The effect of SO2 on CO2 capture by CaO-based pellets prepared with a kaolin derived Al(OH)3 binder

High-temperature CO2 capture by Ca looping cycles was investigated in the presence of SO2. The sorbents tested included natural limestone (HV-P), pellets formed from powdered limestone with a binder (HV-AH) and pellets formed from powdered acetified limestone with a binder (HV10-AH). The binder used in pellet preparation was Al(OH)3 obtained from acid leaching of kaolin. Simulated flue gas with a composition of 15% CO2, 3% O2, 0.45% SO2, and N2 balance was used during the CO2 capture step. Two calcination conditions were employed for sorbent regeneration: pure CO2 at 920°C and pure N2 at 850°C. By five reaction cycles, the well-developed porous texture of HV10-AH acetified by 10vol.% acetic acid did not offer better CO2 capture than that for natural limestone at 650°C for calcination in CO2; instead it promoted a higher SO2 retention than that for the limestone. Unfortunately, the CO2 capture capacity for all sorbents calcined in CO2 in the presence of SO2 was effectively eliminated after 2–3 cycles. Calcination in N2 did not significantly enhance CO2 sorbent capture capacity; but instead it improved the SO2 retention ability of HV10-AH, for which the sulphation level was over twice that of the natural limestone. Increasing the carbonation temperature to 700°C enhanced sulphation rather than carbonation with HV10-AH, but produced the opposite effect with natural limestone. In general, it appears that SO2 impedes CO2 capture, leading to a negligible CO2 capture capacity after only few cycles regardless of how the sorbent morphology is modified, which suggests any method designed to improve sorbent performance for CO2 capture will not be proof against significant SO2 concentrations in flue gas. These results suggest that despite the fact that sorbent performance can be influenced to some extent by altering carbonation and calcination conditions; the presence of SO2 must be avoided if the objective is CO2 capture from flue gas.