Modelling and simulation of chemical looping combustion in a double loop circulating fluidized bed reactor system
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Among the well-known combustion technologies with CO2 capture, Chemical Looping Combustion (CLC) stands out for its potential higher energy efficiency. CLC involves combustion of carbonaceous fuel such as natural gas or syngas via a red-ox chemical reaction with a solid oxygen carrier circulating between two fluidized beds, an air reactor (AR) and a fuel reactor (FR). CLC is a complex system, hence for optimal design a large number of candidate materials must be evaluated, and several reactor configurations may be considered for the process. This thesis presents a study on mathematical modelling and numerical simulation of reactive gas-solid flow in a double loop circulating fluidized bed (DLCFB) system which is proposed by SINTEF Energy and Norwegian University of Science and Technology (NTNU). Different from the typical configuration of the CLC process, the FR in the DLCFB system is designed to be operated in turbulence or fast fluidization regime in order to achieve a better utilization of the upper region of the reactor and thus enhance the contact between the gas and solid phases. A multiphase CFD model has been developed and implemented in FORTRAN. The kinetic theory of granular flow (KTGF) is applied to close the governing equations for the solid phase. The solution strategy is based on the extended SIMPLE algorithm for reactive multiphase flow. The finite volume method is used to discretize the model equations. The fractional time step method is applied for solving reactive flow. The Bi-Conjugate method is used to solve the algebraic equations obtained from discretization. The two reactors are simulated in a sequential way in the model. The connections between the two reactors are modelled through the use of time-dependent inlet-outlet boundary conditions. Two widely used oxygen carriers (OCs), Cu- and Ni-based particles, were selected for the numerical analysis. The model developed in the current study was validated against the experimental data provided by SINTEF Energy within the BIGCLC project and the open literatures. In general, the simulated results were in reasonable agreement with the available experimental data. The flow and chemical process performance of each reactor was examined and the effect of different operating conditions on the chemical process were investigated. Moreover, model sensitivity studies were conducted regarding the inter-phase drag coefficient, the inter-particle restitution coefficient, the radial distribution function and the wall boundary conditions for the solid phase.