Cyclic Carbonation Calcination Studies of Limestone and Dolomite for CO_2 Separation From Combustion Flue Gases

摘要

Naturally occurring limestone and dolomite samples, originating from different geographical locations, were tested as potential sorbents for carbonation/calcination based CO_2 capture from combustion flue gases. Samples have been studied in a thermogravi-metric analyzer under simulated flue gas conditions at three calcination temperatures, viz., 750℃, 875℃, and 930℃ for four carbonation calcination reaction (CCR) cycles. The dolomite sample exhibited the highest rate of carbonation than the tested limestones. At the third cycle, its CO_2 capture capacity per kilogram of the sample was nearly equal to that of Gotland, the highest reacting limestone tested. At the fourth cycle it surpassed Gotland, despite the fact that the CaCO_3 content of the Sibbo dolomite was only 2/3 of that of the Gotland. Decay coefficients were calculated by a curve fitting exercise and its value is lowest for the Sibbo dolomite. That means, most probably its capture capacity per kilogram of the sample would remain higher, well beyond the fourth cycle. There was a strong correlation between the calcination temperature, the specific surface area of the calcined samples, and the degree of carbonation. It was observed that the higher the calcination temperature, the lower the sorbent reactivity. The Brunauer-Emmett-Teller measurements and scanning electron microscope images provided quantitative and qualitative evidences to prove this. For a given limestone/dolomite sample, sorbent's CO_2 capture capacity depended 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 CO_2 capture capacity could be increased by lowering the calcination temperature. According to the equilibrium thermodynamics, the CO_2 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 CO_2 stream from the exit gas mixture of the calciner. A calciner design based on this concept is illustrated.