Investigation On Cyclic Reactivity And Mechanical Strength Of Pelletized Calcium Sorbent During Calcium Looping Process

The calcium looping process is a promising technique for post-combustion CO2 capture.The advantages include operability for existing plants and lower cost than amine scrubbing.But two major chanllenges emerged:(1)the decay of sorbent reactivity due to thermal sintering and(2)the attrition/fragmentation in actual fluidised-bed units,which decreased the lifetime of sorbents.Aiming to handle these problems,synthetic sorbents were pelletized by a mechanical granulator.Cement and biomass were added during granulation in order to enhance carbonation reactivity and mechanical strength.The cyclic CO2 capture capacity and attrition were investigated in a bubbling fluidized bed.Because the bubbling reactor couldn't simulate the high speed impact condition that was frequent in calcium looping system,a vertical particle impact apparatus was employed to evaluate the impact fragmentation resistance of sorbents.Furthermore,the effect of steam hydration on reactivity and strength of spent synthetic sorbents was also focused.Finally,the primary fragmentation of sorbents in calcinator was modelling by numerical methods.The transient thermal stress,pressure and total stress distribution inside particles were calculated.The reactivity and impact fragmentation of synthetic pellets supported by aluminate cement were fully studied.The synthetic sorbents granulated with 10 wt.%aluminum cement during pelletisation showed higher sintering resistance and lower porosity decay than raw limestone,thus more superior cyclic reactivity.Synthetic sorbents had higher attrition resistance.The pellets originated from powder still possessed similar impact breakage resistance to limestone after calcination,proved by the impact tests.Higher calcined temperature declined the strength of pellets,while multiple cycles causing sintering could improve it.A semi-empirical formula for calculating the average diameter after impact breakage based on Rittinger's surface theory was developed.Biomass can serve as pore-enlarging material.The effect of biomass addition in synthetic pellets on cyclic reactivity and strength was focused on the chapter four.The porosity was efficiently expanded by biomass decomposition,thus better CO2 capacity was achieved.However,the resistance to breakage decreased due to biomass addition.Higher calcined temperature declined the strength of those pellets,while multiple cycles could improve it.And the impact breakage of those pellets also meets Rittinger's surface theory.The microstructure analysis of sorbents indicated that the increased porosity and emerging cracks on surface declined the strength.The effect of steam hydration on reactivity and strength of spent cement-supported pellets was investigated.The reactivity of synthetic pellets was elevated significantly by steam hydration.The conversion of CaO for pellets increased from 0.113 to 0.419 after hydration,and that for raw limestone was from 0.089 to 0.278.However,the mechanical strength of synthetic pellets declined severely after reactivation.Large cracks emerged on hydrated limestone,which increased reactivity and impaired strength.Similar appearance was not observed on hydrated synthetic pellets,except improved porosity and expended surface which could enhance CO2 capacity and decline strength.Superheating which allows the annealing of stacking faults and mechanical strain formed by hydration was able to enhance the strength of hydrated pellets.The transient distributions of thermal stress and pressure stress in CaO-based sorbents during calcination were numerically modelled.The effect of particle diameter,porosity and calcination reaction rate on stress distribution was investigated.The maximum of radial thermal and pressure stress was located at the centre of pellets while that of tangential thermal stress was located at the surface.The moment of peak pressure stress fell behind that of thermal stress.With the increase of particle diameter,the moment of peak delayed,while the peak value of thermal and pressure stress increased firstly and then decreased.The critical particle diameters of the peak thermal and pressure stress were different.The increasing porosity and decreasing calcination rate reduced the peak pressure stress.Less cracks emerged on calcined synthetic pellets,which was attributed to lower pressure caused by better porosity and less gas generated based on the model results.