Mesoscale Structure Evolution And Activity Attenuation Mechanism Of Calcium-based Sorbents

Calcium cyclic decarbonation is a method to capture CO₂by using a cyclic calcination/carbonation reaction of natural calcium-based sorbents,which has the advantages of a wide operating temperature range,the ability to handle high temperature flue gases and low capture costs.However,the carbon capture efficiency of calcium-based sorbent decays rapidly during the actual cyclic calcination/carbonation process,which is a key factor preventing it widespread use.The adsorption capacity and cyclic stability of calcium-based sorbents are significantly influenced by their internal pore structure characteristics and grain sintering phenomena.Hence,it is essential to study the evolution of the mesoscale structure of the sintering,the surface interface of regenerated calcium oxide during calcium cycling from the micro perspective,and develop a kinetic model of its intrinsic link with the evolution of the microscopic pore structure and the stability of the macroscopic during calcium-looping cycle.In this study,the formation and evolution of the mesoscale structures of carbonation products and regenerated calcium oxide grains during the calcium cyclic decarbonization reaction were investigated by using natural limestone of different grain sizes,which combined with SEM,XRD,BET,EDS,TGA and other characterization methods.A kinetic model describing the periodic cyclic calcination/carbonation reactions of calcium-based sorbents was developed by relating the mesoscale structural evolution to the carbon oxide transfer behaviour through a multiscale distribution function of the calcium-based sorbents pore structure.The internal relations between the mesoscale structural evolution of the surface interface and periodic reaction kinetics of calcium-based sorbents was clarified.The main results as follows:（1）The matrix of calcium carbonate changes from cleavage surface and dense structure to an uneven partition island-like（2-5μm）structure with isolated macropores about 240-600nm for the first 30 cycles,the specific surface area increased from an initial 0.23 m²/g to 2.17m²/g.The key factors for promoting the sintered fusion of regenerated Ca O was evolution of the uneven partition structure calcium carbonate matrix and the diffused distribution of deactivated calcium oxide.（2）The specific surface area of sorbents decreased from 15.32 m²/g to 1.06 m²/g from1st to 30th cycles,and the peak of the highly reactive pore distribution in the range 20-100nm decreased rapidly within the first 5 cycles.The corresponding sorbents carbonation conversion rate decreased by 29.6%,while the amount of unreacted calcium oxide in the calcium carbonate matrix increased to 31.3%after 2 carbonation reaction,with the increase slowing down during the subsequent cycles.（3）Based on the unreacted shrinking core and grain models,the coupled"structure-transfer-reaction"mathematical models were developed to describe the kinetic characteristics of calcination and carbonation reactions during the calcium cycle,respectively.The maximum relative error in calcination decomposition rate is 10.5%and the maximum relative error in the critical transition point of the carbonation reaction mechanism is 9.62%in 20 calcium cycles.