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Energy consumption of
alternative process
technologies for CO2 capture
Magnus Glosli Jacobsen
Trial Lecture
November 18th, 2011
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Outline
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•
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Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
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Outline
•
•
•
•
•
Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
4
Scope of presentation
• CO2 capture has a big range of applications
• Small-scale:
– Rebreathers for divers, mine workers etc
– Air recirculation in spacecraft and submarines
• Industrial scale:
– CO2 removal from feed gas (e.g. in gas treatment plants). Widely
used today
– CO2 removal from exhaust gas (e.g. in power plants, steel
production etc)
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CO2 capture in industry
• Removal of CO2 from feed gas
– Avoid processing ”worthless” material – compression is costly!
– Reduce corrosion on equipment
– Keep specification on product gas (lower heating value)
• Removal of CO2 from exhaust gas
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Reduce overall emissions of CO2 from power plants and refineries
Various approaches exist:
Pre-combustion CO2 removal
Post-combustion CO2 removal
Oxy-fuel combustion
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Outline
•
•
•
•
•
Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
7
Alternative technologies for CO2
capture
• Where is CO2 captured?
– Post-combustion plants
– Pre-combustion plants
– Oxy-fuel plants
• How is CO2 captured?
– Adsorption
– Absorption
– Membrane separation
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Post-combustion capture
• This is the most conventional technology – fossil fuel
is burned, and carbon dioxide is separated from the
exhaust gas
From coal: C + O2  CO2
From gas: CH4 + 2O2  CO2 + 2H2O
• The CO2 must be separated from the exhaust gas at
low (partial) pressure
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Post-combustion capture
Illustration: Bellona (www.bellona.no)
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Pre-combustion capture
• Fossil fuel is converted to CO2 and H2 by gasification
and water-gas shift:
3C + O2 + H2O  3CO + H2
CO + H2O  CO2 + H2
• Separation of CO2 from H2 is easier than separating it
from N2
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Pre-combustion capture
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Oxy-fuel processes
• Pure oxygen, rather than air, is used in the
combustion
• The exhaust gas is either pure CO2 or a mixture of
CO2 and H2O
• Main advantage: Easy separation of CO2 from
exhaust gas
• Main drawback: Requires separation of O2 from air,
which is costly
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Oxy-fuel processes
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Efficiency loss for power plants
• Post-combustion: Separation
of CO2 dominates energy
consumption
• Pre-combustion: Lower
separation cost for CO2,
requires water-gas shift
• Oxy-fuel: No separation cost
for CO2, high cost for air
separation
Illustration: Davison (2007)
15
Examples of separation
technologies
• Absorption
– Amines
– Chilled ammonia
• Adsorption
– Pressure-swing adsorption (PSA) (physical)
– Thermal swing adsorption (TSA) (physical)
– Calcination/carbonation cycling (chemical)
• Membrane separation
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Outline
•
•
•
•
•
Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
17
Minimum energy requirement
All separation of gases requires energy. For an ideal
gas mixture, the required energy at given T and P is
ΔGseparation = - T ΔSseparation
where, for total separation into pure components,
ΔSseparation = - ΔSmixing = nR Σi (xi ln xi)
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Example: CO2 from exhaust
• Assume stoichiometric ratio between air and
methane, and complete combustion:
8N2 + 2O2 + CH4  8N2 + CO2 + 2H2O
• The composition of the exhaust is xN2=0.73,
xH2O=0.18 and xCO2=0.09
• At 298K, this gives a ΔGseparation of 1.89 kJ/mol
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Example, continued
• We don’t need to separate N2 from H2O. Subtraction
gives a ΔGseparation of 0.76 kJ for separating the CO2
from 1 mole of exhaust.
• This equals 190 kJ/kg CO2 (or 0.190 GJ/ton CO2)
removed from the exhaust stream, for 100% CO2
recovery
20
Outline
•
•
•
•
•
Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
21
What do we compare?
• Papers report different measures of energy
consumption, including:
– Fraction of fuel heating value which is consumed by capture
process
– Energy consumed for a given amount of CO2 captured
– Loss in overall plant efficiency
• Many papers are based on simulation models and
pilot-scale plants
• Some include post-separation compression of CO2,
this is not considered here
– This compression is independent of which separation technology is
used, but can be integrated with separation
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What do we compare?
• Papers report different measures of energy
consumption, including:
– Fraction of fuel heating value which is consumed by capture
process
– Energy consumed for a given amount of CO2 captured
– Loss in overall plant efficiency
• Many papers are based on simulation models and
pilot-scale plants
• Some include post-separation compression of CO2,
this is not considered here
– This compression is independent of which separation technology is
used, but can be integrated with separation
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Absorption processes
• CO2 is absorbed in a liquid solvent in an absorber
and driven off in a stripper
– Amines (MEA, MDEA etc)
– Ammonia
• The stripping stage is the most energy-intensive
• The only technology which has reached to the fullscale testing stage
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Amine absorption processes
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Amine absorption process
• Solvent is usually monoethanolamine (MEA), methyldiethanolamine (MDEA) or a mixture of the two
• The process runs at pressures slightly above
atmospheric and at moderate temperatures
• Well established process for CO2 removal, only
scale-up issues remain
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Chilled ammonia absorption process
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Chilled ammonia absorption process
• Uses less energy for regeneration than the amine
process
• Uses more energy for compression
• Needs more process equipment than the amine
process
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Energy usage in absorption
processes
• Pure MEA: 3,0 GJ/ton CO2 at a CO2 recovery rate of
90% (Abu-Zahra et.al, 2007)
• MEA/MDEA mixture: 2,8 GJ/ton CO2 at 90% recovery
(Rodriguez et.al., 2011)
• Chilled ammonia: About 1,5 GJ/ton CO2, at >90%
recovery (Valenti et.al., 2009)
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Adsorption processes
• CO2 is adsorbed in a porous material
• Uses the fact that adsorption properties change with
temperature, pressure et cetera
• Thermal swing adsorption
• Pressure swing adsorption
• In physical adsorption, CO2 selectivity is generally
lower than for chemical absorption
• Chemical adsorption: CaO/CaCO3 cycle
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Energy usage in adsorption
• Thermal swing adsorption: 3.23 GJ/ton CO2 at a
recovery of 81% and a CO2 purity of 95% (Clause
et.al. (2011))
• Pressure swing adsorption: 0.6457 GJ/ton CO2 for a
recovery of 91% and a CO2 purity of 96% (Liu et.al
(2011))
• Calcination/carbonation: Not found. General remark:
CaO degradation reduces efficiency quickly.
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Membrane separation
• Two approaches:
• Membranes alone
– Pre-combustion: Separate CO2 from H2
– Post-combustion: Separate CO2 from N2
• Membranes in combination with absorption
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Post-combustion separation with
membranes
(numbers are from Zhiao et.al. (2008))
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Energy usage with membranes
• Post-combustion: 0.36 GJ/ton CO2 at 80% recovery
(Zhiao et.al. (2008))
• Pre-combustion: 0.3 GJ/ton CO2 at 85% recovery
(Grainger & Hägg (2007))
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Outline
•
•
•
•
•
Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
36
Summary
• Chemical absorption processes are more energyintensive than membrane-based processes and
pressure-swing adsorption
• However, the former are more mature and closer to
realization
• The potential energy savings in CO2 capture are
huge!
37
Sources:
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Illustrations: www.bellona.no
Abu-Zahra, M.R.M.; Schneiders, L.H.J.; Niederer, J. P. M.; Feron, P. H. M.; Versteeg, G. F.
(2007): CO2 capture from power plants Part I. A parametric study of the technical
performance based on monoethanolamine. International journal of greenhouse gas control,
1, 37–46
Clausse, M.; Merel, J.; Meunier, F. (2011): Numerical parametric study on CO2 capture by
indirect thermal swing adsorption. International journal of greenhouse gas control, 5, 12061213
Davison, John (2007): Performance and costs of power plants with capture and storage of
CO2. Energy 32, 1163–1176
Hägg, M-B.; Grainger, D. (2008): Techno-economic evaluation of a PVAm CO2-selective
membrane in an IGCC power plant with CO2 capture. Fuel, 87, 14-24
Liu, Z.; Grande, C. A.; Li, P.; Yu, J.; Rodrigues, A.E. (2011): Multi-bed Vacuum Pressure
Swing Adsorption for carbon dioxide capture from flue gas. Separation and Purification
Technology, 81, 307-317
Rodriguez, N.; Mussati, S.; Scenna, N. (2011): Optimization of post-combustion CO2
process using DEA-MDEA mixtures. Chemical engineering research and design, 89, 1763–
1773
Valenti, G.; Bonalumi, D.; Macchi, E. (2009): Energy and exergy analyses for the carbon
capture with the Chilled Ammonia Process (CAP). Energy Procedia, 1, 1059–1066
Zhao, L.; Riensche, E.; Menzer, R.; Blum, L.; Stolten, D. (2008): A parametric study of
CO2/N2 gas separation membrane processes for post-combustion capture. Journal of
Membrane Science, 325, 284-294
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