Investigation On CO₂ Capture Properties And Pelletization Modification Mechanism Of CaO-based Sorbent Derived From Calcium-rich Wastes

Based on the need of recycling use of calcium-rich solid wastes and low carbon emission of cement industry,an integrated process route is proposed to prepare CaO-based sorbents from calcium-rich solid wastes used in the calcium looping process for CO₂ removal in the cement industry,and the spent CaO-based sorbents instead of limestone as the raw materials for the production of cement clinker.According to the integrated route,the major reasearchs in this work include the activation of the sulphate and silicate calcium-rich solid wastes,granulation of CaO-based sorbents derived from calcium-rich solid wastes,and modification of CaO-based pellets via self-sacrificial biomass templating technique and the modification mechanism investigation.The activation of phosphogypsum was conducted via a direct aqueous carbonation method.The direct use of the CO₂ derived from cement plant flue gas（20 vol.%CO₂）is able to convert 94.5%of CaSO4 in the phosphogypsum into CaCO₃.However,a long time of 90 min is required for the completion of the conversion.Therefore,we proposed to introduce a part of the highly concentrated CO₂ gas stream separated from the CLP and,hence,to increase the overall CO₂ concentration of the carbonation gas stream.It was found that only 45 min is needed to achieve a comparable carbonation level when the gas stream containing 45 vol.%CO₂ was used.Moreover,the solid carbonation residues derived from phosphogypsum carbonation possess relatively good cyclic CO₂ capture performance,which is superior to the CaCO₃ reagent.The activation of steel slag was conducted via acetic acid acidification.Orthogonal experiments were used to optimize the four main parameters in the acetic acid pretreatment of steel slag（i.e.,extraction time,extraction temperature,acetic acid concentration,and solid-liquid ratio）.The optimal steel slag-derived sorbent is obtained at an extraction time of 2 h,an extraction temperature of 60 ℃,an acid concentration of 30 wt.%,and a solid-liquid ratio of 1:10 g/mL.The CO₂ sorption performance of the optimal steel slag-derived sorbent is more stable than a sorbent made from naturally occurring limestone,with a CO₂ uptake of 0.223 g/g in the 50th cycle.The presence of MgO particles with a high anti-sintering ability and the modification of the pore structure during initial decomposition process of calciumacetate are responsible for the stabile CO₂ capture capacity of the steel slag-derived sorbent.Comparing the cyclic CO₂ sorption performance of the activation products of phosphogypsum and steel slag,white mud and carbide slag,it is found that carbide slag possesses the optimal cyclic CO₂ sorption performance.Therefore,carbide slag was selected as the research object,which was granulated via an extrusion-spheronization method to study the granulation property of CaO-based sorbent derived from calcium-rich wastes.The carbide slag pellets possess the inferior CO₂ sorption performance in comparison with the carbide slag powders.It is mainly attributed to the extrusion-spheronization method results in the compact interior structure of the pellets,increasing the diffusion resistance of CO₂ to react with the free CaO within the pellets.Two types of biomass-based pore-forming materials（microcrystalline cellulose and rice husk）were used to modify the pore structures of the pellets,and thus enhance their CO₂ sorption performance.The addition of microcrystalline cellulose is effective in improving the CO₂ sorption of carbide slag pellets.However,the additions of 10 and 20 wt.%rice husk both result in inferior cyclic CO₂ capture performance owing to the aggravated sorbent sintering caused by the new eutectics,which are from the combination of al μminosilicate with the potassi μm contained in the rice husk.The pellets doped with 20 wt.%pre-washed rice husk（most potassium removed）display the highest 25th carbonation conversion of 51.02%,which is almost 1.46 times that of the pellets doped with raw rice husk.The addition of pore-forming materials weakens the pellets’ mechanical strength,and 5-10 wt.%addition of cement could enhance the mechanical strength and maintain the relatively good CO₂ sorption performance of the pellets.Further,the modification effect and mechanism of five types of conventional biomass materials（microcrystalline cellulose,corn starch,rice husk,sesbania powder and lycopodium powder）and two types of unconventional（microalgae and defated microalgae）on CO₂ sorption of the CaO-based pellets were investigated.For microcrystalline cellulose,corn starch and rice husk,more addition amounts would result in better improvement of CO₂ sorption performance of the CaO-based pellets.However,for sesbania powder and lycopodium powder,a decreasing enhancement tendency of the CO₂ sorption performance was found with the increasing addition amount.It is probably due to the accelerated sintering of the sorbent because of the presence of excessive amounts of alkali metal elements.It indicates that except of the improvement of the pellet’s structure,the compositions of the（?）templatings and their pyrolysis residues also affect the modification effect on CO₂ sorption of the CaO-based pellets.In addition,a small amount（0.5-2 wt.%）of microalgae is able to significantly enhance the CO₂ capture capacity of the pellets.When excessive microalgae（>5 wt.%）were doped,the interaction between CaO and the produced tar and coke from the microalgae during the initial calcination stage leads to the deactivation of the CaO-based pellets;consequently,an inferior CO₂ capture capacity is observed in the initial cycle.Defatted microalgae are superior to microalgae to enhance the CO₂ sorption performance of the CaO-based pellets,because the lipid removal can relieve the deactivation phenomenon of active CaO during intial calcination process.