Preparation And Carbon Fixation Performance Of Eggshell-derived High-efficiency CaO-based Sorbent

Controlling and reducing CO₂ emissions during coal combustion plays an important role in coping with increasing global warming and the greenhouse effect.In view of the coal-based energy structure in China,the development of efficient coal-fired CO₂ capture technology to achieve CO₂ emission reduction targets has great social significance and promotion value.As an efficient coal-fired flue gas CO₂ capture technology,calcium looping technology has significant advantages in CO₂ absorption capacity,capture purity,energy consumption and cost,and has attracted close attention and vigorous research at home and abroad.Exploring low-cost,high-performance CaO-based sorbents is the prerequisite and basis for the industrialization and application of calcium looping technology.As a solid waste in the food industry,discarded eggshells have a huge annual output,and their main component is Ca CO₃.Using it as a raw material for CaO-based sorbents in calcium looping technology has many advantages such as wide resources and low prices.Therefore,making full use of the calcium source in discarded eggshells,exploring the preparation of high-efficiency CaO-based sorbents from eggshells and realizing its resource utilization and efficient utilization in the process of coal-fired flue gas CO₂ capture have significant promotion potential.This thesis first analyzed and described the basic physical and chemical properties of discarded eggshells in detail,and processed them with organic acids to prepare three different eggshell-derived organic calcium,namely calcium acetate from eggshells and citric acid from eggshells.Calcium and calcium gluconate derived from eggshells have been tested and found that the purity of the three types of organic calcium derived from eggshells is above 95%.Comparing three kinds of eggshell-derived organic calcium and the original eggshell as the precursor of CaO sorbent,multiple carbonation/calcination experiments show that CaO made from eggshell-derived calcium gluconate has the largest specific surface area and pore volume,and the most outstanding carbon fixation performance,the highest absorption capacity can reach 0.704g_（CO2）/g_{（sorbent）}.It is the best calcium-based precursor.Aiming at the problem of the performance degradation caused by the agglomeration and sintering of the sorbent after multiple cycles,the calcium gluconate from eggshell was used as the calcium-based precursor,and the effect of the addition of different proportions of inert carrier Al₂O₃ on the eggshell source CaO/Al₂O₃ composite sorbent was investigated.Experimental studies have found that with the increase of the doping ratio,the cycle stability of the sorbent gradually increases,but its CO₂ absorption capacity and cycle conversion rate show a downward trend.This is due to the side reaction of Al₂O₃ and CaO in a high temperature environment.Ca₉Al₆O₁₈ with higher stability is formed,but CaO,the effective absorption component of the sorbent,is also lost.Comprehensive analysis shows that the composite sorbent with a doping mass ratio of 9:1 has the best CO₂ fixation performance and energy storage characteristics.The absorbent exhibits different reaction properties under different calcination atmospheres.In the pure CO₂ calcination atmosphere,the sintering of the sorbent after 20 cycles is more serious,and its CO₂ absorption capacity is reduced to 0.325g_（CO2）/g_{（sorbent）},which is much lower than 0.402g_（CO2）/g_{（sorbent）}in the pure N₂ atmosphere.In order to avoid the occurrence of CaO side reaction,based on the best mass ratio 9:1,the Mg O inert carrier was added on the basis of CaO/Al₂O₃ composite sorbent.The effects of different preparation methods such as wet mixing,sol-gel combustion synthesis method（one-step,two step）and template sol-gel combustion synthesis method on the properties of eggshell source CaO/Mg O/Al₂O₃ composite sorbent were studied.The experimental results show that the MR-DCG-CaO/Mg O/Al₂O₃ sorbent prepared by the template-sol-gel combustion synthesis method has the best cycle performance,and its specific surface area and porosity are higher.This is due to the Al₂O₃/Mg O core-shell structure The generation of not only increases the anti-sintering ability of the sorbent,but also reduces the occurrence of side reactions between Al₂O₃and CaO,and avoids a large loss of reactive CaO in the sorbent.Furthermore,in order to improve the reactivity of the sorbent after the cycle and realize the reactivation of the depleted sorbent,the MR-DCG-CaO/Mg O/Al₂O₃ depleted sorbent after20 cycles was modified by ball milling activation with different ball milling times and ball milling methods.It analyzes its influence on the carbon fixation performance and energy storage characteristics of the spent sorbent.Studies have shown that:ball mill activation greatly improves the performance of the spent sorbent.As the milling time increases,the improvement effect of the spent sorbent is more obvious.The initial CO₂ absorption capacity of the spent sorbent after 54 minutes of dry ball milling can reach 0.674g_（CO2）/g_{（absorbent）},it is found by comparison that the improvement effect of wet ball milling is significantly better than that of dry ball milling.The main reason is that wet ball milling not only reduces the particle size of the spent sorbent,but also increases its specific surface area.The internal pore structure is improved,thereby improving the carbon fixation capacity and heat release capacity of the spent sorbent.To sum up,this thesis uses eggshells waste to prepare CaO-based sorbents from different eggshell sources,and conducts in-depth and systematic research on the carbon fixation performance,reaction characteristics,and structural evolution of the prepared high-efficiency CaO-based sorbents,and on this basis preliminary explorations on the characteristics of energy storage.It has laid a solid foundation for the further promotion and application of calcium looping technology,and at the same time realizes the efficient and clean combustion of coal,the effective and economical capture of CO₂,and the resource utilization of discarded eggshells.