Recent publications demonstrated that human emissions of greenhouse gases (GHG) are very likely warming our planet and therefore actions to mitigate such emissions are needed. Carbon dioxide (CO₂) is the main GHG produced by the combustion of fossil fuels in power and energy production facilities. It is anticipated that until 2030, fossil fuels will be the dominant source of energy, and that's why it is becoming crucial to develop technologies that reduce CO₂ emissions [1,2]. CO₂ can be potentially captured and then sequestered from fuel or flue gas streams using solid-based or solvent-based processes. The solvent-based processes include (1) chemical solvents; (2) physical solvents; and (3) mixed chemical/physical solvents.

The focus of this study is on physical solvents for CO₂ capture from fuel gas streams. Among the criteria for an economically feasible physical-solvent process are: (1) low vapor pressure to prevent solvent loss; (2) high selectivity for CO₂ when compared with those of CH₄, H₂ and CO in the fuel gas stream; (3) low viscosity at the system temperature to minimize solvent pumping cost; (4) thermal and chemical stability to prevent degradation; (5) environmentally benign effects, and (6) non-corrosive behavior. Available literature indicated that only few physical processes, such as Selexol, Rectisol and Morphysorb can be used to capture CO2 from gas streams. In these processes, however, the post water-shifted fuel gas, expected at 505 – 533 K and over 30 bar, has to be cooled to ~ 312 K (Selexol), or ~ 298 K (Morphysorb process), or to ~ 263 K (Rectisol), which are obviously energy-intensive processes. Analysis conducted at National Energy Technology Laboratory (NETL) showed that CO₂ capture and compression using the Selexol as a benchmark process increase the cost of electricity (COE) from a newly built Integrated Gas Combined Cycle (IGCC) power plant by 25% (from 5.5 cents/kWh to 6.5 cents/kWh) [3]. This rise of COE is obviously high, considering the NETL's 2012 programmatic goal is to keep this rise of COE to less than 10% for advanced CO₂ capture and sequestration systems applied to IGCC.

The objective of this study is to investigate the potential of using different ionic liquids as physical solvents for selective capture of CO₂ from post water-gas-shift reactor streams at elevated pressures and temperatures. In order to achieve this objective, an experimental program was devised to obtain the equilibrium gas solubility (C*) and volumetric liquid-side mass transfer coefficients (k_La) for gas mixtures containing H₂, CO₂, Ar, CO, and CH₄ in two different ionic liquids, namely TEGO IL K5 and TEGO IL P9. It seems that ionic liquids could be attractive candidates for CO₂ capture from fuel gas streams, because their vapor pressure has been reported to be immeasurable at relatively high temperatures, eliminating solvent loss under the capture conditions.

In order to obtain the equilibrium solubility and volumetric liquid-side mass transfer coefficient for each component in the gaseous mixture, an online Mass Spectrometer was calibrated and used to continuously monitor the mole fractions of all components during the transient and equilibrium conditions. The partial pressures of each component were then calculated allowing the determination of the respective C* and k_La. The experimental technique was initially tested and validated at low temperature using Selexol solvent and at higher temperature using a fluorinated physical solvent (PP25).

Preliminary data indicated that the solubilities at 300 K of CO₂ in the ionic liquid TEGO IL K5 is greater than those in Selexol and PP25 solvents. The solubility of H2 in the TEGO IL K5 appears to be negligible. Also, the volumetric liquid-side mass transfer coefficients obtained for each component in the gas mixture were slightly lower than those obtained for each single component in the TEGO IL K5 solvent. This behavior can be attributed to the importance of the gas-side resistance (k_Ga) created by the presence of other gaseous components in the mixture during mass transfer of that component through the gas-liquid interface (gas bubble surface). Experimental data of the solubility and mass transfer coefficients obtained under high pressures and temperatures with the gaseous mixture in TEGO IL P9 will be also discussed.

References:

1. Evaluation of Innovative Fossil Fuel Power Plants with CO₂ Removal, EPRI, Palo Alto, CA, U.S. Department of Energy - Office of Fossil Energy, Germantown, MD and U.S. Department of Energy/NETL, Pittsburgh, PA, Report No. 000316, 2000.

2. Gale, J.; Bachu, S.; Bolland, O.; Xue, Z., "To store or not to store?," International Journal of Greenhouse Gas Control, 2007, Vol. 1(1), p. 1.

3. http://www.netl.doe.gov/technologies/carbon_seq/core_rd/co2capture.html (May 2007) Carbon Sequestration - CO₂ Capture.