Validation of a numerical approach for simulation of the thermal decomposition behaviour of biomass in grate combustion plants

The overall objective of the modelling work is to develop CFD aided design tools for optimum grate fired biomass-to-energy (BtE) and waste-to-energy (WtE) plants. An important part of this work has been to develop a flexible detailed transient fuel-bed model taking into account drying, pyrolysis, and char combustion/gasification, for different fuels (i.e. MSW fractions, and softwood and hardwood, including their bark and GROT (branches and treetops)). The fuel-bed model has been implemented in a CFD tool, ANSYS Fluent. The fuel bed consists of a large number of thermally thick particles. In this work, the fuel bed model is made up of representative particles, and the motion of every representative particle is individually tracked (Lagrangian tracking through Fluent's Discrete Phase Model). Thermochemical degradation and conversion of the representative particles are calculated by a thermally thick single particle model (SPM), with boundary conditions obtained from the solutions of the gas phase equations. The SPM model then provides sources to the gas-phase equations. In the modelling approach, the gas phase is solved using the Reynolds Averaged Navier-Stokes (RANS) equations. Under the given conditions, the gas flow in the bed is laminar. The developed model was validated against detailed experimental results from pyrolysis of dried spruce wood pellets in an electrically heated fixed bed reactor, with varying final pyrolysis temperature (600-800°C), heating rate (5-20 K/min) and purge gas composition (none, 100% N2 and 90/10% N2/O2). The experimental results included transient temperature measurements in different locations in the reactor and inside the pellets bed throughout the thermal decomposition process, as well as gas measurements of permanent gases. Through the CFD simulations, the main experimental trends could be reproduced, verifying the validity of the detailed modelling approach. This work is a step towards detailed modelling of biomass grate combustion units, which is required to improve their environmental and energetic performance.