Large eddy simulations of swirl-stabilised gas-assisted oxy-fuel biomass flames under varying oxygen concentrations

Biomass combustion with oxy-fuel technology offers a promising route to negative CO X 2 emissions by enabling efficient energy conversion with carbon capture from a renewable fuel source; however, its operational behaviour and flame stability remain insufficiently understood, requiring further fundamental investigation. This study applies high-fidelity simulations with advanced submodels to examine the impact of varying oxygen concentration on pulverised oxy-fuel biomass combustion. Based on Large Eddy Simulation (LES), a swirl-stabilised, gas-assisted biomass combustion chamber is analysed to enhance the understanding of flame stabilisation, mixing, reaction regions, and particle conversion. The LES employs advanced and fully-coupled models for solid-fuel conversion, radiation, and gas-phase chemistry, previously developed for coal combustion and now extended to pulverised biomass. Walnut shells are selected as a reference fuel due to available characterisations and validated solid-fuel kinetics. Particles are tracked in a Lagrangian framework, and their conversion is computed using state-of-the-art devolatilisation and char models calibrated for the local conditions with detailed seamless biomass kinetic model. Radiative heat transfer is resolved by coupling the finite-volume discrete ordinates method with the weighted-sum-of-grey-gases model and particle radiation, taking emission and forward scattering into account. Gas-phase reactions are modelled using six-dimensional tabulated flamelet manifolds tailored to the respective oxidiser compositions in the investigated operating conditions. Three operating conditions are compared to investigate the effect of varying atmospheres while keeping the equivalence ratio and total oxidiser flow constant: oxy-fuel atmospheres with 27 and 33 vol.% O X 2 and air as a reference case. Validation against an extensive experimental data set, including velocity fields from particle image velocimetry (PIV) and temperature fields from tunable diode laser absorption spectroscopy (TDLAS) and coherent anti-Stokes Raman spectroscopy (CARS), shows good agreement and demonstrates the applicability of the employed models to biomass oxy-fuel combustion. Detailed analyses of flame shape, particle mass release, reaction zones, and particle conversion are combined to explain the interactions between gas assistance, swirl stabilisation, and solid-fuel behaviour. The simulations reveal that the side flame, which stabilises only in the diffuser for certain oxidiser compositions, strongly influences global flame stabilisation mechanisms but has a minor impact on the overall particle conversion. • The study confirms that oxy-fuel coal combustion models can be reliably applied to biomass systems. • Large eddy simulations of swirl-stabilised, gas-assisted biomass combustion under air and oxy-fuel conditions were conducted. • Flame stabilisation and particle conversion were analysed for varying oxygen levels. • The simulations show strong agreement with an extensive experimental dataset.