Trace Na₂CO₃ Addition to Limestone Inducing High-Capacity SO₂ Capture

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.