- 1:30 PM

Pre-Combustion CO2 Capture at High Temperature with Alkaline Promoted Aluminas and Hydrotalcites: The Crucial Role of Alkaline Cations in CO2 Adsorption

Stephane Walspurger, Paul D. Cobden, Wim G. Haije, and Ruud W. van den Brink. Hydrogen production and carbon capture, Energy research Centre of the Netherlands, Westerduinweg 3, Petten, 1755LE, Netherlands

Greenhouse gas mitigation can be achieved in coal gasification power plants by pre-combustion decarbonisation. Alkaline promoted hydrotalcites (HTC, clays) have been found to be a suitable material for sorption enhanced water-gas shift reaction (SEWGS) at temperatures around 400C incurring only a limited plant efficiency penalty. Despite a rather low working capacity, the kinetics of the sorption processes on alkaline promoted HTC have proven to be suitable for use in Pressure Swing Adsorption units. In parallel with bench scale tests at ECN, mechanistic investigation on CO2 adsorption on such sorbents have been carried out to identify active sites and eventually design an optimal sorbent with higher working capacity.

In the present work alkaline promotion effects on CO2 sorption have been characterised by a series of complementary analytical techniques on alumina, magnesia, hydrotalcites and alkaline promoted corresponding materials under conditions representative for WGS conditions. Thermogravimetric analyses (TGA) and Temperature Programmed Desorption (TPD) have clearly confirmed that alkaline carbonates are destabilised when interacting with solid surface, i.e. temperature at which CO2 is released from alkaline promoted hydrotalcites is much lower than the equilibrium decomposition temperature of the corresponding bulk alkaline carbonate. In situ XRD experiments have clearly evidenced structural rearrangements in alkaline promoted hydrotalcite at the temperature used for capture. Moreover, in the presence of CO2, the reconstruction of hydrotalcite cooled at room temperature after the experiment could not be observed most probably due to the formation of new carbonate species at high temperature that are stable while temperature decreases and prevent the reconstruction of crystalline hydrotalcite. In addition, in situ vibrational spectroscopies (Raman and DRIFT) showed that carbonates rearrange while heating and that alkaline ions strongly interact with carbonate groups coordinated to aluminium oxide centres. Indeed both techniques highlight remarkably the loss of symmetry for carbonates group and comparison between parent material's fingerprints evidenced that basic (KAlnO2n/CO3)-like species play a key role in CO2 fixation at 400C.