A Premixed Flamelet−PDF Model for Biomass Combustion in a Grate Furnace

Abstract

In this paper, a new modeling approach for biomass furnaces is presented. A flamelet model is applied for the prediction of combustion and NO x emissions in a pilot-scale low-NO x biomass grate furnace. The model describes the combustion chemistry using premixed flamelets. The chemical system is mapped on three control variables—the mixture fraction, enthalpy, and a reaction progress variable—by means of the flamelet-generated manifold chemical reduction technique (FGM). The density and source terms are tabulated as functions of the control variables in a preprocessing step to speed up the numerical calculations. The turbulence—chemistry interaction is described by an assumed shape probability density function (PDF) approach. Generally, transport equations are solved for mean and variance of mixture fraction, mean progress variable, and the mean enthalpy. The FGM-PDF model is first validated for a diffusion flame: Sandia Flame D. Good agreement of predictions with measurements is found. The model is then applied to a 2D cross section of a biomass combustion grate furnace. FGM-PDF is compared with the standard eddy dissipation concept model (EDC). The predictions of the two models are similar. The FGM-PDF model reduces the calculation time with GRI-Mech2.11 reaction mechanism from weeks to hours, when compared to EDC. Furthermore, in FGM-PDF more physics can be taken into account thanks to the integration of flamelets in the turbulence—chemistry interaction.