Multi-scale analysis and optimization of chemical looping gasification of biomass: Macro-scale simulation
Timo Dymala, M.Sc.
Subproject A: Modelling of fluid dynamics and chemical reactions
1. Problem description
Essential for a sophisticated model of the gasification of biomass inside a plant is the simulation of the hydrodynamics to evaluate the performance of the process. Characteristic for fluidized beds are the complex hydrodynamics as a result of intensive interactions between fluid and particles. Therefore, the computational fluid dynamics (CFD) are needed to allow the numerical simulation of the fluid dynamics. For the investigation of the particle dynamics during the process two main approaches are suitable: The Euler-Euler and the Euler-Lagrange approach. The Euler-Euler approach, or two-fluid model (TFM), assumes the fluid as well as the particle flows as fluids, whereas in the Euler-Lagrange model a discrete particle model (DPM) is used. This results in a multiple higher accuracy of the Euler-Lagrange model but also enormously high computational costs. Therefore, it is currently nearly impossible to accurately simulate industrial sized applications with reasonable computational time.
A modification of the Euler-Lagrange approach results in the Multi Phase - Particle in Cell (MP-PIC) method. The fluid is described by the Navier-Stokes equations and a defined number of particles with the same properties are represented by so-called parcels, which are modelled by using Lagrangian computational particles. This facilitates the implementation of various effects like particle size distributions as well as shrinkage effects and reduces the computational costs compared to the Euler-Lagrange approach while increasing the accuracy of the simulation compared to the Euler-Euler approach. This is expected to allow the simulation of industrial plants with reasonable computational time.
The first step of this subproject is to model the cold plant behavior and to validate the results with the experimental data measured at the Southeast University. For this purpose, the commercial software Barracuda VR® is used and suitable drag models for the different particles species have to be chosen.
In the second step the behavior of the hot plant including the chemical reactions and volatile release as well as breakage and shrinkage of biomass pellets based on experimental data and correlations of subproject B has to modelled. This can be achieved by the implementation of user-defined functions (UDF) using the open source software OpenFOAM®.
The aim of the study is to optimize the CLG process and detect suitable operation conditions, e.g. for the gasification temperature, ratio of bed materials, syngas composition and the OC lifespan.
We gratefully acknowledge for the ﬁnancial support the German Research Foundation (DFG) (Germany).
Project number HE 4526/21-1. Project start: May 2018.