Study On Cyclic Adsorption Performance Of Calcium - Based CO ₂ Adsorbents

In recent years, as the main greenhouse gases, CO2was identified as a major contributor to climate change which has become a global common concern. Coal-fuel-burning power plants are the single-largest CO2emission sources. To mitigate the CO2emissions and the global climate deterioration, large scale CO2capture and sequestration (CCS) is considered as a good approach. A variety of CO2sorbents have widely studied by the researchers at home and abroad. The CO2capture process of calcium oxide based materials and lithium based sorbents is tested during the multiple absorption-desorption cycles at the relatively high temperature. They were considered as good candidate sorbents for energy-saving because of the heat loss reduction of the hot fuel gases during the CO2capture. In this paper, using eggshells and firstly bauxite tailings (BTs) or red mud (Rm) as raw materials, novel low-cost CaO-based sorbents were synthesized. Effects of different BTs or Rm ratios, calcium oxide sources and thermal pretreatment temperature on carbonation conversion of the cyclic capture were investigated. The morphology was investigated by scanning electron microscopy (SEM) and the phase compositions were determined by X-ray diffraction (XRD). The surface area was determined according to N2physisorption data at77K calculated by the Brunauer-Emmer-Teller (BET) model, while the average pore diameter was obtained using the Barrett-Joyner-Halenda (BJH) model. Moreover, the kinetic about the CO2adsorption were also examined. This paper is divided into four parts.(1) When the BTs was used as an additive, the BTs-doping sorbents were tested for carbonation conversions for20carbonation/calcination cycles in a thermogravimetric analyzer (TGA). Effects of different BTs/CaO ratios, calcium oxide sources and thermal pretreatment temperature for the cyclic CO2capture were investigated.(2) When the Rm was used as an additive, the Rm should be subjected to citric acid leaching. Response surface methodology (RSM) was used in the experiment in order to gain the optimum conditions. The Rm-doping sorbents were also tested for carbonation conversions for20carbonation/calcination cycles in the TGA. Effects of different Rm/CaO ratios, calcium oxide sources and thermal pretreatment temperature for the cyclic CO2capture were also investigated.(3) The morphology was investigated by SEM and BET, and the phase compositions were determined by XRD. The reasons of the unfavorable performance of the sorbents during the multiple cycles were explained by their morphology and phase compositions.(4) According to the characteristics of high rate reaction and slow rate reaction during carbonation, kinetic equation was founded by the shrinking unreacted core model and Avrami-Erofeev model. The activation energy and other kinetic parameters were acquired.Results show that when the BTs/CaO ratio is reached at10%, the hydrated eggshell doped with BTs showed superior cyclic stability, with being51.33%conversion after20cycles. The thermal pretreatment temperatures are not found to improve the performance of cyclic CO2capture.The best citric acid leaching conditions are found (8wt%of citric acid concentration,100of liquid to solid ratio,60℃of leaching temperature and leaching time under4h). When the Rm/CaO ratio is reached at10%, the hydrated eggshell doped with BTs showed superior cyclic stability, with being54.22%conversion after20cycles.According to the analysis of XRD, some results are found that Na in the red mud is responsible for the unfavorable performance during the absorption-desorption cycles, while it can be removed by citric acid leaching. Mayenite (Ca12Al14O33) which is formed from the reaction between eggshell and BTs/Rm is considered to be responsible for the excellent CO2absorption performance of CaO-based sorbents. The results of BET showed a trend of surface area decrease with the increase of cyclic numbers. Particle aggregation could be observed by the results of SEM, which indicates particle sintering and porosity decline after multiple cycles.With combining experimental data, shrinking unreacted core and Avrami-Erofeev equation simulation, kinetic equations of high rate reaction and slow rate reaction are found respectively. By kinetic analysis of calcinations in different sorbents, it is found that activation energy of high rate reaction is lower than slow rate reaction stage, and the lower activation energy sorbents have, the higher carbonation conversions it will reach.