Preparation and characterization of lime/coal ash sorbents for sequential CO2 and SO2 capture at high temperature

Extensive research has been done on Ca-based sorbents as a promising way to capture CO2 and SO2 from power plants. Considerable effort has also been directed toward maintaining sorbent activity by means of sorbent modification to deal with activity decay with repeated CO2 capture cycles. Based on the principle of “treating waste with waste” and inspired by the idea that a pozzolanic reaction can enhance the surface area, this paper presents a method of hydrothermal synthesis of lime and coal ash. A small amount of CaSO4 or NaOH was added during the hydration process and the mixture was stired for several hours at about 90oC. The synthesized samples were then characterised by scanning electron microscopy, nitrogen adsorption/desorption spectroscopy and X-ray diffraction. The activity of the synthesized sorbent for CO2 and SO2 capture were then tested in a thermogravimetric analyser. The treated samples demonstrate longer-lasting performance for CO2 cyclic capture, albeit with a slightly reduced capture ability compared to pure lime in the first few cycles due their lower CaO content (25~81% versus 98%). The sample with lime/ash mass ratio of 45:5 showed higher CO2 capture ability after three cycles and much greater stability in terms of their activity. The main product of the pozzolanic reaction is CaSiO3, which has a network structure, whose development is related to the ratio of CaO/coal ash, hydration duration and the amount of CaSO4 and NaOH additives. After high temperature calcination, a new phase, namely Ca3Al2O6 is believed to serve as a skeleton preventing sintering in repeated capture cycles. After experiencing multiple cycles, the synthesized sorbents also have a high SO2 capture capacity. A small amount of added NaOH decreases the cyclic CO2 carrying capacity of the synthesized sorbent but enhances SO2 carrying capacity dramatically. The explanation for this is that the sulphation reaction is controlled not only by gas diffusion but also by solid-state ion diffusion. Na+ ions generate more crystal lattice defects which can accelerate the ion diffusion rate in the product layer, and consequentially enhance overall SO2 carrying capacity.