Transcript Ingen lysbildetittel

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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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Scope of presentation – what is CO2 capture?
Alternative technologies for CO2 capture
Minimum energy consumption
Comparison of technologies
Summary
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Outline
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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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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
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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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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)
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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
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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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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
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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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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
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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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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!
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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