The co-firing of pulverised coal/biomass in power generation plants is receiving considerable attention due to its influence in reducing all forms of emissions. Unlike coal, milled biomass, such as straw, may contain large particles of different sizes and shapes, which can have an impact on the combustion characteristics and emissions. Computational fluid dynamics (CFD) is often used to understand the influence of large biomass particles in a furnace. However, most CFD sub-models simplify heat transfer effects within the particles during combustion. In this paper a particle heat-up model, which considers the influence of thermal gradients within large biomass particles, is applied to a co-firing coal/biomass simulation in a tangentially fired furnace with up to 12% thermal biomass loading. Different sizes of biomass particles of non-spherical shape and their impact on the combustion behavior have been investigated. The influence of the particle size and shape distribution on the combustion characteristics and emissions was found to be significant. The computed results were found to be in good agreement with the experimental data.