Supported modified hydrotalcites as sorbent for CO2 capture

Abstract

The average concentration of CO2 in the atmosphere has been increasing since the start of the industrial revolution in the 18th century from 280 ppm to 385 ppm nowadays, and continues to increase because of the enormous human usage of fossil fuels (oil, gas, coal). This can strongly affect the climate, causing the Earth’s surface to warm up, the so called ‘amplified greenhouse effect’. To alleviate these environmental concerns regarding the current CO2 emissions into the atmosphere, Carbon Capture & Storage (CCS) is investigated as one of the possible routes. Due to the acidic character of CO2, basic oxides are expected to be suitable sorbents. Hydrotalcite, a natural clay, is specifically suitable for pre-combustion capture (250- 400 C), due to its acceptable sorption capacity and facile regeneration. The influence of lateral platelet size (~40 nm- 2 ?m), the use of a support (carbon nanofibers, CNF) and addition of a promoter (alkali carbonate: K2CO3/Na2CO3) on the CO2 capture properties of HT was investigated. There was no significant difference in the CO2 sorption capacities at 523 K fsor all unsupported HTs, regardless the platelet size of the HT precursor (~0.1 mmol.g-1). The use of activated, promoted (alkali carbonate) hydrotalcites showed a much higher capacity (~0.3 mmol.g-1) at 523 K. In addition, the capacities of the activated supported HT at 523 K were significantly increased compared to the activated unsupported HT (1.3- 2.5 mmol.g-1HT). The alkali-loaded supported HTs showed capacities slightly higher than the capacity of supported unpromoted HT. The increase in capacity for the promoted and/or supported HTs points to a higher concentration of defects (low-coordination of oxygen sites) on the surface of the activated (alkali-)loaded HTs compared to the unloaded and unsupported HT. The higher concentration of adsorption for the promoted (supported) HTs, might be caused by the presence of Na+/K+ on the surface of Mg(Al)Ox. Due to the larger size of these alkali ions, incorporation in an activated HT would therefore be difficult and it is proposed that the K+/Na+ are located at the surface and not in the ‘bulk’ of the MgAlOx. The tentative mechanism is that K+ substitutes an Mg2+ and additional oxygen vacancies at the surface are created. Finally, a new developed sorbent, i.e. potassium carbonate (K2CO3) deposited on carbon nanofibers for CO2 capture at low temperatures (373 K, post-combustion capture), was compared with potassium carbonate deposited on activated coal (AC) and alumina (Al2O3). K2CO3 loaded on the CNF support revealed excellent properties as CO2 sorbent compared to the K2CO3-AC and K2CO3-Al2O3 sorbents, having the highest capacity and fast desorption kinetics at low desorption temperatures (423- 523 K). These favorable properties of K2CO3-CNF are considered to originate from relatively small K2CO3 particles combined with a good accessibility of these particles surrounded by the CNF. Moreover, the K2CO3-CNF could be regenerated with a low energy input estimated at 2- 3 MJ/ton CO2, far below the energy needed for the currently used amine-scrubbers, which shows this sorbent's potential to become competitive with established post-combustion sorbents.