Investigation On Cyclic CO₂ Capture Performance And Pelletization Modification Mechanism Of High-temperature Solid Sorbents

The enormous emission of the greenhouse gases,manily containing CO₂,is the key factor leading to a series climate and environmental issues such as global waiming.The technology of carbon capture and storage has been accepted as one of the promising solutions to CO₂reduction.Postcombustion capture based on CaO-based and Li₄SiO₄-based sorbents have exhibited great application prospects for high-temperature CO₂ capture.The main advantages of CaO-based sorbents include abundant and cheap raw materials,fast sorption rate,high theoretical capacity,etc.However,there also exist the problems of poor anti-sintering ability and rapid capacity loss with the increase of the cycle number.Besides,the instinct characteristic of the high regeneration temperatures is also a disadvantage for CaO-based sorbents.The regeneration temperature of Li₄SiO₄-based sorbents is relatively low and Li₄SiO₄-based sorbents also have the advantage of high sorption capacity and good cyclic stability.But the Li₄SiO₄ sorbents derived from the conventional solid-state method usually present dense structures and barrn porosity,which is unfavorable for the sorption reaction.And the reported pure Li₄SiO₄ usually performs poorly under realistically low CO₂concentrations.In addition,both CaO-based and Li₄SiO₄-based sorbents are faced with the problems associated with crushing and abrasion during the cyclic fluidization processes.In view of these problems,this work improves the performance of CaO and Li₄SiO₄ sorbents through different modification and granulation methods,and also investigates the sorbents’cyclic CO₂ sorption and mechanical performance,influencing factors and modification mechanism,in order to acquire the high-temperature sorbents with both good CO₂ capture performance and mechanical properties.First of all,aiming at the poor anti-sintering ability and cyclic stability of CaO sorbents,this work utilized the inert support of Nd₂O₃,which owned a high sintering temperature,to modify the sorbent performance.The results showed that the homogeneously dispersed supports could finely separate the CaO/CaCO₃ grains,thus preventing the agglomeration and sintering of sorbent grains and further mitigating the collapsion of the pore structures and the decay of the cyclic sorption capacity.Furthermore,this work also studied the anti-sintering ability of the commonly used inert supports under the same preparation methods and test conditions.The screening results indicated that under the same condtions,the anti-sintering performance of the inert support for CaO sorbents can be ranked as follows:Y>Al>Mn^Mg^La^Yb^Nd>Ti^Ce^Zr^Si^Pr.And it was also found that the performance of the sorbents modified with inert supports had a close relationship with the melting point（or sintering temperature）of the inert support and sorbent’s specific surface area.To overcome the disadvantage of poor cyclic performance for natural limestone,this work employed _L-（+）-tartaric acid,lactic acid and _L-（-）-malic acid to acidify natural limestone.The screening study of the modificatioin effect for these three acids as well as the commonly used organic acids was also conducted to provide the foundation and basis for the acid selection.The experimental results showed that a large volume of gases was quickly released during the high–temperature decomposition of the acidified limestone,bringing abundant porosity and surface areas for the ultimate sorbents and thus improved cyclic CO₂ sorption capacity.Under the same conditions,the sorbent modified with _L-（+）-tartaric acid exhibited the best performance and even after 26 cycles,it still owned a high carbonation conversion,which was nearly twice as high as that of the unmodified limestone.Unlike the traditional development routine of CaO-based sorbents by two separated steps（modification and subsequent granulation）,this work prepared CaO-based sorbent pellets with one step via a gel-casting technique.The prepared sorbent pellets displayed good cyclic CO₂sorption performance under both mild and severe test conditons.Besides,the cyclic performance of the pellets could be further enhanced by adding the pore-forming material（microcrystalline cellulose）,mainly owing to the improved mcirostructures of the pellets by the combustion of the pore-forming material.The results of the mechanical tests indicated that the prepared pellets owned good anti-attrition properties,which is comparable to that of the granulated sorbents using the commonly used extrusion method.To solve the problem of the dense structures,barren porosity of the conventional Li₄SiO₄and the poor sorption performance under realistically low CO₂ concentrations,this work prepared Li₄SiO₄ sorbents via the precipitation method using the organic lithium precursors.The specific surface area and abundance of the porosity of the prepared sorbents were markedly enhanced.The Li₄SiO₄ sorbent using lithium oxalate as the precursor exhibited a high conversion of 84%after 20 cycles,still holding the conversion of as high as 74%at 100cycles when tested with 15%CO₂.Besides,the sorbent performance was further improved by doping with Na₂CO₃.Moreover,this work also proved the existence,compostions and function of the eutectic melt via the high-temperature in situ X-ray diffraction（XRD）and differential scanning calorimeter（DSC）.The above-prepared excellent Li₄SiO₄ powder was further granulated via the extrusion–spheronization method.The CO₂ sorption performance and mechanical properties of the pellets were also characterized.It was found that compared with Li₄SiO₄ powder,Li₄SiO₄pellets displayed desified structures,decreased surface area and porosity due to the extrusion force during the palletization process,resulting in the reduced cyclic sorption capacity.Therefore,the pore-forming material of microcrystalline cellulose was employed to improve the structures of the pellets.The addition of 20 wt.%microcrystalline celluloses could significantly enhance the pellets’cyclic sorption capapctiy,which was even comparable with that of the sorbent powder.In addition,the results from the anti-attrtion and compression tests demonstrated that the prepared Li₄SiO₄ pellets possessed excellent mechanical strength.In summary,this research on the modification,granulation and related mechanisms has played an important role in promoting the industrial application of CaO-based and Li₄SiO₄-based sorbents for high-temperature CO₂ capture.