Study Of Modified CaO-based Sorbents For CO₂ At High Temperature

The climate has been greatly influenced by the greenhouse gases.As CO₂ was the main ingredient of greenhouse gases,the vast anthropogenic emission of CO₂ has affected the survival environment of human.One-third of the anthropogenic emission of CO₂ comes from the power plants.The reduction and recycle of CO₂ discharged from flue gas has drawn public attention.Currently,the main methods of CO₂ capture are chemical absorption and pressure swing adsorption,which could not be conducted until the flow gas was cooled below100 ^oC.In the cooling period,about 13-37%of the net-generation is wasted.In order to reduce the extensive energy consumption,CO₂ sorbent at high temperature has caused the extensive concern.The CaO-based sorbent is cheap and abundant.With high CO₂ adsorption capacity and fast reaction rate,CaO-based sorbent could also be prepared and regenerated easily.With these advantages,CaO-based sorbent become the promising CO₂ sorbent at high temperature.As the sintering of CaCO₃ easily occurred at high temperature through a long time,the carbonation conversion of CaO-based sorbent decreased with the increase of cycle number.Then,the service life of sorbent became short.In present work,different metallic oxides,coal ash and shale ash were used to modify the CaO derived from different precursors.The cyclic stability was improved by strengthening the channel structure.The sintering was delayed.The adsorption performance of modified sorbent was measured by thermal gravimetric analyzer?TGA?.The effect of carbonation temperature,doping element,doping ratio,pre-calcination,continuous cycle,CaO precursor and doping ash on carbonation conversion and cyclic stability were studied.The kinetic analysis was also conducted.CaO was respectively drived from CaCO₃,CaC₂O₄,Ca?OH?₂ and CaAc₂.The doped element was Mg,Al,Ce,Zr and La,respectively.The channel structure,microstructure and phase composition of modified sorbents were measured by N₂ adsorption-desorption,Scanning Electron Microscope?SEM?,Energy Dispersive Spectrometer?EDS?and X-Ray Diffraction?XRD?,respectively.The changes of structure and composition for modified sorbents in different conditions explained the mechanism of different adsorption performance.The dopant could help to improve the cyclic stability of sorbent,while there was little promotion for carbonation conversion in the 1st cycle,except La.After 22 cycles,the cyclic stability of sorbent doped with Al,Ce or La could be more than 96.2%.After 110 cycles,the cyclic stability of Al-doped CaO derived from Ca Ac₂ was as high as 87.1%.After 132 cycles,the cyclic stability of Ce-doped CaAc₂ was as high as 130.1%.For the CaO derived from CaAc₂,the conversion and cyclic stability through 110 cycles of sorbent doped with both Mg and La was 1.4 times those doped with Mg or La,which was also 1.9 times that without dopant.For the different precursors,modified CaO derived from CaAc₂ showed the greatest conversion and cyclic stability.With pre-calcination,the cyclic stability of modified sorbent was greatly improved although there was some sacrifice of carbonation conversion in the 1st cycle.In the discontinuous cycles,it provided an easy and cheap reactivation method for the spent sorbent by the hydration of air.In continuous cycles,the cyclic stability was better.The carbonation conversions of CaO prepared by different methods increased due to the doping with coal ash and shale ash.The cyclic stability was also improved in different degrees.Furthermore,the kinetic parameters of sorbents were investigated.The most probable mechanism function was random nucleation and growth model.The results showed that the channel structure of sorbent became stable after doping.The sintering was also delayed.The cyclic stability of CaO sorbents derived from different precursors was improved.The modified sorbents,especially for Al-doped and Ce-doped sorbent,showed the good prospect in industrial appilication.