Energy consumption of alternative process technologies for CO 2 capture

Energy consumption of alternative process technologies for CO2 capture Magnus Glosli Jacobsen Trial Lecture November 18th, 2011

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

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)

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 • Reduce overall emissions of CO2 from power plants and refineries • Various approaches exist: • Pre-combustion CO2 removal • Post-combustion CO2 removal • Oxy-fuel combustion

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

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

Post-combustion capture Illustration: Bellona (www.bellona.no)

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

Pre-combustion capture

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

Oxy-fuel processes

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)

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

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)

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

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

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

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 full-scale testing stage

Amine absorption processes

Amine absorption process • Solvent is usually monoethanolamine (MEA), methyl-diethanolamine (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

Chilled ammonia absorption process

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

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)

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

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.

Membrane separation • Two approaches: • Membranes alone • Pre-combustion: Separate CO2 from H2 • Post-combustion: Separate CO2 from N2 • Membranes in combination with absorption

Post-combustion separation with membranes (numbers are from Zhiao et.al. (2008))

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))

Summary • Chemical absorption processes are more energy-intensive 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!

Sources: • 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, 1206-1213 • 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

Energy consumption of alternative process technologies for CO 2 capture