Trace Na2CO3 Addition to Limestone Inducing High-Capacity SO2 Capture

Abstract

Although the literature has reported enhanced indirect sulfation of limestone by adding Na2CO3, the amount of Na2CO3 additive required to achieve high CaO conversion is typically high (∼4.0 mol %), which commonly results in adverse effects in fluidized-bed combustion boiler systems and increased cost of sorbents. In this work, we demonstrate for the first time that trace Na2CO3 (0.1 mol %) can significantly enhance the sulfate conversion of limestone. This enhanced sulfation is attributed to the increased surface area and optimized pore size distribution. The trace Na2CO3 additive splits the pores of the original sorbents peaking at ∼70 nm into pores peaking at ∼4 nm and ∼140 nm due to the slight promotion of sintering. This well-developed pore structure results in a relatively high reactivity for sulfation. Thus, the Na2CO3 additive influences the sorbent reactivity in two ways: (1) at less than 0.5 mol %, tuning its pore structure; (2) at more than 0.5 mol %, promoting the product layer diffusion. We also find that trace amount of other metal salts, such as CaCl2 and NaCl, clearly enhance the sulfation of limestone. The strategy of enhancing limestone sulfation by the addition of trace amount of metal salts offers evident engineering and economic advantage.