Investigation Of Convective Heat Transfer In A Large Circulating Fluidized Bed

This article primarily aims to study the convective heat transfer characteristic of suspended surface and mid-board in circulating fluidized bed (CFB), basing on the background of large supercritical CFB boilers.As a clean combustion technology, CFB boiler has scaled up and its steam parameters have advanced towards supercritical level, in order to improve its generating efficiency. In large CFB boilers, additional heating surface as suspended surface has been installed to balance the extra amount of heat. Consequently, the heat transfer characteristic of suspended surface in large CFB is very important for the design and safe operation of the boiler. However, relevant research is little.Convective heat transfer of water wall, suspended surface and mid-board were studied experimentally in a CFB cold test rig with six cyclones and a pant-leg. Electrical heating copper plate was hung in the top region of the riser to simulate suspended surface and mid-board in furnace. The convective heat transfer coefficient was calculated based on energy-balance. The experimental result shows that the convective heat transfer coefficient of the three different heating surfaces all increase with an increase in the suspension density; the superficial gas velocity has no significant effect on the convective heat transfer coefficient by itself, however it influences the heat transfer coefficient by its impact on suspension density. Among the three different heating surfaces, the water wall has the highest convective heat transfer coefficient and the suspended surface has the lowest.Heat transfer mechanism of water wall, suspended surface and mid-board was further studied by the cluster renewal model. The parameters of the cluster renewal model were revised indifferently according to the experimental result in order to apply to different types of heating surfaces. The calculation with cluster renewal model shows that cluster coverage rate of the heating surface increases with the increment of solid volume fraction, which leads to a higher convective heat transfer coefficient.Lastly, empirical equation has been concluded basing on the experimental result, which will provide reference for the prediction of the heat transfer coefficient of CFB plants.