Clean Coal Technologies - Examples
Clean coal technologies are a family of new technological
innovations that are environmentally superior
to the technologies in common use today.
Clean coal technologies can be new combustion
processes - like fluidized bed combustion and
low-NOx burners - that remove pollutants, or prevent
them from forming, while the coal burns.
Clean coal technologies can be new pollution
control devices - like advanced scrubbers - that
clean pollutants from flue gases before they exit
a plant's smokestack.
Still other clean coal technologies can convert
coal into fuel forms that can be cleaned before
being burned. For example, a clean coal plant
may convert coal into a gas that has the same
environmental characteristics as clean-burning
natural gas.
Examples of clean coal technologies currently
in operation or under development around the world
include:
Stack Gas
Treatment - applied to gaseous emissions from
Pulverised Fuel (PF) Combustion
Pulverised Fuel (PF) combustion
is the most widely used method for burning coal
for power generation. In PF combustion, coal is
milled to a powder and blown into the boiler with
air. As a powder, the coal has a large surface
area and is easily combusted in burners. This
provides the heat which is used to produce superheated
steam to drive turbines and hence generate electricity.
At present, nearly all of the world's coal-fired
electricity is produced using PF combustion systems.
Typical coal-fired power station with Flue Gas
Desulphurisation
Emissions from PF combustion can
be reduced by post-combustion CCTs. Electrostatic
Precipitators and/or fabric filters can remove
more than 99% of fly ash from flue gases. Flue
Gas Desulphurisation (FGD) methods can remove
90-97% of the oxides of sulphur (SOx) from flue
gases, and can convert it into gypsum for use
in the building trade.
Among the key CCTs for PF combustion that reduce
emissions of nitrogen oxides (NOx) are low-NOx
burners, which modify emissions by up to 40%,
and reburning techniques. Together these modify
the combustion process to reduce NOx emissions
by up to 70%, and are being widely adopted as
they can be installed into existing plant. Selective
catalytic NOx reduction, a post-combustion technique,
can achieve reductions of 80-90%.
Advanced
Pulverised Fuel (PF) Combustion
Industry has continuously striven
to increase efficiencies of conventional plant;
for example, the average thermal efficiency of
US power stations has increased from 5% in 1900,
to around 35% currently. In China, most power
plants are relatively small, average efficiency
is about 28% compared to an OECD average of 38%.
New conventional PF power plants achieve above
40% efficiency.
Advanced modern plants use specially developed
high strength alloy steels, which enable the use
of supercritical and ultra-supercritical steam
(pressures >248 bar and temperatures >566°C)
and can achieve, depending on location, close
to 45% efficiency.
Application of new advanced materials to PF power
plant should enable efficiencies of 55% to be
achieved in the future. This results in corresponding
reductions in CO2 emissions as less fuel is used
per unit of electricity generated.
Fluidised
Bed Combustion (FBC)
Fluidised bed combustion is a method
of burning coal in a bed of heated particles suspended
in a gas flow. At sufficient flow rates, the bed
acts as a fluid resulting in rapid mixing of the
particles. Coal is added to the bed and the continuous
mixing encourages complete combustion and a lower
temperature than that of PF combustion.
The advantages of fluidised beds
are they produce less NOx in the outlet gas, because
of lower combustion temperatures, and they produce
less SOx when limestone is continuously added
with the coal. They can also use a wider range
of fuels than PF combustion.
Atmospheric-pressure fluidised beds are commercially
available now as two types, bubbling-bed (known
as Atmospheric Fluidised Bed Combustion - AFBCs)
and circulating-bed (CFBCs). The efficiency of
most fluidised beds used for power generation
is similar to that of conventional plant. However,
use of this technology has been stimulated by
its better environmental performance when utilising
lower grade fuels.
Pressurised fluidised beds, which can achieve
efficiencies of 45%, are now in commercial operation.
As with PF plants, employing higher steam conditions
would further boost efficiency.
Gasification
and Integrated Coal Gasification Combined Cycle
(IGCC)
An alternative to coal combustion
is coal gasification. When coal is brought into
contact with steam and oxygen, thermochemical
reactions produce a fuel gas, largely carbon monoxide
and hydrogen, which when combusted can be used
to power gas turbines.
Integrated Coal
Gasification Combined Cycle Unit
Integrated Coal Gasification Combined
Cycle (IGCC) power generating systems are presently
being developed and operated in Europe and the
USA. These systems give increased efficiencies
by using waste heat from the product gas to produce
steam to drive a steam turbine, in addition to
a gas turbine.
Existing commercial systems achieve efficiencies
close to 45%. With recent advances in gas turbine
technologies these systems are now capable of
reaching above 50%. IGCC systems additionally
produce less solid waste and lower emissions of
SOx, NOx and CO2. Over 99% of the sulphur present
in the coal can be recovered for sale as chemically
pure sulphur.
Hybrid
and Advanced Systems
Hybrid combined cycles are also
under development. These combine the best features
of both gasification and combustion technologies,
using coal in a two-stage process. The first stage
gasifies the majority of the coal and runs a gas
turbine, the second stage combusts the residual
'char' to produce steam. Efficiencies greater
than 50% are possible.
In addition to these CCTs, a development which
can apply to all of the generating systems is
the co-firing with coal of biomass or wastes.
This involves burning or gasifying such materials
together with coal. Benefits can include reductions
in CO2, SOx and NOx emissions relative to coal-only
fired plants, and recovery of useful energy from
biomass and wastes at high efficiencies can be
achieved, without the need for building dedicated
plant. Hence, the coal-fired power industry can
support the renewable energy and waste industries.
(The description of clean coal technologies
above has been drawn from the World Coal Institute
publication - Coal, Power for Progress.
See http://www.worldcoal.org
)
Fuel Cell Technologies
Fuel cells and magnetohydrodynamics (MHD) are
two technologies still in the early development
stage. Fuel cells allow hydrogen from natural
gas, methanol or coal gas to react electrochemically
with oxygen from the air to generate electricity.
 Fuel
cells have the potential for very high power generation
efficiency and low carbon dioxide emissions. ln
a coal-fired magnetohydrodynamics system, coal
is burned to form an extremely hot gas or plasma.
This is given an electric charge by adding a seed
compound like potassium salt.
When the charged gas is passed through a strong
magnetic field, electricity is produced. Heat
from the combustion gases is also used to produce
electricity using a conventional steam turbine.
The use of fuel cells has been demonstrated at
the 2 MWe size and plans are underway to use hydrogen
from coal gasification in this and other technologies.
Together with sequestration of CO2 in isolation
this clean coal technology provides a nil CO2
option. However, lower cost equipment and more
particularly markets for hydrogen need to be developed.
Further references/links:
For further information on Clean Coal and
related technologies visit:
IEA Clean Coal Centre
http://www.iea-coal.org.uk
The Clean Coal Technology Compendium - US National
Energy Technology Laboratory
http://www.netl.doe.gov/cctc/
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