From: Massachusetts Institute of Technology
Published December 2, 2008 09:32 AM
MIT: A quicker, easier way to make coal cleaner
WASHINGTON, DC--Construction of new coal-fired power plants in the United
States is in danger of coming to a standstill, partly due to the high cost
of the requirement — whether existing or anticipated — to capture all
emissions of carbon dioxide, an important greenhouse gas. But an MIT
analysis suggests an intermediate step that could get construction moving
again, allowing the nation to fend off growing electricity shortages using
our most-abundant, least-expensive fuel while also reducing emissions.
Instead of capturing all of its CO2 emissions, plants could capture a
significant fraction of those emissions with less costly changes in plant
design and operation, the MIT analysis shows.
"Our approach — 'partial capture' — can get CO2 emissions from coal-burning
plants down to emissions levels of natural gas power plants," said Ashleigh
Hildebrand, a graduate student in chemical engineering and the Technology
and Policy Program. "Policies such as California's Emissions Performance
Standards could be met by coal plants using partial capture rather than
having to rely solely on natural gas, which is increasingly imported and
subject to high and volatile prices."
Hildebrand will present her findings on Nov. 18 at the 9th International
Conference on Greenhouse Gas Control Technologies in Washington, DC. Her
co-author is Howard J. Herzog, principal research engineer at the MIT Energy
Initiative and chair of the conference organizing committee.
The United States is facing a pressing need for more power plants that
run essentially all the time. Renewable sources aren't suited to the task,
nuclear plants can't be built quickly enough, and expanded reliance on
natural gas raises price and energy-security concerns. Coal, which now
supplies more than half of all U.S. electricity, seems the best option.
But as several states have started to regulate CO2 emissions, and others are
expected to follow suit, some of the luster has come off coal. Amid the
uncertainty, no one wants to be the "first mover" on building a new coal
plant incorporating carbon capture and storage (CCS). Depending on the type
of plant, carbon capture alone can increase the initial capital cost by 30
to 60 percent and decrease plant efficiency so that the cost per
kilowatt-hour rises. That high cost would reduce — or possibly eliminate —
the hours the plant will be called on to run. Plus, CCS hasn't been proved
at full scale, so no one knows exactly what to expect.
In Herzog's view, the call for full carbon capture is "a policy of inaction,
a policy that won't move forward either new coal plants or the CCS
technology." Partial capture could be a viable intermediate step.
The push for full capture (defined as 90 percent of the total) is in part
economic: everyone assumed that 90 percent capture would — due to economies
of scale — yield the lowest cost per ton of CO2 removed. Anything less than
90 percent would mean a higher per-ton cost.
To investigate that assumption, Hildebrand and Herzog modeled the
technological changes and costs involved in capturing fractions ranging from
zero to 90 percent. The model takes into account technological breakpoints.
For example, carbon capture is achieved by a series of devices that absorb
CO2, release it and compress it. Full capture may require two or more
parallel series.
The model confirms that the cost per ton of CO2 removed declines as the
number of captured tons increases. Not surprisingly, when the second series
is added, cost per ton goes up, but it then quickly levels off. Cost per ton
is thus roughly the same at, say, 60 percent capture as it is at 90 percent
capture. Since there are no economies of scale to be gained by going to 90
percent, companies can remove less — and significantly reduce their initial
capital investment as well as the drop in efficiency once the plant is
running.
The researchers conclude that as a near-term measure, partial capture looks
promising. New coal plants with lower CO2 emissions would generate
much-needed electricity while also demonstrating carbon capture and
providing a setting for testing CO2 storage — steps that will accelerate the
large-scale deployment of full capture in the future.
MIT |