Experimental Study Of Cyclic CO₂Capture From Coal Combustion Flue Gases At High Temperature Using CaO-based Sorbents

The increasing concentration of CO₂in the atmosphere due to fossil fuel burning hasbeen identified as a major contributor to global warming. In the predictable future, fossilfuels will continue to be the dominant energy source, which means that CO₂will continueto be released into the atmosphere. To mitigate the related global climate deterioration,large scale CO₂capture and sequestration （CCS） has been proposed and widely studied.With efficient energy recovery, it is well accepted that CaO-containing materials are goodcandidate absorbents for CO₂capture due to their high reactivity for CO₂absorption, highCO₂capacity, low material cost, and, importantly, their high carbonation temperature.In this paper, a screening of potential calcium precursors for the production of CaOsorbents for CO₂capture at high temperature was conducted. The precursors studiedinclude CaO, CaCO₃, CaC₂O₄, CaAc and Ca （NO₃）₂. CaAc exhibited the best capacity forcapturing CO₂with a conversion of87.21%at the first cycle and a conversion of53.69%atthe10thcycle.We also report that the method of incorporating the inert materials has a critical effecton the long-term stability of the CaO-based absorbent. A class of stable MgO-doped CaOabsorbents have been developed using mechanical mixing of small MgO particles with Ca（CH3COO）₂followed by high temperature calcinations. MgO was selected because of itshigh stability, lack of CO₂absorption at the CaO reaction temperature, and lack of reactionwith CaO or CaCO₃under the operating conditions. MgO nanoparticles can be easilyproduced by thermal decomposition of many Mg-containing salts.We examine water reactivation of sorbent to improve the reversibility of multipleCaO-CO₂capture cycles. After reactivation the sorbent had even better characteristics forCO₂capture than that of the original sorbent.Pretreatment of CaO-based sorbent in a CO₂atmosphere at high temperature isinvestigated for its effect on CO₂capture. Pretreatment was done in a tube furnace atdifferent temperatures and for different durations. The results obtained showed significantdecrease of sorbent surface area after pretreatment, but the pore surface area of pretreatedsorbent samples increased after CO₂cycling and SEM images showed the reappearance of smaller CaO grains. This led to an increase in CO₂capture activity, up to68.2%after20cycles.