Improving
efficiency levels increases the amount of energy that can be extracted
from a single unit of coal. Increases in the efficiency of electricity
generation are essential in tackling climate change. A one percentage
point improvement in the efficiency of a conventional pulverised coal
combustion plant results in a 2-3% reduction in CO
2 emissions.
Moving
the current average global efficiency rate of coal-fired power plants
from 33% to 40% by deploying more advanced off-the-shelf technology
could cut two gigatonnes of CO2 emissions now, while allowing affordable
energy for economic development and poverty reduction.
Two gigatonnes of CO2 is equivalent to:
- India's annual CO2 emissions
- Running the European Union's Emissions Trading Scheme for 53 years at its current rate, or
- Running the Kyoto Protocol three times over.
Deploying
high efficiency, low emission (HELE) coal-fired power plants is a key
first step along a pathway to near-zero emissions from coal with carbon
capture, use and storage (CCUS).
Platform for Accelerating Coal Efficiency (PACE)
Given
the huge potential offered by improving efficiencies, the World Coal
Association has published a concept paper on the launch of a global
Platform for Accelerating Coal Efficiency (PACE).
The
vision of PACE would be that for countries choosing to use coal, the
most efficient power plant technology possible is deployed. The
overriding objective would be to raise the global average efficiency of
coal-fired power plants and so minimise CO2 emissions which will
otherwise be emitted while maintaining legitimate economic development
and poverty alleviation efforts.
Technologies
Improvements in the efficiency of coal-fired power plants can be achieved with technologies including:
- Fluidised Bed Combustion
- Supercritical & Ultrasupercritical Boilers
- Integrated Gasification Combined Cycle
Fluidised Bed Combustion
Fluidised
Bed Combustion (FBC) is a very flexible method of electricity
production – most combustible material can be burnt including coal,
biomass and general waste. FBC systems improve the environmental impact
of coal-based electricity, reducing SOx and NOx emissions by 90%.
In
fluidised bed combustion, coal is burned in a reactor comprised of a
bed through which gas is fed to keep the fuel in a turbulent state. This
improves combustion, heat transfer and recovery of waste products. The
higher heat exchanger efficiencies and better mixing of FBC systems
allows them to operate at lower temperatures than conventional
pulverised coal combustion (PCC) systems. By elevating pressures within a
bed, a high-pressure gas stream can be used to drive a gas turbine,
generating electricity.
FBC systems fit into two groups,
non-pressurised systems (FBC) and pressurised systems (PFBC), and two
subgroups, circulating or bubbling fluidised bed.
- Non-pressurised
FBC systems operate at atmospheric pressure and are the most widely
applied type of FBC. They have efficiencies similar to PCC – 30-40%
- Pressurised
FBC systems operate at elevated pressures and produce a high-pressure
gas stream that can drive a gas turbine, creating a more efficient
combined cycle system – over 40%
- Bubbling uses a low fluidising
velocity – so that the particles are held mainly in a bed – and is
generally used with small plants offering a non-pressurised efficiency
of around 30%
- Circulating uses a higher fluidising velocity – so
the particles are constantly held in the flue gases – and are used for
much larger plant offering efficiency of over 40%
The
flexibility of FBC systems allows them to utilise abandoned coal waste
that previously would not be used due to its poor quality.
Supercritical & Ultrasupercritical Technology
New
pulverised coal combustion systems – utilising supercritical and
ultra-supercritical technology – operate at increasingly higher
temperatures and pressures and therefore achieve higher efficiencies
than conventional PCC units and significant CO
2 reductions.
Supercritical
steam cycle technology has been used for decades and is becoming the
system of choice for new commercial coal-fired plants in many countries.
Research
and development is under way for ultra-supercritical units operating at
even higher efficiencies, potentially up to around 50%. The
introduction of ultra-supercritical technology has been driven over
recent years in countries such as Denmark, Germany and Japan, in order
to achieve improved plant efficiencies and reduce fuel costs. Research
is focusing on the development of new steels for boiler tubes and on
high alloy steels that minimise corrosion.
These developments are
expected to result in a dramatic increase in the number of SC plants and
USC units installed over coming years.
Integrated Gasification Combined Cycle (IGCC)
An
alternative to achieving efficiency improvements in conventional
pulverised coal-fired power stations is through the use of gasification
technology. IGCC plants use a gasifier to convert coal (or other
carbon-based materials) to syngas, which drives a combined cycle
turbine.
Coal is combined with oxygen and steam in the gasifier to produce the syngas, which is mainly H
2
and carbon monoxide (CO). The gas is then cleaned to remove impurities,
such as sulphur, and the syngas is used in a gas turbine to produce
electricity. Waste heat from the gas turbine is recovered to create
steam which drives a steam turbine, producing more electricity – hence a
combined cycle system.
By adding a ‘shift’ reaction, additional hydrogen can be produced and the CO can be converted to CO
2
which can then be captured and stored. IGCC efficiencies typically
reach the mid-40s, although plant designs offering around 50%
efficiencies are achievable.
Reliability and availability have
been challenges facing IGCC development and commercialisation. Cost has
also been an issue for the wider uptake of IGCC as they have been
significantly more expensive than conventional coal-fired plant.
Gasification
may also be one of the best ways to produce clean-burning hydrogen for
tomorrow’s cars and power-generating fuel cells. Hydrogen and other coal
gases can be used to fuel power-generating turbines, or as the chemical
building blocks for a wide range of commercial products, including
diesel and other transport fuels.
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