Development of sodium/lithium/fly ash sorbents for high temperature post-combustion CO2 capture

Abstract

CO2 capture from combustion processes faces several challenges including high energy penalty, low CO2 partial pressure, high flow rates and presence of water vapours. Absorption of CO2 at high temperature is recently attracting increasingly attention. Alkali metal based sorbents present clear advantages compared to other high temperature sorbents, such as high CO2 capture capacity, lower regeneration temperatures (<750°C) and excellent stability. In this work, Na/Li-silicates prepared by mixing Na/Li carbonates with fly ash (FA) in various molar ratios were evaluated for their capacity to chemisorb CO2 at 500–700°C and in presence of H2O (2–12vol%), diluted CO2 (14vol%) and CO2 sorption promoters.The results indicate that the carbonate:silica ratio used in the sorbents synthesis significantly affects the CO2 sorption capacity and regeneration temperature. The presence of steam enhances the diffusion of Li and Na ions resulting in higher CO2 uptake. CO2 chemisorption follows a double layer mechanism with formation of carbonate layer on the surface. The simultaneous presence of Li and Na (and K when used as additive) in the formed carbonate layer results in an eutectic melt between 600 and 700°C, which facilitates the diffusion of the ionic species. Li–Na–FA with molar ratio of 0.5:0.5:1 was the best prepared sorbent with a capacity of 2.54 mol CO2/kg sorbent (12% H2O, 14% CO2 at 700°C). Absorption/desorption was completed in 15min with reaction kinetics comparable to that of pure Li4SiO4 sorbents. The tested materials maintained their capacity and absorption/desorption rates after 10 cycles at 700°C. Overall, the Na/Li materials showed a CO2 capture capacity, stability over time and sorption–desorption kinetics comparable to those of other high-temperature sorbents, such as Li–FA (Li4SiO4 prepared using fly ash), and higher stability than CaO and hydrotalcites.