Large eddy simulation of air and oxy-coal combustion

Abstract

The main aim of the present work is to investigate the applicability of Large Eddy Simulations to pulverised coal combustion. The Navier-Stokes equations that describe an incompressible tur- bulent reactive flow are presented, with a source term which ac- counts for the effect of the coal particles on the gas phase. Both a Eulerian and Lagrangian approach are presented to describe the coal particles motion and their heat exchange with the gas phase. The main processes that characterise pulverised coal combustion: devolatilisation, volatile combustion and char combustion are described and the main models to represent them are presented. The performance of the numerical approximation is tested on two main experimental cases: 1) a pulverised coal jet flame surrounded by a methane pilot and 2) a 100 kWth swirling burner operating in an O2/CO2 environment. The results of the simulations are com- pared to qualitative and quantitative experimental measurements for both test cases. Finally a parametric study is performed on both test cases to understand firstly, which combustion processes are dominant and secondly to understand which models perform best for each experimental set-up. The results showed the Lagrangian approach to be more representative of the pulverised coal combustion process. The analysis for the pulverised coal jet flame, showed that the radiation and char combustion processes have almost negligible effect. Instead, the simulation results were highly sensitive to variations in the devolatilisation and volatile combustion models and model parameters. For the second test case, char combustion was dominant throughout most of the domain as the coal particles had a longer time to burn. The devolatilisation and volatile combustion processes were dominant at the initial stages of the combustion process and characterised the initial flame bhevaiour.