This work performs numerical simulations of fluidized beds under different conditions of pressure and temperature and using air and CO2 as fluidizing agents. The conditions of high temperature and pressure tested turn the CO2 into supercritical conditions, so the differences when the fluidizing agent is at supercritical conditions are also tested. The results show that when pressure and temperature are increased, fluidization with air or CO2 shifts from the typical bubbling fluidization characteristic of ambient conditions, to a more homogeneous fluidization where not only bubbles and dense phase are present in the bed, but also a dilute phase of moderate solids concentration. The main consequence is an increase of the lateral motion of gas and solids at high pressure and temperature. A deviation from the classical Two-phase theory occurs because the gas velocity through the dense phase at high pressure and temperature is higher than the corresponding minimum fluidization velocity.