Optimization Of CO₂ Sorbent Pellets Formulation And Investigation On Gas-solid Flow Characteristics Of Serial Fluidized Bed For Calcium Looping Process

In recent years,the greenhouse effect caused by anthropogenic CO₂emissions has become increasingly serious,and the issue of CO₂emission reduction has attracted worldwide attention.Calcium looping（Ca L）technology is widely regarded as one of the most potential CO₂capture technologies due to its wide range of sorbent raw materials,low cost and large sorption capacity.However,this technology also faces two prominent problems:First,the CO₂sorption/desorption cycle process is mainly implemented by a fluidized bed reactor,which requires the powder sorbent to be formed into particles with enough mechanical strength.However,the pelletization process will destroy the pore structure of the sorbent,and during the cycling process,the sintering and aggregation of Ca CO₃is likely to occur due to its low Taman temperature（533°C）,resulting in a rapid decline in the CO₂sorption capacity.Second,the existing fluidized bed reactor can not meet the technical requirements of Ca L.Therefore,a novel serial fluidized bed reactor must be developed,and the design of the new reactor and the gas-solid flow characteristics in the bed need further research.In response to the above problems,this paper first screened three new phosphate binders（aluminum phosphate,magnesium phosphate and calcium phosphate）and prepared the shaped calcium-based sorbent pellets by extrusion-spheronization method.And the optimal formula and process for the preparation of the spherical pellets were obtained.Then,a novel serial fluidized bed reactor for Ca L was designed according to the physicochemical properties of the sorbent pellets,and the gas-solid flow characteristics were understood by numerical simulation,the structure of the reactor was further determined.Finally,the gas-solid flow characteristics of the serial fluidized bed were systematically studied by experiment.That was the effects of inlet air speed,bed material volume and particle size on bed pressure drop and particle circulation coefficient under cold state were investigated.The main innovative results obtained are as follows:The optimal formulation of high-efficiency calcium-based CO₂sorbent pellets was obtained（80 wt.%calcium hydroxide+10 wt.%magnesium phosphate+10 wt.%polyethylene plastic）.And the pre-calcination temperature that was most beneficial to improve the mechanical properties and sorption performance of the sorbent was found out.Magnesium phosphate is the best binder among the three phosphates.The calcium-based sorbent pellets doped with 10 wt.%magnesium phosphate have the better CO₂sorption and mechanical properties.After calcining at 900°C,its maximum compressive strength is 12.14MPa,which is about three times that of pure calcium hydroxide pellets.And the CO₂sorption capacity after 25 sorption/desorption cycles is about 0.23 g-CO₂/g-sorbent,which is increased by 64%compared with pure calcium hydroxide pellets.In addition,900°C and 1000°C are the best pre-calcination temperatures that are conducive to improving the mechanical and sorption properties of the sorbent pellets,respectively.The structure and gas-solid flow characteristics of a novel serial fluidized bed for Ca L were clarified.The novel serial fluidized bed designed by theoretical calculations is mainly composed of an air reactor（fast bed）,a riser,a fuel reactor（bubble bed）,upper（lower）loop-seals with two air inlets and a cyclone,and it meets the conditions of Ca L.The pressure distribution in the air reactor（AR）and the riser is more complex and changeable than that in other areas.And the pressure drop in the AR decreases as the air inlet velocity of the AR increases,and it promotes as the air inlet speed of the upper or lower loop-seal enhances.Experimental research on the cold state gas-solid flow characteristics of the novel serial fluidized bed shows that only the air velocity at the inlet of AR of the serial fluidized bed has a large effect on the pressure drop in the AR,while the air velocity at each inlet has a large effect on the pressure drop in the fuel reactor（FR）and the particle circulation coefficient.In addition,increasing the air velocity at the inlet of the AR,FR and upper loop-seal will increase the fluctuation of the bed pressure and reduce the stability of the pressure distribution in the system.And it can decrease the fluctuation of the bed pressure and improve the stability of the pressure distribution in the serial fluidized bed if the superficial gas velocity in lower loop-seal could be enhanced.The amount of bed material has a significant impact on the gas-solid flow characteristics of the fluidized bed.As the amount of bed material increases,the bed pressure drop and particle circulation coefficient both increase greatly,but the fluctuation range of the bed pressure promotes and the stability of pressure distribution in the system also decreases.While at the same air speed,the particle size of the bed material has little effect on the gas-solid flow characteristics of the serial fluidized bed.In addition,bed materials with different particle sizes need to be matched with appropriate fluidizing air speeds in order to achieve ideal and stable fluidization.This paper has mainly explored the high-efficiency sorbent pellets and serial fluidized bed reaction systems for high-temperature CO₂capture of Ca L,and can provide theoretical guidance for the practical application of CaL in industry.