Radiation modelling considering burnout-dependent properties and cellwise non-uniform particle distributions in the numerical simulation of pulverised solid fuel combustion

The present study evaluates the impact of the gas and particle radiation on flame characteristics of a pulverised solid fuel using Reynolds-averaged Navier–Stokes (RANS) equations. As a reference, a pilot-scale combustor with a 60 kW th flame is used. The burner is fed with pulverised (10 – 180 μ m) Rhenish lignite particles under oxyfuel conditions (25/75 vol % O 2 /CO 2 ). CFD simulations are carried out using Ansys Fluent equipped with user-defined functions (UDFs), e.g., for gas and particle radiative properties and kinetic models (devolatilisation and char conversion) adapted for oxyfuel conditions. Particular focus is placed on evaluating detailed modelling of the particle radiative properties, which are determined with the aid of Mie theory and taken into account in the simulations via UDFs as tabulated data. For this purpose, simulation results for the reference case with constant particle radiative properties (a common assumption in the relevant literature) are compared to those obtained with burnout-dependent particle radiative properties (determined using the Mie theory — Mie case) as well as to those obtained considering the effect of cellwise non-uniform distributed particles on the burnout-dependent particle radiative properties (within the framework of a recently proposed weighted-scaling approach — WSA case). Simulation results show that the role of particle radiation is more pronounced in the near-burner region, such that comparisons of the predicted temperatures in different models signify high local temperature differences. The importance of particle radiation reduces with the axial distance from the burner until the differences in the predicted temperatures in all three cases become negligible. In the near-burner region, predictions of the reference case on the particle radiative fluxes are much smaller than those of the Mie and WSA case. These differences lead to high local temperature differences, especially where strong temperature gradients exist. • Simulation of pulverised fuel combustion with adopted gas and particle radiation. • Tabulated burnout-dependent particle radiative properties obtained by the Mie theory. • Cellwise non-uniform particle distribution effects on particle radiative properties. • Required level of detail when modelling the particle radiative properties. • The locally varying influence of particle radiation on the flame characteristics.