The Economist  [Printer-friendly version]
December 2, 2006


[Rachel's introduction: As the end of the petroleum era peeks over
the horizon, the chemical industry and the energy industry are
planning to shift to "clean" coal. Unfortunately, there is no such
thing as "clean" coal.]

Two new coal-fired power plants will soon appear on the banks of the
Ohio River if American Electric Power (AEP), a utility, gets its way.
There is nothing unusual about that, of course: a rival firm, TXU,
unveiled plans to build 11 new coal-fired plants in Texas earlier this
year. All told, there are some 150 plants on drawing boards around
America, which derives 56% of its power from coal. But AEP's two
plants are different in one critical respect: their design would make
it relatively easy to filter the carbon dioxide out of their
emissions, should the company ever need to do so. America's first
"capture-ready" power plants, to use the industry parlance, are
nearing construction.

Coal has several advantages as a fuel. It is abundant. It is widely
distributed: countries that are short of other fossil fuels, such as
Germany and South Africa, have mountains of it. As a result, it is
cheap. Even though the price has risen in the past few years, it is
still less expensive to run a power plant on coal than on almost
anything else.

But coal is also dirty. It releases lots of soot and various noxious
chemicals as it burns, and so has fallen out of favour in many Western
countries. Worse, coal-fired plants produce roughly twice as much
carbon dioxide per unit of electricity generated than those that run
on natural gas. Power generation contributes more to global warming
than any other industry, and coal is the dirtiest part of it. Coal-
fired plants are responsible for perhaps 8 billion out of the 28
billion tonnes of man-made carbon dioxide released every year, and are
thus a prime target for emissions cuts. If environmentalists had their
way, there would be no coal-fired plants at all: protesters recently
called for the closure of Drax, Britain's biggest coal-fired power

Yet the number of coal-fired plants is growing. The International
Energy Agency (IEA), a think-tank funded by power-hungry countries,
estimates that consumption of coal will increase by 71% between 2004
and 2030. Developing countries, in particular, rely on it. Coal
provides some three-quarters of the power in both India and China.

The obvious solution is to make coal-fired generation cleaner. And
that's what utilities in Western countries have been doing for years,
to comply with ever stiffer air-pollution standards. Many literally
wash coal to remove some of the impurities before burning it. Other
technologies concentrate on purifying the smoke created during
combustion. Small particles of ash, for example, are normally removed
by forcing the flue gases, as the fumes from the furnace are known in
the trade, between electrically charged plates. (Ash particles have a
small electric charge, and are then trapped on one of the plates.)
Other filters and chemical "scrubbers" catch oxides of sulphur and
nitrogen that would otherwise cause acid rain.

Reducing emissions of carbon dioxide, however, is another matter. Most
utilities tackle the problem indirectly, by attempting to improve the
efficiency of their plants, and so to squeeze more electricity from
each tonne of coal consumed or carbon dioxide produced. Most firms
want to make such improvements anyway, since they cut costs and
improve profits. In Britain, as in most rich countries, the average
efficiency of coal-fired power stations is about 35%. But Mitsui
Babcock, an engineering firm, says its most recent designs can achieve
efficiencies as high as 46%. It reckons that switching from an old
design to a new one can cut fuel consumption and emissions by 23%.

Most of the gains in efficiency come from increasing the heat and
pressure of the steam used to turn a plant's turbines. The newest ones
heat the steam to as much as 600 deg. C -- a state physicists call
"supercritical". But there is no reason to stop there. Engineers
believe that hotter boilers would raise yields even more. Such "ultra-
supercritical" boilers, they say, could achieve efficiencies of over
50%, reducing emissions still further.

Substituting biomass for some of the coal burned can also help.
Plants, after all, grow back after being harvested, so burning them
does not add to overall levels of carbon dioxide in the atmosphere. By
replacing 20% of their fuel with biomass, power stations can reduce
their emissions by a further 20%. (Any more than that, says Lars
Stromberg of Vattenfall, a European energy firm, and the ash from
burning it would gum up the works of most furnaces.) Emissions also
fall if biomass fuel is used to pre-heat the steam before it enters
the boiler. All told, Mitsui Babcock calculates, these measures could
cut emissions from coal-fired plants to the same level as those using
natural gas.

The coal burnt in power plants can also be improved, mainly by drying.
Less dense types, such as sub-bituminous and lignite coal, can contain
up to 50% water. When burned, the water escapes as steam up the
chimney, carrying valuable heat with it. Evergreen Energy, an American
firm, is selling heat- and pressure-treated coal which, it claims, is
as much as a third more efficient than ordinary coal.

Such techniques are particularly important in America, where power
plants use a lot of sub-bituminous coal from the Powder River Basin in
Wyoming and Montana. Coal from this region is low in sulphur, and so
burns more cleanly, but has a relatively low energy density unless
treated. Meanwhile, RWE, a German utility, is building a plant which
will use the residual heat of its own flue gas to dry lignite before
burning it. That increases the overall efficiency of the plant at
little cost, since the coal can be treated without having to generate
any extra heat.

These methods can reduce the various emissions produced by coal-fired
power stations, so that they are at least no worse than gas-fired
stations. But technologies also exist to make coal cleaner still, by
filtering out carbon dioxide from the flue gas and storing it somehow.
This is theoretically possible, but expensive. Other pollutants, after
all, are essentially impurities, which can be washed from the coal or
filtered out of the flue gas. But carbon dioxide is not a
contaminant -- it is the inevitable by-product of carbon in the coal
reacting with oxygen in the air. Along with nitrogen, the inert
remainder of the air, carbon dioxide is the chief component of flue

Gases, of course, are bulky and difficult to store. Most plans for
carbon-dioxide storage involve liquefying it and pumping it
underground into former oilfields, gasfields or coal beds -- an
energy- intensive and thus expensive process. Separating the carbon
dioxide from the nitrogen in the flue gas is also expensive, but
necessary, since nitrogen turns liquid at a much lower temperature
than carbon dioxide, and so requires even more energy to liquefy.

Moreover, unlike modifications that improve efficiency, there are no
savings to be had by adding carbon-capture technology to a power
plant. As a result, no such plants have been built. A few firms are
building demonstration projects, while they wait to see whether
governments impose long-term restrictions on carbon-dioxide emissions.
Others, especially in Britain, which is blessed with natural storage
tanks in the form of the declining oil and gasfields of the North
Sea, have announced feasibility studies for clean-coal plants. Some
utilities, such as AEP, are planning commercial plants that will
initially lack carbon-capture facilities, but are designed to allow
the technology to be added fairly easily at a later date. All are
looking for government hand-outs or regulatory incentives to pursue
these experiments, in the absence of any more compelling commercial

How does carbon capture work?

Most utilities are eyeing one of three basic designs. The simplest,
and easiest to bolt on to existing plants, treats carbon dioxide like
any other pollutant, and extracts it from the flue gas. As this gas
passes through a solution of chemicals called amines, the carbon
dioxide is absorbed but the nitrogen is not. The carbon dioxide can
later be released, by heating the solution, for subsequent
liquefaction and storage. Many firms already use this "amine
scrubbing" approach to remove carbon dioxide from natural gas, for
example. But it is not so practical for large- scale uses, since the
amines are expensive, as is heating them to release the captured
carbon dioxide. The extra energy required would reduce a state-of-the-
art supercritical plant's overall efficiency by about 10%, according
to the IEA.

"Oxy-fuel" plants sidestep the difficulties of separating oxygen and
nitrogen in the flue gas by burning coal in pure oxygen rather than
air. The resulting flue gas is almost pure carbon dioxide. But the
energy used to separate oxygen from air before burning is almost as
great as that needed to filter out nitrogen afterwards, leading to a
similar loss of efficiency. Oxy-fuel enthusiasts claim that modern
plants can be easily retro-fitted to operate as oxy-fuel plants.

The third approach, called "integrated gasification combined cycle"
(IGCC), also requires oxygen, but for use in a chemical reaction
rather than for burning. When heated in oxygen, coal reacts to form
carbon dioxide and hydrogen. An amine solution then absorbs the carbon
dioxide, while the hydrogen is burnt in a modified furnace. The amine
scrubbing is cheaper than usual, since the reaction generates carbon
dioxide in a more concentrated form. Engineers are also experimenting
with membranes that would allow hydrogen to pass, but not carbon

There are four IGCC demonstration plants operating in America and
Europe, although none currently captures carbon dioxide permanently;
instead, it is simply released into the atmosphere. AEP's planned new
plants will follow a similar design. The attraction of IGCC plants,
aside from their carbon-capture potential, is that they produce fewer
traditional pollutants and also generate hydrogen, which can either be
put to industrial uses or burnt. But most utilities doubt that IGCC
plants are suitable for mainstream power generation, because of higher
capital costs and frequent breakdowns. Proponents retort that teething
problems are natural, since IGCC is a newer technology, which combines
previously unrelated processes from different industries.

George Bush is a believer, at any rate. In 2003 he unveiled a
subsidised scheme to build a zero-emissions IGCC plant called
"FutureGen" by 2013. The European Union, for its part, is giving money
to utilities dabbling in oxy-fuel, among other schemes. Handouts from
the taxpayer are needed, power firms argue, since the technology in
question is still young. But it is hard to believe that it will ever
grow up unless subsidies give way to stronger measures, such as long-
term caps or taxes on carbon-dioxide emissions. The technology to
eliminate such emissions from coal-fired plants exists, but it will
not be adopted without regulatory incentives from governments.