Fabrication Of Carbon Doped Mesoporous MgO Pellets And Its CO₂ Adsorption Performance

CO₂ emission by the use of primary energy is sharply increasing with the development of economy, and the related greenhouse effect has brought about serious of problems. CO₂ seperation and sequestration is one of the most arduous task in the 21 st century. Flue gas in process industry is the main CO₂ emission source in industrial production. Especially in coal fired power plant, the amount of CO₂ emmision accounts for 40.6% of the total global emissions, therefore the reduction of CO₂ from flue gas is urgent. CO₂ capture and storage?CCS? is well accepted as the most efficientive method to mitigate greenhouse effect. As for the existed power plant, adsorption after post-combustion has a promising future because of its advantages, such as low cost, low energy consumption for regeneration, easy control and stable adsorption performance. Selecting a suitable adsorbent is key to achieving a efficient CO₂ capture.Mesoporous MgO has drawn much attention because of its high specific surface area, low adsorption/desorption temperature and high adsorption capacity. But when MgO powder adsorbent is applied in scaled-up adsorption bed, there exists many disadvantages, such as high pressure drop, easy to cake with water, shorter lifespan and hard to recycle. For this reason, fabrication of MgO pellets with a desirable performance and studying its CO₂ adsorption from pellet scale to fixed-bed scale are significative to promote its industrial application for CO₂ capture. In this view, firstly, in this paper, carbon doped mesoporous MgO pellets with a high specific surface area and appropriate mechanical strength were fabricated by carbonizing pellets under high temperature. The effect of fabrication conditions on the pore structure, CO₂ adsorption performance and mechanical strength were studied. Subsequently, the effect of hydration on the morphology, pore structure and CO₂ adsorption performance were investigated. Furthermore, the relationship of CO₂ adsorption kinetics parameters between pore structure and adsorption temperature were obtained. At last, the behavior of CO₂ adsorption breakthrough curve and relationship between temperature in the axial location of the fixed-bed and flow under different condition were investigated. In this paper, the main research achievements were concluded as follows:?1? Carbon doped mesoporous MgO pellets with a high specific surface area and good mechanical strength were fabricated with basic magnesium carbonate pentahydrate and sodium carboxymethylllcellulose?CMC? by calcining the pellets under high temperature. With the increase of calcination temperature and CMC mass ratio, CO₂ adsorption capacity recovery increased first and then decreased, while and mechanical strength increased all the way. High calcination temperature increased the sintering of the pellets, thus increasing the mechanical strength, but it also caused growth of grain size which leaded to agglomeration. A appropriate amount of activated doped in the pellets could increase mechanical strength and alleviate agglomeration. According to the quantitatively described of MgO pellets texture structure by FHH model, it was found that pore structure got to be regular when the fractal dimension decreased, thus increasing the adsorption efficiency. The micropore was likely to cause uneven of the surface and irregulal of pore structure.?2? Hydration had a great effect on the appearance of MgO pellets. 3D flaky inter-connected network structure appeared after hydration. It largely improved the specific surface area and developed the pore structures of the adsorbent, resulting in a significant improvement in CO₂ adsorption capacity. Pseudo-second order model was good to describe CO₂ adsorption process on MgO pellets by adsorption kinetics analysis. High adsorption temperature contributed to the increase of equilibrium kinetics rate constant, while adverse to the adsorption capacity. Moreover, the initial adsorption rate h was related to the texture structures of the MgO pellets other than with the adsorption temperature. Meanwhile, the hydrated MgO pellets had a good cyclic stability, making it practical for CO₂ capture from power plants.?3? In this work, the performance of CO₂ adsorption on MgO pellets on a fixed bed was studied. In dry condition, with the increasing of gas flow and CO₂ concentration, the breakthrough time got shorter, and length of unused bed got to be longer with a small bed utilization efficiency. CO₂ adsorption on MgO was controlled by chemical reaction and pore diffusion. The adsorption heat had a great effect on the bed temperature.With the advancing of the adsorption wave, the axial temperature in all the locations increased first and then decreased to the original, and the largest temperature rise could reach 14?. The time which peak temperature in all location arrived advanced with the increase of gas flow and CO₂ concentration. In humid condition, the existence of vapor could increase breakthrough time and adsorption saturation time, thus increasing the adsorption capacity. The axial max temperature got higher in humid condition and the temperature decreasing rate got to be slow. What's more, overloaded vapor was bad for the adsorption which would form a thick film in the inner surface of the pellet that increased the diffusion resistance.