Sulfation performance of CaO-based pellets supported by calcium aluminate cements designed for high-temperature CO₂ capture

CaO-based sorbents supported by calcium aluminate cements were originally prepared as sorbents for CO₂ capture in looping cycles. However, their high affinity for CO₂ at high temperatures suggests that they will readily react with any SO₂ present in flue gases to be decarbonated. Thus, the sulfation performance of these pellets was investigated in this study using a synthetic flue gas in a thermogravimetric analyzer (TGA). The results obtained showed that after 6 h in gas containing 0.5% SO₂ at 900 °C the pellets prepared from hydrated lime and cement were >90% sulfated. They showed the highest sulfation affinity among the sorbents tested here. Namely, Cadomin limestone was <30% sulfated and the corresponding hydrated lime <70%. The pellets prepared from limestone powder and cement had significantly lower sulfation (∼65%) in comparison to that for pellets obtained from hydrated lime and cement. The scanning electron microscope (SEM) images of sulfated samples clearly showed the presence of a sulfated shell at the surface of original limestone particles, while the calcium aluminate pellets had porous morphology even after almost 100% sulfation. The X-ray diffraction (XRD) analyses showed that mayenite (Ca₁₂Al₁₄O ₃₃), which is responsible for the good CO₂ capture performance of these pellets, was not present after sulfation. Pellets after 30 carbonation/calcination cycles displayed significantly reduced affinity for SO₂, with sulfation conversions at ∼15%, but they easily recovered this capacity with ∼80% sulfation levels after hydration. These results clearly show that, for the pellets to perform well, the presence of SO₂ must be avoided during looping cycles at least during sorbent regeneration at high temperatures.