Kinetics of CO2 absorption by CaO through pressure swing cycling

CO2 capture by means of CaO cycling represents a cost effective, immediate solution to rising CO2 emissions. The mechanism of loss of utilization efficiency and associated change in carbonation kinetics of CaO particles was examined by conducting calcination/carbonation cycling of 150–250 μm Strasburg limestone precursor, by swinging the pressure from atmospheric (for calcination) to 5, 10 and 20 barg (for carbonation) at constant temperatures of 975–1025 °C and a flow of pure CO2. Increased carbonation pressure led to an increase in utilization over 100 cycles from 0.128 ± 0.005 to 0.271 ± 0.035 for 5 and 20 barg respectively. Samples were examined by scanning electron microscopy and BET. The carbonation kinetics were determined by plotting the rates of reaction for both the kinetically controlled and diffusion controlled carbonation stages. Sintering, grain boundary formation and elimination, and changes in particle surface area are found to all play important roles in causing the initial rapid loss in surface reaction-controlled rate, followed by partial recovery over multiple cycles. The loss of grain boundaries due to sintering caused an increase in the reaction-controlled rate and residual calcium utilization due to a shift in the dynamics of carbonation/calcination nucleation and molecular volume contraction. The rate of the reaction-controlled regime was found to be a function of carbonation pressure. The diffusion-controlled rate is independent of pressure and temperature.