Annex 1
CARBON CAPTURE
AND STORAGE
TECHNOLOGIES
Carbon capture
The separation and capture of carbon dioxide
is neither difficult nor new. Chemical sorbents such mono- and
di-ethanolamine (MEA and DEA) will selectively remove carbon dioxide
from gas streams. The carbon dioxide is released on regeneration
of the sorbent simply by heating. At least four leading systems
are commercially available.
Carbon dioxide can be extracted before or after the
fuel is combusted. The options are sometimes represented as: post-combustion
by retrofitting existing plant; pre-combustion by gasification;
and the use of oxy-fuel combustion with capture. It is possible
to capture carbon dioxide from the flue gases of existing power
plants. However, it is very inefficient and costly because of
the vast volume of gas that has to be scrubbed. The flue gases
are at atmospheric pressure and at a low carbon dioxide concentration—between
3-10%. The efficiency penalty would be about 10 percentage points
from the 35-38% current efficiency level of a coal power generation
plant due to the power consumed in the scrubbing process. While
such systems will achieve the goal of emissions reduction, other
ways of approaching the problem need to be examined if the cost
of CCS is to be reduced.
Carbon dioxide can be captured much more efficiently
in two ways,
Air contains about 80% nitrogen so the bulk
of the flow through a boiler is inert thus substantially diluting
the carbon dioxide content. Hence, if carbon dioxide is to be
captured in bulk, the separation of the nitrogen from the air
is an important option to be assessed.
However, the status of the technologies is such that
a watershed is approaching. If power alone is to be generated
at minimum cost, steam systems may have a cost advantage. If carbon
dioxide capture is required as a major step towards establishing
an equilibrium level in the atmosphere, then gasification appears
to offer the least cost route to capture in conjunction with power
generation.
Carbon dioxide can be transported to a suitable storage
site via pipeline or ship. The former is a mature market, with
approximately 3,000km of land-based carbon dioxide pipelines in
existence, the majority in North America. The use of existing
oil and gas pipeline infrastructure to transport carbon dioxide
is also a possibility; however wet carbon dioxide (plus other
substances such as sulphur dioxide) is corrosive, which may render
these pipelines unusable. Gas dehydration is therefore employed
to ensure minimal corrosion of the pipeline. Carbon dioxide pipelines
are currently designed to transport carbon dioxide at approximately
100bar, with upstream compressors providing the necessary compression
(although some pipelines require intermediate compressor stations).
These compressors and their associated pumps must be purpose designed
to avoid damage due to the poor lubricating characteristics of
dry carbon dioxide. Shipping of carbon dioxide to storage sites
is at an early stage of development. The carbon dioxide is transported
in liquid form (this time at -50C). However the sheer magnitude
of shipping required might prove a considerable challenge.
CARBON DIOXIDE
UTILISATION
Utilisation of carbon dioxide is one means by which
it can be prevented from reaching the atmosphere. At present,
the most common application for carbon dioxide is in Enhanced
Oil Recovery (EOR), this process having been implemented in West
Texas since the 1970s. EOR is the application that has the greatest
potential, with leading oilfield services companies such as Halliburton
and Schlumberger expressing interest. It is also a means of carbon
storage—hence the current interest of oil majors such as
BP.
CARBON STORAGE
In relation to carbon storage clearly the period
of storage needs to be on a very long timescale compared with
human timescales; the cost of storage, including transportation,
needs to be minimised; environmental impact needs to be minimal;
and the storage method should not contravene any national or international
laws and regulations. The three basic mechanisms are geological
storage, ocean storage and ecological sinks. The first two of
these can be linked directly with power plants, however the third
is a storage mechanism which is not tied to any particular carbon
dioxide source.
Figure 1 below indicates the approximate global capacity
of the various storage options. It shows that the ocean has the
highest storage capacity. Deep saline formations, depleted oil
and gas reservoirs and coal seams all offer reasonable storage
potential, but ecological sinks and utilisation have minimal capacity.
The figure also demonstrates the vast global storage potential
for carbon dioxide (1,000s of GtC).
Sources: Carbon Capture and Storage from
Fossil Fuel Use, Howard Herzog and Dan Golomb, Massachusetts Institute
of Technology Laboratory for the Energy and the Environment, July
2005. Solutions for the 21st Century Zero Emissions Technologies
for Fossil Fuels, May 2002, International Energy Agency Working
Party on Fossil Fuels, McKee B., Technology Status Report
UK storage capacity is generally in proportion to
global storage capacity, although depleted oil fields offer a
comparatively greater opportunity for the UK.
THE SLEIPNER
CARBON DIOXIDE
STORAGE PROJECT
The first, and to date only, commercial-scale project
dedicated to geological carbon dioxide storage is in operation
at the Sleipner West gas field, operated by Statoil, located in
the North Sea about 250 km off the coast of Norway. The natural
gas produced at the field has a carbon dioxide content of about
9%. In order to meet commercial specifications, the carbon dioxide
content must be reduced to 2.5%. At Sleipner, the CO2
is compressed and injected via a single well into the Utsira Formation,
a 250 m thick aquifer located at a depth of 800 m below the seabed.
About one million metric tons of carbon dioxide have been stored
annually at Sleipner since October 1996, equivalent to about 3%
of Norway's total annual CO2 emissions. A total of
20 Mt of CO2 is expected to be stored over the lifetime
of the project. One motivation for doing this was the Norwegian
offshore carbon tax, which was then about $50 (USD) per tonne
of CO2 (the tax was lowered to $38 per tonne on January
1, 2000). The incremental investment cost for storage was about
$80 million. Solely on the basis of carbon tax savings, the investment
was paid back in about 1.5 years.
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