Transient simulation using Large-Eddy Simulation of multiphase coal/biomass gasification has been conducted to evaluate the design of an entrained flow reactor to produce syngas. This work considers fast and slow fuel devolatilization, volatile and oxygen transport, chemical reaction and heat release, soot formation, diffuse radiation in the combustion zone and radiation to surrounding objects. Reaction rates and radiation parameters are based upon a local equivalence ratio calibrated against experimental data. This work evaluates reactor design for various co-firing conditions with model verification performed by comparing predictions to measured values of an experiment published by the Advanced Combustion Engineering Research Center. Results confirm the importance of small particles in flame stabilization and overall combustion efficiency. •A multi-physics transient combustion model for coal-biomass co-firing is discussed.•The LES based transient coal-biomass model correctly identified impact small particles have on combustion efficiency.•The LES based coal combustion model runs on standard engineering workstation making it useful for industrial applications.•The model supports increased use of biomass to reduce fossil fuel use.