45% and offers prospects for
an increase to 48%; this technology is the preferred option for large units (and
will probably continue to be so up to 2020). For lignite, supercritical pulverized
firing attains more than 43% (in the so-called BoA unit of the German plant of
Niederaussem), with a target of 50% and higher if pre-drying and new materials
are used (time frame 2020) [15] . An analytical study of CCTs for their energy
consumption, net efficiency, CO 2 emissions and specific CO 2 reduction, in relation
to the conventional pulverized system, is presented in Table 2.3 [46] .
Fluidized bed combustion (FBC) boilers (bubbling, circulating and pressurized
bed boilers) have now been commercialized [45] . FBC boilers, suitable for
smaller capacities and high-ash coals, presently operate at 40% efficiency with
prospects for up to 44% [15] . FBC can reduce the production of NO x and fix sulfur
using limestone. Sulfur released from coal in the form of SO 2 is absorbed
by the limestone, which is injected into the combustion chamber with the coal.
Around 90% of sulfur can be removed as a solid compound with ashes. FBC
boilers operate at a much lower temperature than conventional pulverized coal
boilers. This greatly reduces the amount of NO x formed [9] .
The integrated gasification combustion cycle (IGCC) involves the total gasification
of coal, mostly with oxygen and steam, to produce a high heating value
fuel gas for combustion in a gas turbine [44] . The fuel gas is cleaned and then
burned in the gas turbine to generate electricity and to produce steam for a
Table 2.3 . Energy consumption, net efficiency and CO 2 emission of generation systems .
Technology
Energy
consumption/
(kW � h fuel)�
� 1 (kW� h el )
Net efficiency
(at full load)
CO 2 emissions /
� 1
kg�(kW � h el )
Conventional hard coal-fired
power plant (PC)
2.63
0.38
0.87
–
Combined cycle with PFBC 2.41 0.41 0.80
8
IGCC 2.22 0.42 0.79
9
IGCC with hot gas cleaning 2.22 0.45 0.73 16
( Source : Ref. [46] )
31
Specific CO 2
reduction related
to conventional
systems / %