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Abstract

Bench scale studies were carried out, focusing on the application of calcium-based sorbents in fossil-fuel-fired combustion and gasification systems, with conditions ranging from atmospheric to elevated pressures and at practical combustion and gasification temperatures. In the kinetic study of CaO carbonation, the reaction order changed abruptly from first- to zero-order when the CO₂ partial pressure exceeded the equilibrium value by more than ~10 kPa. A Langmuir mechanism successfully explained the experimental information, with the intermediate complex CaO•CO₂ postulated to saturate CaO sites immediately at high CO₂ partial pressure. The activation energies for rate constants were found to be 29 ± 4 kJ/mol and 24 ± 6 kJ/mol for Strassburg limestone and Arctic dolomite, respectively. A discrete-pore-size-distribution-based model was formulated, with the aid of which the kinetic study was extended to obtain diffusivities through the solid product layer formed during carbonation, with activation energies of 215 and 187 kJ/mol for the limestone and dolomite, respectively. Sorbent cyclic CO₂ removal ability was investigated based on pore size distribution measurements. Several important features observed from measurements could be predicted by a mechanistic model which included simultaneous sintering and calcination in the fixed bed. It was found that the decay in the reversibility of limestone capture/regeneration was insensitive to operating conditions; the achievable carbonation extent of each cycle depends on the