By John Kemp
LONDON Nov 12 New funding for research into
lower-cost ways of capturing carbon dioxide is meant to
emphasise the Obama administration's commitment to a future for
cleaner coal-fired power generation.
But it underlines the continuing cost problems with carbon
capture technology that make it hard to deploy on a commercial
On Nov. 7, the U.S. Department of Energy announced that it
was awarding a total of $84 million to 18 research projects
intended to find a lower cost way of capturing carbon dioxide
(CO2) emissions from power plants.
The projects will test new solvents and sorbents to separate
CO2 from other gases in power plant exhaust streams.
The aim is "improve the efficiency and drive down the costs
of carbon capture processes for new and existing coal-fired
plants," according to officials.
The announcement is designed to underline the
administration's commitment to developing carbon capture and
storage (CCS) technology and refute suggestions it is waging a
"war on coal."
But all the projects are for basic science and very early
stage technologies, underscoring the fact CCS is not yet a
Nonetheless, draft emissions rules published by the U.S.
Environmental Protection Agency in September would require any
new coal-fired power plants built in the United States to be
equipped with CCS.
The agency is now developing rules for existing coal-fired
plants, though officials caution that these will probably be
different from those governing new plants.
Coal producers and the owners of coal-fired power stations
complain that the administration is mandating the use of a
technology which has not yet proven cost effective at commercial
scale, effectively prohibiting new coal-fired generation.
For supporters, the new rules will force the power industry
to devote all its inventiveness to making CCS a reality, using
the regulatory process to spur technological change.
For opponents, the rules are an arbitrary backdoor ban that
puts an important source of fossil energy off limits and makes
the power sector dangerously dependent on natural gas.
The relative immaturity of the capture stage of the CCS
process suggests there is some validity to that criticism.
CCS is a promising technology, which is indispensable to
meeting future energy requirements and ensuring a future for the
coal industry while limiting emissions of greenhouse gases. But
to mandate its use for new power plants, let alone existing
ones, seems premature.
Transporting carbon dioxide over long distances by pipeline
and storing it underground in depleted oil and gas fields are
fairly mature technologies, which have already been implemented
many times over the last 40 years.
Capturing CO2 in the first place is more commercially
challenging. CO2 has been successfully separated from other
gases in industrial processes for nearly a century.
But most of the CO2 separation has occurred at natural gas
processing plants and facilities producing ammonia, where the
volume of gas is comparatively small and the CO2 is found in
fairly high concentrations.
The exhaust stream of a major power plant involves a much
larger volume of gases, perhaps 20-50 times higher, and the
carbon dioxide is much more dilute. CO2 accounts for as little
as 15 percent of the exhaust gases from a power plant. The rest
is mostly nitrogen (N2) as well as small quantities of
pollutants like nitrogen oxides (NOx) and sulphur dioxide (SO2).
Most existing capture systems pass the mixed waste gases
through an absorber, where the CO2 comes into contact with a
solvent and becomes bound to it; the other gases pass through
unaltered. The CO2-rich solvent is then passed through a
stripper, where it is heated to 100-140 degrees Celsius,
releasing the concentrated CO2 and regenerating the solvent so
it can be re-used in the absorber.
The need to heat the solvent to regenerate it and the
enormous quantities of material involved impose a considerable
cost and energy penalty on the process.
Scaling the process up to deal with the much larger volume
of dilute CO2 in the exhaust gases of a major power station
would involve enormous expenditure and impose a severe energy
penalty on the efficiency and competitiveness of the power
So the hunt is on for a more efficient way to capture CO2.
Until now, the most promising routes to reducing the energy
penalty have focused on raising the concentration of CO2 in
power plant exhausts.
One option involves burning coal in nearly pure oxygen (O2)
rather than ordinary air, most of which is nitrogen, which would
produce a stream that is 80-98 percent pure CO2.
The problem with this "oxy-combustion" process is that
oxygen has to be produced from air in the first place in an
oxygen plant, which itself involves large amounts of energy. The
energy penalty is changed rather than eliminated.
A second option is to convert coal into synthesis gas (a
mixture of hydrogen and carbon monoxide) by partially oxidising
it in a gasifier rather than burning it. The carbon monoxide
(CO) is then reacted with steam (H2O) to produce even more
hydrogen and a fairly concentrated stream of CO2.
In an integrated gasification and combined cycle (IGCC)
power plant, the produced hydrogen is first burned in a jet
turbine, then the hot exhaust gases are used to raise steam
which powers a secondary steam turbine.
IGCC plants are extremely efficient and produce concentrated
CO2 streams which are easy to capture. But they are formidably
expensive to build. For the plant to operate efficiently, all
the processes (gasifier, jet turbine and steam turbine) must
operate smoothly together, which poses a severe engineering
In the United States, Southern Company is building an
IGCC power plant with some CCS capability in Kemper County,
Mississippi, which will use local supplies of lignite in the
gasifier. But Kemper has already gone wildly over-budget and it
is not yet in service so its operating efficiency remains
The Dakota Gasification Company has operated 14 enormous
gasifiers at the Great Plains Synfuels Plant in North Dakota for
nearly two decades, producing pipeline natural gas, as well as
ammonia and other specialty products, using lignite as a
feedstock. It also produces CO2, which is captured and sold to
oil producers in Canada for enhanced oil recovery (EOR)
But the Great Plains Synfuels Plant was only built with an
enormous amount of financial assistance from the federal
government, and had to be bailed out by the Department of Energy
when it became insolvent in the mid-1980s, so it is hardly a
model for gasification on a commercial basis.
Policymakers and engineers are now working on a so-called
"second generation" of separation technologies which could be
fitted to new and existing power plants, which would be cheaper
and require much less energy to strip the CO2 from exhaust
Combined membrane-absorbent systems use sophisticated
membranes to make the solvent/absorption process faster and more
Most of the projects for which the Department of Energy
announced funding involve membrane technologies.
Fifteen projects with total funding of just under $70
million are trialling membranes and solvents that could be
employed for post-combustion CO2 capture, so they could be
retrofitted to existing power plants.
The remaining projects focus on pre-combustion CO2 capture
and would support a new generation of IGCC plants, if they are
All the projects are either pilot-scale or bench-scale. They
are the sort of advanced science and technology funding that the
department has successfully sponsored in the past. Even if they
achieve a breakthrough, however, it will be years before they
could be deployed commercially.