Grid-independent Eulerian-Lagrangian approaches for simulations of solid fuel particle combustion

Jingyuan Zhang,Tian Li,Henrik Ström,Terese Løvås

doi:10.1016/j.cej.2019.123964

Abstract

In this study, a computational fluid dynamics (CFD) model with three coarse graining algorithms is developed with the implementation of a layer based thermally thick particle model. Three additional coupling methods, cube averaging method (CAM), two-grid method (TGM) and diffusion-based method (DBM), are implemented. These coupling methods are validated and compared with the widely used particle centroid method (PCM) for combustion of a biomass particle in a single particle combustor. It is shown that the PCM has a strong dependence on the grid size, whereas the CAM and TGM are not only grid independent but also improve the predictability of the simulations. Meanwhile, a new parameter, the coupling length, is introduced. This parameter affects the sampling of the gas phase properties required for the particle model and the distribution of the solid phase properties. A method to estimate the coupling length by using empirical correlations is given. In general, it is found that a too small coupling length underestimates the heating-up rate and devolatilization rate, while a too large coupling length overestimates the O2 concentration at the particle surface. The coupling length also has an influence on the distribution of the gas phase products.

Highlights

Direct combustion of solid fuels, such as coal and biomass, is one of the main routes to generate heat and electricity [1,2]
The three coupling methods, cube averaging method (CAM), two-grid method (TGM) and diffusion-based method (DBM) have all been extended for reacting particles, and are able to improve the grid independence of the computational fluid dynamics (CFD) solver
When linking the single particle model and the Eulerian gas phase model, the interaction between the particle and the gas phase is shown to occur within a certain coupling length scale

Summary

Introduction

Direct combustion of solid fuels, such as coal and biomass, is one of the main routes to generate heat and electricity [1,2]. To model the multi-phase combustion system in CFD simulations, the gas phase is usually described with a continuum approach in the Eulerian framework. Irrespective of the framework, single particle conversion models are normally required as sub-models to describe the thermal decomposition of the solid phase. These include the sub-processes of heating, drying, devolatilization and char burnout. Such single particle models use local operating conditions from the gas phase to predict heat and mass release from the particles as boundary conditions or source terms for the gas phase

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Results

Conclusion