Simulation of Oxy-Coal Combustion in a 100 kWth Test Facility Using RANS and LES: A Validation Study

Oxy-fuel combustion of solid fuels, often performed in a mixture of oxygen and wet or dry recycled carbon dioxide, has gained significant interest in the last two decades as one of the leading carbon capture technologies in power generation. This paper presents a numerical study on oxy-fuel combustion of lignite coal in a 100 kW(th) test facility using Large-Eddy Simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) approaches, along with radiation and char combustion submodels properly modified for the CO2-rich environment. The performance of different turbulence models is investigated by comparing their predictions with experimental measurements of velocity and species concentrations as well as gas and particle temperatures. Results show that although agreeing reasonably with the measured mean axial and tangential velocity, all the RANS turbulence models used in this study underestimate the internal recirculation zone size and the turbulence mixing intensity in the char combustion zone. The standard k-epsilon and RNG k-epsilon models with default model constants fail to predict accurately the flow and mixing process associated with the staging stream and perform poorly on the oxygen concentration prediction. The SST k-omega model captures most of the flow regimes and improves the prediction of oxygen diffusion than other turbulent-viscosity models. LES can resolve some of the large-scale turbulent structures of the swirling flow in the burner quarl and of the staging stream downstream of the burner, better matching the measured internal recirculation zone size, the entrainment of oxygen from the staging stream, and the overall flame length than the RANS approaches. The unsteady LES approach captures the dynamic characteristics of oxy-coal combustion, such as the attached volatiles flame and the mixing between the recirculated hot gases and the unburned burner streams as well as the stable coal particle devolatilization and ignition in the quarl, which indicates good flame stability. The simulation shows that oxidation reactions dominate char consumption in oxy-fuel combustion. However, gasification reactions can be important locally in the fuel-rich zone of the volatiles-flame and the char-combustion regions, where the temperature is high and the oxygen concentration is low.