Heat transfer in standpipe of circulating fluidised bed boiler

The paper presents experimental and theoretical investigations into heat transfer to cross-and vertical tubes in the standpipe of a circulating fluidised bed (CFB) boiler. Two separate test rigs (one to study the heat transfer on the wall and the other to cross-tubes) were used for this study of heat transfer to moving packed beds in the standpipe of a CFB. The height averaged heat transfer coefficient on the wall and the surface averaged heat transfer coefficient on the tubes are comparable at a given solid flux through the standpipe. Heat transfer coefficient on the wall reduced from the top to the bottom, and the rate of decrease reduced with height as generally observed on the furnace wall of a CFB riser. Analysis of heat transfer around the tube showed that heat transfer is the lowest at the bottom of the tube due to the presence of an area of stagnant particles resembling a wake. Owing to the increased residence time of particles on the surface of larger diameter tubes, an increase in the tube diameter caused a decrease in the heat transfer coefficient at all flowrates. The models proposed in this paper show good agreement with the experimental results from both cross- and vertical tubes.