Study on Heat Transfer from a Bubbling Fluidized Bed Combustor to a Membrane Wall

Abstract

The importance of projection and splashing of bubbling bed particles on the heat transfer rate towards a membrane wall heat exchanger placed above the bed was studied.To characterize the heat transferred from a bubbling fluidized bed to a membrane wall heat exchanger placed above the freeboard, a laboratory scale fluidized bed reactor, heated by a 2 kW electric resistance, was used. The reactor, with an internal diameter of 54.5 mm, had two 0.83 m height double pipe heat exchangers placed one above the other. Only the heat exchanger close to the bed surface participated in the heat exchange process. Tests were done without and with combustion.For experiments without combustion the bed was fluidized with air at superficial velocities of 0.2 to 0.5 m/s in the 400-700 °C temperature range. For experiments with combustion, the fluidizing gas was obtained through propane combustion in the bed in 700-720 °C temperature range, for superficial velocities of 0.2 to 0.3 m/s. Five different bed particle sizes were considered: 107.5, 142.5, 180, 282.5 and 357.5 µm.Particle convective heat transfer coefficients were obtained in the range of 2 to 16 W/m²/K and a correlation for the corresponding Nusselt number as a function of the fluidization characteristics and the combustion equivalence ratio is proposed.For the three smaller bed particle sizes, the particle entrainment ratio had a strong influence on the heat transferred towards the membrane wall and the corresponding bed particle entrainment rate was determined and correlated with the fluidization characteristics and the combustion equivalence ratio.