Abstract Fluidisation and mixing of sand and biomass particles have significant effects on product yields in bubbling fluidised bed reactors (BFBR). There is a lower limit for the velocity of fluidising media known as minimum fluidisation velocity. Similarly, an upper limit is defined for velocity where the bio-oil production is maximised – known as the maximum effective velocity (MEV). A computational fluid dynamics (CFD) simulation for biomass fast pyrolysis process in a 2-D lab-scale BFBR is developed to analyse the variation of MEV as the sand particle size varies. The model is first validated using the experimental data. Then, a parametric study is conducted for the carrier gas velocities in a range of 0.3–1.1 m/s where the sand particle sizes vary from 0.4 to 1 mm, and the biomass particles are in a range of 0.2–0.5 mm. Effects of the packing limit are also analysed. For this purpose, the different sand particle sizes and their corresponding MEV are tested at the sand packing limits of 45, 55, 65, and 75 mm in which the optimal packing limit is found to be 55 mm. A detailed thermodynamic study is performed with the focus on the sand particle size and packing limit affecting the heat transfer rates between phases. A decrease in required heat transfer rates is directly linked to an increase in bio-oil yield. It is observed that heat transfer between sand-biomass is much more efficient than the heat transfer between nitrogen-biomass, confirming the importance of the sand particles as the heat carriers.