Uncertainty in Climate Effects of Power from
Coal and Natural Gas with CCS
Greg Schivley, Constantine Samaras,
and Paulina Jaramillo
[email protected]
Fossil fuels are difficult to shake
• Provide baseload power
• ~70% of current install capacity
• Primarily coal and natural gas
• Limiting CO2 emissions will require CCS
• Uncertain when CCS will start
• Does timing affect fuels choice?
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Background on coal vs gas
• Without CCS: Natural gas power has lower GHG
emissions than coal unless CH4 emissions are very high
• With CCS: ~2% CH4 emissions needed for climate benefit
(90% capture)
• Current emission rate is uncertain, likely 1-5%
– Potential for reductions in methane emitted is not covered in
current literature
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Research questions
• How does CCS start time change climate impacts?
• Will reducing methane emissions from NG change
results?
• What if coal plants use CCS at less than 90% capture?
– EPA 111b limits new coal CO2 emissions to 1,400 lbs/MWh
(16% capture)
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Scenarios
Compare radiative forcing from 1 GW of new SCPC or NGCC power
plants over 60 years to determine the effect of uncertainty from:
1. CCS deployment: Now, or retrofit in 20 years
2. CCS capture amount: 90% of CO2 from SCPC and NGCC, or 16% capture
from SCPC (111b)
3. Natural gas CH4 emission rate: 1-5%
4. Reduce CH4 emissions: Constant emission rate or halve over 10 years
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Calculation of radiative forcing
• Emissions take place over 60 years of operation
• Every emission decays according to a response
function (right)
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𝑎𝑖 𝑒𝑥𝑝 − 𝑡 𝜏𝑖
𝑦 𝑡 = 𝑎0 +
• Mass of a species in the atmosphere is calculated
using a convolution of the emission and response
functions (above)
• Code available at github.com/gschivley/co-fire
𝑖=1
𝑦 𝑡 = 𝑒𝑥𝑝 − 𝑡 𝜏
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Results
• SCPC starting with CCS
similar to NGCC with constant
2-3% emission rate (subplot a)
• Delaying CCS for both
increase RF from coal more
than natural gas (subplots b,
d)
• If CH4 from natural gas is
reduced within 10 years, initial
emissions have almost no
effect on the peak RF
(subplots a, c)
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Results
• If only coal has CCS,
the coal plant will have
lower RF over nearly all
time frames (subplot e)
• Delaying CCS for coal
and not using it for
natural gas leads to a
cross-over in forcing
(subplot f)
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Results
• SCPC power plants meeting 111b
regulations have RF close to that of
uncaptured NGCC with constant
4+% emissions, but peak higher
(subplot a)
• Unless it leads to higher capture
rates, SCPC 111b capture will have
higher RF than uncaptured NGCC
in cases where methane emissions
are reduced over time (subplot b)
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Discussion
• Natural gas methane emission rates are important
– Under several scenarios they need to be under 3% to achieve lower radiative forcing than
coal
– If it is possible to reduce methane emissions in the near future, current emission rates are
less important than the new emissions
• It is possible for coal with CCS to have lower RF than
natural gas, but only if methane emissions stay high or CCS
is not available for NGCC
– Constant emissions of 3% or more can lead to higher RF for NGCC in several scenarios
– Immediate implementation of CCS for SCPC (but not NGCC) favors coal as a fuel
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Discussion
• Partial implementation of CCS for SCPC (111b) is useful if it
enables higher capture rates in the future
– SCPC stack emissions of 1,400 lbs/MWh lead to RF higher than uncaptured
NGCC
• CCS is likely to start as capture and use
– Use of CO2 for enhanced oil recovery (EOR) may help to offset cost of capital
equipment and operations
– EOR is not primarily designed for sequestration, and it may lead to higher total oil
production
– Future research should investigate the effects of using CO2 in EOR on RF, and
how quickly operations would need to transition to full storage to provide a
climate benefit
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Acknowledgments
This research was supported in part by the Climate and
Energy Decision Making (CEDM) center through a
cooperative agreement between the National Science
Foundation (SES-0949710) and Carnegie Mellon University.
Valuable data and feedback have been provided by the
NETL LCA Team.
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