Study On Gas-solid Two-phase Flow Characteristics In Circulating Fluidized Bed Under Cold Condition

As China’s economy continues to grow,the demand for energy is increasing.As the main source of energy in China（57.6%of primary energy）,the CO₂emitted during the combustion process of coal is a major contributor to global warming.Traditional coal combustion methods produce CO₂that mixes with air,making it difficult and costly to collect and separate CO₂.Chemical looping combustion technology is a promising fuel utilisation technology that uses oxygen carriers to supply oxygen to the fuel.The CO₂produced by combustion is not mixed with air and can be separated and trapped directly,resulting in a cleaner use of energy and a promising fuel utilization technology in line with current developments.The circulating fluidised bed is the most used reactor in chemical looping technology applications.The fluidised gas is added to the bottom of the conventional reactor to move the particles,increasing the contact between the oxygen carrier and the fuel and improving the fuel conversion efficiency.In this paper,a new circulating fluidised bed device is proposed and investigated using a combination of numerical simulations and experiments.The particle flow inside the circulating fluidised bed unit and its influencing factors are investigated,and suitable operating conditions are selected to provide a reference for the construction of subsequent thermal circulating fluidised bed units.The main work of this paper is in the following areas:1)A circulating fluidised bed device was constructed and numerical simulations were used to initially investigate the particle flow inside the fuel reactor and the fast fluidised bed device,to analyse the factors affecting the particle flow inside the device,and to build a visualised fast fluidised bed cold experiment device to investigate the actual particle flow inside the device and compare it with the simulation results.The results show that in the fuel reactor（at lower flow rates）,the particles move slowly and bubbles appear inside the device;in the fast fluidised bed（at higher flow rates）the bed height of the bed material rises rapidly and the gas-solid contact efficiency increases.The initial bed material is 500 mm and the aperture diameter of the air distribution plate is 0.8 mm,which gives a good fluidisation effect.The flow of particles in the cold experimental setup is in good agreement with the simulation results,indicating that the numerical simulation method can accurately describe the flow of particles inside the setup.2)Thermal experiments based on bubbling fluidised beds were carried out to investigate the effect of different fluidising atmospheres on the combustion products of lignite in bubbling fluidised beds.The results show that compared to a pure Ar atmosphere,mixing either 20%CO₂or 20%H₂O can effectively promote the conversion of char in the fuel reactor stage,and mixing 20%H₂O has a better promotion effect on the conversion of lignite char.3)Numerical simulations of the whole circulating fluidised bed unit were carried out to investigate the flow of particles in the whole circulating fluidised bed unit.The results show that the particles in the unit can be driven by the fluidising medium in a cyclic process from the fuel reactor into the lift tube,cyclone,air reactor and finally back to the fuel reactor in turn.The particles in the fuel reactor in the circulating fluidised bed show a bubbling fluidised state,the residence time of the particles in the reactor is longer and the contact between the oxygen carrier particles and the coal particles is more adequate.