The Effects of Different Dopants and Doping Procedures on the Reactivity of CaO-based Sorbents for CO2 Capture

Calcium-looping technology is a promising CO2-capture technology. The economics and environmental impact of the process are affected by the rapid decay in reactivity of the limestone sorbent, which necessitates a high purge rate from the system to maintain sorbent reactivity. This work investigates techniques to improve long-term reactivity in such a system by doping the limestone with a variety of different dopants. It also demonstrates that the main reason for improvement is that the doping increases the volume of pores around 100 nm in diameter in the calcined material after a significant number of cycles, in comparison to the calcine from an undoped sorbent. Improved reactivity means a lower required purge-rate and potential disposal issues, and less fresh limestone to be used. Doped samples were subject to repeated cycles of carbonation and calcination in a Fluidised Bed Reactor (FBR). The work includes the use of inorganic salts MgCl2, CaCl2, Mg(NO3)2 and the Grignard reagent-isopropylmagnesium chloride. Three types of doping procedures were explored; wet impregnation, solid mixing and quantitative wet impregnation. Experimental results showed that MgCl2, CaCl2 and Grignard reagents as dopants improved the carrying capacity of Havelock limestone. X-ray fluorescence (XRF) analysis of the doped samples after 13 cycles revealed that the number of dopant ions does not increase further with an increase in the doping concentrations above a critical value. Gas adsorption analyses showed that sorbents have a very small surface area-below 4 m²/g, post-cycling. The pore size distribution appears to change significantly upon doping, and it is likely that optimising the pore size distribution upon cycling is one reason for the enhanced reactivity observed.