| Recently, there have been a large amount of pollutants emissed to the atmosphere during the process of industry, and the change of climate was due to the emission of greenhouse gas (CO2),30%of which was developed by the generation of electricity from coal. Thus, there is a growing impetus in the world to control the emission of CO2produced during the process of coal firing in the foreseeable future. Ca-based sorbent is employed as the carrier of CO2, then by heating the CaCO3, CO2is released as a concentrated stream suitable for sequestration in the earth or injection to ocean, the above processes are workable to achieve zero, or near-zero emission of CO2in the power industry. But the adsorption activity with CO2based on CaO decreases markedly with the number of forward and reverse steps undergone in reaction, resulting in the large use of limestone, where natural Ca-based sorbents are being considered. Thus, this dissertation was focused on the characteristics of CO2chemical looping employed the natural, synthetic or thermal pretreated calcium-based sorbents as the carrier of CO2based on fluidized bed, the analysis of microstructure of sorbents based on gas adsorption, SEM and X-ray diffraction, the confirmation of sorbents with better CO2capturing capacity, and the influence of the carbonation parameters and the pretreatment parameters on the reaction activity of sorbents, the propulsion of the overlapping grain model to simulate the CO2chemical looping.Based on fluidized bed system,4different kinds of limestone were employed as sorbents for capturing CO2, and the calcination/carbonation cycles were conducted in different reaction parameters, such as temperature, pressure and atmosphere, then the capacity and activity of CO2were recorded, in addition, the change of particle microstructure was tested. The results showed that the decreased of small pore(dpore<200nm) volume was responsible to de-activity of natural sorbents with the proceeding of CO2capturing cycle.Improved, Ca-based synthetic sorbents have been prepared and tested, the purity was Al2O3or MgO. One of them, a mixed oxide consisting of CaO and Al2O3, met the criteria for an ideal sorbent for CO2, specifically, large uptake of CO2per unit mass of sorbent, reasonably high reactivity during carbonation, stable uptake of CO2after a large number of calcination/carbonation cycles, and good resistance to attrition. Moreover, comparing with limestone, some of synthetic sorbents were represented the increase of uptake with increasing the concentration of CO2during carbonation.The thermal pretreatment of Ca-based sorbent under various conditions (temperature, atmosphere) was completed to modify its activity during long-term calcination/carbonation cycle. The pretreated synthetic sorbents were shown larger overall uptake during long-term CO2capturing cycles comparing with limestone. Noticeably, some of pretreated synthetic sorbents’activity were regenerated, as was related to the regeneration of small pore(dpore<200nm) volume.The overlapping grain model was developed to formulate the process of calcination/carbonation based on Ca-based sorbent. In case of limestone, synthetic sorbent and thermal pretreated sorbent, the assumed grain diameter distributions were revised based on theirs fractal dimension respectively, the simulations were shown the good agreement with experimental results. Moreover, the surface morphology of synthetic sorbent and also thermal pretreated sorbent was found to be impressive agreement to the assumed morphology of sorbent. |
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