Experimental And Simulation Study On Gas-solid Flow Characteristics In A Tower Fluidized Bed For Coal Chemical Looping Combustion

Chemical looping combustion（CLC）is a novel combustion technology which has the ability of high energy conversion and low-cost CO₂ capture,and can achieve the goal of‘carbon neutrality’in China.At present,the CLC process is still facing the problem of low reaction efficiency due to insufficient gas-solid contact.A reasonable reactor structure can not only ensure a stable operation of the system,but is also beneficial to improving the reaction performance.Some strategies like adding an internal circulation path and coupling carbon capture device in the fuel reactor（FR）can improve the carbon conversion efficiency,however,the structure of these systems is quite complex.In order to enhance the gas-solid contact,a new tower bubbling bed reactor with baffles is designed and coupled in a dual circulating fluidized bed with good performance and simple structure.The gas-solid flow characteristics,particle circulation stability,and multiphase chemical reaction performance in the tower bubbling reactor are investigated in detail by using experimental and numerical simulation methods.The structural optimization strategies to improve the reaction performance are also proposed in the paper.Firstly,the fuel reactor is divided into several chambers along the bed height by baffles,and a multi-stage tower reactor is designed.Based on the methods of image processing and signal analysis,the influences of baffle structure,baffle number,and baffle opening ratio on the bubble properties are investigated.The results show that the addition of baffles can effectively reduce the bubble size,decrease the bubble rising velocity,and relieve the bubble energy.Hence,the mass transfer rate from the bubble phase to the emulsion phase is increased,and the insufficient gas-solid contact caused by large bubbles can be avoided.The particles go up through the baffles,and fall down due to gravity,which make full contact with the upward gas.The height to diameter ratio of each chamber is chosen as 1.8:1,and the baffle opening ratio is better to be chosen as 8.05%,which improves the bubble behavior and flow pattern in the baffled fluidized bed reactor.Then,a dual circulating fluidized bed with good performance and simple structure is constructed based on the tower reactor for the chemical looping process.A cold experiment is performed and the system achieves a self-pressure balance with external particle circulation.If the fluidization number in the tower reactor reaches 4.0,a gas plug will be formed in the lower chamber,and it will push the particles upwards to penetrate through the baffle into the upper chamber.It can be seen that the presence of baffles reconstructs the gas-solid flow state and unifies the gas-solid distribution.However,it will also limit the particle circulation.Introducing a secondary air at 3/5height of the tower reactor can improve the particle circulation rate and guarantee the particle residence time.About 80%～90%of gases blown in the bottom of the loop-seal will enter into the reactors due to the sealing effect.There is almost no gas mixing between the air reactor and fuel reactor,which can ensure a safe and stable operation of the system.A three-dimensional numerical model of the dual circulating fluidized bed with tower reactor is established via the computational particle fluid dynamics（CPFD）method.The EMMS-bubbling model is introduced to study the gas-solid distribution,particle velocity,mass flux,and wall erosion.The numerical results show that the wall effect in the small-scale AR leads to an uneven particle distribution and irregular fluidization at a small AR diameter.Increasing the AR diameter to more than 50mm can weaken the influence of the wall effect.The wall erosion is mainly in the inlet area of the cyclone and the baffles due to high particle velocity.It can be improved by increasing the flow area of the cyclone and the opening ratio of baffles.According to the similarity criterion,a scale-up study of the fuel reactor is carried out.The gas-solid dynamics and particle circulation in the tower fuel reactor with pilot-scale are predicted.Finally,the redox reaction test of the hematite oxygen carrier is carried out on a fluidized bed thermogravimetric analyzer（FB-TGA）,and the kinetic parameters are obtained.A 3k W_th tower CLC plant model is established coupled with the gas-solid flow and the chemical reaction.Some significant characteristics,including gas composition,particle residence time,back-mixing rate,carbon conversion efficiency et al.,are investigated in the simulation.Owing to the intensifying effect of baffles on gas-solid reaction,the multi-stage fuel reactor has the high gas conversion efficiency,where the outlet CO₂ concentration is as high as 93.8%,and the oxygen demand is as low as 3.8%.Through tracing and analyzing the path of char particles,it is found that the residence time of most char particles is too short to be fully gasified.The residual char will be entrained into the air reactor,resulting in the reduction of CO₂ capture efficiency,which is only 80.3%.By increasing the number of baffles in the FR,the char residence time in the fuel reactor can be extended.Improving the structure of the loop-seal can control the char transmission to the AR,and the CO₂ capture efficiency is raised up to 90%in the tower fuel reactor.