Dynamics of biomass conversion in a fixed bed — A comparison of different simulation methods based on the Eulerian–Lagrangian approach

The growing demand for eco-friendly and sustainable heat and power generation has sparked interest in small-scale biomass and biowaste conversion technologies. Gasification, a key method, offers low-emission fuel production by converting materials into liquid or gaseous fuels. Owing to limited measurement capabilities and time- and money-consuming experimentation, numerical modeling is an invaluable tool in recognizing the phenomena comprising the process and predicting the gasifier performance. The goal of the work is to analyze the dynamics of biomass conversion in a batch downdraft gasifier while taking into account the movement of particles shrinking due to thermal decomposition. The focus is on the process dynamics in the upper reactor, where the associated change in the particle size and bed structure are significant. Two simulation methods, based on the coupled Euler–Lagrange approach, are utilized. First, we developed a novel Dynamic Coupled Eulerian–Lagrangian (DCEL) method implemented in Fortran and Python. It solves the mass balance for bed sublayer-representative solid particles and the energy balance for the packed bed. Using 2D modeling, this approach effectively captures thermal effects from conduction, radiation, and fuel decomposition. The second one involves 3D modeling and employs the extended discrete element method (XDEM) that, coupled with Computational Fluid Dynamics (CFD), can predict the movement of the solid fuel particles undergoing thermal conversion in an external fluid field. The distribution of temperature and pyrolysis gas yield and the movement of the fuel particles, predicted via utilizing both methods are compared and discussed. Though some discrepancies were observed between the models’ predictions as regards heat transfer and fuel mass loss rates, both approaches demonstrate the importance of accounting for particle shrinkage and their corresponding movement in the simulation of fuel conversion in a fixed bed.