DEVELOPMENT OF AN OPENFOAM SOLVER FOR CARBONIZATION OF PACKED BEDS IN INDUSTRIAL REACTORS

Thermal conversion of biomass is a key process to take energetically benefit from high moisture content biomass and biogenic wastes or recycle them for further applications. Rotary drum reactors are known due to their flexibility in various process conditions and reliability on quality control of the product. Steam assisted carbonization is a kind of slow pyrolysis, in which wet biomass is treated continuously in a superheated steam atmosphere at elevated temperature and atmospheric pressure. A numerical simulation tool using OpenFOAM has been developed to simulate the carbonization process in the reactor resolved in time and space. This study attempts to demonstrate the features of this refined and validated an Eulerian-Lagrangian approach for dense particulate flows in order to simulate efficiently a multiphase flow behavior in large-scale rotary drum reactors. For the particulate flow, a Lagrangian approach is known as Multi-Phase Particle-In-Cell (MP-PIC) model from O’Rourke et. al. is used which is available in OpenFOAM. The model provides two-way-coupling where the energy equation is solved for multicomponent particles. Furthermore, the applied solver has been extended with a four-way-coupling method using an isotropic particle stress gradient model from Harris and Crighton as the core of particle-particle interaction. The latter model is also developed by O’Rourke et. al. and is available in OpenFOAM, however only for isothermal conditions and incompressible formulation of the Navier-Stokes equations. The modified solver enables calculating flows with any particle volume fraction from the dilute to the close-pack limits in the kiln and it includes coupling of heat and mass transfer between phases. An Eulerian method solves the Navier-Stokes equations in compressible formulation for the gas-phase, the energy equation and the balance equations for chemical species considering drying and devolatilization of the particulate phase. For the radiative heat transfer in the kiln, an adjustment is suggested for the existing discrete ordinate model (DOM) in order to consider the shadowing of particles on each other. The high moisture content in the biomass feedstock can create fully saturated or pure steam conditions in the freeboard gas. In this case, existing vaporization models in OpenFOAM, which are based on diffusion law and flash-boil vaporization, needed to be extended to take the constant temperature vaporization into account. Furthermore, an n-th order kinetic devolatilization model is implemented in OpenFOAM for the modeling of devolatilization based on the experimental work of Bockhorn et. al. The validation of the model is performed by a series of simulations of a laboratory-scale rotary kiln for spherical and non-spherical biomass particles. Residence time, mixing degree, convective and radiative heat transfer coefficients and devolatilization are validated individually with experimental measurements. Thereafter, the computational effort has been optimized and a full-scale industrial size rotary kiln has been simulated running the code on a supercomputer. The results have been used to optimize the wall temperature of a 180 kWth rotary kiln based on the characteristics of the biomass throughput.