(276g) Fast Sorbent Mediated Water-Gas Shift (C-SHIFT) Process for Pre-Combustion CO2 Capture

M.E. Boot-Handford, P.S. Fennell, J.A.Z. Pieterse, P.D. Cobden, J. Boon, M.G. Sceats, B.N.C. Sweeney, B. Arstad, R. Blom

The carbonated shift (C-SHIFT) process is part of the ASCENT project aiming at proof of concept of three different advanced CO₂ capture process. C-SHIFT, a sorption enhanced water-gas shift process, uses a highly innovative fluidized bed reactor system to match the heat requirements of reaction and regeneration of medium temperature carbon dioxide acceptor materials (300-600°C) with pressure swing technology. The aim is to produce a fuel stream consisting of high temperature H₂ and H₂O, at a pressure above the compressor temperature of a gas turbine, with a CO₂ capture efficiency greater that 90%. Alkali carbonate promoted Mg/Al-hydrotalcites (HTCs) and magnesium oxides were considered the most promising of the bifunctional CO₂ sorption and water-gas shift reactive materials reviewed. In particular, high MgO containing Mg/Al-hydrotalcites impregnated with K₂CO₃ and high magnesium contents were identified as being strong candidates for high pressure, mid-temperature CO₂ capture in the presence of large quantities of steam. However, in the context of C-SHIFT, the cyclic performance information provided thus far is limited. There is a significant discrepancy between the long carbonation periods under which these materials have been investigated and the short residence time that the CO₂ sorbent material is expected to reside within the proposed C-SHIFT carbonator (< 60 s). At present there are no published studies reporting either the uptake capacity or kinetics for the reaction between CO₂ and HTC-derived materials over time scales relevant to C-SHIFT. The work performed within ASCENT project aims to address this gap in the knowledge and provide information relating to the kinetics and capacities for CO₂ uptake by K₂CO₃-promoted HTC-derived materials at conditions relevant to a C-SHIFT process. This has been achieved using a pressurized spouted fluidized bed and fixed bed reactor and for pressure-swing operation at pressures up to 10-20 bar with large concentrations of steam. Fluidized beds are particularly suited for kinetic studies based on their superior heat and mass transfer characteristics when compared with other reactor configurations such as fixed beds and thermo-gravimetric analysis (TGA). The reactor has now been commissioned for use at pressures up to 10 bar with steam concentrations up to 40 % v/v. Modifications to the water trap design and drainage protocol that enables continuous removal of water from the system without disruption to the system pressure stability. Results from initial testing of the cyclic performance of the selected material over carbonation/ calcination cycles are also provided serving as a proof-of-concept for the pressurized fluidized bed reactor testing. The aim of this initial cyclic test was to provide empirical measurements of the carbonation and calcination kinetics and cyclic capacities of K₂CO₃-promoted HTC-derived mixed metal oxide materials at conditions and within the time period relevant to an industrial C-SHIFT process. The capabilities of the pressurized fluidized bed reactor to test the cyclic performance of candidate mid-temperature CO₂ sorbents with pressure-swing regeneration was demonstrated with carbonation at 10 bar 400 ËšC, 20 % v/v CO₂ and 40 % v/v steam and regeneration at 1 bar, 400 ËšC under mild condition (100% N₂). The presence of steam was also found to dramatically enhance both the carbonation rate and short-term carbonation capacity. Furthermore, the presence of H₂S or operation with syngas did not affect the cyclic performance of either of the sorbent materials tested. The materials reacted reversibly with H₂S presenting the possibly of a consolidating CO₂ and sulfur gas removal into a single step in a sour sorption enhanced water gas shift reaction.

Acknowledgements

The presented work is funded within the ASCENT project as part of the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement nº 608512. Note: "The present publication reflects only the authors’ views and the European Union is not liable for any use that may be made of the information contained therein”.