Coarse-grained CFD-DEM simulation of biomass gasification in a fluidized bed reactor

Abstract

Gas-solid fluidized beds play an important role in many industrial operations. However, there still is a lack of knowledge concerning the processes inside the bed, which impedes proper designing and upscaling of fluidized bed reactors. In this work, biomass steam gasification in a fluidized bed reactor is investigated with a coarse-grained CFD-DEM approach.The numerical model in the Eulerian-Lagrangian framework treats the gas phase as a continuum and describes the particle interactions with the discrete element method (DEM). The non-spherical shape of the particles is accounted for in the momentum exchange calculation with the gas phase. The considered systems consist of steam as the fluidization gas, entering the bottom region of the three dimensional reactor geometry through inclined nozzles in an inner duct at the center axis. The wood pellets are fed into the reactor from the top of the freeboard together with a nitrogen purge gas stream. Over time, different operation modes occur, since there is no solids removal from the gasifier and unconverted material is continuously added to the bed.Three different operation modes are investigated. For the start-up procedure, the reactor is filled with 2.565 kg of sand particles. The inert material is allowed to settle for three seconds and is fluidized for another three seconds until quasi steady-state is achieved. Afterwards, the wood pellets are added for 35 s at a rate of one pellet every two seconds and undergo the processes of heating, drying, pyrolysis and char conversion. From the simulation results and literature correlations, the average time intervals for pyrolysis and char conversion, as well as the respective particle properties (temperature, diameter, density) are estimated. With the knowledge of the average composition of reactive material in the reactor, variable operating points can be simulated.The other two cases investigated are after approximately 20 and 50 h of operating time. Additionally to the start-up procedure, 0.6 and 1.5 kg of inert residue are added to the sand particles. Again, the inert material is allowed to settle for three seconds and is fluidized for another three seconds. Subsequently, the average reactive material obtained from the start-up simulations is patched into the fluidized bed and wood pellets are fed to the freeboard.The following phenomena are implemented in the numerical model: gas-solid momentum exchange, solids collisional behavior, heat and mass transfer, particle shrinkage and change in material properties, pyrolysis, as well as homogeneous and heterogeneous chemical reactions.Simulation results are analyzed both qualitatively and quantitatively. The particle flow fields and the mixing between sand, residue, and wood are investigated for different fillings of the reactor. Bed pressure drop, product gas composition, and temperature are compared to experimental data in order to validate the numerical model.The results show, that the bed pressure drop, the gas composition, and the conversion time obtained with the numerical simulation agree well with experimental observations and literature correlations. This indicates that the proposed model can make a significant contribution towards understanding and improving the internal processes in fluidized bed reactors for biomass gasification and combustion.