CFD modelling of air staged combustion in a wood pellet boiler using the coupled modelling approach

Abstract

Optimal geometric and process parameters related to air staging, which ultimately lead to lower pollutant formation and a higher thermal efficiency, can be found by utilisation of the CFD simulation method. Simplified and computationally efficient methods for the simulation of complex phenomena which occur in a biomass boiler can, therefore, be of great use, provided they offer reliable and reasonably accurate predictions. A previously developed and validated Computational Fluid Dynamics combustion simulation methodology for large-scale waste to energy grate-fired boilers is modified and implemented for the case of a commercial 32 kW wood pellet hot water boiler. An experimental investigation is performed in order to determine process parameters used in the fuel conversion modelling, case setup and, finally, to enable the validation of the numerical modelling approach. The solid fuel conversion is modelled by a coupled empirical 1D bed model which predicts the composition of the combustible gas mixture released from the fuel bed into the freeboard. Compared to the originally proposed fuel bed conversion model, the modified model includes partial char and volatile oxidation, and relies on gas temperatures measured above the fuel bed. Using the modified fuel bed conversion model, several cases were simulated in order to find the most appropriate parameters of the combined finite rate – eddy dissipation combustion model. Results show that by implementing the modified fuel bed conversion model that relies on measurements, reasonably accurate predictions can be achieved of the gas temperature in the combustion chamber and CO concentration at the flue gas outlet.