(John Kemp is a Reuters market analyst. The views expressed are
By John Kemp
LONDON, July 9 According to the popular
narrative, coal is locked in a fight to the death with natural
gas and renewables to supply clean electrical energy.
Promoters of gas, wind and solar often talk about coal as if
it were not just a rival but an enemy. Environmental campaigners
want to phase out coal-fired power plants and leave most of the
world's coal reserves below ground.
Coal-fired power plants produce much higher carbon dioxide
(CO2) emissions and make the largest single contribution to
global warming, so the conflict has a moral dimension.
For their part, coal producers are battling new
environmental regulations which they see as a threat to the
survival of their firms and communities.
All sides employ confrontational rhetoric.
In practice, however, there is no way to meet growing global
demand for electricity that does not rely on large amounts of
coal-fired power generation for the foreseeable future.
Cheap, abundant and widely distributed coal reserves will
remain an essential component of the global energy mix for the
next 50 years.
The challenge is to burn coal more cleanly, producing more
electricity with fewer emissions of CO2 and other harmful
With current technology, that outcome is possible, but it
will be expensive and require heavy capital investment.
In the long term, the goal is to fit coal-fired plants with
carbon capture and storage (CCS) systems, which would separate
nearly all the carbon dioxide from exhaust gases and trap it
underground in saline aquifers and depleted oilfields.
But while CCS has been successfully demonstrated on a small
scale at various facilities, nowhere has it been implemented at
a big utility-scale power plant. The engineering and commercial
challenges are significant. CCS is at least a decade, and maybe
two, away from being a mature commercial technology.
Gasification is another technology that could radically cut
emissions. By converting coal into hydrogen and carbon monoxide,
rather than burning it directly, and using these gases to drive
first a jet turbine and then a steam one, the efficiency of the
process can be improved enormously.
The by-product of coal gasification is a concentrated stream
of carbon dioxide, which is much easier to capture and store.
Coal gasification technology is more than a century old.
Modern plants that integrate gasification with combined-cycle
turbine technology, however, are expensive to build and
difficult to operate.
But there are other technologies already in use that could
cut emissions by as much as 40 percent - mostly by improving the
efficiency with which the coal is turned into steam.
In a conventional coal-fired plant, only a third of the
energy contained in the fuel is turned into electricity.
The rest is lost, mostly as heat, from the steam generator,
turbines, and exhaust system as well as in the cooling water.
The waste of energy is prodigious.
But it is possible to raise the thermal efficiency of a
coal-fired power plant from around 33-37 percent to 40 percent
or even 45 percent using fairly well-established technology.
By squeezing more electrical energy from the same amount of
coal, more-efficient power plants can slash carbon emissions.
Every 1 percentage point gain in thermal efficiency equates to a
2-3 percent reduction in CO2 per kilowatt-hour.
The main efficiency gains come from operating the steam
generator and turbines at higher temperatures and pressures.
In a conventional sub-critical power plant, water is boiled
first and then turned into steam, and the temperature of the
steam is raised further in a superheater.
But in a super-critical power plant, water is converted
directly to steam without passing through the boiling stage,
which is much more efficient.
Thomas Edison's first electric power plant, Pearl Street
Station in New York, employed steam at a pressure of just 60-160
pounds per square inch (psi) and operated at a maximum
temperature of 185 degrees Celsius.
Pearl Street Station was just 2.5 percent efficient. Since
then, steam generation technology has improved enormously.
In a modern sub-critical power plant, steam pressure is
below 3,200 psi and temperature is under 550 degrees Celsius.
In a super-critical plant, however, pressure is raised to
over 3,500 psi and temperature to about 565 degrees.
More than 200 supercritical units were operating worldwide
Even higher pressures and temperatures are possible.
Ultra-supercritical (USC) plants have been installed that
operate at 4,600 psi and 600 degrees.
Siemens, for example, has installed large
ultra-supercritical steam plants in Japan, China, Germany and
the Netherlands since the turn of the century.
Power plant designers now aim to build advanced
ultra-supercritical (A-USC) plants that would operate at 700-730
Boosting power plant efficiency by using supercritical or
ultra-supercritical technology is not a new idea.
The first supercritical power plant was built near
Zanesville, Ohio by American Electric Power, Babcock &
Wilcox and General Electric. Philo Unit 6 began
operating in 1957 and ran until 1975 ("Philo Unit 6: Advancement
of a Technology", August 2003).
A second supercritical plant was built in 1959-60 at
Eddystone, Pennsylvania, by companies that are now part of
Siemens, ABB and Exelon.
The idea of a supercritical steam generator had been around
for decades earlier. Yet in the 1960s, 1970s and 1980s, most
coal-fired power plants built in the United States and around
the world were still installed with sub-critical boilers.
The constraint on supercritical, ultra-supercritical and now
advanced ultra-supercritical systems has always been the state
of technology and cost of materials.
To withstand tremendous pressure and temperature, the steam
generators, turbines and pipework require tremendously strong
metals that are highly corrosion-resistant.
Super-strength steels and alloys of the type needed contain
large amounts of expensive metals such as nickel, chromium and
cobalt, which push up the cost considerably.
For example, in the advanced ultra-supercritical steam
generators of the future, power plant designers are considering
employing a super-alloy called INCO 740.
INCO 740 consists almost entirely of nickel (48 percent),
chromium (25 percent) and cobalt (20 percent) with small amounts
of carbon, molybdenum, aluminium, titanium, niobium, manganese,
iron and silicon added.
INCO 740, being developed by Special Metals Corp, part of
Precision Castparts, is astronomically expensive.
Refined nickel currently costs around $20,000 per tonne. Cobalt
is $30,000 per tonne. And chromium is almost $9,000 per tonne.
The challenge is how to build a power plant using as little
of this expensive alloy as possible. Among other problems that
designers have to solve is how to move the turbines closer to
the steam generator to minimise the amount of expensive piping
needed to carry steam between the two ("Advanced
ultra-supercritical power plant design for Indian coal", October
Steam generators have become progressively more efficient as
the state of materials science and costs have allowed.
The challenge is how to replace the old fleet of ageing and
inefficientsub-critical power plants around the world with the
most modern and efficient ultra-supercritical units.
For example, nearly half the coal-fired power-generating
capacity in the United States dates from before 1973. Most of
those plants are subcritical. The average capacity is just 172
megawatts. Efficiency is generally below 35 percent.
Maximum efficiency is generally thought to require power
plants rated around 800 megawatts in Europe and up to 1,000
megawatts in China.
Trianel's ultra-supercritical power plant at Lunen in
Germany, which began operating in December 2013, is rated at 750
megawatts and is almost 46 percent efficient, making it the most
efficient in Europe.
If coal is to play a useful role in future power generation,
while limiting emissions, the global coal fleet must be upgraded
to supercritical and ultra-supercritical standard, old
subcritical units must close, and eventually the fleet must be
coupled with CCS.
The coal-fired plants of the future will have to compete
with gas, nuclear and increasingly efficient wind and solar.
Modern coal-fired plants will be expensive to build, though
there are efficiency savings, and coal is relatively cheap.
Some commentators imply there will be no role for coal. The
optimum mix (from a cost, security and environmental standpoint)
is unlikely to have a zero coal share, however. Coal will still
supply a significant amount of electricity. But the power plants
of the future will be radically re-engineered to be far more
efficient and much cleaner.
(Editing by Dale Hudson)