CO2 Capture
CO2 capture means separating carbon dioxide from gas streams such as power plant flue gas so it can be stored or used. Absorbents may take up CO2 physically (physisorption, dissolution) or chemically (chemisorption, forming species such as carbamates or bicarbonate). Chemical absorbents reach higher capacity at low CO2 pressure, but the strength of binding must be balanced against the energy needed to regenerate the solvent, and the reaction products can change solvent properties such as viscosity.
Electrochemical CO2 separation coupled to oxy-firing (2010)
Pennline et al. proposed a capture route that avoids both solvent regeneration and cryogenic air separation: a low temperature electrochemical cell placed after conventional SO2, NOx, mercury and particulate cleanup pumps CO2 and O2 out of flue gas as bicarbonate or carbonate ions, and the resulting CO2/O2 stream (at most 4:1 for bicarbonate carriers) feeds an ancillary oxy-fired combustor whose product is essentially CO2 and water (Pennline et al. 2010). A proof-of-concept cell with a Neosepta anion exchange membrane and Ni gas-diffusion electrodes transported CO2 from a 50:50 CO2/O2 feed below 1.23 V, needing about 0.8 V for measurable current (1 mA/cm2); assuming a 4:1 product, the estimated electrical demand was 77 to 116 kJ/mol CO2 at 0.8 to 1.2 V. See also Electrochemical CO2 separation. Summary: pennline2010.
Energy feasibility of electrochemical CO2 pumping for coal plants (2010)
Landon and Kitchin estimated the parasitic power of capturing the roughly 3250 mol/s of CO2 from a 500 MW, 40% efficient coal plant with an electrochemical CO2/O2 separator, as a function of cell voltage and the CO2:O2 product ratio (Landon 2010). Meeting the DOE target of under 35% cost of electricity increase (taken as under 35% power loss) required a ratio of at least 3.5 at about 0.5 V; their anion exchange membrane cell met the ratio (3.56:1) but needed 1.2 V and gave low current densities, so they judged it not yet suitable for coal plants, though possibly useful for CO2 removal from breathing air. Summary: landon2010.
A free-energy window for sorbent capture and regeneration (2010)
Using molecular amidine sorbents with a known number of sites, Alesi et al. converted measured capture efficiencies into equilibrium constants and standard reaction Gibbs energies for bicarbonate formation (about -25 to -28 kJ/mol for DBU and DBN) (Alesi et al. 2010). Plotting predicted efficiency against standard reaction free energy for capture at 30 C and 0.1 bar CO2 and regeneration at 75 C under inert purge gives a feasible window between roughly -20 and -110 kJ/mol: weaker binders capture little, stronger ones cannot be regenerated. The framework predicts how a sorbent behaves under other conditions, for example that the weakly binding amidines gain efficiency at higher CO2 pressure, a trend confirmed experimentally though with a smaller gain than predicted. Summary: alesi2010.
Outlook for improved carbon capture technology (2012)
Rubin et al. reviewed the three main capture routes for power plants (post-combustion amine scrubbing, pre-combustion capture from shifted syngas in IGCC, and oxy-combustion) and placed emerging solvents, sorbents, membranes and looping processes on a five-stage maturity ladder tied to technology readiness levels (Rubin et al. 2012). They report energy penalties (IPCC definition) from about 16% for natural gas combined cycle to 43% for post-combustion capture on an existing subcritical coal plant, with roughly 60% of the penalty due to solvent regeneration heat (or shift losses, or oxygen production) and about 30% to CO2 compression. CCS raised the cost of electricity by about 60 to 80% for new supercritical coal plants and 30 to 50% for IGCC, corresponding to avoided costs of about 60 to 80 and 30 to 50 $/t CO2. Drawing on DOE cost studies, experience curves, industry roadmaps and the history of novel SO2 and NOx control processes (which took two to three decades to reach large-scale demonstration and mostly failed commercially), the authors conclude that large cost reductions need commercial deployment as well as R&D, and therefore a market created by policies that limit CO2 emissions. Summary: rubin2012.
A primary-amine ion-exchange resin as a CO2 sorbent (2012)
Alesi and Kitchin evaluated the commercial resin Lewatit VP OC 1065, a cross-linked polystyrene carrying benzyl amine groups, as a postcombustion solid amine sorbent (Alesi 2012). In a packed bed with 10 vol% CO2 the capacity fell from 1.85 to 1.15 mol CO2/kg between 30 and 70 C, was stable over 18 temperature swing cycles (50 to 120 C), and the resin took up only 1.5 mol H2O/kg of water. Uptake had two parts: CO2 dissolved in the polymer, released by a simple N2 purge, and carbamate species identified by DRIFTS, released on heating. In pure CO2 the capacity reached 2.5 mol/kg at 50 C, and the resin could be fully regenerated in 1 atm CO2 by 200 C; capture at 50 C and regeneration in hot CO2 at 150 C was estimated to give about 1.2 mol/kg of working capacity, versus about 1.5 mol/kg with inert regeneration. The later O2 and SO2 study below uses the same resin. Summary: alesi2012.
Electronic descriptors of amine reactivity with CO2 (2012)
Lee and Kitchin used B3LYP calculations with a continuum water model to compute bicarbonate and carbamate formation energies for primary, secondary and tertiary alkylamines, alkanolamines and trifluoroalkylamines, aiming at descriptors that could guide the design of amine solvents with a target reaction enthalpy (Lee 2012). Ion-pair product complexes were always more stable than separated ions. Electron-donating alkyl groups stabilized the products and electron-withdrawing CF3 and OH groups destabilized them, most strongly when close to nitrogen, and hydrogen bonding added stabilization. The bicarbonate complex energy (spanning roughly 0 to -80 kJ/mol) tracked the electronegativity difference between amine and carbonic acid, while the carbamic acid energy followed the nitrogen local softness only within families. Summary: lee2012.
Weak ion pairing in a physisorbing ionic liquid for CO2 separation (2013)
[Emim][TCB] is used in supported liquid membranes with high CO2 permeance and CO2/N2 selectivity and dissolves roughly 30% more CO2 than [Emim][Tf2N]. Mao et al. computed that its cation-anion interaction is markedly weaker than in [Emim][BF4] and proposed that this weak coordination leaves more free volume that can accommodate CO2 (Mao et al. 2013). Summary: mao2013.
Flue gas impurity tolerance of a primary-amine resin sorbent (2013)
Solid amine sorbents avoid heating the water in aqueous amine solvents, but their cost depends on lifetime in real flue gas. Hallenbeck and Kitchin cycled the primary-amine ion-exchange resin Lewatit VP OC 1065 in a packed bed between 50 and 127 C under continuous simulated flue gas (Hallenbeck 2013). With 4% O2 the capacity stayed near 1.27 mol/kg over 17 cycles, but with 431 ppm SO2 the resin took up 0.98 mol/kg of sulfur and lost 1.31 mol/kg of CO2 capacity in nine cycles. The poisoning was not reversed by heating to 208 C, and 1.5 M NaOH recovered only about 35% of the capacity. Seven days at 120 C in air removed about 1.1 mol amine/kg and cut capacity by 79%. The authors conclude that desulfurization upstream of an amine sorbent is essential. See also Catalyst Poisoning. Summary: hallenbeck2013.
Process-level comparison of amine solvents (2013)
Lee, Eslick, Miller and Kitchin coupled Aspen Plus models of a 90% capture absorber/stripper and compression train for a 550 MW coal plant with a multi-objective genetic algorithm, so that MEA, DEA and AMP were each compared under their own best designs rather than a flowsheet tuned for MEA (Lee et al. 2013). Pareto fronts of net plant power versus capital cost showed that DEA and AMP can beat MEA on both objectives, mainly through lower reboiler heat duty, whereas a fixed MEA-optimal design made them appear worse on power. Summary: lee2013.
Tuning CO2-reactive triazolide ionic liquids (2014)
Thompson et al. studied aprotic 1,2,3-triazolide ionic liquids in which the anion nitrogen reacts with CO2 to form carbamates (Thompson et al. 2014). Capacity at 30 C and about 1 bar ranged from 0.07 to 0.40 mol CO2 per mol IL depending on the 4-substituent, roughly 2 to 12 times the physisorption capacity of [hmim][Tf2N]. Less hindered and electron-donating substituents gave the highest uptake; 13C NMR showed only chemisorbed carbamates, and FTIR showed the binding was reversible under vacuum. DFT calculations (B3LYP) found the N1 site binds CO2 most strongly and that electron donation raises the CO2 interaction energy (from 36 kJ/mol for phenyl to 51 kJ/mol for hexyl in magnitude) more than the competing water interaction. Water up to 54 mol% did not reduce capacity, which the authors attribute to an additional bicarbonate pathway. Summary: thompson2014.
Redox-mediated active membrane for flue gas (2015)
Watkins et al. demonstrated a qualitative proof of concept for an actively pumped liquid membrane in which a quinone redox couple sets up a pH gradient across aqueous bicarbonate, so CO2 can be separated from simulated flue gas (16% CO2, 3% O2, 81% N2) without co-transport of N2 or O2 (Watkins et al. 2015). Sustained transport required both an applied potential (about 1 V with a Pt catalyst) and CO2 in the feed, and the authors point to amine media as a way to raise uptake rate and capacity. Summary: watkins2015.