Experimental And Numerical Study On Chemical Looping Combusiton Of Coal Based On Fuel Staging Oxidization

CO₂ emission caused by fossil fuel is the main reason for global warming.Chemical looping combustion with inherent CO₂ separation is a promising technology.The design of the reactor has an important impact on the efficiency of chemical looping combustion.The chemical looping combustion system mainly consists of air reactor and fuel reactor.Nowadays,most researchers adopt the single-stage reactor as fuel reactor,which limits the fuel conversion.Concerning air reactor,the fast fluidized bed is widely used to oxidize the oxygen carrier and provide enough propulsion for circulation of oxygen carrier.However,the core-annulus gas solid flow in the air reactor strongly affects the mass and heat transfer and chemical reaction conversion.In this study,a new kind of two-stage fuel reactor was proposed to improve the contact between fuel and oxygen carrier.In the first stage fuel reactor,a small proportion of combustible gases produced by coal pyrolysis and gasification was oxidized by the oxygen carrier.After that,fresh oxygen carrier was expected to fully convert remaining combustible gases into pure CO₂ and steam.As a result,the chemical looping combustion efficiency can be increased dramatically.Additionally,a novel ring-type baffle was installed into the riser to improve gas-solid contact between air and oxygen carrier.The gas-solid flow pattern and chemical reaction performance were investigated by employing experiments and numerical simulations.Cold experiment was carried out in the cold-flow apparatus to obtain the solid mass flux and the pressure distribution.Computational fluid dynamics?CFD?simulation based on two fluid model was conducted to model gas-solid flow in the riser and the simulation results showed a good agreement with the experimental results.Three parameters including the baffle opening ratio,baffle thickness and baffle number were investigated based on CFD simulation.The results showed that baffle affected significantly the hydrodynamics in the riser.The baffle opening ratio and the number of baffles were the key factors to decrease the standard deviation of radial solid volume fraction.The baffle with small opening ratio can effectively improve the radial solid distribution.However,the smaller opening ratio may result in a serious erosion to baffle which was not beneficial for industrial application.The axial profile of radial non-uniform index concerning solid volume fraction showed that the baffle with 44.4%opening ratio and 27mm thickness as well as 1.35 aspect ratio can effectively maintain a uniform radial solid distribution in the region away from baffle.Moreover,the combination of three baffles was found to be most effective in improving the radial solid volume distribution in the region away from baffle.The gas-solid flow and chemical reactions characteristics in the fuel reactor were simulated by computational particle fluid dynamics?CPFD?method.The average relative deviation of the simulated gas composition to the experiment was below 12%.The combustion efficiency of the first and second stage fuel reactor was 86.3%and 64.2%respectively at 850?,suggesting that the two-stage fuel reactor can effectively improve the chemical looping combustion efficiency.Concerning the operation parameters,higher temperature led to higher combustion efficiency.Moreover,simulations based on CPFD method were performed to investigate the gas-solid flow pattern and the solid circulation rate in the dual fluidized bed.The pressure distribution agreed well with experimental results.The power spectral analysis of the pressure fluctuations showed that the unsteady feeding to the riser and the bubble behavior in the bottom region contributed to pressure fluctuations along the air reactor.The effect of operation parameters on solid circulation rate of the system was also discussed.The results showed that the global circulation rate increased as gas inlet velocity of loop seal increased.Besides,the internal circulation rate showed an upward trend as the fluidization rate of fuel reactor increased.