Cyclic Carbonation and Calcination Studies of Limestone and Dolomite for CO₂ Separation From Combustion Flue Gases

Naturally occurring limestone and dolomite samples, originating from different geographical locations, were tested as potential sorbents for carbonation/calcination based CO₂ capture from combustion flue gases. Samples have been studied in a thermo gravimetric analyzer under a simulated flue gas conditions at three calcination temperatures, viz., 750°C, 875°C and 930°C for four Carbonation Calcination Reaction (CCR) cycles. The dolomite sample exhibited the highest rate of carbonation than the tested limestones. At 3^rd cycle, its CO₂ capture capacity per kg of sample was nearly equal to that of Gotland, the highest reacting limestone tested. At 4^th cycle it surpassed Gotland, despite the fact that the CaCO₃ content of Sibbo dolomite was only 2/3 of Gotland. Decay coefficients were calculated by a curve fitting exercise and its value is lowest for Sibbo dolomite. That means, most probably its capture capacity per kg of sample would remain higher, well beyond the 4^th cycle. There was a strong correlation between the calcination temperature, specific surface area of the calcined samples and degree of carbonation. It was observed that higher the calcination temperature lower the sorbent reactivity. The BET measurements and SEM images provided quantitative and qualitative evidences to prove this. For a given limestone/dolomite sample, sorbent’s CO₂ capture capacity was depend on the number of CCR cycles and the calcination temperature. In a CCR loop, if the sorbent is utilized only for a certain small number of cycles (<20), the CO₂ capture capacity could be increased by lowering the calcination temperature. According to the equilibrium thermodynamics, the CO₂ partial pressure in the calciner should be lowered to lower the calcination temperature. This can be achieved by additional steam supply into the calciner. Steam could then be condensed in an external condenser to single out the CO₂ stream from the exit gas mixture of the calciner. A calciner design based on this concept is illustrated.