Gas-Solid Flow Dynamics In A Chemical Looping Combustion System With A Moving Bed Air Reactor

Chemical looping combustion(CLC)is a novel combustion technology which has the features of low energy penalty,low NO_x emission and high energy utilization rate.A reasonable system structure is beneficial to achieve high combustion efficiency,stable operation and flexible adjustment of running performance.Currently,the reactor design is still in the exploration stage and many institutes have proposed different structures for a CLC system.In most of the systems,both of the fuel reactor(FR)and air reactor(AR)are fluidized beds,which needs two driving sources to realize continuous solid circulation between the FR and AR.As a result,the system structure and running procedure are relatively complex,which makes it difficult to realize favorable matching of the FR and AR.An effective method to simplify the system is using a moving bed as the AR.However,the conventional AR only has one gas inlet and one gas outlet thereby causing limited carrying capacity of gas flow in the AR and limited thermal power of the CLC system.This cannot meet the requirement of a large-scale thermal power plant.This thesis aims to solve the problems of complex structure of a multi-source system and limited thermal power of a single-source system.A novel AR with increased carrying capacity of gas flow is proposed.The AR is arranged in the middle of a down-comer in a circulating fluidized bed,thereby forming a single-source CLC system with simple structure.The gas-solid flow characteristics,operation stability and adjustment flexibility are investigated in detail by using the experimental and numerical simulation methods.The main research content and conclusions obtained from the work are shown as follows:A quasi-2D CFD-DEM model is first built up for a moving bed AR,and solid movements with stable flow rates are achieved in the reactor.Based on the stable solid flow,the solid residence time distribution(RTD)and effects of reactor angle₁,wedge angle₂ and ratio of down-comer diameter to reactor diameter d/D are investigated by using the tracer particles.Results show that the solid flows in the upper part and lower part of the reactor are mass flow and funnel flow,respectively.With decrease in₁,increase in₂ or increase in d/D,the coefficient of solid residence time's variance decreases and the solid flow is closer to the ideal plug flow.The change of₁ has more effects on the solid velocity distribution in the near-wall region;the change of₂ has a greater impact on the central solid velocity distribution in the bottom of the reactor;the change of d/D affects the distribution of solid velocity in the whole reactor.Further,based on the solid velocity information obtained from the numerical simulation,BP neural network is adopted to establish a quantitative relationship between the solid flow and structure parameters.Optimal geometric parameters(₁=83^o,₂=55^o,d/D=0.2)are finally selected for the moving bed AR by using the preorder traversal algorithm.A multi-stage air distribution structure is then designed and integrated with the moving bed AR.The experimental results prove that compared with a conventional moving bed AR,the novel multi-stage AR has successfully improved the gas carrying capacity per cubic meter from 0.12 Nm³/s to 0.41 Nm³/s.By using the tracer gas,the effects of the inlet gas flow rate and outlet pressure on gas flow paths are investigated.Results show that increasing inlet gas flow rate will increase the amount of gas flowing between different stages of the AR.To optimize the gas flow paths,it is important to obey the rule of“bottom first and top last”which means the outlet pressure should be adjusted from the bottom to the top.A CLC system which consists of a multi-stage moving bed as the AR and a riser as the FR is further established.In the system,quasi-stable solid circulation is successfully built up.The detailed gas-solid flow characteristics under different operating conditions are obtained by analyzing the pressure signal in time domain and frequency domain.When the gas fluidizing number N_f in the FR is decreased,the solid circulation flux J_p is increased or the initial solid bed height H_cis increased,the pressure drop ratio of the AR side to the FR side increases.Periodic plug flow may occur when N_f is too small.To avoid occurence of the phenomenon,the FR should be operated between the lower boundary₁)=1.528×^0.237 and upper boundary₁(9)=2.2×^0.351 of the fast fluidization regime.Based on the stable solid circulation,the operation of AR is then coupled to the running of the whole system.Results demonstrate that the AR operation has little impact on the solid circulation,and there is almost no gas leakage between the FR and AR,indicating good coupling performance between the fast-fluidized bed FR and the moving-bed AR.Further,the operation of carbon stripper indicates that the continuous returning of particles to the FR may lead to the gas leakage from the FR to the second-stage down-comer,which may further adversely affect the steady solid circulation.Finally,a 3D full-loop numerical model is built up for the CLC system by using the Eulerian-Eulerian approach.The simulation results reveal the internal dynamic characteristics in the FR,AR,down-comer,etc.,which broadens the understanding of the complex flow patterns.In the FR,the distributions of gas-solid concentration,velocity and solid mass flux are asymmetrical in the lower and upper sections but symmetrical in the middle part of the reactor because of the single-side solid returning and discharging structures.With the solid particle diameter decreasing from 0.6 mm to 0.3 mm,the cluster frequency increases from 1.21-1.47Hz to 2.27-3.00 Hz;the average cluster duration time decreases from 0.26-0.31 s to 0.07-0.10s;the cluster time fraction decreases from 0.34-0.41 to 0.18-0.24.In the AR,the gas velocity,gas concentration and solid concentration are uniformly distributed with the uniformity index larger than 0.97,while the uniformity index of the solid velocity is relatively small with the value smaller than 0.6.The tower-type structure of the AR effectively eliminates the cavity formed in the upper part of the reactor because of the particle packing.