Oxy-fuel combustion is considered as an important technology option for carbon capture and storage (CCS) from existing and new-build power plants. The understanding of heat transfer aspect underpins oxy-fuel technology demonstration and deployment. The aim of the paper is to address the role of emissivity in oxy-fuel combustion heat transfer as furnace sizes scale up. The emissivity of combustion products including gases and fly ash particles are considered for one coal fired in three different oxy-fuel furnaces, and compared with air fired furnaces. A well stirred reactor model which takes into account the impact of furnace volume and surface area as furnace sizes scale up is used. It is concluded oxy-fuel combustion can achieve the same heat transfer rate as air-firing, and lower volumetric gas flow rate, lower adiabatic flame temperature, lower flue exit gas temperature, due to higher emissivity in oxy-fuel firing. As furnace sizes scale up, combustion gas temperatures in oxy-fuel combustion increases, but lower than gas temperatures met in air-firing in the furnaces of the same scale.