Investigation On CO₂ Cyclic Capture Performance Of Nano-structured CaO/CuO Composites And Its Numerical Modeling

Calcium looping coupled with chemical-looping combustion is an energy-efficient technology for carbon capture.The heat required for calcination of Ca-based sorbents is provided by an exthermic reduction reaction of oxygen carriers,which significantly reduces the energy penalty due to the use of the air separate unit in convertional calcium looping.Thus,this technology possesses great potential in CO₂ capture and utilization.The CaO/CuO composite is the most suitable sorbent for the new concept.CO₂ will be captured through the transformation from CaO into CaCO₃ and the heat required for calcining CaCO₃ is provided by the chemical-looping combustion of CuO,instead of pure O₂,with fuel,which saves energy for separating O₂ from air.However,both CaO and CuO endure severe decay of their capability.Therefore,the most demanding and crucial work is to enhance the cyclic performance of the CaO/CuO composite and to understand its working principle.Considering the modification of sorbent microstructure,micro-emulsion method and solution combustion synthesis were adopted with different synthesis parameters for producing nano-structured CaO/CuO composites,respectively.The composites were experimently investigated on the fixed-bed reactor and the thermal gravimetric analyzer?TGA?to evaluate their performance.The model of a single particle of CaO/CuO composite coupling chemical reactions,heat transfer and mass transfer was established to study the effect of microstructure on the performance.The performance tests of CaO/CuO composites synthesized by micro-emulsion method conducted on a fixed bed reactor showed that the carbonation conversion increased first,and then declined during the cycle experiment.At certain level,with smaller molar ratio of water to surfactant?w?and higher precursor concentration,the CO₂ capture performance was better.According to analysis of carbonation reaction kinetics,comparing with coprecipitation method that shared similar precursor reaction,micro-emulsion method could produce sorbents with lower activation energy of carbonation reaction.The transmission electron microscope?TEM?tests showed that there were many nano-scaled pores in the sorbents synthesized by micro-emulsion method,which improved the microstructure.Besides,all sorbents manufactured by micro-emulsion method showed excellent and stable redox performance,with oxidation conversion over 90%.The tests of CaO/CuO composites synthesized by solution combustion synthesis showed that,self-reactivation occurred in the process of carbonation for all the sorbents.The sorbent with Ca/Cu molar ratio of 1 had better CO₂ capture performance than that of the sorbent with Ca/Cu molar ratio of 3.The CO₂ capture performance was improved with the addition of CeO₂,especially the reaction rate in diffusion-controlled stage.After adding CeO₂ with Ca/Ce molar ratio of 2,the sorbent with Ca/Cu molar ratio of 1 had better CO₂ capture performance and lower carbonation activation energy.The TEM tests showed that there were also many nano-scaled pores in the sorbents synthesized by solution combustion synthesis,which improved the microstructure.Besides,all sorbents synthesized by solution combustion synthesis showed excellent and stable redox performance,with oxidation conversion over 90%.The research on the model of a single CaO/CuO composite coupling chemical reactions,heat transfer and mass transfer showed that reducing grain size could help enhance CO₂ capture and oxidation performance of sorbents.Decreasing reactor temperature in the range of 600-700?would lead to faster carbonation and higher capacity.With larger particle size,the heating rate of sorbent was slower.CO₂ concentration should stay at certain level or the capture efficiency would decrease.CaO/CuO composites manufactured by micro-emulsion method and solution combustion synthesis both exhibited stable microstructure and similar cyclic performance.However,solution combustion synthesis offers particular promise for further development in improving reactivity of CaO/CuO composites due to its simple operation,easy access to raw materials and mass-production potential.