Study On The Decarburization Characteristics Of Potassium-based Absorbent Shaped Particles And The Modification Mechanism Of The Template

Massive CO2 emission from fossil fuel combustion processes has been considered as the dominant contributor to the severe global warming and climate change.Potassium-based sorbents have been deemed as economically and technically promising for capturing CO2 from flue gas stream,because of the stable CO2 sorption performance in repeated cycles and low parasitic energy load for regeneration.However,the sorbents will suffer from significant abrasion and elutriation when operated in dual fluidized-bed reactors,due to the presence of mechanical stress and thermal stress.These have hampered the scale-up industrial application of the technique.Facile preparation of solid adsorbents with excellent CO2 capture capacity and desirable mechanical strength as well as steady fluidization is one of the key solutions to addressing the issue.Sorbent pelletization has been identified as a key technology for synthesizing solid CO2 adsorbents with great mechanical strength and good anti-abrasion performance.However,the pelletization process would adversely affect the microstructure characteristics,CO2 capture capacity and kinetic performance.In this thesis,we prepared potassium-based sorbent pellets using a novel extrusion-spheronization method.To enhance CO2 sorption performance of the pellets,several pore-fonning templates were added to improve the porosity.The CO2 sorption performance and the underlying mechanisms of template modification were investigated with different techniques.Detailed contents and the major conclusions are as follows:Activated alumina(AI2O3),Bayer aluminum hydroxide(BAh),kaolinite clay(KC)and calcium aluminate cement(CA)were selected as supports to prepare K2CO3-based sorbent pellets using a novel extrusion-spheronization method.Urea(UA)and TPR were selected as pore-forming agents to improve the pore structures of the desired K2CO3/AI2O3 pellet.The mechanical properties of the K2CO3-based pellets were tested using a friability tester and a mini-type precision compression tester.The results showed that all the K2CO3-based pellets exhibited relatively high mechanical strength.The maximum weight loss for the K2CO3-based pellets was less than 2%after 4000 rotations,indicating a high anti-attrition ability.Adding pore-forming materials weakens the mechanical strength of the sorbent pellets to some extent.The maximum weight loss for the modified K2CO3/AI2O3 pellets was still less than 3%after 4000 rotations,and the mechanical strengths of the samples were greater than 20 MPa,which could perfectly satisfy the mechanical properties that required in industrial application.The effects of supporting material,K2CO3 loading amount,particle size and calcination tempreture on CO2 sorption performance of K2CO3-based pellets were investigated using a self-made U-type reactor.Physicochemical properties of the pellets were characterized by N2 adsorption-desorption,scanning electron microscopy(SEM)and X-ray diffraction(XRD).Besides,the effects of template type and doping amount on CO2 sorption performance of the template-modified pellets were also demonstrated.The main results were summarized as follows.(1)K2CO3/BAh and K2CO3/AI2O3presented good textural properties,benign surface morphology,abundant basic sites and superior CO2 sorption performance.Their CO2 sorption capacitie were calculated as 1.75 and 1.85 mmol COs/g,and their BET surface areas were measured as 12 and 104 m2/g.K2CO3/KC and K2CO3/CA showed relatively poor CO2 capture capacities and inferor textural properties.(2)With the increasing K2CO3 content from 30 wt.%to 70 wt.%,the BET surface area of K2CO3/Al2O3 decresed from 12 to 3 m2/g,its CO2 capture capacity increased first and then decreased.The maximum CO2 capture capacity of 2.29 mmol CO2/g could be achieved when K2CO3 loading was 50 wt%.(3)The K2CO3/AI2O3 pellets calcined at 200? and 300? possessed stable CO2 capture capacities of 2.3 mmol CO2/g.However,when the calcination tempreture increased to 400? and 500?,CO2 capture capacities of the as-prepared pellets decreased to 1.18 and 0.46 mmol CO2/g.CO2 capture capacity of K2CO3/Al2O3 decreased with the increasing particle size.(4)The addition of 10 wt%UA and TPR could significantly improve the porosity of K2CO3/Al2O3 pellet for enhanced CO2 sorption performance.By comparison,UA exhibited superior pore-expanding effect than TPR,with respect to improving the pellets' CO2 sorption performance.With the increase in urea addition amount in the 1-15 wt.%range,CO2 sorption performance of K2CO3/Al2O3 could be significantly enhanced.CO2 sorption kinetic performance over K1CO3-based pellets and the modified sorbents were investigated by fitting the experimental data to the pseudo-first-order,pseudo-second-order,Avrami,modified Avrami-fractional kinetic equations and the deactivation model.The main results were summarized as follows.(1)The modified Avrami model worked satisfactorily in precisely predicting the CO2 sorption capacities and well describing the CO2 sorption kinetic performance of the K2CO3/Al2O3 pellets.CO2 sorption kinetic performance of the template modified pellets could also be well described by the deactivation model.(2)CO2 sorption process over the K2CO3/Al2O3 pellets was jointly controlled by the chemical reaction at the interface and the internal diflfusion through the product layer.The pore-expanding process not only endowed the sorbent pellets reduced internal diffusion resistance for facilitated COa sorption,but also benefited the pellets enriched active sites for enhanced CO2 sorption capacity.Therefore,CO2 sorption performance of the template-modified pellets would be remarkably improved.