Effect Of Steam On Ca-based Sorbents Calcination/Carbonation Looping Cycle For CO₂Capture

Ca-looping is a potentially effective method for CO2capture from coal firedpower plants. However, there are still issues that need to be studied further; one ofthese issues is the sorbent’s decay of activity in the Ca-looping process. In this work,using a custom made tube furnace experimental system that can measure sampleweight changes at constant temperatures, the effects of steam on calcination andcarbonation of calcium based CO2sorbent are studied. The effects of steam present incalcination or carbonation process alone or present in both of these processes aredetermined. It is found that steam has a positive overall effect on CO2capture in theCa-looping process. However, steam present only in the calcination process leads todeactivation of the CaO, and results in lower CaO carbonation conversion in thefollowing carbonation process. In contrast the presence of steam in carbonation stageonly improves CO2capture. For carbonation stages the influence of steam is relatedto temperature, steam concentration and CO2concentration. One interestingobservation is that there is not apperent particle size effect.Effect of sintering was investigated by considering the decomposition oflimestone and residence time of lime (of CaO). It shows both carbonation rate andfinal conversion ratio is decreased with the increase of sinter duration, especiallyduring the chemical reaction stage. The carbonation of0min sintered CaO degradedby steam, but, for the6min and12min sintered CaO it is contrast. The sintering ofCaO will be accelerated by higher temperature and prolong sintering time, both leadto the less carbonation ratio. There were not apparent particle size effects also. Largeparticle size (355-450μm) sorbent was more resistance of sintering.The pore structure of CaO was measured by N2absorption/desorption methodfor analysis further, and steam affects carbonation conversion during calcinationstage by sintering, while during carbonation stage steam promotes carbonationconversion vin catalysis effect. Regardless of the presence or absence of steam, thesurface area and pore volume of CaO always be decreased by the prolonged sinteringtime. It is interesting, for0min sintered CaO, the surface area and pore volume willbe decreased by steam, but not for6min and12min sintered CaO. Similar carbonationreaction happened for another limestone.